Automobiles are powered and controlled by a complicated interrelationship between several systems. This diagram shows the parts of a car with a gas engine and manual transmission (the air filter and carburetor have been removed to show the parts beneath but usually appear in the space above the intake manifold). The major systems of the automobile are the power plant, the power train, the running gear, and the control system. Each of these major categories includes a number of subsystems, as shown here. The power plant includes the engine, fuel, electrical, exhaust, lubrication, and coolant systems. The power train includes the transmission and drive systems, including the clutch, differential, and drive shaft. Suspension, stabilizers, wheels, and tires are all part of the running gear, or support system. Steering and brake systems are the major components of the control system, by which the driver directs the car.
The basic components of an internal-combustion engine are the engine block, cylinder head, cylinders, pistons, valves, crankshaft, and camshaft. The lower part of the engine, called the engine block, houses the cylinders, pistons, and crankshaft. The components of other engine systems bolt or attach to the engine block. The block is manufactured with internal passageways for lubricants and coolant. Engine blocks are made of cast iron or aluminum alloy and formed with a set of round cylinders.
The upper part of the engine is the cylinder head. Bolted to the top of the block, it seals the tops of the cylinders. Pistons compress air and fuel against the cylinder head prior to ignition. The top of the piston forms the floor of the combustion chamber. A rod connects the bottom of the piston to the crankshaft. Lubricated bearings enable both ends of the connecting rod to pivot, transferring the pistonâ€™s vertical motion into the crankshaftâ€™s rotational force, or torque. The pistonsâ€™ motion rotates the crankshaft at speeds ranging from about 600 to thousands of revolutions per minute (rpm), depending on how much fuel is delivered to the cylinders.
Fuel vapor enters and exhaust gases leave the combustion chamber through openings in the cylinder head controlled by valves. The typical engine valve is a metal shaft with a disk at one end fitted to block the opening. The other end of the shaft is mechanically linked to a camshaft, a round rod with odd-shaped lobes located inside the engine block or in the cylinder head. Inlet valves open to allow fuel to enter the combustion chambers. Outlet valves open to let exhaust gases out.
Depending on the vehicleâ€™s design, engine power is transmitted by the transmission to the front wheels, the rear wheels, or to all four wheels. The wheels receiving power are called drive wheels: they propel the vehicle forward or backward. Most automobiles either are front-wheel or rear-wheel drive. In some vehicles, four-wheel drive is an option the driver selects for certain road conditions; others feature full-time, all-wheel drive.
The differential is a gear assembly in an axle that enables each powered wheel to turn at different speeds when the vehicle makes a turn. The driveshaft connects the transmissionâ€™s output shaft to a differential gear in the axle. Universal joints at both ends of the driveshaft allow it to rotate as the axles move up and down over the road surface.
In rear-wheel drive, the driveshaft runs under the car to a differential gear at the rear axle. In front-wheel drive, the differential is on the front axle and the connections to the transmission are much shorter. Four-wheel-drive vehicles have drive shafts and differentials for both axles
The suspension system, part of the undercarriage of an automobile, contains springs that move up and down to absorb bumps and vibrations. In one type of suspension system, a long tube, or strut, has a shock absorber built into its center section. Shock absorbers control, or dampen, the sudden loading and unloading of suspension springs to reduce wheel bounce and the shock transferred from the road wheels to the body. One shock absorber is installed at each wheel. Modern shock absorbers have telescoping design and use oil, gas, and air, or a combination to absorb energy.
Luxury sedans generally have a soft suspension for comfortable riding. Sports cars and off-road vehicles have firmer suspensions to improve cornering ability and control over rough terrain.
Older automobiles were equipped with one-piece front axles attached to the frame with semi elliptic leaf springs, much like the arrangement on horse-drawn buggies. Front wheels on modern cars roll independently of each other on half-shafts instead of a common axle. Each wheel has its own axle and suspension supports, so the shock of one wheel hitting a bump is not transferred across a common axle to the other wheel or the rest of the car. However, advanced passenger cars, luxury sedans, and sports cars feature independent rear-wheel suspension systems.
Active suspensions are computer-controlled adjustments of the downward force of each wheel as the vehicle corners or rides over uneven terrain. Sensors, pump, and hydraulic cylinders, all monitored and controlled by computer, enable the vehicle to lean into corners and compensate for the dips and dives that accompany emergency stops and rapid acceleration.