Why does most fuel have to be atomized to burn?

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Originally Posted By: clinebarger
Originally Posted By: motor_oil_madman
Is it because it creates more fumes?


Liquid Gasoline is NOT flammable....LOL.
we were taught this in hiskool autoshop!
 
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Chemical reactions such as combustion must happen molecule by molecule. A gas has the molecules already separated for quick reaction. When a quantity of liquid gasoline is on fire, it is actually the layer of vaporized gas rising from the surface burning, not the bulk liquid itself.
 
Surface of the fuel has to evaporate and reach an air/fuel mixture suitable for combustion. smaller drop sizes, more surface to volume ratio, the more homogenous the charge is. Less large particles to impinge on walls and dilute oil as well.

Diesels work similarly, that's what the "knock" is, when enough diesel has evaporated to ignite with a bang. Too big a droplet, and it turns to soot.

Coal does the same thing. Ground to a powder, the volatiles boil off and commence combustion. The air can then get into the coal particle, and start the "char burnout" phase, leaving molten ash particles behind (some of which form tiny hollow bubbles that end up in cosmetics, packaging, and toothpaste).
 
The fuel needs to atomise, then to vapourise. In a carburettored engine, it was atomised leaving the discharge tube, and then vapourised in the manifold by lower pressure and heat. fuel injection moves the fuel closer to the valve, but is atomised better by the injector...it still needs the vapourisation stage to be combustable.
 
Originally Posted By: spasm3
To get more surface area of the fuel exposed to oxygen.


Surface area is basically it. The ability of a fuel to absorb heat, leading to combustion, depends on the surface area to volume ratio. Something small has a lot of surface area for it's volume (or weight). It heats up quickly, reaching combustion temperature.

This is the main factor in dust explosions. A flour mill, or sugar plant explosion, depends on a lot of dust being suspended in the air. A flame starts somewhere and it propagates quickly through the cloud of dust, because each particle of dust virtually instantaneously absorbs the heat and rises to combustion temperature.

In the case of liquid fuels, once atomized, much vaporizes. You can't get a smaller particle than a vaporized molecule of fuel. It's all ready to absorb heat and combust. The small droplets that remain, rapidly absorb heat (due to their high surface area to volume ratio), vaporize and combust with an explosion-like quality.

At least that's my take on it.

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Originally Posted By: madRiver
This is why it is really safe to have 250 gallons of home heating fuel sitting in Your basement.


If home heating oil is like diesel , it is not anywhere as easily ignited as gasoline .

A fuel like natural gas is already a gas and all it needs to do is mix with air / oxygen . Almost the same with propane / butane .

As has been said , gasoline must be atomized and hopefully vaporized .
 
It's the Oxygen that burns, the fuel is just a catalyst to that process. Gasoline is actually classified as a "reluctant combustor" which doesn't mean it's safe to throw around but does mean it requires a very specific range of mixture with air to burn; if that ratio is too high or too low, it just wets the chamber, basically.

So both air and fuel need to be well mixed for proper complete combustion.
 
Originally Posted By: Johnny2Bad
It's the Oxygen that burns, the fuel is just a catalyst to that process. Gasoline is actually classified as a "reluctant combustor" which doesn't mean it's safe to throw around but does mean it requires a very specific range of mixture with air to burn; if that ratio is too high or too low, it just wets the chamber, basically.

What? Where did you learn your chemistry? Oxidizers don't oxidize, the fuel does. Oxygen will not burn unless it is being oxidized by fluorine.

You must have slept through high school chemistry is all I can guess.
 
Originally Posted By: WyrTwister
Originally Posted By: madRiver
This is why it is really safe to have 250 gallons of home heating fuel sitting in Your basement.


If home heating oil is like diesel , it is not anywhere as easily ignited as gasoline .

A fuel like natural gas is already a gas and all it needs to do is mix with air / oxygen . Almost the same with propane / butane .

As has been said , gasoline must be atomized and hopefully vaporized .





