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B How to save petrol in a Petrol Engine

  1. Aug 1, 2018 #1
    Like Gas Engine can we use Petrol Engine working as Gas Engine?
    I mean instead injecting petrol directly into engine can we inject vapours of petrol
    into engine using vacuum pump? Can we save petrol in this way without using stroke oil?
     
  2. jcsd
  3. Aug 1, 2018 #2

    cjl

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    The fuel is vaporized before it is burned - that's why it's sprayed into the intake (or, on newer designs, into the cylinders) as a very fine mist.
     
  4. Aug 1, 2018 #3

    russ_watters

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    Staff: Mentor

    Your post is pretty incoherent, but are you saying you want to literally run a car on fumes? Have you calculated how fast you can get petrol to evaporate? How much power the vacuum pump and compressor takes? How much heat you have to absorb from the environment? My suspicion is this is a net loser because of the compressor/pump energy and loss of the liquid to gas expansion in the cylinder, but I'm not sure.

    Also, by "stroke oil", do you mean motor oil? I'm not sure what that has to do with anything.
     
  5. Aug 2, 2018 #4

    cjl

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    Russ - the fuel is vaporized prior to entering the cylinder in a lot of designs though, and even in direct injection engines, the injection occurs with the intake valve still open (except when running in stratified charge lean burn mode). The liquid to gas expansion doesn't help with cylinder pressure at all, though it is beneficial to help cool the intake charge.
     
  6. Aug 2, 2018 #5

    russ_watters

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    How is that accomplished? By means of a vacuum pump?
    Does the fuel that is injected as a liquid vaporize prior to the intake valve closing?
     
  7. Aug 2, 2018 #6

    cjl

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    Nope, just by spraying the (fairly volatile) fuel in a very fine mist in the intake or cylinder, where it will vaporize nearly instantly due to the very large surface area and hot environment.
    I would tend to assume so, but I don't actually know for sure.
     
  8. Aug 2, 2018 #7

    russ_watters

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    Ok, so I don't think that's what the OP was after.
    It seems I got the mechanism wrong. I was thinking a similar principle to water injection, but looking it up, I see that is also done before closing the inlet valve. The vaporization of the water (or gas) cools the inlet air, allowing more air into the engine. Two sides of the same coin (pressure ratio), but the improvement is in dropping the low end, not increasing the high end.
     
  9. Aug 2, 2018 #8

    cjl

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    It won't really reduce the low end pressure - what it will do is increase air density due to cooling, which lets more oxygen in and therefore allows greater power production (plus it helps if the engine is thermally constrained).
     
  10. Aug 2, 2018 #9
    There are several patents to Charles Pogue that were filed in the late 1920's through early 1930's for vapor carburetors. The Pogue carburetor is the basis for the mythical 100 MPG full size car. There have been consistent rumors from then through even today of this car being driven around until it was suppressed by your choice of the evil (car companies) (oil companies) (other villains). These rumors all ignore the fact that all patents are public information. You can read the patents by going to patents.google.com, and using search terms pogue carburetor.
     
  11. Aug 3, 2018 #10

    sophiecentaur

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    I seem to remember that the burning rate needs to be controlled and not explosive (high octane fuel is used for this reason) so is it really necessary to vaporise all the fuel at the same time? Fuel injection is more effective than a conventional carburettor so there will presumably be some optimum combustion time.
    The proportion of the total stroke that's taken up by combustion is quite small, I believe.
     
  12. Aug 3, 2018 #11

    cjl

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    Yes, you want the burning to occur along a single propagating flame front rather than occurring spontaneously throughout the cylinder, and you want it to not occur until the spark fires. That's a separate matter though - the fuel is all vaporized and the mixture in the cylinder is homogeneous before this ever happens. Fuel injection is more effective than a carburetor, but the reason isn't because of any difference in rate of vaporization or liquid fraction in the cylinder - it's more just about precisely controlling how much fuel makes it to each cylinder at any given time, and being able to control this across a wide range of operating conditions, RPM, and throttle settings. As for the combustion time, yes, it's generally assumed to be quite short, unlike a diesel engine where it takes a significant fraction of the power stroke.
     
  13. Aug 3, 2018 #12

    sophiecentaur

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    I just read this again. Even in a gas engine, there needs to be mixing of the gas and air.
     
    Last edited: Aug 3, 2018
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