Final answer:
In a 4-stroke engine, if the piston is at TDC of the compression stroke, the intake and exhaust valves will be closed. During the compression stroke, the air-fuel mixture is rapidly compressed with the valves closed, preparing it for ignition.
Step-by-step explanation:
In a 4-stroke engine, if the piston is at TDC (Top Dead Center) of the compression stroke, the intake and exhaust valves will be closed. During the compression stroke, the piston rises with the valves closed, rapidly compressing the air-fuel mixture in a nearly adiabatic process. This compression builds up pressure in the cylinder, preparing the mixture for ignition during the power stroke.