Emissions - What they are and Why they Matter
Diesel exhaust is classified by the WHO (World Health Organization) as "carcinogenic to humans", whom attributes prolonged exposure to an increased risk of lung cancer. Additionally, OSHA (United States Department of Labor Occupational Safety and Health Administration) considers prolonged exposure to diesel particulate matter as a considerable occupational hazard. Growing concerns over public health and environmental impacts have driven the Federal Government to impose increasingly stricter regulations on exhaust emissions from diesel engines. The growing popularity of diesel engines in the United States must also be taken into account as a contributing factor in the development of regulations and pursuit of safer levels of emissions. The concerns, risks, and consequent implementation of technologies are relatively identical from manufacturer to manufacturer.
Carbon Dioxide - Carbon dioxide (CO2) is emitted as the result of the combustion of hydrocarbons. Categorized as a "greenhouse gas", carbon dioxide is considered a leading contributor in the phenomenon known as global warming.
Nitrous Oxides - "Nitrous oxides", abbreviated NOx, includes the compounds NO2 and NO. This is not to be confused with nitrous oxide (N2O). Nitrous oxides are formed as the result of the reaction between oxygen and nitrogen gases during combustion. They are a cause of concern regarding diesel emissions because they are produced in relatively large quantities. Nitrous oxides are partially responsible for the formation of visible "smog" in the atmosphere and have been linked to the onset of asthma and bronchitis as well as worsening symptoms in persons with pre-existing medical conditions.
Diesel Particulate Matter (DPM) - Also referred to as diesel exhaust particles (DEP), these are the minute particles that make up the black soot emitted by diesel engines. These partially burned hydrocarbons have been linked to lung cancer and there are many additional health concerns derived from both short and long term exposure.
6.6L Duramax LML Emissions Components and Technologies
HCI - Hydrocarbon Injector
The HCI is located in the turbocharger downpipe. It is simply a fuel injector used to inject diesel fuel into the exhaust stream during active regeneration (cleaning of the diesel particulate filter). This active regeneration strategy is unique to the LML, as the previous LMM allowed fuel to be injected into the cylinder during the exhaust stroke instead of utilizing a separate injector. However, GM found that cylinder washing was a major concern with this method, as the fuel injected during the exhaust stroke would "wash" the oil from the cylinder and lead to a considerably higher level of fuel dilution in the engine oil. By injecting fuel directly into the exhaust stream, cylinder washing is no longer serious a concern. The transition from the late/post injection strategy to the dedicated "9th injector" technique also allowed the LML to receive a B20 biodiesel compatibility rating.
DOC - Diesel Oxidation Catalyst
The diesel oxidation catalyst, or DOC, converts hydrocarbons and carbon monoxide into water and carbon dioxide through an oxidization reaction. For all intents and purposes, the DOC is the diesel equivalent of a gasoline engine's catalytic converter.
DEF (diesel exhaust fluid) is injected downstream of the DOC. DEF is composed of 32.5% urea, its active ingredient, distilled water, and a very small amount of additives. Because of its urea content, some people call the process "urea injection". DEF is required for the selective catalytic reduction process to occur. The heat of the exhaust converts the DEF into carbon dioxide and ammonia.
SCR - Selective Catalytic Reduction
Once DEF is added to the exhaust, it travels through the SCR catalyst. Here, nitrous oxides (NOx) are converted to nitrogen gas (N2) and water (H2O) by means of a reduction reaction. The SCR system significantly reduces NOx emissions, reducing the frequency of active regeneration cycles and allowing for more freedom in the calibration of the engine.
DPF - Diesel Particulate Filter
Once the exhaust stream has been treated by the DOC and SCR, it travels through the diesel particulate filter, where particulate matter (soot) is trapped and stored. The DPF is cleaned through a process known as regeneration, which is divided into two strategies. Passive regeneration occurs at anytime the vehicle is being operated and the exhaust gas temperature is high enough to burn the particulate matter trapped by the filter. It is a naturally occurring process, meaning that it occurs whenever the conditions are met under normal operation. Active regeneration occurs when the engine senses that the DPF needs to be cleaned as it is approaching maximum capacity. When active regeneration occurs, fuel is injected into the exhaust stream via the HCI to increase the exhaust gas temperature so that the particulate matter can be burned off. Active regeneration dramatically reduces fuel economy since fuel is being used for purposes other than moving the vehicle. The exhaust system features a specifically designed air cooled exhaust tip to reduce the heat of the exhaust gases as they are expelled.
EGR - Exhaust Gas Recirculation
Exhaust gas recirculation is used to reduce NOx emissions. Since nitrous oxides form in oxygen rich, high temperature environments, introducing exhaust gases back into the intake air charge reduces the amount of these compounds that form. Exhaust gas recirculation is not a new technology and has been regularly used on diesel engines for many years.