MARINE ENGINEERING KNOWLEDGE (MOTOR) TIME ALLOWED - 3 HOUR

Instructions 1. Answer any SIX questions. 2. All questions carry equal marks. 3. Neatness in handwriting and clarity in expression carries

weightage 4. Illustration of an answer with clear sketches ? diagrams

carries weightage 5. All unused pages of answer script must be cancelled out by

two lines (X) across the page.

Q1. As second engineer you have been requested to obtain a set of indicator card (diagram) from the large slow speed engine of a recently purchased second hand ship. A. Describe your initial checks and preparations; B. State with reason the types of card you would consider necessary and explain the procedure for obtaining these; C. State in order of importance the additional information required with the card; D. State your procedure for analysis of the cards and obtaining cylinder powers.

Answer:- The initial checks and preparation are as follows:

(1)Ensure the weather is fine, RPM steady and constant.

(2) The exhust temperatures are normal and steady.(3) The indicator instrument should be opened and the

cylinder cleaned internally with the brush provided for the same. Before assembling the piston of the instrument the cylinder wall is to be lubricated with a few drops of oil specially provided for the same.

(4) Before connecting the indicator instrument to the indicator cock , blow through the indicator cock to clear the passage through the cock.

(5) Adjust the length of the connecting cord so no slack occurs in the drum connection. This ensures true drum motion. Place the indicator paper on the drum and fix it tight within the paper clips. Draw the atmospheric line by touching the stylus on the paper and turning the drum with the cord.. Operations (3) to (5) should be repeated for each unit.

(b) The types of cards taken are as follows:

(1) Indicator diagram through the instrument for assessing the indicated Power of each unit.

(2)Draw card taken on the same indicator sheet for each unit. The draw card is necessary to show distinctly the difference in compression pressure and peak pressure.

(1) Light spring diagram taken occasionally if any defect is suspected in the scavenging and exhausting operations.

(2) Compression card taken if compression of any cylinder is suspected to be inadequate or defective.

Procedure for taking power card:- The indicator with paper properly mounted is connected to the cock outlet, and cord connected to the actuating lever driven by engine. Open the indicator cock and touch the stylus tip on the paper mounted on drum ,for a short while till one full diagram is drawn. Ensure the stylus is withdrawn immediately when a full diagram is recorded. This will ensure overwriting on the diagram already recorded. Blow through the indicator cock before taking the indicator card. This should be repeated for each unit.

Procedure for taking Draw card.:- Mentally count the firing sounds and in the same timing pull the drum cord with stylus touching the paper. The Draw card is drawn. This comes by practice.

Procedure for taking light spring diagram:- The procedure is same as per the power card, except that in this case the main spring of the instrument is replaced by a light spring.

Procedure for taking compression card.:- The procedure is same as per the power card, except that in this case the the fuel to the unit is cut out when the card is drawn which gives a compression card.

(c)The additional information to be recorded is as follows:-

(1) Exhaust temperatures of all units.

(2) Cooling water outlet temperatures from each cylinder head.

(3) Lub-oil pressure , inlet and outlet temperatures.

(4) Scavenge air pressure and temperature.

(5) T/C temperature across turbine.

(6) Air pressure drop across air cooler.

(7) Air pressure drop across air filter..

(8) Turbine RPM.

(9) Fuel oil consumed during the period of the experiment, taken from fuel flow meter during the period of experiment.

(10) RPM calculated as a mean from counter readings taken at commencement of experiment and end of experiment

(11) engine load control position

(d) The total indicated power is calculated as follows.

The area of each power diagram is found by a planimeter or by use of Simpsons rules. This area is given in mm2 . the area divided by length of the diagram in mm is the mean height of the diagram given in mm. The main spring of the instrument has a spring constant given by 1 mm= x KN/M2 . Hence the mean effective pressure of each cylinder unit can be calculated and the mean effective pressure is given by P in the formula for power. The power produced by each cylinder unit is given by P*L*A*N *2* π /60 kw., where :

(1)P is the mean effective pressure given in KN/M2 (2) L is the length of stroke given in metres.(3) A is the area of bore of each cylinder given in M2. (4)N IS the RPM/60 measured from counter readings .

