# Auto ignition temperature in pure O2

First of all (and I know this is not the place I'm supposed to introduce myself,) I'd like to say hi to the forums.

Now onto my question. For the past few weeks, I've been thinking about an idea I had. That idea was to use pure oxygen to oxidize a combustion reaction in an engine. I'm sure most, if not all, people on this form understand Guy-Lussac's law. I know that as the oxygen content of a fuel/ oxygen mixture is increased, the auto ignition temperature decreases (please correct me if I'm wrong.). Guy-Lussac's law states that p1/t1=p2/t2, meaning that temperature will increase with pressure.

In order to actually design the engine, I need to know the auto ignition temperature of gasoline in a pure (or nearly pure) oxygen atmosphere, otherwise I would end up with knocking.

Thanks.

## Answers and Replies

Im sorry my friend but what you are trying to do has already been done. It is what we call today a diesel engine.

Let me elaborate. A diesel/petrol-air engine works EXACTLY as you described it. In case of a petrol-air engine, a mixture of petrol and air is inserted into one "piston" of the engine. Now this piston starts compressing this mixture to a really small volume. Its basically the same thing you are trying to do, except we use Boyle's Law here. At that really small volume where the pressure is large, the spark plug generates a small spark (its called spark plug, duh) which lights up the petrol. THIS causes the piston to fly back to its initial position from where the whole process repeats. A car engine contains FOUR of these, hence is called a four-stroke engine. A bike contains two.

Now the point is that a spark is required because such incredible pressures too aren't enough to light up the gasoline. I guess that answers your question to an extent. Feel free to interact.

PS: Welcome to PF..!

BvU
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Been watching this thread (because I had no sensible answer). Welcome to PF from me too!

Not overwhelmed by Ninja answer -- sorry about that -- because RP idea is about using pure oxygen and he/she wants to avoid this dieseling effect.

Car makers haven't experimented with this O2 idea (it probably costs a lot of energy to separate the stuff from the N2 in air and then you also have to haul it with you , but never mind). So it's hard to find ignition T data in pure oxygen. And attempting to obtain such data at home is not a good idea at all.

Why not make some assumption and carry on with the design ?

I wonder if the pressure required to actually light the gasoline up will be so high that the ideal gas equation is no longer applicable. Could be the reason why no one tried it so far.

Not overwhelmed by Ninja answer -- sorry about that
No probs mate. We are all here to do science. We wouldnt get anywhere if everyone agreed with everyone. Keep correcting me..!

BvU
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Would be surprised if it matters: regular petrol-air engines (my brand of car calls them Otto engines) can have knocking as well. In pure oxygen T ignition will be lower, so more ideal gas-like.

What is knocking? Im sorry but I have no idea.

BvU
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Spontaneous ignition/explosion ahead of the (spark-ignited) flame front. 4th and 5th paragraph here . Pinging a better word ?
One also has 'dieseling': after switching off the ignition, this spontaneous ignition keeps the (Otto) engine running (briefly)

SteamKing
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Actually, it's pretty easy to compress gasoline and get it to auto-ignite. That's why the compression ratios of gas engines are limited to about 10:1 or so. In the past, gas engines had compression ratios of about 12:1 max., but it took fuel with special anti-knock compounds added to keep these engines from destroying themselves with pre-ignition (this is where the fuel-air mixture wants to auto-ignite before it is fully compressed by the piston in the cylinder. Pre-ignition is bad because, at a minimum, the engine still wants to compress the burning mixture, thus robbing itself of power. At worst, the high forces thus generated can cause damage to the internal parts of the engine, like snapping crankshafts.) Because gasoline engines are limited by the max. compression ratio which gasoline can take before it pre-ignites, they are also limited in the max. theoretical efficiency which they can attain.

Gasoline is not a compound with a fixed chemical formula, like water (H2O) or carbon dioxide (CO2). The gasoline which we put in the tanks of our cars and motorcycles is a complex mixture of different hydrocarbons and other stuff, the formulation of which varies seasonally. ('Winter' gasoline contains more volatile hydrocarbons to make for easier cold starts; 'Summer' gasoline contains fewer volatiles to prevent vapor lock). Additional compounds containing lead were once added to gasoline to keep it from pre-igniting, especially on hot, summer days, but these compounds were phased out decades ago in most countries. What to do?

It was found that gasoline which contained a lot of hydrocarbons with short carbon chains was better at withstanding pre-ignition. Gasoline refineries adjusted their manufacturing process to 'crack' longer chain hydrocarbons into stuff with a lot of short chains. Engineers developed engine sensors and controls which would detect the onset of pre-ignition and adjust the timing of the spark to prevent it from damaging the engine. Different engine designs were also developed which mixed the air and fuel more thoroughly before it was compressed to allow for more even combustion, since advancing flame fronts within the burning fuel mixture in the cylinder can cause engine knock as the fuel no longer burns evenly.

OTOH, diesel fuel is somewhat different from gasoline. You want to make it easy for this stuff to start burning when it is compressed, so diesel fuel has a lot of longer chain carbon molecules in its formulation. This formulation also tends to make diesel fuel less volatile than gasoline, but it also increases the energy content in each gallon or liter of fuel. The higher compression ratios (from 14:1 to 22:1) of a typical diesel engine and the manner in which the diesel fuel is burned tend to make these engines more efficient than comparable gasoline units, but diesel engines are also heavier in order to withstand the higher stresses which are produced.

CrazyNinja and mfb
bigfooted
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