Auto ign point of petroleum gases

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Heckler

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Further to several posts recently and VicS discovering the fact that the auto ign point of petroleum gases is higher than that of diesel, can we discuss why diesels will apparently fire on gases?
I know we used to put inlet manifold flaps on oil field engines, this to stop engine runaways if gas started leaking near them.
PS Detroit Diesels fitted them as norm, this to deal with engine runaways if the super charger oil seals started leaking!
 
Diesels do not have to run on what we commonly called diesel fuel. They are truly multi fuel engines and can be adapted (by fuel pump timing) to run on a number of liquid fuels. Diesel, paraffin, Avgas and In extremis heavy fuel oil (which has to be heated ). I think the only fuel they cannot cope with is petrol and this may be down to the speed of the flame front and exhaust valve temperature.
 
skipper_stu said:
Further to several posts recently and VicS discovering the fact that the auto ign point of petroleum gases is higher than that of diesel, can we discuss why diesels will apparently fire on gases?
I know we used to put inlet manifold flaps on oil field engines, this to stop engine runaways if gas started leaking near them.
PS Detroit Diesels fitted them as norm, this to deal with engine runaways if the super charger oil seals started leaking!
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Ah yes, Chalwyn valves, remember them well.
 
You will find that liquid gas carriers have the ability to "sniff" cargo, with the charters permission. The engine is primarily still run on diesel fuel but a meterd amount of gas is bled into the inlet tract. This provides lower throttle settings saving fuel and cleans up the emissions by providing a better combustion by burning the particulates remaining from the incomplete combustion on diesel alone.
 
RichardS said:
All internal combustion engines will fire on suitable gases, providing that the compression ratio is sufficient to generate enough heat, so I'm not sure what you mean Stu?

Richard
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Sorry. Didnt make myself clear. The assumption has always been that gas is easier to ignite and that it can be used to help start a knackered engine. If the ign point is higher than diesel then this cant be true. Or is there something else going on? Some of the other answers chime with my experiences, we used to use big French built engines as crude shipping pump drivers, they were started on diesel and then could be turned over to gas.
Stu
 
skipper_stu said:
Sorry. Didnt make myself clear. The assumption has always been that gas is easier to ignite and that it can be used to help start a knackered engine.

...

Stu
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Indeed - that assumption is based on the results of attempting to ignite a dish of the relevant fuel at atmospheric pressure - as in lighting an Eberspacher or a drip feed heater. Petrol is very easy to ignite under those circumstances whereas diesel is extremely difficult. But I think that is mostly down to volatility - diesel has a high boiling point compared to things like petrol and there is not enough vapour present above the fluid to ignite.

Diesel engines effectively run by "knocking" - something that requires low grade fuels - the manufacturers go to a lot of trouble to reduce the vulnerability of petrol to knocking.
 
The flashpoint of gas and petrol is much lower (below ambient temps) than diesel/paraffin.
So gas/petrol will ignite with a spark or flame but diesel won't.
The autoignition temp of petrol and diesel is about 80°C so will ignite without an external ignition source at that temp.
Flash point and autoignition temps can be found on MSDS (hazards data sheets).
 
aquaplane said:
The autoignition temp of petrol and diesel is about 80°C so will ignite without an external ignition source at that temp.
Flash point and autoignition temps can be found on MSDS (hazards data sheets).
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Do you know what component auto ignites at 80 deg C?

Prompted by VicS's post a couple of weeks ago I looked up the values. Octane: 220 deg C and Cetane: 215 deg C. By comparison, Propane is 470 deg C.

So it seems the autoignition temp decreases with chain length. Of course, given the flash points, if something else ignites at a lower temp they will burn.
 
lpdsn said:
Do you know what component auto ignites at 80 deg C?

Prompted by VicS's post a couple of weeks ago I looked up the values. Octane: 220 deg C and Cetane: 215 deg C. By comparison, Propane is 470 deg C.

So it seems the autoignition temp decreases with chain length. Of course, given the flash points, if something else ignites at a lower temp they will burn.
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I think there is a degree of confusion here between flash point and autoignition point. The flash point is the lowest temperature at which something can be ignited by an external source - it tends to be quite low - room temperature or not a lot higher. Autoignition point is the temperature at which things simply burst into flames - we are talking temperatures of more than 200C for most hydrocarbons and the temperature increases as the molecule gets smaller. Petrol engines are fired by a spark and only need the fuel temperature to be above the flash point. Diesels fire by autoignition.
 
maby said:
I think there is a degree of confusion here between flash point and autoignition point. The flash point is the lowest temperature at which something can be ignited by an external source - it tends to be quite low - room temperature or not a lot higher. Autoignition point is the temperature at which things simply burst into flames - we are talking temperatures of more than 200C for most hydrocarbons and the temperature increases as the molecule gets smaller. Petrol engines are fired by a spark and only need the fuel temperature to be above the flash point. Diesels fire by autoignition.
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There is also flame point.
Flash point: minimum temperature at which a substance will burn given an external ignition source.
Flame point: minimum temperature at which a substance will keep burning once ignited.
Autoignition point: minimum temperature at which a substance will ignite without an external source.

All these are affected by oxygen concentration, so high pressure and an oxygen rich atmosphere will lower these temperatures. Also internal combustion engines almost always burn vapour or gas*, very different from dropping a match into a dish of liquid fuel.

