Spring 2004 Forward to a Friend



Harmful exhaust-gas emissions from gasoline engines became a social issue in 1970, when the U.S. Congress passed the Clean Air Act. It was aimed at cleaning the atmosphere of carbon monoxide, hydrocarbons and nitrous oxides (NOx). This U.S. law obliged manufacturers to reduce exhaust pollutants to 1/10 of their levels within five years.


Engineers looked to catalytic-converter technology as the best way to meet the Clean Air Act's standards. Catalytic converters look like mufflers in the exhaust system and use chemical reactions to clean emissions. Catalysts inside them promote decomposition of harmful chemicals into water and carbon dioxide. These catalysts are expensive, precious metals such as platinum, palladium and/or rhodium.

With engine combustion efficiency nearing its limits, catalytic converters are almost universally accepted, despite their costs. Since 1970, improvements in engine design combined with catalytic devices have reduced exhaust pollutants to 1/1,000 of their level prior to the Clean
Air Act.

Causing Change

Catalytic converters are part of the exhaust system in every Subaru vehicle. Their shapes and sizes vary, depending on the vehicle.

The heart of a catalytic converter is the honeycomb ceramic or metal base coated with a precious-metal catalyst. The holes of the honeycomb are very small – about 90 holes per square centimeter.

"The principle is simple," explains Masanori Sasaki, in charge of catalytic converter development for the Subaru Engineer Division. "All the walls of the honeycomb structure are coated with a substance that induces a chemical change in the harmful substances. When exhaust gas from the engine passes through these holes, almost 100 percent of the harmful substances are cleansed. We are striving to reduce these harmful substances to zero.


Shielding the exhaust system between the engine and the catalytic converter helps the system to retain heat, which enables the catalytic converter to reach operating temperatures quickly.

"We'd prefer not to use the precious metals as catalysts. They are expensive, valuable underground resources. Since no other materials can be used, we have aluminum and cerium in the base to achieve a large cleaning effect with a small quantity of precious metals." Only a few grams of these metals are needed for the converter to function well.

"In addition, we make the walls of the catalytic converter thin to reduce resistance to the flow of the exhaust gas, improving engine performance," says Masanori Sasaki.

Subaru Technology

Catalytic-converter technology is similar throughout the auto industry. The difference in Subaru technology lies in the placement and function of the catalytic converter.

Turbocharged models pose specific problems. "Catalytic converters need heat to function effectively," explains Masanori Sasaki. "Positioning the catalytic converter near the engine allows hot exhaust to pass through it. However, the engine compartment is small. So rather than installing one large catalytic converter, we arranged two or three small ones. Then we achieved a balance between the heat of the exhaust gas and resistance to the exhaust flow."

The turbocharged engine in the Impreza WRX has a low-restriction metal catalytic converter located in the exhaust-gas flow before the turbocharger. Subaru was the first manufacturer in the world with this design.

"Arranging any device in front of the turbine, which revolves extremely fast, involves a remote chance that the turbine could be destroyed, so development was difficult," states Masanori Sasaki. "However, precise design and elaborate tests made it possible.

"Moreover, metal catalytic converters are about 10 times more expensive than ceramic ones. But if we can achieve both the driving pleasure brought about by turbo engines and clean exhaust gas by adopting a metal catalytic converter, the effort is worthwhile.

"Catalytic-converter technology not only provides enjoyable driving performance, it protects the earth's environment. That, in turn, safeguards human life. We want to leave a clean environment for our children and grandchildren."