1. Emission standards
Emission standards are requirements that set specific limits to the amount of pollutants that can be released into the environment. Many emissions standards focus on regulating pollutants released by automobiles (motor cars) and other powered vehicles but they can also regulate emissions from industry, power plants, small equipment such as lawn mowers and diesel generators.
Vehicle emission performance standard: An emission performance standard is a limit that sets thresholds above which a different type of emission control technology might be needed.

2. In the United States, emissions standards are managed by the Environmental Protection Agency (EPA). The state of California has special dispensation to promulgate more stringent vehicle emissions standards, and other states may choose to follow either the national or California standards.
California's emissions standards are set by the California Air Resources Board, known locally by its acronym "CARB". Given that California's automotive market is one of the largest in the world, CARB wields enormous influence over the emissions requirements that major automakers must meet if they wish to sell into that market. In addition, several other U.S. states also choose to follow the CARB standards, so their rulemaking has broader implications within the U.S.

3. European Emission Standards
European emission standards define the acceptable limits for exhaust emissions of new vehicles sold in EU member states.
Currently, emissions of nitrogen oxides (NOx), total hydrocarbon (THC), non-methane hydrocarbons (NMHC), carbon monoxide (CO) and particulate matter (PM) are regulated for most vehicle types, including cars, lorries, trains, tractors and similar machinery,  but excluding seagoing ships and aeroplanes.
For each vehicle type, different standards apply. Compliance is determined by running the engine at a standardised test cycle. Non-compliant vehicles cannot be sold in the EU, but new standards do not apply to vehicles already on the roads. No use of specific technologies is mandated to meet the standards, though available technology is considered when setting the standards. New models introduced must meet current or planned standards,
CO2 emission
Within the European Union, road transport is responsible for about 20% of all CO2 emissions, with passenger cars contributing about 12%.
The target fixed at Kyoto Protocol was an 8% reduction of emissions in all sectors of the economy compared to 1990 levels by
Relative CO2 emissions from transport have risen rapidly in recent years, from 21% of the total in 1990 to 28% in 2004,

4. Toxic emission: stages and legal framework
The stages are typically referred to as Euro 1, Euro 2, Euro 3, Euro 4 and Euro 5 for Light Duty Vehicle standards. The corresponding series of standards for Heavy Duty Vehicles use Roman, rather than Arabic numerals (Euro I, Euro II, etc.) The following is a summary list of the standards, when they come into force, what they apply to, and which EU directives provide the definition of the standard.
Euro 1 (1993):
For passenger cars - 91/441/EEC.
Also for passenger cars and light trucks - 93/59/EEC.
Euro 2 (1996) for passenger cars - 94/12/EC (& 96/69/EC)
For motorcycle /51/EC (row A) - 2006/120/EC
Euro 3 (2000) for any vehicle - 98/69/EC
For motorcycle /51/EC (row B) - 2006/120/EC
Euro 4 (2005) for any vehicle - 98/69/EC (& 2002/80/EC)
Euro 5 (2008/9) and Euro 6 (2014) for light passenger and commercial vehicles - 715/2007/EC

5. Emission standards for passenger cars
Since the Euro 2 stage, EU regulations introduce different emission limits for diesel and petrol vehicles. Diesels have more stringent CO standards but are allowed higher NOx emissions.
Petrol-powered vehicles are exempted from particulate matter (PM) standards through to the Euro 4 stage, but vehicles with direct injection engines will be subject to a limit of 0.005 g/km for Euro 5 and Euro 6.

9. Bharat Stage Emission Standards
Bharat stage emission standards are emission standards instituted by the Government of India to regulate the output of air pollutants from internal combustion engine equipment, including motor vehicles. The standards and the timeline for implementation are set by the Central Pollution Control Board under the Ministry of Environment & Forests.

