In an internal-combustion engine, a combustible material, a fuel, usually some kind of petroleum product, is made to burn rapidly at the top of a chamber called a cylinder. Engines can have anywhere from 1 to 12 or more cylinders and automobile engines typically have 6 or 8. The explosive burning of the fuel produces hot gases, which expand and push a mass of metal, called a piston, down the cylinder. A system of rods and shafts in the engine uses the work done by pushing down the cylinder to spin a crankshaft. This spinning can then be applied to do other work, such as turning the wheels of a motor vehicle.
To make an engine operate continuously, the piston has to be returned to its original position at the top of the cylinder. Not only that, but the hot exhaust gases have to be expelled, fresh air brought back in, and new fuel supplied.
The pistons are attached by way of a connecting rod to the crankshaft. The crankshaft is not straight but contains a series of U-shaped bends through which the back-and forth motion of the piston rod is converted into rotary motion. When the piston reaches the bottom of the cylinder and the rod is fully extended, the rotating crankshaft pushes it all the way up to the top again.
At the top of each cylinder are two valves. An exhaust valve lets the burnt gases out after they have pushed the piston all the way down, and an intake valve lets in fresh air and new fuel. These valves open and shut with the aid of a system of rods and springs that operates according to the commands of a spinning shaft, camshaft, which is in turn driven by the crankshaft.
The engine ordinarily used in an automobile is a four-stroke engine. During one cycle, each piston moves back and forth twice, completing four strokes.
In the first, or intake, stroke, the piston moves from the top of the cylinder to the bottom. During this stroke, air and fuel are drawn into the cylinder through the open intake valve. In the second, or compression stroke, the piston moves up to the top of the cylinder. Both valves are closed, and the fuel mixture is compressed. At the end of the stroke, the fuel mixture is ignited by an electric spark or spontaneously by the heat inside the cylinder.
The resulting explosion creates hot, expanding gases that drive the piston back down the cylinder. This is called the power stroke. During the exhaust stroke, the piston returns to the top of the cylinder, pushing the gases out through the open exhaust valve.
In ordinary gasoline-powered engines, the fuel is fed into the carburetor, where it is mixed with air before entering the cylinder. This mixture is then ignited by a spark created by a spark plug. In diesel engines, the fuel is sprayed directly into the cylinder (called fuel injection). The heat generated when the air inside the cylinder is compressed during the upstroke of the piston ignites the diesel fuel spontaneously.
In an automobile engine, combustion temperatures are extremely high, for example, to 6000.degree. F. This heat and the heat caused by the friction of moving parts must be dissipated. In a water cooled engine, water is circulated in jackets or pockets that surround the cylinder walls and the cylinder heads. The water carries away the heat from the engine to a radiator. The radiator dissipates the heat by the air that circulates past the radiator. A water pump driven by a belt keeps the water in circulation along a closed loop including the radiator,engine, and the passenger compartment heater. In a standard vehicle internal combustion engine, the engine dissipates about 60 percent of the heat it generates, and converts only the remaining 40 percent to mechanical energy for driving the car. Thus, it can be seen that a substantial portion of potential heat energy is being wasted.
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