The Engine Improvements That Are Making Diesel Cleaner

The implementation of a new diesel fuel standard in the US has considerably improved the environmental performance of diesel vehicles. The Environmental Protection Agency (EPA) says that the new ultra-low sulfur diesel (ULSD) fuel has combined with other improved technologies to reduce diesel's particulate matter (PM) emissions by 99%.

Aside from the ULSD fuel itself, associated technological improvements appear in two primary areas: engine efficiency and emissions control. In recent years, says the US Dept. of Energy (DoE), better fuel injection and electronic engine controls have increased the power and efficiency of diesel engines and improved acceleration. Newer engine designs are quieter and smoother and start better in cold weather than in years past.

How are automotive engineers improving on diesel engine design to build on the environmental benefits of ULSD?

Diesel is inherently more energy-efficient than gasoline. According to the DoE, side-by-side, diesel engines "are more fuel-efficient and have more low-end torque than similar-sized gasoline engines." Diesel fuel contains about 10% to 15% more energy than gasoline, meaning that "diesel vehicles often go about 20% to 35% farther on a gallon of fuel than their gasoline counterparts." As German automaker Audi puts it, "One gallon of diesel contains, on the average, 147,000 BTUs, versus 125,000 in the same amount of gasoline."

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A diesel engine burns fuel using compression rather than spark ignition, as in a traditional gasoline engine. The diesel engine produces heat by compressing air in the combustion chamber, then injecting the fuel into the chamber, where it vaporizes and ignites. Diesel engines tend to operate at a higher compression ratio and thus efficiency. Newer diesel engines improve even more on this efficiency by employing electronically controlled direct injection (DI) systems that deliver fuel directly and more quickly to the cylinder. An array of sensors throughout the engine delivers feedback so the control systems can optimize performance.

The efficiency improvements, coupled with the phase-in of ULSD fuel, have given rise to increasing numbers of light-duty diesel vehicles on the road. According to the Diesel Technology Forum automakers have already introduced 47 models of diesel cars, pickups, SUVs, and vans in the US market, with 15 more officially announced for release in the next two years. The organization says there are already more than 7 million diesel cars and light trucks on US roads today.

Such vehicles are eating into the market share of gasoline vehicles and contributing to the effort to meet US air-quality standards. A somewhat longer-term challenge, though, is presented by the older fleet of trucks and buses tied to legacy diesel technologies. Retrofit and engine-replacement programs are being rolled out to improve the efficiency and environmental performance of these older vehicles, but that's admittedly less sexy than designing new, sleek, high-performance diesel vehicles, meaning clean diesel's transition is going to take some time.

Next week: How advanced emissions control systems are making diesel cleaner.

Al Bredenberg is a writer, analyst, consultant, and communicator. He writes about technology, design, innovation, management, and sustainable business, and specializes in investigating and explaining complex topics. He holds a master's degree in organization and management from Antioch University New England. He has served as an editor for print and online content and currently serves as senior analyst at the Institute for Innovation in Large Organizations.