What occurs during ignition in a 4-stroke engine?

During ignition in a 4-stroke engine, the correct process is that fuel is injected just before the piston reaches the top dead center (TDC), and combustion occurs shortly thereafter. This sequence is crucial because it ensures that the air-fuel mixture is at the optimal pressure and temperature for ignition. As the piston approaches TDC, the compression of the air-fuel mixture increases, and once the spark plug ignites it, combustion begins. This combustion generates high-pressure gas that forces the piston down, creating power for the engine's operation.

The other options describe different aspects of the 4-stroke engine cycle. The maximum downward position of the piston pertains to the power stroke, not ignition. The expulsion of exhaust gases corresponds to the exhaust stroke, which follows the power stroke. Lastly, the crankshaft rotation speed varies throughout the engine cycle, but maximum speed does not specifically coincide with the ignition process. Understanding these different stages helps clarify the roles they play in the overall function of a 4-stroke engine.