The first gasoline-powered engine was modeled on the steam engine, and was the first practical alternative to it as a power source. In the steam engine, expanding steam inside a cylinder pushed up a piston, which then returns to its original position when the steam is allowed to escape and the steam supply is cut off. A steam engine is said to have a two-stroke cycle, since the piston moves twice (through one cycle) with each blast of steam.

The two-stroke gasoline engine used a mixture of ignited gasoline and air in place of the hot steam above. The piston was attached to a crankshaft with U-shaped sections called cranks via connecting rods attached to the bends. With these cranks, the crankshaft converted the up-and-down motion of the piston into rotating mechanical energy. A two-stroke engine did not have much power, however, because it required a...