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  1. The inlet valve opens as the piston is about to descend, sucking in the mixture of fuel and air.
  2. With both valves now closed, the piston rises upwards, squeezing the mix of fuel and air.
  3. Just before the piston reaches the top, the fuel and air mix is ignited by the spark plug.
  4. The exhaust valve opens and the piston rises back to the top, expelling the burnt gases.