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Petrol flash point

Discussion in 'Technical and Troubleshooting Torque' started by duncanp, May 11, 2006.

  1. I had 7 litres of petrol leak from the fuel selector tap thread onto a hot engine on the go. I stopped or rather was stopped (as it went putt, putt, putt), turned the ignition off as fast as I could, jumped off and ran across the road fully expecting an explosion to get me airborne as I bolted for it. Not to be. An eerie silence ensued.

    So, what is the flash point of petrol? How hot is a hot engine? How safe/unsafe is it to investigate the cause of the leak with a mobile phone in one's pocket?

  2. Dripping like that in the open air there is almost 0 chance that it will explode.
    Maybe small amounts could ignite but more likely it will just vaporise and be blown away by the breeze (or movement of the bike)

    Despite what the movies say petrol is very dificult to make explode.
    An almost empty tank sitting in the sun for a couple of hours then apply a match, and you could do it... (Maybee)
    Anything short of that, and not a chance.
  3. Petrol (gasoline) is designed for use in an engine which is driven by a spark. The fuel should be premixed with air within its flammable limits and heated above its flash point, then ignited by the spark plug. The fuel should not preignite in the hot engine. Therefore, gasoline is required to have a low flash point and a high autoignition temperature.

    Flash point: >45 °C
    Autoignition temperature: 246 °C

    Flash point: >62 °C
    Autoignition temperature: 210 °C

    Jet Fuel:
    Flash Point: >50 °C
    Autoignition Temperature: 210 °C
  4. Sorry Falcon-Lord, petrol does go Boom fairly easily (in the right circumstances). Which is why you shouldn't mess with it :grin:


    You can put a match out in a cup of petrol. But only in the open air where vapours are not allowed to build up and create the correct fuel/air mix.

    Stick a match in a partialy full fuel tank = Boom! The flame will ignite the vapour (because when you opened the cap you allowed air to enter and the vapour + air + mixing + ignition source = Boom!

    Why do you think people who check fuel tanks with lighters have so much re-constructive surgery? It might not happen every time but you can bet it does happen. I've seen the results, not pretty.

    Simplified explanation:
    The bike didn't explode because there was no ignition source. The fuel was evaporated by the hot engine and when mixed with the surrounding air would have been very Boomy. But there was no spark / naked flame around so no Boom!. All you needed is a spark at the right (wrong) time and the bike would have been an insurance job.

    There was a passenger plane which exploded and crashed because of Jet fuel vapour + air in the right mix and some faulty wiring.

    I can get you the exact figures for fuel / air mix percentages to get the best bang for buck :wink: but I'd have to head to to the station to get them cause my memory is crap :roll:

    But yeah in real life cars don't go boom like the movies. That is Bullshit, most accidents do not produce large fireballs or even any fire.

    Mobiles could possibly set off vapour however it's not all that common but it does happen.

    Trust me, I'm a FireFighter :grin:
  5. Origin:


    You should quote your references if you copy other's work. It's the polite thing to do.
  6. Isn't the mobile fear that you'll need to drop it and have the battery disconnect and spark? I remember some show on TV (Brainiac i think) throwing petrol all though a caravan, they then rang 8 or 10 mobiles in there all at once and no boom.... They then used a small static spark from some dude in a funny looking suit and BOOM!
  7. I think that was mythbusters right? Or they did the same sort of thing anyway. I rememer the guys from mythbusters putting fuel in a contained box and having a mobile ring but nothing went off. I think they used several mobiles etc with no success.
    Their conclusion was that the mobile / petrol myth was busted. But you always have to take their experiments with a pinch of salt. :)
  8. Doggy...
    What are you talking about???
    Not questioning your conclusions, just questioning how you decided i was wrong when your conclusions, and my conclusions were the same...
    In the open it wont go boom, in a tank where there is vapor it will go boom, laying against the engine it will most likely evaporate...
    That is what we both said.

