What you should know about BS6-compliant diesel engines, before making your next purchase

There has been a steady shift in customer preference towards our TSI powered-vehicles for the unparalleled driving experience they offer.

It would be an understatement to say diesel powertrains are no longer viewed as faithful workhorses, but instead are seen as the glacier-melting root of all environmental evil. With the advent of BS6 emission norms, diesels in India, which still remain a popular choice for customers, got a much needed shot in the arm. There were now in-built safeguards which could, theoretically give the polar bear a stay of execution. However, recent lines of enquiry on automotive forums like Team-BHP, have brought to light the flip side of those safeguards and how they are hindering the functioning of the diesel powertrain.

So, why are BS6-compliant diesel powertrains more prone to issues than BSIV ones?

It all comes down to a technology called the “Diesel Particulate Filter” that fits into all diesel powered exhaust systems to make them BS6 compliant. In order to limit the amount of particulate matter discharged by diesel motors, brands are required to fit it with a “Diesel Particulate Filter” or DPF.

The DPF essentially traps the soot that, prior to the enforcement of stricter emission norms, was freely discharged from the exhaust. Now, however, it gets burnt off, and broken down into much smaller particles, which are then dispersed in the air, rendered comparatively less harmful than before.

However, there is a catch. When driven at low speeds, or short distances, the DPF can get clogged with the particulate matter leading to the car becoming undriveable, because the exhaust is not hot enough to initiate burning off the soot. The issue, while not prevalent, has been common enough among Japanese and European vehicles sold internationally, in countries which had the same emission standards in place as we do now.

It is also far from a recent occurrence, with multiple Indian automotive portals having addressed the potential issue that the DPF might cause when BS6 was enforced. Apart from having a DPF, the filter also has thermal regeneration, which burns off the collected soot.

However, the burning off of the soot is far more conducive to long drives, when enough heat has built-up in the exhaust system to burn off the soot. While BS6 diesels have safeguards to artificially heat-up the exhaust and burn the soot, failure to do so, can cause considerable inconvenience and the potential replacement of a DPF, which is not cheap.

Recent concerns pertaining to the efficacy of diesel SUVs in high-altitude areas could emerge from one of two issues: first, that of the speeds being too low to help burn off the soot. And second, the cold climate, mixed with the staccato throttle inputs not heating the exhaust sufficiently enough to burn off the soot.

What’s the solution?

It goes without saying that brands have taken cognisance of this issue, long before BS6 norms were implemented nationwide. DPFs were introduced as part of Euro 5 emission norms. Once a DPF reaches its maximum capacity, the ECU in a car is programmed to allow the burning of the soot, if not naturally then with additional fuel injected to heat the system.

According to Veluswamy R, Chief of Global Product Development, Mahindra & Mahindra, their models, such as the diesel Thar, among others, have in-built countermeasures. “We select the DPF very carefully after assessing its trapping efficiency and size to avoid clogging and ensure swift calibration deployment, preventing extreme pressure drop and failures”.

Veluswamy also adds that the customers are warned before any serious driving impediment presents itself. “The customer receives four warnings after careful evaluation of the conditions” adding that “Mahindra diesel powertrains go through an intense validation process to ensure we deliver powerful performances across a variety of terrains”.

It would appear, there is no way around avoiding the warnings then to keep the exhaust temperatures warm enough to initiate burning. Either through prolonged driving or through frequent redlining, the latter being inadvisable while the former is unlikely, particularly during short commutes or long stops at traffic junctions.

While most diesel buyers pay the added premium for the long-distances they cover, the DPF issue (which does not occur for most diesel cars, but is prone to clogging) is another element to be taken into consideration, if long-distance commutes are not on the cards. That said, most manufacturers have in-built mechanisms to artificially raise the exhaust temperature and therefore allow the successful burning of fuel. What is advisable is to check about such countermeasures, before making the purchase.

Is BS6 the final nail in the diesel powertrain’s coffin?

While it would be premature to predict the culling of the diesel from the Indian automotive landscape, it would appear that diesels would be the first to go in the road to electrification. Škoda Auto Volkswagen India, which has injected Rs 8000 crore into its massive localisation efforts, is perhaps the biggest automotive entity operating in India, to have discontinued diesels entirely.

However, according to the brand, the reasons for doing so stem, not from issues pertaining to the efficacy of BS6 compliant powertrains, but because it was simply not deemed cost-effective. The technological overhaul required for BS6 fitments, particularly given the new found parity between the everyday costs of petrol and diesel, meant the choice was clear: Focus on electrification, while continuing to sell petrol cars.

According to a spokesperson of the Škoda Auto Volkswagen India group, “There has been a steady shift in customer preference towards our TSI powered-vehicles for the unparalleled driving experience they offer. The narrowing gap between petrol and diesel prices also supports the shift of our volume products to petrol powertrains”.

At present the group manufactures TSI engines in various denominations, some of which provide the maximum power in their respective segments.

Their reasons notwithstanding, Škoda Auto VW India is far from the only one to have discontinued diesels. Several manufacturers, while not having removed diesels from their lineup completely, have discontinued several popular diesel engines which were just not worth the costs and efforts required for conversion. This included the Maruti Suzuki 1.3-litre diesel, Tata’s Revotorq range of diesel motors and Renault’s 1.5-litre diesel.

However, there are other points to be taken into consideration before the next diesel purchase is made, other than rising cost of the fuel in question. At present there are two types of DPF systems, one is pretty standard issue, burning soot directly using heat. The other one, found in more expensive and powerful diesel engines uses a component called Selective Catalytic Reduction system which also reduces nitrogen dioxide from emissions.

Multiple Indian automotive portals have addressed the potential issue that the DPF might cause when BS6 was enforced.

Ignoring the notification and running without the fluid for prolonged periods can cause performance issues with the car, and more importantly cause it to fail emission tests. AdBlue rates can differ and are much cheaper than that of fuel. However, it is another added expense to be considered while making a diesel car purchase. And that doesn’t bode well for the diesel powertrain in the long run.