Starting & Reversing Problems in Marine Engineswritten by: Willie Scott edited by: KennethSleight updated: 11/14/2011There are a number of reasons for starting and reversing problems in marine engines. This malfunction is one of the most frightening and dangerous situations to encounter when maneuvering a ships main diesel engine, but it can be avoided through regular maintenance of the air start components. A ships main marine diesel engine is started on compressed air that is controlled by various components of the air start system. It is a well-tried and tested reliable system, but it can go wrong if not properly maintained.The following sections examine a typical air start system, with the first section providing an overview of the system. Overview of SystemThe air start system looks rather complicated, but it is quite simple when you examine it without the safeguards. These are put in place to prevent such occurrences as starting the engine without having a signal from the engine room telegraph, trying to start the engine with the turning gear engaged, or trying to start ahead when the telegraph asks for astern. There are also safety systems incorporated such as a bursting disk and numerous non-return valves in the event of a leaking air start valve.The next section lists some of the problems that can be encountered when maneuvering. Problems in Air Start SystemsWe shall look at two common problems encountered when maneuvering the main engine: not starting and starting in the wrong direction (reversing instead of starting ahead). Not Starting As we have seen, there are various interlocks in place to prevent the engine being started until certain criteria are met. If the engine wont turn over on air, the bridge should be informed then the following checks should be carried out.3. Check air start supply valves from air receivers are open and that the pressure is 30 bar.3. Check that the turning gear is disengaged3. Check that the turning gear and telegraph solenoid valves have actuated. This will supply air to the automatic valve, air distributer, the air manifold, and air start valve.These are the initial checks that can be quickly carried out. If these are all satisfactory, then the problem lies in the controls ahead/astern solenoids. The air distributer or the air start valve itself may be stuck in the closed position. The ship will need to anchor or be towed alongside for these checks to be carried out.Engine starts in wrong directionIf the engine starts in the astern instead of ahead direction, the following checks should be carried out.3. Ensure the air start control moves to reverse mode at the control station. This is a visual check and can be observed when the telegraph rings from ahead to stop then astern. If this does not happen, the solenoid valve may be stuck.3. The oil and air supply to and from the reversing valve should be checked. A blockage of either will stop the reversing servo motor operating and allowing change over from the astern to ahead position.This again will take further investigation, so the ship should anchor or remain tied up to the quay.As this ahead/astern changeover is controlled by lube oil and compressed air and is interlocked with the fuel pumps. These are the usual culprits and the starting point of a thorough investigation. I have experienced this situation only once and fortunately we were leaving port and still tied to quay by stern spring. Once the bridge was informed, a rope from the focsle was thrown ashore and made fast. This gave us the chance to check for the fault, which turned out to be the oil supply from the crosshead oil supply pipe being blocked.As I have said before, the maintenance of the air start system components is paramount to the operation of the system.1. Main Components of the SystemAir supply system4. Two air compressors4. Two air start vessels4. Numerous non-return valves4. Numerous drain valvesControl system4. Turning gear out sol v/v4. Telegraph signal sol v/v4. Automatic valve4. Ahead and astern change-over4. Air distributer4. Air start valveAnti-explosion components4. Air supply to manifold from air vessels non-return valve- this prevents hot gasses from returning to air receivers.4. Air manifold pressure relief valve this operates if pressure rises due to heat from gasses.4. Air supply to air start valve bursting disk this disk ruptures under increased pressure caused an air start valve leaking back.1. Mandatory Safety PrecautionsBefore we get into the operation of the system in the next section, this is an opportune moment to make a closer examination of the precaution against explosion, which is a very real threat even in todays modern engines that incorporate the latest in engine management systems.5. CompressorsThe compressor air inlet filters should be positioned in an oil-free zone, i.e. no oil fumes should be present.The compressed air supply lines to the air receivers must be protected by non-return valves.5. The air receiversThere are two air receivers, linked by a common discharge pipe to the system. The air from the compressors will contain oil and water (there is no way around this). This mixture ends up in the air vessels as a mist, eventually settling to the bottom of the vessel. It is imperative, and I cannot overstress this, that the mixture be drained from the vessels after every charge, and regularly when maneuvering. The oil also coats the internal of the supply pipes; this too can be reduced by draining the air vessels.These actions, as well as checking by hand for heat in the air supply pipe between the air start valve and the air manifold, form part of the watch keepers duties. Any excess heat here, and the fuel and air to that particular cylinder should be isolated, and the bridge made aware of the situation.Before we leave the precautions there are many examples of air start system explosions. One of worst ones occurred on the MV Capetown Castle, killing seven engineers. Lloyds register recorded 11 such explosions between 1987 and 1998; all down to oil gathering in the receivers and piping and ignited by exhaust gasses. One a year speaks for itself: drain the air vessels regularily and maintain the system.A sketch showing an air start system where the air start valve is leaking is shown below. Note the pipe that should be checked by hand for overheating;1. 1. The Operating Principles of Marine Engine Air Start SystemsI have sailed on quite a few marine diesel engines, including B&W, Sulzer, and Stork/Werkspoor. All had variations of the system I am about to describe, but the principles are much the same.I drew a sketch from memory (45 years ago) but updated it from a very good website referenced at the end of the section. The sketch also appears at the end of the section and can be referred to during the reading of the notes.We begin then with the bridge ringing down standby on the engine room telegraph. (We used to change over fuel from Heavy Fuel Oil to Modified Diesel Oil for maneuvering.)1. If in port, ensure turning gear is not engaged.2. Open both air receivers isolation valves and start up a compressor to fill receivers to maximum; drain oily water of reservoirs and also from dead leg on supply pipe work.3. This allows the compressed air to flow as far as the turning gear solenoid valve. Provided the turning gear is disengaged, this will allow the supply of air at 30 bar to the automatic valve passing though the non-return valve and into the manifold. From here the air is piped to the air chamber in the air start valve. (This is the pipe that will get hot if you have a leaking air start valve.) The valve is held in the shut position by the spring tension.4. When an ahead or astern movement is rung and answered on the engine room telegraph, the telegraph start signal sol v/v is activated allowing air to the ahead and astern solenoid valves mechanism.5. The air is now directed to the starting air distributer that is fitted on the end of the camshaft. This enables it to select the appropriate cylinder(s) to supply air to. This will be the relevant cylinder that is just passed TDC and on the downward stroke.6. The air from the starting air distributer is at 30 bar, and this is injected into the air start valve top piston. This overcomes the spring tension and forces the piston downwards thus opening the valve and introducing the air at 30 bar to the cylinder(s) having been supplied earlier to the air chamber.7. Depending on the engine make and model, air can be supplied to several cylinders to assist starting. A "slow start" supply can be used if there has been a lapse of half an hour between movements when maneuvering.1. 1. Maintenance of System Components9. CompressorsRegular inspection of filters, suction and discharge valves, as well as piston and ring checks should be performed at the manufacturers recommended periods. Intercooler tube nests should be cleaned ensuring optimum air flow.9. Air supply Manifold Relief ValveThis should be regularly inspected to ensure that the spring is operating correctly, with the complete overhaul being to manufacturers instructions.9. Air Start ValvesThis is the most important component and if not maintained, will begin to stick due to a weak/badly adjusted spring or worn piston rings allowing hot combustion gasses into the compressed air piping.The valve should be replaced regularly with an overhauled and tested spare, the spare then being stripped and spring, pistons, and rings inspected. The valve is ground into the seat using fine lapping paste before rebuild and bench pressure testing.STARTING AIR VALVEAIR STARTING VALVE:The valve is fitted into the cylinder head. It is opened by control air from the starting air distributor. The valve shown is from a slow speed MAN-B&W two stroke engine but a lot of modern engines have valves working on similar principles and design.MaterialsThe body of the valve could be of mild steel, the spindle of high tensile or stainless steel, and the valve and seat could have the contact faces hardened.How it worksMain starting air at about 30 bar from the manifold enters the chamber above the valve via the circumferential ports in the valve body. The air pressure will not open the valve because a spring is holding the valve shut, an the area of the balance piston is the same as that of the valve lid so the valve is pneumatically balanced.When the valve is required to open, air at 30 bar from the air start distributor enters the the top of the valve body and acts on a piston which is bigger in size, This pressure overcomes the spring force holding the valve shut and pressure of trapped air from the main starting air valve, and the valve opens. When the air signal from the air start distributor is vented, the spring closes the valveWhen the start sequence is finished the main air start pressure is vented through holes in the main start air manifold.

