Why are BS6 motorcycles getting heavier?

By Shivank Bhatt | on April 10, 2020

A simple and straightforward answer is a bigger catalytic converter. But why's that needed? Now, that needs some explaining...

BS6 vehicles have been coming thick and fast in the Indian market, lately. We believe you already have a fair bit of idea on what BS-VI stands for. Still, if you’ve any doubt, here’s a quick description – BS-VI norms are the latest regulatory norms for vehicular emissions in India, and they replace the BS-IV regulations. Put simply, the emission norms for new vehicles sold in India have become stricter and tighter than ever before.

In order to make their products meet the new & stringent emission regulations, automakers had to carry out some small yet significant tweaks to the powertrain of their vehicles. On the surface, it may appear simple, but in reality, there’s a heck of a lot more to it. The BS-VI norms have turned out to be a nightmare for a lot of manufacturers, especially for diesel carmakers and producer of commuter motorcycles. And that is because the transition from BS-IV to BS-VI has resulted in a huge price hike for diesel cars and 100-125cc bikes. The prices of petrol cars or premium motorcycles have also risen, but they remain relatively low. The prices have gone up due to the incorporation of newer emission control technology, of course. 

In this piece, though, we’re going to talk about a very small but significant factor about BS-VI motorcycles, which most of you might have also noticed. You see, BS-VI motorcycles – especially the premium and more powerful ones – have gotten significantly heavier than their respective BS-IV counterparts. For instance, the Royal Enfield Himalayan BS6 is 5kg heavier than before, the KTM 390 Duke and TVS Apache RR 310 have gained 4kg each with their BS-VI updates, while the Bajaj Dominar 400 BS6 and Hero XPulse 200 BS6 are now heavier by around 3 kilos over their previous versions. And these are just a few examples, if you check other bikes, you’ll notice a similar trend.

Where’s the extra weight coming from?

The straightforward answer is a bigger catalytic converter or ‘cat-con’.

2020 TVS Apache RR 310 Rear Side

Why is a bigger cat-con needed?

To understand this, you’ve to understand new BS-VI emission guidelines in details. It’s a bit complicated but not as complex as quantum physics.

First off, let’s get the fundamentals right. Exhaust gases from a motorcycle engine, or any other petrol vehicle, contain three toxic gases – Oxides of nitrogen (NOX), Hydrocarbons (HC), and Carbon Monoxide (CO).

In BS-IV regulation, the limit for NOX emission was 0.20g/km, and for CO, it was 1.970g/km. And there was a combined limit for NOX+HC emissions – either 0.40g/km or 0.20g/km. The NOX+HC limit for a motorcycle depended on its evaporative emissions (petrol evaporating from the fuel tank or fuel line). The evaporative emissions were decided based on a product's compliance with 2g/test and 6g/test. For instance, if a motorcycle met 2g/test evaporative emissions, it’d have to follow the less stringent NOX+HC limit of 0.40g/km, whereas if it’d comply with 6g/ test, the NOX+HC limit will get tighter at 0.20g/km. The combined NOX+HC parameter also allowed manufacturers a little bit of freedom to increase or reduce the quantity of one pollutant at the expense of others, so as to meet the norms.

Now, in BS-VI norms, things have gotten more serious. The NOX limit has been reduced by up to 85%, and it’s down to 0.06g/km! CO emission limit is down to 1g/km, and there isn’t a combined NOX+HC standard anymore. Instead, there’s a new independent HC emission limit that’s been introduced, and it’s set at 0.10g/km. Under the new BS-VI regulation, the evaporative emissions (mainly hydrocarbons) have also been set at 1.5g/test, and they don’t allow the ‘flexibility’ to be included with tail-pipe NOX emissions, which used to be the case with BS-IV emissions. And this is precisely why a bigger cat-con is incorporated, as it cuts down emissions more efficiently.

And with that out of the way, it brings us to the next question…

How does a bigger catalytic converter help reduce emissions?

A catalytic converter is basically an exhaust control device that sits between the engine and the exhaust pipe/muffler. Its job is to reduce the volume of toxic gases (CO, NOX, HC) coming from the engine by converting them into less harmful pollutants (CO2, Nitrogen, Oxygen, and water or steam) before they get released out in the atmosphere. 

This is basically done by a series of chemical reactions using 'catalysts, which are usually elements like platinum, rhodium, and palladium. These catalysts are coated on a honeycomb structure inside the cat-con. The exhaust gases coming out from the engine pass through the vents of the said honeycomb structure and two chemical reactions take place – Oxidation (adding oxygen, as in from CO to CO2 or HC to H2O) and Reduction (removing oxygen, like NO to N2). The job of the catalysts here is to simply accelerate the rate of these two chemical reactions. So, if there are more layers of catalysts, the reaction will be faster, and the amount of NOX, CO or HC coming out of the exhaust of a motorcycle will be lower. And that explains the need for a bigger cat-con in BS-VI motorcycles.

But is a bigger catalytic converter enough?

To meet the tighter norms, there are more factors involved, which mainly requires better combustion. The hydrocarbons found in an engine's emission (fumes before they meet the cat-con) is basically the unburnt fuel that escapes the engine during combustion. So, if you improve the combustion process, you automatically reduce HC emissions to a great extent. This is where fuel-injection and technologies like variable valve timing come into the picture. A cat-con is also virtually ineffective during cold starts as they work at very high temperatures (above 300 degrees), and that means during the first 3-5 minutes of idling or riding, the emissions are generally higher. Since the BS-VI regulations now require the vehicles to pass real-world tests, cold-starts are also a part of the testing parameter, so a cat-con, in this case, won't alone solve the problem.

What are the disadvantages of putting a bigger cat-con?

A bigger catalytic converter increases the exhaust backpressure, which, in turn, affects the performance and efficiency of the engine. In case you were wondering, it is one of the prime reasons for the slight drop in power or fuel efficiency figures in some BS-VI motorcycles.

Also read,

Royal Enfield Himalayan BS-VI Review 

2020 TVS Apache RR 310 Review

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