This gives the product in KNM/ sec , which is KW The total power is sum of all individual powers produced in each unit

The specific fuel oil consumption (SFOC) is calculated as follows :

From the fuel oil clock readings we get the fuel consumed by the engine in litres.

If it is v litres consumed in the time duration of the experiment given in hours, say t hours . Then the mass of fuel oil consumed is given by v* ρ, where  ρ is the density of the fuel oil at the corresponding temperature. Given in grams . The SFOC is now

given by , say for a six cylinder engine as ( P1 + P2+P3 +P4 + P5 +P6

) / V* ρ grams per KW hour, where ( P1 +P2 + P3 + P4 +P5 +P6 ) is the total power produced by the engine.

Quality analysis:- Apart from estimation of Indicated power and SFOC , the Draw card is used to check if peak pressures and compression pressures of all units are the same. If they are not suitable corrective action should be taken as follows :

(a)For uniform peak pressure the individual fuel pump rack settings may be adjusted.

(b) If individual compression pressure are low, first confirm by compression card and then the unit may have to be opened to renew broken or worn out piston rings.

(c) It is also essential that the power produced in each unit is uniform.

By doing this adjustment exercise the engine works vibration free and efficiently

Q2. A. How is the cylinder lubricators fitted in a large marine diesel engine that has a water jacket? B. Where is the location of the lubricator outlet in relation to the piston rings on the piston? C. Why is this position necessary?

Answer:- The cylinder lubricator quill is fitted on the jacket and crosses the water space between liner and jacket. The quill end rests within the liner and oil is fed through an oil hole drilled at the location of the quill, The arrangement is shown in sketch given under. The necessary expansion allowance is also shown in sketch, The location and timing of injection depends on type of engine, The ideal delivery of lubricating Oil would be to locate the feed points and time the delivery so that oil is delivered at colder region of the liner and at a time when the top ring just covers the feed points during the travel of the piston in the compression stroke. The same principle is followed in four stroke and two stroke uniflow scavenged engines. In two stroke port engines the feed points however , must be located above the exhaust ports for obvious reasons of oil loss through ports. A

groove is cut to act as storage of a quantity of oil.. only one point may be fed in one revolution so that 8 points ( around the liner assumed ) are fed in eight revolutions of the engine.

Q3. A. Describe the causes and effects of bacterial attack on

diesel engine lubricating oil. B. Bacterial activity has been detected in the lubricating oil of the main engine fitted in the ship aboard which you are serving as 2nd eng. Write a letter to the owner operator of the ship indicating the action you intent to take and offer suggestion to the avoidance of future incidents

Answer:- -(a) The bacterial degradation of lubricating oil is caused by the following influences and environmental conditions affecting the lubricating oil.;

(1) Microbes need water and food to survive. Ingredients provided by the hydro carbons and water in the oil is sufficient to allow them to breed and multiply.

(2) They breed in a favourable temperature of anywhere between 150 c and 400 c

(3) Water leakage into lubricating oil could be from leaky coolers and even otherwise condensation of moisture from atmosphere in storage tanks and sumps is a constant source of water entering the oil.

(4) A stagnant condition within the sump or storage tank is also favourable for microbial breeding.

Hence to prevent this degradation conttinuoys purification of the oil is essential even when the engine is stopped during port stay. By continuous purification the oil in the sump is in a state of movement and its ambient temperature is higher since a higher temperature is needed for purification.. The water in the oil is also being removed continuously, and so continuous purification of the oil is a good and reliable method to prevent microbial attack. The symptoms of biodegradation or microbial attack are :

(1) A bad odour emanating from the sump resembling the smell of rotten eggs.

(2)The oil becomes more opaque and darker(3) It forms an emulsion with water and is very

difficult to separate oil from water by standing as well as purification.

The effects of microbial attack are as follows ;

(1) The oil loses its viscosity considerably and hence is unfit for further use.

