In previous article, emission, we discussed about various emissions and their sources. One of major sources is I.C.Engine. So, here we will discuss the emission control in I.C.Engines.
Emission Control in S.I. Engine:
Spark Ignition Engine emissions mainly contains, Carbon Monoxide, Oxides of Nitrogen and Hydrocarbons.
Methods of Emission Control:
As mentioned before, various ingredients of pollution are result of various processes in the engine and hence various types of emission control measures are used.
1. Controlling Emission of Unburnt Hydrocarbons and Carbon Monoxide:
1. Air Fuel Ratio Method:
Combustion needs both air and fuel in proper amount. Air fuel ratio must be so adjusted that it is neither a rich mixture nor the lean one. So, 17:1 is good air fuel ratio which is result of overall experiments.
When A/F ratio is rich HC remain unburnt or half burnt which releases the dangerous pollutants in the air. When A/F ratio moves towards the leaner side, emissions rises due to quenching of flame or it can be said that flame can not propagate in the engine cylinder.
2. Power Output:
This is factor which affects on HC concentration in the emission and not CO. As power output of engine increases the amount of HC increases in the exhaust. So, light and efficient cars or vehicles are better choice in the case.
3. Engine Speed:
Again, this factor matters only to HC. The amount of HC in exhaust decreases as the engine speed increases. The reason behind it is here. As the engine speed increases it increase the turbulence in cylinder and exhaust which is desirable for complete combustion of HC and hence desirable.
4. Spark Timing:
This is major factor of HC control. Whenever there is retard of 10 degree in spark timing the amount of HC decreases in the emission upto 100 ppm. But this reduction is achieved on the behalf of fuel economy. More retardation cause more decrease in HC but more fuel consumption.
5. Valve Overlap:
6. Intake Manifold Pressure:
7. Compression Ratio:
2. Emission Control of Oxides of Nitrogen (NOX):
1. Manifold Pressure:
An increase in manifold vaccum decreases load and temperature and increases the mass of residual gas. As a result the ignition delay is increased and the flame speed is reduced. Both these factors increase the time of combustion. This would reduce the maximum cycle temperature reducing the NO concentration in the exhaust.
2. Coolant Temperature:
As increase in the coolant temperature results in a reduction of heat losses to the cylinder walls and an increase in the maximum gas temperature. This results in an increase in NO concentration. An increase in deposit thickness causes an increase in CR, reduction in heat losses to the coolant and an increase in NO concentration.
3. Humidity:
The reduction in NO formation caused by an increase in mixture humidity is mainly due to the drop in maximum flame temperature. Test on hydrogen -air and ethylene -air mixtures indicated that 1% of water vapour reduced the flame temperature by 20ºc. This reduces the initial rate of NO production by about 25%.
So, these are the ways to control or reduce emissions from S.I.Engine. In next article we will see the emission control measures for C.I.Engine.
References:
[1] http://www.sae.org/technical/papers/2002-01-2147
[2] http://www.emissioncontrol.com/
[3]http://en.wikipedia.org/wiki/Automobile_emissions_control
