| T O P I C R E V I E W |
| YS |
Posted - Mar 25 2003 : 5:36:56 PM
Hi guys,
I am thinking of fuel economy meter, or rather an indicator. Fuel becomes expensive and I wonder which speed is optimal for commuting. Of course, no info is available from car manufacturers.. I know that economy driving is no fun, but I wonder if I can find a style to combine both - so I need an indicator.
I do not know much about cars, but it seems the instrument needs to measure the amount of fuel going to the engine - and probably take speed into account - well, that part I can do myself just looking at speedometer :-)
My idea was to measure the time ratio while fuel injector is opened/closed - I assume that fuel pressure is more or less constant. I do not want to connect anything to the car's system - it is under warranty - so maybe a coil over injector wire.. and a sensitive amplifier, as signal will be weak - I do not want to make a big coil. To measure the ratio is pretty simple after you restore the signal to logic level - it's just a capacitor and a switch, and two current generators.. and an analog voltmeter (again, at the moment I need just an indication)
Any suggestions?
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| 15 L A T E S T R E P L I E S (Newest First) |
| Aaron Cake |
Posted - Apr 08 2005 : 09:56:36 AM
OBD-II is a requirement for all cars sold in North America since 1996.
Mileage is not a standard function of OBD-II. It depends on the vehicle, and the software. The software generally "guesses" based on other parameters.
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| Kevinlekiller |
Posted - Apr 07 2005 : 12:33:42 PM
Hi YS, why dont you get an OBD-II connector and some software and hook it up to your laptop (if you have one). OBD-II is on all american cars that where made after 1995 or 96. Im not 100% sure if it tells you how much gallons of gas your burning per mile. Heres a site to start you off: http://www.obdii.com/
Bab Himself |
| GMP |
Posted - Apr 02 2005 : 12:03:38 PM
Looks like I am entering this discussion in the end zone, but I wanted to add that if the system is computer controller and has an ALDL connector, you might be able to get the info out to a laptop from there. I have found an excellent site regarding this here:
http://www.techedge.com.au/vehicle/aldl8192/8192hw.htm
It is a simple RS232 interface to read several parameters from the on-board computer.
-George |
| n/a |
Posted - Sep 27 2004 : 9:23:49 PM
Most cars change the fuel pressure (line pressure) with a vacuum pod on the fuel pressure regulator that is connected to the intake manifold. When the vacuum drops down the fuel pressure (line pressure) goes up, this is to compensate for the change in manifold pressure (which effects the fuel flow rate). Theoretically this keeps the fuel flow through the injector the same whether the fuel pressure is 45psi and manifold vacuum is 20" or if the fuel pressure is 52psi and manifold vacuum is 0" and everywhere in between. I made up the numbers but they are actually pretty close to what a typical car would be.
BTW most BMW's come with a MPG gauge in the car, I think for the most part it just uses Pulse width and distance (or it could be speed) for the calculation.
Ramon
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| Aaron Cake |
Posted - Sep 10 2004 : 08:47:46 AM
From pulsewidth you can easily get duty cycle. So, to sum up, you need:
1. Measure pulsewidth/duty cycle of injectors
2. Measure speed and distance covered
You need to know:
1. Your fuel pressure. This can change (some cars reduce voltage to the fuel pump at idle, thus reducing pressure) so confirm that your car uses a single fuel pressure.
2. The size of your injectors. Normally in CC or LB/Hour. This tells you the flow rate if the injector is wide open. This changes with fuel pressure, which is why pressure is important. With pressure and your injector size, you can calculate fuel flow if the injector is wide open.
3. Injector duty cycle/pulsewidth. This tells you how long the injector is open. You can now calculate fuel injected per event, then use your distance and speed to determine your average MPG.
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| n/a |
Posted - Sep 09 2004 : 10:54:39 PM
1. You cannot determine fuel economy by using any kind of GPH flow from your fuel pump. The fuel pump provides as much fuel as it can through the fuel pressure regulator. Fuel not used by the injectors is returned to the tank. This returned fuel would be included in your GPH meter at the fuel pump. Caveat: A signal from both the fuel pump and fuel return lines could be subtracted to give you an amount consumed.