(2) The oil becomes acidic by microbial attack and hence can cause corrosion of pins and journals which come in contact with this oi.

b) To , The Engineer superintendent.,

The Commonwealth shipping co ltd.

Subject;:- Decomposition of sump lub - oil

Dear Sir,

I wish to bring to your notice that the main engine sump lub-oil is contaminated by microbial attack. There is a foul smell coming from the sump through the crank case. I have also checked a sample of the oil from the sump and found it is discoloured being darker and opaque. The ship was laid up for about 40 days undergoing her third special survey and came out of dry dock on 5th October 2014. The ship is likely to sail out on the 10th of October when the survey will be completed

We have about 15000 litres of fresh crankcase oil in the storage tank which we will transfer to the sump after transferring the contaminated oil from sump to renovating tank, The sump will be cleaned thoroughly before transferring the fresh oil. I intend to recover the contaminated oil by heating and settling the oil in the renovating tank and further purifying it. I however do not assure of success in this attempt. I request you to supply 15000litres of fresh crank case oil, the indent for which is enclosed herewith

This problem could have been avoided if purification of the sump oil was continued during the layup I signed on the ship on 3rd October when i noticed foul smell from the crank case. I would suggest that you issue a suitable notice to all ships in the fleet .

Yours faithfully,.

Q4: With reference to Turbo-Chargers; A. Explain why when surging occurs, the speed off the turbocharger is seen to change slightly; B. Give examples of defects likely to result in surging; C. Explain in simple terms what occurs in a turbocharger and the diesel engine as the speed / power increases. Based on the explanation what is meant by matching of T/C with an engine? D. State the advantage of fitting uncooled turbochargers.

Answer :- (a)Surging is a phenomenon that affects centrifugal compressors, when the continuous flow of air is obstructed in its passage through the vanes of the compressor. This obstruction causes return flow of air through the same vanes causing interference in the air flow through the vanes. This is accompanied by loud sounds of ‘ howling and banging’. When the obstruction is cleared the flow is resumed and this is seen as a change in RPM. The RPM fluctuates during minor surging as the obstruction occurs and clears.

(b) When surging occurs , the defects likely to occur are as follows ;

(1) Howling and banging noise heard.

(2) Turbine RPM fluctuates and in worse cases it is reduced.

(3) Pressure drop across air cooler increases due to obstruction in free passage of air

(4) Scavenge pressure drops.

(c) When the speed /power ratio increases the turbocharger has to cope with the increased mass of air required for the proper combustion of increased fuel and for this purpose the turbine

should be capable of turning the rotor at a higher speed required and this is possible by the increased mass of exhaust gas flow. How ever this requirement has to be tried out practically and this test is called turbocharger matching with engine .To obtain efficient and stable operation of the charging system it is essential that the combined characteristic of the engine and blower are carefully matched. . The engine operating line , as indicated on the figure is mainly a function of these characteristics and taking into account the fact that blower efficiency decreases as the distance between surge and operating lines increases, the matching of blower to engine becomes a compromise between acceptable blower efficiency and a reasonable safety margin against surge. An accepted practice is to provide a safety margin of 15 to 20 percent to allow for deterioration of service conditions. Such as fouling and contamination of turbochargers.

Cooling media for cooled exhaust gas casings is generally from the engine jacket water cooling system although in some cases sea water is employed.. In both cases anti corrosive plugs are fitted to prevent or inhibit corrosion on the water side.. With water cooled casings experience has shown that under light load condition s when low exhaust temperatures are encountered it is possible that precipitation of corrosive forming products --mainly

sulphuric acid will occur on the gas side of the casing..This results in serious corrosive attack which is more marked at the casing. This results in serious corrosive attack which is more marked at the outlet casing because of lower temperatures. Methods of prevention such as enamelling and plastic coatings ,etc have been tried to alleviate this problem with varying degrees of success. A particularly effective approach to the problem is the use of air as the cooling media with the result that this particular instance of corrosive attack is virtually eliminated.

