AIRPOLLUTION DUE TO AUTOMOBILES

INTRODUCTION Automotive emissions in aggravating the air

pollution and various causes of the genesis and exodus of these pollutants have been identified and methods to control them have been outlined.

The three main types of automotive vehicles being used in our country are: Passenger cars powered by four stroke gasoline

engines Motor cycles scooters and autorickshaws powered

mostly by small two stroke engines, Large buses and trucks powered mostly by four

stroke diesel engines,

Contd… Emissions from gasoline powered vehicles are

generally classified as: Exhaust emissions Crank case emissions Evaporative emissions

The amount of pollutants that an automobile emits depends on a number of factors including the design and operation.

Diesel powered engines create relatively minor pollution problems compared to gasoline powered ones.

The major problems of diesel engines are smoke and odor.

EXHAUST EMISSIONS The important exhaust emissions from a gasoline

engine are carbon monoxide, unburnt hydrocarbons, nitrogen oxide and particulates.

Automobile manufactures have used two basic methods:

The first is to inject manifold near the exhaust valves, where exhaust gas temperature is highest, thus inducing further oxidation of unoxidised or partially oxidised substances.

The second basic method is to design cylinders and adjust fuel-air ratio, spark timing and other varaibles to reduce the amounts of hydrocarbons and CO

Contd…. Devices and methods to control hydrocarbon

emissions fall into three classes Devices that modify engine operating conditions

such as intake manifold vacum breakers, carburation mixture improvers, throttle retraders,

Devices that ‘Treat” exhaust gases such as after burners, catalytic converters, absorbers and filters

Use of modifies or alternate fuels.

CRANK CASE EMISSIONS Crank case emissions consist of engine blow

by which leaks past the piston mainly during the compression stroke and of oil vapours generated into the crank case.

Contd… The gases mainly contain hydrocarbons and

account nearly for 25% of the total hydrocarbon emissions from a passenger car.

Emissions of hydrocarbons from the crank case of automobiles can be largely eliminated by the positive crank case ventilation(PCV) systems.

These systems recycle crank case ventilation air and blowby gases to the engine intake instead of venting them to the atmosphere.

EVAPORATIVE EMISSIONS It has been estimated that an average Indian

passenger car would emit about 20kg of hydrocarbons through evaporation annually.

It might also be dealt with by changing the properties of gasoline such as reducing the volatility of fuel and replacing the C4 and C5 olefinic hydrocarbon in the fuel with the less reactive C4 and C5 paraffinic hydrocarbons.

Mechanical methods can also be used to control evaporative emissions.

The exhaust gas pollutants comprise of hydrocarbons, carbon monoxide, nitrogen oxides and lead compounds.

FORMATION OF PHOTOCHEMICAL SMOG The deleterious effects of automotive pollutants

include toxic effects of CO and lead compounds and the formation of photochemical smog.

The chief culprits in the smog dilemma are the volumetrically lower concentrations of unburnt or partially burnt hydrocarbons and nitrogen oxides.

The necessary conditions for smog formation are: Sufficient quantity and concentration of unburnt

hydrocarbons and nitrogen oxides in the atmosphere Stagnant atmospheric conditions produced by

meteorological thermal inversions Strong sunlight.

AIR-FUEL RATIO A decrease in the AF ratio increases the HC

content in the exhausts of passenger cars at idle, but does not have any effect at part throttle.

Methane and acetyleene are the two hydrocarbons most greatly affected by the AF ratio.

Spark Timing: The HC emission generally decreases as the spark

is retrated at constant power. Combined Effects of AF Ratio and Spark

Timing: The reductions in HC emission due to leaner AF

ratios and due to retarted spark timing are additive.

COMBUSTION CHAMBER SURFACE-VOLUME RATIO

The effects of combustion chamber S/V ratio on the exhaust emissions have been investigated by Scheffer.

Engines with S/V covering a wide range were installed in cars and tested according to the California schedule.

Contd… It has been found that when S/V is decreased

by decreasing CR, a greater reduction in HC emission is obtained that when S/V is decreased by changing other variables.

COMBUSTION CHAMBER DEPOSITS HC exhaust emissions are significantly

enhanced with accumulation of chamber deposits.

During combustion, these pores and their interconnecting passsages are filled with unburnt HC and escape burning.

