Pollutants produced by Automobiles

Air pollution can be defined as addition to our atmosphere of any material which will have a deleterious effect on life upon our planet. The main pollutants contributes by automobiles are carbon monoxide (CO), unburned hydrocarbons (UBHC), oxides of nitrogen (NOx) and lead and other particulate emissions. Automobiles are not the only source of air pollution, other sources such as electric power generating stations (which mainly emit sulphur oxides, nitrogen oxides, and particulates), industrial and domestic fuel consumption, refuse burning, industrial processing etc., also contribute heavily to contamination of our environment. 

Automobiles are responsible for:

–18% of HC

–31% of CO

–11% of NOx     released into the atmosphere

•Automotive emissions have three sources:

* Evaporative Fuels - 15 to 25% of HC

* Crankcase Dilution / Blow by gas - 20 to 35% of HC 

* Exhaust Gases - 50 to 60% of HC and almost all CO and     NOx

Evaporative Losses 

Evaporative emissions account for 15 to 25 percent of total hydrocarbon emission from a gasoline engine. The two main sources of evaporative emissions are the fuel tank and the carburetor.

(i) Fuel tank losses: Fuel tank losses occur by displacement of vapour during filling of petrol tank, or by vaporization of fuel in the tank, forcing the vapour through a breather vent to the atmosphere. Where the temperature is low the fuel tank breathes in air. When the temperature goes high it 'breathes out' air loaded with petrol vapour. Fuel tank losses occur because the tank temperature is increased during the vehicle operation which causes an increase in the vapour pressure and thermal expansion of tank vapour. The mechanism of tank loss is as follows:   When a partially filled fuel tank is open to atmosphere the partial pressure of the vapour phase hydrocarbons and vapour pressure of the liquid are equal and they are in equilibrium. If the temperature of the liquid is increased, say by engine operation, the vapour pressure of the liquid will increase and it will vaporize in an attempt to restore equilibrium. As additional liquid vaporizes, the total pressure of the tank increases and since the tank is open to atmosphere the vapour will flow out of the tank. This outflow to the vapour will increase if in addition to liquid temperature rise, the vapour temperature is also increased.

   The evaporation from the tank is affected by a large number of variables of which the ambient and fuel tank temperature, the mode of vehicle operation, the amount of fuel hi the tank and the volatility of the fuel are important. Other significant factors are the capacity, design and location of the fuel tank with respect to the exhaust system and the flow pattern of the heated air underneath the vehicle.

   Less the tank fill, greater is the evaporation loss. This reflects the difference in the tank vapour space. Also when a car is parked in a hot location the evaporation of the gasoline in the tank accelerates, so the evaporation loss is greater. The operational modes substantially affect the evaporation loss. When the tank temperature rises the loss increases. The vapour which vent from a partially filled tank during vehicle operation called soak, is a mixture of air and hydrocarbon. After a prolonged high speed operation the HC per cent in the soak is as high as 60 per cent as compared to about 30 per cent after an overnight soak.

(ii) Carburetor losses: Carburetor losses result from (a) external venting of the float bowl relieving the internal pressure as the carburetor heats, and (b) 'hot soak1 losses which occur after the engine has been stopped, as a result of evaporation of petrol stored in the bowl, loss being through vent pipe or through the air cleaner. Most of the loss from the carburetor occurs due to direct boiling of the fuel in the carburetor bowl during hot soak. Carburetor bowl temperature during hot, soak rises 15°C to 45°C above the ambient. This can cause fuel boiling and the front end gasoline components. In some designs the small passage from bowl leading to the throat after 'heating causes siphon action leading to HC loss.

  If the pressure in the fuel line becomes greater than the pressure holding the needle valve closed, after supply will occur. One of the possible reasons may be fuel evaporation pressure in the carburetor bowl which presses down the bowl and increase pressure in the fuel line. If the after-supply is more than the bowl volume the losses from the carburetor will change drastically. Thus bowl volume and maximum bowl temperature both significantly affect the evaporative losses from the carburetor.

Crankcase/Blow by gases:

The blow by is the phenomenon of leakage past the piston and piston rings from the cylinder to the crankcase. The blow by HC emissions are about 20 per cent of the total HC emission from the engine. This is increased to about 30 per cent if the rings are worn.

   The mechanism of leakage past the piston is as follows Air-fuel mixture trapped in the top land clearance and behind the top ring is unable to burn due to wall quenching effect. The cylinder forces this quenched gas past the piston ring and into the crankcase, along with some burned gases. In the blow by gas about 85 per cent carbureted mixture in the form of raw HC is present and rest 15 per cent is the burned gases.

   The blow by rate is greatly affected by the top land clearance and the position of the top ring because some of quenched gas is recycled in the combustion chamber and the ability of this to burn will depend on nearness to spark plug and the flame speed, etc., and it will burn only when favourable conditions are there, otherwise, it will go in the form of HC.

   Exhaust Emissions:

        Tail pipe exhaust emissions are the major source of automotive emissions. Petrol consists of a mixture of various hydrocarbons and if we could get perfect combustion then the exhaust would consist only of carbon dioxide and water vapors plus air that did not enter into the combustion process. However, for several reasons combustion is incomplete and hence we also get carbon monoxide, a deadly poisonous gas, and unburnt hydrocarbons (UBHC) in exhaust. Hydrocarbons play an active part in the formation of smog.

Exhaust gas or flue gases is emitted as a result of the

combustion of fuels such as petrol, diesel or fuel oil.

•A car's exhaust system is responsible for transporting

the burned exhaust, or combustion gases, from its engine

and out through the tail pipe.

•An exhaust system is usually tubing used to guide

reaction exhaust gases away from a controlled combustion

inside an engine . The entire system conveys burnt gases

from the engine and includes one or more exhaust pipes,

depending on the overall system design.

Pollutants from Exhaust

•CO — carbon monoxide. The main source of CO in cities is the internal combustion engine, where it is produced by incomplete combustion.

•CO2 — carbon dioxide. The internal combustion engine contributes to the increased concentrations of CO2 in the atmosphere.

•NOx — nitrogen oxide forms when fuels are burned at high temperatures.

•HC — hydrocarbons. Much of the hydrocarbon fuel passes through the process unconsumed and is expelled into the atmosphere along with other exhaust fumes.

•Other pollutants such as C6H6 — Benzene and its derivatives

•SO2 — sulphur dioxide are also present