Wednesday, August 25, 2010

Chevy Volt and the Future of Cars

The Chevy Volt will probably be a flop, but the technology behind it is the future of cars.

A little primer on cars and how they're powered:

Standard car w/recip engine: Reciprocating engine as the prime mover mechanically coupled to the wheels with the engine using either the Otto cycle (gasoline) or Diesel cycle. Toyoto Avalon is an example of this type of car.

Hybrid car w/recip engine:

Gas Electric Parallel Hybrid w/ recip engine: has a battery connected to an electric motor which is coupled to a transmission and then to the wheels and an independent reciprocating engine coupled to a transmission and then to the wheels. The transmission is necessary for torque conversion for the reciprocating engine. There is weight saving from a smaller engine, hower the engine is underpowered and still requires transmission and weight is added from the battery. Toyota Prius is an example of this type of car.

Gas Electric Series Hybrid w/ recip engine: has a battery connected to an electric motor connected to the wheels, with no transmission needed for torque control (simply controlled electronically and electrically), and the prime mover is a reciprocating engine attached to an electric generator which hooks into both the battery and to the electric motor. The benefit here is that the IC engine can operate without discharging the battery. Chevy Volt is an example of this type of car.

Before I get to the future of cars, we need to go over prime mover.

The two main prime movers for transportation are reciprocating engines and turbines. Reciprocating engines are used primarily in cars, trains, cargo ships while turbines are used in planes and warships. Standard turbines have much higher power to weight ratios and similar efficiencies to standard reciprocating engines, however, they have very poor partial load performance whereas reciprocating engiens have relatively constant performance and turbines also have extremely poor torque. They also have to spin up (anyone with a turbocharger has a turbine in their car and knows exactly what I'm talking about) with sufficient mass flowing throught them to provide power, but once that happens their power output increases dramatically (e.g. turbo lag).

The future:
Gas Electric Series Hybrid w/ microturbine: has a battery connected to an electric motor connected to the wheels, with no transmission needed for torque control (simply controlled electronically and electrically), and the prime mover is a microturbine attached to an electric generator which hooks into both the battery and to the electric motor.

The torque issue is solved by the electric motor and the microturbine spooling issue is solved through the battery. At the car start: while the microturbine is spinning up the battery is providing the power to the electric drive. Once the car is started either the battery or the microturbine provides the power to drive the electric motor. For in-city stop-and-go driving the car could be set to use the battery or the battery and microturbine. For highway driving the car would use the microturbine. No heavy transmission needed. No heavy reciprocating engine needed (power to weight ratio is 5x that of a reciprocating engine).

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