    Parafrasing me who prety much said, Petrol wont go boom easily (unless you provide the right circumstances).
  9. for an explosion you need pressure - spilling fuel from a hose onto a hot surface at best will just evapourate, at worst, it'll just burn.

    Setting fire to a jerry can or fuel tank on the other hand...
  10. nahh that just burns as well, I epected it to go boom once the fule volume got down and the temp got up, but nope it didn't.

    You need heated compressed vapor. So a closed fuel tank with only a little fuel at the point of opening can go boom, but within a few seconds i would expect (Now this bit is assumption) that the major risk will have substantually deminished
  11. I tried the jerry back in school, just burnt out the top for a long time...

    Throwing a sealed one on a campfire however can cause bigger flamage ;)
  12. Yep fuel/air mix is an important thing. Get the ratio right and wheat flour is incredibly explosive - but you don't have to worry about bread exploding in an oven.
  13. Whats the definition between flash point and Autoignition anybody?

    Does it mean that with say petrol,at 44degrees C and below,no amount of spark will make go boom now?

    I know as mentioned above in grain mills or wheat,any fine powder,causes potential booms in large chimnemyextraction towers,and blows normally coz of huge extraction fans spontaneously combusting the very stuff its supposed to be extracting if the motor in the fan causes a spark ofthere CDI ignition source.
  14. exactly the same way that if you light a rag in the neck of a car's fuel tank, it'll just burn away at the top. The vapor mix inside the pipe will be too rich to burn.
  15. the reasone why you turn your mobile off at petrol stations is so that you don't get someone calling you and get distracted from what you're doing. forget that you have a pump in your car and drive off. :LOL:

    It's not the mobile phone, it's your own stupid self they're protecting you from. why do people buy ringtones anyway?
  16. Yeah mythbusters definately covered the mobile phone thing and worked out that no amount of ringing them will set off an explosion, even in perfect air/fuel ration conditions.

    The also had trouble throwing a lit cigarette onto a puddle of petrol to ignite it too. The burning embers on the end of a ciggy weren't hot enough to ignite it apparently. They tried several times and couldn't get it to light.
  17. You would most likely get liquid petrol evaporating and forming a vapor cloud which if it came into contact with a iginition source, plenty of them on bikes, would have made a flash fire which would have ignited the liquid spilling out the tank, but probably not the fuel in the tank itself, as the flame path probably would have been quenched due to the small hole the petrol was coming from.

    But there are about a million different scenarios for this, but that is probably the most likely one which would be bad situation. Or liquid fuel fire if it ignited, and normally you assume that it will find an ignition source on a vehicle
  18. Yep - in fact both of these scenario's have been covered in that Dr Karl myths thing in the Weekend Magazine (the Age). The conclusion from both of them was that it is possible but would very rarely happen in a realistic situation (automotive situation here, not people playing with petrol in general).

    To explode, if I remember correctly, the petrol vapour has to be between 2 and 8 per cent of atmosphere or thereabouts which is relatively difficult to achieve perfectly: to create those explode-on-impact cars in the movies they apparently have to fill a water bed inside the car with some petrol and ignite it at a pre-determined point.

    The mobile phones fires were even less likely - there haven't been any reported cases of it in any petrol station fires... and for the obvious reasons these fires are investigated fairly thoroughly. It is possible but, again, requires circumstances that are even less likely to happen in everyday life.

  19. Thats all well n good,but still doesnt answer my question :-k ,essentially Im being lazy and couldnt be bothered trolling through the net for answers.

    So,does any bright spark :roll: budding scientists :idea: ,pyromaniacs :butt: or fireman :demon: out there know?
    Whats the definition between flash point :furious: and Autoignition :soapbox: anybody?

    Does it mean that with say petrol,at 44degrees C and below,no amount of spark will make go boom now?
  20. I was told that the flashpoint of petrol is actually around 43°C below zero :shock: . Meaning that even on the coldest day you can possibly think of, petrol can still ignite very easily.

    Just had a fire warden refresher course, the guys at chubb provided this tit bit of information during the course!