In a gasoline engine, the air-fuel mixture is ignited to cause combustion, and the force that is generated by the explosion causes the piston to push downward. The thermal energy can be most efficiently converted into a motive force when the maximum combustion force is generated at a crankshaft position of 1ATDC (After Top Dead Center). An engine does not produce the maximum combustion force simultaneously with ignition; instead, it generates the maximum combustion force slightly after ignition has occurred. Therefore, ignition takes place in advance so that the maximum combustion force is generated at 10ATDC. The ignition timing that enables the engine to generate the maximum combustion force at 10ATDC changes every moment, depending on the operating conditions of the engine. Therefore, the ignition system must be able to ignite the air-fuel mixture at a timing that enables the engine to generate an explosive force in the most efficient manner in accordance with the operating conditions.
1. Ignition delay period
Combustion of the air-fuel mixture does not occur instantly after ignition. Instead, a small area (flame nucleus) in the immediate vicinity of the spark starts to burn, and this process eventually expands to the surrounding area. The period from the time when the air-fuel mixture is ignited until it is burned is called the ignition delay period (between A and B in the diagram). The ignition delay period is practically constant, and is not affected by the changes in the conditions of the engine.
2. Flame propagation period
After the flame nucleus is formed, the flame gradually expands outward. The speed at which the flame expands is called the flame propagation speed, and its period is called the flame propagation period (B~C~D in the diagram). When there is a large amount of the intake air, the airfuel mixture becomes denser. For this reason, the distance between the particles in the air-fuel mixture decreases, thus accelerating the flame propagation. Also, the stronger the swirl of the air-fuel mixture, the faster the flame propagation speed will be. When the flame propagation speed is fast, it is necessary to advance the ignition timing. Therefore, it is necessary to control the ignition timing according to the engine condition.
Ignition timing control
The ignition system controls the ignition timing in accordance with the engine speed and load so that the maximum combustion force occurs at 10ATDC.
HINT:
In the past, ignition systems used a governor advancer and vacuum advancer to control timing advancing and retarding. However, most ignition systems today use the ESA system.
1. Engine speed control
(1) It is considered an engine to output power most efficiently when the maximum combustion force occurs at 10 ATDC, on which the optimal ignition timing is set to 10BTDC (Before Top Dead Center) at a speed of 1,00rpm.
(2) It is supposed that the engine speed is increased to 2,00rpm. The duration for the ignition delay is practically constant regardless of the engine speed. Therefore, the crankshaft rotational angle increases, as compared to when the engine is running at 1,00rpm. If the same ignition timing described in (1) is used at 2,00rpm, the timing at which the engine produces the maximum combustion force will be retarded more than 10 ATDC.
(3) Therefore, to produce the maximum combustion force at 10ATDC while the engine is running at 2,00rpm, the ignition timing must be advanced in order to compensate for the crankshaft rotational angle that was retarded in (2). This process for advancing the ignition timing is called timing advance, and for retarding the ignition timing is called timing retard.
T – Duration for ignition delay
1 – Ignition timing
2 – Timing that produces the maximum combustion force
3 – Boundary between the ignition delay period and flame propagation speed
A – Ignition delay period
B – Flame propagation period
C – Timing retard
D – Crankshaft rotational angle
2. Engine load control
(1) It is considered when the maximum combustion force occurs at 10 ATDC, on which the optimal ignition timing is set to 20BTDC when the engine load is low.
(2) As the engine load increases, the air density increases and the flame propagation period decreases. Therefore, if the same ignition timing described in (1) is used when the engine load is high, the timing at which the engine produces the maximum combustion force will be more advanced than 10ATDC.
(3) To produce the maximum combustion force at 10ATDC when the engine load is high, the ignition timing must be retarded in order to compensate for the crankshaft rotational angle that was advanced in (2). Conversely, when the engine load is low, the timing must be advanced. (When the engine is idling, however, the amount of timing advance must be kept small or zero, to prevent unstable combustion.)
Knocking control
Knocking in the engine is caused by spontaneous combustion that occurs when the air-fuel mixture selfignites in the combustion chamber. An engine becomes more susceptible to knocking as its ignition timing is advanced. Excessive knocking negatively affects the performance of the engine, such as by causing poor fuel economy or reduced power output. On the other hand, slight knocking has the opposite effect of improving both fuel economy and power output. Recent ignition systems effect ignition timing control to retard the timing when a knock sensor detects knocking, and advance the timing when the knocking is no longer detected. By preventing the engine from knocking in this manner, these systems improve the power output and fuel economy.
1- Ignition timing
2 – Timing that produces the maximum combustion force
3 – Boundary between the ignition delay period and flame propagation
speed
A – Ignition delay period
B – Flame propagation period
C – Timing retard
D – Crankshaft rotational angle