When running an engine at idle speed, which would typically be the optimum location to peak the cylinder pressure?

When evaluating the optimal location to peak cylinder pressure during engine operation at idle speed, it is crucial to understand the combustion process and the timing of ignition in relation to the piston position.

The correct answer, which indicates 15 degrees after top dead center (ATDC), reflects the point at which combustion is most effective for maintaining a smooth idle, especially in a four-stroke gasoline engine. At this point, the pressure in the cylinder is high enough to ensure that the engine runs smoothly without misfiring or stalling.

As the piston completes the intake and compression strokes, ignition occurs just before reaching top dead center (TDC). After TDC, the combustion gases continue to expand, reaching maximum pressure shortly thereafter, around 15 degrees ATDC. This timing allows for optimal energy transfer to the crankshaft when the piston begins its downward stroke, thereby enhancing engine performance at idle.

In contrast, other options such as 20 degrees BTDC, 10 degrees BTDC, and TDC would not provide the same benefits. Igniting too early (like 20 or 10 degrees BTDC) can lead to pre-ignition or knocking, while TDC may not maximize the force exerted on the piston, resulting in a less