As more automakers are working on extended range electric vehicles, the issue of NVH is becoming even more of an issue than in existing parallel hybrid vehicles. With vehicles that operate predominantly in electric mode minimizing the impact of starting up the range extender or auxiliary power unit is critical. Companies like General Motors are working hard to make sure the range extenders operate as quietly and smoothly as possible, but small piston engines inherently have some degree of vibration.

Engineering services company FEV has been studying the problem and has decided to propose an unusual solution. FEV has built an experimental range extended electric version of the Fiat 500 that uses a small Wankel rotary engine as the APU. The Wankel has a number of potential advantages, including much higher power density than piston engines and virtually vibration free operation. The high specific power output means that a very small engine can be used that reduces weight and adds packaging flexibility. The downside is that Wankel's traditionally aren't very fuel efficient. However, FEV feels that most ER-EVs will be driven predominantly in EV mode and the limited use of the range extender will limit the effect of the lower engine efficiency. New Wankel engines can also be made more efficient through the adoption of technology like direct fuel injection. It remains to be seen if anyone will adopt a Wankel range extender in a production vehicle, but it's an interesting idea.

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