The Sulzer air start valve uses air on both sides of the operating piston to maintain positive closing. The piston is stepped. The reason for this is so the starting air valve will not open when the gas pressure in the cylinder is higher than the starting air pressure; i.e. when the cylinder is firing. Once the valve starts to open then the opening is accelerated when the larger diameter piston has the opening air acting on it.MAINTENANCEAfter certain periods of service starting air valves are changed and overhauled. If piston rings are fitted, care must be taken to ensure that they are free in their grooves. Should it be necessary to fit new rings, the butt clearances of the rings must be carefully checked by placing the ring into the operating cylinder and measuring the clearance. This is especially important if they are usually made of brass which has a larger coefficient of expansion than the other parts of the valve. The valve and valve seat are ground with grinding paste and finished to a fine surface with lapping paste. It is essential to ensure that all parts of the valve are scrupulously clean before reassembly. Lubricate all sliding surfaces sparingly with a molybdenum disulphide grease.HOW TO TELL IF AN AIR START VALVE IS LEAKING OR HAS JAMMED OPEN?When an engine is in operation leakage of starting air valves is shown by overheating of the branch pipe connecting the starting air valve to the starting air rail. The heating occurs due to the leakage of hot gases from the engine cylinder into the starting air line connected to the starting air rail.STARTING AIR LINE EXPLOSIONLubricating oil from the compressor will under normal operation pass along the air lines and deposit on them. In the event of a cylinder air starting valve leaking, hot gases would pass into the air pipes and ignite the lubricating oil. If starting air is supplied to the engine this would further feed the fire and could lead to an explosion in the pipelines. In order to prevent such an occurrence, cylinder starting valves should be properly maintained and the pipelines regularly drained. Also oil discharged from compressors should be kept to a minimum, by careful maintenance.SAFETY DEVICES IN STARTING AIR LINEFlame ArrestorsThe flame trap is manufactured from brass or aluminium alloy which both have a high specific heat capacity. A number of holes are bored through the thick circular form to allow the air to pass through. They are fitted in the main air line immediately before the air start valve to restrict the risk of a flame in the cylinder propagating back to the main air start manifold, by dissipating the heat energy in the flame.

FLAME ARRESTORBursting Disks:The safety cap consists of a bursting disk enclosed by a perforated cylinder and a perforated cover in order to protect any bystanders, in the event of a burst. The cover is fitted with a tell tale, which shows if the bursting disc has been damaged. If the bursting disc of the safety cap is damaged due to excessive pressure in the starting air line, overhaul or replace the starting valve which caused the burst, and mount a new disk If a new disk is not available, or cannot be fitted immediately, then the cover can be turned in relation to the perforated cylinder, in order to reduce the leakage of starting air.

Relief ValveThe sketch shows a relief valve as fitted to the air start manifold of Sulzer RTA 2 stroke engines. Its purpose is to relieve excess pressure in the air start manifold. It consists of a spring loaded valve disk which locates on a mating seat which is bolted to the end of the air start manifold. When the force exerted on the disk due to excessive pressure is greater than the spring force holding the valve closed, the valve will open. NON RETURN VALVE:Non return valve is provided after the Automatic air starting valve to prevent any flow of air/ hot gas to the air reservoir.PRECAUTIONS:Starting Air System Precautions

Great care is to be exercised in the operation and maintenance of starting air systems. The hazard of compressed air and lubricating oil forming an explosive mixture must be avoided. Oil from any source must be excluded from the starting air system.

Air compressor, starting air reservoir blow down drains are to be operated at regular intervals, and if automatic, their function verified.

Periodic inspection of air starting system pipelines is to be carried out to ensure that no build up of oil is occurring. Highly flammable cleaning fluids must never be used in any part of the starting air system. Any residue of liquid or vapours could result in an explosion.

Routine duties must include the manual checking of the main air starting valve pipes for any increase in temperature, which would indicate leakage of combustion gasses into the system. It is particularly relevant during manoeuvring when the main air starting reservoirs are open to the system despite the existence of non-return valves and other devices.

It is absolutely essential that if an air starting valve is in any way suspect that immediate action is taken i.e. shutting the fuel off the unit in accordance with the manufacturer's instructions, "gagging" the valve shut, and replacing the valve at the first opportunity.AIR STARTING SYSTEM:Diesel engines are started by supplying compressed air into the cylinders in the appropriate sequence for the required direction. A supply of compressed air is stored in air reservoirs or 'bottles' ready for immediate use. Up to 12 starts are possible with the stored quantity of compressed air. The starting air system usually has interlocks to prevent starting if everything is not in order. Compressed air is supplied by air compressors to the air receivers. The compressed air is then supplied by a large bore pipe to a remote operating non-return or automatic valve and then to the cylinder air start valve. Opening of the cylinder air start valve will admit compressed air into the cylinder. The opening of the cylinder valve and the remote operating valve is controlled by a pilot air system. The pilot air is drawn from the large pipe and passes to a pilot air control valve which is operated by the engine air start lever.When the air start lever is operated, a supply of pilot air enables the remote/ automatic starting valve to open. Pilot air for the appropriate direction of operation is also supplied to an air distributor. Distributor is usually driven by the engine camshaft and supplies pilot air to the control cylinders of the cylinder air start valves. The pilot air is then supplied in the appropriate sequence for the direction of operation required. The cylinder air start valves are held closed by springs when not in use and opened by the pilot air enabling the compressed air direct from the receivers to enter the engine cylinder.An interlock(Turning Gear interlock) is shown in the remote operating valve line which stops the valve opening when the engine turning gear is engaged. This is a simplified diagram. Engine starting and reversing system has numerous small components and interlocks for safe and positive starting and reversing and fuel admission, which is given in Engine Instruction manual and is commonly known as Manoeuvering Diagram.

a trammel can be as simple as a flat bar bent at 90 degrees to form a pointer that rotates on a fixed point.The rim of a flywheel is usually marked into 360 degrees, showing the top and bottom positions of the various cranks. This is for convenience of setting the various valve timings. A stationary index arrow points at the flywheelIf the flywheel was not marked, the following procedure can be used:First turn the engine up to near the top for that cylinder and mark the guide and shoeThen with a trammel fixed on the column and long enough to reach the crank, mark the top of the crankNow turn the engine over the center until the marks on the crosshead guide and show come together and again mark the crank top with the trammelFind the center between the two marks on the top of crank and make a center markThen turn the engine until this mark comes into line with the trammel point, the engine will now be at top dead center for that cylinder

compressor: I think that the question may not be phrased correctly but I think it has to with changing out worn parts to decrease the volumetric clearanceThe wear of moving parts (main bearings, rings/liner) will increase volumetric clearance and reduces the volumetric efficiency of the cylinderAlso for an air compressor with a larger clearance volume, the outward travel of the piston will be greater before the pressure is low enough within the cylinder to allow the suction valves to openIn consequence, a large part of the suction stroke is made ineffective and the amount of air taken into the cylinder during each suction stroke is reducedA lot of research was done years ago into stopping supertankers when they were introduced, if engine speed or pitch was reduced too quickly on large ships the propeller cavitated and lost its grip on the water so you got no breaking affect at all. Generally, the bigger the ship, the lower the power to weight ratio and therefore the bigger the problem with stopping. Hence the idea of reducing engine speed in steps, half ahead, slow ahead etc however, the time / distance at each speed setting would depend on the ship and propeller design as well as the loaded condition. If you have a Torsion meter on the shaft you could attempt to gradually reduce the engine speed settings in a way that keeps the Torque from the propeller, into the engine at a maximum.On some engines you could select Astern while the engine was being driven ahead by the ship's momentume (but the fuel was off) you could then apply starting air, without fuel, to bring the engine to a stop, so that it could be started astern.On large ships if you put the directly to full aster you would again have cavitation problems and lose all your thrust so it is better to increase the astern power slowly, again a Torsion meter would be a great help although in practical terms in an emergency situation it might be difficult to be sure if the Torque was reducing because the ship was catching up with the engine or the opposite and cavitation was increasing!As Jolly Jack says frequent reversal of rudder direction would also slow the ship, although on some ships putting the rudder hard over at Full Sea Speed would probably rip it off! Hoever a steady Port 15, Starboard 15, Port 15 would have a significant braking effect without allowing the ship to go to far off course. This is also used as a measure to prevent pirates boarding from small craft, it increases the bow and stern waves making it harder to get alongside without being swamped, and absorbing lots of power.in an emergency how would you use start air to assist in braking a slow speed reversing engine when going from AHD to AST. HERE IS WHAT YOU DO!

For SULZER : - telegraph reply is moved immediately to AST- This will shut off fuel automatically- lever is moved to START- this helps to BRAKE ENGINE-ONCE DIRECTION IS REVERSED, fuel is put back on

For B & W :- ANSWER telegraph- Put fuel lever to STOP- When rev falls to about 40% - Air is put to assist in breaking- Once rev is seen in ASTERN DIRECTION- Fuel is put on

Essentially you need to look at everything that could give you grief, or that could get you detained at a future port. Think of it as doing your own port state or USCG inspection. In reality you would judge what is necessary by the state of the ship when joining.

