Saturday, February 13, 2010

Turbine Cars

Capstone Turbine is planning limited production of a plug-in hybrid, the CMT-380 that uses a small turbine engine to extend range. The car looks great, as it should because it's a basically just a Factory Five GTM with batteries, an electric motor, and a turbine powered generator to keep the batteries topped up on long trips.

My guess is that the weight of the batteries hurts handling and acceleration somewhat.However, I'm still really excited by this car because I'm persuaded that putting turbines in cars is a good thing.

Turbines have several advantages over piston-engines:

  1. They are smaller and lighter-weight for a given horsepower.
  2. They are much more efficient - more of the energy of the fuel can be turned into usable power.
  3. They are much cleaner burning, producing less NOx because they can run a leaner (more oxygen rich) burn.
  4. They are naturally flex-fuel. Though turbines are usually optimized for one particular fuel they are generally much less fuel sensitive than piston-engines, making flex-fuel designs easy to implement.
  5. They are much more reliable than piston-engines.
For essentially these reasons, turbines are used to power some special purpose ground vehicles, like the M1 Abrams.

Here's another turbine diagram:

So if turbines are so great, why aren't they already being used in cars? They have two serious limitations that have made them impractical until now. The first is that turbines are not good at changing speed. They lose much of their efficiency advantage over piston engines if they are forced frequently change RPM. Also, they don't change speed very rapidly - after all they have a lot of rotating mass that doesn't want to slow down or speed up. The second limitation is that they are high cost.

The brilliant thing about using a turbine in a plug-in hybrid is that it solves the problem of changing RPM. The CMT-380 uses the turbine to spin a generator that charges the batteries when they get depleted. That way, the turbine can spin along at the one RPM where it is most efficient, regardless of how fast the car is moving.

The problem of cost can also be overcome but it depends on economies of scale, and possibly the regulatory environment. For example, tighter emissions requirements may give turbines an advantage over piston-engines. It's certainly the case that if turbines were widely adopted for automotive use that their price would come down somewhat.


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