Last week Volkswagen announced it has stopped selling its diesel cars in the United States because it cheated on U.S. emissions testing. This news, according to some, threatens to cripple diesel’s inroads into the U.S. market following years of steady growth.  

Whether hyperbole or harbinger, diesel engines are one tool in the fight to reduce auto emissions. The percentage of cars sold with diesel engines in the U.S. is less than 3 percent, according to WardsAuto. But the number of diesel cars sold since 2010 has been rising steadily. From 2010 to 2012, for example, registrations of diesel cars rose by 24.3 percent, even though total vehicle registrations rose only 2.75 percent over the same period.

Why did diesels begin to grow in popularity in the U.S.? And do they have a role to play going forward? The answers lie in understanding the plusses and minuses diesels bring.

Clean diesel engines offer good fuel economy and great torque, but they cost more upfront. Photo courtesy of General Motors

Upside: Fuel Economy, Power and Clean Energy

Superior fuel economy has been a selling point for diesel cars in the United States over the past half-decade. In fact, as a whole, diesels average about 30 percent better fuel economy than gasoline models.

The 2015 Chevy Cruze, for example, gets good combined EPA fuel economy estimates of 27 and 30 mpg with the 1.8-liter and 1.4-liter engine, respectfully. The 2.0-liter diesel model earns a combined EPA rating of 33 mpg, and its mileage on the highway hits a very impressive 46 mpg.

[Read Diesels Save Fuel, But Do They Save Money?]

Diesels also have more power, in the form of torque, than many gasoline engines. Torque is simply the amount of twisting force the engine creates. More torque produces quicker acceleration, and more towing power.

Trucks have long used diesel engines because of the superior torque they provide. The Ram 1500, for example, has a standard 3.6-liter V6 gasoline engine that makes 269 pound-feet of torque. But the optional 3.0-liter turbodiesel makes 420 pound-feet. Automakers have used diesel engines with impressive torque in cars to improve acceleration from a stop. Critics, for example, comment that the turbodiesel Chevy Cruze has a lot of torque and accelerates rapidly.

In 2008, Volkswagen introduced so-called clean-diesel technology. This advancement used filters to supposedly capture and destroy gases that were previously responsible for the heavy exhaust diesel cars were notorious for producing. With this advancement, automakers began positioning diesels as blueprints for reducing some greenhouse gases. Hence, the fallout over VW’s announcement that its clean-diesel emission numbers were fraudulent.

Nonetheless, the Chevrolet Cruze diesel engine, as well as clean diesel engines in BMW and Mercedes-Benz products, have not been implicated in the cheating scandal and appear to successfully produce much lower emission levels than earlier diesels.

The potential remains that clean diesels will have a role to play in low-emission vehicles.

Downside: Cost

Diesels are more expensive to purchase. The base 2015 Chevrolet Cruze model, for example, can be purchased for $16,170. The better-equipped ECO model with an automatic transmission is $21,370. The diesel model retails for $25,660. That's a difference of almost $10,000.

Further complicating matters is the price of fuel. As of September 30, 2015, the average price of a gallon of gas in the United States is $2.28. Diesel is a full 21 cents higher at $2.49 per gallon. Lower fuel prices make it less of an incentive to buy a more fuel-efficient diesel model, because you aren't necessarily saving enough money over time at the pump to justify spending more upfront.

[Read Upsides and Downsides of Driving Diesel]

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