The observational evidence would suggest that unleaded is much more likely to catch fire.
In the days of 4-star; it was such a rarity to see a charred car in a breakers, that you went and had a look. Around 1990, it suddenly became commonplace - and not just on injected cars.
It was widely remarked on by recovery and garage staff.
I've dumped the contents of a blocked-jet Viva's carb on to the exhaust manifold so many times that I never gave it a second though. It always boiled off as a cloud of petrol vapour - I wouldn't try it with unleaded [and not just because of the benzine.]
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The observational evidence would suggest that unleaded is much more likely to catch fire.
Around 1990 it suddenly became commonplace ...........
Perhaps there was an increase in the number of cars being stolen and subsequently set on fire by the perps.
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Perhaps there was an increase in the number of cars being stolen and subsequently set on fire by the perps.
In the slow lane of the m40? or the a14?
dont think so
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Leaded has (generally) a higher octane rating than unleaded, so I would expect it to be more flammable, not less.
A quick check of sites which have done flammability tests suggests than leaded and unleaded are the same at the same octane rating, so whatever you are observing isn't due to the petrol changing.
More likely to be things such as use of fuel injection (and hence more fueld sparying about), hot catalytic convertors converting liquid petrol to flammable vapour, and such like, I would think.
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>Leaded has (generally) a higher octane rating than unleaded, so I would expect it to be more flammable <
My two penn'orth: if the difference between the flammability of leaded and unleaded is as noticeable as has been suggested, I doubt it is due to the hydrocarbon content - which as has been said, is controlled at the refinery to suit the climate where it is used. The purpose of tetraethyl lead is to control the speed of combustion in the engine (pre-ignition), so perhaps it has a certain flame-retardant effect 'out in the open' too?
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Flammable and inflammable both have the same meaning ie the worrds are interchangeable, according to my pedants dictionary!
Edited by Armitage Shanks {p} on 10/09/2008 at 07:34
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Maybe modern cars go up more easily because of the increased use of plastics throughout the car. I had a car burn out on me and that was due to oil from a leak coming into contact with the exhaust.
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The exhaust manifolds of petrol engines runnning close to stoichiometric are hotter, contemporary ECU controlled engines acheive optimum AFR better than previously and need to do so to protect catalytic converters that came along at the same time as unleaded.
Hence spilt fuel or brake fluid hitting a exhaust manifold of a contemporary petrol car designed for unleaded is more likely to combust.
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As the "lead" or benzene content of petrol is small - wikipedia suggests up to 5% - (benzene being the additive that prevents knocking in these post-tetra-ethyl-lead days) I find it difficult to believe that it would change the "flammability" whatever that means.
Flammability isn't something that is measured on an absolute scale. It is a product of volatilty (ease of vapourising), flashpoint (temperature at which catches fire WITH a spark) and autoignition temperature (temperature at which catches fire WITHOUT a spark).
Screwloose who happily allowed his petrol to evaporate off a hot manifold (thereby hoping that the manifold was below the autoignition temperature of petrol (246 degrees C - Wikipedia) should note that diesel has an autoignition temperature that is lower than that of petrol - 210 degrees C - (Wikipedia). This is why Diesel engines don't need spark plugs.
The volatility of petrol is quite separate from its octane level and can be (is) manipulated. Petrol sold in the Arctic has a different composition from that sold in the desert in order that the volatilities of the two blends are similar.
So, sorry. Don't believe a word of it.
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>>So, sorry. Don't believe a word of it.>>
Agreed, see my post above yours.
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As it was in the days long before thermolasers; the exact temperature of those, recently-running lean, manifolds are lost in the mists of time - I can accept that repeated experimental evidence shows that they must have been below that 246 o C threshold.
Whatever the cause [and I can tell the difference between torching and a fire subsequent to impact] workshop whoopsies also showed that there was a marked difference in the accidental flammability of petrol at that time. I always understood that a higher octane petrol was less prone to auto-ignition? 0.15g/ltr of TEL isn't flammable, but just how flammable are benzine fumes?
As most injected cars had inertia switches and no vulnerable bowl-full of petrol held in a carb; I wouldn't think that injection was a tenable reason for the very marked increase in fires following accidents noted at that time.
Brake fluid is indeed auto-ignitable; but there was no significant change in braking systems in that period.
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Brake fluid is indeed auto-ignitable; but there was no significant change in braking systems in that period.
Hotter manifolds make auto ignition more likely be it brake fluid or fuel.
Also IIRC DOT3 fluid is more flammable than DOT2 and DOT4 more so than DOT3.
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I doubt that there was a significant change in brake-fluid flammability in the same six-month period - or that spilled brake fluid is that common in RTAs.
The sudden jump in crash fires was dramatic; we're not talking a few percent here, this was in the scale of a 50-fold jump.
I'd attended dozens of RTAs where there was leaded petrol running down the road and had learned to be merely wary of it - unleaded leaks seemed to have always already caught fire. I'm not sure that hot manifolds are necessarily involved; there are a lot of other potential sources of ignition in a heavy impact.
