Are High Efficiency Clean Diesel Automobiles a Myth?

By Ramanan Krishnamoorti, UH Chief Energy Officer & William S. Epling, Cullen College of Engineering

The controversy over Volkswagen’s admission that it rigged pollution tests in the United States – causing the tests to show fewer emissions than the company’s highly efficient diesel vehicles produced during normal operations – has raised many questions about the global automaker’s operations. The future of high-efficiency clean diesel automobiles shouldn’t be one of them.

Diesel automobile engines offer inherent benefits in fuel efficiency, allowing drivers to go farther on less fuel. That will be critical in coming decades, as efficiency plays an increasingly important part in our energy future.

Environmental concerns should not be ignored, and technological advances make it possible to take advantage of that increased efficiency without sacrificing air quality.

Here’s why diesel should continue to have a role as a transportation fuel: Diesel automobiles fundamentally operate at higher efficiency than gasoline engines – that is, diesel cars can be driven farther on a gallon of fuel – because the combustion process happens at a higher engine compression ratio and consequently higher engine temperature, resulting in greater efficiency in converting chemical energy in the diesel molecules to mechanical movement.

There is a downside. This higher compression ratio and the combustion propagation in the engine cylinder also produces more pollution than that emitted by gasoline engines, as several unwanted chemical reactions combine with the simple combustion of the diesel and result in the release of soot and nitrous oxide compounds, known as NOx. NOx is a key ingredient of smog and is linked to respiratory illnesses, including asthma.

Still, the efficiency benefits of diesel engines are substantial. A gallon of diesel has about 10 to 15 percent more energy than a gallon of gasoline. Add to that the higher efficiency typical of a conventional diesel engine, and a diesel engine’s fuel efficiency can be as much as 35 percent higher than that of a gasoline engine. Simply put, diesel engines can get 35 percent more miles per gallon than a comparable gasoline engine.

That shouldn’t be ignored as automakers work to meet the higher automobile efficiency standards – known as the Corporate Average Fuel Economy (CAFE) standards – set by the U.S. Environmental Protection Agency and Department of Transportation. First enacted by Congress in 1975 in the wake of the Arab Oil Embargo, the standards are intended to reduce energy consumption by requiring greater fuel economy for the nation’s cars and trucks. Using less fuel also helps to meet clean air goals by lowering carbon pollution.

Current standards call for cars to average 54.5 miles per gallon by 2025, and converting gasoline engines to diesel engines might prove to be the fastest way to get there.  As a point of reference, transportation fuels account for a little more than a quarter of U.S. energy needs.

That, along with recent high prices for transportation fuels, made Volkswagen’s low-emission diesel engine cars resonate with a sector of the car buying public, both in the United States and in Europe, where consumers traditionally have valued fuel efficiency. The German company’s admission that it used software that fooled emissions testers in the United States puts that appeal in jeopardy.

It shouldn’t. Technology makes it possible to have both the efficiency of diesel engines and lower emissions.

Advances over the last four to five decades have reduced the pollution from diesel engine exhaust. These have come from improvements in engine design, cleaner diesel fuel produced by reducing the sulfur content and additional emission controls technology, including diesel particulate filters, exhaust gas re-circulation, selective catalyst reduction and diesel oxidation catalysts.

The emissions controls technologies have lowered the conventional expected engine efficiencies of diesel, as some fuel is used to power the technology and the exhaust from the engine encounters higher back-pressure from the additional emissions control devices.

Emission control technologies also increase vehicle weight, and installation and maintenance can add significant expense.

Nevertheless, as numerous heavy and light duty engine manufacturers have demonstrated, these losses in efficiency are more than offset by increases in efficiencies resulting from improved engine design and strategies for fuel injection.

These technologies have resulted in diesel engines with increased power, acceleration and cold weather performance.

Those improvements in fuel, engine design and emissions controls technology have resulted in remarkably clean diesel vehicles that maintain their efficiency advantage over comparable gasoline engines and are clearly an important part of the portfolio of solutions to develop environmentally friendly and high efficiency automobiles.

Allowing the scandal over Volkswagen’s actions to overshadow the promises of clean diesel engines would be an unnecessary setback in the global push for both energy conservation and cleaner air.

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