Test of lambda used in petrol passenger cars
Naresh Kukkar
An Overview of Test of Lambda used in Petrol 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. 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 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 petrol, the stoichiometric ratio is about 14.7:1, meaning that for each unit of petrol, 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 colorless and odourless 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 petrol 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 analyzer. This four gas analyzer 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.: |
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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. 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 stabilize 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 %)
IX Indian Scenario In India, the idle emission test has been introduced since mid eighties. Only CO was tested (% Vol) from petrol 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. Lambda testing may be considered for petrol 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. |
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The emission norms for petrol vehicles are as: |
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Naresh Kumar Kukkar
Jalalabad( W), Punjab
nku...@gmail.com
01638-254054
09814706491