But working towards a worst case scenario

If you have access to the internet before you join you can check the vessel for any history of detention etc... check for previous names and check detention history of these as well - vessel could have recently changed owners and/or names.

You should satisfy yourself that all the emergency equipment is working as required;Main and emergency fire pumps - ready to run and a hose test to check pressure and power from each pump. Suggest one hose at the furthest hydrant from the pump and one from the highest point simultaineously.Emergency bilge suction - valve is free and that bilges are clear of anything that could block the valve or the pump.Machinery space bilge alarms are workingEmergency D/alt starts and can take the expected load.Emergency batteries are fully charged and will start/run what is required.Emergency air compressor arrangement is capable of filling the bottle.Fire alarm system is working and that nothing is inhibited.Fixed firefighting system is operational and ready for use.All portable fire fighting equipment is in place and ready for use. Lifeboats and engines are in good working order.Sound powered phones at each locations to every other location.

You should satisfy yourself that all the machinery is in good working order;LO analysis tests of Main Engine (including camshaft system if B&W) steering gear, stern tube, auxiliary engines and may be a few more depending upon the ship in question. If this is not possible there should be equipment onboard to do a water in oil test, samples should at least be drawn to visually check oil condition.Is the main engine scavenge space clean?Any white metal lying at the bottom of the main engine sump?If the fuel has been onboard for a while fuel analysis may also be a sensible test to have carried out.Auxiliary engines will autostart and that they can take the load expected.Bilges are oil free and that there are no machinery leaks (oil or water).Spare parts on board for essential equipment, main engine, auxiliary engines, fire pumps etc...Essential tools for overhaul or removal of main engine cylinder unit.OWS is operational and that the bilge and sludge tanks are at acceptable levels.Fuel and LO purifiers are working correctly and not producing too much sludge.Alarm system is working and alarms have not be inhibited.Does the engine room operate UMS? Test the ER dead man alarm and cabin alarms. "Engineers call" does this work.

Talk to the people you would be working with in the engine room department, ask them about their experience either on the present vessel or previous - you can soon judge their competence levels - and ability to speak the working language.

Familiarise yourself with the emergency controls for the main engine, steering gear, lifeboats, lifeboat engines, and various means of starting the emergency d/alt. Change over proceedures for engine control - bridge to engine control room, engine control room to local and back again. Check the emergency escapes and where they come out, making sure doors are not locked or blocked. Make sure the rest of the engine room staff know this too.

For my own satisfaction I would take a walk along the jetty looking for any large indents, and oil leaks from stern tube and, if fitted, thrusters or stabilisers. Walk along the main deck checking the condition of any cranes or cargo gear and the hatches and covers. Also check funnel for smoke quantity and colour.

You can start the engine and test it ahead and astern whilst alongside, you will need to stop cargo and raise the gangway. It may be necessary to place additional mooring lines out. But a start test of the engine takes roughly 30 - 40 seconds and as soon as it can be seen that the governor is controlling the rpm the engine can be stopped. If the engine is tested in the reverse direction for a similar period the ship should end up where it started You should test from all control stations, bridge, engine control room and local control position.

If the vessel is fitted with Bridge control and it is proved ok at the start ahead and astern tests then there should be no trouble to give them control. If there is any doubt leave on engine control room control and change over once in clear waters.

Documentation:You could check discharge books and certificates of the other engineers onboard if the Master will let you.I would ask to see the latest class status report and check for any conditions of class and any overdue survey work.You could ask to sight all the statutory certificates;Load LineSafety ConstructionSafety EquipmentRadioMARPOL OilMARPOL SewageMARPOL AirDangerous Goods (if applicable)ISMVessels maintainence history, defects list and outstanding work list.Main engine parts running hoursLast main engine bearing clearances and crankshaft deflections.Outstanding orders for machinery parts.Read through the engine room log book, and check that all parameters are what you would consider normal.If possible read through past Chief, third and fourth engineers handover notes.Who is the designated person ashore, who is the emergency contact? Any parts of the ISPS plan that you are allowed to see, and your role/responsibility onboard. Muster list and your duties in an emergency

I'm sure I will think of more as the day goes on, and that the others in the forum will also add some relevent points or comments to the above.

Steering gear and main engine points I will reply with separately, when the boss is not around!

Taken from one of my assignments as a cadet, so some of this may be a bit basic, but I think that this covers most things for a older two stroke engine.

Preparing a Diesel to Start After Long Periods of Inactivity.

Before a diesel is started after a period of inactivity i.e. a drydock, it must be very gently be coaxed up to a operational standard to reduce mechanical and thermal stresses which would be present, not only in the engine but in the systems involved in running the engine. A rough guide is given below.1. Check the liquid level, and condition, of all tanks belonging to the engine, i.e. jacket cooling tank, the piston cooling water tank, the L.O. sump, and governor L.O. level, and refill if necessary.2. Line up and start cooling water pumps, vent the systems, and check for, any leaks, correct operating pressures and flow rates.3. Line up and start the S.W. cooling pumps, vent the system, check for any leaks, correct operating pressure and flow rates.4.The cylinders and pistons must be warmed up gradually over a period of at least eight hours. To a final temperature between 65 and 75oC. This is done by passing steam through the heating coils, either in the tank or in the line. Steam must never be injected into the water system as this can lead to corrosion problems. 5. Make one last inspection of the crankcase to ensure nothing has been left inside, and that everything appears normal. Line up and start the L.O. pump, vent the system, and check for, any leaks, correct pressure, flow and that the oil is flowing uniformly from all the bearings and guides, i.e. there are no blocked lubricating points. Close up the crankcase.6. With the indicator cocks open the engine should be turned over (by means of the turning gear). To check that there is no hindrance to the turning, or water leaking out of the indicator cocks. Whilst turning the cylinder lubricators should be turned to ensure correct lubrication of the liners.7. Start to heat up, to their correct temperatures, the wing tanks, the F.O. settling and service tanks. When up to the correct temperature drain all tanks of any water that has collected. Line up and start both the F.O. purifier and clarifier. Leaving the F.O. to recirc between the tanks.8. Line up and start F.O. booster pump, vent the system and check for, any leaks, correct pressure and flow. Crank the fuel pumps over to ensure that there is diesel right up to the injectors.9. Start the air compressors, and fill up the air reservoirs to their correct pressure (30 Kg/cm2). Drain off any water that has collected. Line up the rest of the air system, check for any leaks, water that may of collected in the lines, correct pressures at all points, and that the starting interlocks are working, i.e. the turning gear interlock.10. Check the reversing servomotor, regulating linkage, running direction safety device, the emergency fuel lever, engine telegraph, telephone link to the bridge, and emergency telephone link with the bridge for proper functioning.11. Drain off the air side of the air cooler, charge air receiver and piston skirt space for any water that may have collected.12. Disengage the turning gear, inform the bridge, and turn the engine over on air, from both the control room and the engine side controls. To ensure that everything is free and operating correctly.13. Inform the bridge and turn the engine over on fuel, both ahead and astern, from both the control room and the engine side control, to ensure that there is diesel right up to the injectors and no air in the lines and that everything is free and working correctly.

The engine is now ready to be run. The above description is only a brief outline of the procedures needed to get a diesel engine to a state of readiness after an overhaul. I have made several assumptions above and these include:1. That the engine has been reassembled correctly and that any faults have been already found and corrected.2. That the F.O. system has been lined up and started with diesel in the lines.3. That if any leaks or mistakes have been found, whilst checking the engine, they have been corrected before continuing

Running a Diesel for the First Time After a Long Period of Inactivity.

Before a diesel can be run as `normal', after a period of inactivity, it has to be run up slowly so that everything can be checked for correct operation, i.e. the coolers, the governor, the trips, etc... A rough guide is given below.1. The first run should be limited to a low speed, low power run. The engine should only be run for a period of about 30 minutes, with a constant watch kept, for any abnormalities, on all pressures and temperatures. After The first 30 minutes the engine should be stopped and all external surfaces checked for overheating. If there is no sign of overheating the crankcase should be opened up and a check made on internal bearings and running gear to ensure that all temperatures are normal.2. If all is satisfactory it may be restarted and run for a little longer, again at low speed and power. This time all the trips and safety devices should be checked for correct operation, i.e. the low L.O. pressure trip, overspeed trip, automatic pump starts, etc...3. Once everything has been checked and is found to be satisfactory, the engine can be brought up to full speed in small steps taking about 6 hours to reach full speed. Whilst running the engine up to full speed, the fuel oil system can be changed over from diesel to heavy.4. When the engine has reached full speed and all pressures and temperatures are satisfactory, when compared to the original trial data, a set of indicator cards should be taken to check the timing and condition of each unit.

The engine is now ready for normal use. The above description is only brief and several assumptions have been made, these include :1. That any defects found when preparing the engine, were corrected.2. That any defects found whilst running up were corrected.

Running a Diesel Engine on Heavy Fuel Oil.

