AN OVERVIEW AND CONTROL OF ENGINE-OUT EMISSIONS

During 1950s the road vehicles were found to be the principal source of air pollution in the US cities. Carbon monoxide, unburned fuel (hydrocarbons), nitrogen oxides and smoke particulates were identified as the main air pollutants. Now, carbon dioxide has been added to the list of harmful gaseous emissions due to its global warming effect.  Initially, to solve the local air pollution problem during 1960s efforts were mainly focused on reduction of CO from gasoline vehicles and black smoke emissions from diesel vehicles. Another area of priority attention was the prevention of blue smoke emissions caused by excessive consumption of engine lubricating oil which resulted from worn out piston rings, cylinder bore etc.
The first emission control for the spark ignition engines involved adjustments of air-fuel ratio. It was followed by control and adjustment of other engine parameters such as mixture control under idling, acceleration and deceleration, spark timing, precision manufacturing of key engine components such as piston, rings, cylinder head gasket to minimize crevice volume, cams, valves etc. Positive crankcase ventilation (PCV) system was introduced on gasoline vehicles during mid 1960’s to prevent release into atmosphere of hydrocarbon-rich crankcase blow by gases
As the emission standards were tightened, exhaust aftertreatment devices such as catalytic converters were introduced for the first time in 1974-75 and more advanced modifications in engine design and fuel system were employed. Electronic fuel and engine management become necessary during 1980s to meet the then emission regulations. Further advancements in engine, fuel system and emission control technology have emerged in the meantime.  Multi-valve cylinder engines became common and variable valve actuation was applied in production vehicles during late 1980s. In mid 1990s, gasoline direct injection stratified charge (DISC) engines were put into production by Japanese auto-manufacturers. 

Besides all-round advancements in engine technology and aftertreatment systems happening all the time, in the past few years alternative power trains also for vehicles have been developed which provide a higher fuel efficiency in addition to low emissions. Hybrid electric vehicles (HEV) are already in market place. The HEV has IC engine as a primary source of power but employ electric propulsion powered by storage batteries as the main propulsion unit. Fuel cell vehicles using hydrogen as energy source are in an advanced stage of development and they completely eliminate the use of IC engines as a propulsion system

Categorization of Emission Control Techniques
The emission control techniques may be grouped into the following broad categories:

• Engine design and fuel system parameters
• Engine add-ons to enable reduction of engine-out emissions and
• Exhaust aftertreatment