To make existing engines cleaner, there are a number of retrofit activities available including: aftertreatment devices such as particulate traps and catalytic converters; replacement of older engines with newer, cleaner engines; and use of cleaner fuel (including ultra-low sulfur) or fuel additives.

Aftertreatment technologies (post-combustion) break down or capture emissions. Two examples of these devices are catalytic converters and particulate traps. Catalysts convert damaging pollutants to less-harmful products. A catalytic converter consists of a metal housing filled with a hard material which is covered with a catalytic compound. The presence of the catalytic converter in the engine exhaust system breaks down the chemicals in the exhaust and reduces harmful pollutant emissions.

Particulate traps collect and burn away particulate emissions. A particulate trap filters or "traps" diesel particulate matter (PM) from engine exhaust until the trap becomes filled to the point that a regeneration cycle is started to burn off the trapped particulate matter. Dust, dirt, soot, smoke, etc. is thereby prevented from being emitted directly into the air.

There are also efforts to improve the emission characteristics of diesel fuel by modifying fuel properties such as sulfur content. Fuel additives may also contribute to reduced NOx and PM emissions and provide engine manufacturers with greater ease in using new emission control technologies.

Retrofit Technology

EPA Verified Technologies

New -- Particulate Matter Control Devices Verified for Additional Engines
The Air Resources Board (ARB) verified the Engelhard DPX and the Johnson Matthey CRT diesel particulate filters for use with most 1994-2001 model year diesel engines in on-road applications. All of these engines are four-stroke, turbocharged, and were certified in California to the 0.1 gram per brake horsepower-hour (g/bhp-hr) particulate matter (PM) emission standard when new.