Changing from Diesel to Heavy.Before changing from diesel to heavy fuel operation, it is necessary to heat the diesel oil in the mixing column, fuel pumps, lines and pre-heater up to about 50C.The system must be heated through slowly as abrupt changes in temperature may cause the fuel pump plunger to stick. It is advisable to heat only the fuel lines, the mixing column, and the filters, to begin with and if the diesel has not reached 50C to use the pre-heater.If the fuel oil temperature is normally controlled by a viscotherm this must be by-passed, when preheating the diesel, and the temperature regulated by hand.The engine must be kept running on preheated diesel oil until the fuel oil pump blocks feel warm to the hand. If this is the case, change over from diesel to heavy can be effected by way of the three way cock.After change over the fuel temperature is brought up to the `required preheating temperature' for the heavy oil. If the temperature is regulated by a viscotherm, the latter can be put back on line.During change over from diesel to heavy and until the `required preheating temperature' has been reached, it is advisable to not exceed 75% of the nominal power.

Changing from Heavy to Diesel.To change over from heavy fuel oil to diesel oil, the three way cock is firstly to be changed over. The heavy oil and diesel oil will then mix in the mixing column. The viscosity of the circulating mixture for a given temperature will correspondingly drop as the proportion of diesel oil increases. The temperature in of the fuel circuit can gradually be reduced and the heating may be completely shut off after a while.

Manoeuvring on Heavy Fuel Oil.When manoeuvring on heavy fuel care must be taken that the `required preheating temperature' is maintained at all times. The high-pressure fuel pipes must be kept heated and the fuel valve cooling kept at its required temperature.

Stopping on Heavy Fuel Oil.The fuel oil booster pump should be left running such that the fuel circulates through all the fuel pumps and back to the mixing column. The high-pressure fuel pipes must be continuously heated, the fuel valve cooling water heated to its normal operating temperature, and the fuel oil kept at its `required preheating temperature'.

Starting on Heavy Fuel oil.Because the fuel oil has been kept at the required temperature no special conditions have to be met when restarting the engine.

Steering gear must be tested within 12 hours prior to leaving port. This is a SOLAS requirement. If the ship is on short voyages (dont ask me what that is defined as) then the tests can be done weekly.

The following items need to be checked, if fitted:Main steering gearEmergency steering gearRemote steering gear systemEmergency power supplyRudder angle indicators compared to rudder angleSteering gear power failure alarmsSteering gear control system power failure alarmsAutomatic isolating valves and systemOil level in the emergency storage tank

The full movement of the rudder should be checked, a visual inspection of all linkages, and the operation of the emergency communications with the bridge should be tested. If a gyro repeater is fitted in the steering gear compartment the heading shown should be compared with that on the bridge.

At deepest draught, running ahead, the rudder should move from 35 degrees to 30 degrees (both port to starboard and starboard to port) within 28 seconds. Using emergency steering system the rudder should be capable of moving 15 degrees to 15 degrees within 60 seconds when running ahead at one half normal service speed or seven knots which ever is the greatest. Whilst you can not simulate that in port, the rudder must be able to complete these movements within the times redquired. Ifthe rudder is almost at the maximum time whilst operating in port it could be considered a good posibility that it wont acheive the required times on voyage.

Generally the steering gear tests used to go like this;Duty engineer phones the bridge from the steering gear this proves the emergency communication line. Bridge starts one steering gear, and moves rudder from hard over to hard over whilst timing and back to midships. Once rudder back at midships, take out breaker for running unit. Power fail alarm will sound and second unit wil start.Replace first breakerBridge will repeat movement of the rudder, stopping at a 10 and 20 degrees (port and starboard) to verify actual rudder position versus reported position.Take out second breaker to verify power fail alarm and autostart of first motorReplace second breakerEnsure that both systems are running and check full movement of rudder again. Confirm gyro repeater headingLeave bridge to test follow up, non follow up and remote steering positions and return to ER.

I think all the previous posts have covered everything except Fuel.UK Regulations used to be that you should have sufficient fuel on board for the estimated consumption to the next Port plus a safety margin of 3 days or 3% whichever was less.

You can always be cought out by the unexpected. I was told about a passenger cruise liner that had been sold, and a full Deck and Engine room crew joined to sail the ship the same day.When they tried to start the main engines ( 4 Pielstick V12s) none of them would start from the control room, if they started them locally, the WABCO control system did not recognise that they were running, so it was impossible to clutch them in remotely. If they were clutched in locally, the WABCO system still did not recognise that they were running or clutched in, so it was impossible to control pitch or RPM remotely!

Eventually they found that the there was a start interlock on all the main engines linked to the ventilation fans!!All the machinery space supply and exhaust fans had to be running before you could start the main engines.

When transitioning between HFO and MGO it is assumed that the diesel and HFO mix in the mixing tower or tank.

But is this the case?

During trials of a new digital viscometer, significantly more accurate and much faster responding than the conventional twin capillary, and logging the data, some anomalies were noticed.

Normally, at full load, the fuel flow to the engine is at about twice the maximum consumption rate. This gives a good heat flow from the engine back to the mixing tank and a 50/50 mix of hot fuel (eg 140C) from the engine and cooler make up HFO (98degC?) will result in fuel arriving at the heaters not far below the target temperature (120degC) and the heaters can work efficiently and under relatively stable control.

Usually the transition between MGO and HFO, during shutdown or start sequences, takes place with the engine under very low load conditions with very lttle fuel consumed. Hence at low load the proportion of fuel returned from the engine is much higher and the MGO is only a small proportion leading to a progressive step sequence of small viscosity and temperature changes easily managed by the heater.This is seen as small drops in viscosity and temperature and the heater gradually ramps down (shutting down sequence). Ideally at the start of the sequence the 90% (say) return flow of neat HFO mixes with 10% MGO resulting in a small drop in viscosity and temperature. This interface between the neat HFO and the mix of HFO and MGO will circulate to the heater when the small step change is easily managed. When the interface between the neat HFO and 90/10 HFO/MGO reaches the mixing tank a new interface is created between 90/10 and 80/20 and another incremental step change in heater control is effected. (the step increment and interval also depends on the capacity of the high pressure circuit).

However, it was found that cold MGO introduced into the mixing tank instead of mixing with the HFO from the engine, remained in the mixing tank while the higher density HFO was tunnelling through the MGO and flowing to the engine neat and hot. After a while the mixing tank was flooded with MGO which then flowed as a slug of cold low viscosity fuel. This wasn't a clean transition and the cycle repeated a few times with alternate slugs of hot and cold fuel.

This prompted the engine manufacturer to re-assess the mixing tank design.

Apart from the undesirability of neat cold diesel reaching the engine (seen as a step change in viscosity and temperature at the viscometer) it meant that the heater control was unsettled. The low viscosity MGO (which was heated to the HFO temperature before the heater could respond - the viscometer is in the outlet) and then caused the heater to be ramped down so that when the next interface with hot HFO arrived the heater was unable to respond fast enough to prevent the HFO going to the engine at too high a viscosity. This aggravated the problem at the mixing tank when hot MGO was even lower density than the fresh MGO.

Always interesting to monitor the data and see what is happening and why. And it is surprising what insights even the two usual measurements of temperature and viscosity will yield.

Of course, when complying with MARPOL sulphur directives transitioning takes place under full load which requires a different management strategy but which still depends on an efficient and effective mixing tower or tank.

Be interesting to see how many have noticed these odd effects and who may still have ineffective mixer designs. I wonder, by the way, why mixing tanks are used rather than inline static mixers which would deliver a nice homogeneous mixture. Headloss isn't a problem since this is the high pressure circuit with a decent PD pump.

PS I understand that samples are often collected from the mixing tank. I suggest that this layering/tunnelling effect may make such samples rather less useful during shutdown and start sequences and especially when making a full load transition between low sulphur and high sulphur fuels - even when switching between LSHFO and HSHFO rather than LSMGO and HSHFO?I don't see how you would have time to check all this things when you sign on. I always demand a handover/overlap time on a new ship so i can make my opinion if to stay or not. I believe any serious company should give a weeks handover at least. And stopping cargo operation for testing the engine, were i work it would be unheard of and the chief engineer would get phone call from the office fast!

As a help for calculating LSHFO - HSHFO i use this FOBAS calculator. It is pretty accurate we have taken samples at diffident times and been close in sulphur content. Its good to play around with the numbers and see the different changeover times you get, with different fuel consumption and sulphur content.

