What determines if a fuel is explosive or not?



So specifically I’m interested in gas vs diesel, but I would love to learn further about the specific property that determines this. It’s obvious that an explosion occurs when pressure builds up, and when gas ignites, it propels outwards and expands, but why isn’t diesel like that? Why isn’t every flammable gas explosive in nature?

In: Chemistry

All fuels are potentially explosive if you put enough vapor in a small enough chamber. The question is whether the vapors are warm enough to ignite. Gasoline has a flash point of -40C/-40F. This means as long as gasoline is above -40C/-40F it’ll ignite if you have an ignition source. Hell, if you have gasoline vapors and an ignition source in close proximity you can probably get it going by accident. Diesel on the the other hand has a flash point of >52C/125F. This means diesel vapors won’t ignite until they’re over that temperature. Try lighting a cup of room temperature diesel with a lighter. It won’t work.

In the case of an engine this temperature requirement is easily met by the cylinder itself which is why diesel is still capable of being used in an engine. In open air? You won’t get temperatures that high so trying to ignite the vapors won’t actually work. Gasoline? It’s heavier than air and probably above its flash point so it’ll pool around the floor and then ignite if you even look at it funny.

Explosions only happen when a lot of energy is released quickly. This usually requires the fuel and oxidizer to be well-mixed prior to ignition. A bucket of gasoline will not explode, but gasoline vapor mixed with air in a canister will.

In the shortest possible way of explaining it, the more a reaction can occur in a shorter time, the more abrupt/explosive the reaction.

For flammable reactions, the reaction is fueled by air and whatever matter is catching fire. Increased surface area of burn material with increased exposure to air yields larger reactions.

Pressure generally can channel the flammable explosion into a more directed energy of greater concentration. Fire cracker in air vs tube. It doesn’t necessarily increase or decrease explosiveness.

A number of answers are missing the point that what is exploding is not gasoline itself but a gasoline + air **mixture**. Of a certain range of concentration which is why it’s dependent on the flashpoint. Fill a container with pure gasoline vapour and nothing will happen when you set off a spark because there is no air. Fill it with air with a trace of gasoline vapour and nothing will happen either because the molecules are too spread out for the oxidation of one to affect others. Like all fires, even explosive ones depend on the triangle of fuel – oxygen – heat reduce one below a certain level and the fire isn’t sustained. The difference between gasoline and diesel is that the former is easier to get to a suitable concentration

Everything that can oxidise is explosive under correct conditions. First you need what will “burn”, oxygen, then enough energy. Now you can choose whether you add energy, like with gasoline-air mixture with a spark to start the combustion. Or you can compress something under pressure until there is enough energy to combust.

What is important to understand is that if you compress something, it has the same amount of energy as it had before it was compressed, but in less space. But more energy in smaller space basically means that thing is hotter. Heat is just movement. If you compress it you get same amount of movement in smaller space.