These are discharged back to the atmosphere during exhaust in the same state, increasing emission levels considerably.

Uniform distribution of deposits is likely to provide more void space for the adsorption on unburnt HC during combustion thereby raising exhaust levels

CONTROL OF EXHAUST EMISSIONS Two main approaches to minimize exhaust emissions

are: Modifications in the engine design and operating variables. Treatment of exhaust gases after emission from the

engine. The following modifications may help in cleaner

exhaust Use of leaner idle mixture Use of leaner possible mixture and maximum spark retard

compatible with good power output and drivability. Use of minimum valve-over-lap necessary Pre-treatment of the mixture to improve vaporization and

mixing of fuel with air. Low quench combustion chambers Piston and ring variables

Contd… Among the methods employed for the

purpose are: Use of narrow venturies to produce higher air

speeds and better fuel atomisation Provision of exhaust heated hot spots to vaporise

the heavier liquid droplets in the airstream, Use of exhaust het to pre heat the mixture at part

loads Use of automatic transmission Special devices for reducing or cutting off fuel

supply during deceleration.

Contd…

Exhaust Treatment Devices: Promotion of after burning of the pollutants by exhaust

heat conservation, introduction of additional air and by poviding sufficient volume to ensure adequate reaction time.

Use of catalytic convertors Catalytic converters depend on the action of a

catalyst containing certain exotic chemicals to convert HC and CO emissions to their oxidised products.

How ever they have certain limitations: Poisoning of catalyst by lead compounds in fuel Deterioration with time Pressure loss and noise problems

Other Methods: Petrol Injection:

The petrol injection system provides the flexibility needed to meet the engine requirements and eliminates the problem of mixture distribution altogether and hence has lower levels of exhaust emissions and better specific fuel consumption.

Stratified Charge Engine: It operates with very lean air petrol mixtures depending

on a localised rich mixture region near the spark plug to initiate combustion

NITROGEN OXIDES IN EXHAUST GAS The concentration of NOx in exhaust gas is

found to depend mainly on the peak temperature and pressure and the supplied air fuel mixture composition.

The NOx concentration rises rapidly as the mixture is made leaner around the stoichiometric ratio of 14.6.

Contd… The following are the methods have been

tried to reduce the concentration of oxides of nitrogen: Exhaust gas recycling Water injection into inlet manifold Exhaust manifold reactor with recycling Catalytic reduction Lean operation

CONTROL OF EVAPORATIVE EMISSIONS There are two sources of evaporative

emissions viz., the fuel tank and the carburettor.

The principal factors governing tank emissions are: Fuel volatility Ambient temperature Tank emissions

Carburettor emissions may be divided into two categories: Running losses occurring during engine operation

and Hot soak losses occurring when the vehicle is

parked.

CONTROL OF CRANK-CASE EMISSIONS These consist of engine blowby gases,

ventilation air and crank-case lubricant fumes. For air pollution blowby is most important and

the principal constituents in blowby gases are hydrocarbons.

Designers are shifting crank-case exhaust vents from simple open ending to a feed back.

New engines equipped with this positive crank-case ventilation (PCV) system return crank-case vapours through a vacuum valve, back to the downstream side of the carburettor.

Contd…

ALTERNATIVES Electric car Natural gas Wankel Engine Gas Turbine Ammonia-fueled SI Engine Unleaded-gasoline Powered SI Engine

AIR POLLUTION DUE TO AUTOMOBILES IN CALCUTTA

NEERI carried out a short-term survey in Calcutta to collect information on the level of CO pollution due to automobile exhausts.

High traffic density streets in Calcutta were studied by measuring CO levels during different phases of traffic; Heavy traffic Traffic jams with slow movement, Normal and light traffic

It was observed that the level of CO due to auto exhausts at street level varied from a high as 35ppm to less than 10 ppm at all selected streets depending upon the traffic conditions.

Contd…

The Calcutta data shows that although the number of vehicles in Indian cities is smaller compared to western cities, yet the intensity of pollution caused by them is apparently the same, if not more.

This can be attributed to old vehicles with poor engine performance

City Maximum 1 hr values – ppm

London 58

Chicago 46

Los Angeles 43

Washington 41

New York 27

Calcutta 35