During which stroke of the Otto cycle engine does the spark plugs fire?

In the Otto cycle engine, the spark plugs fire during the power stroke. This is the phase where the air-fuel mixture, having been compressed in the previous stroke, is ignited by the spark from the spark plugs. The ignition causes a rapid increase in temperature and pressure, leading to an expansion of gases that pushes the piston down, which generates the power needed to turn the crankshaft and ultimately drive the vehicle.

The firing of the spark plugs is crucial during this stroke because it initiates the combustion process. This energy release is what propels the vehicle and is essential for the engine's overall function. Ignition occurring at this point ensures that the maximum force is applied to the piston, which enhances the efficiency and effectiveness of the engine.