At which position is the instantaneous speed of the piston maximum?

The instantaneous speed of the piston is maximized at the middle of the stroke, specifically between top dead center (TDC) and bottom dead center (BDC). This is due to the piston's motion being driven by the rotating crankshaft. As the piston moves away from TDC towards BDC, it accelerates until it reaches its maximum speed approximately halfway through the stroke. This point represents the balance between acceleration and the piston’s velocity before it begins to decelerate again as it approaches BDC.

In contrast, at the start of the stroke, the piston is beginning to move from a stationary position, leading to lower initial speeds. Similarly, at the end of the stroke, the momentum begins to decrease as the piston approaches TDC again, resulting in slower speeds. During the exhaust phase, while the piston is moving, it is typically under different pressures and conditions that don't favor maximum speed. The design of the engine and the timing of events contribute to the fact that maximum speed occurs in that middle region of the stroke cycle, resulting in the correct answer being the position midway between TDC and BDC.