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Ratio of air and fuel in SI

  1. Apr 26, 2010 #1
    what is the average air to fuel ratio in spark ignition engines?
  2. jcsd
  3. Apr 26, 2010 #2


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  4. Apr 26, 2010 #3
    It depends on the current condition.

    When an engine is cold/first being started up the temperature of the cylinders and the fuel is quite low thus fuel atomization is poor. To compensate for the low temp the computer must inject more fuel than necessary due to a lower percentage of the fuel being vaporized. So at start up - depending on temp - air fuel ratios can vary between roughly 7:1-9:1. During idle and cruise after the engine has warmed up the computer oscillates the air fuel ratio very rapidly (many times per second) between rich and lean with an average of 14.7:1. The computer oscillates the air fuel ratio at idle and cruise to ensure proper catalytic converter operation. At wide open throttle the computer enriches the air fuel mixture to combat detonation and produce max torque/horsepower, this ratio can vary between 9:1-13:1 depending on the compression ratio, detonation resistance of the combustion chamber, etc.

    This is describing Electronic fuel injection but the principles are very similar across other types of fuel metering systems.
  5. Apr 27, 2010 #4
    I think your numbers a possibly a bit out there. simply becuase a ratio of 7:1 is getting to the point where combustion won't occur. You will certainly be getting enormous clouds of smoke from the exhaust full of raw fuel.

    Typcially you get max power from an equivilance ratio of 1.1 - 1.2 . So thats approx 12.25:1 AFR. Anything below this you will start to hinder combustion on a naturally aspirated engine. On blown engines, you can drop the AFR below this, but the excess fuel simply acts as charge cooling.

    For cruising you can lean the engine slightly beyond stoichiometric.

    Out of curiosity, where did you get your figures for AFR from?
  6. Apr 27, 2010 #5
    I'm an auto enthusiast/gear head and those were rough figures that were pulled from the anal (I apologize I should have looked the figures up) area to give the OP an idea of what is required to run an engine under various conditions. You have to remember that at lower temps volatility of the fuel is lower so more fuel than necessary has to be injected just to create a burnable mixture (a lot in extreme temps). At temperatures of -10 degrees F and below 7:1 may not be entirely out of the question when you're trying to start an engine.
  7. May 1, 2010 #6

    jack action

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    The reason why the AFR is so rich when the engine is cold is not because the engine needs more fuel for the combustion, but because the fuel "sticks" to the manifold's walls (like water on a cold window), which means less fuel going to the combustion chamber. As the engine heats up, the "left behind" fuel evaporates and get mixed in the air flow again. That's why on older engines with carburetor, they often have a passage heated by exhaust gas under the intake manifold to accelerate the process while the engine warms up.
  8. May 1, 2010 #7
    In a fuel injected engine the fuel is sprayed directly through the intake valve so the fuel will barley touch the intake manifold. The reason that the AFR is so low when the engine is cold is because much of the fuel won't be vaporized at the low temperatures and thus won't be burnt.

    In a carburetored engine though, your quite right.
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