The inlet valve must be open to allow the air-fuel mixture into the cylinder while the exhaust valve remains closed. This stroke takes place with the piston moving down from TDC to BDC. The purpose of the intake or induction stroke is to draw a mixture of air and fuel into the cylinder. The engine will always complete the cycle in the same order: Figure 2: Elements of the Four Stroke Cycle Intake or Induction Since it takes two crankshaft revolutions to complete one four-stroke cycle, the complete cycle will be completed at half the engine RPM e.g engine running at 3000 RPM will complete 1500 complete cycles in one minute. Each step in the cycle represents a 180° movement of the piston which corresponds to a half-revolution of the crankshaft. \( S.V.: \) Swept Volume The Four Stroke CycleĪs long as the engine is running, it will continue to repeat the four steps in a four-stroke cycle continuously. This can be calculated by multiplying the bore by the stroke: Swept Volume – this is the difference between the volume of the cylinder with the piston at TDC and at BDC. As an example, an engine with a compression ratio equal to 9 has a volume in the cylinder nine times greater with the piston at BDC than at TDC. The ratio between the two give the compression ratio. Stroke – the stroke of the engine is the reciprocating distance that the piston moves in the cylinder from BDC to TDC.īore – this refers to the inside diameter of the cylinder.Ĭompression Ratio – the volume of the space in the cylinder can be determined with the piston at BDC and at TDC. The piston is situated near the top of the cylinder head and the crankpin is at its uppermost position.īottom Dead Center (BDC) – this refers to the point in the cycle where the piston is at the bottom of its travel and the crankpin is at its lowest position. Top Dead Center (TDC) – this refers to the position of the piston when it is at the top of its travel. Figure 1: The bore and stroke of a piston moving in a cylinder
Refer to the image below and the definitions underneath the image. There are a number of definitions that should be well understood before continuing with the details of the four-stroke cycle. A four-stroke cycle includes an intake and compression step (one crankshaft revolution) and a power and exhaust step (one crankshaft revolution).
A four-stroke engine takes two crankshaft revolutions to complete one engine cycle with the piston moving through 180° to complete each step in the cycle. Two stroke engines are common on microlights and some smaller ultralight aircraft, as these engines have a lower part count making them simpler to operate and cheaper to procure and maintain.įour-stroke engines are the most common engine type in use in the general aviation industry and it is this engine type that we will study further. Two-stroke engines complete a cycle in one revolution of the crankshaft with two movements an up and down stroke of the piston which incorporates intake, compression, combustion and exhaust.
Piston engines are classified according to the number of distinct steps that the engine performs in one complete engine cycle.