How do I calculate the amount of fuel injected (both gasoline and diesel cars). I want an instantaneous value. http://www.rceng.com/technical.aspx Websites like these ask for 'Desired Crankshaft Power' which is not the actual power from the car, but a desired value by the user.


You can do this through the OBD-II protocol (SAE J1979).:

| Mode | PID | Description       |
|  01  |  5E | Engine fuel rate  |  ( liters/hour )
|  01  |  43 | Engine RPM        |

You can use the RPM to figure out how much fuel is injected per engine cycle as well.


If you know what the fuel pressure is at the injector, what size of injector you have (flow rate), and you pull out the pulse width (how long the injector stays on), you can figure out how much fuel is being injected each and every time it fires. You should be able to pull out the pulse width at any given time from the ECU, if you pull the right PID. It's going to be hard to read (visually), though, because things will be firing at quite a quick rate, usually. Since every vehicle is going to be different, it would be hard pressed to give you one distinct answer.

The calculations from the page you have linked can be used to help you with that flow rate. Those calculations do not ask for "Desired Crankshaft Power", they expect you to know the approximate value of the engine combination you have built or what the dyno numbers are for a given engine. The idea is to see if the injector size you have (or are looking at) can support the amount of power which an engine produces.

  • This is the ground-up approach. I don't think there is a generic PID for injector pulsewidth, though I do know that BMW has one squirreled away somewhere on its older models
    – Zaid
    Jan 10 '16 at 12:01

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