A trivial automotive question

A cow-orker just bought a new (to him) 1998 Dodge Durango. Not my kind of car, but suitable to his needs -- wife, three kids, and a travel trailer.

It has a feature I have heard of but not actually seen myself -- a gauge that tells him how many miles per gallon he is getting. His Durango is equipped with the 5.2 liter V-8 with automatic transmission and four wheel drive, and the mileage is appalling. According to his gauge, he is getting less than 10 miles per gallon around town. Since this vehicle will be driven fewer than 5,000 miles per year, he can stand it.

Here's the question: How does that mileage gauge work? I am guessing that it is metering the fuel delivered to the injectors, and comparing that to distance travelled. But is the readout an instantaneous calculation, or an average over some pre-determined time (30 seconds? A minute? An hour?), or an average over some pre-determined distance, or is it a cumulative figure since the most recent time the engine was started, or is it a check on all the fuel that has passed through it since the car came out of the factory? What happens when you stop the car and leave the engine running? Does your fuel mileage display then drop to zero for the duration of the stop?

Who knows how this works and can explain it to me?

tanstaafl.

I had one of these features in my 1998 Volkswagen GTI. It was built into the car's LCD trip computer readout and controlled from one of the stalks on the steering wheel. I miss this feature terribly, I wish all cars had it.

Interestingly, the car's own owner's manual gave me the hint that I needed to figure out how it worked. It gave me a list of all the things that the trip computer recorded, and on the list was the following item:

"Amount of fuel used (Not displayed)."

So it was hooking into the electronic fuel injection system and keeping track of the amount of fuel that got squirted through the injectors since turning on the key. It took the "current trip" distance (a value that was similarly reset when I turned on the key), and did some sort of complicated newfangled high-tech math (I think it's called "division" whatever that is) to give me the MPG readout.

I also had the option to reset the trip counter by hand of course, and when I did this I assume it also reset the fuel consumption counter. I used to amuse myself by resetting it when I would go downhill or uphill, or when I was coasting in neutral, just to see how much difference there really was with various engine loads. Hours of fun.

I also had the option to reset the trip counter by hand of course, and when I did this I assume it also reset the fuel consumption counter. I used to amuse myself by resetting it when I would go downhill or uphill, or when I was coasting in neutral, just to see how much difference there really was with various engine loads. Hours of fun.

Hah! And they said that the Empeg's visuals were distracting!

Outlaw car gauges now!!

on bosch systems (lh, trionic, motronic) there is a pin that goes outbound to the system the displays such things as fuel consumption, miles to empty, and so on.
the display system has the choice of giving you real time figures, or average over time (most are 10 minutes)
they are rather accurate.
because all (known to me) EFI systems use a fuel pressure regulator that keeps the fuel behind the injector at a constant pressure relative to the (ever changing) manifold pressure, the amount of fuel injected will be dependent only on the time the injector is open (~2.4 ms at idle, 2.8 at cruise, max ~8ms under throttle snap or heavy load)

the system dates back to the mid 80's

I miss this feature terribly, I wish all cars had it.

/jim (87 GTI16V), too, *bigtime*. Sniff!

The Insight has an instataneous MPG display, average MPG trip-meter, and an average lifetime MPG display. As Tony stated the amount of fuel used is calculated from the fuel injection system. The calculations seem to be very accurate and are usually off by no more than +/- one percent based on my experience.

(GIS image)


When the car is sitting at idle the instantanous MPG reads zero (as expected), and the MPG/trip average will start to decrease as well. Alternatly, when coasting down a hill, the instantaneous MPG will max out at 150mpg. If you switch to metric displays zero L/100km.

I agree with Tony, all cars should be equiped with these gadgets.

-Jeff

all cars should be equiped with these gadgets

Even better. Have them use WAP to download the current price of petrol or diesel and display how much you're spending...

that would be funny. imagine getting a spending ticket.

sir, you were doing $6.50 in a $3.75 zone.
step away from the car.

That would seriously cut down on the amount i drive if i had to watch the pennies fly.

And heres me happy to get a fuel gauge but i might buy an emerald which would keep you techy types happy enough to burst http://www.emeraldm3d.com/em_m3dk.html fully mappable fuel injection