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It's hard to imagine living in a world where there are no cars, buses, or trucks. When it comes to important inventions, the internal combustion engine has to be near the top of the list. Unlike most steam engines that preceded them, internal combustion engines are small enough to fit in personal vehicles, such as cars. Unlike electric motors, these little powerhouses can travel a long distance on a compact fuel source. As with most inventions, the internal combustion engine is really the product of many individuals working over a long period of time. Early experiments with engines that burned liquid fuel started back in 1838, but it wasn't until 1876 that a German engineer named Nikolaus Otto created one that actually worked. Little has changed in the Otto-cycle engine over the last 120 years. The key element behind its power involves igniting a small amount of gasoline inside a confined space called the cylinder. As the fuel explodes, it produces a great deal of hot gas, which presses against the face of a piston, pushing it down in the cylinder. The other end of the piston is connected to a piston rod that turns a rotating crankshaft, which in turn is linked to the car wheels. The up-and-down motion of the piston makes the crankshaft turn, just as the up-and-down pedaling turns the crank of a bike. It is this rotary motion of the crankshaft that runs the car's engine. For all this motion to take place smoothly, four distinct actions or strokes occur in the engine. During the intake stroke, the piston moves down in the cylinder and a mixture of air and fuel enters the cylinder through a valve in the top. In the compression stroke, the valve closes and the piston begins to move back up the cylinder, compressing the mixture. Once the piston reaches the top of the cylinder, the spark plug ignites the fuel, which drives the piston back down. This is called the power stroke because it's where the power comes from. In the final exhaust stroke, the piston moves back up again and a second valve opens to allow the spent gas to escape. Then the cycle starts all over again. Typical car engines have either four, six, or eight cylinders. It is important that all of the piston movements are timed to move in an orderly way. Otherwise, the engine won't run smoothly.