Bking:
So how come if you put petrol in a diesel engine fuel tank it wont er go.
Petrol is more flammable than crappy old engine oil but for some reason the old diesel motor just cant be assed running anymore even when you inject it at the right time!odd eh!
Hi Bking,
It’s not odd at all once one understands a couple of words and then use them correctly. 
You’re correct that petrol is more flammable than diesel.
Flammability is based on flashpoint.
Flashpoint is a temperature.
The flashpoint for a flammable liquid is the temperature at which it just begins to evaporate an ignitable flammable vapour.
The various flammable liquids have quite varying flashpoints.
The flashpoint for petrol is approx -40 deg C
The flashpoint for diesel is approx +65 deg C
However, the flashpoint has absolutely NOTHING to do with the reason that a diesel engine can’t run on petrol.
The next phrase one needs to understand is “Automatic Ignition Temperature” (AIT.)
An easy way to think of AIT is that it is the temperature (=amount of heat) needed to ignite something.
For instance, the AIT for paper is a lower figure than the AIT for coal.
The AIT for petrol is approx 380 deg C
The AIT for diesel is approx 220 deg C
A diesel engine can easily achieve 220 deg C all on its own and therefore can ignite its own fuel/air mix without the use of a spark plug, provided it’s injected at the right time as you’ve said.
A petrol engine cannot achieve the 380 deg C needed to ignite its fuel/air mix, hence the need for spark plugs, which let combustion take place at a lower engine temperature.
Given the march of technology, wouldn’t you think that, if it were possible, the engine manufacturers would have done away with spark plugs already?
For any given flammable liquid, there is absolutely NO connection between its flashpoint (flammability) and its AIT (the temperature needed start its vapour/air mixture burning.)
The answer lies in knowledge of the properties of the fuel, rather than any mechanical knowledge. 
