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4 Stroke Procedures Detailed Annotation of Diesel Generator Units

 Date:2019/02/26


Diesel generator units normally use 4 stroke diesel engines to rotate generator motor. 4 stroke diesel engine working procedure is composed of intake, compression, power and exhaust. Piston will finish one complete working circle by finish these four procedures.

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Intake stroke

Regarding the first stroke (intake), its mission is to fill cylinder with diesel mist and air mixture. When intake begins, the piston starts from top dead center (T. D. C.); there remains few exhaust gas inside of cylinder. During crankshaft rotating, rod pulls down piston, crankshaft transmitting to open intake valve.

During the piston moving down, decreasing air pressure will suck diesel mist and air mixture to fill in cylinder. During this procedure, the cylinder air pressure will fluctuate according to cylinder volume change. Due to few remnant exhausts remain from previous working circle, the air pressure will be higher than atmospheric pressure; due to flush resistance while air passing intake valve and pipes, the intake pressure is about 0.085-0.095MPa, which is lower than atmospheric pressure, during the whole intake procedure, the pressure inside of cylinder almost keep steady.
While piston reaches bottom dead center (B. D. C.), air still flushing in the cylinder with very high speed and high inertia, the valve will be closed just a little bit after piston sweep B. D. C., to take full advantage of inertia and get more air in the cylinder. So even the piston start to move up, some air can still fill in the cylinder.

Compression stroke

Second stroke (compression), piston move from B. D. C. to T. D. C., this stroke achieves two functions: 1. warm up air temperature, get ready for burst, 2. get ready for air expand.

While piston moving up, valves are closed, fuel mist is compressed, and the pressure and temperature are keeping increasing. The pressure & temperature on T. D. C. point is relative to compression rate, normally Pc=4~8MPa, Tc750~950K.

The diesel self ignition temperature is 543-563K, and the temperature of T. D. C. is much higher than that, which is more than enough to ensure diesel mist to self ignition inside of the cylinder. However the diesel mist will not self ignition right away, after physical & chemical reaction, which request 0.001-0.005 second (ignition delay period), the diesel mist will then burst.

So the diesel mist must be injected to cylinder while crankshaft 10-35° ahead of the T. D. C., after crankshaft rotate 5-10° behind the T. D. C., the cylinder achieve highest burst pressure, to force piston move down.

Power stroke

The third stroke (power stroke). When this stroke begins, most fuel mist burst in the cylinder. Massive heat energy is released, temperature and air pressure keep increasing severely to push piston move down, and crankshaft is rotated by rod, to release power, that’s why this stroke also called working stroke.

Piston keep moving down, cylinder volume keep increasing, air pressure also decreasing, power stroke ends while piston reach B. D. C., exhaust valve open at the same time.

Study the pressure curve, the rising part means pressure severely rise during fuel mixture bursts, the highest point means highest burst pressure Pz = 6-15MPa, Tz = 1800-2200K. the ratio between Pz/Pc is called pressure rising ratio, which represented by λ. The λ value range at highest power out put is λ = Pz/Pc1.2-2.5, which according to different diesel engine type.

exhaust

The exhaust stroke will discharge exhaust gas after burst, ready for next refill of diesel mis. During the power stroke, when piston move down and near BDC, then discharge valve will be opened, driven by crankshaft and rod, piston keep moving up and discharge exhaust gas.

Due to the resistance of exhaust system, at the beginning of the exhaust stroke, the air pressure inside of cylinder is 0.025-0.035MPa than atmospheric pressure, temperature around Tb=1000-1200K. exhaust valve will be opened a little bit before piston move to the BDC, to reduce resistance of piston movement while discharge exhaust. Once the valve opening, high pressure exhaust air flush out of cylinder, air pressure suddenly reduce, piston keep moving up, to discharge most exhaust air.

In order to discharge as much as exhaust gas, exhaust valve will be closed right after piston move over TDC, to take complete advantage of air flush inertia. The exhaust stroke curve showing that the air pressure almost keep steady, but a little bit higher than atmospheric pressure.


After exhaust stroke finish, intake stroke start again, the whole working circle repeat according to above procedures. The working circle is composed of four piston strokes and crankshaft rotate two turns, so it was named as stroke diesel engine.

Only the third stroke (power stroke) generating and external power, the rest three strokes are prepare for power stroke and consuming power.