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In the effort to design reliable diesel engines which meet the strict US Federal Regulations for emissions, considerable progress has been made by engine manufacturers. Particulate emissions are now below 0.25 g/BHPh and after 1994 will be below 0.1 g/BHPh. Diesel fuel has a revised specification limit of 0.05% sulfur as a means to assist diesel engine manufacturers in complying with the 1994 standard. Diesel oxidation catalysts (DOC) have been chosen as another means. A DOC can efficiently oxidize soluble organic particulate matter (SOF) and gaseous hydrocarbons while easily oxidizing SO2 to SO3-the latter being a particulate and undesirable. Selective DOCs have been developed which maintain the activity for SOF and minimize the undesirable SO2 oxidation step. However, performance for gaseous hydrocarbons may be negatively affected.

The performance of platinum/rhodium based TWC catalysts was compared to that of palladium/rhodium based TWC catalysts for the control of emissions from an M-85 fueled vehicle. The catalysts were artificially aged on an engine test stand using a simulated fuel cut aging cycle. The evaluation test cycle was the US FTP-75 test. A REGA 7000 FTIR system was used to specifically monitor methanol and formaldehyde emissions. Double layered palladium/rhodium based TWC catalysts exhibited better methanol and formaldehyde removal as well as superior overall performance. Essentially all of the hydrocarbon emission occurred during cold start. It was demonstrated that significant reduction in formaldehyde and methanol emission could be achieved by presenting a hotter exhaust gas to the converter by use of a small starter catalyst located near the engine.

Environmental concerns and regulations have stimulated the study of applying catalytic emission control to 4-stroke air cooled utility engines of less than 25 Hp. These engines require air/fuel mixtures considerably richer than those of automotive engines, entailing different catalytic solutions. In addition, small utility engines are subjected to a variety of unique operating modes. Factors discussed for this new catalyst system application are space velocity, temperature, test cycle, operating modes, lube oil consumption, engine control systems, engine life, and operating efficiency as well as other factors unique to this engine. An unexpected effect of this catalyst application, after-ignition of unburned exhaust components in a classical diffusion flame, is also discussed. It appears that catalytic emission control of small 4-stroke utility engines can be effective.