Air Starting Systems

1) INTRODUCTION

Various types of starting and reversing systems are found on motor ships. Large engines are usually started by using air pressure. However other starting systems are used on smaller engines.The usual methods of starting an engine are:

1) Hand - cranking (e.g. lifeboat engine, emergency air compressor engine, emergency fire pump)

2) Electric batteries (e.g. emergency generator engines, main and auxiliary engines for small craft, lifeboat engine)

3) Air motors (small engines)

4) Hydraulic motor (e.g. emergency generator engine)

5) Air starting system (large engines)

1.1) Small Engine Starting Systems

Some smaller engines are started electrically. Usually the starting motor is a series wound d.c. motor with the voltage across the motor varying between 6 and 120 volt. Series - wound motors have a very high starting torque, which, of course, is necessary to move main engine from stationary position, overcoming high static friction, to a moving state. Where the frictional forces are less. Air starting motors, too, may be found on the small engines. The air starting motor, attached to the engine through gearing, rotates the engine to start a compression ignition process. There are two types of air starting motors, the piston type and the vane type. This system is quite common on medium speed engines. The third type of small engine starting system is hydraulic. This system is more common than the other two, particularly, on small and medium size high-speed engines. These starting systems use a piston type hydraulic motor. The hydraulic pressure in the system is about 200 bars and the system cut off pressure, when no more starting is possible, is around 100 bar. Typically, there is a hand pump and an electrical or engine driven pump. One of the pumps should be hand driven to allow an emergency startup when there is no power. System has accumulators, which are charged with nitrogen. As the hydraulic motor is activated, the accumulators maintain a high pressure throughout system so that there is not an immediate pressure drop.

1.2) Large Marine Engines Starting Systems

Almost all the marine diesel engines are started by admission of compressed air in that cylinder which is in the starting position with respect to the direction of rotation. The compressed air acts on top of the piston and reduces a starting torque at the crankshaft. The shaft is set to rotate upto a minimum firing speed when air is compressed in some other cylinder producing a temperature sufficiently high, to ignite the fuel.

Ques: How do the physical conditions contribute towards the starting of an engine?

Ans: When a diesel engine is started after a period of idleness a high initial torque at low revolutions is necessary to accelerate the engine rotating and reciprocating masses. One of the principle factors is to overcome the forces of adhesion between the bearing surfaces due to the presences of cold lubricating oil between them. In slow speed engines with the propeller directly coupled with the shaft, the load on the engine cannot be taken completely off at the time of starting. The walls of combustion chamber being cold there will be a greater heat flow to the lower temperature of compression at the time of start. An engine, which has been warmed up by the metallic surfaces of the combustion chamber, viscosity of lubricating oil lowered, will have the starting speed reached earlier.

In a two stroke main propulsion engine the starting air should be admitted in such a way that the engine can be set to motion in either direction from any position, to satisfy this condition the minimum cranking angle possible is the angle by which the cranks are displaced plus a period of overlap. The starting phase is most effective during a part of the expansion stroke. The overlap required for the period when one cylinder is at the end of starting range and phasing out the other cylinder is just entering in the starting torque. Unless a large overlap is given the starting torque, during this period, is inadequate. The concept of STARTING AIR OVERLAP will be dealt in detail later in the paper.

Quesn: Why are marine engines preferred to be started on AIR than on ELECTRICITY?

Ans: If we take the example of an ordinary car engine, we observe that the size of battery used to crank the engine before the engine picks up on fuel is almost equivalent to that of the engine itself. Considering the huge sizes of large engines (specially, marine engines) and the amount of electrical power requirements to set such engines in motion, it becomes clear that the size of battery required for the purpose would either be equal to the size of engine or perhaps greater than that. The increased space and maintenance a requirement of such a large battery thus makes it unfit for the marine applications. Instead an easily obtainable, hazard-free & economical solution is preferred i.e. AIR.

2) CONDITIONS FOR STARTING A DIESEL ENGINE

The starting of a diesel engine is one of the crucial operations dependent upon which is the normal service of the engine. A reliable starting is also a factor that materially influences the ship's maneuverability. The starting is effected by setting a crankshaft to rotate at a speed that will produce the ignition temperature in the engine cylinders. The starting falls into three stages which are cranking the engine by means of compressed air until some cylinders fire, picking up the cycle on fuel without the engine's misfires an acceleration to a speed in accordance with the fuel injection pump setting and the warming up of the engine with the gradual increase in load to a given value.

Marine practice favors the starting with compressed air. High-speed diesel engines may employ the starting either by air or electricity. The main requirement is that the crankshaft must be rotated at the minimum cranking speed to effect the starting by air irrespectively of the position of the crankshaft, therefore a two-stroke engine must have at least four cylinders and a four stroke six cylinders. (For further understanding go to section- Starting Air Overlap). To obtain a trouble free starting, the cranking period may vary with the condition of air fuel inter mixture, fuel self-ignition and the state of the engine. So much so, that the USSR register calls for the builders guarantee that the engine will startup from cold failure free at an engine room temperature of at least 8 degrees C. The period, which elapses, before the engine is under its own power on being cranked by air is between 2 and 8 seconds. At this stage the running is irregular and the exhaust is smoky.

The irregular running is due to the fact that some of the cylinders misfire and the engine gains speed in jerks as the cylinders pickup the cycle on fuel one after another.

3.) THE STARTING PROCESS

Starting air comes directly from the ships medium-pressure (MP) or high-pressure (HP) air service line or from starting air flasks, which are included in some systems for the purpose of storing starting air. From either source, the air, on its way to the starting system, must pass through a pressure-reducing valve, which reduces the higher pressure to the operating pressure required to start a particular engine.

A relief valve is installed in the line between the reducing valve and the starting system. The relief valve is normally set to open at 12 percent above the required starting air pressure. If the air pressure leaving the reducing valve is too high, the relief valve will protect the system by releasing air in excess of a preset value and permit air only at safe pressure to reach the starting system of the engine. In the following section, we will discuss two common types of systems that use air as a power source for starting diesel engines-the air starting motor system and the compressed air admission system.

3.1) AIR STARTING MOTOR SYSTEM Some larger engines and several small engines are cranked over by starting motors that use compressed air. Air starting motors are usually driven by air pressures varying from 90 to 200 psi. In figure 1, starting air enters through piping into the top of the air starter housing (1) and flows into the top of the cylinder (2). The bore (3) of the cylinder has a larger diameter than the rotor. The rotor (4) in side the cylinder is a slotted rotating member, which is offset with the bore of the cylinder. The rotor carries the vanes (5) in slots, allowing the vanes to maintain contact with the bore of the cylinder. The pressure of the starting air against the vanes forces the rotating member to turn approximately halfway around the core of the cylinder, where exhaust ports (6) allow the air to escape to the atmosphere. A shaft and a reduction gear connect the rotating member to a Bendix drive, which engages the ring gear of the flywheel to crank the engine.

3.2) COMPRESSED AIR ADMISSION SYSTEMThe compressed air starting system comprises air compressors, air bottles, high pressure pipes, main maneuvering valve, reduction valve, starting air distributor, starting valves on engine cylinders, etc.

The compressed air starting systems vary with the type of engine and may comprise low-pressure air bottles at 20 to 30 kg/cm2, MP air bottles at 60 to 80 kg/cm2 and HP air bottles at 150 to 250 kg/cm2. In the latter two cases, reducing valves are used for reducing the air to 20 to 30 kg/cm2. Starting air pressures range from a minimum of 6 to 10 kg/cm2 to a maximum of 25 to 30 kg/cm2. Control air pressures are in the range of 5 to 10 kg/cm2.Methods of Starting with Compressed Air:

This can be done in the following ways: Supplying starting air and fuel in sequence to the engine cylinders Simultaneous supply of starting air and fuel in sequence to the engine cylinders. This is generally used on medium and high speed powered engines. Supply of fuel in sequence to two groups of cylinders of the engine, e.g. to 4 out of 8 cylinders at a time. Simultaneous supply of fuel to all engine cylinders. This method is generally used for starting. Ques: Is there any fixed rpm to which the engine should be accelerated with compressed air before it picks up the cycle on fuel?

Ans: . In general, starting is achieved by cranking the engine to a shaft speed of between 15-25% of the nominal speed, which results in a mean piston speed of between 0.7 to 1.5 m/s.

Cranking is done by hand on small engines upto 25 to 30 hp by using decompression arrangements to ease cranking effort. Electric starters can be used on high-powered engines. Some low powered engines have both systems, of which one is a back up system.The starting shaft speed is dependant upon the design features of the engine, combustion chamber design, method of fuel injection, moment of inertial of the rotating assemblies and temperature of water and air.

The shaft speed at which starting occurs is largely influenced by the type of fuel and the manner of its delivery and depends upon the following factors:- Type of fuel & its cetane no.; hence engines burning heavy fuel, use a lighter fuel such as diesel oil, while starting Selection of the optimum angle of fuel injection

Quantity of fuel delivered; this should be around 50-75% of the full delivery. Quesn: Comprehensively describe, how an engine picks up its cycle on fuel after being accelerated by the starting air?

