Exhaust Smoke and Other Emissions:
Smoke and other exhaust emissions such as oxides of nitrogen, unburned hydrocarbons, etc. are nuisance for the public environment. With increasing emphasis on air pollution control all efforts are being made to keep them as minimum as it could be. Smoke is an indication of incomplete combustion. It limits the output of an engine if air pollution control is the consideration.
Emission Formation Mechanisms: (S.I) This section discusses the formation of HC, CO, Nox, CO2, and aldehydes and explains the effects of design parameters.
HC emissions are various compounds of hydrogen, carbon, and sometimes oxygen. They are burned or partially burned fuel and/or oil. HC emissions contribute to photochemical smog, ozone, and eye irritation.
There are several formation mechanisms for HC, and it is convenient to think about ways HC can avoid combustion and ways HC can be removed; we will discuss each below. Of course, most of the HC input is fuel, and most of it is burned during “normal” combustion. However, some HC avoids oxidation during this process. The processes by which fuel compounds escape burning during normal S.I. combustion are:
1. Fuel vapor-air mixture is compressed into the combustion chamber crevice volumes.
Fuel compounds are absorbed into oil layers on the cylinder liner. 3. Fuel is absorbed by and/or contained within deposits on the piston head and piston crown. 4. Quench layers on the combustion chamber wall are left as the flame extinguishes close to the walls. 5. Fuel vapor-air mixture can be left unburned if the flame extinguishes before reaching the walls. 6. Liquid fuel within the cylinder may not evaporate and mix with sufficient air to burn prior to the end of combustion. 7. The mixture may leak through the exhaust valve seat. (ii) Carbon Monoxide Formation of CO is well established. Under some conditions, there is not enough O2 available for complete oxidation and some of the carbon in the fuel ends up as CO. The amount of CO, for a range of fuel composition and C/H ratios, is a function of the relative air-fuel ratio. Even when enough oxygen is present, high peak temperatures can cause dissociation – chemical combustion reactions in which carbon dioxide and water vapor separate into CO, H2, and O2. Conversion of CO to CO2 is governed by reaction CO + OH ↔ CO2 + H Dissociated CO may freeze during the expansion stroke. (iii) Oxides of Nitrogen Nox is a generic term for the compounds NO and NO2. Both are present to some degree in the exhaust, and NO oxidizes to NO2 in the atmosphere. Nox contributes to acid rain and photochemical smog; it is also thought to cause respiratory health problems at atmospheric concentrations found in some parts of the world. To understand Nox formation, we must recognize several factors that affect Nox equilibrium. Remember that all chemical reactions proceed toward equilibrium at some reaction rate. Equilibrium NO (which comprises most of the Nox formation) is formed at a rate that varies strongly with temperature and equivalence ratio. (iv) Carbon Dioxide While not normally considered a pollutant, CO2 may contribute to the greenhouse effect. Proposals to reduce CO2 emissions have been made. CO2 controls strongly influence fuel economy requirements. (v) Aldehydes Aldehydes are the result of partial oxidation of alcohols. They are not usually present in significant quantities in gasoline-fueled engines, but they are an issue when alcohol fuels are used. Aldehydes are thought to cause lung problems. So far, little information of engine calibration effects on aldehyde formation is available.
Emission Formation in C.I. Engine:
For many years, diesel engines have had a reputation of giving poor performance and producing black smoke, an unpleasant odor, and considerable noise. However, it would find it difficult to distinguish today‟s modern diesel car from its gasoline counterpart. For diesel engines the emphasis is to reduce emissions of Nox and particulates, where these emissions are typically higher than those from equivalent port injected gasoline engines equipped with three-way catalysts. Catalyst of diesel exhaust remains a problem insofar as researchhas not yet been able to come up with an effective converter that eliminates both particulate matter (PM) and oxide of nitrogen.