Jump to content
SAU Community

Recommended Posts

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?

Link to comment
https://www.sau.com.au/forums/topic/331742-injector-duty-and-fuel-consumption/
Share on other sites

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

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.

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 by gwilkinson34

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.

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.

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

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...

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

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.

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.

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.

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

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! :)

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


  • Similar Content

  • Latest Posts

    • As discussed in the previous post, the bushes in the 110 needed replacing. I took this opportunity to replace the castor bushes, the front lower control arm, lower the car and get the alignment dialled in with new tyres. I took it down to Alignment Motorsports on the GC to get this work done and also get more out of the Shockworks as I felt like I wasn't getting the full use out of them.  To cut a very long story short, it ended up being the case the passenger side castor arm wouldn't accept the brand new bush as the sleeve had worn badly enough to the point you could push the new bush in by hand and completely through. Trying a pair of TRD bushes didn't fix the issue either (I had originally gone with Hardrace bushes). We needed to urgently source another castor arm, and thankfully this was sourced and the guys at the shop worked on my car until 7pm on a Saturday to get everything done. The car rides a lot nicer now with the suspension dialled in properly. Lowered the car a little as well to suit the lower profile front tyres, and just bring the car down generally. Eternally thankful for the guys down at the shop to get the car sorted, we both pulled big favours from our contacts to get it done on the Saturday.  Also plugged in the new Stedi foglights into the S15, and even from a quick test in the garage I'm keen to see how they look out on the road. I had some concerns about the length of the LED body and whether it'd fit in the foglight housing but it's fine.  I've got a small window coming up next month where I'll likely get a little paint work done on the 110 to remove the rear wing, add a boot wing and roof wing, get the side skirt fixed up and colour match the little panel on the tail lights so that I can install some badges that I've kept in storage. I'm also tempted to put in a new pair of headlights on the 110.  Until then, here's some more pictures from Easter this year. 
    • I would put a fuel pressure gauge between the filter and the fuel rail, see if it's maintaining good fuel pressure at idle going up to the point when it stalls. Do you see any strange behavior in commanded fuel leading up to the point when it stalls? You might have to start going through the service manual and doing a long list of sensor tests if it's not the fuel system for whatever reason.
    • Hi,  Just joined the forum so I could share my "fix" of this problem. Might be of use to someone. Had the same hunting at idle issue on my V36 with VQ35HR engine after swapping the engine because the original one got overheated.  While changing the engine I made the mistake of cleaning the throttle bodies and tried all the tricks i could find to do a throttle relearn with no luck. Gave in and took it to a shop and they couldn't sort it. Then took it to my local Nissan dealership and they couldn't get it to idle properly. They said I'd need to replace the throttle bodies and the ecu probably costing more than the car is worth. So I had the idea of replacing the carbon I cleaned out with a thin layer of super glue and it's back to normal idle now. Bit rough but saved the car from the wreckers 🤣
    • After my last update, I went ahead with cleaning and restoring the entire fuel system. This included removing the tank and cleaning it with the Beyond Balistics solution, power washing it multiple times, drying it thoroughly, rinsing with IPA, drying again with heat gun and compressed air. Also, cleaning out the lines, fuel rail, and replacing the fuel pump with an OEM-style one. During the cleaning process, I replaced several hoses - including the breather hose on the fuel tank, which turned out to be the cause of the earlier fuel leak. This is what the old fuel filter looked like: Fuel tank before cleaning: Dirty Fuel Tank.mp4   Fuel tank after cleaning (some staining remains): Clean Fuel Tank.mp4 Both the OEM 270cc and new DeatschWerks 550cc injectors were cleaned professionally by a shop. Before reassembling everything, I tested the fuel flow by running the pump output into a container at the fuel filter location - flow looked good. I then fitted the new fuel filter and reassembled the rest of the system. Fuel Flow Test.mp4 Test 1 - 550cc injectors Ran the new fuel pump with its supplied diagonal strainer (different from OEM’s flat strainer) and my 550cc injectors using the same resized-injector map I had successfully used before. At first, it idled roughly and stalled when I applied throttle. Checked the spark plugs and found that they were fouled with carbon (likely from the earlier overly rich running when the injectors were clogged). After cleaning the plugs, the car started fine. However, it would only idle for 30–60 seconds before stalling, and while driving it would feel like a “fuel cut” after a few seconds - though it wouldn’t fully stall. Test 2 – Strainer swap Suspecting the diagonal strainer might not be reaching the tank bottom, I swapped it for the original flat strainer and filled the tank with ~45L of fuel. The issue persisted exactly the same. Test 3 – OEM injectors To eliminate tuning variables, I reinstalled the OEM 270cc injectors and reverted to the original map. Cleaned the spark plugs again just in-case. The stalling and “fuel cut” still remained.   At this stage, I suspect an intermittent power or connection fault at the fuel pump hanger, caused during the cleaning process. This has led me to look into getting Frenchy’s fuel hanger and replacing the unit entirely. TL;DR: Cleaned and restored the fuel system (tank, lines, rail, pump). Tested 550cc injectors with the same resized-injector map as before, but the car stalls at idle and experiences what feels like “fuel cut” after a few seconds of driving. Swapped back to OEM injectors with original map to rule out tuning, but the issue persists. Now suspecting an intermittent power or connection fault at the fuel pump hanger, possibly cause by the cleaning process.  
×
×
  • Create New...