MAZDA ANNOUNCES WORLD'S FIRST THREE-WAY CATALYST EFFECTIVE FOR LEAN-BURN ENGINES
Mazda has announced that it has found the holy grail of emissions control: a three-way catalyst capable of purifying hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxides (NOx) from lean-burn engines. This long-sought-after technology should lead to further fuel economy improvements, both by increasing the usage of lean-burn technology, and improving the fuel economy of existing lean-burn engines.
Mazda claims that it has confirmed the durability of the new catalyst in laboratory bench-tests and that it intends to use this new three-way catalyst for a lean-burn engine installed in a subcompact model within a couple of years.
This new three-way catalyst employs zeolite as a base material, which is coated with precious metals. The new catalyst can purify the emission gases even in oxygen-rich emission atmospheres which occur during lean-burn combustion.
A conventional three-way catalyst can purify HC, CO and NOx emissions when the lean-burn engine runs on a stoichiometric air-fuel mixture (14.6:1) where oxygen hardly remains in the exhaust. However, with the lean mixture, the exhaust gas becomes oxygen-rich and the excess oxygen interferes with the deoxidization of NOx. This is the reason why a conventional catalyst cannot effectively reduce the NOx from a lean-burn engine. And a stoichiometric mixture is invariably too rich for best fuel economy.
Mazda's newly developed three-way catalyst improves fuel economy of a lean-burn engine, because lean-burn combustion can be applied during any driving condition except full acceleration, while meeting severe Japanese regulations for NOx emissions. As a result, practical fuel economy improves by 5 percent to 8 percent, even when compared to a lean-burn engine with a conventional three-way catalyst (based on Japanese 10-15 mode standard).
The lean-burn engine was developed to enhance fuel economy. However, with a conventional catalyst, it has resulted in minor fuel improvements because lean-burn combustion can be applied only during constant cruising. The reason for such a limited application is that constant cruising under light throttle load is the only driving condition where the emission meets severe NOx regulations.
There is another virtue. There is no need to equip an additional NOx decomposition catalyst which is required for a conventional three-way catalyst, because the three emissions (HC, CO and NOx) can be effectively purified with this new catalyst alone even during lean-burn combustion. This makes the emission control system simple, which will result in a small light-weight system.
As it stands, the Mazda catalyst is claimed to be effective only for small displacement engines, though the company intends to further improve the NOx purifying efficiency of the new catalyst for application to larger lean-burn engines and diesel engines. Though Mazda isn't actively researching two-stroke engines, such a catalyst would also be ideal for these powerplants, and may hasten the advent of two-strokes in small economy cars.