BS3 and BS4 are two different emission norms set by the Indian government to regulate the emissions from vehicles. BS3 stands for Bharat Stage 3, while BS4 stands for Bharat Stage 4. Both these emission norms are aimed at reducing the harmful emissions from vehicles and improving the air quality. The basic difference between a BS3 and a BS4 engine lies in the technology used to reduce emissions.

1. Fuel Quality: The primary difference between a BS3 and a BS4 engine is the quality of fuel used. BS4 engines require high-quality fuel with lower sulfur content to meet the stringent emission norms. BS3 engines, on the other hand, can run on lower quality fuel with higher sulfur content.
2. Technology: BS4 engines use advanced engine technology, such as electronic control units, which helps in better control over emissions. The advanced technology also helps in better combustion of fuel, resulting in higher efficiency and lower emissions. BS3 engines do not use such advanced technology, and hence, they emit higher levels of harmful gases.
3. Emissions: BS4 engines emit significantly lower levels of pollutants such as nitrogen oxides (NOx), particulate matter (PM), carbon monoxide (CO), and hydrocarbons (HC) as compared to BS3 engines. BS4 engines have to meet much stricter emission norms as compared to BS3 engines, which has led to the development of advanced emission control technologies.
4. Cost: The cost of a BS4 engine is higher than that of a BS3 engine. The advanced technology and the need for high-quality fuel add to the cost of a BS4 engine. However, the higher cost is justified by the improved efficiency and lower emissions.
5. Availability: BS4 engines were introduced in India in 2010, and since then, all new vehicles have to comply with BS4 norms. However, BS3 engines were phased out in 2017, and it is illegal to sell or register a new vehicle with a BS3 engine in India.

In conclusion, the primary difference between a BS3 and a BS4 engine lies in the technology used to reduce emissions. BS4 engines use advanced technology and require high-quality fuel to meet the stringent emission norms, while BS3 engines emit higher levels of pollutants and can run on lower quality fuel. BS4 engines emit significantly lower levels of harmful gases as compared to BS3 engines and are more efficient. However, the higher cost of a BS4 engine is justified by the improved efficiency and lower emissions.

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