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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.
Step 2: Compression Stroke. Just after the piston moves to the bottom of its travel (bottom dead center), the cylinder bore contains the maximum air-fuel mixture possible. The intake valve closes and the piston returns back up the cylinder bore. This is called the compression stroke of the 4-cycle engine process.