The 6.7L Cummins, introduced midway through the 2007 model year, was designed to meet strict emissions requirements for diesel engines as outlined by the EPA. Sophisticated tuning and a complex system of components maintain acceptable levels of CO2, NOx, and new for 2007, particulate emissions. An advanced exhaust gas recirculation (EGR), diesel oxidation catalyst (DOC), and diesel particulate filter (DPF) completed the emissions system on 2007 to 2012 model year trucks. Selective catalytic reduction (SCR) became standard in 2013, when Ram/Cummins launched more powerful versions of the 6.7L Cummins (up to 385 horsepower and 850 lb-ft). The SCR system requires the use of diesel exhaust fluid (DEF), a urea based substance, to further cut nitrous oxide emissions (NOx).
Diesel Exhaust Emissions Products
Carbon Dioxide - Carbon dioxide (CO2) is produced naturally from the combustion of hydrocarbons (fuels). It is believed to be a leading contributor to global warming and is categorized as a "greenhouse gas".
Nitrous Oxides - Nitric oxide (NO) and nitrogen dioxide (NO2) are generalized simply as nitrous oxides (NOx). Both are produced during combustion as a result of the reaction between oxygen and nitrogen. These are not to be confused with nitrous oxide, which is identified by the chemical formula N2O. NOx emissions have been connected with the formation of "smog" in the lower atmosphere. Additionally, nitrous oxides may cause or worsen preexisting medical conditions such as asthma and bronchitis.
Particulates - Diesel particulate matter (DPM), diesel exhaust particles (DEP), or simply particulates refers to the microscopic particles that make up a portion of diesel exhaust. They have been classified as carcinogenic by the WHO (World Health Organization). Exposure to the small abrasive particles, which cling to the lining of the lungs, can have both short and long term health effects.
Exhaust Gas Recirculation (EGR)
(2007.5 - current engines)
The EGR system reduces nitrous oxide emissions by strategically introducing and metering exhaust gases through the engine intake and into the combustion chamber. Nitrous oxides are primarily formed in oxygen rich environments subjected to high combustion temperatures. Thus, in theory they are primarily produced under light load conditions when the air-to-fuel ratio tends to be on the lean side. By supplementing a quantity of the intake air charge with spent exhaust gas, the formation of nitrous oxides during the combustion process can be reduced considerably. In the EGR system, exhaust gas is allowed to flow through an EGR cooler, then the EGR valve, and into the intake manifold. The EGR cooler relies on engine coolant to remove heat from the spent exhaust gas. The obvious disadvantage of the EGR system is a small reduction in efficiency and performance - for a quantity of exhaust gas recirculated into the combustion chamber, there is an equal reduction in the amount of clean, combustible oxygen present. In the grand scheme of things, a normally functioning EGR system has only a minor effect on fuel economy and performance.
Diesel Oxidation Catalyst (DOC)
(2007.5 - current engines)
Essentially a catalytic converter for diesels, the DOC is employed to convert carbon monoxide, hydrocarbons, and other exhaust gases into water and carbon dioxide. Palladium and platinum are used as catalysts during the oxidation process. The DOC is integrated into the turbocharger downpipe on the 6.7L Cummins.
NOx Absorption Catalyst (NAC)
(2007.5 - 2012 model year engines)
The NOx absorption catalyst predates the selective catalytic reduction (SCR) system. In ideal conditions, the NAC captures and stores harmful NOx emissions during lean engine operation and converts them into harmless nitrogen and oxygen gas during rich operation through a catalytic process. The NAC requires a short 3-5 second regeneration process to occur every few minutes in order to remove sulfur deposits from the catalyst.
Diesel Particulate Filter (DPF)
(2007.5 - current engines)
The DPF captures particulate matter (soot) from the exhaust stream via a honeycomb shaped element. It is said to reduce diesel particulate and unburned hydrocarbon emissions by up to 90 percent. Two regeneration modes clean the filter when it nears maximum capacity - passive and active regeneration. Passive regeneration occurs naturally anytime the exhaust gas temperature reaches the required threshold to burn the collected particulates from the filter (950+ degrees F). Since the conditions that allow for passive regeneration don't always occur during everyday use, the engine enters the active regeneration state to periodically burn off particulates that have accumulated in the filter. A differential pressure sensor measures the pressure difference between the inlet and outlet of the DPF in order to monitor the filter's capacity. An exhaust gas temperature and oxygen sensor mounted in the inlet of the DPF provide additionally information to the engine control module. During active regeneration, commonly referred to as "reburn" or "regen", diesel fuel is introduced into the exhaust stream by an injection event occurring late in the power stroke and/or through the exhaust stroke.
Early regen strategies proved widely unreliable and DPF clogging issues were common. Most of these concerns have since been alleviated due to a more effective regeneration strategy. The disadvantage of the DPF system is significant reductions in fuel economy, although the introduction of selective catalytic reduction for the 2013 model year has proven to offset some of the efficiency concerns by reducing the frequency of active regenerations.
Selective Catalytic Reduction (SCR)
(2013 - current engines)
Cummins first required SCR for the 6.7 liter for the 2011 model year Ram chassis cab pickup line. The technology later became standard on all 2013+ model year Ram pickups equipped with any version of the 6.7L Cummins turbodiesel. The SCR system requires the use of diesel exhaust fluid (DEF), a substance comprised primarily of urea and water. DEF is injected into the SCR catalyst where a reduction reaction occurs, converting nitrous oxides into harmless water and nitrogen gas. While it adds significant cost to the price of a new vehicle, it reduces both the EGR and DPF regeneration duty cycles. Ideally, a tank of DEF should last between oil change intervals, though towing and stop-and-go driving may require more frequent refills.