Delaying Autoignition: 10:1 Air-Petrol Mix at 20°C

[daveyjones97]

can you delay autoignition of petrol air mixture by preventing it from heating above 20 degree celsius as its compressed. how high pressure could be reached before autoignition? assume 10 to 1 (10:1) air to petrol mix.

 

[berkeman]

can you delay autoignition of petrol air mixture by preventing it from heating above 20 degree celsius as its compressed. how high pressure could be reached before autoignition? assume 10 to 1 (10:1) air to petrol mix.

Welcome to the PF.

What is the application?

 

[mrspeedybob]

Yes. Heat from compression is what causes autoignition, If you keep the heat down you could probably compress the mixture all the way to a liquid without igniting it.

Diesel engines rely on autoignition, they have no spark plugs. Older diesels were hard to start in cold weather because the air started out too cold to get hot enough during compression to ignite the fuel. That is why modern diesels have glow-plugs and intake air heaters.

 

[rcgldr]

I doubt it's possible to extract a significant amount of heat from the fuel-air mixture during the short time of a compression cycle in an engine. Water vapor injection can be used to cool the intake air.

http://en.wikipedia.org/wiki/Water_injection_(engines)

 

[mrspeedybob]

If he's designing the engine he could design it to run at 1 revolution per hour if he wanted it to. At that speed it would be easy to remove heat from the air-fuel mixture. This would obviously not be practical for an automobile but he didn't say what the application was.

 

[xxChrisxx]

Lets not get carried away with this. Before we even start let's go through the basics.

Firstly with fuels there is a certain ratio that will give a chemically 'correct' burn. Called the stoichiometric point. For petrol the ratio is 14.7:1 (Air to fuel).

Anthing with less than this ratio is fuel rich (fuel left over). Anything more than this is fuel lean (air left over).

As you compress the air mixture it will increase in pressure and temperature. It will also absorb heat from the surroundings (ie the hot block).

Petrol autoignites due to temperature, pressure and mix conditions at point of ignition. (In reality there are hundreds of factors, but we can lump them into those three areas).So what can we do to prevent autoignition?
We can reduce the pressure.

(got to go to work, i'll finish later)

 

[FlexGunship]

Yes. Heat from compression is what causes autoignition, If you keep the heat down you could probably compress the mixture all the way to a liquid without igniting it.

Diesel engines rely on autoignition, they have no spark plugs. Older diesels were hard to start in cold weather because the air started out too cold to get hot enough during compression to ignite the fuel. That is why modern diesels have glow-plugs and intake air heaters.

I doubt it's possible to extract a significant amount of heat from the fuel-air mixture during the short time of a compression cycle in an engine. Water vapor injection can be used to cool the intake air.

http://en.wikipedia.org/wiki/Water_injection_(engines)

Intercoolers are a common solution to this problem. When you compress the air in the compressor wheel of a turbocharger, that compressed air is then fed through an air-to-air passive cooler.

Proper intercooler function can keep auto-ignition from happening at higher compression levels. I used to have a Subaru STi which ran 18psi of positive pressure on the compressor exit side. If I didn't have an intercooler my compression ratio in the engine would have had to have been much lower than the 9.1:1 it was.

http://en.wikipedia.org/wiki/Intercooler

This only cools the air portion of the air-fuel mixture, but your fuel it only 1 part in 15 of the mixture.

 

[daveyjones97]

thanks guys, was daydreaming about compressed air energy storage with the ability to ignite the mix as a power boost. Lots of time to daydream as I am a trucker. Thanks

 

[FlexGunship]

thanks guys, was daydreaming about compressed air energy storage with the ability to ignite the mix as a power boost. Lots of time to daydream as I am a trucker. Thanks

I've had the EXACT same idea. Run a compressor off of your belt (or by exhaust, to fill a tank with compressed air. It's a similar premise to using nitrous oxide (the only goal being to add more O₂ to the mixture so that more fuel can be burnt). Good for passing if you have a diesel.

There's nothing fundamentally wrong with it. Run the compressed air through an intercooler, or other heat exchanger, and be sure to compensate in your fuel map for the increased air.

 

[carmatic]

so let me try to understand this... it is possible to have an air compressor tank which is pumped by something like the turbo or supercharger, building up pressure...
which upon release will release a burst of cold air which can be blown at the intercooler/heat exchanger to increase the cooling simultaneously as the engine is being fed more pressure...

how about, the tank pressure is so high that being released into the intake manifold (i think) is enough to cause considerable adiabatic cooling as well as a direct boost in air pressure

the downside is if the tank and the pump will add enough weight and suck enough power to make all this not worth it... the high air pressure tank could take the place of a radiator, for instance, provided that it has enough internal volume and is structurally capable of handling the pressure in this shape... many cars are capable of high performance despite having the weight of a radiator and reservoir full of coolant
but it would also need to have all parts of the fuel and air line capable of withstanding the added pressure...
but if it works, it could be used analogously with a turbocharger, except without the turbo lag...you can have solenoid valves releasing the compressed air at certain engine performance conditions... the extra responsiveness should translate to not having as much extrapolation for the engine computer to anticipate wrongly, thereby saving fuel and increasing the perceived performance of the engine

but in any case, the tank's primary purpose is to be thermally conductive and hold air at such a high pressure that a meaningful amount of heat can be dissipated isobaricly , which makes it different from a supercharger which does not have the cooling function