Controlling tailpipe emissions is a central challenge in modern engine design, with distinct strategies applied to diesel and gasoline engines due to their different combustion principles.

Diesel engine emissions

Diesel engines run on a lean air-to-fuel mixture for high efficiency, but this combustion process inherently produces high levels of two key pollutants: nitrogen oxides (NO_x) and particulate matter (PM). To combat these, a multi-stage after-treatment strategy is required, two primary technologies are used. Exhaust Gas Recirculation (EGR) recycles some exhaust back into the engine to lower combustion temperatures, which thereby reduces NO_x formation. Selective Catalytic Reduction (SCR) is an after-treatment system that injects a reducing agent (like urea) into the exhaust to convert NO_x into harmless nitrogen and water.

Gasoline engine emissions

Gasoline-powered, spark-ignition (SI) engines have a significant advantage in emissions control due to their operation at a stoichiometric (chemically balanced) air-to-fuel ratio. This allows for the use of a highly effective Three-Way Catalyst (TWC), which simultaneously oxidizes harmful carbon monoxide (CO) and unburned hydrocarbons (HC) while reducing NO_x. A TWC can achieve over 90% reduction in these pollutants, meaning significant emissions typically only occur during specific conditions like cold starts or heavy acceleration.

However, the TWC is not a perfect solution. It operates efficiently only within a very narrow air-to-fuel ratio window, making it unsuitable for lean-burn engines. Under certain conditions, it can also produce secondary emissions like ammonia (NH₃) and nitrous oxide (N₂O). Furthermore, as direct-injection (DI) technology has become more common in gasoline engines, particulate matter (PM) emissions have increased, potentially requiring the future use of gasoline particulate filters.