An engine question

[phoenix99]

Hi guys

I am a mechanical & aerospace eng student and I am doing an assignment on aircraft piston engines. Prior combustion the liquid fuel becomes vapor. My question is how does it become vapour? I know there must be heat transfer prior to fuel/air sprayed into the cylinder.Is there some sort of heat exchanger which pre-heats the fuel to its flash point? Or is thre something else going on.

Any help would be great

Cheers

[holmsy]

if i see my bro anytime soon ill ask him but i prob wont for a few days.
good chance he will know but yeh

[Piotr]

Wouldn't the heat come from the compression of the air and heat from previous combustions?

[Outbackjack]

Have a poke around here....

http://www.chris-longhurst.com/carbibles/index.html

Lots of good info and dispells a few myths.

[BJ TE50]

Are you referring to atomisation of the fuel through the injectors/carby or something after this?

[Interceptor]

Originally Posted by phoenix99

Hi guys

I am a mechanical & aerospace eng student and I am doing an assignment on aircraft piston engines. Prior combustion the liquid fuel becomes vapor. My question is how does it become vapour? I know there must be heat transfer prior to fuel/air sprayed into the cylinder.Is there some sort of heat exchanger which pre-heats the fuel to its flash point? Or is thre something else going on.

Any help would be great

Cheers

the fuel isnt a vapour, its a fine mist of liquid suspended in the airflow into the combustion chamber, atomised either by a carburettor or by a fuel injector

[Ringo]

^^ Agreed.

Then with a petrol engine there is the spark plug which ignites the fuel molicules...

Diesel engine have a variation on this....but that another thread

[phoenix99]

thanks for the help yep the I was a bit confused, it is a fine mist(liquid) and in the compression stage the temp rises before combustion. The temp rise may be quite significant dependng on the compression ratio.

Cheers

[jzab]

The heat comes from engine itself and the intake air. The heat from the engine comes from the inlet ports, the inlet valves and the combustion chamber. When the engine is running and warm there is ample heat around to vaporise the fuel. Fuel injectors and carburetors attempt to atomise the fuel so that there is a large surface area to assist the heat transfer from the air to the fuel. Most fuel injected engines also aim the fuel at the back if the inlet valve so that the heat of the valve vaporises the fuel. Some carby engines also have heated inlet manifolds using coolant or even the heat transferred from the exhaust manifold to ensure that the fuel is in a vapour state. Contrary to popular belief, most of the fuel needs to be vaporised prior to entering the combustion chamber so that mixing of the fuel and air can take place properly. If the fuel has to vaporise and mix in the combustion chamber quite often there isn't time to do both. I know there are exceptions to this rule, eg direct injection petrol engines, however these engines have had a lot of engineering work done to ensure that both vaporisation and mixing are done in the combustion chamber.
If you look at some racing engines, they try to take advantage of the latent heat of vaporisation by mounting the injectors away from the inlet valves and further back up the inlet runners. What they're trying to do here is to cool the inlet air with the vaporising fuel, which makes the air-charge more dense increasing performance. Mounting the injectors this way also allows more time for the fuel to vaporise before entering the combustion chamber.

When an engine is cold, quite often tha fuel doesn't vaporise and actually sits around in the inlet manifold on the walls and in puddles. It can even go through the combustion chamber and not even take part in the combustion process! Modern fuel injection systems compensate for both these situations by modelling the processes (Google "fuel puddle modeling x and tau" and you'll find heaps of ways of modelling the fuel puddle)

I hope this helps
jzab