Argonne Labs is a national resource, part of the Department of Energy,
that conducts well documented engineering studies including the
So here are the datafiles of interest:
|61504034||SSS 0-80-0 0% Grade||6.24|| || |
|61504059||UDDS #1, Ph 1+2||7.43||45||0.165|
|61504060||UDDS #2, Ph 1+2||7.48||51.6||0.145|
|61504061||UDDS #3, Ph 1+2||7.48||50.7||0.147|
|61504062||Hwyx2 with Coastdowns Ph 2||10.24||50.4||0.203|
|61504063||US06x2, Ph 3+4||8.03||36.4||0.221|
For vehicle efficiency, test 61504034, as the speed sweeps from 0-80-0 mph:
The dyno merics provides the kW as a function of speed.
So we can map the miles per kWh, exactly the same metric reported
by the BMW i3-REx.
This chart was used to adjust earlier benchmarks and has
been confirmed in the field.
To looks at the range extender (REx) efficiency, I used 61504062, a highway test
with the longest distance:
The available REx power is a function of oil temperature:
The REx initally runs at a reduced power level for ~250 seconds (just under 5 minutes.)
Once the engine oil temperature reaches 94C, it can output full power, ~20.5kW.
This suggests the car should be operated in reduced speed, enough to keep the REx on, ~6kW but
not so high as to have the REx turn off.
- 6kW @40mph under 92C
- 16kW @62mph under 94C
- 20kW @70mph over 94C
A typical EPA test consists of two phases: warm-up and for the numbers.
So the 600 seconds covers the warm-up and after a 100 second pause,
it is repeated starting at seconds 700.
The data suggests there are finite number of power bands.
The "W/cc" is the watts divided by the fuel cc/second, a measure of power
per unit of fuel.
But these are not standard metrics such as Brake Specific Fuel Consumption (BSFC).
So plotting the BSFC over the test duration gives:
Converting the "cc/sec" to "g/sec" and the power gives
gives BSFC grams per second, divided by the Watts.
So scatter plotting the power versus BSFC, we get:
So we see four distinct power groups:
What we can do is identify the flat-land, speed that requires 17kW. Using
the earlier 0-80-0 metrics:
- ~4kW - the lowest power, close to an idle
- ~6kW - a low power band
- ~17kW - a high power band with peak efficiency, least fuel per kWh
- ~20.5kW - the highest power band and least efficient
- 13,554W @60 mph (0-80-0 test)
- 16,607W @64 mph (roll-down model)
- 17,849W @66 mph (roll-down model)
- 19,263W @70 mph (0-80-0 test)
Now these are the measured electrical output which is typically
96% of the mechanical energy.
This suggests the actual BSFC is about 5% higher.
Regardless, this is in the 30% thermoldynamic efficiency range
suggesting there may be more, low-hanging fruit.
BMW has already announced going to 0-20W engine oil.
But cooled Exhaust Gas Recirculation (EGR) is an obvious trick and one
I might try.
My first Prius modification was a 1 kW inverter to provide emergency
power which we used every year since 2005. So I started research how to do the same
modification with our 2014 BMW i3-REx:
- 2500W 12V inverter - per the training PDF
- ~500W overhead - worst-case, 12V overhead
- keeping car in READY - ordinarily leaving the driver seat turns the car off. But it the seat belt is buckled behind the driver, the car remains in READY.
Looking at the REx output, it looks good except for the 440W vehicle overhead.
Assuming 1kW of 120VAC output and the vehicle overhead, we're looking at generating
1.4kW. Assuming we're looking at 10kWh/gal, 1.0kW / 1.4kW ~= 7.1 kWh/gal. To keep things in perspective, this is the 2003 Prius performance: