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Gas (hog) Turbine
Cool idea, but a gas turbine has terrible fuel efficiency even for the big units. I worked for General Electric as a Field Engineer and installed and overhauled them in some nasty parts of the world. They are a quick solution to power needs. They can run on natural gas (best) or distillate oil, heating oil, even heavy fuel oil which cuts the hot gas path parts life severely. But a gas turbine peaking plant usually has a fuel storage tank of several million gallons....
When you make a turbine small like the APU on eBay, the thermal efficiency drops sharply. That unit probably has a 3 to 1 pressure ratio. Try running your Metro engine on 3:1 compression and see what that does to your fuel consumption. An APU is also not designed to run for long periods of time. Gas turbines work best at base loading and do not like severe load swings as encountered in a car. They are fantastic for helicopter service as a unit such as the Allison 250 can run at 318 rated HP for 100% of the time as long as you can afford to feed clean fuel to it. The turbine is about the same weight as a Geo Metro engine and trans.
The secret to making a gas turbine efficient is to reclaim the waste exhaust heat. Chrysler tried a regenerator but the fuel economy was still miserable in its turbine car project. GE builds combined cycle power plants that use 2 gas turbine generators exhausting into an unfired boiler that makes free steam to run a single steam turbine generator. Now thats efficient. Too complicated to be scaled down to car size, though.
The pic: Thats me putting the rotor into the lower half....
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