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Marine Diesel Engines

Power Calculation

Power Cards

A power card is a graph of cylinder pressure against time, it was originally drawn using a mechanically driven pen onto graph paper mounted on a drum. The drum was rotated by string, via a cam on the camshaft and pushrod. As the drum rotated the pen mounted on the linkages was pressed up to the paper. For clarity the pen is released once a single cycle has passed otherwise slight fluctuations in power demand could lead to several cycles being superimposed on one another blurring the image.

The indicator is a sensitive piece of equipment which can malfunction and so it must be treated with care. It can only be used effectively on an engine operating below 200 rpm due to the difficulty involved in getting only a single line on the card. In addition the inertia in the drum can lead to delays distorting the shape. For higher speed diesels either peak pressure indicators are used, or sophisticated electronic monitoring equipment is required with oscilloscope type displays. The time base for these is off transducers mounted on the flywheel.

It is important that the indicator is kept well lubricated with a light high quality oil . Prior to mounting the indicator the indicator cock is blown

through to ensure it is clear. Compression cards are then first taken to check for errors caused by wear or friction/stiction in the instrument.

Compression curves

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Two stroke cycle power card

Bottom dead centre

1. scavenge port closed 2. exhaust port shut-commence of compression 3. fuel injection 4. top dead centre 5. 7post combustion expansion 6. exhaust port opens

Four stroke cycle

Shown above are typical power cards for 4 stroke engine. The lower one shows the effect of improving turbocharger efficiency. That is some mechanical effort is made by the charge air pressure lowering fuel consumption. Poor timing can negate this effect.

Shown below is a power card drawing taken from an exercise book. it should be noted that the 3rd and 4th stroke indicate power is being abosrbed. It is probable that this drawing was made for a non

turbocharged engine although the source is forgotten. The atmospheric line would split the 3rd and 4th stroke

o 3-4-5 fuel injection and combustion o 5-6 expansion o 6-7-8-Exhaust valve open o 8-9-10 overlap, exhaust remains open whilst air enters o 10-1 aspiration and exhaust valve closes

Power calculation

The area swept out by the power stroke will give the power developed by the engine. It should be noted on a four stroke most of the non-power stroke occurs below atmospheric on a naturally aspirated engine and so gives a net loss of power.

Power = p.A.L.n p - mean average pressure in the cylinderA-area of piston[m3]L-stroke [m]n-revolutions per second

From a power card this is altered to

Power = area of diagram/length of diagram x Indicator spring constant

By use of an instrument called a Planimeter the area scribed out by the pen could be measured giving the power generated by the cylinder. In addition, through experience, certain problems could be diagnosed by looking at the shape drawn.

Fault diagnosis

As indicated there are practical difficulties with use of the power indicator instrument on a high speed four stroke engine. Therefore the following is based around the two stroke

The light spring diagram For this, the spring is replaced with one of much lower spring constant. In this way the operation at the lower pressures, i.e. around bottom dead, may be examined. In particular this gives indication of blocked or restricted scavenge and exhausts. To further clarify, the motive effort for rotating the drum is often by hand so only a small part at the end of the stroke is covered.

Draw card (90o out of phase)

Scavenge port opens at 140 degrees after top dead and closes 140 degrees before top dead.

Early injection

Early injection can be caused by incorrect fuel timing, broken or wrongly set up fuel injector, incorrect fuel condition, overheating of parts around the combustion space.

Its effect is to increase the maximum cylinder pressure. There will be an increase in combustion efficiency but the increased peak pressure leads to overload of the bearings and shock to pressure parts.

Late injection

Late injection can be caused by loss of compression, insufficient scavenging, delayed timing, incorrect fuel condition and atomisation, undercooled parts around the combustion space. It results in a condition called diesel knock where the flame front travels rapidly down the liner to strike the receding piston. In addition, leads to afterburning and high exhausts

Afterburning

Causes loss of power, smoke and high exhaust temperatures. Can lead to damage to exhaust valves and seats as well as piston crowns. Fouled turbocharger and waste heat recovery units. High cylinder temperatures causes problems with lubrication

Leaking fuel injector

Detected by loss of power, smoky exhaust and high temperatures. A knock can be heard on the fuel supply system. Can lead to after burning .