Fuel Injector theory
Discussion
Hi Guys,
Sorry for my usual dullness
BUT
I have been struggling with this topic and wondered if any of you had a few mnutes to slap me into line ?
First prob: As the revs increase, does the injection period become longer or more powerful ?
My prob is that I can't understand what happens as the revs increase:
The time to get the fuel into the manifold will reduce as the volume requirement increases.
Odd ramblings:
My personal solution is to have a second set of injectors, mounted in the port, to kick-in when the relevant ECU signal says
"OK lads, we're gonna exceed 4,500 rpm, clear your throats !".
I have talked, at length, with a certain Mr Melling, who complained of throttle bodies and valves drowning in fuel,
which is blocked from entering the port, thereby giving a fuel droplet size far too large to reach the ideal combustion size.
He extolled the virtue of the reactive, carburettor system, rather than the pro-active, injection system which, in my mind, cannot be benefical, as every carburettor system has been (or is being) replaced by injector systems, which are designed to produce more power.
As I always say, I await your intelligent and/or amusing responses with beery-baited breath.
p.s. I was just about to suggest "Anyone up for having a natter about this at next week's NEC show ?" when I discovered that it is, in fact, this weekend.
What a Dullo !
PJ
Sorry for my usual dullness
BUT
I have been struggling with this topic and wondered if any of you had a few mnutes to slap me into line ?
First prob: As the revs increase, does the injection period become longer or more powerful ?
My prob is that I can't understand what happens as the revs increase:
The time to get the fuel into the manifold will reduce as the volume requirement increases.
Odd ramblings:
My personal solution is to have a second set of injectors, mounted in the port, to kick-in when the relevant ECU signal says
"OK lads, we're gonna exceed 4,500 rpm, clear your throats !".
I have talked, at length, with a certain Mr Melling, who complained of throttle bodies and valves drowning in fuel,
which is blocked from entering the port, thereby giving a fuel droplet size far too large to reach the ideal combustion size.
He extolled the virtue of the reactive, carburettor system, rather than the pro-active, injection system which, in my mind, cannot be benefical, as every carburettor system has been (or is being) replaced by injector systems, which are designed to produce more power.
As I always say, I await your intelligent and/or amusing responses with beery-baited breath.
p.s. I was just about to suggest "Anyone up for having a natter about this at next week's NEC show ?" when I discovered that it is, in fact, this weekend.
What a Dullo !
PJ
Edited by Mr Cerbera on Friday 10th November 17:46
Hi Paul - the injectors are an on/off switch, they’re either spraying fuel or not.
The amount of fuel sprayed per millisecond is a function of the size of the injector and the pressure from the pump.
As revs increase, the injector duty-cycle increases (they stay open for longer).
The design is pretty crap (at least on the 4.5), as the injectors spray onto the back of the throttle butterfly. On one bank, the butterfly opens to reveal an ever larger aperture for the injector to spray into. On the other bank, the butterfly opens in the same direction, which results in something of an obstruction for the jet of fuel.
Hope that helps.
Always enjoy reading your posts, keep em coming :-)
The amount of fuel sprayed per millisecond is a function of the size of the injector and the pressure from the pump.
As revs increase, the injector duty-cycle increases (they stay open for longer).
The design is pretty crap (at least on the 4.5), as the injectors spray onto the back of the throttle butterfly. On one bank, the butterfly opens to reveal an ever larger aperture for the injector to spray into. On the other bank, the butterfly opens in the same direction, which results in something of an obstruction for the jet of fuel.
Hope that helps.
Always enjoy reading your posts, keep em coming :-)
Almost right Imran.
The injector duration is a function also of cylinder filling, which closely resembles the torque output. As such the injector opening on a TPS based map will look initially quite odd.
On closed throttle the injector duration decreases as revs rise (imagine going downhill on a closed throttle and revs increasing) the closed butterfly becomes more and more of a restriction as revs rise so the cylinder fills less and less on each induction stroke, so the fuel required gets less and less also.
On full throttle the injector duration will rise up to around peak torque (peak cylinder filling) and then decrease from that point onwards. On a short induction ajp8 that will put max injector duration around 5500rpm.
On any part throttle the max injector duration will be at reducing revs as the throttle is closed down, because on part throttle the peak torque occurs at lower and lower revs as the butterflies close down (taking foot off the pedal)
The injector duration is a function also of cylinder filling, which closely resembles the torque output. As such the injector opening on a TPS based map will look initially quite odd.
On closed throttle the injector duration decreases as revs rise (imagine going downhill on a closed throttle and revs increasing) the closed butterfly becomes more and more of a restriction as revs rise so the cylinder fills less and less on each induction stroke, so the fuel required gets less and less also.
