Poised to make a comeback in the United States, diesel-powered cars are a proven way to save fuel. Best of all, unlike many other alternatives, the technology and infrastructure are here already. But as automakers ready several new diesel models for North America, escalating fuel costs on top of an already existing price premium make the ownership equation a little tougher to justify.

Long the preferred motive power for big trucks and heavy equipment, automakers are quick to point out diesel-powered cars make up the majority of Europe’s fleet. Diesel was gaining ground here, particularly in the 1970s after that decade’s oil crisis. Indeed, around three-quarters of Mercedes-Benz autos coming to America were diesels at that time. After U.S. diesel sales peaked in 1981, diesel engines basically disappeared from showroom floors with the exception of heavy-duty pickup trucks.

One can’t help but place some of the blame on the infamous Oldsmobile 350 diesel, rushed to market to capitalize on this craze; its epic problems sullied the diesel name in the United States for years to come. Years of cheap gasoline and tightening emissions requirements helped finish off that phase of diesel popularity.

A diesel engine works on a different principle and a different fuel than a gasoline engine. While both look alike inside with rotating assemblies of pistons on a crankshaft and intake and exhaust valves that open and close, the similarity ends there. A gasoline engine uses an ignition system to generate electricity to ignite a mixture of gasoline and air via spark plugs in the cylinders. Diesels have no ignition and use the heat produced by compressing air in the cylinders to ignite fuel injected into the cylinders.

Diesel engines use a higher compression ratio to achieve this and hence are built stronger to withstand those pressures. This higher compression is one reason diesels are more powerful than similarly sized gasoline engines. Diesel fuel itself is also denser than gasoline and therefore contains more energy. Hence, most automakers cite a 20 percent to 30 percent fuel mileage advantage over comparable gasoline-powered models.

If your impression of diesels is of noisy, stinky, pokey and smoky vehicles, that is no longer the case. While we were away from diesel, refinements continued and today’s diesel are quiet, quick and clean. Cold-weather starting difficulty was an issue that also has been overcome.

One historic advantage of a diesel engine was its lack of ignition system; ignition systems are often to blame when cars will not start, so older, properly engineered diesels had a reputation for dependability and no need for tune-ups meant less maintenance. The stouter internals of the engines also added to longevity. Of course, gasoline engines have been evolving as well, so those differences have become negligible in today’s cars.

In addition to better mileage, diesel engines produce more power at lower engine speeds. This makes them better for heavy hauling in trucks, but in cars also results in a more responsive feeling that many drivers enjoy.

So what’s not to like? These advantages are not without cost. These heavier-duty engines are more costly to build. They are typically heavier than their gasoline counterparts, hence other parts of the vehicle need modification. While older diesels used simple mechanical fuel-injection pumps, today’s diesels use high-tech electronic common rail injection systems that deliver fuel at extremely high pressures.

Emissions regulations are continuing to be tightened, with elaborate after-treatment systems and/or particulate filters required on most new diesels. All this naturally adds to the price of the vehicle and in the case of emission controls, results in some occasional additional maintenance.

There was a time when the lower cost of diesel fuel along with the improved mileage resulted in a low break-even point. However, diesel fuel prices have been running around 20 percent higher than regular gasoline this year.

According to the Energy Information Administration, higher diesel prices are the result of increased crude oil prices, a higher federal tax than gasoline and costs incurred from the transition to ultra-low-sulfur diesel fuel. This change began in 2007 as an effort to reduce vehicle emissions with cleaner fuel. Also, U.S. refining capacity cannot meet the demand for diesel, so some refined diesel fuel has to be imported. Increasing worldwide demand for diesel keeps driving prices up.

How does this impact the purchase decision?

A diesel vehicle will still be cheaper to operate. A Jeep Grand Cherokee Limited CRD with a 3.0-liter V-6 diesel costs about $1,000 more than one equipped with a 5.7-liter Hemi V-8, with the same towing capacity and similar torque. Using $4 per gallon gas and $4.80 per gallon diesel as a baseline, a rough calculation averaging estimated mileage to 15 miles per gallon for gas and 20 miles per gallon diesel, it looks like it would take around 70,000 miles to break even at those prices.

It might take even longer in a Volkswagen Jetta TDI, which costs around $2,000 more for the diesel engine. Using 25 and 35 miles per gallon estimates, it could take nearly 90,000 miles to start saving money. Your mileage will vary, literally, as will fuel prices, but if you do a lot of driving a diesel still makes economic sense.