Tuesday, February 27, 2007

What is the difference between gasoline and diesel fuel?

Who's asking: Claire Bea, Montreal, Quebec

The nicer waiting area at O'Connor Volkswagen, where I got my Beetle fixed a couple of weeks ago, is in the sales showroom -- of course. So as I sat there, a nice salesman asked whether I was interested in buying a new car, given the fact that my own vehicle has now hit seven years and 75,000 miles.

I asked whether VW had any plans to manufacture a hybrid car. The salesman said he didn't think so; "VW has a very fuel-efficient diesel engine option," he said. "I think they're committed to that approach."

That raised this question, which Claire asked over the weekend. What's the difference between gas and diesel engines -- and (my question) -- why is diesel the more fuel-efficient option?

Gasoline and diesel fuel are both refined petroleum products. Gasoline is a mix of hydrocarbon chains that range from seven carbons/16 hydrogens to 11 carbons/24 hydrogens. Diesel chains are heavier, with chains of 14 carbons/30 hydrogens. Gasoline vaporizes at temperatures below the boiling point of water; diesel boils at temperatures higher than 212F.

It's an oversimplification, but gasoline engines burn mostly gasoline, and diesel engines burn mostly air. Cylinders in a gasoline engine fill with a mixture of gasoline and air, which ignites with a spark from a spark plug. In a diesel engine, cylinders compress air until it becomes very hot; fuel injectors then add a little bit of fuel, which ignites and drives the engine. Thus, diesel engines use considerably less fuel than gasoline engines, anywhere from 15 to 40 percent less.

Does this mean that diesel is a greener choice than gasoline, then? I'd have to be an econometrician to figure that out. Diesel engines burn less fuel than gasoline engines, but refining diesel is a lengthier process than refining gasoline. Does the energy put into refining diesel equal the energy saved when diesel fuel burns? I have no idea. If anyone knows, post a response below.


JIM LAMB said...

And diesel is dirtier. Just follow one of the big 18 wheeler down the highway and watch the sooty smoke coming out. The diesels have been cleaned up by regulation lately but there are still a lot of dirty older trucks on the road.

What the VW spokeman didn't mention is that VW is still having trouble meeting US emission standards. The cleanup measure decrease efficiency.

The jury is still out on hybrid cars also.

Still my yacht will be a deisel because I can go much farther and longer between fuelings on the same tankage.

AnswerGirl said...

I'd expect no less, but those standards are a minimum. Electric cars are cleanest, then hybrids.