Range extender efficiency – a study of a study

January 3, 2008

With the concept of series hybrids/range-extended EVs growing in popularity, I thought it might be interesting to take a look at a study that was put out a few years ago by San Dimas, CA-based AC Propulsion. AC Propulsion constructed a towable trailer with a range-extender for their tZero prototype car. This genset was based off of a small motorcycle engine – something GM has suggested they might use for some eFlex vehicles.

AC propulsion’s used a 500cc Kawasaki engine rated at 20kW output to maintain their tZero’s battery at around 80% SOC. Without any battery supplementation, the device could achieve between 32-38 mpg. Though an interesting experiment, I think that this study was flawed in a few ways – or at least not completely applicable to future efforts.



1) The “trailer” concept-

The range extender was carried within a separate trailer that added its own aerodynamic and rolling resistance. AC Prop themselves noted that changing the tires of the trailer increased efficiency by 2%. By contrast, a self-contained genset would probably reduce weight, aero, and friction resistance.

2) The engine-

AC Prop chose their motorcycle engine based on compactness and power density, rather than on strict thermodynamic efficiency. Their tank-> battery efficiency was around 22%. Considering that most petrol ICE’s top out at around 25% efficiency under the very best circumstances, that would indicate an alternator efficiency of around 88%, which is fairly reasonable.

0.25 X 0.88=0.22

If an ICE designed from the ground up to be a genset could be used, one would probably expect a base thermodynamic efficiency of around 35-40%. For example, Argonne National Laboratories found the Toy prius’s engine achieves around 37% at its most efficient point of operation. Diesels regularly achieve efficiencies up to 40%, and alternative external combustion engines such as the Cyclone are in this general range as well. Use of a such a device in conjunction with an optimized generator would probably see much better results in a test similar to what AC prop devised.

3) “Transmission” efficiency-

Generator and motor efficiency has come a long way in recent years – optimized permanent magnet generators achieve very high efficiency (90-95%). Compact traction motors such as that employed in the FCX clarity or Equinox Equinox fuel cell have almost no gear reduction at all apart from the the differential and CV joints. A dramatic example of efficiency is that used by the Nuna 3 solar racer, which achieves 98%.

By contrast the AC prop system – despite its sophisticated power electronics – seems to use a greater degree of gear reduction – e.g. even their recent eBox project necessitates the use of the Scion xB’s manual transmission components.

The ultimate goal for the range extender system is for the genset and traction motor to get the energy from the tank to the wheels with an efficiency comparable to that of a mechanical connection. That’s the main reason why series hybrids haven’t been used outside of diesel-electric trains or old submarines.

4) Speed-

AC Prop has noted that efficiency dropped if the ICE was heavily throttled – not surprising at all. Future gensets should be kept at maximum efficiency at all times, while using the battery to cope with greater power demands as well as regen.

In summary, I applaud AC Prop for their pioneering study, but the disappointing mileage of their genset trailer should in no way be regarded of a harbinger of problems to come.


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