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Meeting Higher Emission Norms Using Lower-Grade Fuel: How Mercedes-Benz Did It

Abhirup is currently a student in his 3rd year of his course for acquiring his B. Tech degree in Automobile Engineering.

This article will break down how Mercedes-Benz managed to meet BS6 norms with their S350d model while still using BS4 fuel.

This article will break down how Mercedes-Benz managed to meet BS6 norms with their S350d model while still using BS4 fuel.

What Are BS4 and BS6 (Bharat Standard 4 and Bharat Standard 6)?

Bharat Stage 4 and Bharat Stage 6 are technically the Indian equivalents of the Euro 4 and Euro 6 emission norms. The Bharat Stage 4 emission norms were recently introduced in India, taking effect from January 1, 2017. All vehicles to be registered after April 1, 2017 were mandated to meet Bharat Stage 4 emission norms.

These norms were made mandatory under short notice in view of the recent rise in pollution, mostly in metropolitan cities such as Delhi NCR, Mumbai, Chennai, and Kolkata.

On November 15, 2017, the petroleum ministry of India decided to bring forward the date of introduction of BS6 norms in Delhi, with effect from April 1, 2018 instead of April 1, 2020. This caused a huge turmoil for the automakers in India, as they were just upgrading to the BS4 norms.

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How Did Mercedes-Benz India Design the S350d Engine to Meet BS6 Norms Without Even Using BS6 Fuel?

The 2018 Mercedes-Benz S350d is the first car developed and made in India and for India. The engineers at Mercedes-Benz extensively tested all qualities of diesel available across the country's 29 states and were pleasantly surprised by the quality of fuel available throughout the country.

The BS4 emission norms currently in effect allow the diesel to only have maximum sulfur content of 50 ppm (parts per molecule). The engineers found out that these levels were around 35–40 ppm, which further fire-powered their research. Now, having checked up with the quality, the next big hurdle was to reduce the emissions to BS6 levels using a BS4 engine running on standard BS4 fuel, as India is skipping BS5 altogether, which makes the problems harder.

To meet the very stringent BS6 norms, an engine requires certain hardware like a Diesel Particulate Filter (DPF) to reduce particulate matter exiting from the exhaust and a Selective Catalyst Reduction (SCR) catalyst to reduce the NOx levels of the exhaust gas. These are the two key exhaust pollutants that need to be controlled to reach BS6 levels. The roadblock to meet these norms was not the technology, however, but the current grade of fuel.

BS6 fuel has a sulfur content of 10 ppm, as compared to BS4 having 50 ppm, which is a huge gap. Because of this, the particulate filter cannot work at all with BS4 fuel, as the higher sulfur content results in more soot. This will eventually result in the clogging up of the filter, which in turn will drastically affect the engine's performance and efficiency and may even damage it.

Mercedes-Benz's Genius Innovations

Therefore, to overcome this hurdle the new engine code-named OM656 uses a special combination of materials in the DPF that reduces clogging. The main genius is the innovative way in which the excessive soot in the filter is cleaned. The accumulated soot has to be burnt off at very high temperatures to clean and regenerate the filter. To do that, Mercedes-Benz devised a precise, electronically controlled injection system that squirts neat diesel into the hot DPF, which then ignites and burns off the residue and keeps the filter functioning optimally. The only downside to this technology is that uses a bit more diesel than a regular BS4 engine.

The OM656 also uses a host of other innovative exhaust gas purification technologies, such as a huge catalytic converter and a Selective Catalytic Converter (SCR) with an Adblue dosing tank. (Adblue is the market name for liquid urea.) While the particulate filter trap reduces particulate emission by as much as 98%, the SCR system injects Adblue into the exhaust flow, which triggers a chemical redox reaction, converting nasty NOx into harmless nitrogen and water.

The injection of urea, or Adblue, has implications for the consumer, who has to keep it topped up for the SCR to do its job seamlessly. Topping it up is as easy as filling coolant. The S350d has a 25-liter Adblue tank with a port next to the fuel filler port. Should the level of Adblue fall lower than safe limits, the car will show a warning. If the warning is ignored and the car is allowed to run out of Adblue, the car will automatically stop running until Adblue is topped up.

All of these technologies and innovations implemented for a cleaner diesel car have made the 2018 Mercedes-Benz S350d a mobile chemical factory, scrubbing the exhaust gases to make them as clean as possible!

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This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.