The timing of an Engine

Overview

Engine timing also refers to the ignition timing of an engine as well.

•      In case SI (spark ignition) engine it is an angle measured in crankshaft degrees relative to piston position at which the spark plug ignites the air-fuel mixture before TDC (top dead center).

   

•      In case CI (compression ignition) engine it is the angle measured in crankshaft degrees relative to piston position at which the fuel injector will inject fuel in the combustion chamber before TDC.

The environment in the combustion chamber is extremely violent. Therefore, if the ignition timing goes wrong the controlled combustion turns into a violent explosion and destroys anything on its way. As the mixture burns, it expands and creates pressure in the combustion chamber. The goal is to have the maximum cylinder pressure occurring after TDC when the piston starts to make its way back down the bore.

If ignition timing goes wrong?

If the ignition occurs too soon the expansion of the mixtures collides with the piston in a violent explosion known as detonation. Detonation is caused by ignition timing that is too far advanced and it is extremely detrimental to the engine. As a result, power generated is reduced. Detonation causes blown head gaskets and piston failure.

The amount of time it takes for air-fuel mixture to burn remains same.

How to detect the timing?

Engine timings can be checked by timing light. A timing light is a stroboscope used to dynamically set the ignition timing of an SI engine equipped with a distributor. The timing light connected to the ignition circuit and used to illuminate the timing marks on engine’s flywheel. The position of the marks, frozen by the stroboscopic effect, indicates the current timing of the spark. On most automotive engines, timing is set according to the timing of No.1 cylinder in the engine block.

Setting the timing

Setting the timing of an engine means the adjustment of the crankshaft angle at which spark ignites the air-fuel mixture so that we have maximum pressure in the combustion chamber when the piston starts to make its way down the bore. It should be kept in mind that knocking must be avoided while setting the timing. The setting of the timing depends on the type of engine and it involves timing advance and timing retardation.

Timing Advance

Timing advance refers to changing the timing so that fuel ignition in CI engine or spark in SI engine happens earlier than the manufacturer’s specified time. For example, if an engine is manufactured to have the timing of 12^o before TDC, then if it is to be set at 15^o, this process is timing advance.

Timing Retardation

Timing retardation is defined as changing the timing so that fuel ignition in CI engine or spark in SI engine happens later than the manufacturer’s specified time. For example, if an engine is manufactured to have the timing of 20^o before TDC, then if it is to be set at 15^o, this process is a retardation of timing.

SI Engine

In SI engines, a distributor controls the timing of an engine. And the timing can be advanced or retarded by changing the relative position of the rotor with respect to the engine in the distributor. Usually, the distribution is camshaft driven.

SI Engines

With an increasing engine speed there is a need to advance the timing of an engine because the ignition time remains the same but due to the increased speed of the engine, position reaches at TDC before the mixture is burnt completely. So, there are two ways of achieving this goal.

1.      Mechanical Timing Advance

By using the law of inertia, it is possible to increase the timing with increasing engine speeds automatically. Weights and springs inside the distributor rotate and affect the engine timing advancement according to speed. This type of timing advance also called as centrifugal timing advance. The amount of timing advance depends on centrifugal force on the weights, which is dependent on the engine rpm

2.      Vacuum Timing Advance

The vacuum advance system consist of a vacuum diaphragm mounted on the distributor body, the diaphragm is spring loaded in the zero advance position and has a rod which connects to a hole in the breaker plate, which is a movable plate on which points are mounted. When vacuum is applied to the diaphragm, it pulls on the rod, which in turn pulls on the plate rotating it with respect to the cam on distributer shaft. When viewed from the top, the distributor shaft turns clockwise, when the vacuum advance rod pulls on the breaker plate, it rotates the breaker plates anticlockwise; when advance the timing. Hence by increasing engine speed, intake air increases which ultimately increases the advancing of the timing of an engine.

In Case of CI Engine

In case of compression ignition engines, timing refers to the No. of degrees before TDC, when the fuel injector injects the fuel. To change the timing of a CI engines, we have to set the fuel injection pump to inject fuel at a proper time. Usually, the fuel injection pumps housing has the marks on it. There are some bolts on the driven side of the pump. By rotating this plate, anti-clockwise, we can advance the timing, and by rotating the plate clockwise we can retard the timing of a CI engine.

Factors affecting Engine Timing

There are several which greatly affect the engine timing.

·         Type of engine

·         Type of fuel

·         Engine temperature

·         Geometry

·         RPM (speed of engine)

·         Air-Fuel ratio