Pollution from Diesel Engines and Control
POLLUTION FROM DIESEL ENGINES AND ITS CONTROL
Diesel engines and gasoline engines are the most widely used primary sources of power in automobiles and the sheer number of them makes the issue of emissions from these engines an important one. Table 1 gives a comparison of the amounts of different pollutants in the exhaust of diesel and petrol engines. FORMATION OF NOX AND CONTROL METHODS: The main source of NOx in diesel engine emissions is the oxidation of the nitrogen in the air at the high temperatures in the cylinder during combustion. As the reaction mixture cools during the expansion stroke and exhaust blowdown, the reactions of NO freeze and NO is left in concentrations much higher than its equilibrium concentration. The following reactions are responsible for NO formation: N2 + O2 + N + O N OH = = = NO NO NO + + + N O H
It is evident from the kinetics of NO formation that the formation of NO can be suppressed by reducing the oxygen available which may be done by using a richer air fuel mixture but this is not the case in Diesel engines which are run very lean to ensure that the spontaneous combustion occurs efficiently when the fuel is injected just before TDC. However the rate of NO formation may be checked by implementing Exhaust Gas Recirculation. Furthermore, the amount of NOx in the exhaust gases may also be reduced using catalytic converters or SCR (Selective Catalytic Reduction) technique.
EXHAUST GAS RECIRCULATION (EGR) EGR works by recirculating a portion of an engine 's exhaust gas back to the engine cylinders. This inert exhaust reduces the mass of combustible material in the cylinder, thus reducing the peak temperatures inside the cylinder and consequently lowering NO x production rates effectively. In modern diesel engines, the EGR gas is cooled through a heat exchanger to allow the introduction of a greater mass of recirculated gas. Unlike SI engines, diesels are not limited by the need for a contiguous flamefront; furthermore, since
Diesel engines and gasoline engines are the most widely used primary sources of power in automobiles and the sheer number of them makes the issue of emissions from these engines an important one. Table 1 gives a comparison of the amounts of different pollutants in the exhaust of diesel and petrol engines. FORMATION OF NOX AND CONTROL METHODS: The main source of NOx in diesel engine emissions is the oxidation of the nitrogen in the air at the high temperatures in the cylinder during combustion. As the reaction mixture cools during the expansion stroke and exhaust blowdown, the reactions of NO freeze and NO is left in concentrations much higher than its equilibrium concentration. The following reactions are responsible for NO formation: N2 + O2 + N + O N OH = = = NO NO NO + + + N O H
It is evident from the kinetics of NO formation that the formation of NO can be suppressed by reducing the oxygen available which may be done by using a richer air fuel mixture but this is not the case in Diesel engines which are run very lean to ensure that the spontaneous combustion occurs efficiently when the fuel is injected just before TDC. However the rate of NO formation may be checked by implementing Exhaust Gas Recirculation. Furthermore, the amount of NOx in the exhaust gases may also be reduced using catalytic converters or SCR (Selective Catalytic Reduction) technique.
EXHAUST GAS RECIRCULATION (EGR) EGR works by recirculating a portion of an engine 's exhaust gas back to the engine cylinders. This inert exhaust reduces the mass of combustible material in the cylinder, thus reducing the peak temperatures inside the cylinder and consequently lowering NO x production rates effectively. In modern diesel engines, the EGR gas is cooled through a heat exchanger to allow the introduction of a greater mass of recirculated gas. Unlike SI engines, diesels are not limited by the need for a contiguous flamefront; furthermore, since
References: 1) Engineering Fundamentals of the Internal Combustion Engine – Willard W. Pulkrabrek 2) Internal Combustion Engine Fundamentals – John B. Heywood 3) Wikipedia