Although the engine is major system by itself (figure 1-3), its output should be considered a component of the drive train. The engine provides the power to drive the wheels of the vehicle. An engine develops a rotary motion or torque that, when multiplied by the transmission gears, will move the car under a variety of conditions. The engine produces power by burning a mixture of fuel and air in its combustion chambers. Combustion cases a high pressure in the cylinders, which forces the pistons downward. Connecting rods transfer the downward movement of the pistons to the crankshaft, which rotates by the force on the pistons.
All automobile engines, both gasoline and diesel, are classified as internal combustion engines because the combustion or burning that creates energy takes place inside the engine. Combustion is the burning of an air and fuel mixture. As a result of combustion, large amounts of pressure are generated in the engine. This pressure or energy is used to power the car. The engine must be built strong enough to hold the pressure and temperatures formed by combustion.
Diesel engines have been around a long time and are mostly found in big heavy-duty trucks. However, they are also used in some pick-up trucks and will become more common in automobiles in the future ( figure 1-4). Although the construction of a gasoline and diesel engine are similar, their operation is quite different.
A gasoline engine relies on a mixture of fuel and air that is ignited by a spark to produce power. A diesel engine also uses fuel and air, but does not need a spark to cause ignition. Diesel engines are often called compression ignition engines. This is because its incoming air is tightly compresses air. The heat of the compressed air ignites the fuel and combustion takes place. The following sections cover the basic parts and the major systems of a gasoline engine.
Most automotive engines are four-stroke cycle engines. The opening and closing of the intake and exhaust valves are timed to the movement of the piston. As a result, the engine passes through four different events or strokes during one combustion cycle. These four the intake, compression, power, and exhaust strokes.
One the intake stroke, the piston moves downward, and charge of air/fuel mixture is introduced into cylinder. As the piston travels upward, the air/fuel mixture is compressed in preparation for burning. Just before the piston reaches the top of the cylinder, ignition occurs and combustion starts. The pressure of expanding gases forces the piston downward on its power stroke. When it reciprocates, or moves upward again, the piston is on the exhaust stroke. During the exhaust stroke, the piston pushes the burned gases out of the cylinder. As long as the engine is running, the cycle of event repeats itself, resulting in the production of engine torque.