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Old 08-14-2008, 09:41 PM   #1 (permalink)
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Does a given throttle position always mean consistant, predictable fuel usage?

If the throttle position is precisely 20% at six o'clock on Thursday cruising on the freeway, and it's also precisely 20% at seven o'clock accelerating out of a stoplight, is the same amount of fuel being used (gal/hour) in both instances? If you are idling in neutral, would throttle position be 0% or would it reflect fuel usage? Is fuel usage linear from throttle position 0% (none) to throttle position 100% (maximum possible fuel injection)? I am asking in reference to the OBD-II parameter ID 01 11 (OBD-II PIDs - Wikipedia, the free encyclopedia). If you have a glimmer of an idea, please reply!

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Old 08-14-2008, 10:06 PM   #2 (permalink)
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Throttle position is one of many inputs the ECU uses to determine how much fuel to inject into the engine. So, the answer is no.
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Old 08-15-2008, 12:32 PM   #3 (permalink)
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I would say combine throttle position, load on engine, and what gear your in and you will get consistent numbers
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Old 08-15-2008, 12:34 PM   #4 (permalink)
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also throttle position doesnt go to 0% when idling as far as i know. I believe mine starts at 10
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Old 08-15-2008, 04:20 PM   #5 (permalink)
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Fuel consumption depends on more than just throttle position. Engine RPM is also very important. The intake air pressure and temperature and air/fuel mixture ratio are also factors. Fuel usage probably does not have a linear relationship with throttle position.

I made a crude formula to let you estimate the fuel consumption rate. It assumes an intake air temperature of about 100 degrees Fahrenheit and a normal air/fuel ratio.

GPH=D*RPM*VE*PA/18130

GPH: gallons per hour of gasoline used in engine
D: engine displacement in liters
RPM: engine revolutions per minute
VE: volumetric efficiency of engine which changes with throttle position
PA: intake air pressure in psi (about 14.7 psi at sea level)

The volumetric efficiency is a ratio of volume of air taken into the engine in one intake stroke to the displacement of one cylinder. It can be estimated by this formula:

VE=1-(VAC/PA)

VAC: intake manifold vacuum gauge reading in mm of mercury
PA: atmospheric pressure in mm of mercury (about 30 at sea level)

The actual VE will probably be lower than this because of the restriction of intake valves and exhaust gas recirculation which displaces some of the intake air. Operating the engine at speeds far away from the torque peak also lowers VE. Engines with a turbocharger or supercharger can have a VE greater than one. When idling or running without a load, the VE is usually near 0.30 or 30% but it varies according to compression ratio and valve timing.

Last edited by Andyman; 08-15-2008 at 07:58 PM.. Reason: mistake in VE definition
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Old 08-15-2008, 06:37 PM   #6 (permalink)
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i can set mine to 1.8--1.9 gph and if i hit a hill and my odo drops below 45 my fuel lowers also, but that is like 1500ish (+/-200 rpm) but once it speed back up it goes back to where my foot was.
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Old 08-15-2008, 09:08 PM   #7 (permalink)
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In a diesel engine a constant throttle position means a constant volume of fuel is consumed per revolution. Only an engine RPM change will change your fuel used in GPH.

In a gas engine, there is almost no discernible correlation between throttle position and gallons of fuel consumed per hour. There are far too many variables to give a consistent answer, and most of the folks have already pointed them out.

I don't know how good Andyman's formula is, but his variables are inconsistent. The PA cannot both be manifold pressure and atmospheric pressure. The VAC is actually your manifold gage pressure. Volumetric efficiency varies with engine speed and throttle position. Even if the formula does work over a certain range of variables, it would only be meaningful at steady-state vehicle operation, and would have to be recalculated for a different set of parameters and would most assuredly yield a different answer.
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Old 08-16-2008, 01:32 AM   #8 (permalink)
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Quote:
Originally Posted by MechEngVT View Post
I don't know how good Andyman's formula is, but his variables are inconsistent. The PA cannot both be manifold pressure and atmospheric pressure. The VAC is actually your manifold gage pressure. Volumetric efficiency varies with engine speed and throttle position. Even if the formula does work over a certain range of variables, it would only be meaningful at steady-state vehicle operation, and would have to be recalculated for a different set of parameters and would most assuredly yield a different answer.
When I said the PA variable is intake air pressure I meant the pressure of the air before it goes through the air filter and throttle, not manifold pressure. The VAC variable is supposed to be the number you read on a vacuum gauge connected to the intake manifold. I was inconsistent with the units to measure it in the two formulas. I decided to use mm of mercury for the VE calculation because a vacuum gauge is normally calibrated that way. As an example, if you have 12 inches of mercury vacuum in the intake manifold then the VE should be about 1-(12/30) = 60%. There probably are some standard variable names for the parameters but I don't know what they are.

I don't claim the formula is perfect. It comes from bits of data of collected from various places. For example, a pound of gasoline is supposed to make 210 cubic feet of fuel mixture after mixing with the correct amount of air. Because of changes in the composition of gasoline, my numbers might be a few percent off. With alcohol added to the gas, the volume of air mixed with a pound of gasoline would be decreased a little and the fuel GPH would go up. I could make a more complicated formula to account for temperature and fuel type but I thought it was best to keep things simple so the formula would be easy to use and easy for me to create.
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Old 08-17-2008, 07:50 PM   #9 (permalink)
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Quote:
Originally Posted by MechEngVT View Post
In a diesel engine a constant throttle position means a constant volume of fuel is consumed per revolution. Only an engine RPM change will change your fuel used in GPH.
Not in my diesel!
My focus is fly by wire throttle. The fuelling is determined by throttle %, engine speed and vehicle speed. The fuelling is further limited by the air density limit map, which is relative to boost so smoke is reduced to reasonable levels.

There are 4 throttle position and engine speed maps, with vehicle speed interpolating between these maps.

Oh, and there's a temperature dependant map too plus lots more

So, no. There is not much correlation between throttle position and fuelling unless the conditions are exactly the same.

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