Diesel Emission Components

Like their gasoline engine counterparts, diesel engine vehicles create harmful products during combustion, which must be processed with exhaust aftertreatment components to minimize pollutants and ensure the vehicle complies with exhaust emissions regulations. While the pollutants generated by each engine type - Hydrocarbons (HC), Carbon Monoxide (CO), Nitrogen Oxides (NOx), and Particulate Matter (PM) – are similar, their percentages within the total exhaust stream content vary a great deal because of the different way that combustion is initiated in each engine. Diesel engines produce relatively little HC and CO compared to gasoline engines, but they emit as much as 10 times the PM and a much greater amount of NOx, therefore their exhaust emission control components are unique to the demands of processing these combustion products.

A typical light-duty diesel engine vehicle exhaust emission control system consists of an EGR (Exhaust Gas Recirculation) system, an SCR (Selective Catalytic Reduction) system, a DPF (Diesel Particulate Filter), and a DOC (Diesel Oxidation Catalyst). The EGR and SCR systems substantially reduce NOx emissions, while the DOC reduces HC and CO emissions and the DPF captures particulate matter, generally known as “soot”. NOx forms when nitrogen combines with oxygen at extremely high combustion chamber temperatures. NOx reacts with VOCs (Volatile Organic Compounds) in the presence of sunlight to form harmful smog. Diesel engines generate large amounts of NOx because there must be highly compressed, extremely hot air in the combustion chambers to ignite the atomized diesel fuel when it’s injected.

The EGR system meters exhaust gas into the intake air stream to lower combustion chamber temperature and reduce NOx emissions. Diluting the incoming air with exhaust gas lowers combustion temperature, reducing the amount of nitrogen molecules that bond with oxygen molecules, thereby diminishing the formation of NOx. Exhaust is used because it’s readily available and contains no oxygen because it was consumed during combustion, so exhaust gas will not burn. The exhaust gas occupies space in the combustion chamber that would otherwise be taken up by the intake air. Air is comprised of 21% oxygen, so the exhaust gas reduces the amount of oxygen in the combustion chamber that would be available for combustion and raise the temperature. Therefore, substituting the exhaust gas for the oxygen reduces combustion temperature. In the EGR system a valve is mounted in a passage that connects the exhaust system to the intake. When it opens it allows exhaust to mix with the incoming air charge to the combustion chambers. EGR valve opening is regulated by the engine control computer. Most diesel engines employ an EGR cooler to lower the temperature of the exhaust before it mixes with the air charge to further assist with combustion chamber cooling.

However, while the EGR system decreases NOx output, further reduction is needed, which takes place in the SCR system. The SCR system converts NOx to water and harmless nitrogen using DEF (Diesel Exhaust Fluid) and the SCR catalyst. DEF is a solution of urea and purified water that is contained in an on-board tank. A filler pipe and cap allow the tank to be easily replenished. A pump transfers the DEF via a supply line to a point in the exhaust stream ahead of the SCR catalyst where it’s injected and vaporized by an injector nozzle. The DEF and exhaust flow through the catalyst, where the chemical reaction and conversion takes place. A heater is included in the DEF tank to ensure the solution doesn’t freeze in cold weather.

Diesel PM is comprised of unburned fuel, lubricating oil and compounds formed during combustion. Long term exposure to PM can cause respiratory and cardiovascular disease and lung cancer. The internal channels and honeycomb structure and filter material in the DPF traps most PM as the exhaust passes through the component. However, as the accumulated PM builds up the DPF can become clogged and restrict exhaust flow. To prevent restriction that can cause undesirable backpressure and reduced fuel economy, the DPF must be regenerated by burning off the PM. Regeneration takes place passively, actively, or manually. DPFs contain precious metals that act as a catalyst and when the exhaust reaches a sufficient temperature, passive regeneration occurs, oxidizing the PM. This typically occurs when the vehicle is operating under a load. If operating conditions aren’t optimal for passive regeneration, the engine control computer may initiate active regeneration by injecting fuel into the exhaust stream to raise the temperature. On some vehicles active regeneration can be initiated manually by the driver or by a technician using a scan tool.

The DOC functions like the catalytic converter on a gasoline engine vehicle. Inside the DOC is a substrate which contains precious metals that act as a catalyst, oxidizing the CO and HC to convert them to CO2 and water vapor. No matter what vehicle you’re working on or the type of diesel emission control problems you’re having, we have the replacement components to restore proper performance and emissions compliance. We offer components that are manufactured to original equipment specifications, so when repairs are completed you can count on the performance and emission system operation your vehicle was designed to deliver. In addition to the SCR components described above, we also offer parts that are often needed to achieve a complete repair including DEF tank brackets, insulators, shields, and straps, DEF pump wiring harnesses and O-rings, DEF temperature sensors and more. And you’ll find replacement EGR valves, coolers and related components in our EGR Valves & Components section, and replacement DOCs and DPFs in our Catalytic Converters section.