Injector Duty And Fuel Consumption

[gwilkinson34]

For example, if cruising at 60kmh in 3rd at say 2.5k rpm and injector duty is at 4.5-5% compared to cruising in 4th at 2k rpm and injector duty is at 3.5-4% - does this mean that the car is using less fuel in 4th? And even less in 5th if the duty is lower again? (I'm assuming here that the injector duty I see is directly related to the fuel in the load/rpm cell I am cruising in for a given gear...)

Assuming that each injector is flowing roughly 1% less each time it fires and that it is firing 500 times less per minute - is there a direct correlation between duty and fuel consumption that is this easy to draw, or is it more complicated?

[djr81]

No there isn't a direct correlation, sorry.

There are too many variables to work it out - to list just some - ignition advance, engine speed, combustion efficiency, pumping losses etc etc etc.

A better guide is the AFR, but even that is not very good.

[JonnoHR31]

it is related. timing, efficiency etc have no effect on the injector duty itself.

regardless of duty the amount of times it fires per minute is only affected by engine rpm. inj duty is the % of time the injector is open in every pulse, ie at 0% it never opens, at 50% its open for the same amount of time as it is closed, so half the time, at 100% its constantly open.

its not exact but yes in 4th you would be using around 20% less fuel than you would be in 3rd in your example

[gwilkinson34]

it is related. timing, efficiency etc have no effect on the injector duty itself.

regardless of duty the amount of times it fires per minute is only affected by engine rpm. inj duty is the % of time the injector is open in every pulse, ie at 0% it never opens, at 50% its open for the same amount of time as it is closed, so half the time, at 100% its constantly open.

its not exact but yes in 4th you would be using around 20% less fuel than you would be in 3rd in your example

That's what i figured - surely the time the injector was open and the RPM must relate to petrol consumption.

All the other factors mentioned above would then have an effect on injector duty itself, as you would need to put your foot down more if you weren't making efficient power, hence increasing the duty.

I think i'll try cruising in 5th and watching my inj duty for a tank and see if I can notice any difference.

[gwilkinson34]

I guess another variable though is that at 2k rpm '100%' injector duty is going to be longer than at 2.5k rpm - meaning that the actual inj duties i'm reading may not be directly comparable? Is that right?

Ie in 4th it's 3.5% of X (@ 2k rpm) and in 3rd it's 4.5% of 1.2x X (@ 2.5k rpm)....?

Or is '100%' duty a constant regardless of RPM?

Edited August 6, 2010 by gwilkinson34

[Moodles2]

I'm just going to use 444 cc injectors as an example (stock GTR injectors). 100% duty cycle means each injector is pushing 444 cc's of fuel per minute which is 26.64 litres per hour (0.0074 litres per second) each.. since you have 6 injectors times that by 6 and you have 159.84 litres per hour (0.0444 litres per second). I admit it's not exact but it's a good idea.

[MrStabby]

My 2c.

Injector duty will give you a very accurate idea of fuel consumption. The fuel pressure regulator means that the injectors will see the same pressure differential across the tip, so flow rate should be pretty much the same regardless of vacuum/boost. All other stuff is not relevant AFAICT.

[JonnoHR31]

My 2c.

Injector duty will give you a very accurate idea of fuel consumption. The fuel pressure regulator means that the injectors will see the same pressure differential across the tip, so flow rate should be pretty much the same regardless of vacuum/boost. All other stuff is not relevant AFAICT.

exactly a 1:1 fuel reg gets rid of any flow difference due to pressures.

inj duty is not affected by rpm directly, yes you'll usually see a higher duty at a higher rpm but thats because you need generally need more fuel at higher rpm's. if you have 444cc injectors at 50% your using 222cc of fuel per minute per injector ragardless of whether its at 2000rpm or 6000rpm. obviously its never going to be exact but it gives you a good idea of how much fuel your using

[gwilkinson34]

exactly a 1:1 fuel reg gets rid of any flow difference due to pressures.

inj duty is not affected by rpm directly, yes you'll usually see a higher duty at a higher rpm but thats because you need generally need more fuel at higher rpm's. if you have 444cc injectors at 50% your using 222cc of fuel per minute per injector ragardless of whether its at 2000rpm or 6000rpm. obviously its never going to be exact but it gives you a good idea of how much fuel your using

I thought that injector duty was the amount of time it could be open on each cycle - ie in ms - 50% duty meant the injector was open for half the available time etc - therefore the higher the revs the less time available for it to be open (i would assume)?

Read it on some EFI website somewhere...

[JonnoHR31]

yer thats true. at double the rpm the same injector duty is going to see half the fuel injected each pulse, but theres twice as many pulses so you end up using the same amount of fuel relative to time

[tridentt150v]

Sorry if I misunderstood, but........

If you are under load then your rpm won't go up but your IDC will increase. ie going up a hill so rpm and IDC will only be comparable on a flat surface with the least amount of friction.

This could also be true - and logic says it is - for wet vs dry, gravel vs tar vs concrete and heavy atmosphere vs a bright sunny day, but to what extent I don't know.

So I say that rpm and IDC can at times and under certain conditions be correlated, but the relationship would not be consistent nor for all situations.

[MBS206]

Sorry if I misunderstood, but........

If you are under load then your rpm won't go up but your IDC will increase. ie going up a hill so rpm and IDC will only be comparable on a flat surface with the least amount of friction.

This could also be true - and logic says it is - for wet vs dry, gravel vs tar vs concrete and heavy atmosphere vs a bright sunny day, but to what extent I don't know.

So I say that rpm and IDC can at times and under certain conditions be correlated, but the relationship would not be consistent nor for all situations.

IDC will ALWAYS be comparable at a given RPM.

What you really need though, is a read out of how long the injector is open for, per minute.

The less it's open per minute, at any given speed, the lower your fuel consumption. But then you can only compare at the same speed. (This removes engine RPM from the mixture)

Then if you want to remove time, you need to replace time with distance, and suddenly, you have a working Fuel Economy gauge.

[MrStabby]

wikipedia: "the duty cycle is the fraction of time that a system is in an "active" state".

IDC will ALWAYS be comparable at a given RPM.

No, the frequency will be comparable to RPM (half the rpm?), the duty cycle will be dependent on load.

To calculate fuel consumption you will need injector flow rate, injector open and close latency difference, duty cycle and car speed. I think.

[JonnoHR31]

when your talking about fuel consumption and injector duty rpm is rather irrelevant, yes i know it makes a difference due to latency but its fairly insignifigant, even at idle (the lowest fuel consumption apart from fuel cut) the latency for most injectors is still less than the actual open time.

for a rough guide you can use inj duty to determine what gears are more efficient for certain speeds

[gwilkinson34]

yer thats true. at double the rpm the same injector duty is going to see half the fuel injected each pulse, but theres twice as many pulses so you end up using the same amount of fuel relative to time

touche

when your talking about fuel consumption and injector duty rpm is rather irrelevant, yes i know it makes a difference due to latency but its fairly insignifigant, even at idle (the lowest fuel consumption apart from fuel cut) the latency for most injectors is still less than the actual open time.

for a rough guide you can use inj duty to determine what gears are more efficient for certain speeds

And that's pretty much exactly what I was after!