Nitrous Oxides - Nitrous oxides, typically simplified as NOx, refers to the compounds nitrogen dioxide and nitric acid. These are not to be confused with nitrous oxide. NOx emissions contribute to the development of acid rain and smog. In addition, NOx may worsen pre-existing medical conditions such as asthma and bronchitis. Nitrous oxides are produced when combustion occurs in the existence of nitrogen. Such is the case in the combustion of fuel in a diesel engine, as nitrogen gas is a naturally occurring element in atmospheric air.
Diesel Particulate Matter - Diesel particulate matter, often abbreviated DPM, refers to the emittance of soot following the combustion of diesel fuel. The WHO (World Health Organization) has classified diesel particulates as carcinogenic, meaning prolonged exposure may cause cancer. Additional health concerns, including difficulty breathing, have been linked to both short and long exposure to DPM.
Carbon Monoxide - Carbon monoxide is a colorless, odorless, highly toxic gas produced during the combustion process.
Carbon Dioxide - Carbon dioxide is formed as a result of the combustion of various hydrocarbon compounds that make up petroleum based fuels, including diesel. Carbon dioxide is classified as a "greenhouse gas" and CO2 emissions are believed to be a leading contributor of global warming.
Exhaust Gas Recirculation (EGR)
Exhaust gas recirculation (EGR) is a technique employed to reduce NOx emissions in combustion engines - the process is not specific to diesel engines. On an EGR equipped such as the 6.7L Power Stroke, a metered portion of exhaust gases are recirculated through the intake where they are mixed with the incoming air charge. While the concept may seem counterintuitive, NOx emissions are significantly reduced as a result of 1) reduced oxygen concentration in the combustion chamber and 2) lower combustion temperatures.
The flowrate of exhaust gases in the recirculation circuit is controlled by the EGR valve. Its position is directed by the PCM and varies with various parameters, including load, speed, and manifold pressure. As exhaust gases are relatively hot after leaving the combustion chamber, they are cooled prior to recirculation by means of an EGR cooler. The EGR is a heat exchanger that uses engine coolant as a medium to remove heat from the hot exhaust gases. The 2011 - 2014 MY 6.7L Power Stroke is unique in that it has two dedicated cooling circuits. The primary or high temperature cooling circuit is reserved for engine cooling only. A secondary cooling circuit, referred to by Ford as the low temperature circuit, provides cooling to the EGR cooler, charge-air-cooler, transmission cooler, and fuel cooler. For the 2015 MY, the EGR cooler was moved to the high temperature cooling circuit. Do in part to the effectiveness of the SCR system, the 6.7L Power Stroke uses a lower EGR duty cycle than previous generation engines and therefore the system is not inherently problematic.
The 6.7L Power Stroke features a highly complex exhaust aftertreatment system, including a diesel oxidation catalyst (DOC), diesel particulate filter (DPF), and selective catalytic reduction system. The arrangement of the aftertreatment equipment differs between pickup and chassis cab model trucks, although the operation of the system remains the same.
Diesel Oxidation Catalyst (DOC)
The diesel oxidation catalyst, or DOC, is essentially the diesel equivalent of a gasoline engine's catalytic converter. As exhaust gases flow through the catalyst, residual hydrocarbons and carbon monoxide gas is converted into carbon dioxide gas and water vapor by means of an oxidation reaction.
Diesel Particulate Filter (DPF)
The diesel particulate filter, or DPF, filters particulate matter (soot) from the exhaust stream. Diesel particulate filters can capture up to 100% or particulate emissions, and are responsible for the absence of dingy black buildup on the tailpipes of modern diesel engines such as the 6.7L Power Stroke. The PCM constantly monitors DPF loading, or the quantity of particulates captured in the filter. When the DPF is "full", the the filter is cleaned by a process known as regeneration. Under ideal conditions, removal of the DPF for manual cleaning should never be required.
Regeneration Process (Regen, Reburn)
Regeneration is the process in which accumulated particulate matter is burned off in the DPF. Since diesel soot is comprised of partially combusted hydrocarbons they can be further combusted, at which point the resulting gases are expelled through the tailpipe and into the atmosphere. In order for regeneration to occur, the exhaust gas temperature must be increased to the point that particulates in the filter combust. This temperature is typically between 950° and 1050° F. The three regeneration strategies are passive, active, and manual. By nature of the process, regeneration may also be referred to as "reburn", or just simply "regen".
Passive regeneration is the naturally occurring regen strategy, requiring input from neither the driver nor engine control module. Passive regeneration occurs when operating conditions, most notably exhaust gas temperature, reach or exceed the minimum range in which regeneration can occur. Passive regen often occurs under relatively high loads at constant speed; long periods of highway driving while towing, for example. Passive regeneration rarely occurs with adequate incidence to completely unload a DPF, although it may reduce the frequency of initiated active regeneration cycles.
Active regeneration is the process by which the control module initiates a regeneration cycle. During this period, fuel is introduced into the exhaust stream where it combusts, raising exhaust gas temperatures such that the burning of particulate matter in the filter is facilitated. The 6.7L Power Stroke uses a post-injection technique to introduce fuel into the exhaust stream. This strategy injects diesel fuel late during the exhaust stroke where it is expelled with the outgoing exhaust gases. This method is somewhat controversial in contrast to the "9th injector" technique, as it contributes to cylinder washing and fuel dilution of the engine oil. The frequency in which active regen is initialed will depend on driving patterns - lengthy periods of idling and stop-and-go driving will require more frequent cleaning, while consistent highway driving will reduce DPF loading and negate the need for frequent active regeneration cycles. By nature of injecting raw diesel fuel into the exhaust stream instead of combusting it for means of propulsion, the active regeneration process significantly impacts fuel economy.
Manual or static regeneration is a regen strategy that can be initiated by a technician through the vehicle's on-board diagnostic system. The 6.7L Power Stroke does not have a manual regeneration strategy that can be initiated by the driver. A manual regeneration is essentially an active regeneration cycle that is manually initiated, typically in order to repair a malfunction of the system (i.e. vehicle that failed to regen as needed).
Selective Catalytic Reduction (SCR)
The selective catalytic reduction (SCR) process employs the use of a diesel exhaust fluid (DEF) in order to convert NOx emissions into harmless nitrogen gas and water vapor through a reduction reaction. The system is highly efficient and the use of SCR systems on DPF equipped vehicles greatly reduces active regeneration frequency and therefore positively impacts fuel economy. DEF is a solution comprised of urea (~32%), distilled water, and additives which increase its shelf life. It is highly corrosive and spills should be immediately cleaned from paint. The DEF fill spout on the 6.7L Power Stroke is located next the diesel fuel neck and is covered by a blue cap.
DEF is constantly injected into the exhaust stream through the dosing nozzle located at the front of the SCR device. There, the DEF is mixed with the exhaust stream by means of a spiral mixer. The homogeneous mixture then flows through the catalyst itself, there the reduction reaction occurs.
It is necessary to maintain the DEF fluid level and quality, i.e. you cannot put anything but DEF in the tank. The system is smart and will recognize, for example, if pure water were to be used in place of DEF. If the DEF level is depleted, speed will be limited, and eventually the engine will enter "idle-only" mode. Normal operation will resume once the DEF tank has been refilled. These penalties are required by the United States Environmental Protection Agency (EPA) and are standard operating procedures for all SCR equipped pickups regardless of make.