A full tank of fuel lit from the filler neck will burn - if you are insane enough to try it. The reason it burns and doesnt go up and take the garage with it is because in order for petrol to burn at all it needs to vaporise at the flash point.
In a motor there are two ratios that are important - mixture and compression. the stoichiometric pressure for petrol is 14.1:1, ie a compression ratio of 14.1:1 will give you the maximum power release - however this is hard to obtain and requires high pressure injection systems so most engine designers tend to go the easy way and go for a medium high compression (around 10-11:1) and increase the rpms using such tricks as desmo valves or pneumatic valve springs (formula 1). The other method is increasing the swept displacement by boring out and fitting oversize pistons and hardware. In the case of the Rootes group 1725cc engine a compression of 9.7:1 and rebore giving 1800cc is possible which gives up to another 10hp or so
Then there is the matter of mixture. In order for any fuel to burn it needs to be mixed with the right amount of oxidant - in this case oxygen from the air. Too lean and the mixture burns hot, you get cooked plugs and if left too long valves and general overheating. Too rich and you get a cold burn which in older engines caused coking up and that blackish smoke when throttled. Both of these conditions reduce the power of the flash burn and therefore engine power.
Now, if you have a petrol tank on your MB and you leave the machine in the open in the sun you will notice that if you open the cap (if unvented) you will hear a hiss and smell fuel. What you have in that situation is both a degree of compression and a fuel air mixture... and its this that is the explosive combination.
It is a myth that only an almost empty fuel tank is the only one that will explode, it merely means that when it does go up you get the most bang for your buck as it were and it does the most damage to anything in its immediate surroundings. As long as there is petrol vapour in air there will be some sort of explosive reaction - it is merely how much fuel/air mix that controls how bad the bang will be.
If you have seen aircraft in films that are on fire for whatever reason you will notice that alot of the time at first the fire seems merely to spread and flow back - even when its location means its likely to be fuel fed - this is because the high speed airstream keeps the burn area away from where the pressurised fuel vapour is escaping - the explosion usually happens when the airstream slows enough for the burn point to enter the area containing pressurised vapour and creates a flash burn or explosion.
This is a reason why if you have a fuel leak on a MB at speed it is more likely to burn with a flame if it ignites - and why given the same situation you are more likely to get a much more unpleasant explosion when the bike is at standstill...
sorry for the essay
Jemma xx