Ans: Figure 2 shows a typical indicator diagram during starting, for a mechanical operated starting valve. From the moment air is admitted to the cylinder till the piston commences it motion (point a), the pressure of the cylinder rises rapidly from the ambient pressure to the max starting pressure (point b) the pressure of air entering the chamber of varying volume, is almost constt. When the starting valve is shut (point c) the polytropic expansion of air commences, followed by the exhaust, compression, etc.

After a few strokes on air (curves 2,3) fuel is supplied and if the engine has developed the necessary speed of rotation, the first working diagram of the engine on diesel fuel, is obtained (curve 4). The mean indicated pressure of the first diagram is around 3-7 kg/cm2, with pressure after the first ignition reaching more than 1.5 times the maximum combustion pressure. During the start process, the angular velocity of the shaft increases sharply, during the first few revolutions of the shaft; hence each successive indicator diagram of the starting differs considerably from its predecessor. In addition, the start process does not complete with the first ignition, hence it is essential that during each succeeding working stroke, the excess gas energy (after accounting for friction losses) be used for increasing the shaft speed and accelerating the engine. As the engine accelerates, the mean indicated pressure of air initially decreases rapidly and later slowly. The effectiveness of the start process, as mentioned earlier, is dependant upon the thermal state of the engine and the method of fuel supply.

Quesn: Elucidate, how engine starting is affected as a function of the thermal state of an engine?

Ans: (Ref. Figure3, 5,6-above) Figure 3,above shows the variation of shaft speed and the fuel rack position for a 2-S engine for a cold start (curve 1), a warm start (curve 2) and the next warm start (curve 3). The intensity of acceleration of the engine increases with thermal state of the engine, while the acceleration process is dependant upon the manner of fuel supply.

The temperature of the air, cooling water, lubricating oil and the cylinder walls, determines the thermal state of the engine. These temperatures are almost equal to each other, when the diesel is started in cold condition. The starting diagram for the 2-S engine referred to in fig3 at various cooling water temperatures, is shown in Figure 5. It is evident that the duration of start process is longer and the acceleration of the process is slower, at lower cooling water temperatures. The starting indicator diagrams for the same engine at 5,15 and 30 deg C show the increase in work done by gases with increase in temperatures (Fig 6).

A cold diesel engine is warmed through for easy starting in the following manner: Circulation of hot water or steam through the cooling water system, generally from the ships services. Circulation of warm oil through the engine.

4) ELEMENTS OF COMPRESSED AIR ADMISSION SYSTEM

The compressed air starting system comprises air compressors, air bottles, high pressure pipes, main maneuvering valve, reduction valve, starting air distributor, starting valves on engine cylinders, etc. Lets understand the function of each component involved, before we proceed to understand the starting air circuit, as a whole:

4.1) The Main Air Starting Valve/ Automatic Valve

The main air starting valve, installed on the air starting line, downstream of the starting air bottles, enables repeated starts when the starting air bottles are opened. Besides it also synchronizes the moment of air delivery to the starting system with respect to the reversing mechanism and the fuel supply. Main air starting valves (see FIGURE 7 & 8) are divided into balanced and unbalanced types as per their mode of operation and could be controlled either mechanically or pneumatically controlled. Balanced type valves are more reliable in service as compared with the unbalanced type and hence widely used. The main starting valve is operated from the control position through a control valve, which could either be of the pressure or relief type .The relief type of control valve is used in conjunction with the unbalanced main starting valves, while the pressure type is used with balanced main starting valves. An example of an unbalanced main air-starting valve used on the sulzer SD72 engine is depicted AT FIG.7. Compressed airflows around the external spaces of the valve, passes through the gap 2 between the valve spindle3 and the internal body, to space 7. Valve 4 remains shut under the combined action of the spring and the air pressure. On opening the relief valve 8, located at the control post, air from space 7 is released and the valve 4 opens under the action of air pressure in space 6, which overcomes the pressure of spring1.Balanced pneumatically operated valves of the type depicted at fig 8 are widely used. Valve 2 located within the casing 1(SEE FIG 8) prevents air from space A (connected to the air bottle) passing to space B (connected to the starting valve). Valve 2 is maintained in a closed position by the combined action of spring 3 and the starting air pressure acting above the valve. The space above valve disc A and below space B is connected through pipe 7, to the relief type control valve shown at Fig 8b, while pipe 6 is led to the atmosphere. During starting, the control valve is opened and air from space A passes to the control valve. Air from the control valve flows through 4, to the connector 5 in the main valve and to space a in the pneumatic cylinder. Due to the differential pressure on the valve disc, valve 2 lifts and starting air from space A passes to space B and to the starting valves. Valve 2 also shuts off the atmospheric outlet 6, when in the open position. On completion of the starting process, air to the pneumatic valve is shut off, air within leaks to atmosphere and the main valve shuts under the action of the spring. Air in the pipeline is released to atmosphere through opening 6, while that in the pneumatic cylinder is released through opening 4.

After the first injection of fuel in the engine, the fuel control lever is placed in the position corresponding to the required regime. With this, pneumatic valve shuts automatically and air to the main starting valve stops and engine runs on fuel.

4.2) Starting Air Distributor:

Starting air DISTRIBUTORS (SEE figures 9 & 10) control the sequence of opening and closing the pneumatic operated starting valves mounted on the cylinder heads in accordance to the firing order thus admitting air into the engine cylinder. These are of 2 types-disc and slide valve construction. In the slide valve type, the number of slide valves equal the number of starting cylinder valves and could be grouped in a single casing or installed individually for each cylinder.

4.2.1) Disc Type Air Distributor(MAN B & W)

. It consists of body 4containing cylindrical distribution block 5 with vertical drilling 10 in its upper end face and rotor 2 fitted to the vertical shaft 7 driven from the camshaft through bevel gear 8. Distribution block 5 has as many passages being connected each to corresponding drillings 10. Rotor 2 is provided with a passage of a diameter equal to that of the vertical drillings 10. On opening the starting air master valve, valve, pressing the rotor registers with drillings 10 connected to the starting valve of the engine cylinder which is in a position ready for cranking, air continuous its travel into this cylinder. Thus, the rotor, actuated from the camshaft, becomes connected to the rest of the drillings in the distribution block which, in their turn, are connected by lines to starting valves in a way prescribed by the firing order.

While the rotor is a sliding fit on a key, permitting vertical displacement along shaft 7, the distribution block is arranged to turn about the same shaft from one exchange setting into another, corresponding either to right hand or left hand rotation of the crankshaft at starting. Thus, the distribution block provides for a firing order in running ahead and one for running astern depending on the setting which can be changed with the aid of rack 9 meshing gears on a side surface of the distribution block and linked to the reversing handle by way of tie rod.

On the delivery of starting air into one cylinder, its starting valve closes, and the process of feeding starting air into rest of the cylinders is repeated cylinder by cylinder in accordance with the firing order.

When the engine has picked up the cycle on fuel, the rotor goes on rotating because of being rigidly linked to the camshaft. To minimize wear on the end face of the rotor, it is set out of contact with the distribution block by spring 3. At the same time, the drilling of the block registering with the opening in the rotor is connected to a relief valve to relieve the starting valve of pressure.Flanged bracket 6 serves to mount the starting air distribution valve on the engine.

4.2.2) Slide Valve Type Distributor-SULZER DESIGN (RND-M, RTA)

The starting air valves in the cylinder heads are pneumatically controlled by pilot valves 3 arranged in the housing 4. Each individual pilot valve is connected to a starting air valve by a closing pipe CP and an opening pipe OP.

The pilot valves 3 are arranged radially according to the engine firing order. They are actuated by starting cams5, which are fitted to the upper end of the vertical shaft 1.

As long as the starting control air distributor receives no control air the pilot air valves3 are held off the starting cams 5 by the tension springs3a. (CLEARENCE S FIGURE 5A).

In this position of the pilot valves the opening pipes OP are vented to the vent space VS. When the starting air shut-off valve is opened, the distributor space DS and the closing pipe CP are filled with air from the starting air receivers upto the starting air valves. All the starting air valves remain closed.

When reversing the engine, the shaft 1 and the starting cams 5 are turned relative to the crankshaft by the reversing servomotor in such a way that the timing remains equal with the new turning direction of the crankshaft.1 Shaft CA Control Air pipe (8 kg/cm2)2 Sleeve SA Air Inlet (30/25 kg/cm2)3 Pilot Valve RS Annular Space3a Tension Spring DS Distributor Space4 Housing VS Venting space5 Starting Cam CC Base circle of the cam6 Roller P Pressure SpaceS Clearance

Mode of Operation

Assumption: Engine at a standstill. The starting air shut-off valve is opened.

When the starting button is pressed or when by controlling the engine at the auxiliary maneuvering stands,The starting lever is put into the START position; control air flows through pipe CA into annular space RS and space P and pushes the pilot valves inwards until the rollers 6 land on the starting cams 5.

The roller 6 of at least one pilot valve 3 now lies on the base circle BC of the starting cam 5. (FIGURE C).

