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Home >>Technical Regulations>>Lambda Measurement
 
An Overview of Test of Lambda from In- Use Gasoline Passenger Cars

The modern day car is a result of several technological improvements that have happened over the years and would continue to do so to meet the performance demands of Exhaust-Gas Emissions, Fuel Consumption, Power Output, Convenience and Safety.

ICRA looked at two states, Maharashtra and Tamil Nadu, which have automotive hubs and had estimated in July 2003 that the quantum of embedded tax amounts to around 12% of manufacturing cost.

I. Engine Management System

Engine Management ensures that driver commands are translated into appropriate engine performance. It regulates all engine functions in such a way that the engine delivers the required level of torque, but fuel consumption and emissions are kept low. The power output is determined by the torque and the engine speed and the torque is generated by the combustion process.

Combustion torque is mainly determined by:
· The available air mass
· The available fuel mass
· The point at which the combustion occurs

The primary function of engine management is to coordinate the various subsystems in order to adjust the torque generated by the engine and simultaneously meet the performance demands.

II. Pollutants Produced by a Car Engine

In order to reduce emissions, modern car engines carefully control the amount of fuel they burn. They try to keep the air-to-fuel ratio very close to the stoichiometric point, which is the calculated ideal ratio of air to fuel. Theoretically, at this ratio, all of the fuel will be burned using all of the oxygen in the air. For gasoline, the stoichiometric ratio is about 14.7:1, meaning that for each unit of gasoline, 14.7 unit of air will be burned. The fuel mixture actually varies from the ideal ratio quite a bit during driving. Sometimes the mixture can be lean (an air-to-fuel ratio higher than 14.7), and other times the mixture can be rich (an air-to-fuel ratio lower than 14.7).

The main emissions of a car engine are:

· Nitrogen gas (N2) - Air is 78-percent nitrogen gas, and most of this passes right through the car engine.

· Carbon dioxide (CO2) - This is one product of combustion. The carbon in the fuel bonds with the oxygen in the air.

· Water vapour (H2O) - This is another product of combustion. The hydrogen in the fuel bonds with the oxygen in the air.

These emissions are mostly begin (although carbon dioxide emissions are believed to contribute to global warming). But because the combustion process is never perfect, some smaller amounts of more harmful emissions are also produced in car engines:

· Carbon monoxide (CO) - a poisonous gas that is colourless and odourless
· Hydrocarbons or volatile organic compounds (VOCs) - produced mostly from unburned fuel that evaporates
· Nitrogen oxides (NO and NO2, together called NOx) - combines with Oxygen at high temperature to form NO

These are the three main regulated emissions, catalytic converters are designed to reduce these emissions.

III. Emission Control

The emission control system in modern cars consists of a catalytic converter, a collection of sensors and actuators, and a computer to monitor and adjust everything. For example, the catalytic converter uses a catalyst and oxygen to burn off any unused fuel and certain other chemicals in the exhaust. An oxygen sensor in the exhaust stream makes sure there is enough oxygen available for the catalyst to work and adjusts things if necessary. Catalytic converter treats the exhaust before it leaves the car and removes a lot of the pollution.

IV. The Control System

The closed loop control system in the cars monitors the exhaust stream, and uses this information to control the fuel injection system. There is an oxygen sensor mounted upstream of the catalytic converter, meaning it is closer to the engine than the converter is. This sensor tells the engine computer how much oxygen is in the exhaust. The engine computer can increase or decrease the amount of oxygen in the exhaust by adjusting the air-to-fuel ratio. This control scheme allows the engine computer to make sure that the engine is running at close to the stoichiometric point, and also to make sure that there is enough oxygen in the exhaust to allow the oxidization catalyst to burn the unburned hydrocarbons and CO.

In India the electronic engine management system is increasingly being used. These system controls the above parameters through electronic sensors fitted at various parts of the engine system. A simple schematic of the same is given below .

As soon as the Accelerator Pedal (4) is pressed the flow of air into the engine takes place. Simultaneously, fuel is injected into the engine which is stored in fuel tank (2) and is fed through the injector (3); the exhaust comes out from the tale pipe (8) after passing through the catalytic converter (7). Before the exhaust passes through

the catalytic converter an oxygen sensor (6) measures the amount of oxygen available in the exhaust and suitable feedback is given to the electronic control unit (5) to control the ratio of air and fuel entering into the engine.

This is known as the closed loop system wherein continuous feed back is given to the inlet of the engine depending upon the quality of exhaust for controlling the air fuel ratio.

