What is it?
Fuel is what keeps your car running. It's that smelly liquid that gas stations sell for such an outrageous price!
What does it do?
The two common types of fuel: diesel and gasoline. Diesel fuel is used to power diesel engines.
Are there any differences between the two?
Yes, there are quite a few differences. They are both derivatives of crude oil, but due to being derived at a different temperature during the refining process, they have very different molecular structures.
Gasoline Engines: These engines use spark plugs on the end of each cylinder to ignite the fuel. This is due to the fact that this type of engine has a lower compression ratio. The steps of a typical stroke cycle are
Diesel Engines: Diesel engines have a higher compression ratio, therefore they don't need spark plugs. When gasses are compressed, they heat up. In a diesel engine, they are compressed so much that the heat caused by the compression causes them to explode. A diesel engine's typical stroke cycle is as such:
Is there a difference in fuel economy?
Diesel engines are more efficient. It's because the fuel is denser than gasoline and releases more energy when it explodes. The engines themselves are more efficient by design, as they have less moving parts than gasoline.
However, being more efficient doesn't mean they're better for the environment. Gasoline burns cleaner than diesel. When a large tractor-trailer truck starts moving, did you ever notice the big black cloud of exhaust it leaves behind?