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the cylinder (known as Bottom Dead Centre or BDC) the exhaust valve starts to open. As the piston now moves up the cylinder, the hot gases (consisting mostly of nitrogen, carbon dioxide, water vapour and unused oxygen) are expelled from the cylinder. As the Piston approaches TDC again the inlet valve starts to open and the cycle repeats itself.
As the piston in an engine moves down, a mixture of fuel and air gets drawn into the cylinder, which causes compression. This compression is ignited by the spark plugs, causing the exhaust valve to open to allow the burned fuel mixture to be sent to the exhaust system.