NOTE :-Shown below are two relevant sketches of how turbocharger matching is done. These are for information and understanding the process and hence need not be produced in the exam.

Q5. A. Describe briefly the operation of an electrical or hydraulic main engine governor B. For the type described indicate how failure can occur and the action to be taken if immediate correction cannot be achieved and the engine must be operated.

Answer:-The main propulsion engine works at a constant load setting which varies nearly as the square of the propeller rotational speed and this is the commonly referred propeller

law. Since it is working at a constant load and speed as long as the sea is calm, the ordinary proportional governor with an amplification system for the controlling force should suffice to take care of the variations in the sea load condition caused by heavy seas. However at times when the variations in speed are too fast especially with the weather turning bad , the governor is cut off and the engine controlled manually since the proportional action is slow.

A proportional action governor is diagrammatically shown in Figure.. The centrifugal speed measuring unit is fitted with a conically shaped spring. This gives a spring rate which varies as the Square of the speed.. This gives linearity to the speed measuring system, i.e. the response is directly proportional to the change in speed. If we consider an increase in load on the engine, the pilot valve will move down due to the speed drop. The piston in the servo-amplifier will move up and increase the fuel supply to the engine. The feedback link reduces the force in the speeder spring so that the flyweights can move outwards to a new position, thus raising the pilot valve and closing off the oil supply. If for some reason the oil supply system should fail then the spring loaded piston in the servo- cylinder would be moved down and fuel to the engine cut off. This is called fail safe. Any oil that leaks past the servo-piston will be drained off to the oil sump tank. If this were not so the servo-piston would eventually lock in position.

Q6. With reference to a particular make of main propulsion unit, describe how the engine is reversed manually and discuss the problems involved and the safety precautions which would be required if the control were operated remote from the machinery space.

Answer:- The engine is reversed manually in the MAN B&W engine by an actuating lever connected to a screwed spindle which can be connected to the actuating link for emergency use and manually operated from the local manoevring station. The system is shown in the sketch given under .

(b)When the control is operated remote from the local emergency control station say either the Bridge or Engine control room, the reversing interlock which is activated when engine starts to turn can only be unlocked when engine stops. In this condition the reversing lever can be moved to the desired direction. This protective device is provided to safeguard the camshaft and the cams mounted on the camshaft. This also provides safety to the engine components if reversal is accomplished whilst turning has not stopped . In extreme cases it can cause slippage of journal web shrink fit.

PLAN AT XX

Q7. With respect to Diesel Engines, describe the cause of – i. Piston rings that is too tight in the piston grooves; ii. Corrosion of piston rings; iii. Piston grooves that are worn; iv. Piston rings that is too slack with piston grooves. B. Describe the effect of each of the faults listed above and the remedial action for each.

Answer:- (1) The recommended axial clearance with rings fitted in the grooves is important. This clearance allows free movement of the ring in the groove as the piston travels along the stroke. This clearance reduces at working temperature and this clearance can become nil if the thickness of the ring is more than the maximum allowable thickness. Restriction of free movement of the ring in the groove causes the ring to stick in one position and this leads to breakage of the ring. Improper combustion causes accumulation of un burnt carbon in ring grooves causing the rings to stick in the grooves and break. This is especially true for top ring grooves where un burnt carbon can collect in the grooves

(2)Corrosion of piston rings is caused by the burning of sulphur in the fuel oil to sulphur-di-oxide and combining with the water vapour ,formed by the burning of hydrogen in the fuel oil , to form a weak acid which causes corrosion of the piston ring which comes in contact with the products of combustion.

(3)Piston ring grooves wear caused by abrasion of ring grooves top and bottom surfaces caused by mixture of unburnt carbon and cylinder oil which acts as an abrasive substance coming in contact with the surfaces due to movement of rings in their grooves.

(4)New piston rings if found having excessive axial clearance after fitting indicates that the grooves are worn out excessively and this will lead to ring breakage.

