| Author |
 Topic  |
|
YS
Nobel Prize Winner
USA
1132 Posts |
 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?
|
|
|
ecm
Apprentice
Canada
89 Posts |
Posted - Mar 25 2003 : 7:28:28 PM
|
a simple vacuum gauge will help you know when the engine is under load by indicating a low vacuum and when the vacuum is high a normal load. when the vacuum is high you are driving your car at a normal rate wich means you are saving on gas and when the vacuum is low you are using a lot of gas. so if you keep a steady speed or vacuum you would get a good consumption of fuel and save on the cost fuel and go further.
to do or not to do |
 |
|
|
BEatonNo1
Nobel Prize Winner
USA
1133 Posts |
Posted - Mar 25 2003 : 8:24:22 PM
|
you could get a Gallons per minute meter and figure out your miles per gallon. there are many "computers" that calculate this and more for you. we used to get a magazine that carried things like this, I remember that they werent to expensive.
|
|
|
|
Aaron Cake
Administrator
Canada
6718 Posts |
Posted - Mar 26 2003 : 3:44:53 PM
|
A real "MPG" guage is very difficult to make...You need to know injector flow rate, resistrance of the vehicle through the air, mixture, throttle position, speed, RPM, etc. and make calculations based on these factors. It's quite complicated.
My suggestion is to pick up an Air/Fuel guage and a vacuum guage. Drive to keep the A/F guage in stoich (closed loop) and the vacuum high. Also get a set of low-rolling-resistance tires, and ditch all the extra weight.
|
|
|
|
BEatonNo1
Nobel Prize Winner
USA
1133 Posts |
Posted - Mar 26 2003 : 3:57:21 PM
|
couldnt you just put a flow meter (gph) by the fuel pump and then use your speed to calculate it giving you gallons per mile
anyways it would still be easier to buy one of those computers there pretty cheap (if i can remember right) and they do more than just calculate your fuel consumption
P.S. Arron I have a feeling that some of the forces that you added in there are pretty nominal when you calculate it unless its a really windy day your resistance isnt going to affect much in the tenths place. also would it matter if your gas is richer or leaner if your still burning 1gph and travling 50 mph
|
|
|
|
YS
Nobel Prize Winner
USA
1132 Posts |
Posted - Mar 26 2003 : 3:59:34 PM
|
Well, extra weight is simple - I'll just vacuum inside.. :-)
New set of tires probably would not justify the fuel economy, though. It's like buying electric car to "keep environment clean and save on fuel" - but who knows what impact on environment was made with all these batteries manufacturing/disposal, not mentioning the price?!
I just thought that engine should have some RPM area with the lowest fuel consumption - and wanted to find it.
|
|
|
|
BEatonNo1
Nobel Prize Winner
USA
1133 Posts |
Posted - Mar 26 2003 : 4:38:47 PM
|
I found that my truck gets the best mileage between 25 and 30 rpm
|
|
|
|
Aaron Cake
Administrator
Canada
6718 Posts |
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.
|
|
|
|
BEatonNo1
Nobel Prize Winner
USA
1133 Posts |
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
|
|
|
|
YS
Nobel Prize Winner
USA
1132 Posts |
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.
|
|
|
|
BEatonNo1
Nobel Prize Winner
USA
1133 Posts |
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
|
|
|
|
Aaron Cake
Administrator
Canada
6718 Posts |
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.
|
|
|
|
BEatonNo1
Nobel Prize Winner
USA
1133 Posts |
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
|
|
|
|
Aaron Cake
Administrator
Canada
6718 Posts |
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.
|
|
|
|
n/a
DELETED (Inactive)
4 Posts |
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.
|
|
|
|
n/a
DELETED (Inactive)
56 Posts |
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.
|
|
|
|
Topic |
|
Fuel economy
