Due to stringent diesel emission regulations adopted by countries worldwide,e.g., Tier 4 in USA, BS IV in India, their emission control has become mandatory for every automobile and non-road vehicle manufacturer.
Diesel engines are preferred owing to their higher thermal efficiency, durability and low fuel consumption. But, they have their drawbacks as well by emitting more oxides of nitrogen and particulate matters.
Causes of Diesel emissions:
The main causes can be attributed to the following. Incomplete combustion of fuel resulting in emission of toxic gases and soot(particulate matter)
- Rich air-fuel mixture leading to gas emissions during engine start (cold start)
- Time elapsed for Catalytic converter to function to its potential (only after 4-6 miles of vehicle operation catalytic converter performs to its full potential).
Let us take a look at the diesel emission control methods commonly employed by automobile and non-road vehicle manufacturers.
- Exhaust Gas Recirculation (EGR)
- Selective Catalytic Reduction (SCR)
- Diesel Particulate Filters (DPF)
Exhaust Gas Recirculation (EGR):
EGR system is designed to regulate the amount of NOx (nitrogen oxides) created during engine operation resulting in higher combustion temperatures. When engine operation exceeds 2500°F/1370°C, a higher concentration of NOx is released to the atmosphere.
It works by re-circulating a portion of exhaust gas back to the cylinders, thus decreasing the oxygen content in the intake system, thereby lowering the peak combustion temperature inside the engine cylinder.
Selective Catalytic Reduction (SCR):
SCR is an active emission control that injects a liquid agent (ammonia or urea) into the exhaust line of a diesel engine, which acts as a catalyst. This will in turn reduce Nitrogen oxides into Nitrogen, water and a tiny amount of carbon-di-oxide. The liquid injected is the reducing agent. Particulate matter is controlled within engine cylinder
In commercial trucks conforming to BS-IV (Also Euro IV & V) in India, SCR is used. It consists of Ad Blue tank in which a liquid agent is injected to reduce the NOx and soot resulting in regulating emissions from tailpipe.
Both EGR and SCR are NOx regulating systems.
EGR vs. SCR:
|EGR in first place doesn’t allow the engine to produce NOx||SCR treats or reduces the NOx produced from the engine.|
|Diesel combustion is less efficient due to decreased combustion temperature.||More optimized combustion resulting in controlling of soot/particulates.|
|Decreases engine fuel efficiency / durability||Can enable better fuel efficiency / power|
|No liquid storage required & no decrease in payload||Heated storage space required as urea freezes @12°C & additional hardware required|
Diesel Particulate Filter (DPF):
A DPF is effectively a soot trap in the exhaust system, mostly an after treatment device filtering out black smoke (seen on old cars & trucks) under heavy acceleration. In simple terms, it uses a filter to restrict gas flow and burns off particulate matter in the filter during the start up of an engine. As more and more soot particles get trapped in the filter it creates back pressure resulting in less power and reduced engine efficiency.
To overcome this, regeneration of DPF i.e. cleaning the filter takes place by two methods.
- Passive regeneration (Cleans itself when sufficient exhaust temperature is present enabling continuous oxidation – Vehicle in continuous operation)
- Active regeneration ( When sufficient temperature in snot available, a small quantity of fuel is injected into the combustion chamber enabling hotter gases to flow through DPF filter)