While California wants car companies to make more electric cars, automakers are trying all sorts of things to meet tough fuel economy standards. They're not just turning to new technologies. They're also taking a second look at an old one. NPR's Sonari Glinton reports on the return of the diesel engine.

SONARI GLINTON, BYLINE: I want to find out exactly what a diesel engine is, so I've come here to the College of Engineering at the University of Michigan, and I'm here with Professor Margaret Wooldridge.


GLINTON: So tell me what a diesel engine is.

WOOLDRIDGE: OK. A diesel engine is an engine that runs on diesel fuel. And because it runs on that fuel, the strategy for introducing the fuel into the engine is you inject it directly into the engine. And for a gasoline engine, it needs a spark plug to ignite the fuel. A diesel engine doesn't have a spark plug.

GLINTON: The spark plug, however that works, is not the only difference between the two engines.

WOOLDRIDGE: Pound for pound, in terms of fuel economy, the diesel engine wins. So that's the primary advantage. So if you sit down and do the math and look at the fuel cost - and it really depends on where diesel fuel is relative to gasoline prices - you can come out with a benefit.

GLINTON: Diesel fuel is a cousin of gasoline. It's still a fossil fuel.

WOOLDRIDGE: They all come from the same sweet light crude - that's what we like most - and same with jet fuels. It's just a different refining process to get you to a different mixture, a different set of hydrocarbons.

GLINTON: In passenger cars, the difference in technology between diesel and gasoline is essentially cost. Diesel engines cost more to engineer and build. And right now, the cost of diesel fuel in the U.S. is higher, and their exhaust contains more soot. Most trucks use diesel already. Wooldridge says diesel has several advantages to gas-powered cars. They go from zero to 30 faster, they tend to last longer, and there's the fuel efficiency. So if diesel has all these good qualities, why are there so few diesel passenger vehicles in the U.S.?

The answer is history. During the oil crisis in the '70s, car companies looked to diesel to help solve fuel economy problems. John O'Dell is with He'll pick up the story from here.

JOHN O'DELL: GM took an internal combustion gasoline engine and heavily modified it to make a diesel out of it very quickly. It was an absolute disaster of an engine. It broke, it smelled bad, it was noisy, it was unreliable, and it left most Americans with an incredibly bad taste in their mouth or in their mouth memory about diesels.

GLINTON: The U.S. essentially gave up on diesel cars. The European carmakers, though, kept making them and making them better. In Europe, diesel is one of the primary ways of getting more miles to the gallon, about 20 percent more. A lot has changed since GM first launched its diesel. The kind of diesel fuel sold for passenger cars in the U.S. is now cleaner, and diesel could be a way carmakers gets a higher fuel economy. The Obama administration has proposed aggressive new fuel rules. They give incentives for hybrids and electrics but not diesel.

David Geanacopoulos is general counsel with Volkswagen Group of America. VW has bet a lot on diesel. Geanacopoulos says the new fuel standards shouldn't favor one technology over another.

DAVID GEANACOPOULOS: Let the customers and the marketplace and future technical and scientific developments determine which are the winners. In a technology-neutral approach, the regulations can maximize innovation and improve our chances of achieving efficiency throughout the product range.

GLINTON: Meanwhile, Gina McCarthy with the U.S. Environmental Protection Agency says diesel engines don't get the same incentives because they're already in the marketplace.

GINA MCCARTHY: They are available, and the infrastructure is there to support them. So we think they're available to customers right now. We want to get the customers an ability to get these other advanced vehicles as well and get them into the market sooner, and that's the reason for the incentives.

GLINTON: Even without government incentives, you're going to see more and more diesel cars on the road. The biggest car company in the world, General Motors, is making a diesel version of its best-selling car, the Chevy Cruze. This time, the executives say, they'll get it right. Sonari Glinton, NPR News.

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