Internal Combustion Engine

The Otto Cycle

Each movement of the cylinder up or down the cylinder is one stroke of the four stroke combustion cycle or Otto cycle. Most modern internal combustion engines use the four stroke cycle. The four stroke cycle consists of an induction stroke where air and fuel are taken into the cylinder as the piston moves downwards, a compression stroke where the air and fuel are compressed by the upstroke of the cylinder, the ignition or power stroke where the compressed mixture is ignited and the expansion forces the cylinder downwards, and an exhaust stroke where the waste gases are forced out of the cylinder. The intake and outlet ports open and close to allow air to be drawn into the cylinder and exhaust gases to be expelled. The cycle is illustrated in the picture shown below:

During the intake stroke the inlet valve opens at the top of the cylinder, as the piston moves down air and fuel are drawn into the cylinder. As the piston reaches its lowest position the inlet valve closes and the piston travels upwards compressing the air-fuel mixture. As the piston reaches its highest position at maximum compression a spark ignites the mixture causing a rapid expansion of gas raising the pressure in the cylinder and forcing the piston downwards. Once the cylinder has reached its lowest position the outlet port opens and as the piston rises up the cylinder the exhaust gases are forced out. The valves which open and close the port are sprung to make them naturally close. The valves are opened by a system of rotating cams and pushrods driven by a camshaft which in turn is timed and driven from the crankshaft. The valve timings vary between engines depending on the setup, generally there is some overlap to speed the flow of gases as shown below.

Ref: [15],[K]

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