Compression is the result of a piston moving up and down in a cylinder bore of the engine block. A cylinder head is bolted to the engine block to contain the pressure create by the pistons upward movement. The piston is sealed against the cylinder bore using piston rings. The seal between the head and block is called a head gasket.
Each cylinder piston is attached to a rotating crankshaft within the block by a connecting rod and travels downward in the bore as the intake valve is opens to allow air into the cylinder. Once this process is complete the piston will start to travel upwards in the bore as the intake valve closes which in turn starts the compression process. Once the ignition system has ignited the fuel air mixture inside the cylinder the piston is trusted downward. The exhaust valve then allows the spent fuel to exit the chamber so the process can start all over again.
Here is a typical engine while running which displays the combustion process.
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