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With the introduction of stringent emission norms for two wheeler applications, and need to reduce green house gases emission, it is a challenging task to improve the performance of the engine, especially at part throttle conditions. The typical drawback of spark ignition engine is poor part throttle performance and the Indian driving cycle (IDC) predominantly covers these part throttle operations. Contrary to popular belief, carbureted two wheelers with engine capacity up to 250cc are able to meet regulation Euro III with add-ons like secondary valve and catalyst. Vehicles with EFI systems are more expensive and they are not able to provide any fuel economy benefit or improvement in drivability. Under these circumstances, carburetor designers are working continuously to improve its performance. In order to improve the performance, optimization of slow speed to high speed fueling circuit dividing ratio is of immense importance.

Emission standards have been one of the driving forces behind many of the technological changes in the automotive industry. Crankcase breathing of engine is necessary in order to prevent build up of pressure and explosions due to leakage of combustion gas past the piston rings. Breathing means admitting outside air when the pistons move up creating a partial vacuum in the crankcase and releasing the gas when the pistons move down pressurising the crankcase. The gases let out will consist mostly of hydrocarbons. 20% of the total hydrocarbons emitted in a passenger car is due to crankcase gases. By introducing the Positive Crankcase Ventilation system, which is essentially sealing the crankcase and allowing the gas through the engine to be burnt off, the hydrocarbon emissions due to crankcase breathing is totally eliminated. Positive Crankcase Ventilation (PCV) system using variable flow PCV valve is used in most of the automobile gasoline engine applications.

Carburetors are widely used for two wheeler applications in India as well as countries like Taiwan, Indonesia and China. These carburetors are characterized by simple design and low cost. As the emission norms are becoming more and more stringent, matching the carburetors for vehicle application becomes very challenging. Earlier it was widely believed that, for meeting the Euro-3 regulations, even two wheelers with small engine capacity had to employ electronic fuel injection. However, many vehicle manufacturers have successfully developed carburetors for meeting Euro-3 norms by employing catalytic converters without any electronic control. This development has been essentially motivated by the need for low cost. Flow variation is inherent in mass produced carburetors because there are many parts, which contribute to the air-fuel ratio delivered by the carburetor. Carburetors with the mixture control pilot systems were mass-produced and compared for results with air control system.

The simple design and low cost of the carburator makes it an attractive proposition to be used for two-wheeler applications in India and other countries. However, stringent emission standards pose greater challenge in carburator matching for different vehicle applications. The use of electronics for controlling the air-fuel ratio in a carburator with closed-loop control to meet Euro-III standards is the driving force to undertake this project. The goal is to achieve the best catalytic converter efficiency when the air-fuel-ratio is maintained at stoichiometric within a close band. Inherent variations in manufacturing of carburators can cause the air-fuel ratio to drift from stoichiometric and have an adverse effect on catalytic converter efficiency. The most efficient method of maintaining air-fuel ratio within a close band near stoichiometric is through closed-loop control. This method is widely used in passenger cars with fuel injection.

Carburettors are widely used for control of Air/fuel mixture in 2/3 wheeler vehicles in India as well as other countries. Carburettor technology has improved over the years to meet stringent emission requirements as well as fuel economy demands of customer. With the introduction of Euro-III regulations in 2006 and proposed BS IV requirements in India, emphasis is laid on the transient control capability of carburettor. A project was undertaken at Ucal Fuel Systems to understand the transient characteristics of carburettor and develop electronic control to achieve programmable control of Air/Fuel ratio on the driving cycle. Several tests were conducted on carburettor flow test bench and on chassis dynamometer to understand the transient control response and develop control strategy. It is observed that the Air/fuel ratio measured at steady speed trials, have a marked influence on the transient Air/Fuel ratio behavior as measured on drive cycles.