Friday, October 3, 2008

The Joys of Free Piston Engines

Free piston engines are combustion engines where the output is electrical, not mechanical energy. The system work by using induction to harvest the moving piston's power.

Technology Review has an article that explains the concept better than I can:
High-Efficiency Generators for Hybrid Vehicles

The article sites a number of advantages such as:

  • Dynamic compression control
  • Flexi fuel
  • Low friction

In addition it could probably be added that the electrical energy system allows an optimal combustion process by carefully adjusting the current in the induction cylinder. This again allows the system to operate efficiently in different load conditions. A mechanical system have a quite fixed acceleration cycle for the piston, whereas the free piston (as it's name suggest) can accelerate and move freely throughout it's faces. Another great feature is that multiple pistons can be arranged much more flexible than in a conventional engine. As they will always be run under ideal conditions, the number of cylinders can probably be reduced, but if more is desired they don't have to stand in a row. They could also be "programmed" to fire simultaniously, in opposite directions - to counter vibrations.

This is interesting in a number of eras. Most notably for hybrid vehicles. However the concept of "electric drive" seems to be gaining popularity among engineers and can be seen in ship design as well as in the (far) future of air transport. I'm not sure how well the free piston compares with a gas turbine on power and weight but it's interesting anyway.

Another usage for the concept could be as suspension in cars. High end cars today typically employ some means of adjusting the shock absorber's physical characteristics. This must be much simpler to do with a an electrical system. The piston would be replaced with the rod from the wheel and the cylinder would be mounted on the car. If the car happens to be a hybrid vehicle as well, it could possibly utilize the some of the moving electrons when traveling on bumpy roads...

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