Ignition timing

Ignition timing in an internal combustion engine is the process of setting the time that a spark will occur in the combustion chamber (during the power stroke) relative to piston position and crankshaft angular velocity. Setting the correct ignition timing is crucial in the performance of an engine. The ignition timing affects many variables including engine longevity, fuel economy, and engine power. Modern engines that are controlled by an engine control unit use a computer to control the timing throughout the engine's RPM range. Older engines that use mechanical spark distributors rely on inertia (by using rotating weights and springs) and manifold vacuum in order to set the ignition timing throughout the engine's RPM range. There are many factors that influence ignition timing. These include which type of ignition system is used, engine speed and load, which components are used in the ignition system, and the settings of the ignition system components. Usually, any major engine changes or upgrades will require a change to the ignition timing settings of the engine.



History

The ignition systems of older, non-computer controlled engines consists of a mechanical spark distributor which distributes spark to cylinders based on an initial timing advance, mechanical timing advance, and vacuum timing advance. In 1972, Chrysler introduced the electronic ignition system that controlled ignition timing and fuel delivery, replacing the mechanical spark distributor and therefore mechanical and vacuum timing advance. This system led to the advanced automobile electronic systems of today’s automobiles. Ignition systems will continue to improve as ignition components become more technologically advanced.


Setting the Ignition Timing

“Timing advance” refers to the number of degrees before top dead center (BTDC) that the spark will ignite the air-fuel mixture in the combustion chamber during the power stroke. “Timing retard” refers to the number of degrees after top dead center (ATDC) that the spark will ignite the air-fuel mixture during the power stroke.

Timing advance is required because it takes time to burn the air-fuel mixture. Igniting the mixture before the piston reaches top dead center (TDC) will allow the mixture to become fully burnt soon after the piston reaches TDC. If the air-fuel mixture is ignited at the correct time, maximum pressure in the cylinder will occur sometime after the piston reaches TDC allowing the ignited mixture to push the piston down the cylinder. Ideally, the time at which the mixture should be fully burnt is about 20 degrees ATDC. This will utilize the engines power producing potential. If the ignition spark occurs at a position that is too advanced relative to piston position, the rapidly expanding air-fuel mixture can actually push against the piston causing detonation and lost power. If the spark occurs too retarded relative to the piston position, maximum cylinder pressure will occur after the piston is already traveling too far down the cylinder. This results in lost power, high emissions, and unburned fuel.

The ignition timing will need to become increasingly advanced (relative to TDC) as the engine speed increases so that the air-fuel mixture has the correct amount of time to fully burn. Another reason for advancing the timing is because as the engine speed increases, fuel consumption increases. Since more fuel is present in the cylinder, the time required to fully burn the air-fuel mixture will be longer. Poor volumetric efficiency at lower engine speeds also requires increased advancement of ignition timing. The correct timing advance for a given engine speed will allow for maximum cylinder pressure to be achieved at the correct crankshaft angular position. When setting the timing for an automobile engine, the factory timing setting can usually be found on a sticker in the engine bay.


Dyno Tuning

Setting the ignition timing while monitoring engine power output with a dynamometer is an excellent way to correctly set the ignition timing. After advancing or retarding the timing, a corresponding change in power output will usually occur. Using a knock sensor to find the correct timing is one method used to tune an engine. In this method, the timing is advanced until knock occurs. The timing is then retarded one or two degrees and set there. After achieving the desired power characteristics for a given engine load/rpm, the spark plugs should be inspected for signs of engine detonation. If there are signs of detonation, the ignition timing should be retarded until there are no signs of detonation.


Mechanical Ignition Systems

Mechanical ignition systems use a mechanical spark distributor to distribute a high voltage current to the correct spark plug at the correct time. In order to set an initial timing advance or timing retard for an engine, the engine is allowed to idle and the distributor is adjusted to achieve the best ignition timing for the engine at idle speed. This process is called 'setting the base advance'. There are two methods of increasing timing advance past the base advance. The advances achieved by these methods are added to the base advance number in order to achieve a total timing advance number.


Mechanical Timing Advance

An increasing mechanical advancement of the timing takes place with increasing engine speed. This is possible by using the law of inertia. Weights and springs inside the distributor rotate and affect the timing advance according to engine speed. This type of timing advance is also referred to as centrifugal timing advance. The amount of mechanical advance is dependant solely on the speed at which the distributor is rotating. In a 2-stroke engine, this is the same as engine RPM. In a 4-stroke engine, this is half the engine RPM. The relationship between advance in degrees and distributor RPM can be drawn as a simple 2-dimensional graph.

Lighter weights or heavier springs can be used to reduce the timing advance at lower engine rpm's. Heavier weights or lighter springs can be used to advance the timing at lower engine rpm's.

Most vehicle manufacturers specify that the vacuum line for the vacuum advance (if equipped) should be disconnected and plugged when setting the initial advance. Be careful when turning the distributor while the engine is running because deteriorated spark plug wires can deliver a dangerous shock.


Vacuum Timing Advance

The second method used to advance the ignition timing is called vacuum timing advance. This method is sometimes (depending on the engine manufacturer) used in addition to mechanical timing advance. Usually, this method is used to increase fuel economy. Vacuum advance works by using a vacuum source to advance the timing at mid engine load conditions. Vacuum advance is diminished at wide open throttle (WOT), causing the timing advance to return to the base advance in addition to the mechanical advance.

The most common vacuum source for vacuum advance is a small port located in the wall of the throttle body or carburetor adjacent to but slightly upstream of the edge of the throttle plate. In carburetors having primary and secondary throttle plates, the port is located in the primary. The effect of having a hole here is that there is little or no vacuum at idle or at wide open throttle, with the vacuum signal peaking at part throttle opening.

On some vehicles, a temperature sensing switch will apply manifold vacuum to the vacuum advance system when the engine is overheating. The purpose of this is to increase engine speed to turn the engine fan faster to try to overcome the overheating.

On some vehicles, some form of switches may be used to prevent vacuum advance under certain conditions such as when the engine is cold or depending on what gear the vehicle is in. This is usually because these are necessary to comply with emission standards.


Electronic Ignition Systems

Newer engines typically use electronic ignition systems (ignition controlled by a computer). The computer has a timing map which is a chart with engine speed on one axis and engine load on another axis. Timing advance numbers are inserted in this chart. The computer will send a signal to the ignition coil at the indicated time in the timing map in order to spark the spark plug. Most computers from original equipment manufacturers (OEM) are not able to be modified so changing the timing advance is not possible. Aftermarket engine control units allow the user to make changes to the timing map. This allows the timing to be advanced or retarded based on various engine applications.