Reducing diesel emissions

Global warming and the need for greater energy security is fuelling the search for even more the fuel-efficient vehicles. The most readily available option is the diesel engine.

November 4, 2011
by Robert Hayes

While the modern automobile plays a key role in our society, the downside of the combustion engine – emissions of carbon monoxide (CO), hydrocarbons (HC) and oxides of nitrogen (NOx) – came to a head in 1970 with the passage of the Clean Air Act by the US Congress.

Early catalytic converters, used from 1976 to 1979, controlled only the CO and HC, which were oxidized to carbon dioxide and water using a platinum catalyst. The second generation controls CO, HC and NOx. Thanks to this three-way catalyst, a modern gasoline-powered vehicle emits less toxic emissions than a smoker.

But global warming and the need for greater energy security is fuelling the search for even more the fuel-efficient vehicles, and the most readily available option is the diesel engine, which offers much better fuel economy than the gasoline engine. Although diesel suffers from a poor image in North America, the fuel has long been recognized in Europe as a viable alternative to gasoline for passenger cars. Controlling the emissions from a diesel engine is more complicated than it is for a gasoline engine because diesel runs with significantly more oxygen (typically 5% ends up in the exhaust), which makes the removal of NOx a real challenge. Not surprisingly, R&D in this area is focused on NOx control.

Although there are many systems under development or in use, the most popular uses a process called selective catalytic reduction (SCR), which relies on the injection of ammonia into the exhaust. It has several parts. The first is the diesel oxidation catalyst that removes the CO and HC, and converts NO to NO2. Next is the diesel particulate filter that traps and burns small particles of carbon emitted from the engine. Then a small amount of a urea and water solution is injected into the exhaust gas stream. The urea is converted into ammonia, which then reacts with NO and NO2 to produce nitrogen gas. Finally, an ammonia slip catalyst is used to eliminate any excess ammonia present in the exhaust.

These complex catalytic systems have required intensive R&D efforts from key global players such as Umicore, Johnson-Matthey and BASF. Most of the work is done in the US, Germany and the UK, although each of these companies has manufacturing plants around the world, including plants in Canada.

Because government regulations are becoming more stringent, there’s a pressing need to improve converter performance. Computer-aided design and simulation tools provide a better understanding of the physical and chemical processes, and allow virtual prototyping to develop a better product.

In Canada, research funded by the AUTO21 Network of Centres of Excellence is developing sophisticated and efficient computer tools that will realize this goal. The team, which includes members from the Universities of Alberta and Waterloo, and Ecole Polytechnique de Montreal, is also developing chemical kinetic models for the diesel oxidation catalyst to improve converter performance. This work will be used in the development of the next generation of catalytic devices.

The next big R&D challenge will be to design converters for alternative fuels, especially natural gas and its emissions, which are more difficult to destroy.

Robert Hayes is a professor at the University of Alberta and project leader for the AUTO21 Network of Centres of Excellence.