Internal Combustion Engines: Liquid Gasoline or Vapor?

[Mike V]

In an Internal Combustion Engine, does liquid gasoline itself burn/combust, or is it the gasoline VAPOR emitted from the liquid gasoline by evaporation that burns/ combusts? (assuming that Oxygen is present in the Air and Fuel mixture)

 

[Baluncore]

Welcome to PF.
The vapor burns with oxygen in the air.
If there is too much, or too little fuel vapor, then the flame will not propagate.
Liquid fuel will be vaporised by the heat of existing vapor burning.

 

[Drakkith]

As far as I know, ideally the fuel in an internal combustion engine will be vaporized prior to, or shortly after, entering the cylinder. This ensures that the fuel molecules and air molecules are mixed well and makes the combustion as complete as possible given the particular fuel-air ratio for the engine. Vaporization is usually done either in the carburetor or by the fuel injector.

I don't think drops of liquid fuel burn well since only the surface molecules can react with the air. This is why a puddle of gasoline doesn't simply explode when lit on fire. The liquid puddle has little to no air to react with, so only the surface molecules, which quickly vaporize from the heat of the ignition source and then the flame, can burn. In essence, the puddle evaporates and the vapor is what is burning.

If I remember something I read correctly, one of the ways the damage control teams on board WW2 warships kept fires from breaking out after being hit was to spray some kind of foam or something on top of any spilled fuel. This sealed in the fuel vapors and kept the fuel from mixing with oxygen, leaving no way for it to burn.

 

[Mike V]

I ask this question to verify that liquid gasoline will not burn/ combust. I have made a statement that the liquid gasoline must be vaporized and Oxygen must be present for combustion to occur, and that gasoline in a liquid state will not combust. My statement was aggressively called out and refuted online in front of an audience of 225,000 people in this video: . This mechanic insists that a liquid will burn. He then tries to demonstrate this by pouring liquid gasoline down a cars carburetor and then starting the car up. I know that gasoline starts to vaporize at very low temperatures, (-45C), and that when the liquid gasoline comes into contact with engines components that are warmer than -45C, that some vaporization will occur. I am asking the members of the Physics Forum to confirm that Liquid Gasoline will not Burn/combust, but only the vapor emitted by the liquid gasoline, where the gasoline is exposed to heat above -45 degrees causing evaporization to occur, and which is mixed with Oxygen will combust.

 

[Baluncore]

I don't know why you waste your, or our time, on someone who is clearly a victim of the Dunning-Kruger effect. He is insufficiently educated to know the difference between burning a liquid or a vapor.

Don't try this at home. It is an experiment from 75 years ago. Fit a spark plug into the bung of a fuel drum, half-filled with gasoline. Close the drum and wait for a couple of minutes. Then fire the plug, and nothing will happen. That is because the air - fuel vapor mix is too rich to burn. Notice that the liquid does not burn either. The danger comes while venting an empty tank.

 

[Mike V]

Thanks Baluncore.

 

[jack action]

If someone wants to go to youtube for info, some more serious sources are available. The following one is much better. The image shown at 1:30 simply show the process:

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The only place combustion can happen is at the fuel-air boundary. The key to getting good power from fuel (i.e. releasing its energy as quickly as possible) is to make this boundary as large as possible. Vaporizing does that. That alone is not even enough within an engine: You also need to whirl the fuel mixture within the cylinder to make sure every air molecule finds its fuel molecule fast enough, before the piston goes down.

In the video from post #4, that whirl does happen and still partly helps burn that fuel. That being said, the guy in the video did not measure the power output of that engine and did not show what was coming out of the exhaust which was most likely black smoke due to unburnt fuel.

But I enjoyed the video anyway. Good laugh.

 

[LT Judd]

What was happening when the guy poured liquid gasoline into the manifold was that a very small amount would have vaporised. When it dripped into the cylinder a little more would have vaporised. When he started cranking the engine a little more would have vaporised. But you notice it took quite a lot of cranking to get it to fire and it ran pretty roughly. Once it had fired and misfired a few times there was enough heat to get it running reasonably smoothly, but it probably would not have held any load, because the fuel cannot vaporise fast enough to get sufficient vapor burning to provide the energy to hold the load.
When running as designed, the carburetor jets in conjunction with the intake manifold and combustion chamber , break the fuel into very small droplets, such that they have a very large surface area in proportion to their total mass and thus can evaporate very quickly and sustain higher powers.
Even inside the engine they are not fully vaporsied when combustion starts, but they first start burning just at the surface (actually just off the surface) of the droplets and then extra heat accelerates the process . (We are talking small sizes and time intervals here).
You can verify your self that the vapour, not the liquid is burning by pouring some gasoline into a metal bottle top lid , putting it in a place with no wind and hold a match at the surface to get a flame going. If you look very carefully from the side, you will see that the flame actually starts a few millimeteres above the surface of the liquid.
So technically and scientifically you are correct , only gasoline vapour burns , not liquid.

 

[Baluncore]

What was happening when the guy poured liquid gasoline into the manifold was that a very small amount would have vaporised. When it dripped into the cylinder a little more would have vaporised. When he started cranking the engine a little more would have vaporised. But you notice it took quite a lot of cranking to get it to fire and it ran pretty roughly.

An alternative explanation is that; when he flooded the manifold it was too rich to fire. He had to crank the engine until the over-rich mixture was diluted with air to a combustible mix, then it fired a few times before too much air had been pulled through, when the mix became too lean to ignite. At no time did liquid fuel burn. The engine only fired when the air-fuel ratio was in the narrow combustible range.

He then tries to demonstrate this by pouring liquid gasoline down a cars carburetor and then starting the car up.

That guy is not as stupid as he looks. He makes money from hits to his videos. The more you try to explain, or help others understand, the richer the troll gets and the more time you waste. It is in his interest to make contentious claims, to stoke up the dispute, and to keep that fire rageing.

 

[jack action]

So technically and scientifically you are correct , only gasoline vapour burns , not liquid.

Technically and scientifically, only a fuel-oxygen mixture burns. For example, gasoline vapors formed within an argon atmosphere wouldn't burn.