Today: The real difference between diesel and gasoline.

Dear Car Talk

Dear Car Talk | Sep 01, 2011

Dear Tom and Ray:

What is the difference between diesel and regular gasoline? My boyfriend thinks that diesel is mixed with oil, and he can't explain what is in regular gasoline that makes it different from diesel. Please explain.

-- Corie

TOM: The main difference is that in diesel fuel, you can still see the dinosaur bones floating around.

RAY: Actually, gasoline and diesel fuel both are products that are made from crude oil. When a barrel of crude oil comes into the refinery, it's distilled into its heavier and lighter components.

TOM: The lighter stuff is used to make gasoline. The next-heaviest stuff becomes jet fuel. After that on the scale comes diesel. And below that is the stuff they use to fuel ships, run power plants and nourish my brother's hair plugs.

RAY: So, gasoline is lighter, less dense, more flammable and more volatile. When you spray gasoline into a cylinder, it starts to vaporize immediately, so that as soon as the spark plug fires, the gasoline detonates and powers the engine.

TOM: Diesel fuel is heavier, denser, less flammable and less volatile. So in order to detonate it, it has to be compressed in a cylinder to a very high pressure and temperature, at which point it detonates without a spark.

RAY: The upside of diesel fuel is that, because it's denser (like my brother), it has more energy per gallon. That's one reason why diesel-powered vehicles get more miles per gallon.

TOM: The downside is that diesel fuel requires a very-high-compression engine, which is more expensive to build. And because it relies on temperature to detonate, diesel engines traditionally have more trouble starting in cold temperatures.

RAY: That combination is why northern Minnesotans like diesels so much. They can't start them on winter mornings, but because the gas mileage is so great, they can leave them running the night before.

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