Turbo-Electric Hybrids

Wednesday, July 14th, 2010

A series-hybrid, or Range-Extended Electric Vehicle (REEV), like the Chevy Volt, uses electric motors — which have a high power-to-weight ratio and provide adequate torque over a wide range of speeds — to spin the wheels.

The hybrid’s internal combustion engine isn’t connected to the wheels by a heavy transmission. It simply runs a generator and then sends the generated electricity to the electric motors, which turn the wheels. The only transmission is electric.

This isn’t a new idea. So-called Diesel locomotives are actually Diesel-electric locomotives. Eliminating the mechanical transmission is especially helpful when you have four or more axles, and you need dozens of different gears to keep the engine within its power band.

What a modern hybrid adds is significant battery storage between the generator and the motors, so the engine can always run at its optimal speed, either storing extra energy in the batteries or drawing some off, as demand fluctuates. But even this isn’t new; pre-nuclear diesel submarines used their engines to charge batteries while on the surface and ran all-electric when submerged without air.

Of course, once you’ve switched to electric motors, you’re no longer tied to any one kind of generator, and you may decide to switch away from a conventional, reciprocating, piston engine to a gas turbine engine, which has its own strengths and weaknesses:

Advantages of gas turbine engines

  • Very high power-to-weight ratio, compared to reciprocating engines;
  • Smaller than most reciprocating engines of the same power rating.
  • Moves in one direction only, with far less vibration than a reciprocating engine.
  • Fewer moving parts than reciprocating engines.
  • Low operating pressures.
  • High operation speeds.
  • Low lubricating oil cost and consumption.

Disadvantages of gas turbine engines

  • Cost
  • Less efficient than reciprocating engines at idle
  • Longer startup than reciprocating engines
  • Less responsive to changes in power demand compared to reciprocating engines

Those disadvantages are significant in a passenger road vehicle — as Chrysler found out — until you disconnect the turbine from the wheels and switch to a turbo-electric hybrid, with batteries to buffer power demand.

GM apparently did just that, if only briefly, in 1999, with an experimental version of its EV1, which used a 100-kg (220-lb) gas turbine to deliver 40 kW of electrical power, enough to achieve speeds up to 80 mph (128.8 km/h) and to return the car’s 44 NiMH cells to a 50% charge level:

A fuel tank capacity of 6.5 US gal (24.6 L; 5.4 imp gal) and fuel economy of 60 mpg-US (3.9 L/100 km; 72 mpg-imp) to 100 mpg-US (2.4 L/100 km; 120 mpg-imp) in hybrid mode, depending on the driving conditions, allowed for a highway range of more than 390 miles (627.6 km). The car accelerated to 0-60 mph (96.6 km/h) in 9 seconds.

Now Capstone Turbine Corporation is putting its MicroTurbines in hybrid electric buses, trolleys, and supercars:

The CMT-380 plug-in hybrid is powered by lithium-polymer batteries that can be charged at home or at public recharging stations. The car can operate on battery power alone for up to 80 miles. Then, when batteries reach a predetermined state of discharge, the Capstone C30 microturbine starts up to recharge the batteries and extend driving range up to 500 miles. The C-30 provides 30 kW of electric power and can run on a variety of fuels including natural gas, landfill gas, biodiesel, diesel, kerosene, and propane.

Interestingly, the CMT-380 was developed by Richard Hilleman, Chief Creative Director at video game producer Electronic Arts, with support from Capstone Turbine. The series hybrid system is installed in a Factory Five Racing GTM body and chassis that’s normally powered by a Chevy Corvette V-8. The result is a supercar with a 0-60 mph acceleration time of 3.9 seconds and 150 mph top speed.

The CMT-380′s microturbine features an electric generator and turbine components mounted on a single shaft supported by air bearings. Capstone microturbines operate at extremely high speeds of up to 96,000 rpm, resulting in a very high power-to-weight ratio. No liquids are needed to lubricate or cool the microturbine so little maintenance is required.

While this microturbine hybrid was developed primarily to showcase the potential of microturbines in hybrid range extender applications, substantial interest has prompted the company to plan for the production of a limited number of these high-tech supercars for discerning customers. No price has been identified.

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