frequently used engine specification. Displacement indicates engine size, and compression ratio cor the total cylinder volume to compression chamber volume The term“ stroke" describe the movement of the pis ton within the cylinder. The oper- ating cycle may require either two or four strokes to complete. Most &l four stroke cycle als known as Otto cycle, after the name of its inventor, Nikolaus Ot to,who first applied the principle Fig 1-2 The four strokes cycle a)intake stroke b)compression stroke c)power stroke d)exhaust stroke in 1876. In the 4-stroke engine, four strokes of the piston in the cylinder are required to complete one full operating cyele. Each stroke is named after the action. It performs intake, compression, power, and exhaust in that order, shown in ig.12 The piston moves downward to the bottom dead center, a vacuum is created in the cylinder. The intake valve opens and air-fuel mixture comes into cylinder. To obtain the maximum filing of the cylinder the intake valve opens about 10 before t.d. c. giving 20 overlap. The inlet valve remain open until some 50 after b.d. c. to take advantage of mixture 2. Compression strok The air-fuel mixture is compressed within the combustion chamber. While the pres se to about IMP, depending on various factors including the compression ratio, throttle opening and engine speed. The spark plug is fired to ignite the air-fuel mixture prior to the piston being at the t d. c.. Note that both valves are closed 3. Power stroke The air-fuel mixture expands, which creates the power to force the piston downward. The exhaust valve opens near the bottom of the stroke 4. Exhaust stroke As the piston starts to move upward, the exhaust valve is opened The piston moving up forces the exhaust gases out of the cylinder. The intake valve usually opens just before the exhaust stroke This 4-stroke cycle is continuously repeated in every cylinder as long as the engine remains running A2-stroke engine also goes through four actions to complete one operating cycle. However, the intake and the compression actions are combined in one stroke, and the power and exhaust actions combined in the other stroke. The term 2-stroke cycle is preferred to the term 2-cycle, which is really not accurate