Fuel consumption display

[sirvo]

The consumption meter won't work as you have it described. I've tried it ('98 Nissan and '01 Dodge). The numbers end up out in the weeds.

TPI, multiport, (sequential fire, batch fire, doesn't matter) etc. fuel injection systems have a fuel pressure regulator referenced to manifold vacuum which means the rail pressure changes as the manifold pressure changes. Now, yes, this does mean that the pressure across the tip of the injector remains relatively constant, but it's not nearly as precise as a person would think.
Then you have to throw in the fact that most autos don't spend all of their time in perfect closed loop. Hit the gas, and you're burning roughly a 12-13 to 1 Air/Fuel ratio, get off the gas for a stop sign and you might only be burning 20 to 1, idling will tend to be on the rich side of stroich (14.7:1).

BUT! Don't let all of that negativity stop you! It's a fun project to play around with. And as I learned, you get to play with figuring out how to hook up to various inputs, condition the noise out of them, and do math in the relatively restrictive environment of a PIC. Even if the fuel meter part doesn't work so well, you can log your speed, rpms, pulse width, etc. and look at it later to see what you were doing. Just don't forget to keep your eyes on the road while you're testing the system, and not on the LCD to see what's going on!

JDG

Hello you All..

Skimask, are you there?

I built my car fuel consumption and something is not going well. It works like this:
The software does the measuring of the injector's on_time and also the distance run. TMR1 ticks every 100us and the injector's on_time keeps being added up by 1 at each tick in a WORD variable. The distance is also being added up in a word variable. (my car 'distance' sensor ticks 8,31 pulses per meter). The software keeps counting both ticks and on every kilometer (8310 pulses) it does the math and calculates the km/L by a running average of 10km.

So, the thing is. When driving at a low rate of speed it shows a very low km/L. When driving at high speed, it shows a very high km/L. For example: Driving in the city, it shows 5~7 km/L (it should show 7~9 km/L) and driving at the roadway, it shows 13~20 km/L (it should show 9~11km/L).

I read what you wrote about pressure's difference and stuffs like this. But, what I can not understand is: Isn't the injector's on_time proportional to its output? Do you know why this is happening?

It would be great any tips now..

Thanks in advance!

Regards.

[Archangel]

Isn't the injector's on_time proportional to its output?

Hi Sirvio,
The short answer is, likely not. The OEM computer in your car keeps adjusting injector on time based upon sensor input from the oxegen sensor in the exhaust, the throttle position sensor, the mass airflow sensor, if equipped, a MAP sensor, if no MAF sensor, temp sensors in the air and water, a BARO sensor, a data lookup table, Knock sensor, and probably the engineer's astrologer too. So there is no design requirement for the injectors output to be linear, in fact most computers have what they refer to as BLOCK LEARN, which is most likely flash or eeprom memory to keep track of fuel curves so as to alter the computer's calibration around your driving habits. I notice my old exCHP car sure runs better after the wife drives it for a while.

[sirvo]

Hi Sirvio,
The short answer is, likely not. The OEM computer in your car keeps adjusting injector on time based upon sensor input from the oxegen sensor in the exhaust, the throttle position sensor, the mass airflow sensor, if equipped, a MAP sensor, if no MAF sensor, temp sensors in the air and water, a BARO sensor, a data lookup table, Knock sensor, and probably the engineer's astrologer too. So there is no design requirement for the injectors output to be linear, in fact most computers have what they refer to as BLOCK LEARN, which is most likely flash or eeprom memory to keep track of fuel curves so as to alter the computer's calibration around your driving habits. I notice my old exCHP car sure runs better after the wife drives it for a while.

Hi Joe.. thanks for answering..
I agree with you but, let me ask you in another point of view. Lets think that the pressure on the gas line is constant (~3bar). The same way, lets think that if you let 1 injector 1s on, it will output 15ml, it means, the injector outputs 15ml/s. If I can measure the injector on time, It does not matter the fuel curves and engine maps, only the on time. Isn't that write? In other words, the sensors, maps, curves, engineer's astrologer (heheh) and so... only change the on time, but not the 15ml/s. Could you get it?

I did not mean to be rude or anything. Don't get me wrong please! I'm not english neither north american so sometimes my english does not help me to be clear.

Thanks a lot..!

Regards..

[Acetronics2]

Hi, Servo

You're right, fuel pressure keeps steady ... so, just measure how long your injectors are open, make a "real life" calibration ( how much fuel vs total injectors "bleeding" time ) and that's it ...

It's very simple ... just look at fluid mechanics laws for quasi-ideal liquids, and you'll see volume is a function of time and pressure @ 99.99% !!!

so, you just have to measure the pulses driving your injectors ... just 5 wires to wire !!!

Alain

[calzaman]

I am trying to calculate fuel consumption as a function of RPM. So for a given rpm and an engine size I can fetch the approximate fuel consumtion (liters per minute). Does any one know of an equation that can achieve that.

Also your equation which you have used (Fuel consumption (liters/100km) = (RPM * injector open time * injector flow) / speed * 100000) is quite good and it is working for me. However, do you have a reference for where you got the equation for. Also is the 8 ms a standard for flow open time for all engines.

Thank you kindly for your help

[Tobias]

You will be closer at guessing the next lottery numbers than using just RPM to measure fuel consumption. You need to know BSFC and AF ratio as a starting point.

[rshaver]

Just a thought but if you can get 2 fuel flow sensors you can get a pretty accurate reading. You have to get the flow in and out of the system and then just do a little subtraction. Can't remember where I saw it. But someone did this and it worked over all driving conditions. If you have a flow of 5 gallons per hour(GPH) in and 3GPH out then you are using 2GPH. Or what ever measure you use. You get the in and out because the injection system has a return line to the tank for the fuel not used. Only problem with this is trying to find the flow sensor. I have been trying to find just one for my old truck and they seem to be non-existent now. They used to be every where.