In this position of the pilot valve the distributor space DS is connected by the opening pipe OP and the vent space VS with the closing pipe CP. The upper side of the control piston of the starting air valve in the cylinder head is supplied with air whereby the valve opens. Starting air flows into the respective cylinder. The crankshaft begins to turn. The starting cams 5, driven by the camshaft and shaft1, also begin to turn. The pilot valve 3 moves outwards following the starting cam 5 profile. The opening pipe OP is now connected to the distributor space DS with the vent space VS and the closing pipe CP. The starting air valve in the cylinder head closes (FIGURE B). It remains closed as long as the roller 6 of the pilot valve 3 lies on the cam circleCC of the starting cam 5. As soon as the roller leaves the cam circle and moves towards the base circle BC the previously described process is repeated.

When the starting button is released or when, by controlling the engine from the auxiliary maneuvering stand, the auxiliary starting lever is released, the control air supply to the starting control air distributor is interrupted (space RS and P is vented) and the pilot valves 3 are pulled back to their original; position by the tension springs (FIGURE A).

4.3) Air Starting Valve (MAN B & W, SULZER-RTA)

These valves fitted in the engine cylinder covers, supply starting air to each cylinder. The timing for the opening and closing of starting air valves is controlled by the cam(s) on the engine camshaft which actuate the distributor valves. The supply of starting air from the automatic valve is led through a common starting air rail fitted along the length of the engine with separate branch connections to each cylinder air starting valve.The valve is fitted into the cylinder head. It is opened by control air from the starting air distributor.The valve shown is from a slow speed MAN-B&W two stroke engine but a lot of modern engines have valves working on similar principles and design.MaterialsThe body of the valve could be of mild steel, the spindle of high tensile or stainless steel, and the valve and seat could have the contact faces stellited or hardened.How it worksMain starting air at about 30 bar from the manifold enters the chamber above the valve via the circumferential ports in the valve body.

The air pressure will not open the valve because a spring is holding the valve shut, and the area of the balance piston is the same as that of the valve lid so the valve is pneumatically balanced.

When the valve is required to open, air at 30 bar from the air start distributor enters the top of the valve body and acts on a piston. This force overcomes the spring force holding the valve shut, and the valve opens. When the air signal from the air start distributor is vented, the spring closes the valveWhen the start sequence is finished the main air start pressure is vented through holes in the main start air manifold.

The Sulzer air start valve uses air on both sides of the operating piston to maintain positive closing. The piston is stepped. The reason for this is so the starting air valve will not open when the gas pressure in the cylinder is higher than the starting air pressure; i.e. when the cylinder is firing. Once the valve starts to open then the opening is accelerated when the larger diameter piston has the opening air acting on itThe stepped piston also means that closing of the valve is damped as air gets trapped in the annular space formed when the smaller diameter piston enters the upper part of the cylinder.

The air to operate the valve comes from the main air start supply. The distributor pilot air operates the pneumatic change over valve.

MAINTENANCEAfter certain periods of service starting air valves are changed and overhauled. If piston rings are fitted, care must be taken to ensure that they are free in their grooves. Should it be necessary to fit new rings, the butt clearances of the rings must be carefully checked by placing the ring into the operating cylinder and measuring the clearance. This is especially important if they are usually made of brass, which has a larger coefficient of expansion than the other parts of the valve. The valve and valve seat are ground with grinding paste and finished to a fine surface with lapping paste. It is essential to ensure that all parts of the valve are scrupulously clean before reassembly. Lubricate all sliding surfaces sparingly with a molybdenum disulphide grease. Quesn: What are the indications of leaking starting air valves?Ans: When an engine is in operation leakage of starting air valves is shown by overheating of the branch pipe connecting the starting air valve to the starting air rail. The heating occurs due to leakage of hot gases from the engine cylinder into the starting air line connected to the starting air rail. During periods of maneuvering the temperature of each supply pipe from the air rail to the starting air valve should be checked by feeling the pipe-as close to the valve as possible.Ques: What are the usual causes of starting air valve leakage?Ans: Leakage of a starting air valve is usually caused by sluggish valve action preventing fast closure of the valve, or by dirt or foreign particles from the starting air supply lodging on the valve seat and so preventing the valve from closing fully. Sluggish valve action can be caused by dirty pistons or valve spindle guides and the like. In newly overhauled valves sluggish valve action may be caused by parts fitted with inadequate clearances.

Quesn: What causes an Air Start Valve to stick?

Ans: Leakage of a starting air valve is usually caused by sluggish valve action preventing fast closure of the valve, or by dirt or foreign particles from the starting air supply lodging on the valve seat and so preventing the valve from closing fully. Sluggish valve action may be caused by dirty pistons or valve spindle guides and the like. In newly overhauled valves sluggish valve action may be caused by parts fitted with inadequate clearances.

Quesn: How can I tell if an Air Start Valve is leaking or has jammed open?

Ans: When an engine is in operation leakage of starting air valves is shown by overheating of the branch pipe connecting the starting air valve to the starting air rail. The heating occurs due to the leakage of hot gases from the engine cylinder into the starting air line connected to the starting air rail. During periods of manoeuvring the temperature of each supply pipe from the air rail to the starting air valve should be checked by feeling the pipe as close to the valve as possible.

Quesn: What should I do If an Air Start Valve jams open whilst manoeuvring?

Ans: The fuel pump should be lifted (fuel rack zeroed, puncture valve operated or whatever) on the affected unit and the bridge informed. The load should be kept at a minimum, as one unit is now out of operation. As soon as safe to do so, the engine should be stopped and the air start valve replaced.the figure shown below gives an idea as to how the starting air distributor line is connected to the starting air valve, mounted on the cylinder head.

Connection of Starting Air Distributor to the Starting Air Valve

5) STARTING AIR CIRCUIT

6) HOW AN ENGINE STARTS ON AIR

Large Marine Diesel Engines are started using high-pressure compressed air. The air is admitted into the cylinder when the piston is just past TDC and continued until just before the exhaust valve opens. There is always more than one air start valve open: - a situation known as overlap. This ensures that the engine will start in any position. The opening of the main air start valves is controlled by a set of pilot valves, which in turn are timed to operate by a drive linked to the main camshaft. In the example shown, a small camshaft is used to control the opening and closing of the air start pilot valves.The drawing shows the principle of operation of an air start system. Large air receivers are used to store the compressed air. The diagram shows the isolating valve open so air is being allowed as far as the automatic valve and the air start control valve.When the engine is required to start, a low-pressure air signal is sent to the air start control valve (which can also be hand operated in an emergency). The air pushes a piston down, which opens the valve and allows high-pressure air to flow to the pilot valve and the automatic valve operating pistons. The pilot valve is forced down onto the cam profile and the automatic valve opens and high-pressure air is led to the main air start valves and the pilot valve. When the pilot valve cam follower is on the lowest point on the cam, air flows to the operating piston of the main air start valve for that particular cylinder, opening the valve and allowing high-pressure air to flow into the cylinder.

When the cam lifts the pilot valve, the pilot valve vents and the main air start valve closes. When the start air signal is taken off the air start control valve, the system vents and the automatic valve shuts.

The figure below shows the second phase of operations.

An interlock blocking valve will operate, for instance if the turning gear is left in, and this will stop high pressure air from reaching the air start control valve and thus either the automatic valve or the pilot valve.

A slow turning valve is fitted. This will open instead of the main automatic valve if the engine has been stopped for more than 30 minutes during maneuvering. It will only supply enough air to turn the engine over very slowly; this is a precaution incase a cylinder has had oil or water leak into it which would cause damage to the engine when starting. If the engine completes a full revolution on the slow turn, then the main automatic valve opens and the engine will start. (Note: The operating system for the slow turning has been omitted for simplicity).

The system is provided with a non-return valve, flame trap and relief valve for safety purposes.

Quesn: What do you understand by the terms interlocks and blocking devices in a starting and reversing mechanisms?Ans: The function of the interlocking gear is to assure that the engine runs in the desired direction and no fuel is injected prior to the completion of the reversing operation. The engine can be started only after the interlocking gear has performed its function, i.e. the reversing gear has been set into position for running ahead or astern.

An interlocking gear provides for correct functioning of the reversing gear and assures safe sailing of the ship. It is an indispensable element of the reversing gear for all direct-reversing diesel engines in marine applications. Most of the modern interlocking gears are of the mechanical type, which is simple and reliable.

7) THE CONCEPT OF STARTING AIR OVERLAPSome overlap of the timing of starting air valves must be provided so that as one cylinder valve is closing another one is opening. This is essential so as to ensure no angular position of the crankshaft with insufficient air turning moment to gibe a positive start. The usual minimum amount of overlap provided in practice is 15 deg. Starting Air is admitted on the working stroke and the period of opening is governed by practical considerations with three main factors to consider: The firing interval of the engine:

Firing interval = (Number of degrees in engines cycle)/(Number Of Cylinders)

E.g. with a four cylinder Doxford engine (2-S) the firing interval is 90 degs, i.e. (360/4) and if each cylinder valve covered 90 degs of the cycle then the engine would not start if it had come to rest in the critical position with one valve fractionally off closure and another valve just about to start opening.

