I've started calibrating our ZVW30 indicated versus actual mileage. The first part is to define the standard, the actual mileage, and then look at the dependent variable, the indicated mileage. For our standard:
- same pump - #6 at the Shell station on South Parkway in Huntsville AL
- same fill-up protocol - manually listening for the rising tone when the gas backs up into the filler tube. Pause at least 5 seconds and then top to 'one click' manually
- same fuel - Shell 87 octane, E10
- quantity - at least 1 gallon since pump reports to 1/1000th of a gallon
- temperature - not recorded
There needs to be a model for how mileage is calculated and assuming the distance measurements are valid, there are two methods for measuring fuel consumption: (1) injector timing and summing, or (2) mass flow stoichiometric operation.
Injector timing, as Ken@Japan pointed out, is subject to mechanical delay and linear output outside of these two. When the injector is first turned on, it takes a finite amount of time until fuel begins to flow, the delay. This means, as Ken@Japan pointed out, a fixed overhead has to be 'taken out' of the fuel measurement.
The second approach is to use mass flow at the 14.7 air to 1 unit of gas ratio. This works when the engine operates in the closed-loop, feedback system range. But this range does not start working until the O{2} sensors are operational, which typically takes the first 45 seconds in my NHW11. At high power settings, the ZVW30 engine begins using cooled, exhaust gas to moderate exhaust temperatures. In contrast, the NHW11 used a richer mixture to avoid burning out the catalytic converter. In this case, one would expect the error to change as a function of power used, something proportional to vehicle speed.
To figure out the calibration curve, we need to know which mechanism, injector timing or mass flow, is being used. I don't know but errors in the data by looking at average speed, gives an indication of which mechanism is being used. I'm still gathering data and only have four samples.
PRELIMINARY INJECTOR COUNTER
Assuming injector timing is used to calculate fuel consumed, the correction curve should be a line with possibly a fixed offset:
The "14.467" constant in the linear trend-line would correspond to a fixed overhead, the delay that Ken@Japan reported for the 1.5L engine. With just four records, I don't see a speed related factor, which suggests mass flow is not being used.
After I get enough samples, the trend-line values will become more and more accurate. I can then solve the inverse function and convert the indicated MPG to true MPG. But I will only be able to do this for my particular vehicle. Others will need to gather their own calibration data. The inverse function:
my_true_MPG = (indicated_MPG - 14.467) / 0.8376
my_true_MPG = (indicated_MPG - 14.467) * 1.194
So if I use this correction formula on my 'on flash' measurements:
60.8 MPG -> 55.32 MPG
132.6 / 55.32 -> 2.40 gallons residual fuel versus 2.18 gallons
This would add ~.22 gallons to the tank absolute capacity or nearly 12.3 gallons.
If we were to gather enough data points, we could do a population study. This would allow us to test different hypothesis of the source of the error. But that is beyond the scope of this study as I'm just using this, my raw data:
| pump MPG | indicated MPG | avg mph | gallons |
| 54.4 | 61.0 | 33 | 2.28 |
| 54.8 | 60.2 | 33 | 1.444 |
| 49.8 | 56.4 | 35 | 9.65 |
| 48.5 | 55.0 | 32 | 9.8 |
.
Impact of Calibrated MPG
Once I'm sure of the calibration formula, I'll update my earlier mph vs MPG chart to include both the indicated and calibrated MPG. I'll also have to correct the hill climb data, again showing both calibrated and indicated fuel consumption. This will serve the purpose of showing folks why their indicated mileage can be wrong as well as a resource for those who claim to have a true MPG reading, a reference curve to use for their data.
Bob Wilson