Internal Combustion Engine Fundamentals [best]

Simpler, lighter, higher power-to-weight ratio, no valvetrain. Cons: Lower fuel efficiency, higher pollution (oil mixed with fuel for lubrication), shorter lifespan. Used in chainsaws, mopeds, and outboard motors.

An ICE is composed of several high-precision parts that must work in perfect synchronization: Internal Combustion Engine Basics | Department of Energy internal combustion engine fundamentals