Injector Firing Angle

 

Injector Firing Angle

 

The Injector firing angle is the angle at which the injector will SHUT after the injection even has occured
(relitive to a full camshaft revolution).
The injection angle depends on things such as camshaft profile, intake manifold design, fuel type, injector location as well as other external factors.
Injection angle is also used in certain applications to cool in th...e inlet valves.
When tuning the end of injection angle (with the exception of above) you want to make sure all injected fuel is taken in by the motor
and not "blown" back up the inlet manifold.

In practice the easiest way to do this is by tuning your engine to a known AFR. Next, adjust the injector firing angle until the engine becomes as rich as possible, now we know that the engine is burning the maximum amount of injected fuel. Do this for each RPM site you require.
You will notice that the engine will be slightly more responsive at lower RPM with the correct injection angle.
Please note, if the engine is not tuned to accpetable mixtures (rich and lean spots all over the place) that injection angle will not make much of a diffrence and you should spend more time on the tune.

It depends on where you injectors are located. Stock values represent stock ports and injector locations,
and you also need to consider the secondary injectors which have an offset angle and are fired earlier than primaries.
Injectors mounted further than stock need to be advanced, especially at lower rpm areas.
Bigger ports will need to fire a little more retarded, down to around 50-60 at idle.

We have different requirements for different engine conditions.
For example at very low engine speeds you probably want to fire the fuel just after the intake valve opens,
this will ensure that a high percentage of the fuel delivered as it gets caught up in the moving air stream
(because the engine speed is low the valve is open for alot longer than the injector is open so this is easily achieved).

At part throttle and higher RPM it is often of benefit to spray the fuel onto a closed intake valve.
...because the valve is hot the fuel will hit the valve and evaporate giving a better air/fuel distribution due to the smaller droplet size of fuel
(ie its now evaporated) - this of course assumes that the injector actually sprays fuel directed at the valve,
if you have a throttle body injection setup then there is much less benefit to this.

Of course at high duty cycles there is much less importance.

Bore washing is rarely a problem under any condition apart from startup because of the heat in the cylinder walls
(once the engine is warm the temperature of the liner is well higher than the fuels evaporation temperature so bore washing wont occur -the fuel just evaporates).
This is exactly why you would want to fire the fuel into the moving airstream on startup and low engine speeds.
Get the fuel droplets suspended in the airstream and move them into the combustion chamber,
if you spray fuel into the valve when the engine is cold it will liquify,
pool and then run into the combustion chamber - this is where bore washing occurs.