Since the advent of emissions standards for internal combustion engines for cars, trucks, and other vehicles, emissions of hydrocarbons (HC), carbon monoxide (CO), and nitrogen oxides (NO.sub.x) have markedly declined. This decline has been brought about through the use of a variety of techniques including electronic fuel injection (EFI), electronic (computerized) engine control (EEC), and the use of a variety of catalytic converters to oxidize hydrocarbons and carbon monoxide, and to reduce NO.sub.x to nitrogen. However, increasingly more stringent emissions standards, particularly those promulgated by the California Air Control Board, require still further emissions reductions over extended periods of engine operation.
For example, California's ULEV emissions levels of 0.04 g/mile HC, 1.7 g/mile CO, and 0.2 g/mile NO.sub.x ; or the European Stage III requirements of 0.1 g/Km HC, 1.5 g/Km CO, and 0.1 g/Km NO.sub.x are difficult to meet with current equipment, and yet more difficult to sustain over 50,000 mile and 100,000 mile use periods. During emissions testing, a considerable amount of total emissions occur during engine warm-up after a room temperature soak. During this time period, the emissions-reducing catalysts located in the catalytic converter(s) are largely ineffectual due to the fact that they have not reached a temperature at which significant catalytic activity can be maintained (light-off). Thus, particular attempts have been made to decrease emissions during engine warm-up.
Among the devices used to decrease cold-start emissions are electrically heated catalysts to ensure rapid light-off; exhaust system burners, either EGI or fuel burners; close-coupled catalysts, and cold start spark retard and enleanment (CSSRE) or hydrocarbon traps. The first four of these are means of more rapidly heating the exhaust catalyst, thus reducing the light-off time and resultant cold-start emissions. The last are means of collecting hydrocarbons prior to exiting the system, and reusing them as fuel. In general, the above devices require additional hardware, packaging space, and cost.
For example, U.S. Pat. No. 5,349,816 discloses an internal combustion engine having a closely coupled catalyst to reduce HC and further downstream catalysts for reducing NO.sub.x. The first catalyst is active during warm-up, following which a flapper valve bypasses the first catalyst, directing the exhaust exclusively to the downstream catalysts. The use of the electronically controlled flapper valve adds to component cost as well as introducing additional components capable of failure into the vehicle. The closely coupled catalyst is used during cold-start only, representing a non-economical application of expensive catalyst components.
In U.S. Pat. No. 5,332,554, a two-stage catalytic converter is disclosed, the first converter containing an unspecified pretreatment catalyst, the second converter having multiple monolithic catalyst elements, a first element having deposited on its upstream end a multi-layer platinum/palladium catalyst containing 0.35 g/l to 1.0 g/l (9.9 g/ft.sup.3 to 28.3 g/ft.sup.3) of palladium. Located further downstream is a further oxidizing catalyst. The necessity for a pretreatment catalyst increases the cost and complexity of the system. The U.S. Pat. No. 5,332,554 system is not believed capable of meeting upcoming, more stringent emissions standards.
In U.S. Pat. No. 5,179,059 are disclosed catalysts having improved light-off behavior prepared by impregnating active aluminum oxide, containing customary promoters, with platinum, palladium, and rhodium, followed by associating the precious metal catalysts with up to five times their mass of base metal. Amounts of palladium of c.a. 6.5 g/ft.sup.3 are disclosed.
It has also been proposed to further lower hydrocarbon emissions by injection of air into the exhaust stream by engine-driven or electrically driven air pump. For example, U.S. Pat. No. 5,410,872 discloses supplying air to maintain a stoichiometric increase in oxygen in the range of 0.5 to 1.5 volume percent excess oxygen. However, addition of air during cold-start conditions has the effect of reducing the temperature of the exhaust gases, thus increasing the time for catalyst light-off.
It would be desirable to provide a catalyst system which is capable of meeting ultra-low emissions levels by lowering cold-start emissions without the use of active components such as electrical heaters, fuel burners, flapper valves, and the like. It would be desirable, also, to minimize the light-off of such catalyst systems by employing a cold-start, light-off-assisting engine strategy. It would be further desirable to lower emissions through the optional use of air injection without significantly delaying catalyst light-off. It would be yet further desirable to maintain lowered emissions during warmed-up engine operation by eliminating spikes and break-through of CO and HC during this phase of engine operation.