Exhaust Gas Recirculation (EGR)
The exhaust gas recirculation (EGR) system directs a metered portion of exhaust gases through the intake manifold, reducing the concentration of fresh air forced into the combustion chamber. The EGR system helps to drastically reduce the amount of nitrogen oxides (NOx) that the 6.4L Power Stroke emits. Nitrogen oxides are primarily formed when nitrogen and oxygen are subjected to high temperatures, such is the case in the combustion of diesel fuel. Recirculating a portion of the exhaust gas helps to reduce the amount of oxygen entering the combustion chamber as well as reduce peak combustion temperature. Both actions characteristically reduce the formation of NOx during the combustion process.
As a byproduct of reducing the quantity of air available during a given combustion cycle, engine efficiency is reduced along with maximum performance potential. A typical EGR system includes an EGR valve and EGR cooler. The EGR valve is responsible for metering the flowrate of exhaust gas through the recirculation circuit. The amount of exhaust gas recirculated depends on various operating parameters, such as speed, load, driver demand, etc. Before being introduced into the incoming air stream, exhaust gases are cooled via the EGR cooler. The EGR cooler is a heat exchanger that uses engine coolant as a medium to remove heat from the hot exhaust gases prior to recirculation. The 6.4L Power Stroke is one of few engines to feature dual EGR coolers.
Exhaust Aftertreatment (DOC, DPF)
The 6.4L Power Stroke features two exhaust aftertreament processes. The first is performed by the diesel oxidation catalyst, or DOC (1). The DOC is not a new technology and is essentially the diesel equivalent of a catalytic converter. As exhaust gases flow through the DOC, hydrocarbons and carbon monoxide are converted into water vapor and carbon dioxide through an oxidation reaction. On the 6.4L Power Stroke, the DOC is located at the exit of the turbocharger downpipe, upstream from the DPF.
The second aftertreatment process is the filtration of particulate matter across the diesel particulate filter, or DPF (2). Particulate matter is partially burnt hydrocarbons that would otherwise exit the tailpipe as soot. You'll notice that the exhaust tip of a 6.4L Power Stroke lacks the buildup of dingy black residue typical of non-DPF pickups; this is due to the absence of soot being emitted out the tailpipe. The DPF is capable of capturing from 85% to 100% of particulate emissions.
Because the DPF is constantly building with soot, the 6.4L Power Stroke requires routine cleaning. Cleaning is done by a process called regeneration, which can be broken down into two types - passive regeneration and active regeneration. Passive regeneration occurs naturally anytime engine exhaust temperatures exceed the minimum temperature required to burn particulate matter in the filter. Once broken down into smaller compounds and gases, they exit through the tailpipe. Passive regeneration requires no input from the driver nor the engine's control module.
The PCM constantly monitors the differential pressure across the inlet and outlet of the DPF. When the differential pressure reaches a predetermined maximum allowed limit and driving patterns/conditions have impeded passive regeneration from occurring, the PCM will enter the active regeneration mode to clean the filter. When active regeneration is in progress, fuel is introduced into the exhaust stream, combusting and increasing the exhaust gas temperature to the point that particulate matter in the filter burns off. Because of the nature of this process, regeneration is also sometimes referred to as "reburn". Unfortunately, the active regeneration process greatly diminishes the efficiency potential of the engine, as fuel is being injected directly into the exhaust stream and the resulting energy does not contribute to the propulsion of the vehicle.
The 6.4L Power Stroke uses a "post-injection" technique for introducing fuel into the exhaust stream. During this injection strategy, fuel is injected late during the exhaust stroke, ultimately exiting into the exhaust manifold. This technique is somewhat controversial and less desirable than the dedicated "9th injector" strategy due to concerns regarding cylinder washing and fuel dilution of the engine oil.