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Emission Norms and their importance BS II, III, a...

01 February 2018, 11:35 AM

In our previous articles we had dealt with topics such as the working of petrol and diesel engines, suspension systems etc. Another interesting technical element is the emission norm, known in India as the BS norm. These norms control the emission of the car and are of large importance for car manufacturers. Today we take a close look at these norms.


In the earlier days of development of automobiles, the main issue was to get the car moving in the fastest and safest manner. With globalization and improved economic conditions, there has been a huge increase in number of cars and other vehicles coming out of manufacturing plants. With increase in number of vehicles, the amount of exhaust gasses emitted is also increasing exponentially, thus having significant influence on the environment. If this is not controlled, companies may sacrifice the environment for profits, thus harming the environment structurally. It is for this reason that Governments across the world have implemented emission standards to ensure that companies ensure that emissions are not too much. In India, these standards have been captured under the name Bharat Stage, BS for short.

First, it is important to know the contents of exhaust gases and necessity of controlling it before knowing how the Government has decided on implementing different Emission norms

Exhaust Fumes

Emission control on cars and trucks have one purpose i.e. to reduce amount of pollutants and environmentally damaging substances released by vehicles. The consequences of pollutants are dangerous. The air we breathe and water we drink may become contaminated with chemicals that adversely affect our health.

Initially, during the development of automobiles not much attention was given to emissions. However, this changed when winter day in England, smog was spotted. In simple words, SMOG is the name given to the combination of FOG and Smoke, thus standing for dirty air.

However, smog not only appears as dirty air, it is also an irritant to a person’s eyes, nose, and throat. The key elements in smog are Hydro-carbons, NOx (oxides of nitrogen) and Sulphur-oxides. These particles react with each other and cause polluting smog.

Pollutants in Automotive exhaust fumes

The main automotive pollutants – Hydro-carbon, Carbon monoxide, and Oxides of nitrogen - emissions are also present in engine exhaust fumes. These emissions are caused by different reasons.

Hydro carbon emissions are caused largely by unburned fuel from combustion chambers, and can also originate from evaporative sources such as a petrol tank. CO emissions are a by-product of combustion process and they result from incorrect air/fuel mixtures. NOX emissions are caused when cylinder temperatures exceed 1371 degree centigrade; in this case, nitrogen and oxygen combine to form NOx.

All these components are harmful in their own way and thus are subject to government scrutiny.

Emission Norms in India

It was in 1991 that first time emission norms were introduced in India for petrol cars; diesel cars followed in 1992. From then onwards, new cars manufactured in India had to adhere to these standards; their exhaust fumes could not contain more than specified quantity of pollutants.

These standards were compounded with the implementation of mandatory catalytic converters in 1995 for the 4 Metro cities, thus reducing pollution further.

From 2000, India introduced stricter Emission standards modeled on the European ones. This meant the birth of Bharat Norms, with the first set of norms known as Bharat Stage II, followed by BS III, and BS IV (BS I was the earlier, Indian standard).

The tables given below give details of Emission norms at different stages and area of implementation. Here we are focusing on petrol engines, but diesel engines have similar norms also. Interestingly, you will see that initially HC and NOx were considered in one category.


Similar rules as for petrol cars apply to CNG and LPG versions, however the maximum norm for HC is altered. We won’t go into the calculation here.

Emissions Testing

Given the norms, obviously the government needs to have methods to test the exhaust fumes also. For this purpose a gas analyzer is used. A typical exhaust gas analyzer has a long sample hose with a probe at the end of hose. The probe is inserted to the vehicle’s tailpipe. When the analyzer is turned on, an internal pump moves an exhaust sample from tail pipe through the sample hose and the analyzer. A water trap and filter in the hose removes moisture and carbon particles.

The pump forces an exhaust sample through a sample cell in the analyzer. In the sample cell, a beam of infra red light passes through the exhaust sample. Using light spectography, the analyzer then determines the quantities of HC and CO (if the analyzer is a two gas analyzer) or HC, CO, CO2 and O2 if it’s a four gas analyzer. Some analyzers called five gas analyzers can also measure NOX. Nearly all analyzers currently used are four or five gas machines. Most of gas analyzers measure the gases in percentages or parts per million.

The Maximum limits for the measures gases are ser by government for particular vehicle as mentioned in tables above.

Norms Applied to Fuels

Besides norms applied to exhaust fumes (and hence the car’s engineering), fuels are also subject to certain rules. In India, the Ministry of Environment and Forest notified fuel specifications. Herein maximum limits for critical ingredients (like benzene) have been specified for engine fuel.

Automotive Engineering and Emission Norms

Given engine norms, auto manufactures need to do adaptations in their engineering to achieve these norms.

While a discussion on this will be very technical, here are some points that manufacturers will focus on:

-          Temperature in the cylinder as this affects completeness of combustion

-          Recirculating exhaust fumes

-          Better catalytic technology

-          Changes in proportions of fuel and air

At some points, introducing more expensive technology in order to comply with stricter norms will not make sense to manufacturers. It is for this reason that we have seen localized phase out of certain models, such as the Maruti 800.

Non Structural Emission Influences

Besides changes in auto engineering structure, emissions can also vary across cars for situational reasons. The below is an analysis of this variation, listing certain reasons for changes in emissions.

Excessive HC emissions may be caused by

  • Ignition system misfiring
  • Improper ignition timing
  • Excessively lean or rich air/Fuel ratio
  • Low cylinder compression
  • Defective valves, guides, or filters
  • Defective rings, pistons or cylinders
  • Vacuum leaks

All these issues lead to incomplete combustion, thus releasing HC into the air.

Excessive CO emissions are caused by

  • Rich air/fuel mixtures
  • Dirty air filter
  • Faulty injectors
  • Higher than normal fuel pressures
  • Defective system input sensor

Excessive HC and CO emissions caused by

  • Plugged Positive Crank case Ventilation system (PCV system)
  • Excessively rich air/Fuel ratio
  • Stuck open heat riser valve
  • AIR pump inoperative or disconnected
  • Engine oil diluted with gasoline

Higher than normal NOX emissions may be caused by

  • An overheated engine
  • Lean air/fuel mixtures
  • Vacuum leaks
  • Over advanced ignition timing
  • Defective EGR system


Emission norms are controlled by government and the same implemented at manufacturer plant itself. The same applied to fuels also. Hence customers need not worry much about whether their vehicle is BS III version or BS IV version, since any new car they purchase will conform to existing norms. An already registered BS III version car can be used in city where BS IV norms are introduced, until or unless government declares specific restrictions about the same.

That being said, the aim of this article is to give brief information about the pollutants from an automobile perspective and explain the necessity of introducing norms from time to time and causes for difference in emissions.

Do please submit your questions for further information on BS norms and pollutants

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