(b) Remedial measures :- (1) Ensure that new rings when fitted have the correct axial clearance in cold condition. T he cylinder lubrication oil feed is correctly adjusted so that over lubrication is avoided.

(2) The cylinder oil used should have the correct TBN rating to avoid corrosion.

(3) Over lubrication of cylinder oil causes formation of the abrasive mixture leading to wear of grooves. Abrasive mixture of un burnt carbon and oil is also corrected by improving the quality of combustion so that un burnt carbon is avoided.

(4) The width of ring grooves should be measured and calibrated for future reference. If the widths have increased, the piston ring grooves should be reconditioned by a standard process of building up the grooves and machining the grooves to the standard width. The reconditioning is done by licensed and approved engine component reconditioners.

Q8. A. Describe the precaution necessary during the initial running-in of an Auxiliary Engine run on which is newly installed or has a

major overhaul; B. Explain the possible causes of oxidation of lubricating oil. C. State the frequency with which oil samples should be taken for analysis.

Answer:- (A) When an auxiliary engine has under gone a major overhaul which includes the renewal of all the piston rings as well as some liners, The engine needs to be run at half the rated speed and no load for at least 3 to six hours so that the high spots on the new rings and the new liners are gradually smoothened and ground off by the cylinder lubrication which is existing in the cylinder when the engine is running.

(B) Oxidation is the reaction of oil molecules with oxygen molecules. It can lead to an increase in viscosity and the formation of varnish, sludge and sediment. Additive depletion and a breakdown in the base oil can also result. Once an oil starts to oxidize, you may see an increase in the acid number. In addition, rust and corrosion can form on the equipment due to oxidation.

Thermal BreakdownThe temperature of the lubricant should be a primary concern. Besides separating the moving parts within a piece of machinery, a lubricant must also dissipate heat. This means the lubricant can and will be heated above its recommended stable temperature. The Arrhenius rate rule for temperature states that for every 18 degrees F (10 degrees C), the chemical reaction doubles. In other words, for every increase of 18 degrees F for your oil, the life of the oil is cut in half. Keeping the oil as cool as possible when in use will extend its life and reduce the reaction of thermal breakdown.Also known as pressure-induced thermal breakdown (degradation), micro-dieseling is a process in which an air bubble transitions from a low-pressure region in a system to a

high-pressure zone. This is very common in hydraulic systems. Micro-dieseling results in adiabatic compression of the air bubble within the oil, which then cooks the surrounding oil molecules, causing instant oxidation of those molecules.

(C) The frequency for removal of samples for analysis is once every three months.

Q9. With reference to main engine tie rod explain. A. Function; B. Effect on main engine in case slack; C. Identifying slackness; D. Material; E. Tightening procedure.

Answer:- (a) The function of the tie-rod is to bind together the cylinder entablature or block, the ‘A’ frame and the bedplate together with a high tensile force, so that these three components are at all times , during a cycle of operation, in compressive mode. To enable this requirement , the tie-rods are prestressed to a high tensile force during the assembly of the components. (b) If any tie-rod is slack it upsets the distribution of the prestressed tensile forces on the components and if prolonged for long running periods in this condition it will lead to fatigue defects on these components exhibited as fatigue cracks. (c) Whilst on watch during sailing , the top tie-rod nuts should be felt regularly to feel the vibration at the tip of the tie-rod. If any rod end exhibits a higher vibration amplitude which is noted by feeling , then that tie-rod is slack. This recognition comes only if the rod ends are felt everyday during sailing. (d)The tie-rod is made of medium carbon steel and the material composition is as follows: Carbon-------0.60 percent max. Silicon --------0.45 percent max. Manganese-----0.30---1.50 percent Sulphur ---------0.40 percent max

Phosphorus -------0.40 percent max Residual elements; Copper --------------0.30 percent max. Chromium ------------0.30 percent max Molybdenum ------------0.15 percent max. Nickel -------------------0.40 percent max.

(e)The tightening procedure is by use of hydraulic tightening device , tightening in one or two stages as recommended by builders and proceeded in a sequence as shown in the sketches given below.