The valve must close before the exhaust commences. It is rather pointless blowing high-pressure air straight to exhaust and it could be dangerous.

The cylinder starting air valve should open after firing dead center to give a positive turning moment in the correct direction. In fact some values are arranged to start open as much as 10 degs before the dead center because the engine is past this position before the valve is efficiently well open, and in fact any reverse turning effect is negligible as the turning moment exerted on a crank very near dead center is small indeed.

Consider FIGURE 7A for a 4-S Engine: -

With the timings as shown the air-starting valve opens 15 degrees after dead center and closes before exhaust begins. The air start period is then 125 degs. Firing interval for a 6-cylinder 4-S engine=(720/6)=120 degs. The period of overlap is 5 deg, which is insufficient. Although this example could easily be modified so as to give sufficient (say 15 degs) overlap by reducing the 15 degs after dead center and the 10deg before exhaust opening on turbo-charged engines and a 7-cylinder 4-S engine is much easier to arrange.

Consider FIGURE 7(b) for a 2-S engine;

This has an air start period of 115deg. Firing interval for a 3-cylinder 2-S engine=(360/3)=120 deg. This means no overlap. Modification can arrange to give satisfactory stating with this example but for modern turbo-charged 2-stroke engines having exhaust openinh as early as 75deg before BDC it becomes virtually impossible. A 4-cylinder 2-S engine is much easier to arrange and would be adopted but in fact for modern practice requiring such high powers the number of cylinders is increasing and 4-cylinder engines in increasing and 4-cylinder engines are becoming in the minority so that there are virtually no air period starting problems.

Consider FIGURE 7C, which is a cam diagram for a 2-S engine with 4-cylinders. The air open period is 15 digs after dead center to 130 degs after dead center, i.e. a period of 115 degs. This gives 25 deg of overlap [115-(360/4)], which is most satisfactory. Take care to note the direction of rotation and that this is a cam diagram so that for example No.1 crank is 15 degs after dead center when the cam, would arrange to directly or indirectly open the air start valve. The firing sequence for this engine is 1432. This is very much related to engine balancing and no hard and fast rules can be laid down about crank firing sequences, as each case must be treated on its merits.

It may be useful to note that for 6-cylinder, 2-S engines a very common firing order is153624 and similarly for 7 and 8 cylinders 1725436 and 16428357 respectively are often used.

The cam on no.1 cylinder is shown for illustration, as it would be for operating say cam operated valves, obviously the other profiles could be shown for the remaining three cylinders in a similar way. The air period for Nos. 1, 4, 3 and 2 cylinders are shown respectively in full, chain dotted, short dotted and long dotted lines and the overlap are shown shaded.

8.) AIR CONSUMPTION DURING STARTING & REVERSING

The consumption of air during starting is independent on a number of factors, the important ones being: The thermal state of the engine, Starting Air Pressure Duration of the Start Process, Experience of the operators starting the engine.

The actual air consumption observer during trials on the engine can be expressed as:

Q = (Pstair1- Pstair2)Vb/n starts, Nm3

Where,p-st air1 and p-st air2 are the pressures in the air bottle before and after the start process,Vb is the volume of the air bottle in m3,n-is the number of starts.

The pressure of air in the bottle at the commencement of start is around 25 to 30 kg/cm2.

The mean specific air consumption for the slow speed engines is 1.6 to 5 litres/litre of the engine cylinder for each start and 4 to 8 litres/litre of the engine cylinder for each reverse. On medium speed engines this is around 1 to 2 litres/litre of the engine cylinder for each start.

Design of the compressed air line of the starting system comprises determining the overall pressure loss between the air bottle and the starting valve. This data indicated that this is of the order of 40-60%. Hence, for air bottle pressures in the region of 20-30 kg/cm2 , the pressure in the engine cylinder before start is of the order of 8-18 kg/cm2. The pressure loss in the line is not only dependent upon the flow losses, but also on the starting pressure itself.

For example,. for p-st air = 30 kg/cm2 , the loss is 20 kg/cm2 ( 60%) while for p-st air = 30 kg/ cm2, the loss reduces to just 5 kg/cm2 (25%).

While carrying out the strength check on the starting valve, the following is done: check the specific pressure on the taper section, which should not exceed 250-300 kg/cm2. Determine the thickness of the valve disc using the expression;

= 0.5d (Pz/ Rb)0.5 cmWhere,d smallest diameter of the valve disc, cm.Pzcombustion pressure, kg/cm2.Rbpermissible bending stress for various metals in kg/cm2 .

Reversing Systems

A marine diesel engine coupled directly with the propeller must be reversible. The reversing system must include means for turning the engine in the opposite direction by compressed air. To state in simplistic words, reversing can be achieved by admitting air in the unit in which piston stands marginally short of the TDC position. The fuel injection system should therefore adopt itself for astern running. It is therefore necessary to reposition the camshaft or a new set of cam to be brought into action in order to set the engine to a firing sequence commensurate the reversed running direction.

The conditions, which must be satisfied, are:

a change in the sequence of starting air admission repositioning of camshaft for reversed running firing sequence.

There are 5 solutions to reversal of the engine timing, which are as follows:

Reversing Servos on all camshaft-such as old SULZER Separate ahead and astern cams with axial movement. Timing of fuel pumps, exhaust valve symmetrical. Fit air distributor as per DOXFORD where reversal is performed internally by airflow. Fill fuel pumps as per B&W new design where the follower is repositioned relative to the camshaft and this retimes pump for new direction. Exhaust timing is symmetrical.( Dealt in detail later) In this paper we shall confine our discussion to direct propulsion marine engines reversing, which basically entails three types of reversing systems:

Sliding Camshaft Reversing Latch Rotating Latch

The rotating camshaft is used only on two stroke cycle ported engines.

A sliding camshaft-( MITSUBISHI DESIGN) is illustrated in the adjoining figure . The rocker arm roller is in constant contact with the cam, either the ahead cam or the astern cam. When the engine stops, the camshaft moves in a lengthwise direction. A ramp between the head and the astern cams allows for constant rocker arm roller contact. There may be cam followers with pushrods that lead to rocker arms. This type of reversing system is used on two-stroke & four-stroke cycle engines. The reversing latch method of moving the camshaft is also illustrated in figure 13.The side view in figure shows that there are two rollers on the reversing latch cart. the camshaft does not move, but the reversing latch as it is called does. By moving the reversing latch, the roller is allowed to engage with the ca. The cam follower, which would actuate the push rod, acts on the middle of the reversing block all the times.The reversing latch itself may be either manually or hydraulically actuated. This system is found on two and four stroke cycle engines. In a two-stroke cycle engines, the same fuel cams may be used for ahead and astern operations of the engine. This is accomplished by rotating the camshaft by means of a reversing servomotor, actuated by lubricating oil pressure, as illustrated in Fig. 14. To reverse the engine, the control lubricating oil is directed to the proper side of the rotary vane, which is connected to the camshaft. The pressure of the oil against the vane then forces the vane to move through an arc to the astern position. The number of cylinders on the engine determines the arc of travel.

To go ahead again the lubricating oil pressure shifts the vane back to the ahead position. The same starting air cam is used for both the ahead and the astern positions, and the same fuel cam must also be used. This is possible only if the cams are symmetrical. To prevent oil leakage from the housing, the whole unit is sealed.

Summary

The reversing mechanism system should be able to perform several functions:

1. it must stop the fuel to the cylinders when the engine stops.

2. When the engine is put in the ahead position, the fuel pumps should be in a position to supply fuel; then the starting air supply should be activated.

3. When maneuvering from the ahead to the astern position, the fuel must be stopped and the cams moved to the astern position. During this period a brake is often applied to the shaft to stop the engine. If the engine is still rotating, it may continue to run in the same direction even though the cams are shifted. The exhaust valves would then act as inlet valves, and the engine would exhaust through the intake valves into the engine room. As the engine goes to astern, fuel is supplied and then the air is started. There are usually a variety of interlocks associated with engine reversing. If the cam position is not correct, the engine will not start. If the lube oil pressure is not correct, the engine will not start. There may be other interlocks, depending upon the manufacturer.

GENERAL REVERSING DETAILS

Most main engines are of the direct-coupled 2-S type, medium or high-speed engines require reduction gearing and the trend here is to unidirectional types. Hence the need for the reversing mechanisms for 4- S engines is reducing. For these reason the 2-S reversing mechanism will be considered in greatest detail.

2-STROKE REVERSING GEAR

It is usually necessary to reposi