Understanding the Key Differences between BS IV and BS VI Engine

Jun 09, 2023 By : vikash

The early pandemic period was a testament to the lower pollution levels across the country. It painted a ‘clear’ picture of the impact vehicular emissions have on the environment. But that’s not new information. The ill effects of vehicular emissions were evident as early as the 1990s. So, what is new in the latest BS VI engine?

Over the years, the number of vehicles plying on the roads has increased manifold. And so did the pollution levels! Several reports show that India accounts for 39 out of 50 of the most polluted cities. In an attempt to address the rising levels, several BS norms have been introduced over the years in India, with the latest being BS VI.

Complying with the standards, BS VI OBD I (Phase I) engines were introduced, which came into effect on 1st April 2020 for petrol, natural gas and diesel vehicles and latest BS VI OBD II (Phase II) emission norms implemented from 1st April 2023. But how is it different from its predecessor, the BS IV norms? Let’s discuss the differences in detail in this blog.

The Latest Engines as per Emission Norms

BS VI Engine | Weichai India

Before we dive into the differences, let’s look back on the differences. The Indian Government introduced the Bharat Stage or BS norms to reduce emission levels. In 1999 the government made it mandatory for all vehicles to conform to the BS norms, which were similar to the Euro-I standards.  the first emission control norms introduced in India was India 2000.

In India, the transition from Bharat Stage IV (BS IV) to Bharat Stage VI (BS VI) emission norms has brought about a major shift in the engines used in vehicles. In light of the latest BS VI engine technology, we will explore the key differences between BS VI and its predecessor, the BS-IV engines.

BS IV versus BS VI engines: Primary difference

The primary difference between BS VI and BS IV engines lies in the emission standards they adhere to as well as the fuel they are fed. One of the main distinguishing features of the BS IV is that they were designed to meet the previous generation’s emission norms. The issue with the emission standard was that it allowed for higher levels of pollutants such as carbon monoxide (CO), nitrogen oxides (NOx), particulate matter (PM), and hydrocarbons (HC).

To bring in restrictions on such emissions, BS VI engines have been introduced. These engines comply with much stricter emission standards, effectively reducing the permissible limits of these harmful pollutants. The transition from a BS IV to a BS VI engine aims to significantly reduce the impact of vehicle emissions on air quality and public health.

Reduction in Emission Limits

The primary difference between the two engines is in the emission limits. In comparison to BS IV engines, a BS-VI engine massively cuts down on the emissions released. The Sulphur concentration in BS IV fuel is 50ppm, Indian Govt. reduced significantly five times in BS-VI fuel to 10ppm. Therefore, BS-VI engines are comparatively cleaner than BS-IV.

For instance, the allowable limit for NOx emissions has been reduced by 70% in diesel engines and 25% in petrol engines. Similarly, the limit for PM emissions in diesel engines has been reduced by a staggering 80%. These stricter limits necessitate the use of advanced technologies and components to control and reduce emissions, resulting in cleaner exhaust gasses.

Advanced Exhaust Gas After-treatment Systems

To meet stringent emission standards, a BS VI engine is equipped with advanced exhaust gas after-treatment systems. These systems include catalysts such as diesel particulate filters (DPF), Diesel Oxidation Catalysts (DOC), selective catalytic reduction (SCR) systems and ammonia slip catalyst (ASC). These after-treatment devices are designed to trap and remove particulate matter from the exhaust gasses, while SCR systems use a urea-based solution (AdBlue) to convert NOx emissions into harmless nitrogen and water vapour. These after-treatment systems play a crucial role in achieving the desired emission levels of BS VI engines.

Improved Fuel Quality

Another key difference between BS VI and BS IV engines lies in the quality of fuel they require. BS VI engines are designed to operate optimally with ultra-low sulfur fuel, which contains significantly less sulfur content compared to BS-IV fuel. Now, how does this benefit? Well, the reduction in sulfur content helps prevent the formation of sulfur dioxide during combustion, which is a major contributor to air pollution. Therefore, the implementation of BS VI norms necessitates an upgrade in fuel quality as well, ensuring the compatibility of engines with cleaner and lower sulfur fuels.

Impact on Vehicle Prices

The transition from BS IV to BS VI engines involves significant changes in engine design and the addition of advanced emission control technologies. As a result, the manufacturing costs of vehicles have increased to an extent. Naturally, this increase in costs reflects on the ultimate purchase, making BS VI vehicles generally more expensive compared to their BS-IV counterparts. However, it is important to consider the long-term benefits of cleaner air and improved public health that come with the adoption of these new-age engines.

BS VI Engines by Weichai

The shift from BS IV to BS VI engines has several positive implications for the environment as well as human health. The stricter emission standards and improved after-treatment systems result in significantly reduced levels of harmful pollutants being released into the atmosphere (and of course being breathed in).

Weichai’s engines are BS VI OBD II certified, however BS VI Phase II focuses on monitoring real drive emission hence they adhere to the latest emission norms issued by the Indian Government. With an exceptional build and advanced combustion design, a Weichai BS VI engine is engineered to improve air quality without compromising on efficiency.

BS VI engines by Weichai are already in use in cities across India, with Delhi buses being among them. Weichai is on the journey to cater to improved air quality, especially in densely populated areas where vehicular emissions are a major contributor to pollution and contribute to a greener and healthier India.


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