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Stoichiometric air/fuel ratio of gasoline vapor

  1. Jul 6, 2016 #1
    Hello, Can someone tell me the optimum stoichiometric air/fuel ratio of gasoline VAPOR? I know that with liquid gasoline the optimum stoichiometric air/fuel ratio is 14.7 parts ambient air to 1 part gasoline. Thank you in advance for your help.
     
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  3. Jul 7, 2016 #2

    SteamKing

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    Why do you think the ratio for gasoline vapor would be any different for liquid gasoline? It's all the same stuff.
     
  4. Jul 7, 2016 #3
    Thank you for responding.The gasoline is heated and turned from a liquid into a fully gaseous state. The commonly accepted stoichiometric atmospheric air to atomized liquid gasoline ratio is 14.7 to 1. Typically, converting liquid gasoline to a gaseous state leans out the air fuel mixture in an internal combustion engine.
    I am conferring with you and the Physics Forums to determine if there would be a change in the stoichiometric ratio if liquid gasoline is converted into Gasoline vapor. Kind regards, Michael Vannozzi
     
  5. Jul 7, 2016 #4

    SteamKing

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    You can't burn liquid gasoline in an engine. The carburetor/fuel injection system atomizes the liquid gasoline into vapor so that it can mix easily with the combustion air.

    I don't know what you mean by "Typically, converting liquid gasoline to a gaseous state leans out the air fuel mixture in an IC engine." This can only happen if some of the gasoline vapor is lost to the atmosphere before it enters the engine, or if your engine is running with some percentage of excess air, above the amount required for combustion.

    https://en.wikipedia.org/wiki/Air–fuel_ratio
     
  6. Jul 7, 2016 #5
    Atomization of a liquid means that liquid is segmented into tiny droplets, but it is still in liquid form. You are correct in your statement that liquid gasoline doesn't burn. Only that small percentage of the atomized gasoline that comes into contact with the heated surfaces of the top of the hot piston, the hot bottom of the intake manifold tract, and the engineered overlap of the camshaft that allows the exhaust valve to be open for a split second at the end of the exhaust stroke while simultaneously, the Intake valve starts to open, allowing intimate contact between a residue of hot exhaust gases and the intake charge of atmospheric air and an atomized gasoline mixture. This system converts only a small portion,(20-30%), of the atomized gasoline into a vapor or true gaseous state, which in turn is consumed by the internal combustion process of the engine. My question is: are you saying conclusively that the stoichiometric ratio of atmospheric air to gasoline ratio does not change when liquid gasoline is converted from a liquid into a gas,(vapor)?
     
  7. Jul 7, 2016 #6

    SteamKing

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    Not unless you have a leak somewhere in your intake system.

    Read the article I attached about air-fuel ratios
     
  8. Jul 7, 2016 #7
    Will do. Thanks, Mike
     
  9. Jul 7, 2016 #8

    Mech_Engineer

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    Correct me if I'm wrong, but it's my understanding the stoichiometric air/fuel ratio is a mass ratio, in which case it doesn't matter whether the gasoline is in a vapor or gaseous state because the mass will be equivalent regardless of phase...
     
  10. Jul 7, 2016 #9
    Referring to this article, the lean situation in pre-vaporized gasoline probably occurs, because of a perfect stoichiometric ratio prior to the air/fuel mixture entering into the intake manifold of the engine. In a normal carbureted or fuel injected engine, their could be an engineered imbalance in the mixture of atmospheric air and atomized gasoline droplets, where only a small percentage of atomized gasoline are then converted into gasoline vapor to run the engine. Possibly the percentage of atomized gasoline droplets are engineered to be "over-supplied", creating an overly rich fuel curve, and therefore an inefficiency. This extra un-burnt gasoline, aka Hydrocarbons, are mixed with a secondary source of atmospheric air from an air injection pump, and then flows to the catalytic converter to be re-burned.
     
  11. Jul 7, 2016 #10

    SteamKing

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    It's generally not a good idea to dump raw gasoline into a catalytic converter to be burned. The catalyst is already operating at high temperature and doesn't need any additional heat.

    Engines fitted with converters also carry quite a few different engine controls to make sure that at whatever operating condition the engine is running, the proper AF ratio can be looked up in a stored AF ratio map. The engine controls use this AF ratio data to adjust the metering of the fuel. Oxygen sensors in the exhaust stream help determine if the AF ratio is too lean or too rich, and fuel metering is also adjusted accordingly.

    https://en.wikipedia.org/wiki/Oxygen_sensor
     
  12. Jul 7, 2016 #11

    Mech_Engineer

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    But the mass of the fuel stays the same regardless of whether its vaporized or droplet, and the 14.7:1 ratio is a mass ratio.

