What forces burned gases out of the cylinder during the exhaust stroke?

During the exhaust stroke of a small engine, the primary mechanism that expels burned gases from the cylinder is the upward movement of the piston. As the piston moves upward in the cylinder, it creates higher pressure inside the combustion chamber, forcing the burnt gases out through the exhaust valve. This movement is crucial for clearing out the spent gases from the combustion process, allowing fresh air and fuel to be drawn in during the subsequent intake stroke.

The other options do not contribute directly to the expulsion of gases during this specific phase. The opening of the intake valve, for example, pertains to the intake phase, allowing fresh fuel and air mixture into the cylinder, rather than contributing to the exhaust process. The cooling system's purpose is to manage engine temperature and does not interact with gas expulsion. Similarly, while the crankshaft rotation is essential for causing the piston's movement, it does not directly perform the act of burning gases out during the exhaust stroke; it is merely responsible for the piston's motion. Thus, the most accurate response highlights the piston's upward movement as the driving force behind expelling exhaust gases during this critical engine cycle.