How is SAE J1939 fuel consumption calculated?

[pietro82]

Hi all,

I am analysing fuel consumption data got from the CAN-Bus and available into SAE J1939 database. Does anyone have any reference material about its calculation? I have not found that much. The most detailed document I have found is the following, where it is stated: "Fuel rate is based on Command-Fuel-Quantity. This is based on the Injector-Command to Fuel-Quantity-Calibration curve. Verified with fuel-rail-pressure and fly-wheel feedback.". However, I do not know what this means.

thanks

 

[BrianPetersen]

Explain to the rest of us, "the big picture" of what you are attempting to do.

In modern vehicles ... Fuel injectors have very well defined flow characteristics in response to the pressure across them. They are a known pressure drop across an orifice of known size with a fluid of known properties and under known conditions, and the uncertainty in each of these is quite small. The injectors have an "open" time which is related to the injector pulse duration. If your car has instantaneous and average fuel consumption displays (mine does), this is how it is doing it.

Which piece of this don't you understand?

 

[pietro82]

Thanks Brian for your reply. That is what I was thinking about. Since, I read special terms, I thought something more detailed could be found in literature. Moreover, what does "Verified with fuel-rail-pressure and fly-wheel feedback" mean? Speaking about the big picture, I am analysing real-world data of the calculated fuel consumption available in SAE J1939 and I have some concerns about the data precision. I have found a paper where a vehicle was tested and the data precision was farly high (around 5%). Is there any significan differences among the differente vehicle manufacturers?

 

[CWB1]

It’s saying that the fuel trim curve is confirmed within the ecu by a combination of rail pressure (flow) vs torque.

Every manufacturer is going to be very similarly accurate, the logic and electronics are all very similar.

 

[BrianPetersen]

I am thinking that "fly-wheel feedback" is a bizarre way of saying engine RPM. Every fuel injected engine has a crank position sensor, since knowing the crank position is fundamental to timing the injection and ignition events.

The flow through the injector is dependent on an injector-specific flow characteristic and on fuel rail pressure (and, where applicable, intake manifold pressure, since in most applications the total pressure across the orifice - i.e. the injector - is the rail pressure minus the intake manifold pressure).

The rail pressure has an influence on how fast the injector responds to opening and closing requests - there's a correction to the actual injector pulse duration to reflect the effective actual injector nozzle open time.

The above gives the fuel injection quantity in milligrams per stroke. (In reality, the calculation is the other way around. The electronics know how much injection quantity they want based on driver requested torque and various air flow and temperature measurements and calculations, and they back out what the pulse duration needs to be in order to achieve that.)

Number of strokes per unit time, which is engine RPM x number of cylinders, x quantity per stroke ... is the fuel flow rate.

Fuel consumption measurements on board the vehicle should be within a few percent accurate. They're affected by all the same things that affect your speedo/odo (notably actual tire size) in addition to minor inaccuracies in all of the various measurements being made.

 

[TugboatEng]

It's even easier with diesel engine electronic unit injectors. It's a positive displacement pump so pressure isn't even measured. The number of degrees of camshaft rotation while the injector is energized is the amount of fuel injected. It varies slightly so injectors get bench tested and assigned a trim code prior to installation.