The main purpose of Emissions Regulations is to limit, reduce and control air pollution. Emission control is a function of engine technology, fuel quality and traffic speed (congestion). The objective of clean air is achieved only when all these three come together.
On July 15 the MoSRTH issued the draft notification for the next stage of emissions regulations. According to draft notification GSR 522, effective April 1, 2010, Bharat Stage IV norms will be applicable to 11 metros while Bharat Stage III will be applicable to the rest of the country.
Trucks and buses are predominantly powered by heavy duty diesel engines. Modern diesel engines are lighter and are more powerful, fuel efficient and environment-friendly. Bharat Stage IV diesel engines in 2010 will have particulate matter (PM) emissions that are over 95 per cent lower than the Bharat 2000 (BS 1) diesel engines manufactured in the early part of this decade.
This emission reduction is achieved by a combination of improved diesel engine technology and low sulphur (50 ppm) diesel fuel. Combined with reduced traffic congestion this has the potential to check air pollution dramatically.
Today there are two proven approaches to Euro IV emissions compliance. High pressure common rail fuel injection forms the foundation and is common to either approach. Then, to reduce oxides of nitrogen (NOx) and particulate matter in diesel exhaust, the first option is the selective catalytic reduction (SCR) after-treatment approach and the second is exhaust gas recirculation (EGR) with diesel oxidation catalyst or open filter.
SCR is based on running an optimized combustion in the engine that allows it to operate at more optimal combustion temperatures providing better power, fuel efficiency and lower soot (PM) generation. But this process produces higher NOx. To reduce NOx to levels required by emissions standard, a synthetic urea solution such as Adblue is injected into the exhaust stream. In the presence of a catalyst, Adblue turns into ammonia and carbon-dioxide, which then reacts with the NOx to create nitrogen and water vapour, or 2N2 + 3H2O.
The SCR catalyst also acts to reduce soot to an extent. Thus the SCR process reduces soot or PM in engine combustion and Nox and further PM reduction in the after-treatment system.
Now about cooled exhaust gas recirculation (EGR) + DOC / open filter. Cooled EGR introduces cooled exhaust gas, which is low in oxygen, back into the engine, depriving the combustion event of some of its oxygen. This reduces the combustion temperature and lowers NOx production. The downside is that the lower-temperature diesel combustion is less efficient, so it creates more particulate matter and burns more fuel.
The high soot or particulate matter produced during combustion is reduced using oxidation catalyst or open filter. This process results in significantly increased engine heat rejection. In contrast to the SCR process, this process reduces NOx in the engine and soot in the after-treatment process.
As with all technology comparisons aimed at proving the same solution, there are pros and cons to consider for both the technologies that enable compliance to Euro IV emissions. However, both solutions are proven technologies for markets to choose from.
The accompanying table shows a comparison of both technologies across various parameters.
Manufacturers in Europe have implemented both technologies successfully. European production numbers indicate that vehicles with SCR outnumber those with EGR + DOC/ open filter nearly two to one.
Prior to the launch of Euro IV in Europe, there was concern about availability of Adblue distribution infrastructure. Today nobody even talks about this issue as there are various brands like Dureal, Air1, BlueCat, Greenox, etc., supplied by industrial chemical distribution companies like Univar, Brenntag, J & H Bunn Ltd., Kemira Growhow, etc., in partnership with chemicals manufacturers like BASF and Yara ensuring adequate availability of Adblue.
Euro IV regulations became mandatory in Australia with effect from January last. Even though Australia has very little urea production, and most of its urea requirements are being met through imports, SCR technology has been embraced by the Australian commercial vehicle industry. Industrial chemical distribution companies have taken the lead to install manned and unmanned Adblue pumps at remote truck refuelling sites. These and the tie-ups with OEM networks also have ensured adequate distribution networks for supplying Adblue in Australia.
In India
Effective April 1, 2010, Bharat Stage IV norms will be applicable to 11 metros while Bharat Stage III will be applicable to the rest of the regions in India. This implies that BS IV diesel, with 50 ppm sulphur, will be available in the metros, while BS III diesel, with 350 ppm sulphur, will be available in the rest of the country. As commercial vehicles ply across the country, it will be impossible to ensure that BS IV vehicles fill only BS IV fuel.
The negative impact of repeated filling with BS III diesel can be fairly significant for BS IV engines based on cooled EGR + DOC / open filter technology.
The EGR cooler and other engine components are quite vulnerable to the sulphuric acid nuclei formed during the EGR process. Modern coatings offer a fair degree of protection against this situation. However, long-term durability using diesel with higher sulphur content is suspect.
Multiple SAE papers documented that the effectiveness of the DOC / open filter to reduce soot or particulate matter strongly depends on the soot burden and the engine operating point. Repeated fills with BS III diesel is expected to increase the soot burden significantly, raising doubts about long-term emission stability due to catalyst aging and irreversible adhesion of ash particles.
The durability of the SCR system, on the other hand, is not negatively impacted by the usage of BS III diesel. But the system needs an external reducing agent, Adblue, to function effectively. Key concerns on SCR technology center around availability of Adblue in India and vehicle operator’s effectiveness in refilling it on depletion.
The European and Australian experience suggests that Adblue distribution network is unlikely to be a problem. It is expected that industrial chemical distribution companies will take the lead to distribute Adblue through their channels and / or through the vehicle manufacturer channels.
The bigger issue is to ensure that the vehicle operator takes responsibility for refilling Adblue at required intervals. This can be achieved through a combination of driver warning systems, emissions monitoring system and driver inducement system.
The driver warning system (dashboard indication) informs the vehicle operator that Adblue is low and must soon be replenished. The driver inducement system acts as a back-up by triggering engine shutdown or reducing power to a “limp mode” in case a driver attempts to operate the vehicle without proper reple
nishment. The emission monitoring system, consisting of a NOx sensor at the tail pipe, measures actual NOx emissions. In case the operator uses diluted reducing agents or no reducing agent the sensor will detect high NOx emissions and trigger action by the driver inducement system.
Thus, in the Indian context, the choice of EGR + DOC / open filter or SCR distills down to a question of balancing two opposing risks, i.e., the risk of reduced engine durability and emissions durability on account of high sulphur / adulterated fuel usage for EGR + DOC / open filter versus operator effectiveness in refilling Adblue for the SCR system.
It is expected that the SCR system, coupled with the driver warning system, emissions monitoring system and the driver inducement system, is a lower risk from the standpoint of engine durability and pollution reduction.
In India, fuel cost is said to contribute as much as 50-60 per cent of operating costs, and SCR delivers better fuel efficiency compared to cooled EGR + DOC / open filter. This tips the balance in favour of SCR as the preferred solution in the Indian context, especially when viewed through the lens of lower risk on engine durability, and emissions durability and better fuel economy.
