The electric system

TEXT 13

HOW A TWO-CYCLE DIESEL WORKS

TEXT 12

You have learned how a four-cycle diesel works. You remember that it needs four full strokes of the piston to go through one cycle intake, compression, power, and exhaust. Since there are two strokes for each complete rotation or revolution of the crankshaft, it takes two revolutions to complete a cycle. Thus there is only one power stroke for every two revolutions.

The idea of a two-cycle engine is to complete the cycle in only two strokes of the piston, that is, in only one revolution of the crankshaft. Thus a two-cycle engine running at the same speed («revolutions per minute») as a four-cycle engine will have twice as many power strokes. So here's another way of getting more power out of the same size cylinders, or of getting an engine weighing less for the same power.

Function of Air Pump. The two strokes we must have in any diesel engine are the compression and power strokes. The other two strokes are really wasted strokes, which we would like to get rid of. They are the intake and exhaust strokes. During these two strokes, the piston of the four-cycle engine is really acting as an air pump. The way we get rid of the pumping strokes in the two-cycle engine is by using a separate air pump, which takes in atmospheric air, compresses it slightly and feeds it to the engine cylinder. This separate air pump on a two-cycle engine is sometimes a rotary blower, and might be mistaken for a supercharger on a four-cycle engine, which you learned about in the previous article. However, there's an important difference: a supercharger for a four-cycle engine is just an accessory added to give the engine more power, whereas an air pump must be used on a two-cycle engine - otherwise it won't get any air and therefore won't run at all. Air pumps for two-cycle engines are made in many forms, but we won't go into details now. The important point to remember is that a blower on a two-cycle engine is simply an air pump.

The electric current, as it moves through various wires, conductors, and electrically operated devices, performs several jobs in the car. It cranks the engine, furnishes electric sparks at the spark plugs that ignite the compressed mixture in the cylinders, operates the radio and heater motor, supplies lights for night driving, and operates gauges on the dash of the car that indicate charging rate in amperes, oil pressure, engine temperature, and the amount of fuel in the tank.

Components of Electric System. The electric system consists of the storage battery, cranking motor, generator, regulator, ignition distributor, coil, and spark plugs as well as the wires and switches that connect these various units. Lights, radio, heater, indicating gauges, and other electrically operated devices, while a part of the electric system, are usually considered as accessory devices, since they are not absolutely necessary to the operation of the car.

a. Storage Battery. The storage battery is an electrochemical device, which means that its operation depends on both chemical and electrical actions. The battery is a source of electric current when the engine is being cranked with the electric cranking motor and also during the times when the generator is not supplying enough current for connected electric units. When current is withdrawn from the battery, chemical actions take place that produce the flow of current. The chemicals in the battery are, in a sense, used up by this chemical action. Thus, after a certain amount of current has been withdrawn, the battery becomes «discharged». To «recharge» the battery, current from some external source, such as a generator, must be forced through it in the opposite direction.

b. Cranking Motor. The cranking motor is a special direct-current electric motor that starts the engine by rotating the crankshaft when the chancing-motor switch is closed. Closing the switch connects the motor to the battery. There is a special gearing arrangement between the cranking-motor drive pinion and the engine flywheel that will be discussed below.

c. Generator. The generator is a device that converts mechanical energy (from the automobile engine) into a flow of electric current. This current restores the battery to a charged condition when it has become discharged and also operates electrical devices, such as the ignition system, lights, radio, etc. The generator is usually mounted to the side of the engine block and is driven by the engine fan belt.

d. Regulator. Under some conditions, the generator could produce too much current, and this would damage the various electrical devices turned on. To prevent such damage a generator-output regulator is used. The regulator controls the amount of current the generator produces, allowing the generator to produce a high current when the battery is in a discharged condition and electric units are turned on. When the battery becomes charged and electric units are turned off, the regulator cuts down the current produced to the amount needed to meet the operating requirement of the system.

e. Ignition System. The ignition system provides the electric sparks that ignite or set fire to the compressed fuel air mixture in the engine cylinders. Each spark is timed by the ignition system to appear at the correct instant with respect to the piston as it approaches the end of the compression stroke.

f. Lights, Heater, Radio, Indicating Devices. The lights, radio, and heater add to the flexibility, comfort, and convenience of the car, while indicating devices keep the driver informed as to engine temperature, oil pressure, amount of fuel, and battery charging rate.

g. Wiring and Switches. The wiring that connects the various electric units serves as paths or circuits through' which the current can flow from one to another unit. Switches, placed in these circuits, are forms of valves that can close or open the circuits to permit or prevent the flow of current. The wires are made up of conducting materials, such as copper, that freely conduct the current between the electric units. Some materials, such as rubber or glass, are nonconductors or insulators that will not allow current to flow through them. Such substances are used to cover and insulate the wires so that the current will be kept within, the proper circuits or paths.

Дата добавления: 2014-10-17