There are at least one each intake and exhaust valve per cylinder. On the intake stroke the intake valve is open and the piston goes down, drawing air fuel mixture in. The following up stroke both valves are shut and the piston comes up, compressing the mixture. When the piston is near the top the mixture is ignited, driving the piston down and making power. The exhaust valve is open as the piston drives the exhaust gas out.
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