Ignition systems

There are two general types of ignition: the compression [69], and the spark [70] methods.

Compression ignition. The compression type utilizes the heat of compressed air to ignite the fuel as it is introduced to the combustion or precombustion chamber[71]. The temperature of this air may be as high as 1000 F and sometimes may be higher. If fuel was mixed with the air before compression, preignition would occur; that is, the mixture would ignite before the piston was in the most favorable position to receive the thrust of the expanding gases. This would not be desirable. Ignition is timed in the compression-ignition engine by timing the injection of the fuel. In an engine operating at a constant speed, the need for variance of the timing would not be present. The truck diesel engine, which must operate under a large range of speed conditions, must have a governor system which can control the injection[72] starting point and the injection period. In a cold engine, some trouble is usually experienced in bringing the compression temperature up to the ignition temperature of the fuel. To assist the process, glow plugs are sometimes used. These are operated electrically and are turned off when fuel ignition begins.

Most diesel (compression-ignition) engines utilize heavy-duty electrical starters, powered by 12 volts or more from storage batteries, or gasoline engines to turn the CI engine over fast enough to bring the temperature up to the ignition point. When the CI engine has reached temperatures that ignite the fuel, no further trouble is experienced with ignition. There are no wires[73], coils, and plugs to cause trouble. Some diesel engines under light load or at idle may cool sufficiently to produce poor ignition of the fuel. This condition is overcome as more fuel is burned under operating conditions.

The other ignition system, the spark type, is the one which is more complicated, and therefore it is the frequent cause of poor engine performance.

Spark Ignition. The purpose of the spark-ignition system is to deliver a perfectly timed surge of electricity across a spark-plug gap in each cylinder at the exact moment when explode the cylinder's charge of compressed gasoline and air with maximum power efficiency. The distinguishing feature of the SI (spark-ignition) engine is that there is a spark plug in the head. The plug projects into the combustion chamber. In most modern SI engines the gap of the plug remains fixed while the engine operates.

Considering the source of electric current, there are two of spark ignition. They are the battery-ignition and the magneto-ignition types. With the battery, current is produced by chemical reactions within the battery; with the magneto, currents are induced or created by rapidly moving coils or magnets.

Battery-ignition Types. There are two types of battery ignition according to the type of battery. At one time, dry-cell[74] batteries, such as are used in flashlights[75] today, were used to furnish the electric power. Four of these batteries fastened together in series would produce a force of 6 volts or more, until they began to deteriorate[76]. Once deterioration set in, the batteries had to be discarded. Storage batteries have almost entirely replaced the dry-cell batteries for ignition purposes. The storage battery can readily be charged while the engine is operating by means of an accessory called the generator.

Trends [77]. Magnetos are used on small one-cylinder engines powering motorboats and lawn mowers and on airplane engines and farm tractors. The above vehicles have little need for lights and therefore do not need a battery, and so magnetos have been retained. But as farm tractors have been designed to use battery conveniences, such as lights and starters, there has been a trend to use the same battery for ignition purposes. Also, there has been some trend away from magneto ignition because of the initial high cost and because of the difficulty in finding mechanics capable of servicing magnetos. As most automobile mechanics are familiar with battery-ignition systems found on trucks and cars, better service can be obtained with the battery system. The magneto and battery systems are almost identical in theory and operation. The main difference lies in the production of the current. The battery produces current and voltage by chemical action. The magneto produces current and voltage in one of two ways: either a coil rotates within a magnet, or a magnet rotates near a coil.

SPARK PLUG [78]

Вариант №7