2. Injector pulsewidth is only a part of the equation. The pulsewidth is per event, the frequency of which varies by RPM. Duty cycle is more complete, as it specifies the percentage of time which the injector is flowing, yet still not enough. The fuel pressure is relative, therefore the flow rate is variable. Pressure is always measured as a differential, and fuel pressure is set as varying from atmoshperic pressure. Pressure inside the intake manifold is decidedly NOT equal to atmospheric in most cases (Always lower, unless you have a turbo or supercharged engine) which means that at high vacuum, the injectors will flow more at a given duty cycle than they will during low vacuum. At best, measuring the injectors would be inaccurate.
3. Fuel economy is VERY CLOSELY linked to airspeed. Atmospheric drag quickly becomes the most important component in determining power, and therefore fuel, used to keep you at a given speed. Driving into a headwind of 10mph will affect your fuel economy by almost the same amount as increasing your speed by 10mph. This is why drafting large vehicles on the highway can save you a LOT of fuel. (Not necessarily the best for your safety, nor for the paint on the front end of your car.)
But more to the point...
If you can get a pulsed signal for both MPH and GPH, then the problem is simplified. A counter will count the number of pulses from the speedometer. When a pulse is received from the fuel line (If these are too rapid for consistent results, you can use a counter network to slow this down to every 10th or 100th pulse etc.) the total count is passed on to your display driver, which could be anything from a simple bar graph generator (No need to know units) to a numerical LED/LCD display. Counter ICs are readily available, and are usually easy to interface to things like bar graph generators, which are also readily avaialble.
NOTE: While this circuit will work fine in most normal circumstances (indicating zero when you are not moving) it will not indicate "infinite" mileage, if the engine stalls (thus using no fuel) while you are moving, as there will be no fuel signal pulse. I figure that this fits within the design scope.
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| n/a |
Posted - Sep 08 2004 : 12:39:28 PM
OK guys, I got into this MPG discussion late, but all I want to know is the amount of gas that goes to the engine in a given time. I can easily obtain the flow rate, GPH and the speed, MPH. MPH divided by GPH = MPG. What I need is a circuit that will electronically divide MPH signal by GPH signal. My mechanical engineering training leaves me short on the electronics side of this thing.
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| Aaron Cake |
Posted - Mar 30 2003 : 10:40:08 AM
Yeah, but you'd want to take into account wasted fuel. While this is not necessary for a MPG guage, it would be a great "efficiency" guage. Speed doesn't really need to be measured, only distance.
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| BEatonNo1 |
Posted - Mar 27 2003 : 4:58:44 PM
I hate to be annoying but you cannot really use the gas once it goes past the O2 sensor burnt or not
and although there are other factors all of them influence your speed so really all you need to know is how much gas your sucking and how far your going with it (speed could be used) that is the easiest way to do it because you dont need as much calculating
so that means that you know how many gallons you are using in an hour and how far your going in an hour
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| Aaron Cake |
Posted - Mar 27 2003 : 3:14:04 PM
OK, I'm going to try to clear this up... To build a VERY BASIC miles-per gallon guage, you would need to monitor:
-fuel pressure
-injector pulsewidth
-distance
You would also need to know:
-size of your injectors
Based on the size of the injectors, the pulsewidth and the fuel pressure, you can calculate how much fuel is going into the engine. You would then corelate that with distance to determine your MPG. If you wanted to get fancier, you could monitor O2 sensor voltage, so you can tell how much fuel is actually being used to MOVE the car (instead of just pushed back out the exhaust) and make corrections for that.
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| BEatonNo1 |
Posted - Mar 27 2003 : 2:49:27 PM
In that case the best option would be to use a flow meter on the fuel line that way you would know the gallons per hour
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| YS |
Posted - Mar 27 2003 : 1:51:15 PM
I do not think so. Throttle seems to be connected to air intake, and fuel is delivered under computer control, so injector timing measurement is a key. I wonder if fuel pressure is stable or if it can change significantly.
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| BEatonNo1 |
Posted - Mar 27 2003 : 12:03:05 PM
The harder the wind blows the more throttle it takes to maintain your speed. so monitering the throttle and speed should be all you need
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| Aaron Cake |
Posted - Mar 27 2003 : 10:22:36 AM
Wind makes a huge difference. In my Insight (which has a MPG guage) I can see a difference of 4 or 5 MPG based on wind alone.
Probably just taking flow rate (use injector pulsewidth and fuel pressure) versus distance is good enough in this application.
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| BEatonNo1 |
Posted - Mar 26 2003 : 4:38:47 PM
I found that my truck gets the best mileage between 25 and 30 rpm
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