It boils down to break even points.... and how much you are willing to give up for the convience for those rare trips it might be useful.
That Generator costs will weigh ~100 kg... and will only provide ~6.2 kW at a continuous rate.
This generator is using ~100 kg of weight... with LiFePO4 giving ~100Wh/kg ... in order to put this generator on board you are giving up 10kWh of battery capacity worth of weight.
If that 10 kwh you gave up would have allowed you to just use the EV and no gasoline ... you would have been better off / more ahead if you had just used the 10 Kwh of battery instead.
This generator will not provide for the peak power surges a EV will need during acceleration and such... so an additional battery pack would still be needed for those peak power times.
Generators like this could be used to extend the range of an EV ... how much depends on the efficiency of the EV and the average speed of travel ... even a 6 mile / kwh EV which is a pretty good number to reach ... could only travel at ~35 MPH under the power of this generator... The Volt is looking to use a ~50kW generator to be able to provide enough juice... which still does not handle the peak loads they are expecting of over ~110 kW... so an additional ~16 or so kWh battery is also included to handle the peak power needs / loads.
Range extention gasoline powered generators would be better used ( in my opinion ) as built into an easily detachable trailer... that way the extra 100+ kg of weight can be left at home on those trips that the extra range is not needed... as in ~80% or more of ~80% of the trips ~80% of the population makes.
The attraction to having it built in as GM is doing with the Volt is just for convience ... they are giving up allot ... in order to have that conveince ... for a small % of trips ... but it might work to sell it to the public.
Hopefully once the public starts to buy some of these REEVs & some of the PHEV like the Volt and such ... hopefully people will begin to see a bit of the reality of how rarely they ever use the extended range feature ... hopefully it will open the eyes of the public to see that it isn't needed... and then like pulling other trailers for boats and things ... people will leave the range extention generator at home like they leave boats and such ... when they won't be needed them... and just drive normally as a EV for the majority of thier trips.
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As for the efficiency ... a generator can be more efficient ... but it might not be ... it will depend on exactly how it was built... that particular generator is not particularly efficient.
For that Generator to use ~1 Gallon of gasoline in ~1 Hour .. and put out ~6.2 kWh in that ~1 Hour ... that generator is converting the ~36.6kwh of energy that 1 gallon of gasoline only at ~17% efficiency... even if your electric motor is ~90% efficient that means you are only ~15% efficient from gas tank to wheel... Modern efficient gasoline engines are allot more efficient than this... it isn't that hard to get a modern gasoline engine to get closer to ~30% efficient.
Even a modern ~30% efficient ICE attached to only a ~80% efficient generator / alternator ... would still yield ... ~24% efficiency gasoline to electricty ... which is still significnatly better than this example generator.
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I seriously doubt that generator would pass emissions tests ...