V. Idle Emission Test

The emission measurement is done in gasoline cars to test the amount of Carbon monoxide (CO) (in % volume) and Hydro Carbon (HC) (in PPM) present in the exhaust. Additional measurement of Oxygen (O2) and Carbon dioxide (CO2) is also done wherever required using a four gas analyser. This four gas analyser also calculates the value of lambda. Lambda is the calculated value and theoretically it is the ratio of amount of air available for combustion to the amount of air required for combustion to be stoichiometric. The lambda test is generally done at high idle because the catalytic converter achieves the light of temperature and the engine performance is also optimal at high idle. The desired value of lambda is one (1) which indicates that the combustion is perfect.

VI. International Scenario

European Union (EU) directive mandates lambda testing for all 25 EU member states. Although, timing of the testing may vary from one country to another coinciding with the idle emission test. Eg.:

Country First Test Frequency
UK After Three Years Every Year
Ireland After Four Years Once in two Years
Germany Every Year
France After Four Years Once in 2 Years
 

In most countries of Europe, where it can be established that the vehicle manufacturer's recommendations on exhaust emissions are higher than the limits set by Government for failure, then the manufacturers' limits should be the criteria used when deciding whether or not the vehicle passes.

Relevant extract of EU directive

Where the exhaust emissions are controlled by an advanced emission control system, such as a three way catalytic converter that is lambda-probe controlled:

1. Visual inspection of the exhaust system is done in order to check that it is complete and in a satisfactory condition and that there are no leaks.

2. Visual inspection of any emission control equipment fitted by the manufacturer in order to check that it is complete and in a satisfactory condition and that there are no leaks.

3. Determination of the efficiency of the vehicle's emission control system by measuring the lambda value and the CO content of the exhaust gases as per standard procedure or with the procedures proposed by the manufacturers and approved at the time of type-approval. For each of the tests the engine is conditioned in accordance with the vehicle manufacturer's recommendations.

4. Exhaust pipe emissions - limit values

The maximum permissible CO content in the exhaust gases is that stated by the vehicle manufacturer. Where this information is not available the CO content must not exceed the following:

(i) Measurement at engine idling speed: The maximum permissible CO content in the exhaust gases must not exceed 0.5 % vol.
(ii) Measurement at high idle speed (no load), engine speed to be at least 2 000 min-1:
CO content: maximum 0.3 % vol. and for vehicles that have been type-approved according to the limit values. Lambda: 1± 0.03 or in accordance with the manufacturer's specifications.
(iii) For motor vehicles equipped with on-board diagnostic systems (OBD) in accordance with Directive 70/220/EEC (as amended by Directive 98/69/EC and subsequent amendments) Member States may as an alternative to the test specified in item (i) establish the correct functioning of the emission system through the appropriate reading of the OBD device and the simultaneous checking of the proper functioning of the OBD system.

If the lambda value is not within certain bandwidth for e.g. 1+ 0.03 it indicates atleast one or more of the following:
1. The engine controls system is not working properly. Hence a stoichiometric air fuel ratio is not achieved;
2. The exhaust system might have a leakage
3. Since lambda is the calculated value the accuracy of the gas analyser may not be correct;
4. Other factors viz. improper insertion of testing probe in the exhaust or inferior quality of fuel could also result in variation of the lambda system.

Lambda measurement is increasingly being recommended in Asian cities and it is being recommended for two-wheeler fitted with 3-way closed loop catalytic converter.

VII Method of Testing

1. Check visually in the case of 4 stroke spark ignition engines that the emission control system is complete and properly connected and that there are no leaks in the exhaust system.

2. Raise the engine speed to above 2,000 R.P.M. or to a speed specified by the vehicle manufacturer and hold for 30 seconds. Check the HC, CO and Lambda values. If the exhaust emissions are not within the specified limits with the vehicle engine at normal operating temperature raise the engine speed to above 2,000 R.P.M. or to a speed specified by the vehicle manufacturer and hold for 3 minutes. Note HC, CO and Lambda values. Allow the vehicle engine to return to normal idle speed and the exhaust reading to stabilise and note the CO reading.

When checking exhaust emissions the vehicle must be conditioned in accordance with the vehicle manufacturer's recommendations.

VIII Calculation of Lambda

Lambda is a derived parameter. Its calculation requires four components of the exhaust gas to be measured simultaneously viz.:

1. Carbon monoxide (CO %)
2. Hydrocarbons (HC ppm)
3. Carbon dioxide (CO2 %)
4. Oxygen (O2 %).
and the following Brettschneider Equation) is used to arrive at the final value.

