Functions of Ignition Apparatuses and Engine Systems
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Hot Bulb Ignition
Further refinement resulted in the design of the hot bulb ignition, which operates much the same as the hot tube ignition. Its main weakness consisted in the time required to get the engine under way from six to fifteen minutes to get up to speed without load and longer to get to the point of taking full load.
Operation necessitating the open flame of a gasoline torch to start and get under way is a distinctly undesirable system when frequent starts and stops are made, where widely varying loads are frequent, where fires are made possible, or where close regulation is desirable.
Ignition by Compression
Compression ignition was introduced with the Diesel and semi-Diesel types of engine and consists in a reduction of the compression space to a degree where sufficient pressure is produced and heat enough generated to cause complete burning of the fuel injected into the highly compressed air. Compression of from 250 lb. to 500 lb. is used, fuel being injected under pressures above those of compression. None of the types of ignition suit the requirements of tractors as well as electric ignition.
Two systems of electric ignition are used to deliver a spark to ignite the fuel mixture in the cylinder of an engine low tension or 'make-and-break' ignition and high tension or "jump spark" ignition. The source of electricity for either system can be a battery or a magneto. Low tension means low voltage; high tension means high voltage.
Low Tension Ignition
The low tension system requires an igniter which must have some mechanical means of opening or breaking apart the igniter points to produce a spark gap. To get a hot spark at this gap requires a large volume of current and a low pressure or a low tension. This kind of current requires only a primary winding in the spark coil or magneto.
High Tension Ignition
The high tension or jump spark system always uses a spark plug which has a permanent spark gap that the spark must jump across.
The current produced by a low tension system while of large volume does not have enough pressure to jump across this spark gap. To produce the high pressure necessary we add a secondary winding to the primary winding of the spark coil or magneto and thus induce in this secondary winding and circuit a voltage high enough to bridge the spark gap of a spark plug. This secondary winding makes a high tension or high voltage system of a low tension system.
A "Good" Spark
By a "good" spark we ordinarily mean a large, hot spark. Igniting the charge in a cylinder may be likened to kindling a fire. By using plenty of kindling and a hot blow torch flame all over it at once, the fire would burn much quicker than if it were lighted with an ordinary match. Just so with lighting a fuel mixture in the engine cylinder. A big, hot, fat spark that will make the mixture burn as fast as possible is most desirable.
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