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How The 4-Stroke Spark Ignition Engine Got Its Name
All SI engines are essentially air pumps, which means they breath. This breathing takes 4 strokes (720 degrees of crankshaft rotation) to complete a full power operation (or cycle) per cylinder.
Above, you should be able to see a simple animation of the pumping process which all internal combustion engines perform.
Stroke 1) The intake stroke in which the piston is forced down creating a pressure differential between the intake manifold (higher pressure) and the cylinder sleeve (lower pressure) This effectively sucks the air/fuel mixture into the cylinder sleeve from the intake manifold (on the right). Take note that th intake valve is also proportionally larger than the exhaust valve, this is a common design feature of modern IC engines which we later cover in EFI 102.
Stroke 2) The compression stroke in which the piston is forced towards the head, compressing the combustion gases (air/fuel) in order to make them more volatile so they are able to release more energy when ignited.
Stroke 3) The combustion stroke occurs after the ignition event which releases a pressure wave forcing the piston away from the head.
Stroke 4) The exhaust stroke in which the spent combustion gases are pumped out of the cylinder sleeve into the exhaust manifold (exits the cylinder sleeve to the left). With each of these strokes, the pumping process is not 100% efficient due to friction, reciprocal movement, etc. This is where the pumping losses of an engine are realized.
