Computing Average Miles Per Gallon: How ?

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The technique used isn't based on changes in fuel level. It's based on volume flow, but probably not in the way you'd expect. I don't know the actual terms, but in layman's words, fuel is "pumped" (or injected) into the engine according to commands from a small computer that naturally knows how much fuel it has commanded to be injected. Hence amount of fuel consumed at each moment is known and can be used to calculate the consumption.

P.V.

The amount of fuel is approximately know, within some reasonable error, at any rate.

There are very accurate flow measuring devices, but these tend to be quite expensive. I am surprised that the implemented mileage computers give the accuracy levels that they do.

It's known pretty precisely, because the computer knows to squirt a very precise amount of fuel into the cylinder.

Assuming the fuel pressure remains constant, the fuel volume is directly proportional to the duration of each squirt.

Since you know the duration you're commanding, it's very easy to integrate that and show how much fuel you're using over a given amount of time.

There will be some error in the reading, and that error will mostly be a result of the fuel pressure regulation not being quite precise.

They do very well. Back in the seventies lots of folks added aftermarket manifold pressure gauges that claimed to be fuel consumption gauges. But they really weren't and the information they gave often did more harm than good. Injection has changed everything. I wouldn't want to go back, and I'm the worst Luddite around.

--scott

Yes, but the fuel pressure does not remain constant (manifold vacuum feedback to the fuel pressure regulator), which is why it's surprising that the MPG readings are as accurate as they are.

At x rpm the engine pumps x lbs. of air. x lbs. of fuel are added and held to a very close ratio. The only "guess" is the weight of a gallon of gas. (Hot or cold.)

so you are saying that at say 2500 RPM the engine consumes a fuel at a fixed rate regadless of the throttle setting?

Mark

True, and that was my point.. There DO exist metering systems that can do the job over a wide range of pressures, viscosities, etc. But they are expensive.

Back in the 790's I bought a driving computer and cruise control system from JC Whitney for about 100 bucks. It included a fuel flow meter that was surprisingly accurate for the price.

It is all done using the engine computer and the existing sensors. Nothing special is added except the display.

The computer does use the amount of fuel in the tank, not for the instant readings but for the average readings.

It also knows what amount of fuel is being injected while the engine is in operation. It monitors the injector pulses, engine temperature, intake air temperature, timing and throttle position, engine rpm and vehicle speed and on flex fuel vehicles it monitors fuel composition as well. Then it does a bit of math using the above numbers and shows you what the mileage is predicted to be, based on a table of expected fuel used Vs. what it computes you used.

The fuel regulator is exactly why it remains constant. It is not constant in absolute pressure, but it is constant relative to the injector. If the manifold pressure drops, so does the fuel pressure. The pressure is therefore constant across the injector, and thus the amount of fuel injected is only dependent of the injector duty cycle.

I have been studying the Ford EEC-IV EFI as I have adapted this to my

351C, and all fuel calculations in the EFI is done by fuel weight internally, so it is pretty easy to calculate the mpg.

Bingo! And if you actually measure the thing, you'll find it stays pretty constant.

--scott

Right... We can be sure that if extremely accurate methods were easily adaptable, the Formula 1 cars would not run out of fuel a lap or two from the end of the race.

The ones we have built into our cars are still very helpful to improve our driving style for best economy.

I don't have a differential pressure gauge to test the true delta, but certainly driving my truck around with a fuel pressure gauge attached shows pretty significant pressure changes from ~50psi to ~65psi, and the pressure increases are under acceleration.

Relatively easy in the days of low pressure single fuel line carburated stuff. More difficult now with FI with remote regulators and return lines.

I have a device I bought at a Goodwill store many years ago.It has a glass container which holds exactly one quart of gasoline, there are a few vlalves and a metal bracket for hanging the device from a car window or whatever and a flexible tube for connecting to a carburetor.The idea is to run a vehicle in traffic/on the road untill the one quart of gasoline is used up and then multiply that by four to get the average miles per gallon. cuhulin

Of course the pressure increases under acceleration, that is when you have low manifold vacuum == higher absolute manifold pressure -> higher fuel pressure == constant pressure difference across the injector.

You will have the lowest fuel pressure at high engine rpm with the throttle closed (coasting downhill), that is when you have the highest manifold vacuum == lowest absolute manifold pressure.

Exactly, yes! If you measured the manifold pressure at the same time you would find they track. When you accelerate, the manifold pressure (which is normally below atmospheric) increases until it is nearly atmospheric. The total change isn't _quite_ a whole atmosphere but it's a good fraction.

--scott

They've been doing this for 20-plus years now. You're right, precisely measuring very small fluid flow rates from first principles, without impacting the flow itself, is tricky. But fuel injected engines already have injectors that are very precisely calibrated in terms of how many pounds of fuel they flow per hour at a given pressure. And the computer knows EXACTLY how many milliseconds per firing each injector is open because it commands that value. So voila, you multiply the calibrated flow rate by the amount of time the injectors are open, do the appropriate unit conversions, and divide by the distance traveled.

Its VERY accurate in general.

Robert11 wrote:

But the computer also monitors manifold pressure, so applying the known correction factor for fuel pressure is easy. Even if the regulator is analog mechanical and independent of the computer itself.