On full throttle the injector duration will rise up to around peak torque (peak cylinder filling) and then decrease from that point onwards. On a short induction ajp8 that will put max injector duration around 5500rpm.
On any part throttle the max injector duration will be at reducing revs as the throttle is closed down, because on part throttle the peak torque occurs at lower and lower revs as the butterflies close down (taking foot off the pedal)
Mr Cerbera said:
My prob is that I can't understand what happens as the revs increase:
The time to get the fuel into the manifold will reduce as the volume requirement increases.
Yes, and this is one of the main parameters behind sizing your injectors .. the time available to get all the required fuel injected decreases as revs rise (on full throttle)The time to get the fuel into the manifold will reduce as the volume requirement increases.
Multi-set staged injectors is one way of having a small sized injector for light load duty, and phase in another set of injectors for full load .
Imran999 said:
Hi Paul - the injectors are an on/off switch, they’re either spraying fuel or not.
The amount of fuel sprayed per millisecond is a function of the size of the injector and the pressure from the pump.
As revs increase, the injector duty-cycle increases (they stay open for longer).
The design is pretty crap (at least on the 4.5), as the injectors spray onto the back of the throttle butterfly. On one bank, the butterfly opens to reveal an ever larger aperture for the injector to spray into. On the other bank, the butterfly opens in the same direction, which results in something of an obstruction for the jet of fuel.
Hope that helps.
Always enjoy reading your posts, keep em coming :-)
Awwww, cheers Imran, that's very kind of you...The amount of fuel sprayed per millisecond is a function of the size of the injector and the pressure from the pump.
As revs increase, the injector duty-cycle increases (they stay open for longer).
The design is pretty crap (at least on the 4.5), as the injectors spray onto the back of the throttle butterfly. On one bank, the butterfly opens to reveal an ever larger aperture for the injector to spray into. On the other bank, the butterfly opens in the same direction, which results in something of an obstruction for the jet of fuel.
Hope that helps.
Always enjoy reading your posts, keep em coming :-)
My stupidity will guarantee the endless proliferation of my comments
Talking of stupidity, Joolz , my mind is completely blown
BUT
Thnaks, so much, for taking the time to explain.
Stunned Monkey said:
Just to add another 2p. The amount of fuel injected per cycle depends on fuel pressure, injector duty length AND inlet vacuum, since on our engines it is not corrected mechanically (that's what the unused vacuum port on the pressure regulator is usually for).
Cheers Martin !(
) Mr Cerbera said:
Cheers Martin !
( )
Fuel pressure is given relative to atmosphere. eg your typical 3 bar becomes effectively 3.5 bar if your intake is drawing a 500mBar vacuum. (
) Or if your turbo is boosting to 1 bar, your fuel injectors would only have 2 bar to work with.
On most engines, the fuel pressure regulator has a hose connecting it to the intake manifold and corrects on a 1:1 basis, so the same injector pulse produces the same amount of fuel regardless of what the intake is doing.
We don't have an intake manifold, and the engine is simply mapped around it.
Can anyone confirm the sequence of injector firing? It's my understanding that the injectors are connected in 'banks' - so that all the odd injectors open at the same time, and all the even injectors open at the same time.
Is this backed up by the statement "Mr Melling, who complained of throttle bodies and valves drowning in fuel, which is blocked from entering the port"?
Is this backed up by the statement "Mr Melling, who complained of throttle bodies and valves drowning in fuel, which is blocked from entering the port"?
notaping said:
Can anyone confirm the sequence of injector firing? It's my understanding that the injectors are connected in 'banks' - so that all the odd injectors open at the same time, and all the even injectors open at the same time.
Is this backed up by the statement "Mr Melling, who complained of throttle bodies and valves drowning in fuel, which is blocked from entering the port"?
The standard MBE runs batch injection, so yes all 4 injectors per side fire at the same time. My Emerald runs semi-sequential. I think Jools experimented a bit with injector timing when he had my car for mapping (its been a few years now) and found it made no difference.Is this backed up by the statement "Mr Melling, who complained of throttle bodies and valves drowning in fuel, which is blocked from entering the port"?
Hi Will .. yes i couldn't detect any meaningful measurable difference in power when changing the timing (or change in mixture afr at the tailpipe) .. that *might* change if fully sequential (after all in semi sequential you are still injecting half the fuel into a stalled airstream).
Also during injector testing i've never measured any meaningful power changes with fine mist injectors either, possibly for the same reason.
I know that mixture presentation is an important consideration and top flight race teams will investigate that fully, but just from a simple fine mist injector change there seems to be 4/5ths of eff all difference.
Also during injector testing i've never measured any meaningful power changes with fine mist injectors either, possibly for the same reason.
I know that mixture presentation is an important consideration and top flight race teams will investigate that fully, but just from a simple fine mist injector change there seems to be 4/5ths of eff all difference.
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