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Expansion ratio of burned Fuel

  1. Jul 23, 2009 #1
    When a fuel/air mixture burns it expands. How much does it expand? Is there a formula or known volume increase?

    After a fuel burns it contracts. How much does it contract? Is there a formula or known volume that it contracts?

    Gasoline/Air mixture ratio is 15 to 1 for jet engines.

    A fuel is injected into a pulse jet combustion chamber and mixes with the air in the chamber at atmosphereic pressure 14.7 psi. The fuel ignites and burns and the expanding gas goes out the exhaust tail pipe. The burned fuel then produces a vacuum that sucks fresh air in through the intake valves that mixes with more fuel and it ignites and burns. The cycle repeats over and over. The combustion chamber is typically 2 times the diameter of the exhaust pipe. A 2" diameter exhaust pipe will have a 4" diameter combustion chamber. The cross sectional area of the combustion chamber is 3 times larger than the exhaust pipe. Pretty simple so far but now we need to calculate the volume of the combustion chamber???
    Last edited: Jul 23, 2009
  2. jcsd
  3. Jul 23, 2009 #2


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    Depends on the temperature at which the given fuel burns at the particular pressure in the cylinder. You can get a fairly accurate result by applying the ideal gas law, PV = nRT.
  4. Jul 23, 2009 #3

    Ranger Mike

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    don't know about jets but
    do not omit the fact that in the IC engine, once the fuel/ air mix is lit, it begins to do work by moving the piston. this expands the combustion chamber volume, drops temperature, reduces initial ignition pressure immediately.
  5. Jul 23, 2009 #4


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    ...and the volume to which the gas expands is, by definition, dictated by the engine design.
  6. Jul 23, 2009 #5

    Ranger Mike

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    brewnog..you are a valuable asset on this forumn
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