PULSING INJECTORS

[marcus1970]

Hello all,

Having had the pipes off the top and my head in the engine bay numerous times, I have noticed that the injectors kind of pulse and spit their juice onto the butterflies.

The question is, do they all receive the same pulse rate and hence the same fuel pass rate and what controls the pulsing?

Does the pulsing get quicker the harder the accelerator is pushed, or does the volume of fuel just increase but at he same pulse rate.

Cheers

MB

[The AJP Griff]

Very basically,the fuel pressure going to the injectors is fixed,the injectors will open for a given period of time,dependant on what the ecu 'tells them' to,this is known as the 'duty cycle'.The ecu gets its information to determine this period of time,from the throttle position,engine revs,and sometimes the lambda sensor feedback.Each bank of four injectors is operated at the same time.

[marcus1970]

So, does the fuel pressure regulator just keep the fuel coming at a constant pressure/speed?

and...

What actually causes/is responsible for the mechanical pulsing of the injectors - ie what part of the system?

MB

[ridds]

Fuel pressure is constant (ish) it can vary slightly depending on manifold depression (not on the Cerb though).

The quantity of fuel delivered is defined by the duration that the injector is open. This is controlled by a solenoid within the injector. The solenoid is controlled by an electrical signal sent from the ECU. The small signal issued by the ECU is amplified by a driver within the ECU itself. The duration it is open for is commonly referred to as the Pulse Width.

The Pulsing is the rapid opening and closing of the solenoid.

[marcus1970]

ridds said:

Fuel pressure is constant (ish) it can vary slightly depending on manifold depression (not on the Cerb though).

The quantity of fuel delivered is defined by the duration that the injector is open. This is controlled by a solenoid within the injector. The solenoid is controlled by an electrical signal sent from the ECU. The small signal issued by the ECU is amplified by a driver within the ECU itself. The duration it is open for is commonly referred to as the Pulse Width.

The Pulsing is the rapid opening and closing of the solenoid.

Do the solenoids all operate at the same frequency for each different cylinder? - ie when you put your foot down and just say the solenoid opened for 2 seconds and it pulsed 30 times in a minute, would all the injectors behave in exactly the same way, like a set of drones?

and..

Do the solenoids make an audible noise?

MB

[FarmyardPants]

2 seconds is a bit on the long side, the time they are open is usually measured in milliseconds, but yes, they do tick, some more than others.

I expect wikipedia has a good write-up on how they work

[GreenV8S]

marcus1970 said:

would all the injectors behave in exactly the same way, like a set of drones?

yes

marcus1970 said:

Do the solenoids make an audible noise?

Yes, they sound just like an electrical relay closing. They aren't very loud though and can easily be drowned out by other noises from the running engine.

[marcus1970]

So,

does the pulsing of the injectors increase/quicken as the engine revs go up?

and...

do all injectors rely on a solenoid type arrangement, or are there some AJP cerbs that don't have this system?

MB

[FarmyardPants]

marcus1970 said:

So,

does the pulsing of the injectors increase/quicken as the engine revs go up?

The simplest way for an ECU to control its injectors is to have them all synchronised to the same fixed frequency, like a heartbeat. At the time of each beat, the injectors open. They will close again sometime before the next beat. The amount of time they stay open dictates how much fuel is delivered. At idle, when the fuel requirement is low, the injector might only stay open for a very short time (say) 5ms (0.005s) before closing again. Once closed they won't open again until the next beat. So if the cycle/heartbeat was 1s, the injector would be running at 0.5% of its theoretical maximum capacity at idle.

If 'X' is open and '.' is closed, the injector state over time would look like this:

X..........X..........X..........X..........X.......... = low fuel demand
XXXXX......XXXXX......XXXXX......XXXXX......XXXXX...... = medium fuel demand
XXXXXXXXX..XXXXXXXXX..XXXXXXXXX..XXXXXXXXX..XXXXXXXXX.. = high fuel demand

The amount of fuel delivered per unit time is also a function of the size of the injector (measured in cc/min I think) and the fuel pressure difference between the fuel pump outlet and the intake manifold. As said before, if you have forced induction, this differential decreases as boost goes up.

This is not the most efficient model because the fuel will be sprayed regardless of the what the inlet valve is doing at the time - some of the injectors will pulse just after the inlet has closed. So a better system, called sequential injection, is to time the pulse of each injector to align with the intake pulse of the cylinder it is delivering fuel to. This gives better vaporisation and probably better economy. Other systems are in between (eg all injectors in a bank fire at once).

The coils and magnets in an injector need to open and close it very quickly, which means plenty of current and hence heat generated in the coils. They are not designed to stay open for long periods, they need to cool down in between beats. So injectors should not be run at high pulse widths (say more than 85% or so). If you need so much fuel that your injectors are nearing this level, you can add more injectors, use larger injectors, or increase the fuel pressure (and in all cases re-map the ECU so that it knows how long to operate the injectors in your new setup).

marcus1970 said:

and...

do all injectors rely on a solenoid type arrangement, or are there some AJP cerbs that don't have this system?

MB

All "EFI" systems are broadly similar, certainly all AJPs use the same type of system, in fact all cars today use the same basic principle AFAIK.


Edit: this is probably not the right definition of sequential injection

Edited by FarmyardPants on Friday 7th November 20:46

[marcus1970]

Ok,

Lastly then...

What actually physically opens and closes and how does it open and close.


and...


Is the ticking noise from the 8 solenoids the noise I can hear coming from the rear of the engine nearest the windscreen, and how the blazing squad do I get rid of it?

MB

[GreenV8S]

FarmyardPants said:

The simplest way for an ECU to control its injectors is to have them all synchronised to the same fixed frequency, like a heartbeat. At the time of each beat, the injectors open. They will close again sometime before the next beat.

Just to clarify that comment, the 'frequency' is determined by the engine speed - the injectors would usually fire once per engine cycle, some system are set up to fire twice per engine cycle, but it's always based on the engine speed and not a fixed frequency.

In contrast, water injection systems very often *do* use a fixed frequency.

FarmyardPants said:

The coils and magnets in an injector need to open and close it very quickly, which means plenty of current and hence heat generated in the coils. They are not designed to stay open for long periods, they need to cool down in between beats. So injectors should not be run at high pulse widths (say more than 85% or so). If you need so much fuel that your injectors are nearing this level, you can add more injectors, use larger injectors, or increase the fuel pressure (and in all cases re-map the ECU so that it knows how long to operate the injectors in your new setup).

That's not likely to be an issue in practice; injectors can be left 'on' indefinitely without damaging them. However, once the injector is permanently on you have reached the maximum amount of fuel that can be supplied to the engine and that's not a happy situation to be in. In practice the maximum flow of the injectors will never exactly match the peak requirements of the engine so you always go oversize, even if only slightly. The alternative, of risking the engine going lean under full power, is a very very bad idea.

[marcus1970]

marcus1970 said:

Ok,

Lastly then...

What actually physically opens and closes and how does it open and close.


and...


Is the ticking noise from the 8 solenoids the noise I can hear coming from the rear of the engine nearest the windscreen, and how the blazing squad do I get rid of it?

MB

anyone?

[GreenV8S]

marcus1970 said:

Is the ticking noise from the 8 solenoids the noise I can hear coming from the rear of the engine nearest the windscreen, and how the blazing squad do I get rid of it?

Hard to diagnose funny noises over the internet but if the noise is noticeable it's probably not injectors that you're hearing. It may be that you're hearing valve train noises, but there are so many other sources of noise that it's really not practical to guess without hearing it.