•The piston descends from the top of the cylinder to thebottom of the cylinder, reducing the pressure inside thecylinder.• A mixture of fuel and air, or just air in a diesel engine, isforced by atmospheric (or greater) pressure into thecylinder through the intake port. The intake valve(s) thenclose.•The volume of air/fuel mixture that is drawn into thecylinder, relative to the volume of the cylinder is called,the volumetric efficiency of the engine.
During the intake stroke, the piston moves downward, drawing afresh charge of vaporized fuel/air mixture. The illustrated engine features a poppet intake valve which is drawn open by the vacuum produced by the intake stroke.
•The piston returns to the top ofthe cylinder compressing the air,or fuel-air mixture intothe combustion chamber of thecylinder head.
As the piston rises, the poppet valve is forced shut by the increased cylinder pressure. Flywheel momentum drives the piston upward, compressing the fuel/air mixture.
•While the piston is close to Top Dead Center, the compressedair–fuel mixture in a gasoline engine is ignited, usually bya spark plug, or fuel is injected into the diesel engine, whichignites due to the heat generated in the air during thecompression stroke.•The resulting massive pressure from the combustion of thecompressed fuel-air mixture forces the piston back downtoward bottom dead centre.
At the top of the compression stroke, the sparkplug fires, igniting the compressed fuel. As the fuel burns it expands,driving the piston downward.
•The piston once again returns to top dead centerwhile the exhaust valve is open.•This action evacuates the burnt products ofcombustion from the cylinder by expelling thespent fuel-air mixture out through the exhaustvalve(s).
At the bottom of the power stroke, the exhaust valve is opened by the cam/lifter mechanism. Theupward stroke of the piston drives theexhausted fuel out of the cylinder.