IX Indian Scenario

In India, the idle emission test has been introduced since mid eighties. Only CO was tested (% Vol) from gasoline cars and vehicle had to meet a limit of 3.0% for passing the test. The idle CO emission checking process is a world-wide accepted quick and easy method to indicate if the vehicles emission control is within the acceptable performance requirement. A vehicle with improper combustion is in most cases likely to fail the idle CO test. Thus the vehicle could be advised to go to a workshop for suitable maintenance.

This process has been in force for about 20 years in the country. In view of introduction of new technology vehicles, the idle emission limits for were revised from October 2004 including additional measurement of HC.

The introduction of new norms from October 2004 necessitates upgradation / recertification of all the idle emission measuring equipment operating in the country. Presently the credibility of the PUC Centres is not very high and many of them are prone to various levels of malpractices. There needs to be a very strong enforcement system with severe penalties for defaulters, first for the PUC operators and then the vehicle owners. This would significantly enhance the compliance level and the quality of checks.

Introduction of mandatory Lambda measurement for gasoline vehicles fitted with closed loop catalytic converter, would need additional upgradation of the equipment at the PUC Centres. A rpm sensor has to be attached to the machine and all the machines should be certified for rpm and Lambda measurement. A defined test procedure needs to be developed and the PUC operators are to be trained for measuring lambda at high idle.

Several in-use vehicles were checked covering all the major manufacturers. The raw data shows that a lot of variation and errors in lambda values. The variation is mainly due to measurement and calculation error.

· Measurement error includes impractical readings of gas concentrations such as negative values of O2, zero values of CO2. · Calculation error is on account of difference in the value of Lambda as indicated by the analyser and the value given by the formula using the gas concentrations.

Other factors like the use of improper fuel, leakages in testing tubes, etc. can also lead to improper readings.

Lambda testing may be considered for gasoline vehicles fitted with three way closed loop catalytic converter only after addressing several problems relating to measurement of Lambda. As several electronic controls are used in new vehicles the performance of the vehicle does not deteriorate for a much longer period therefore lambda should be checked not more than once every year. But these measurements are very sensitive and therefore, needs to be done by trained mechanics with testing in controlled conditions, so that suitable maintenance measures, if any, could be diagnosed for such vehicles.

In view of the above studies should continue with data generation from the field. The data should be analysed and after discussion with all stakeholders a test procedure should be finalised. The implementation date should take into consideration the, time required to upgrade the machines in the field, training of the operators, declaration time that is required to be given to the vehicle manufacturers for their individual models and time required for issuing the draft and final notification.

 
Vehicle Type CO(%) HC (ppm)
Bharat Stage -II compliant Four wheelers 0.5 750
Four wheelers other than Bharat Stage -II compliant 3.0 1500

The introduction of new norms from October 2004 necessitates upgradation / recertification of all the idle emission measuring equipment operating in the country. Presently the credibility of the PUC Centres is not very high and many of them are prone to various levels of malpractices. There needs to be a very strong enforcement system with severe penalties for defaulters, first for the PUC operators and then the vehicle owners. This would significantly enhance the compliance level and the quality of checks.

Introduction of mandatory Lambda measurement for gasoline vehicles fitted with closed loop catalytic converter, would need additional upgradation of the equipment at the PUC Centres. A rpm sensor has to be attached to the machine and all the machines should be certified for rpm and Lambda measurement. A defined test procedure needs to be developed and the PUC operators are to be trained for measuring lambda at high idle.

Several in-use vehicles were checked covering all the major manufacturers. The raw data shows that a lot of variation and errors in lambda values. The variation is mainly due to measurement and calculation error.

· Measurement error includes impractical readings of gas concentrations such as negative values of O2, zero values of CO2.
· Calculation error is on account of difference in the value of Lambda as indicated by the analyser and the value given by the formula using the gas concentrations.

Other factors like the use of improper fuel, leakages in testing tubes, etc. can also lead to improper readings.

Lambda testing may be considered for gasoline vehicles fitted with three way closed loop catalytic converter only after addressing several problems relating to measurement of Lambda. As several electronic controls are used in new vehicles the performance of the vehicle does not deteriorate for a much longer period therefore lambda should be checked not more than once every year. But these measurements are very sensitive and therefore, needs to be done by trained mechanics with testing in controlled conditions, so that suitable maintenance measures, if any, could be diagnosed for such vehicles.

In view of the above studies should continue with data generation from the field. The data should be analysed and after discussion with all stakeholders a test procedure should be finalised. The implementation date should take into consideration the, time required to upgrade the machines in the field, training of the operators, declaration time that is required to be given to the vehicle manufacturers for their individual models and time required for issuing the draft and final notification.

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