No quibble with your facts, just an addition:

Natural Gas is safer than Propane because it is lighter than air; it dissipates rapidly and completely into the atmosphere and can reach a non-cumbustive ratio quite quickly. If there is a continuous source of gas, then you have a problem, because there will be a variety of mixture ratios available, only one has to light to create combustion, but if the source of the leak is stopped quickly, or the escaping gas is small volume (like with a Natural Gas stove) not a problem, really (you need a pretty continuous leak in an enclosed area to create an explosion risk with NG).

With Propane, which is heavier than air, it tends to pool at the lower level of an enclosed or open space; think of an low lying fog of explosive gas around your feet and legs. This layer of combustible material will mix with air above it, there will always be a combustible ratio available.

For a real-world example, think of a propane bar-b-q that lights up explosively if you are having problems lighting the thing but eventually introduce a working spark. With a natural gas bar-b-q or stove, the gas being on isn't going to cause that kind of reaction, the thing just lights when the working spark is introduced. Naturally there is always a danger when dealing with any explosive gas and enclosed spaces, but outdoors the risk or a reaction like the propane bar-b-q is near zero; the lid being open insures rapid dilution of the gas (versus it hanging around in the bottom of the grill with propane).

Butane, MAP Gas, etc are similar to Natural Gas in that respect. It's also why some Parkades ban Propane powered vehicles.
 
Originally Posted By: kschachn
Originally Posted By: Johnny2Bad
It's the Oxygen that burns, the fuel is just a catalyst to that process. Gasoline is actually classified as a "reluctant combustor" which doesn't mean it's safe to throw around but does mean it requires a very specific range of mixture with air to burn; if that ratio is too high or too low, it just wets the chamber, basically.

What? Where did you learn your chemistry? Oxidizers don't oxidize, the fuel does. Oxygen will not burn unless it is being oxidized by fluorine.

You must have slept through high school chemistry is all I can guess.


There is a difference between being technically correct and practical layman's advice. Electricity doesn't move like water in pipes and electrons don't move from the positive to negative terminals in a battery either, but for some, it helps to look at it that way. You don't need to know chemistry to get severely burned smoking near an oxygen breathing apparatus.

And you might like this one ... my High School Chemistry teacher (after learning I got 97 in Physics and above 90's in my Math, Trig and Geometry classes, he gave me an 8 ... that's not a typo ... in Chem). "If you were my son, I would kill you."

I think if I had just slept through Chem I could have managed a 51.
 
Originally Posted By: Johnny2Bad
There is a difference between being technically correct and practical advice. You don't need to know chemistry to get severely burned smoking near an oxygen breathing apparatus.

Well saying categorically (as you did) that it is the oxygen that burns is pretty far from "practical advice". Oxygen is an oxidizer, without fuel there will be no combustion.

If you got an 8 in chemistry then you may wish to avoid making statements about that subject.
 
Originally Posted By: kschachn
Originally Posted By: Johnny2Bad
There is a difference between being technically correct and practical advice. You don't need to know chemistry to get severely burned smoking near an oxygen breathing apparatus.

Well saying categorically (as you did) that it is the oxygen that burns is pretty far from "practical advice". Oxygen is an oxidizer, without fuel there will be no combustion.

If you got an 8 in chemistry then you may wish to avoid making statements about that subject.


Gasoline is a fuel; I thought we had established that.

High School taught me a bit; lifetime learning taught me ten times more. Why don't you suggest a book: I guarantee I'll read it.
 
Originally Posted By: kschachn
Originally Posted By: Johnny2Bad
It's the Oxygen that burns, the fuel is just a catalyst to that process. Gasoline is actually classified as a "reluctant combustor" which doesn't mean it's safe to throw around but does mean it requires a very specific range of mixture with air to burn; if that ratio is too high or too low, it just wets the chamber, basically.

What? Where did you learn your chemistry? Oxidizers don't oxidize, the fuel does. Oxygen will not burn unless it is being oxidized by fluorine.


Phlogiston theory ?
 
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