A good illustration was an artic tanker-driver's whoopsie at the top of Handcross hill. He dropped-off his loaded 30ton trailer in the lay-by for the night [those were the days!] without securing the brake and, some time later, it rolled forward and the legs collapsed, puncturing the front section.
10,000 litres of four-star spilled right across the road and ran all the way down to the stream at the bottom of the hill. From the bridge [I wasn't getting any closer!] it was possible to see hundreds of cars driving through the flood of fuel and leaving clouds of spray like a wet road - and, somehow, it didn't catch fire?
The recovery of the remainder of the fuel closed the road for 12 hours; but I wouldn't have liked to try that with unleaded. Anyone got the figures for the flammability of benzine vapour, as that is my best guess for the difference?
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Screwloose who happily allowed his petrol to evaporate off a hot manifold (thereby hoping that the manifold was below the autoignition temperature of petrol (246 degrees C - Wikipedia) should note that diesel has an autoignition temperature that is lower than that of petrol - 210 degrees C - (Wikipedia). This is why Diesel engines don't need spark plugs.
I'm getting a bit confused here. I thought that diesel was less flammable, and hence safer than petrol, due to the fact that the vapour from diesel doesn't ignite as easily.
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I thought that diesel was less flammable
Flammable just means that something will burn. It is not a formal measure of chance of of its catching fire.
Autoignition: the temperature at which something will spontaneously burst into flames without a spark.
Flashpoint: the temperature at which something will catch fire with a spark.
Petrol has a lower flashpoint than does diesel. Put a match to diesel, and the match will go soggy and go out; put a match to petrol and you need new eyebrows.
However, drop petrol onto a piece of hot metal at 230 degrees C and it will evaporate (don't smoke near it...). Drop diesel onto the same piece of hot metal and it will burst into flames.
That is why petrol engines have spark plugs - to provide a spark, but diesel don't bother - instead relying on compressing the diesel until it is hot enough to ignite.
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Thanks for the explanation Mapmaker, very interesting.
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I thought flashpoint is the temperature at which a liquid will produced enough vapour to sustain combustion - and it is usually well below its boiling point.
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I'm not sure that I can accept the "hotter manifolds" theory.
There was no reason for a non-cat car to run as rich as a cat-equipped car - and running leaner makes the manifold hotter; so the rich pre-cat mixture should be colder.
Anyway; the sudden increase in post-RTA fires came immediately unleaded was introduced [at a price discount] and long before ECUs and cats were in widespread use.
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There was no reason for a non-cat car to run as rich as a cat-equipped car - and running leaner makes the manifold hotter; so the rich pre-cat mixture should be colder.
The point is that fuel injected ECU equipped cars run leaner and thus their manifolds are hotter, in this regard there was mass adoption of FI and ECU control coinciding with the introduction of unleaded petrol and catalytic converter because running too rich will ruin a cat.
I also remember there being a feature on TV about plastic brake fluid resovoirs breaking away in accidents and the then new (I guess DOT4) fluid being more flammable thus more fires ensuing.
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The point is that fuel injected ECU equipped cars run leaner
Err - beg to differ on that; ECU-equipped cars [with cat] run significantly too rich to leave some unburned elements to fuel the cat.
The sudden appearance of post-RTA fires was too swift to be the - fairly slow - introduction of cats and their associated injection systems. The only significant change around those few months was the widespread availability of unleaded.
Plastic brake-fluid reservoirs had been around for decades - certainly since the '60s.
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ECU-equipped cars [with cat] run significantly too rich to leave some unburned elements to fuel the cat.
I hesitate to take issue with you on such a matter Screwloose, but are you sure this is right? I seem to remember that unburnt fuel in a car's exhaust will soak into the cat and start to burn when the temperature rises towards normal, cooking the catalyser and rendering it useless (for its intended purpose). This is why for example they tell you not to try to push-start cars with catalysers, because if the engine is fuelling but refusing to start the fuel will end up in the cat and destroy it.
Personally I hate the things, although they have never done me any harm.
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>>Err - beg to differ on that; ECU-equipped cars [with cat] run significantly too rich to leave some unburned elements to fuel the cat.
Really?
That's not my understanding of how catalysts "work". They are entirely passive surfaces, which provide a place for a reaction to happen.
My understanding is that a measured quantity of air (according to the quantity of unburnt fuel in the exhaust) is taken into the exhaust upstream of the CAT in order to oxidise unburnt fuel.
The cat is "fuelled" by being hot. It gets hot by being warmed up by exhaust gases, though I dare say the action of oxidising unburnt fuel warms it further.
Edited by Mapmaker on 11/09/2008 at 14:21
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My understanding is that a measured quantity of air (according to the quantity of unburnt fuel in the exhaust) is taken into the exhaust upstream of the CAT in order to oxidise unburnt fuel.
