Request for Data from TheHolyCow
 

Was wondering if you could gather (or report it, if you have it) some injector duty cycle data.

Specifically, was wondering what the duty cycle is for the same rpm but different gears and/or throttle angle.

I other words, would like to compare duty cycle if you're driving at 2000 rpm in 2nd gear versus 2000 rpm in 5th gear.

Not sure you have the capability for throttle angle measurement, though.

you can always send him a PM. I think you would get a faster response that way.

well, at the same rpm's intake air flow should be the same regardless of gearing, so I'd have to figure the duty cycle would also be the same.

Intake air flow will vary based not only on RPM, but on throttle opening too. Duty cycle should mainly follow air flow, with minor variations, and at some times larger variations.

Your request is very timely, actually. I've got an ELM327 coming in the mail, and I've acquired an old laptop to keep in the car at all times. A couple days ago, while waiting for my ELM327, I started thinking about fumesucker's hookup logging fuel injector pulses with his sound card. A project to combine that data with OBDII logged data may be far beyond my abilities but I'm going to try.

I will have to try to test and observe for those things. I have been concerned mainly with speed, acceleration rate, throttle opening, and gear, but not so much with RPM. My VW consistently does better at lower RPM under every condition or circumstance so I haven't had much reason to test same RPM with different conditions; instead, I just keep the RPM as low as possible.

I can answer one piece, though. I previously was able to log OBDII data with an expensive VW-specific tool, so I was able to see throttle angle; its DBW is usually obedient, giving me throttle angle proprtional to gas pedal position. So...
Same RPM, different throttle angle: As you might intuit, fuel injector duty cycle goes up with the throttle more open -- UNTIL you get over a certain point. I'm not exactly sure what that point is; it's somewhere between 50 and 80 percent throttle. At WOT and the same RPM, it actually uses less fuel than at ~70% throttle. I believe this is not a universal behavior, but I'm not sure.

I'm not sure what you are after here, perhaps a comparison with the same RPM but different aerodynamic drag?

I can watch injector duty cycle on my car, and throttle position is easily watched by the TPS (throttle position sensor).

-Bob C.

What I'm after is this: is there a difference in fuel flow rate at the same RPM but different speeds (i.e. gears)? Further, how does the flow rate vary with throttle angle while holding rpm constant?

Since we can't measure flow rate directly, the next best thing is FI duty cycle.

Another reason I'm wondering has to do with P&G technique - how can I improve my mileage at lower speeds (city driving)? Have I hit a lower limit? If the flow rates are pretty much the same at any given rpm, my commute mileage will never get better (one-off long highway trips will show better mileage, but my main concern is daily driving). 2000 rpm is a handy number to kick around for me, as I use that as my 'cutoff.' When accelerating I try not to exceed 2000 rpm, I can cruise at highway speeds at 2000 rpm without much problem, etc.

Theoretically, the answer should be "it depends" due to the effects of BSFC. I'm trying to confirm it ... ... within reason. For ME I would need to setup a similar gage system as HolyBovine has, but at the moment my curiousity is killing me.

I don't have data to prove this but my thinkings are, at any given rpm you are going to use X amount of gas as a minimum and that will go up with throttle opening and load on the vehicle. I would say that your MPG (miles per gallon) would change but your GPM (gallons per minute) would stay constant with constant rpm regardless of gear. that is given every other factor being the same (and aerodynamic drag falls under engine load).

given this, you want to lower your rpm level during city driving. possibly use pulse and glide (in neutral) or even EOCing (engine off coasting) these techniques requre some research to make sure they won't damage your transmission but several people on here are claiming big differences after these techniques are implemented.

there again, I have no hard evidence to prove this and I have been wrong before and will be wrong again (unless I die a very untimely death)

Hmm. Fuel varies approximately linearly with airflow. So if you have a certain RPM but more airflow (more throttle position), yes there will obviously be more fuel flow (higher DC%).

I have all of the tools for this on my "other" car, but it'll be a few weeks before I'm ready to drive that (lapsed inspection sticker etc.) so I can't get you a nice pretty graph in the near future. Maybe in 2-3 weeks.

The unfortunate part about the whole low-RPM, low-throttle position thing is this: When coming off idle, a very slight touch of the throttle will quickly increase the amount of airflow- if you move the throttle angle from 3degrees to 4 degrees that will probably double your airflow. However at higher throttle positions, a lot of throttle movement only makes a small difference- going from 45 degrees throttle position to 90 degrees throttle position (WOT) only increase airflow about 10% even at high RPM. It's one of those obnoxious things where I wish I could modify the laws of physics a bit.

I was going to type a whole bunch of long boring paragraphs about engine load, volumetric efficiency, air/fuel ratios, etc., to attempt to answer your real question, but I'll let HC answer first because a picture could be worth a thousand words.

-BC

Okay, I experimented with 5th vs. 4th using cruise control for constant speed. It was as I expected.

In 5th at 50mph going up a long somewhat shallow grade in a torrential downpour, my meter was reading ~5.5. In 4th it read ~7.5.

In 5th on level ground at 70mph, it read ~9. In 4th, ~12.5.

I didn't get a chance to try lower speeds in consistent conditions, but in my experience, the observations above apply to any speed and any two usable gears. The effect is probably more pronounced between any two other gears, since 4th and 5th are nearly the same in my car.

What about rpm? Using the cruise keeps speed constant, so the ECU varies rpm to maintain speed.

Could you try this but look at constant rpm versus gear?

Again, if my car uses "X" amount of fuel at 2000 rpm (regardless of gear), I'm cooked in city/local driving due to the low speed of the car.