Blasphemy. While dual fuel systems are common in engineering, when it comes to Electric Vehicles, it is a religious matter. Even a generator for emergency use only is sinful.
Start with a 2wd Model 70s. Put a 75kW generator in the Frunk. It will be allowed 200 miles of gas and still be considered an EV. It would weigh the same or less than a 90D. It would have no geographic or climate limits at all. The cost would the same as a 90D. For most owners, it would never use gasoline. The generator is there just for emergencies and to pretend you love driving 2500 miles across the US instead of flying or taking a train.
I'm not sure why you'd need a 75kW generator - even 75 mph is only a little over 20 kW in the steady state, and any sane system design/user would bring the ICE on well before the charge is completely drained, which will handle acceleration and hill climbing power demands.
Of course, that's a lot of weight you'll be carrying around to not use 99% of the time - and a lot of maintenance requirements. Also a lot of manufacturing cost, with catalytic converters and a modern direct injected automotive type engine, too.
Of course, as other folks have mentioned, Tesla isn't going to do this. It runs contrary to their mission and message, and they'd rather spend their money making it possible to drive an EV anywhere instead. If Tesla made an fossil fuel range extender, lots of folks would take it as an admission their way can't work.
One of the more intriguing speculations I've seen has centered around a Frunk mounted metal-air "Primary" battery. At the currently demonstrated 1.3 kWh/kg, an Aluminum-Air battery weighing 60 kg/~130 pounds in the Frunk would double the range of a 90D (since it doesn't need anti-bricking protection.)
The theoretical limits are some 6 times that - a couple thousand miles of range extension from that same 60 kg. Of course, it's a one shot system - you're oxidizing the Aluminum plate into a slurry as you go, and it can't be recharged easily - you'd recycle the slurry and buy new plates and electrolyte instead. Some folks believe the cost of doing that could get down into the range of gas if a good recycling system is instituted (and of course, you'd only use it a bit at a time, to cover places you couldn't charge, stopping to recharge the car where you can.) Tesla apparently has taken some patents related to integrating a metal-air battery into a rechargeable EV...
If someone did decide to do a fossil fuel range extender, the right way would be with a ~30 kW microturbine. No coolant, no oil, no timing, no maintenance and it'll run on any liquid fuel (or any gaseous fuel - gotta choose one or the other for burner design I believe) - and cleanly enough that it doesn't need a catalyst. Much lighter and potentially somewhat smaller than a comparable ICE. A little expensive at the moment, mostly due to a lack of economies of scale. Propane is likely the ideal fuel for it - it stores at low pressures (mostly as a liquid, even,) won't degrade over time, and is widely available in the correct form, either in self contained bottles or at filling stations.
But I'm really thinking this approach isn't necessary now, short of a zombie apocalypse - unless you want to drive a Tesla to Alaska or something like that (which I still imagine will become feasible in just a few years if nothing derails Tesla.)
For the Zombie Apocalypse folks, you want a gasifier to feed the turbine, too - then you can put just about anything into the car (the gasifier takes solid (or possibly liquid) hydrocarbons and breaks them into much shorter chains at high temperatures in an oxygen free environment (generated by burning a small amount of the fuel) yielding a combustible gas mixture to feed the ICE/Turbine. Most folks who use them run them on chipped wood or pellets made from wood/grass/hay I think.)
Walter
