By Andrew Connor
on 8.4.14

Much like soccer or the metric system, diesel cars seem to have caught on almost everywhere around the globe except for the good ol’ U.S. of A. Consider our fancy European friends, who seem to love diesels; they account for roughly 50 percent of all auto sales across the pond. Back in America, diesels only make up around 3 percent of auto sales; but while that may seem like a paltry piece of the pie, their sales have increased 25 percent since last year, eclipsing the 4.2 percent increase in overall car sales since 2013. Thanks to new advances in diesel tech, the fuel has come a long way to become a suitable alternative to gasoline, but still has some drawbacks to overcome.

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To better understand why diesels have so much potential, a bit of an engineering lesson is necessary. In a normal car, combustion is initiated by spark plugs, but in a diesel the heat of compression is used instead. The result is a higher compression ratio — the difference in the volume of the cylinder and compression chamber when the piston is at the bottom of its cycle, and when the piston is at the top. This in turn means more highly compressed air, which begets a more powerful explosion and higher thermal efficiency. If you’re lost, focus on this: the whole process means that more energy can be derived from diesel fuel than gasoline. The result is greater fuel efficiency, and it’s generally accepted that diesel cars are around 30 percent more fuel efficient than their gasoline equivalents.

Don’t think that higher mileage come at a price of performance; after all, diesels do rule Le Mans. While generally the horsepower numbers for diesel engines are lower than gasoline engines, diesels make up for it with higher torque figures; horsepower numbers are all fine and good, but torque has a big impact on power off the line. Consider the Volkswagen Jetta TDI. While the 2.5-liter variant produces 30 more horsepower, the TDI (diesel) engine generates 236 lb-ft of torque compared to the gasoline’s 177 lb-ft figure. In fact, when comparing the two, the Diesel Jetta has a quicker 0-60 time of 8.4 seconds versus the 8.9-second time of the 2.5 version.

Gasoline and Diesel: What’s the Difference?

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While diesel fuel may seem incredibly foreign to those who’ve never driven a diesel car, it’s actually very similar to gasoline. Though we’d love to give you another lecture, we’ll just refer you to Tom and Ray Magliozzi of Car Talk, who’ve explained the nuances between the two fuels in the only way they know how — with equal parts snark and sense. Read their explanation here.

But despite the potential benefits to the consumer, diesels still need to overcome a stigma. To the average U.S. consumer, the diesel engine is noisy, smelly, and dirty — a smoky semi or an old, clapped-out Mercedes sedan are usually what comes to mind. U.S. consumers certainly can’t be blamed for seeing diesels as dirty and inefficient; historically, diesels have produced soot and smoke that have been known to cause health issues to those who inhale it. However, the new generation of diesel-fueled passenger cars are a far cry from the older commercial vehicles and “rolling coal” trucks that have been giving diesel a bad name in the U.S. In fact, some of these new diesel cars are as efficient, and in some ways even more efficient, than their gasoline counterparts.

The most impactful change has been to diesel fuel itself — specifically sulfur content. Sulfur is a natural element of crude oil, from which diesel fuel is derived, and it’s the cause of soot particulates that have been known to cause health issues like lung cancer and bronchitis. Sulfur is the main cause of the smoke and smell of diesel engines, but they’ve been greatly reduced by the creation of Ultra Low Sulfur Diesel (ULSD), which is approximately 97 percent cleaner than traditional diesel fuel. By 2006 the majority of diesel fuel sold in the U.S. was ULSD, and by 2010 all highway vehicles used ULSD.

The new generation of diesel-fueled passenger cars are a far cry from the older commercial vehicles and “rolling coal” trucks that have been giving diesel a bad name.

But 97 percent isn’t 100 percent. There’s still sulfur in diesel fuel, so to further eliminate soot particulates, diesel cars are required to be fitted with a Diesel Particulate Filter (DPF). The filter traps the emitted smoke until a computer determines it needs to be removed. Extra fuel is injected into the combustion chamber, resulting in an increase in heat and oxygen. This triggers a catalyst to burn off the excess soot, thus nearly eliminating soot particulate emissions.

So how do diesels stack up against their gasoline burning counterparts in terms of emissions? There isn’t one clear-cut answer, unfortunately — both engines have their merits and downfalls. CO2 is one of the most important measures for emissions testing: according to the EIA (Energy Information Administration), gasoline engines produce 19.64 pounds of CO2 per gallon of fuel burned; diesel fuel on the other hand has a higher carbon content, resulting in 22.38 pounds of CO2 per gallon burned. But despite the fact that diesels emit more CO2 proportionally, their superior mpg figures allow them to emit less CO2 over their lifetime.

But CO2 emissions are only a part of the equation. Nitrogen oxides (NOx) are a factor in the creation of acid rain and smog, and while new diesel technologies have greatly reduced the amount of NOx emitted from diesel engines, they’re still slightly higher than gasoline engines. Also, by nature gasoline engines do not create the particulates that diesels do, so while the reductions in diesel engines have been impressive, they are still worse than gasoline engines in this regard. It should also be noted that the development of biodiesels from vegetable, animal and recycled fat might be able to further reduce the emissions produced by diesel vehicles, either by being mixed with traditional diesel (somewhat like E85 fuel) or used as a substitute all together. Though promising, the production of biofuels is both expensive and requires using equipment not suited to run on biodiesel fuel itself.

In the end, the argument is moot. Both gasoline and diesel fuel have a negative impact on the environment and each has benefits over the other. The point, really, is that gasoline is no longer the clear-cut winner. Diesel’s advances in the last few years have changed the game. Who knows where diesel cars will be in a decade in terms of cleanliness, efficiency and performance? The inherent efficiency of diesel gives us a great platform to create cars that not only benefit the earth but also the consumer.

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