Fig. 50.—An Ignition Coil, showing the Trembler Mechanism.
We have mentioned that normal ignition occurs when the crank is exactly on the dead-centre and the piston at the top of its stroke. If we set the magneto when the engine is at rest so that ignition ought to occur on dead-centre when the arm K is in its mid position the actual sparking will be late on account of the time lag of the electric current. The current takes time to flow and in that brief element of time the crank has moved a few degrees off the dead-centre, at high speeds. Hence the ignition must be advanced if the charge is to be correctly fired when the engine is running fast. If the ignition is too far advanced it will cause the engine to “knock,” especially under heavy loads. If the ignition is retarded the charge is not fired at the commencement of the stroke so that a portion of the power theoretically available in the fuel is lost to exhaust at the end of the stroke. Retarded ignition always causes overheating of the exhaust system.
If the arm K is fixed mechanically in its mid position so that the ignition can neither be advanced nor retarded, we have what is known as fixed ignition.
Fig. 51.—Ignition Coil Case.
