Ignition System: Description and Operation

IGNITION SYSTEM - GENERAL

Providing the proper air/fuel mixture is an important factor in promoting efficient engine operation. However, once the air/fuel mixture is introduced into the cylinder, it must be burned efficiently. The combustion process on a gasoline engine can't begin until a spark is introduced. Accurate timing of the spark in relation to piston position and can provide the difference between peak performance and inefficient operation.

When a spark is introduced to the air/fuel mixture in the cylinder, a flame front is generated. With proper ignition timing, the flame front exerts force on the piston just as it begins its downward movement. To allow time for the force to reach the piston, ignition occurs before the piston reaches Top Dead Center (TDC). However, if ignition occurs too soon, the force of combustion contacts the piston on its way up. This produces engine knock. If ignition takes place too late, engine performance lags.

The natural rotation of the crankshaft keeps the piston at or near TDC for approximately 46° of its rotation (23° Before TDC to 23° After TDC). The crankshaft rotates at a constant speed, but the piston moves very little at this point. Once past 23° ATDC, the piston begins to move very rapidly. For best results, the burn should be completed as close to 23° ATDC as possible.





As engine speed increases, spark must be further ahead of TDC to allow adequate time for the air/fuel mixture to burn completely. Because of this, increasingly sophisticated methods of advancing and retarding ignition have been implemented.

DIRECT IGNITION SYSTEM





Vehicles equipped with the Generic Engine Management System (GEMS) use a Direct Ignition System (DIS). The system is controlled by the Engine Control Module (ECM) which receives inputs from a variety of sensors. This information is then processed to provide the optimum spark advance for every operating condition.