Methods used for controlling of gasoline engine emission by engine design modification are:-
1. Use of leaner air-fuel ratios: The carburetor may be modified to provide relatively lean air fuel mixtures during idling and cruise operation. With this modification, idle speed needs to be increased to prevent stalling and rough idle. Fuel distribution is improved by better manifold design, Inlet air heating, raising of coolant temperature and use of electronic fuel injection system.
2. Retarding Ignition timing: The controls are designed to retard the spark timing at idle and providing normal spark advance during acceleration and cruising. Retarding spark reduces NOX. Emission. It also reduces HC emission.
3. Modification of combustion chamber: Modification in combustion chamber is attempted to avoid flame quenching zones, resulting in HC emission. This includes reducing surface to volume ratio, reduced squish area, reduced deal space around piston ring and reduced distance of the top piston ring from the top of the piston.
4. Lower compression ratio: The lower compression ratio reduces the quenching effect by reducing quenching area reducing HC. It also reduces NOX. Emission. Reducing compression ratio results in some loss of power and fuel economy.
5. Reduced valve overlap: Increased valve overlap allows some mixture to escape directly to increase emission level. This can be controlled by reducing valve overlap.
6. Alterations in induction system: The supply of designed air fuel ratio to all cylinders under all operating conditions can be affected by alterations in induction. This includes inlet air heating, use of carburetor with closer tolerances and using special type of carburetors.
This also includes fuel injection in manifold.
Treatment of exhaust gas:-
Catalytic converter Catalytic converter is used in a vehicle (exhaust system) to convert undesirable exhaust gases into harmless gases. As part of the exhaust system, it also helps reduce the noise level of the exhaust. The three-way or selective catalytic converter with lambda closedloop control has proven to be an effective concept for exhaust –gas after treatment. It is capable of providing the required reduction of all three pollutants (NOx, CO and HC) The converter if operated at temperature of approx. 400…800°C, provided the engine is operated with a nearly stoichiometric mixture (λ= 0.99 to 1.) gives maximum conversion efficiency and extended Service life.
Exhaust Gas Recirculation:- EGR System control by the ECM. A pressure sensor monitors the exhaust system pressure. The sensor signals this information to the ECM. The ECM sends the signal to electronic vacuum regulator valve (EVR) to open and close the EGR valve. Thus it controls the amount of exhaust gas recalculated.