Simplify the following text for a Grade 8 native English-speaking person.
Original: The term internal combustion engine usually refers to an engine in which combustion is intermittent , such as the more familiar four-stroke and two-stroke piston engines , along with variants , such as the six-stroke piston engine and the Wankel rotary engine .
Simplified: Most road vehicles use the internal combustion engine today , and most of those use the four-stroke engine . Another type of internal combustion engine is the Wankel engine .