Which of the following calculations is the most accurate representation of mass air flow in a VE system?

[TnF]

Hello. I am building a curve to help me compare compressor maps and i am wondering which of the following is the most correct way to calculate the mass air flow.

Consider the following logged parameters at one point of time in WOT:

RPM: 6171

MGP: 168kPa (BAP: 100kPa->MAP: 268kPa)

Lambda 1: 11.17 AFR

Air per Cyl Estimated: 1.566g/cyl

Inst. Fuel Consumption: 2110.2cc/min

Fuel density: 0.7447g/cc

Differential Fuel Pressure: 297 kPa

Inj Duty Cycle: 53.4%

Batt Voltage: 14.56V

Fuel: 98RON petrol

IAT: 47oC

 

Calculation using "Air per Cyl Estimated":

Mass air flow rate (kg/min) = 1.566 * 4 (cylinders) * 6171 (RPM) / 2 (4 stroke) /1000 (kg) = 19.33kg/min = 42.61 lb/min

Calculation using "Inst. Fuel Consumption":

Mass air flow rate (kg/min) = 2110.2 * 0.7447 (fuel density) * 11.17 (AFR) / 1000 (kg) = 17.55kg/min = 38.7 lb/min

There is a 10% difference between the results here which is quite significant. 

Which would be the best way to get the most realistic value?

1st calculation is totally depended on the parameters "Air per Cyl Estimated" is derived from of which i don't know.

2nd calculation is depended on the density of the fuel which should be quite close and the parameters from where "Inst. Fuel Consumption" is derived from.

Note that i use Injector Dynamics 1050x of which i have fully modeled inside the map and i also run a fuel pressure sensor which allows for realtime differential fuel pressure calculation. So injector quantity calculation should be really close to actual.

[Adamw]

There is no real way of knowing.  I would probably average the two and use that for your calcs.  The lambda measurement device accuracy and transient effects are other considerations.

Just for your interest, log below showing measured and estimated air from our subaru which still has the factory maf in place.  This car is not particularly well tuned, I havent flow tested injectors or measured fuel pressure or anything, just guessed or went off assumptions, but I was surprised how well the modelled calc matches the maf.  The large error on overrun is just where I havent tuned the overrun row of the fuel table since it uses ORFC but apart from that the biggest error is the spike just coming off idle and I think the estimated actually looks more realistic than the measured in that area.   

[TnF]

Thanks a lot for the reply. I do understand all the other variables and for some there are ways to process the data in order to eliminate some types of errors.

However i am more interested finding out the actual calculation taking place for these 2 parameters so that a more educated choice can be made between those two. Maybe one of the software engineers can give some input here.

From what you show however (which is if i understood correctly an engine that is VE (speed-density) tuned but still happen to have the MAF sensor installed but it's not used in the load calculation) is that the "Air per Cyl Estimated" to be the closest to the realistic flowrate in my opinion, due to the very close similarity of the curve. If there was a systematic error present it would show here as an offset, but if there was a characteristic error the curves would show noticeable differences.

Which brings me to the point that i should generate the calculations for some period of time because only checking at one point is extremely likely to show big errors like the 10% i calculated. I knew about this, but i figured to ask first and just give it as an example.

Lastly, i've asked in the past about this but it would be nice if we could setup custom parameters with algebraic functions in the PClink software so these calculations can be done without having to export the data csv and do it in excel. Also you should allow us to make a custom gauge graph where we can put any parameter on the axes with a stretchable underlay image so that the compressor curve can be viewed in realtime. I know you put functions in G4x but i think these are functions for the ECU, this is something that can be done on the PC software side and should be easy to put as an update for both PClink versions. Please:)  

[TnF]

I think Inst. Fuel Consumption is the way to go. These are data pulled directly from a log file, where TP and AP are over 90%, essentially WOT. Air per Cyl Estimated looks to be directly related to the VE map, where's fuel consumption is based only on the duty cycle and dead time of the injectors which are known values. Ideally these should be identical but because of not perfect mapping there is quite the difference at the top end. Compressor map is of the exact turbo used a GT2871R 52 trim. This shows also that some overspeed must be happening.