Automobile Construction

Automobile Construction.

The automobile consists of five basic mechanisms, or components. These are:

(1) The engine, which is the source of power, including the fuel, lubricating, cooling, and electric systems.

(2) The frame, which supports the engine, wheels, steering and brake systems, and body

(3) The power train, which carries the power from the engine (through the clutch, transmission, drive shaft, differential, and axles) to the car wheels.

(4) The car body

Car-body accessories, including heater, lights, windshield wipers, and so forth.

       The engine is the source of power that makes the car move. It is usually called an internal-combustion engine because gasoline is burned inside the engine cylinders, or combustion chambers. This is in contrast to external-combustion engines such as steam engines, where the combustion takes place outside the engine. The burning of gasoline in the engine cylinders produces the power. The power is then carried from the engine through the power train to the car wheels so that the wheels turn and the car moves.

       The fuel system plays a vital part in the power-producing process since it supplies the gasoline to the engine cylinders. In each engine cylinder, a mixture of gasoline vapor and air enters the cylinder, the piston pushes up into the cylinder to compress the mixture, and then an electric spark ignites the compressed mixture so that the piston is forced downward. Of course, in the engine the piston is not blown completely out of the cylinder, the piston simply moves up and down in the cylinder-up to com-press the mixture, down as the mixture burns. The piston straight-line motion must be changed to rotary motion before it can be used to make the car wheels rotate. A connecting rod and a crank on the engine crankshaft make this change.

The engine valves get the burned gasoline vapor out of the engine cylinder and bringing gresh charges of gasoline vapor and air into the cylinder. There are two openings, or ports, in the enclosed end of the cylinder, each containing a valve. The valves are accurately machined plugs on long stems. When they are closed, or seated (that moved up into the ports), the ports are sealed off and gas cannot pass through the ports. When the valve is open gas can pass through the port.

       The valves are opened by cams on the engine camshaft. The cam has a high point, or lobe; every time the cam rotates, the lobe comes around under the valve lifter and move it up-ward. The lifter then carries this upward movement through the pushrod to the rocker arm. The rocker arm pivots on tis support and pushes down on the valve stem, causing the valve to move down, that is, to open. After the cam has turned enough to move the lobe out from under the lifter, the heavy valve spring pulls the valve back into its seat. The spring is attached to the upper end of thevalve stem by a spring retainer and lock. There is a cam for each valve (two cams per cylinder) on the engine camshaft. The camshaft is driven off the crankshaft by gears or by sprock-ets and a chain.

       When the entire cycle of events requires four piston strokes (two crankshaft revolutions), the engine is called a four-stroke-cycle engine, or a four-cycle engine. The four strokes are intake, compression, power, and exhaust.

On the intake stroke, the intake valve is open. The piston moves down, pulled by the rotation of the crankshaft. This piston movement creates a partial vacuum in the cylinder, and air rushes into the cylinder past the intake valve to ”fill up” this vacuum. As the air moves toward the cylinder, it must pass through the car-buretor. There it is charged with gasoline vapor. Thus, it is a mixture of air and gasoline vapor that rushes into the cylinder as the piston moves down on the intake stroke.

After the piston moves down to the bottom dead center on the intake stroke, the intake valve closes. The lobe on the cam controlling the intake valve has moved out from under the valve lifter. Since the other valve is also closed, the upper end of the cylinder is sealed. Now, as the piston is pushed up by the rotating crankshaft, the mixture of air and gasoline vapor that has been drawn into the cylinder is compressed. By the time the piston has moved up to the top dead center, the mixture is compressed to a seventh or an eighth of its original volume. That is like taking a gallon of air ad compressing it to a pint. The result is high pressure in the cylinder.

       About the time the piston reaches the top dead center on the compression stroke, an electric spark occurs at the cylinder spark plug. The spark plug is essentially two heavy wire electrodes; the spark jumps between these electrodes. The spark is produced by the ignition system. It ignites, or sets fire to, the compressed air-gasoline-vapor mixture. Rapid combustion takes place; high temperatures and pressures result. At this instant the downward pressure on the top of the piston may amount to as much as 2 tons. This powerful push forces the piston down, and power impulse is transmitted to the crankshaft through the connecting rod and the crank.

       The piston is forced down by the pressure of the burning gasoline vapor during the power stroke. When the piston reaches the bottom dead center, the exhaust valve opens. Now, as the piston starts back up again, if forces the burned gases from the cylinder. By the time the piston has reached the top dead center the cylinder is cleared of the burned gases. The exhaust valve closes and the intake valve opens. Then, the piston starts back down again on the next intake stroke. The four cycles, or piston strokes, are continuously repeated while the engine is running.