10. History
The first emission norms were introduced in India in 1991 for petrol and 1992 for diesel vehicles. These were followed by making the Catalytic converter mandatory for petrol vehicles and the introduction of unleaded petrol in the market.
On April 29, 1999 the Supreme Court of India ruled that all vehicles in India have to meet Euro I or India 2000 norms by June 1, 1999 and Euro II will be mandatory in the NCR  by April Car makers were not prepared for this transition and in a subsequent judgment the implementation date for Euro II was not enforced...
In 2002, the Indian government accepted the report submitted by the Mashelkar committee. The committee proposed a road map for the roll out of Euro based emission norms for India. It also recommended a phased implementation of future norms with the regulations being implemented in major cities first and extended to the rest of the country after a few years.
Based on the recommendations of the committee, the National Auto Fuel policy was announced officially in The roadmap for implementation of the Bharat Stage norms were laid out till The policy also created guidelines for auto fuels, reduction of pollution from older vehicles and R&D for air quality data creation and health administration

11. The standards, based on European regulations were first introduced in Progressively stringent norms have been rolled out since then. All new vehicles manufactured after the implementation of the norms have to be compliant with the regulations. Since October 2010, Bharat stage III norms have been enforced across the country. In 13 major cities, Bharat stage IV emission norms are in place since April 2010.
The phasing out of 2 stroke engine for two wheelers, the stoppage of production of Maruti 800 & introduction of electronic controls have been due to the regulations related to vehicular emissions.
While the norms help in bringing down pollution levels, it invariably results in increased vehicle cost due to the improved technology & higher fuel prices. However, this increase in private cost is offset by savings in health costs for the public, as there is lesser amount of disease causing particulate matter and pollution in the air.

12. For 2-and 3-wheelers, Bharat Stage II (Euro 2) was made applicable from April 1, 2005 and Stage III (Euro 3) standards came in force from April 1, 2010

14. Overview of the Emission Norms in India
Idle CO Limits for Gasoline Vehicles and Free Acceleration Smoke for Diesel Vehicles, Mass Emission Norms for Gasoline Vehicles.
Mass Emission Norms for Diesel Vehicles.
Revision of Mass Emission Norms for Gasoline and Diesel Vehicles, mandatory fitment of Catalytic Converter for Cars in Metros on Unleaded Gasoline.
Cold Start Norms Introduced.
India 2000 (Equivalent to Euro I) Norms, Modified IDC (Indian Driving Cycle), Bharat Stage II Norms for Delhi.
Bharat Stage II (Equivalent to Euro II) Norms for All Metros, Emission Norms for CNG & LPG Vehicles.
Bharat Stage II (Equivalent to Euro II) Norms for 13 major cities.
From 1 April Bharat Stage III (Equivalent to Euro III) Norms for 13 major cities.
Bharat Stage III Emission Norms for 4-wheelers for entire country whereas Bharat Stage - IV (Equivalent to Euro IV) for 13 major cities.

15. CO2 Emission
India’s auto sector accounts for about 18 per cent of the total CO2 emissions in the country. Relative CO2 emissions from transport have risen rapidly in recent years, but like the EU, currently there are no standards for CO2 emission limits for pollution from vehicles.

16. Emission Test Cycles
The ECE+EUDC test cycle is performed on a chassis dynamometer. The cycle—also known as the MVEG-A cycle—is used for EU type arroval testing of emissions and fuel consumption from light duty vehicles. The entire cycle includes four ECE segments, Figure 1, repeated without interruption, followed by one EUDC segment, Figure 2. Before the test, the vehicle is allowed to soak for at least 6 hours at a test temperature of 20-30°C. It is then started and allowed to idle for 40s.
Effective year 2000, that idling period has been eliminated, i.e., engine starts at 0 s and the emission sampling begins at the same time. This modified cold-start procedure is also referred to as the New European Driving Cycle or NEDC.
Emissions are sampled during the cycle according the the “Constant Volume Sampling” technique, analyzed, and expressed in g/km for each of the pollutants.

17. The ECE cycle is an urban driving cycle, also known as UDC
The ECE cycle is an urban driving cycle, also known as UDC. It was devised to represent city driving conditions, e.g. in Paris or Rome. It is characterized by low vehicle speed, low engine load, and low exhaust gas temperature.
The above urban driving cycle represents Type I test, as defined by the original ECE 15 emissions procedure. Type II test is a warmed-up idle tailpipe CO test conducted immediately after the fourth cycle of the Type I test. Type III test is a two-mode (idle and 50 km/h) chassis dynamometer procedure for crankcase emission determination.

19. The EUDC (Extra Urban Driving Cycle) segment has been added after the fourth ECE cycle to account for more aggressive, high speed driving modes. The maximum speed of the EUDC cycle is 120 km/h. An alternative EUDC cycle for low-powered vehicles has been also defined with a maximum speed limited to 90 km/h (Figure 3).