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Originally Posted by Isaac Zachary
I don't know if this is possible for gasoline vapor, but I read once somewhere on the all-knowing internet that propane (and natural gas) engines can run much leaner than liquid gasoline engines.
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I'm not so used to LPG as a motor fuel, since it's actually only allowed in my country for forklifts and other material-handling equipment not really meant to operate on public roads, but the first vehicle with automatic transmission I drove had been upfitted to also run on CNG besides gasoline. It's possible to run leaner on CNG, since its anti-knock properties are better even than ethanol. But with an excessively lean AFR, power goes down considerably.
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This also allows for kind of a diesel-like throttling where you can run super lean, keep the throttle valve wide open and adjust throttle by means of introducing more gas (gas as in propane).
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It's not so simple to get such a Diesel-like operation, as it remains relying on the spark ignition. It might be also dependent on which generation the CNG conversion kit is based, or if the engine is converted to dedicated-CNG which might enable some different approaches with a stepper motor and a so-called "mixer" instead of a carburettor.
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On another note, wouldn't the air also have to be very hot to keep the gasoline completely vaporized and dry?
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A hotter air intake would eventually require more gasoline to keep its charge cool enough to prevent knocking.
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Otherwise wouldn't some of the gasoline just condense back into small droplets in the intake charge?
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With a colder intake, a smaller amount of gasoline evaporates. Considering the modern engines fitted with EFI, they set a lower AFR exactly to keep the engine at its standard operating temperatures. Notice it in a rainy day, the fuel consumption usually goes down due to the colder air with more moisture on it