This action expels the spent fuel-air mixture through the exhaust valve(s). EXHAUST: during the exhaust stroke, the piston once again returns to top dead center while the exhaust valve is open.The resulting pressure from the combustion of the compressed fuel-air mixture forces the piston back down toward Bottom Dead Center (BDC). While the piston is close to Top Dead Centre (TDC), the compressed air–fuel mixture in a gasoline engine is ignited, by a spark plug in gasoline engines, or which ignites due to the heat generated by compression in a diesel engine. POWER: this is the start of the second revolution of the cycle.COMPRESSION: with both intake and exhaust valves closed, the piston returns to the top of the cylinder compressing the air or fuel-air mixture into the cylinder head.A mixture of fuel and air is forced by atmospheric (or greater) pressure into the cylinder through the intake port. The piston descends from the top of the cylinder to the bottom of the cylinder, increasing the volume of the cylinder. INTAKE: this stroke of the piston begins at top dead center.A stroke refers to the full travel of the piston along the cylinder, in either direction. A four-stroke engine (also known as four-cycle) is an internal combustion engine in which the piston completes four separate strokes which comprise a single thermodynamic cycle.
