How Is Fuel Flow Measured?

[CaspersCruiser]

This is an academic question. I use the fuel monitoring feature of the Garmin chartplotter to keep track of how much fuel is burned and therefore how much fuel remains on my Yanmar 4BY2-180 powered R27. How is the fuel flow measured before it is displayed on the chartplotter?

On the airplanes I fly for a living, there is a transducer in the fuel line to each engine that measures the actual flow of fuel. Is the fuel flow on these small diesels a function of pressure? I doubt there is a transducer because, if there was, threads related to failure of those transducers would show up here.

I have no reason to doubt the validity of the fuel GPH display on my chartplotter. I’d just like to know how it is derived.

 

[NorthernFocus]

As I understand it gph is derived from the corrected mass flow rate that the ECU calculates to determine injection rate. Injection rate is then electronically controlled by how long the injector is held open. So the "measured" fuel flow is actually not a measurement but rather a calculated value. Barring problems with the injectors is should be pretty accurate.

 

[CaspersCruiser]

Thanks. I was certain the ECU was the source of the GPH. I just didn’t know how it calculated it. I have found the GPH and the total fuel burned values to be very accurate.

On fuel injected aircraft piston engines I have flown, fuel flow was displayed in the cockpit on an instrument that was actually a fuel pressure gauge calibrated to display in GPH. Some of those airplanes also had a fuel flow meter that had a transducer in the fuel line that measured the actual flow of fuel. They were usually pretty close in GPH value. The advantage of the fuel flow meter was it had a totalizer function that kept track of fuel consumed, much like the Garmin chart plotters.

 

[NorthernFocus]

On a common rail fuel system a meter in the supply line isn't a useful measurement due to fuel returned to the tank. I suppose they could put one in the return as well and subtract the return flow. The Mercruiser Smartdata control panel has a trip fuel totalizer. I find that actual vs indicated fuel burn runs between 0.75-0.85 depending on how much time we have run at low rpm. Error is greater with more time spent at low revs. For fuel management purposes the error is on the conservative side since indicated burn is greater than actual.