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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.
During the intake stroke, the piston is pulled down by the crankshaft. While this is happening, the inlet valve opens and allows fuel and air to mix. For the compression stroke that follows, the inlet valve closes and the piston moves back up the cylinder to compress the mixture of fuel and air. During the power stroke, the spark plug ignites the fuel and air mixture, which pushes the piston back down. For the final phase, the exhaust stroke, the outlet valve opens.