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Injector Duty Cycle

5K views 19 replies 6 participants last post by  Austin V70R 
#1 ·
Ok guys, riddle me this..... how can injector duty cycle be greater than 100%? Or am I missing something?

I know from many data logs with Hilton that max injector duty cycle during a 3rd gear pull occurs at approximately 6200 RPM, so if this VIDA data is correct and my max injector "on time" is 22ms, then my calculated injector duty cycle is on the order on the order of 112%

Stealthy... I seem to recall you had reported higher than 100% duty cycles at one point also, but I just don't understand how this can occur other than the injectors are wide open and the system is managing AFR by modulating fuel pressure. This theory might explain the 692kPa fuel pressure.

Thoughts?



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#2 ·
I can't really tell from that log, but what rpm is your 22ms IPW at? Our caRs don't have static fuel pressure right? if so, then max IPW may not be at max RPM.

Also, what injectors are you running? I think max safe limit is around 80-90% IDC.
 
#3 ·
I know from many logs with Hilton that max injector duty cycle occurs at around 6200 RPM.

The injectors are the Bosch 630cc 16 hole units shown on Hilton's website.

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#4 ·
The fuel injector duty cycle (IDC) is the percentage of time the injector is supplied with power. The time during which the injector is powered (or activated) is called the injector pulse width (IPW). During normal engine operation, the fuel injector fires once during the four strokes of the Otto cycle, which last for 2 revolutions of the engine. As an example, at 3000 rpm it takes 0.040 seconds or 40 milliseconds (ms) for the engine to complete 2 revolutions (3000 rpm divided by 60 equals 50 revs per second; invert to get 0.02 sec per rev or 0.04 second for 2 revs). At 6000 rpm it takes 20 ms for two revolutions. If a fuel injector is activated for 15 ms (the IPW) at 3000 rpm the duty cycle is 37.5% (15 ms/40 ms), or rpm times IPW divided by 1200 equals IDC in percent. If an injector is powered for 15 ms at 6000 rpm, then IDC is 75% (15 ms/20 ms). If you know the engine speed (rpm) and the IPW (dataloggers can provide this information), then it is easy to calculate the IDC.
http://www.stealth316.com/2-calc-idc.htm

Your injectors can only open 100% despite what the duty cycle calls for. You need bigger injectors and a re-tune.
 
#5 ·
On closer inspection, it looks like your 22ms duty cycle is occurring right at the "00:00:16" mark on your graph. In the section right before that, it looks like your RPM is staying roughly around the same (~6-6.5k it seems), but the weird thing is that your IDC is steadily going up at the same time your fuel pressure is steadily going down while your fuel pump dudty cycle is staying rather high (and climbing). It almost appears that your fuel pump is inadequate for sustained high rpm high load runs (assuming this was full load).

However, that's just my noob visual analysis... I really have no idea if I'm interpreting this graph correctly.
 
#6 ·
It's nice to have this attention to detail on how the car actually performs when modified. I'm wondering if in using the CM5 this we can program it to overcome a situation in ME7 ? Also I'm not sure a retune would fix it since the tune is based on Vidas data, althoug it might be some type of protection built into ME7
 
#7 ·
Not sure how accurate some of the graph data is since those flat lines indicate the ECU to VIDA communication may have had some issues. Would have also helped if you could have put some scaling lines on there too.

By using the only really clean data in the middle of the graph, it looks like you maxed out around a 15 ms pulse at 7K rpm, which is close to 85% duty cycle, which is about the highest I've seen when logging my own data.
 
#8 ·
I made an Excel sheet to calculate crank horsepower depending on fuel.
Change the flow rate according to your fuel pressure and pick an rpm like 5500-6500 range. And pick injector pulsewidth value from that point to calculate duty cycle. Rest of the fields are obvious.
People who use Hiltons logger, must use the value from "injection opening time bank 1 (tii_b1)"
VIDA users select "Engine speed" and "Injection on time" parameters.

https://www.dropbox.com/s/5x5wxjjyya851p5/HPcalc.xlsx?dl=0
 
#9 ·
This is cool - thank you.

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#10 · (Edited)
Thanks for your input guys. I created this post to get some collective thought on IDC above 100% as it's inherently non-nonsensical that an injector can be open greater than 100% of an intake cycle.

My current tune is stable with 650cc single hole injectors and I'm experimenting with these 630cc 6 hole injectors to evaluate changes in idle and low speed driving characteristics, which are SIGNIFICANTLY improved over the single hole injectors. I took this data last night to look at IDC of these 630cc injectors compared to the 650's that run around 73% IDC in my setup.

My conclusion based on this discussion is that an IDC above 100% essentially says the injector are running close to full open and the fuel pump is commanded to make up the difference, but can't keep up as we see in the decreasing fuel pressure when the injector open time is 22ms. The IDC greater than 100% is an error in the calculation because its dominated by RPM, which continues to increase as fuel pressure attempts to compensate; you can see this in the extremely high fuel pressure. This is clearly not a safe condition.

Interesting note..... my baseline setup normally pulls MAF airflow of 1440 kg/hr +/- a small 5 kg/hr, but it pulled 1468 with these smaller injectors. I need to noodle this one a little more, but based on these results I'm upgrading to 750cc 6 hole injectors and will be speaking with Hilton about a revision to the tune. Seems that this little EFR6758 is more capable than advertised!

Some of you may have read my post on installation of the TorqByte PM3 fuel pump current driver to eliminate the over current issue with the stock PEM that has been manifesting itself with extremely hard starting after spirited driving and CEM fuel pump relay & ECM fuel pressure high / low codes. Those issues are resolved with installation of the PM3 and the additional available current may also help explain the extreme duty cycle as it allows the pump to compensate for the wide open injector. The fuel pump is an Aeromotive 340.
 
#12 ·
You're correct, my statement was a theory because I didn't have the AFR data available.

I really like your IDC & HP calculator and the IDC matches data from my baseline Hilton logs perfectly. I think your BHP calculation may be conservative though, my baseline is 650cc injectors at 73% IDC, but the car made 392AWHP on a Mustang dyno, whereas your calculator shows 376BHP even at .6 BSFC.
SS common knowledge calculates my MAF values to 414AWHP, which is in the ballpark with the heartbreaker dyno numbers and supported by the EFR matchbot that calculates 488BHP.

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#15 ·
It only matters in the flow pattern and atomization.... 1cc / 6 holes as opposed to 1cc / 1 hole.

750cc seems to be the cutoff to achieve flow thought a 6 hole plate. Larger volume appears to require a single hole to support the flow.

I'm far from an expert and this is only my observation.

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