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 diesels always operate with excess air, they benefit from EGR rates as high as 50% (at idle, where there is otherwise a very large amount of excess air) in controlling NOx emissions. Implementation: Recirculation of exhaust gas by piping it from the exhaust manifold to the inlet manifold (external EGR); an EGR control valve regulates and times the flow
trapping exhaust gas within the cylinder by not fully expelling it during the exhaust stroke (internal EGR) Using a Variable geometry turbocharger(VGT) which uses variable inlet guide vanes to build sufficient backpressure in the exhaust manifold creating a favourable pressure difference for the flow of exhaust into the intake manifold Using a throttle in a turbocharged diesel engine to decrease the intake pressure, thereby initiating EGR flow Drawbacks:
Reduction in combustion efficiency Increase in levels of unburned hydrocarbon, particulates and soot The exhaust gas reduces the effective γ of the gaseous mixture in the cylinder thus reducing the amount of power that can be extracted by the piston during the power stroke SELECTIVE CATALYTIC REDUCTION (SCR) Selective catalytic reduction (SCR) is a means of converting nitrogen oxides with the aid of a catalyst into diatomic nitrogen, N2 and water, H2O. A gaseous reductant, typically anhydrous ammonia, aqueous ammonia or urea, is added to the exhaust gas stream and is absorbed onto a catalyst (oxides of titanium, tungsten or vanadium; zeolites). CO2 is a reaction product when urea is used as the reductant. 4NO + 4NH3 + O2 → 4N2 + 6H2O 2NO2 + 4NH3 + O2 → 3N2 + 6H2O NO + NO2 + 2NH3 → 2N2 + 3H2O Drawbacks: The catalysts used oxidize SO2 to SO3 which is very damaging (acidic properties) The release of unreacted ammonia called Ammonia Slip Very difficult to tune
FORMATION OF PARTICULATE POLLUTANTS AND CONTROL METHODS:
The exhaust of CI engines contains solid carbon soot particles that are generated in the fuel-rich zones within the...