    As mentioned previously, fuel injected engines utilize oxygen sensors to control the fuel delivery, trying to maintain the correct AFR. For most operating conditions a typical ECU will aim for 14.7:1, although its common to increase fuel delivery to about 12.0:1 for wide open throttle to increase power.
     
  13. Jul 7, 2016 #12
    Yes, but the fact remains that a portion of gasoline does go unburned in modern gasoline internal combustion engines, despite the computerized controls, and any unburned hydrocarbons are then re-burned a second time by the catalytic converter to support cleaner exhaust emissions. This represents waste and inefficiency. The real problem is that gasoline cannot be completely vaporized and combusted in the cylinder in the milliseconds of time during the compression stroke. Their is simply not enough time to burn all of the gasoline.
     
  14. Jul 7, 2016 #13

    SteamKing

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    And yer point is ... ?

    The various inefficiencies of the gasoline engine are well known.
     
  15. Jul 7, 2016 #14

    Mech_Engineer

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    I think you may be overestimating the amount of unburned gasoline being exhausted, do you have any publications or references which quantify how much gas is leftover after the combustion cycle?

    http://www.explainthatstuff.com/catalyticconverters.html
     
    Last edited: Jul 7, 2016
  16. Jul 9, 2016 #15
    That would be the power stroke (not to nitpick).
     
  17. Jul 12, 2016 #16
    Yes, I do, Mech_Engineer. Here are a few:

    http://www.consumerenergycenter.org/transportation/consumer_tips/vehicle_energy_losses.html [Broken]

    https://www.fueleconomy.gov/feg/atv.shtml

    http://ffden-2.phys.uaf.edu/102spri...tes/zach's web project folder/eice - main.htm

    Thanks for your interest.
    Kind regards,
    Michael Vannozzi
     
    Last edited by a moderator: May 8, 2017
  18. Jul 12, 2016 #17

    Mech_Engineer

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    None of those references specify the percentage of unburned fuel which is exhausted as a result of incomplete combustion. Try again.

    I agree that overall combustion engines have relatively low efficiency, but the #1 loss is heat loss (radiator & exhaust), not unburned fuel.
     
  19. Jul 12, 2016 #18
    The "EXHAUST LOSS" is the point. The majority of the un-burnt fuel is expelled through the "EXHAUST" port as the piston moves upward on the exhaust stroke, it expels the un-burnt fuel, along with the exhaust gasses from the burning/expansion process. Also, some of the un-burnt fuel mixed with carbon from the combustion process, and then it mixes in with the motor oil, in the gap between the compression rings, and this is what turns your engine oil black.
    That un-burnt Gasoline is not used to propel the vehicle, therefore, you have the inefficiency.
     
  20. Jul 12, 2016 #19

    Mech_Engineer

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    The exhaust loss is mainly thermal in nature, with some kinetic loss as well due to pumping the air. There is a negligible amount of fuel in it as long as the engine is running at peak combustion efficiency (e.g. Stoichiometric ratio). See here:

    https://en.m.wikipedia.org/wiki/Exhaust_heat_recovery_system
     
  21. Jul 12, 2016 #20
    Hello again Mech_Engineer,
    Thank you for taking the time to talk to me, appreciated. Just to let you know what I am doing, I am in the process of building a Gasoline Vapor System for an automobile, (1978 Chevy Camaro, 350 cu.in. SBC, automatic), I am on my 9th prototype and I am getting very close to success. The system does work and run.
    I am using a stainless steel Shell & Tube Heat Exchanger, (3" X 14"), and I have plumbed exhaust tubes into the shell sides, with the system having it's own independent exhaust system. I am using a motorcycle carburetor to supply air and fuel. The Air/Fuel mixture flows through the 30 straight tubes, and then through a 2.5" 90,(elbow), into the Open Plenum Edelbrock intake manifold. The Stoichiometric ratio remains mostly in the 14:7 range, but goes down to the 12:1 range at lower RPM, (by Air/Fuel Gauge). I have been doing a lot of studying and research, and it seems that their are many of my predecessor's who believe that the 14.7:1 Stoichiometric is not correct for Vaporized Gasoline, and should be much leaner. What are your thoughts? Thanks, Mike
     
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