Advanced Air Pollution Engineering Air pollution due to automobiles

Introduction Three main types of automotive vehicles being used are: (a) Passenger cars powered by four stroke gasoline engines, (b) motor cycles powered mostly by small two stroke gasoline engines, and (c) large buses and trucks powered mostly by four stroke diesel engines. Emissions from gasoline powered vehicles are generally classified as : (a) Exhaust emissions, (b) Crank – case emissions and, ( c ) Evaporative emissions. The amount of pollutants, that an automobile emits depends on a number of factors, including the design and operation of hydrocarbons emitted by a car with no controls. Carbon monoxide, nitrogen oxides and lead compounds are emitted exclusively in the exhaust gases.

Introduction Diesel powered vehicles create relatively minor pollution problems compared to gasoline powered ones. Diesel engine exhaust only about a tenth of the amount of carbon monoxide exhausted by a gasoline engine, although the HC emissions may approach those of the gasoline engine. Blow by is negligible in the diesel since the cylinders contain only air on the compression stroke. Evaporative emissions are also low because the diesel engine used a closed injection fuel system and because the fuel is less volatile than gasoline. The major problem with diesel engines is smoke.

1- Exhaust Emissions The important exhaust emission from a gasoline engine are carbon monoxide, unburnt HC, nitrogen oxides, and particulates containing lead compounds. Theses emissions vary with air-fuel ratio, spark timing and the engine operating conditions. To meet the exhaust emission standards for carbon monoxide and hydrocarbons, automobile measurements have used two basic methods. Ist method is to inject air into the exhaust manifold near the exhaust valves, where exhaust gas temperature is highest, thus inducing further oxidation of an-oxidized or partially oxidized substances. 2nd moethod is to design cylinders and adjust the fuel-air ratio, spark timing and other variables to reduce the amounts of hydrocarbons and carbon monoxide in the exhaust to the point where air injection is not required.

2- Crank-Case Emissions Crank-case emissions consist of engine blow-by which leaks past the piston mainly during the compression stroke, and of oil vapors generated into the crank-case. The quantity of blow-by depends on engine design and operating conditions. Worn out piston rings and cylinder liner may greatly increase blow-by. These gases mainly contain hydrocarbons and account nearly for 25% of the total hydrocarbon emissions from a passenger car.

2- Crank-Case Emissions Emissions of hydrocarbons from crank-case of automobiles can be eliminated by positive crank-case ventilation system. These systems recycle crank-case ventilation air and blow by gasses to the engine intake instead of venting them to the atmosphere.

2- Evaporative Emissions Passenger car would be emit about 20 Kg of hydrocarbons through evaporation. The exhaust gas pollutants comprise of hydrocarbons, carbon monoxide, nitrogen oxides and lead compounds. Evaporative emissions essentially constitute fuel evaporation from the fuel tank and carburettor and consist of hydrocarbons alone. The typical break up is exhaust 55%, evaporative emissions 20% and crank – case emission 25%. Exhaust and crank – case emissions are depend upon engine load conditions.

Formation of Petrochemical Smog The chemical reactions involved in smog production are extremely complex and not fully understood yet. The necessary conditions for smog formation are: 1- Sufficient quantity and concentration of unburnt hydrocarbons and nitrogen oxides in the atmosphere. 2- Stagnant atmospheric conditions produced by meteorological thermal inversions. The formation of petrochemical smog is ascribed to the following reaction: Hydrocarbons + nitric oxide + sunlight petrochemical smog

Formation of Petrochemical Smog Exhaust hydrocarbons are composed of a large number of individual hydrocarbons and partially oxidises hydrocarbons produced during the combustion process. The effect of various operating variables on exhaust emissions are discussed below:

Air-Fuel Ratio A decrease in the AF ratio increases the HC content (expresses as wt% of supplied fuel) in the exhausts of passenger cars at idle, but does not have any effect at part throttle. HC concentration decreased with an increasing AF ratio and reached a minimum at an AF ratio leaner than stoichiometric. Methane and acetylene are the two hydrocarbons most greatly affected by the AF ratio.

Combustion Chamber Surface - Volume Ratio The effect of combustion chamber S/V ration discussed in figure below: Engines having low s/v ratio give lower HC emissions but s/v has no effect on CO concentration.

Combustion Chamber Surface - Volume Ratio s/v ratio can be change by altering the compression ratio (CR), number of cylinders, stroke-bore ratio or the displacement. s/v proportion directly to the CR ratio . A greater reduction in HC emission is obtained than when s/v decreased by changing other variables

The bore and stroke of the engine decides the displacement of the engine. The larger the bore and stroke, the bigger the displacement and thus a more power engine.

Combustion Chamber Deposits HC exhaust emissions are significantly enhanced with accumulation of chamber deposits. Quantity of deposits affects the CR of the engine by lowering exhaust temperatures and reducing clearances volume and, exhaust emission increases. Residual gases contain about 11 times more HC concentrations as compared to exhaust gases. Thus , as CR increases with accumulation of more deposits, the clearance volume become small and more residual gases with very high HC concentration are expelled out of the exhaust manifold. This increases HC emission level.

Combustion Chamber Deposits The volume of combustion chamber deposits rather than their weight seems to be more important, it is the porosity of the deposits (void space) and degree of pore interconnections which affect exhaust emissions. During combustion, these pores and their interconnecting passages are filled with unburnt HC and escape burning. These are discharged back to the atmosphere during exhaust in the same state, increasing emission levels. Uniform distribution of deposits is likely to provide more void space for the absorption of unburnt HC during combustion thereby raising exhaust emission levels.

Next lecture Quiz & control of exhaust emissions