Diesel engines emit a complex mixture of air pollutants, composed of gaseous and solid material. Diesel particulate matter (PM), also refereed to as soot, consists of small carbon particles that are coated with several compounds which are formed during the engine combustion process and the subsequent travel of these particulates down the exhaust stream.
Nitrogen oxide (NOx) - the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts - is formed in small amounts when fuel is burned at high temperatures and pressures during an engine’s combustion process.
On the federal level, the U.S. EPA has regulated on-highway diesel engine emissions standards through the Clean Air Act since 1970. Specific requirements for medium and heavy duty engine applications were established in 2001, to be phased in over a number of years, gradually reducing the legal levels of emissions for various sizes of diesel engines.
Meeting those requirements has been a daunting challenge.
The main problem areas for diesel engines are NOx and particulate emissions. These two pollutants are traded against each other in many aspects of engine design.
Very high temperatures in the combustion chamber help reduce PM emissions but produce higher levels of NOx. Lowering the peak temperatures in the combustion chamber reduces the amount of NOx produced but increases the likelihood of soot formation.
In order to meet the EPA’s 2007 diesel engine emission standards, engine and truck manufacturers began using high pressure common rail fuel systems which utilize higher pressures to create a finer atomization of the fuel for combustion to reduce emissions.
To help meet the mandated PM emissions reductions, the manufacturers installed diesel particulate filters (DPFs), also known as particulate traps, as part of the exhaust aftertreatment system to catch PM as it passes through. Over time, the soot builds up and the DPF requires periodic service to ensure it functions properly. Servicing involves regeneration (burning) of trapped particulates.
The U.S. EPA 2010 emissions standards, the world’s most stringent diesel engine emissions standards, mandated emissions no greater than 0.2 g/bhp-hr (grams per brake horsepower-hour) for NOx and 0.01 g/bhp-hr for PM. To comply, engine and truck manufacturers settled on two types of emissions control technologies: selective catalytic reduction (SCR) and exhaust gas recirculation (EGR).
EGR, also referred to as in-cylinder EGR and enhanced, advanced or massive EGR, reduces emissions in the engine cylinder.
SCR is an aftertreatment technology that uses a urea-based diesel exhaust fluid (DEF) and a catalyst to significantly reduce nitrogen oxide (NOx) emissions.
There are several major components of the SCR system, all of which are integrated into the exhaust system:
- DEF, often referred to simply by the name of its active component, urea.
- DEF tank to store the fluid.
- DEF dosing system used to deliver the DEF as required.
- Diesel oxidation catalyst (DOC).
- SCR catalyst.
The DOC and DPF, usually referred to as simply the DPF, are mounted together in one container. Exhaust gases pass through the DOC, where chemical process occur, and then through the DPF, where the particulate matter is collected on the filter medium.
In essence, the SCR system works through a chemical reaction triggered by heat. As the exhaust leaves the engine with the NOx and PM pollutants, it travels downstream into the aftertreatment devices, where a fine mist of DEF is injected upon system demand only. The DEF is rapidly hydrolyzed, producing the oxidizing ammonia needed by the SCR catalyst to convert the NOx into harmless levels of simple nitrogen and water vapor that are released into the atmosphere through the vehicle’s exhaust pipe.