Eh??? There is no air introduced in to the exhaust at all - the slightest amount would send the oxygen sensor scatty. The "unburnt fuel" [an over-rich mixture] is my point exactly; fuel must be wasted for the cat to function. Without a cat; engines would [and did] run far leaner - and the manifolds were thus much hotter.
[And yes; too much of a good thing - neat petrol - can cause damage too.]
The cat is "fuelled" by being hot. It gets hot by being warmed up by exhaust gases though I dare say the action of oxidising unburnt fuel warms it further.
A cat "lights" by initiating combustion of the unburned fuel. They typically add 50 o C to the exhaust gas temp.
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>The cat is "fuelled" by being hot. It gets hot by being warmed up by exhaust gases though I dare say the action of oxidising unburnt fuel warms it further.<
I also hesitate to take issue, but would it be that the cat is 'got going' with some extra fuel only while the engine is warming up? It would seem silly to run deliberately a bit rich, just to keep the cat hot burning stuff which shouldn't be reaching it?
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It would seem silly to run deliberately a bit rich just to keep the cat hot burning stuff which shouldn't be reaching it?
Yes; it is. Wasting up to 15% of fuel just to feed the cat is something that is a nonsense as oil starts to run short. [Let alone the CO 2 argument...]
The only reason that there is a cat is EU regs state that there must be one - even if the car could meet the required emission levels without one.
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My understanding is that a measured quantity of air (according to the quantity of unburnt >> fuel in the exhaust) is taken into the exhaust upstream of the CAT in order to oxidise unburnt fuel.
Eh??? There is no air introduced in to the exhaust at all - the slightest amount would send >> the oxygen sensor scatty. The "unburnt fuel" [an over-rich mixture] is my point exactly; fuel must be wasted for the cat to function. Without a cat; engines would [and did] run far leaner - and the manifolds were thus much hotter.
Although it is common for motorcycles to do exactly this, usually given a fancy name such as Active Radical something or other, they have CAT's and could also be fuel injected.
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Although it is common for motorcycles to do exactly this
That sounds like secondary air injection; some cars use that too - pre going closed-loop - to get the cats fired-up.
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>> Although it is common for motorcycles to do exactly this That sounds like secondary air injection; some cars use that too - pre going closed-loop to get the cats fired-up.
My bike had this though with no cat in sight, Kawasaki call it Kleen Air, I removed the gubbins, runs better without it, it was supposed to inject air bled from the air box into the exhaust ports so as to burn of any unburnt fuel in the exhaust headers.
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Err - beg to differ on that; ECU-equipped cars [with cat] run significantly too rich to leave some unburned elements to fuel the cat.
Carbed car engines would run at AFR around 10:1 to perhaps 12:1 where as ECU control and the focus on emissions mean that EFI engine can run much nearer stoichiometric (14.7:1) albeit not as lean as they could if that did not have cats.
Plastic brake-fluid reservoirs had been around for decades - certainly since the '60s.
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There was certainly a Watchdog type featue on flamability of brake fluid on hot manifolds, I will try to Google.
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Carbed car engines would run at AFR around 10:1 to perhaps 12:1
Not if they were correctly adjusted they wouldn't! Nor am I convinced that hot manifolds were the likeliest ignition source.
There was certainly a Watchdog type featue on flammability of brake fluid on hot manifolds
Yes; I recall watching it. The anecdotal evidence from the period of sudden increase suggested that the cars exploded on, or shortly after, impact; suggesting that it was leaking fuel that caught fire, rather than the slower fire spread that would be associated with brake fluid.
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Flammable and inflammable both have the same meaning ie the worrds are interchangeable according to my pedants dictionary!
**pedant alert**
Well... inflammable is a proper word - from the Latin inflamare - to set fire to. However, it is viewed by some ignorant types as being a negative - like inaudible, inadequate etc., thus from a safety perspective the in has been dropped to generate the non-word flammable.
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Flammable IS a proper word. Just because you dont like it, does not mean it is a non word.
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Just because you dont like it does not mean it is a non word.
Piffle. Bunkum! Of course it does. xxx
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Plenty of interesting theories there, so it could be any number or combination of factors which cause a fire after a RTA really, not just unleaded fuel.
It's brought to mind something which happened years ago. Back in the 70s I was in a band and we were on our way to a gig one night and I saw a car with its engine on fire. I stopped the van (twin wheel tranny, remember those?), grabbed the fire extinguisher I carried with us, ran back to the car, put the fire out and ran back to the van to continue on our way to the gig. The driver who was just watching his engine burn couldn't believe his eyes!
Anyway, it's got me thinking that maybe we should carry fire extinguishers in our cars. Does anyone do that these days?
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Anyway it's got me thinking that maybe we should carry fire extinguishers in our cars. Does anyone do that these days?
Yep, after a gf's car caught fire, I have one in the driver's door pocket. Although thinking about it, I should probably check if it's still in date - it must be f-6 years old by now...
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