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Diesel engines are making a comeback in America and are very popular in Europe. So prospective car buyers are wondering: which is better? From a mechanical standpoint, the difference will be in cost of maintenance. From an ownership standpoint, the overall benefits and costs of one versus the other is important.

Which Costs More to Maintain?

Gasoline cars generally cost more to maintain over their lifetimes, but cost less per service than do diesels. Most shops charge more for hourly labor on diesel engines and parts are often more expensive as well. However, diesels have a better track record in terms of reliability and some maintenance (such as oil changes) are done less often.

In the end, the average diesel car owner will likely spend slightly less than the average gasoline car owner in maintenance costs.

Fuel costs

Today's diesel engines are generally more fuel efficient in terms of gallons per mile than gasoline engines are. A Volkswagen Jetta, for instance, gets over 15mpg better in its diesel rendition than its gasoline option.

Diesel has a higher energy density than does gasoline, so moving the same amount of mass (vehicle) requires less diesel than it does gasoline. The difference is not insignificant with gasoline having a density of 125,000 BTUs versus 147,000 in diesel. On top of this, diesel engines are direct injected versus the gasoline mix-injection (fuel mixed with air, then injected), which means less fuel waste in diesel. In idling, a diesel engine uses about 1/3 the fuel as does a gasoline.

Diesel fuel costs a little more, however, so fuel savings will need to be calculated on a vehicle-to-vehicle basis.

Engine differences

For the mechanic, this is where the nitty gritty is. Both gas and diesel are four-stroke processes (intake, compression, combustion, exhaust). The largest difference, fundamentally, being that diesel engines have no spark, but instead compress air until it's hot, then inject fuel which ignites in the hot air inside the chamber.

Diesel engines are generally more robust because of this higher-compression work. This makes the engine heavier, but is offset by simplifying much of the air intake/mixing components and electronics that aren't needed compared to gasoline injection.

In terms of power, today's diesels versus gasoline engines are about the same in passenger car comparisons. Diesel excels at delivering torque while gasoline excels at horsepower. Most of the time, the typical driver will notice little difference unless looking for specific performance points (as in racing, hill climbing, or trailer pulling).

Mechanically, working on a diesel engine versus gasoline is roughly the same. Diesel engines are usually built heavier, with more robust components, and with more tolerance compared to gasoline. Diesel engines also have fewer seals and often fewer components as well.

In terms of efficiency and cost, diesel beats gasoline on most fronts. There are, however, some areas where gasoline is more feasible. Up-front cost on gasoline is generally lower (gas cars just cost less to build) and gas cars are often less noisy and their exhaust smells less offensive than even the most modern "clean" diesels do (though the days of the black smog coming out of the diesel tailpipe are long gone). Cold weather starting is also easier in gasoline due to the direct ignition process and better fuel temperature tolerances.