05 CI vs SI engine and 2 vs 4 stroke engine

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Four-stroke engines The majority of reciprocating engines operate on a four-stroke cycle: each cylinder requires four strokes of its piston (two revolutions of the crankshaft) to complete the sequence of events which produce one power stroke. Both SI and CI engines use this cycle which comprises: 1) An intake stroke, which starts with the piston at TDC and ends with the piston at BDC, which draws fresh mixture into the cylinder. To increase the mass induced, the inlet valve opens shortly before the stroke starts and closes after it ends. 2) A compression stroke, when both the valves are closed and the mixture inside the cylinder is compressed to a small fraction of its initial volume. Towards the end of the compression stroke, combustion is initiated and the cylinder pressure rises more rapidly. 3) A power stroke, which starts with the piston at TDC and ends at BDC as the high temperature, high pressure gases push the piston down and force the crank to rotate. About five times as much work is done on the piston during the power stroke as the piston had to do during compression. As the piston approaches TDC the exhaust valve opens to initiate the exhaust process and drop the cylinder pressure to close to the exhaust pressure. 4) An exhaust stroke, where the remaining burned gases exit the cylinder: first, because the cylinder pressure may be substantially higher than the exhaust pressure; then as they are swept out by the piston as it moves toward TDC. As the piston approaches TDC the inlet valve opens. Just after TDC the exhaust valve closes and the cycle starts again. The four-stroke cycle requires, for each engine cylinder, two crankshafts revolutions for each power stroke. To obtain a higher power output from a given engine size, and a simpler valve design, the…