Internal combustion engine
From Memory Alpha, the free Star Trek reference
An internal-combustion engine is a heat engine in which combustion occurs at high temperatures and/or pressures. Automobiles in the 20th and 21st centuries were powered by these engines, which were fueled by gasoline. A side-effect of the use of these engines was the production of smog. (ENT: "Carpenter Street"; TOS: "Bread and Circuses"; VOY: "The 37's")
