What opens the valves on the compressor stroke of a four-stroke single-cylinder petrol engine?

In a four-stroke single-cylinder petrol engine, the opening of the valves during the compression stroke is controlled by the camshaft. The camshaft is a crucial component that has lobes, or cams, which push against the rocker arms to open the intake and exhaust valves at the appropriate times in the engine cycle.

During the compression stroke, the camshaft rotates and its lobes engage the rocker arms, which then pivot or move to push the valves open. This allows for the precise timing needed for the intake of air-fuel mixture and the expulsion of exhaust gases during the respective strokes of the engine cycle.

The crankshaft, while essential for converting linear motion into rotational motion, does not directly control the valve operation. Similarly, fuel injectors are responsible for delivering fuel into the engine's combustion chamber but do not participate in the opening and closing of valves. Thus, the rocker arms serve as the mechanism that translates the motion of the camshaft into the movement of the valves, making this the correct answer.