Diesel cars: 7 worries, 7 answers

Just as diesel cars are preparing to enter the U.S. market in a big way, misconceptions may be causing interest to wane.

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By Peter Valdes-Dapena, CNNMoney.com staff writer

Mercedes-Benz sells diesel versions of its SUVs and its mid-range E-class sedan in the United States.
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NEW YORK (CNNMoney.com) -- Diesel engines power half the passenger vehicles sold in Europe but almost none here. Stricter U.S. clean air regulations have meant that - until recently - they were simply too dirty to be sold in the most populous U.S. states, including California, New York and New Jersey.

This virtual shut-out means that American drivers have little experience with modern diesel engines. The last time diesel cars were sold here in any significant number was in the 1980s, and the ones we saw then were noisy, belched black smoke and were slow to move.

Recent survey data from Kelley Blue Book indicates that diesel perceptions haven't improved much. In fact, views seem to be getting worse just as car companies are preparing to introduce us to better performing, cleaner diesel engines.

Interest in diesels among new car buyers is trending downward, according to Kelley Blue Book. It now stands at 39% compared to 60% for gas-electric hybrids. It's even lower than consideration for hydrogen fuel cells, which are still decades away from hitting the market.

But a close look at seven common worries about diesel engines reveals that most of them don't reflect today's reality.

1. I just don't get the difference: Like gasoline engines, diesels burn liquid fuel derived from petroleum. But unlike gas engines, diesels don't use a spark to ignite the fuel. Instead, air is squeezed inside a hot cylinder. At just the right time, fuel is squirted into the cylinder where it instantly ignites in the super-heated compressed air. That releases energy that pushes the cylinder down, powering the vehicle.

That might seem like a minor difference - either way, stuff burns and pushes pistons - but it ultimately means that diesel engines behave differently from gas engines in a number of ways. Diesels have mostly been used in trucks and trains which benefit from the engine's low-speed pulling power and greater fuel efficiency.

Gas engines have traditionally been used in cars because they tend to be quieter, offer quicker acceleration and run more easily when cold. But newer diesel engines are closing - and even eliminating - differences in these areas.

2. Diesel engines get worse fuel economy: Surprisingly, fewer than half of respondents in Kelley Blue Book's survey agreed with the statement that diesels engines "are fuel efficient." Actually, that's their main selling point.

Because of how the engine uses fuel - creating lots of power at low engine speeds - and because the fuel contains more energy per gallon, diesels get much better fuel economy than gas-powered cars. In many cases, diesels get mileage similar to gasoline-hybrid cars of the same size and type.

3. Diesels are old technology: It's true that diesel is an old engine technology. Rudolf Diesel patented his engine design in the 1890s. But the spark-ignited gas-powered engine that powers most of our cars is even older.

Like the gas engine, diesel has been undergoing a lot of improvements in the last few decades as fuel economy demands have increased, so a modern diesel engine is no less "modern" than anything else.

4. Diesel engines are noisy: Old diesel engines used to make loud clanking and popping noises caused by the sudden, high-pressure ignition of fuel inside the engine. Gas engines ignite fuel under pressure, too, but the much greater pressures used in diesels resulted in a popping and banging noises.

New diesels create a more gentle increase in pressure by injecting small amounts of fuel into the cylinder even as the piston is rising. Those small ignitions aren't enough to push the piston back down, but they keep things warm and increase pressure slightly.

Then, when the piston reaches the top, a bigger push of fuel creates the big burst of energy that pushes the piston back down. Another series of smaller fuel injections then keep the piston moving down, easing decompression. The result is gentler, quieter combustion cycle that eliminates most of the noise.

5. Diesel engines spew filth into the air: This has been the real culprit keeping diesels out of the U.S. market. America's strict clean air rules simply couldn't be met.

The extremely high temperatures and pressures inside diesel engines create nasty pollutants like nitrous oxides and so-called "diesel particulate matter." In order to get diesels to meet the same requirements as gas engines, car companies are now using a variety of emission cleaning technologies including filters and devices that release cleaning substances into the exhaust stream.

To make all this possible, gas stations that sell diesel fuel are now required to sell "ultra clean diesel" that has much less sulfur. Sulfur used to be included to help lubricate the engine, but its presence would have interfered with these new emissions-cleaning technologies. With that sulfur taken out, diesel exhaust can now be held to the same standards as gasoline exhaust.

6. Diesel-powered cars are slow: While diesel engines have loads of pulling power - called torque - acceleration has been a traditional weak point. All modern diesel engines now have turbochargers that provide additional boost, which has made a big difference in performance.

Diesels have a gentler take-off before the turbochargers start adding boost. Once things get rolling, they can provide a surprising rush of acceleration. Most drivers would never miss the performance of a gasoline-powered car. In fact, diesel engines with less high-end power - or horsepower - can deliver similar, or even better, real-world performance.

7. They cost too much: While diesel offers a less technologically complex way to get hybrid-like fuel economy, it's not without added costs. Diesel engines cost more to make than gas engines because of the need to withstand high internal pressures, and because they require turbocharging to provide adequate performance.

Diesels also require expensive emission-cleaning technology. Like gasoline/electric hybrid cars, diesel cars will cost American buyers thousands more than comparable gas-powered versions.

Diesels could make up much of that added cost in better resale value, though. Diesel cars. They hold their value much better than gas-powered cars or hybrids, according to Kelley Blue Book.

--Bonus question: Due to a strong reader response, we added this worry to the list:

Diesels are hard to start in cold weather: Because they rely on engine heat and because diesel fuel tends to thicken in cold temperatures, diesels have traditionally been hard to start in cold temperatures.

Improvements in diesel engines and diesel fuels have helped eliminate these problems. Like gasoline, diesel fuels are reformulated for cold weather.

Improved fuel injectors help by better "atomizing" the fuel, spraying out a fine mist that ignites more easily. Newer "glow plugs" - internal heating elements that warm diesel engine cylinders - also heat up right away, providing quick starts even on cold days. To top of page

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