# Fuel consumption

Is there a basic formula for figuring how much fuel used while at idle? I realize how many variables are involved, ie. RPM,in gear or not, etc.

My guess is that it’s too make dependent and would have to be evaluated on a car by car basis like road MPG. Diesels are in a class by themselves for example…some gasoline cars have cylinder shut offs if that makes a difference…etc.

Figuring out how much fuel is used while idling is not really the point. Minimizing the amount of fuel used while idling is. Reduce idling time by shutting off the engine whenever it is safe and practical to do so.

A good scan tool would show you injector pulsewidth (and sometimes duty cycle %), which could be combined with injector size and RPM to give you fuel consumption…

Fill the tank and let it idle until it is about half empty or more than refill. You will get a good idea of idle usage, but then you will have put a lot of hard wear on you car by idling that much.

The only rule (a generalization really) is that with perhaps the exception of variable cylinder systems (like the 4/6/8 engines) the bigger the motor, the more fuel it will use.

Why wonder about it? You can’t adjust it anyway and idling isn’t where most of the fuel goes.

Not knowing what your motivations are for wanting to calculate minimum required fuel for and an engine, I?ll summarize the most basic method below: The best thing to do is set up a MathCADD sheet so you can play around with the variables. The below method is similar to how most ECM programs determining required fuel on a fundamental level. I also assume that you have some math/engineering background.

ReqFuel(ms) = (CID*(air density)*3600000) / (Ncyl * Ninj * AFR inj * 1728 )

from this you determine injector pulse width as follows:

injpw (ms) = ReqFuel* (MAP/14.7) VEinj_open (it’s a little different if you use a MAF sensor instead of MAP sensor)

This gives you the injector pulse width per cylinder firing event, from there it is pretty straight forward to calculate fueling rate in GPH

Variables are as follows:
CID - engine displacement (in3)
Ncyl - number of cylinders
AFR - air fuel ratio (use 14.7)
inj - injector flow rating (lb/hr)
Ninj - injections per engine cycle
MAP - manifold absolute pressure (psi)
VE - volumetric efficiency (very low at idle)
inj_open - injector open time (ms) this is how long it takes the injector to open