Growing automobile population has resulted in the imbalance of the environment. Air pollution has reached a new level. Thus emissions from automobiles have become an area of concern which needs to be controlled. Major emissions released from the internal combustion engines are unburnt hydrocarbons, nitrogen oxides, carbon monoxide, carbon dioxide and sulphur dioxides. Stringent emission norms have resulted in various emission reduction technologies. In India , presently BS IV Emission norms are imposed in 23 major cities and also there is a possibility of future regulations on carbon dioxide emissions and minimum fuel economy depending on the vehicle category. There are various emission reduction technologies depending on the type of fuel used i.e petrol or diesel. It is the fuel chemical composition and properties which lead to the emission formation during combustion inside the cylinder.
Major emission form the petrol driven engines are unburnt hydrocarbons and carbon mono oxide. In such vehicles a catalytic converter is installed in the exhaust pipe of the vehicle.
Catalytic converter consists of an outer metal casing in which a substrate is fitted. The substrate is made of a porous material in which huge number of cells are cut throughout the length of substrate. The exhaust from the engine passes through these cells and finally to the atmosphere. These cells are internally coated with precious metals like platinum, palladium, rhodium in a certain ratio. These materials act as oxidation and reduction catalyst. The exhaust gas on reaction with these catalysts undergoes oxidation and reduction which results in lower emissions.
In most of the modern cars, a three way catalytic converter is installed. Catalytic converters need to be cleaned at certain time intervals for allowing proper flow of exhaust without any blockage. Also if not cleaned regularly it might result in higher catalytic converter temperatures which reduces catalyst efficiency. For petrol engines, a stoichiometric air fuel ratio of 14:1 is important for least emissions. An oxygen sensor is also installed in converter for monitoring the amount of oxygen in the exhaust. The signal from the sensor is then fed into the ECU(Electric control unit) and finally to the engine to maintain the stoichiometric air fuel ratio.A new technology in petrol engines is to allow petrol engines run with a lean mixture which reduces emissions due to lower fuel content . It is achieved by stratified combustion which means a formation of a cloud of air-fuel near spark plug. This cloud being lean in mixture produces lesser emissions. Secondary air injection is also used to allow certain amount of air to enter into the exhaust pipe and allow further combustion of unburnt hydrocarbons.
Major emissions from the diesel engines are nitrogen oxides, soot. In diesel engines the in-cylinder peak temperature is higher, this results in the formation of nitrogen oxides. Heterogeneous combustion is the main cause for the formation of soot in diesel engines. In order to reduce nitrogen oxides, simplest technology used is EGR (Exhaust gas recirculation) where a part of exhaust gas is introduced into the intake air. This results in lesser oxygen content and hence peak combustion temperature reduces and nitrogen oxide levels come down. This method has a disadvantage of lowering the power output from the engine and thus requires precise control of exhaust gas flow into the intake air.
Selective catalytic reduction is another technique to reduce nitrogen oxides where reductants like urea or ammonia are introduced into the exhaust gases. These reductants get adsorbed on the catalyst where the reaction occurs and nitrogen oxides are converted into nitrogen.Diesel particulate filters are used to trap the soot particles. The filters have micron level openings placed in the direction of exhaust flow which then traps the soot particles. The filters get blocked by continuous usage and needs to be cleaned before any further usage to improve efficiency.
CNG fuelled vehicles are gaining popularity among the people . However there is still lack of CNG supply to the various regions of the country and this has inhibited its growth. Today in Delhi all the buses are running on CNG fuel, such initiatives really help in emission reduction and makes cities pollution free.In order to control the emissions from the vehicles, the Government has planned subsidy for electric vehicles. Electric vehicles represent a clean mode of transport.
In India as of today, Mahindra Reva Company has been providing two vehicles in these segment: Mahindra Reva and Mahindra E2O. The electric technology is definitely among the future mode of transport. Also there have been breakthroughs in the solar car concepts, fuel cell driven cars etc. To make the environment clean we need to go for clean technologies in automobiles.