Automobile

The engine operates on internal combustion; that is, the fuel used for its power is burned inside the engine. This burning occurs inside cylinders. Within the cylinder is a piston. When the fuel is burned, it creates an explosive force that causes the piston to move up and down. The piston is attached, via a connecting rod, to a crankshaft, where the up and down movement of the piston converts to a circular motion. When bicycling, the upper part of a person's leg is akin to the piston. Power from the leg is passed through the pedal in order to turn the crank.

Gasoline is the most common automobile fuel. It is pulled into the cylinder by the vacuum created as the piston moves down through the cylinder. The gasoline is then compressed up into the cylinder by the next movement of the piston. A spark is introduced through a spark plug placed at the end of the cylinder. The spark causes the gasoline to explode, and the explosion drives the piston down again into the cylinder. This movement, called the power stroke, turns the crankshaft. A final movement of the piston upward again forces the exhaust gases, the byproducts of the fuel's combustion, from the cylinder. These four movements—intake, compression, power, and exhaust—are called strokes. The four-stroke engine is the most common type of automobile engine.

Most automobiles have from four to eight cylinders, although there are also two-cylinder and 12-cylinder automobiles. The cylinders work together in a sequence to turn the crankshaft, so that while one cylinder is in its intake stroke, another is in the compression stroke, and so forth. Generally, the more cylinders, the more smoothly the engine will run. The size of the automobile will affect the number of cylinders the engine uses. Smaller cars generally have the smaller four-cylinder engine. Mid-sized cars will generally require a six-cylinder engine, while larger cars need the power of an eight-cylinder engine.

The number of cylinders, however, is less important to the level of an engine's power than is its displacement. Displacement is a measure of the total volume of fuel mixture moved by all the pistons working together. The more fuel burned at one time, the more explosive the force, and thus, the power will be. Displacement is often expressed as cubic centimeters (cc) or as liters. A smaller engine will displace 1,200 cc (1.2 L) for 60 horsepower, while a larger engine may displace as much as 4,000 cc (4 L), generating more than 100 horsepower. Horsepower is the measurement of the engine's ability to perform work. The size and weight of the car also affects its power. It takes less work to propel a lighter car than a heavier car, even if they have the same engine, just as a horse carrying a single rider can go faster with less effort than a horse drawing a cart.