What is the difference between diesel and petrol, and why can’t cars take both?
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One explodes one expands. Explosions in places they aren’t meant to is bad as well as having an inert mist that will eat your everything is also bad.
diesel needs compression to ignite, petrol doesn’t. if you compress gas before you
ignite it in a diesel engine, you’ll generate more force than it is built for. diesel in a petrol engine won’t ignite without the compression and will just make a mess.
edit: you can design an engine to use both, but it’s more complex and therefore expensive so people generally don’t.
Petrol is the common name, in some countries, for Unleaded Gasoline.
As the name suggests, unleaded gas is gas without a certain type of lead-based additive, called Tetraethyl Lead, which boosted performance of cars at a serious detriment to the environment. Environmental lead causes very nasty things, like birth defects and high crime rates.
Unleaded gasoline requires a spark and a fuel injector, and is made from petroleum.
Diesel fuel is not necessarily made with petroleum, such as biodiesel fuel, and uses higher compression forces for its fuel injection.
The differences are numerous, but the important thing is that one engine can not use fuel designed for the other without the potential for damage.
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Each fuel was designed to operate in a different type of engine. That is, it is more that the fuel was optimized for the type of engine rather than the engine being optimized for the fuel.
Spark ignition engines — which is what petrol engines are — generally have the mixed fuel and air in the cylinder before ignition occurs. It’s not good if this mixture ignites before the spark occurs. Because of this, petrol has additives that decrease how fast it will auto-combust under compression. This is what the octane rating measures. A higher octane rating means more delay before auto-combustion occurs.
Diesel engines inject the fuel at the last instant. Here it is best if the fuel auto-combusts immediately. Diesel has additives that make auto-combustion occur faster. Diesel has a cetane number. A higher cetane number means less delay.
Because of the difference in operation between the two engines (spark ignition vs auto-ignition), the two fuels need somewhat opposite traits — fast vs slow auto-ignition. Thus, they don’t work well in each others engines.
Bonus note: Gas turbine engines (aka jet engines) have a continuous flame burning in them after they are started. Thus how quickly the fuel starts to burn (in tiny fractions of a second) is not important. This type of engine can be fairly easily designed to work on diesel, gasoline, and other flammable liquids.
Gasoline (as we Americans call it) explodes. Diesel ignites.
It has to do with the volatility of the fuel. Imagine the difference between lighting a candle versus lighting a sparkler. They could both release the same amount of energy, but the sparkler will do it much, much faster.
The difference between the two is how easy they are to ignite.
To ignite a hydrocarbon, first you need it to evaporate into a gas and mix with the surrounding air to create a suitable fuel to oxygen mixture that will burn.
Petrol evaporates very easily – and does this at a fairly normal temperatures and atmospheric pressures. Leave petrol in an open container and it will start to evaporate naturally and form a slowly expanding explosive atmosphere around that container. Allow a source of ignition to touch this cloud of vapour and it will ignite – it doesn’t have to touch the liquid petrol at all.
This is why dousing a bonfire in petrol to light it is so dangerous – leave that petrol time to evaporate and your pile of wood will be sitting inside an explosive cloud that will all light very suddenly and explosively.
Diesel on the other hand doesn’t evaporate until a higher temperature. So to get diesel to ignite you first need to heat it up until it starts to evaporate, and only then will ignition occur.
To can continue each way up the grades of hydrocarbon – something like propane or butane evaporates into a gas at very low temperatures so we will rarely see a liquid form (and is suitably explosive), while heavier oils won’t evaporate until much, much higher temperatures.
What this all means is that we need to account for how the fuel ignites when we design an engine.
A petrol engine works because the fuel will vaporise on its own, and all you need is a spark to ignite it – so we squirt fuel into the engine cylinders and light it with the electrical spark created by a spark plug.
A diesel engine first has to turn the fuel into vapour, so does this by compressing the fuel – as it is compressed it heats up rapidly, causing the fuel to first vaporise, and then ignite itself from the temperature.
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