Another thing is that in a spark ignition engine, autoignition is bad. Since all the fuel/air mixture is in the chamber, autoignition would cause an explosion instead of a burn. The preferred operation is to heat/compress the mixture to above flash point and, once the combustion has started, keep it above flame point but below autoignition. Compression ignition engines (diesels) avoid autoignition explosions by restricting the availability of fuel by injecting it (relatively) slowly. If, on the other hand, the fuel is all available at the start of ignition then there will be an explosion which is not desirable.

So, LPG to aid a stubborn engine will ignite later than the engine is designed for and explode rather than burn (since by that time most, if not all of the diesel will be present as well)

*the first "diesel" engine burnt atomised lycopodium
 
Spirit of Glenans said:
Apologies for stretching this thread to a very obscure place, but forumites might be interested to know that in 1945, when the RAF was removing a number of Luftwaffe jet fighters and prototypes from Germany to Farnborough for evaluation, there was no jet fuel available, so they were initially flown to the Dutch coast on "good quality diesel fuel"!
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The Alouette 3 choppers we flew in Angola would burn common diesel in an emergency, but the pilot said they could only use 85% power to avoid overheating.
Stu
 
Dougal Tolan said:
There is also flame point.
Flash point: minimum temperature at which a substance will burn given an external ignition source.
Flame point: minimum temperature at which a substance will keep burning once ignited.
Autoignition point: minimum temperature at which a substance will ignite without an external source.

All these are affected by oxygen concentration, so high pressure and an oxygen rich atmosphere will lower these temperatures. Also internal combustion engines almost always burn vapour or gas*, very different from dropping a match into a dish of liquid fuel.

Another thing is that in a spark ignition engine, autoignition is bad. Since all the fuel/air mixture is in the chamber, autoignition would cause an explosion instead of a burn. The preferred operation is to heat/compress the mixture to above flash point and, once the combustion has started, keep it above flame point but below autoignition. Compression ignition engines (diesels) avoid autoignition explosions by restricting the availability of fuel by injecting it (relatively) slowly. If, on the other hand, the fuel is all available at the start of ignition then there will be an explosion which is not desirable.

So, LPG to aid a stubborn engine will ignite later than the engine is designed for and explode rather than burn (since by that time most, if not all of the diesel will be present as well)

*the first "diesel" engine burnt atomised lycopodium
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Which neatly brings us back to the first question, do petroleum gases burn easier in knackered engines to help start them?
Stu
 
Intuitively NO, if you are trying to get a diesel engine started with eg. propane/ether.

There is no spark to ignite the low boilers at their flash point, and presumably not enough compression to ignite the diesel at it's auto ignition point.

But if it works, why question it?
 
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Don't know if this helps as I think the vague memory came from discussion with refinery chemists. I worked in a different area but used to chat to these guys over lunch. Their firm did cheap subsidized 3 course meals and didn't check ID. Our R&D lab. wasn't far away, guess where we went for lunch. :D

I remember that the auto-ignition temperature for hydrocarbon/air mixtures does decrease with increasing molecular weight and increasing chain length. Pretty certain that it is also higher for branched chain hydrocarbons than straight chain hydrocarbons.

I don't think that I ever got a straight answer about the mechanism, don't think they really knew for certain at that time. I think it related to ease of radical formation by breaking the chain. Longer chain, more places to break and large number of resulting shorter chains with radicals which probably reduce the activation energy for further reaction. I think it was actually rather a complex subject and I didn't follow it up at the time, not much application for type of chemistry I was paid to look into.

OP asked about runaway if gas got into diesel engines. I guess the answer is that the lighter, shorter chain hydrocarbon gas has a higher Specific Energy than longer chain diesel. I think Specific Energy usually increases with shorter chain length. The methane, ethane etc. gas will auto-ignite inside a hot mod. high pressure diesel engine, so lower auto-ignition temp. not important.
 
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skipper_stu said:
Which neatly brings us back to the first question, do petroleum gases burn easier in knackered engines to help start them?
Stu
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If by petroleum gases you mean propane and butane on the face of it no. Their autoignition temperatures are higher than that of diesel, but I assume the figures I find are at standard pressure ( 760mm Hg ) or at 1 bar. There use may overcome some diesel fuel delivery / injection problem. They may, or may not, have a broader explosive composition range.
 
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VicS said:
If by petroleum gases you mean propane and butane on the face of it no. Their autoignition temperatures are higher than that of diesel, but I assume the figures I find are at standard pressure ( 760mm Hg ) or at 1 bar. There use may overcome some diesel fuel delivery / injection problem. They may, or may not, have a broader explosive composition range.
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I dicd the same as you, checked the figures and you are quite right, but the reason for asking is of course, why does WD40 work? The answer must be the atomised white spirit.
Stu
 
skipper_stu said:
I dicd the same as you, checked the figures and you are quite right, but the reason for asking is of course, why does WD40 work? The answer must be the atomised white spirit.
Stu
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That seems very likely.

Since the autoignition temp is lower than diesel but no where near as low as ether, it should make it safer than ether esp if the problem is due to some fuel system deficiency. Ether perhaps really only to be considered when the compression is so low that nothing else is going to help.
 
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