In the face of growing concerns about air pollution and the impact the burning of fossil fuels has on our planet, the Environmental Protection Agency (EPA) enacted the first Clean Air Act in 1963. Amended in 1970 and again in 1993, one of its key provisions regulates vehicle emissions.
Diesel-burning engines are a particular area of focus for this legislation, so diesel engine manufacturers have been forced to find ways to reduce the pollutants they produce via diesel emissions systems (DES).
In this piece, we’ll dive into diesel exhaust systems, providing an overview of how they work and discussing some of the different types of systems you may encounter.
What do Diesel Emissions Systems do?
The purpose of the DES is to reduce the amount of pollutants generated by burning diesel. EPA legislation targets two types in particular:
- Particulate Matter (PM) – A result of fuel that isn’t completely burned in the combustion process, PM contains sulphates, elemental carbon, heavy metals and carcinogenic compounds.
- Nitrogen Oxides (NOx) – When oxygen combines with nitrogen released during combustion, it creates NOx, including nitrogen dioxide and nitric oxide. In addition to potentially causing health problems, NOx also impacts the environment.
Components of the DES
Diesel engines use a combination of the following components and technologies to clean exhaust before its vented into the environment:
The Diesel Oxidation Catalyst (DOC) – Using rare metals like platinum, palladium and rhodium, the DOC, a type of catalytic converter, acts as a filter for exhaust gases, triggering chemical reactions that convert pollutants like carbon monoxide, hydrocarbons and NOx into carbon dioxide, water and nitrogen. Modern catalytic converters operate at up to 90% efficiency, significantly reducing diesel odor and soot.
The Diesel Particulate Filter (DPF) – The diesel particulate filter uses porous walls to trap soot particles, while still allowing exhaust gases to pass through. Its function is to remove any remaining visible particles that made it through the catalytic converter. To prevent clogging, particulate filters undergo a process called regeneration, in which trapped soot is burned off. This can either occur passively via the engine heat or actively, using a heating element or fuel injection.
Selective Catalytic Reduction (SCR) – An advanced active emissions control technology for diesel engines, the SCR injects diesel exhaust fluid into the exhaust before it enters a special catalyst downstream from the DPF. Similar in appearance to the DOC, the SCR uses a catalytic coating to reduce NOx emissions.
Exhaust Gas Recirculation (EGR) – The higher combustion temperatures used to control PM are counteractive to controlling NOx. To cool in-cylinder temps and reduce NOx, the EGR uses a high-pressure loop to recirculate air. Because EGR gases have very low oxygen content, this prevents the development of NOx.
DES Varieties
Aftertreatment systems that clean exhaust gases to meet emissions requirements for diesel engines come in two primary types, one box and standard systems.
One Box System – Developed by Freightliner, the one box emissions control system integrates multiple components like the DOC, DPF and SCR into a single unit. Unlike other emissions systems, the one box does not allow you to remove and clean the DOC and SCR, as these are built in. Only the DPF can be removed for cleaning.
Standard DES – Used in heavy-duty trucks, standard DES requires more space on the vehicle but offers greater cross-manufacturer compatibility. Maintenance and service are usually performed separately, making standard systems less complex to work on.
Become a Diesel Specialist
Diesel engines remain the standard for ground shipping, construction equipment and anywhere else a big, powerful and reliable engine is needed. And there is always a need for skilled professionals to service and maintain them.
If that’s a job that interests you, Automotive Training Center is the perfect place to get started. Our Automotive and Diesel Technology program is designed to give you hands-on experience, so you can start your career with confidence. Contact us today to find out more.
Q&A
What does a diesel emissions system do?
Diesel emissions systems reduce pollutants produced during combustion, particularly particulate matter (PM) and nitrogen oxides (NOx), before exhaust is released into the environment.
What are the main components of a diesel emissions system?
Common components include the Diesel Oxidation Catalyst (DOC), Diesel Particulate Filter (DPF), Selective Catalytic Reduction (SCR), and Exhaust Gas Recirculation (EGR), which work together to reduce soot and harmful gases.
What types of diesel emissions systems are used?
Two common designs are one-box systems, which combine the DOC, DPF, and SCR into one unit, and standard systems, which use separate components that can be serviced individually.
