by John B

Created on: August 21, 2008

The four stroke engine is a more specific term for the type of internal combustion engine that you would find in the typical automobile. A two stroke engine is typical of small motors such as outboard motors and lawn mowers. But even these applications are moving to four stroke for environmental reasons.

The four-stroke engine derives it's name from the fact that there are four distinct stages in the cycle. The cycle repeats every two revolutions of the motor. The cycles are: intake, compression, power, and exhaust.

Intake: This stage starts when the piston is at the top of the cylinder. The intake valve is open and the exhaust valve is closed. As the piston travels down the cylinder, air and fuel is drawn in through the valve and the piston fills with the air fuel mixture which is fed through the intake manifold. If the engine is turbocharged or supercharged, the air is blown in under pressure rather than drawn in.

Compression: This stage starts when the piston is at the bottom of the cylinder. The intake valve closes and the exhaust valve remains closed. As the piston travels up the cylinder, air and fuel are compressed to a volatile mixture ready to explode at the slightest encouragement. The volume of air at the beginning of the cycle divided by the volume at the end of this cycle is known as the compression ratio.

Power: This stage starts when the piston is at the top of the cylinder. The intake valve is closed and the exhaust valve is still closed. As the piston reaches the top of the cylinder, the spark plug is timed to ignite the air and fuel mixture causing it to rapidly expand and thus delivering the power stroke. If the mixture ignites prematurely due to poorly adjusted timing or low octane fuel the engine will be said to be pinging.

Exhaust: This stage starts when the piston is at the bottom of the cylinder. The intake valve is closed and the exhaust valve is open. As the piston travels up the cylinder, the remnants of the combustion are pushed out through the open valve. When the piston reaches the top, the exhaust valve closes and the intake stroke starts again.

As you can see it takes two full revolutions to complete one cycle. Therefore we can assert that a 4 cylinder engine has 2 power strokes per revolution. An 8 cylinder engine has 4 power strokes per revolution etcetera. Diesels work in a similar manner but without a spark plug. Diesels typically have a high compression ratio and the compression stroke alone is enough to generate enough heat to cause ignition.

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