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Old 27th January 2008, 04:12 PM   #1
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Default DIY MPG Gauge/ Fuel consumption meter?

Has anybody built a DIY MPG (miles per gallon) gauge, for knowing fuel consumption of a motorcar/motorcycle?

There are some projects in the net, but they are for new cars (>1995), digital based DIY project, usually connected to ODB-II (16 pin on-board diagnostic) connector. It is for computer-controlled cars.

If the car/motorcyle don't have any digital diagnostic (still using carburators, using analog speedo cable, old cars/motorcycle), is it possible to make a real-time digital-displayed MPG gauge? Maybe we can monitor it and change the way we drive, because the gasoline price is high now

For a certain time span, it need to measure how many gallon of fuel consumed (putting sensor on gasoline hose?), how many miles have been travelled (putting sensor on speedo cable?), and compare these two. The idea seems simple, but I don't know where to start.
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Old 27th January 2008, 04:34 PM   #2
Eva is offline Eva  Spain
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I would start considering a small microcontroller to do the math and an LCD display to show the data. Good old HD44780-compatible text displays are quite easy to use and may be driven with 3 pins only with the help of a shift register.

Then I would consider a magnetic sensor in one of the wheels or in the transmission, which should produce one or more pulses per turn. It may be as simple as a iron piece attached to the rotating surface and a fixed small coil whose inductance will become much higher when the iron piece is near it. There are plenty of ready-made sensors available.

Thet it comes fuel consumption sensing. A high precision approach would involve a fluid flux sensor in the fuel path, which would produce one pulse everytime a certain amount of fuel passes though it. In case there is a fuel pump of the kind that operate in variable frequency bursts, a medium precision approach would be to spy the coil and count the bursts. A low precision approach would just use the signal from fuel tank gauge.

Then it comes calibration... The firmware must know the relationship between pulses and distance travelled, and between fuel sense signal and the actual amount of fuel dealt with.
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Old 27th January 2008, 04:53 PM   #3
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My 95 BMW has a vacuum operated mpg gauge. Maybe you can monitor vacuum (air sucked into the intake manifold) with a pressure sensor and use that combined with speed to get an indication of mpg.

I_F
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Old 27th January 2008, 05:07 PM   #4
poobah is offline poobah  United States
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Fuel efficiency is inversely proportional to throttle position... more throttle... less efficiency.

The best gauge is in your pocket. Record your odometer reading, and the volume of fuel required at each fill-up. This will give you consumption/efficiency values with the advanced feature of high-tech averaging over a period of days or weeks!

You will need a PooTech XLT-1000 Personal Palm Recorder... pictured below. These are available from PooTech for $199 U.S. (free shipping within 48 states).
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Old 27th January 2008, 05:54 PM   #5
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All a vacuum MPG gauge really tells you is your throttle position, which you can measure just as accurately with a device connected to the end of your leg. They're basically useless, AFAIK modern mpg gauges use injector pulse width, a speedometer connection and knowledge of injector size and fuel pressure to calculate MPG. Of course, these won't work in your case, EVA's ideas look viable. Seems a lot of work for something you can figure with pen and paper though.
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Old 27th January 2008, 07:45 PM   #6
Bungle is offline Bungle  United Kingdom
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You are correct in your thinking.
Maplin published a project for this in the Maplin Electronic magazine March-May 1982 in the UK and reproduced it in the Maplin project Book 2 I've still got the cover page but not the rest of the magazine unfortunately. Might be worth contacting them.

You are correct in your thinking.
I can recall that they cut a section out of the outer speedometer cable and glued a magnet to the inner cable and used a hall sensor and then put a small flow sensor with a plastic impellor in the fuel line.

It was a frequency counter circuit with the time base gated by the output of the speedometer sensor and then it counted the output pulses of the fuel sensor between gate pulses
It was all TTL logic with adjustable chain of dividers ( set by links) on both the sensor outputs to set it up correctly for different cars
So I guess you start with a frequncy counter circuit.
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Old 27th January 2008, 07:45 PM   #7
Bungle is offline Bungle  United Kingdom
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Whoops!
Got the sensors wrong way around.
The time base would be gated by the fuel sensorand you would count the pulses from the speedo sensor
unless you drive a tank and want gallons per mile.
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Old 28th January 2008, 01:47 AM   #8
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I read that the biggest devil in saving fuel is the driver's style, not the car, not the ECU system.
For every car/motorcycle, there is a sweetspot where the fuel efficiency is the highest, without having to modify the car itself, just adjusting the driving style.

There is a digital based system
http://www.lightner.net/lightner/bruce/Lightner-183.pdf
http://www.circuitcellar.com/AVR2004...05abstract.pdf
A nice display, can fit anywhere
http://www.thinkgeek.com/gadgets/electronic/8426/
A vacum system?
http://www.jcwhitney.com/autoparts/P...atalogId=10101

Is it possible to make something more advanced, without digital data from the vehicle (sensing analog way), can get real-time digital displayed MPG gauge?
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