What causes the movement of the piston in a combustion engine?

The movement of the piston in a combustion engine is primarily caused by a controlled explosion within the engine's cylinders. When the air-fuel mixture is compressed and ignited by the spark plug, a rapid expansion of gases occurs. This explosion exerts pressure on the piston, pushing it down the cylinder. This action is part of the four-stroke cycle, which includes intake, compression, power (the explosion), and exhaust strokes.

This is fundamental to the operation of internal combustion engines as the energy released from the explosion translates into mechanical energy, which can then be harnessed to move vehicles or perform work. The other options, while related to engine operation in various contexts, do not directly cause the piston movement in the same way. For instance, electricity is used to ignite the mixture but doesn't move the piston itself. Liquid fuel injection is a process that delivers fuel to the engine but does not account for the explosion that creates the force driving the piston. Mechanical friction exists within the engine but is more of a byproduct of the moving parts rather than a primary cause of piston's motion.