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Air fuel Mix Ambient psi vs. compressed energy ouput

  1. May 30, 2008 #1
    Internal combustion engines, Ram Jet, Turban engines etc. require that the air fuel mix be compressed. Is the compression of the air fuel mixture required to give a higher or more efficient energy output? Otherwise if the air fuel mix were at a static or ambient pressure vs. compressed would the energy output be different?
  2. jcsd
  3. May 30, 2008 #2


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    For piston engines higher compression is better (as long as the octane is high enough to avoid pre-detonation). I'm not sure about turbines, ram and scram jets. Compressing the air first increases the amount of oxygen available to mixs with the fuel in jet engines, so it's more efficient in terms of power versus size and/or weight to compress the air.

    For rocket engines, it probably doesn't matter much, for example, solid fuel rockets are pressurized by the thrust from the engine, but the rate of compression is very small. For liquid fuel rocket engine, the nozzle pressure just has to be greater than the pressure due to thrust from the rocket engine.
  4. May 30, 2008 #3


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    Pretty much, yes. The fuel has a specific amount of energy it can release on a unit mass or volume basis. Therefore, to get more energy, you need to burn more fuel. That more fuel requires a larger mass of air to keep the combustion near the optimal stoichometric air-fuel ratio. Not only that, but the thrust created by an engine is a result of the change in momentum of the air stream. The more air the engine pumps, the greater the momentum change that is possible.
  5. May 31, 2008 #4
    Thanks Jeff and Fred that helps me a lot. I am sure I will have more questions later. I am quite curious about sterling engines such as delta t vs. torque out. But I need to formulate a proper question. Thanks again
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