Thursday, May 05, 2005

Injector Duty Cycle and Volumetric Efficiency

In few previous posts I was playing a lot with injector sizing, and figuring out duty cycles, to gauge what kind of injectors do you need for your application.

Few days ago I came across this . Basically it outlines how we can convert injector duty cycles and air fuel ratios into a fairly close approximation of volumetric efficiency.

First you start out with converting injector pulse width into duty cycle. That I covered few posts ago. Once you know how much fuel gets injected, you calculate a total fuel flow. That takes the duty cycle, number of injectors and fuel pressure into consideration.
Airflow is not that far off, since you know your AFR (if you have a wideband; if you don't, you just go by the target/commandered AFR from PE tables). The problem with this one is units. You got lbs/hr and you somehow have to end up with cubic feet per minute. That's just math though, and not a complicated one, you just have to watch it.
The actual calculation of VE is a division of airflow vs displacement, as that's theoretically the maximum of what you're supposed to be able to flow though it. Of course forced induction and big cam/NA applications can go over 100% VE.

So DOWNLOAD spreadsheet. All you really need is RPM and Injector Pulse Width. If you have a wideband, then you can also go off that for your AFR muliplier. This spreadsheet is set up with 12.2AFR in mind, so if you have a different goal, please change it.

In the spreadsheet you downloaded, you'll see two tabs with two different sets of data. One is for stock '99 F-body 26.4lbs/hr injectors during a high speed run with 'good air' where the Injector duty went over 100%. The other graph is for SVO 30lb/hr injectors with a much more healthy Injector Duty Cycle. The weird part is that the VE changes from one sheet to the other quite a bit. Both FuelFlow and AirFlow are actually higher on the smaller injectors. So what makes the big difference? Is it:
1. weather conditions--the 'good air' day was really spectacular, car ran like mad. Just before I swapped the injectors I did some runs just so I know where I was on Duty Cycle that day, and it showed 96% max, while on the good day I've seen it as high as 109%.
2. Old injectors were being pushed beyond their limits as far as flow was concerned, and the flow might have been actually misreported, so that whole set of data as far as VE is concerned is just plain wrong.
3. different day, different injectors, injectors going static, too many variables, stop worrying, you can't compare the two?


At 2:21 PM, May 06, 2005, Blogger PJ said...


At 2:33 PM, May 14, 2005, Anonymous Anonymous said...

3- when injectors are >80% they start acting flaky, let alone when they are static. The PCM was expecting a certain injector output and subsequent calculations were based on this output.
Don't worry about it!


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