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Old 05-12-2008, 08:42 PM   #1 (permalink)
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Duty cycle of fuel injectors

Is the duty cycle (ON time relative to OFF time) of fuel injectors directly proportional to fuel being used by the engine?

In other words, if I can see a 10% ON time of my injectors on an o'scope can I assume that when it shows a 20% ON time that it is using twice as much fuel per unit of time?

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Old 05-12-2008, 11:03 PM   #2 (permalink)
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yes, (duty cycle) times (fuel flow rating of your injector per hour) times (number of injectors) = fuel flow per hour

correct me if i'm wrong it's late.

my 91F250 has bank fire injection, 4 at one time every other revolution.

Last edited by diesel_john; 05-13-2008 at 07:55 PM..
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Old 05-13-2008, 07:17 AM   #3 (permalink)
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Thanks diesel_john,

Another question: the wiring diagram of my 2.2 litre 4 cyl. Sonoma shows that the 4 fuel injectors are wired such that #1 and #4 are in parallel with each other and that #2 and #3 are in parallel with each other.

This seems very odd. This would cause each injector to deliver fuel two times during one cycle. If it delivered fuel during the intake stroke of its cylinder it would also deliver fuel during the power stroke of its cylinder since pistons #1 and #4 go up and down together but #1 is on intake while #4 is on power stroke.

Is my Chilton's manual showing a misprint or is this the way multiport injection is done?

One thing that makes me think 'misprint' is that this same wiring diagram page (2.2L 4 Cyl. engine) shows 6 spark plugs, which is obviously an error.
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Old 05-13-2008, 08:21 AM   #4 (permalink)
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That kind of setup is not uncommon. Its called batch or group injection as opposed to sequential injection. It makes a small difference at low rpm and next to no difference as the rpm increases.
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Old 05-13-2008, 11:04 AM   #5 (permalink)
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Thanks Daox. Much appreciated.
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Old 05-13-2008, 02:56 PM   #6 (permalink)
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Quote:
Another question: the wiring diagram of my 2.2 litre 4 cyl. Sonoma shows that the 4 fuel injectors are wired such that #1 and #4 are in parallel with each other and that #2 and #3 are in parallel with each other.
This is the "Iron Duke" not the "2200", correct? The 2200 has SFI and as far as i know does fire the injectors one by one. There's nothing wrong with batch injection though. Due to intake manifold resonance, air is still moving within the runners even though the intake valve is shut, so there's no worry of fuel pooling up. I've got a V8 Porsche in my driveway that runs 100% injector duty cycles constantly. It varies fuel delivery by modulating the line pressure.
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Old 05-17-2008, 10:51 AM   #7 (permalink)
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Quote:
Originally Posted by JohnnyGrey View Post
This is the "Iron Duke" not the "2200", correct?
I guess I don't have any knowledge of either of these you mentioned. I am referring to my GMC Sonoma 2.2L 4 cyl.

Anyway, I just finished building a little fuel injector monitor which I can now use to visually monitor exactly how much time the injectors are injecting fuel. It has 10 LEDs, each one representing an injector ON time of approximately 2.3% of total time.It was very interesting to learn how much a higher gear reduced fuel consumption. I have been driving in too low of a gear climbing hills. I have always thought that pressing the accelerator pedal down further, as required in higher gears, was worse for mileage, but I am surprised to see how that is not quite true.

My LED monitor just converts the pulses from the injectors to a voltage which the LEDs correspond to.
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Last edited by Gregte; 05-17-2008 at 11:17 AM..
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Old 05-17-2008, 10:59 AM   #8 (permalink)
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That's really cool Greg! Would you be kind enough to hack up a schematic in paintbrush and a parts list? Maybe a little description of what to do with the potentiometers?
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Old 05-17-2008, 11:59 AM   #9 (permalink)
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Quote:
Originally Posted by dcb View Post
That's really cool Greg! Would you be kind enough to hack up a schematic in paintbrush and a parts list? Maybe a little description of what to do with the potentiometers?
The pots are to set the voltage range that the LED drive will see. I set VR1 so the first LED lights up when the car is warmed up and idling. This turns out to be about 1 mili second of ON time of the injectors. I then set the total range, using VR2, such that each LED represents another 1 mili second of ON time. This of course requires an O'scope. What I did was monitored the actual fuel injectors with a scope while driving, then I made a circuit (555 timer IC) to replicate the range of square pulse width modulated waves for use on the bench (it's harder than hell to build and test a circuit while driving!).

EDIT: The schematic does not show it but the anodes of the 10 LEDs are connected to the car's 14.x volts. The shotkey diode (D1) can be any diode, I used it because it has a lower voltage drop and that made adjusting the pots easier, to get a range (completely arbitrary) that I wanted to be in.

for instance, If I floor the pedal all 10 LEDs will light, and beyond. I wanted a good resolution with the mere 10 distinct voltage ranges that the LEDs can show so I chose to have the 10th one lit at the 24% injector duty cycle. The engine will go as high as 45%, at least that's what the scope appeared to show while I was driving hard and watching its display, and the road, at the same time.
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Last edited by Gregte; 05-17-2008 at 12:45 PM..
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Old 05-22-2008, 09:21 PM   #10 (permalink)
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Very clever device you created. I like the sleek way you integrated it into your dash.

Quote:
Originally Posted by Gregte View Post
I have always thought that pressing the accelerator pedal down further, as required in higher gears, was worse for mileage, but I am surprised to see how that is not quite true.
I think one of the most useful concepts in the world of hypermiling is P&G. And I think the essence of P&G is large throttle settings. They are more efficient because they minimize pumping losses. But this is perversely counterintuitive.

More on this subject: http://autospeed.com/cms/A_110216/article.html

Monitoring instantaneous consumption is helpful, but I think it's fundamentally important to not be misled by the short interval of measurement. In P&G, you spend extra fuel now in order to save fuel later. The fuel has been converted to kinetic and potential energy that you'll soon call on (while gliding).

Focusing only on lowest instantaneous consumption is at odds with maximizing efficiency. It will guide you into a steady cruise, which is less efficient than P&G. Maximizing efficiency requires a broader interval of analysis, like a trip, or at least portions of a trip. Because your analysis has to encompass the entire P&G cycle, as a complete unit.

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