Here's why:
* Intake valve closed: This prevents the incoming air-fuel mixture from escaping out of the cylinder during the compression and combustion process.
* Exhaust valve closed: This prevents the burnt exhaust gases from being released prematurely, allowing the pressure generated by the burning fuel to push the piston down and generate power.
The power stroke occurs after the ignition of the air-fuel mixture, which is the result of the spark plug firing. This creates a rapid increase in pressure inside the cylinder, forcing the piston down and turning the crankshaft.
