CATALYTIC TRANSFORMERS
Before 2002, some diesel truck and bus engines utilized catalytic converters, but currently all diesel engines need to use oxidation converters; some employ both oxidation and reduction converters. A catalyst is a material that, by definition, promotes a chemical process without changing itself. Only since 2007 have truck diesels been equipped with two-stage devices, as opposed to the many years that vehicle catalytic converters have had twostage, three-way converters. These are the two phases:
- Oxidation stage: HC and CO that were not oxidized in the engine cylinder are attempted to be oxidized.
- Reduction stage: When NOx compounds have generated during combustion, attempts are made to reduce NOx back to elemental nitrogen and oxygen.
Stage of Oxidation
The catalytic converter’s oxidizing step works to convert HC and CO found in cylinder exhaust gas into less dangerous H2O and CO2. Oxidation catalysts include both platinum and palladium. They are pricey. Peak temperatures for catalytic afterburning at oxidizing catalysts vary from 800°C to 1,000°C (1,472°F to 1,832°F), occasionally helped by the addition of fresh air to the combustion. It is common for an oxidation converter to glow red while working at night due to the high operating temperatures. An oxidation converter’s red light during nighttime operation is common, especially when engines are working hard.
Reduction Stage
A reduction stage aims to return oxidized nitrogen compounds (collectively referred to as NOx) from the exhaust gas to their elemental forms, nitrogen and oxygen. Since the 1970s, automakers have relied on rhodium as their standard reduction catalyst. It can separate NOx molecules back into their native oxygen and nitrogen forms. Rhodium may, however, only act as a reduction catalyst when the cylinder burn is controlled almost stoichiometrically. This means that the AFR mixture concentration in the converter must be near to stoichiometric in order to employ a rhodium catalytic converter in a low burn diesel engine. OEMs who utilize rhodium-based catalytic converters accomplish this by “dosing,” or injecting extra fuel into the cylinder toward the end of the combustion cycle or into the exhaust stream before to the converter, to create a momentary rich burn condition. Today, small-bore engines without SCR are more likely to use reduction catalysts.
Dosing
Dosing can be carried out in one of two ways:
- By sending a pulse of diesel into the engine cylinder so late in the combustion phase that it cannot completely burn in the cylinder, this is accomplished in common rail (CR) fuel systems.
- Aftertreatment dosing: This technique necessitates the installation of a special injector that sends fuel into the exhaust gas upstream of the reduction catalyst.
Be careful to prevent misunderstandings because the word “dosing” is also used to describe the injection of urea or diesel into the exhaust aftertreatment circuit to regenerate DPFs.
Dosing Control
There are three possible techniques to control the dosage of the aftertreatment circuit:
- By a dosing control module (DCM) connected to the engine’s electronic control module (ECM) through a special bus
- By the engine ECM directly
- Through a dosing control or aftertreatment module with a J1939 bus address.
Dosing management refers to the practice of adding fuel only when necessary to the exhaust gas stream.
Intelligent Dosing Actuators
Actuators for smart dosing immediately sense the emission of exhaust gases, process the information, and adjust the dose. They are linked to the data bus, but their operation is independent of a local ECM or dosing module broadcast. As OEMs work to satisfy the 2013 and 2016 emissions requirements, their utilization will rise.