Motor vehicle emissions are composed of the by-products that come out of the exhaust systems or other emissions such as gasoline evaporation. These emissions contribute to air pollution and are a major ingredient in the creation of smog in some large cities. A 2013 study by MIT indicates that 53,000 early deaths per year occur because of vehicle emissions.
The three main types of automotive vehicles used in our country are: (a) passenger cars powered by four stroke gasoline engines, (b) scooters and auto rickshaws powered by small two stroke gasoline engines, and (c) large buses and trucks powered mostly by four stroke diesel engines. Emissions from gasoline powered engines are generally classified as:
1. (a) Exhaust emissions (b) Crank-case emissions and (c) Evaporative emissions
Of the hydrocarbons emitted by a car with no controls, the exhaust emission gases account for roughly 65%, evaporation from the fuel tank and carburettor for roughly 15% and blowby or crank-case emission about 20%. CO, nitrogen oxides and lead compounds are emitted almost exclusively in exhaust gases. Diesel powered vehicles create relatively less pollution problems than gasoline engines. It exhausts only about one tenth of CO released by gasoline engine. Blowby is negligible in diesel engine since cylinder contains only air in compression stroke. Evaporative emissions are also low because the diesel engine uses closed injection fuel system and because the fuel is less volatile than gasoline. The major problem of diesel engine is smoke and odour.
The important exhaust emissions from gasoline engine are carbon monoxide, unburnt hydrocarbons, nitrogen oxides and particulates containing lead compounds. The emissions vary with air fuel ratio, spark timings and the engine operating conditions. To meet the exhaust emission standards for carbon monoxide and hydrocarbons, automobile manufacturers have used two basic methods.
1. Inject air into the exhaust manifold near the exhaust valves, where exhaust gas temperature is highest, thus inducing further oxidation of unoxidised or partially oxidised substances.
2. Design cylinders and adjust air-fuel ratio, spark timing and other variables to reduce the amount of hydrocarbons and carbon monoxide in the exhaust to the point where air injection is not required.
Device used to control hydrocarbon emissions falls in three classes
1. Devices that modify engine operating conditions such as intake manifold, vaccum breakers, carburation mixture improvers, throttle retarders etc
2. Devices that treat exhaust gases such as after burners, catalytic converters, absorbers and adsorbers and filters
3. Use of modified or alternate fuels
Crank-case emissions consist of engine blowby which leaks past the piston mainly during the compression stroke and of oil vapours generated into the crank-case. Worn out piston rings and cylinder liner may greatly increase the blowby. These gases mainly contain hydrocarbons and account nearly for 25% of total hydrocarbon emission from a passenger car. It can be reduced by eliminating the positive crank-case ventilation (PCV) system. These systems recycle crank-case ventilation air and blowby gases to the engine intake instead of venting them to the atmosphere.
An average Indian passenger car would emit about 20 kg of hydrocarbon through evaporation annually. For controlling the evaporation of fuel from the carburettor and fuel system, systems are being developed that store vapours in the crank-case or in a charcoal canister that absorbs hydrocarbons for recycling to the engine. Mechanical methods are used to control evaporative emissions.
The exhaust gas pollutants comprise of HCs, carbon monoxide, nitrogen oxides and lead compounds. It essentially constitutes the fuel evaporation from the fuel tank and carburettor and consists of HCs alone.
The deleterious effects of automobile pollutants include toxic effects of CO and lead compounds and the formation of photochemical smog. The chief culprits in the smog are the volumetrically lower concentrations of unburnt or partially burnt HCs and nitrogen oxides. The relative concentration of either pollutant varies with the engine operation.
The necessary conditions for smog formation are:
1. Sufficient quantities and concentration of unburnt HCs and nitrogen oxides in the atmosphere.
2. Stagnant atmospheric conditions produced by meteorological thermal inversions
3. Strong sunlight
AIR- FUEL RATIO
The decrease in air-fuel ratio increases the HC content (expressed as wt % of supplied fuel) in the exhausts of passenger cars at idle, but does not have any effect at part throttle. On the basis of experiments conducted on single cylinder engine operating at full throttle on propane, Daniel reported that ‘relative HC concentration’ measured with a dispersive infra-red analyser decreased with an increasing AF ratio and reached a minimum at an AF ratio leaner than stoichiometric. Methane and acetylene are two HCs most greatly affected by AF ratio.
The HC emission generally decreases as the spark is retarded at constant power. A 100 retard from the optimum economy value causes 7-18% reduction as measured by a flame-ionisation analyser.
Combined effect of AF ratio and Spark Timing
The reductions in HC emission due to leaner AF ratios and due to retarded spark timing are additive, but while the former improves fuel economy, the latter impairs it. However, these fuel economy effects tend to balance each other when both methods are employed.
This chart shows the resultant gases from burning petrol at different AFRs. Rich mixtures are cooler but you can see the increased Hydrocarbon emissions as the excess fuel is unused. Nitrogen oxides are low from the cooler temps, but Carbon Monoxide is far higher with the lack of free oxygen to convert the CO to CO2. Lean mixtures around 16:1 AFR produce the best economy, but the extra heat oxidises the Nitrogen in the air increasing air pollution, but with low CO levels.
CONTROL OF EXHAUST EMISSIONS
Two main approaches to minimize exhaust emissions are:
1. Modification in the engine design and operating variables
2. Treatment of exhaust gases after emission from the engine
Modification in the engine design and operating variables
1. Use of leaner idle mixtures
2. Use of leanest possible mixture and maximum spark retard compatible with good power output and drivability
3. Use of minimum valve-over-lap necessary
4. Pre-treatment of mixture to improve vaporisation and mixing of fuel with air
Exhaust treatment devices
The basic technique is to provide oxidation of HC and CO emission from the engine. Exhaust oxidation devices fall into two categories:
1. Promotion of after burning of pollutants by exhaust heat conservation, introduction of additional air and by providing sufficient volume to ensure adequate reaction time,
2. Use of catalytic converters
Catalytic converters depend on the action of a catalyst containing certain exotic chemicals to convert HC and CO emissions to their oxidised products. Extra air is introduced by an engine driver blower. Vanadium pentoxide (V2O5) is one of the successful catalysts used so far.
CONTROL OF EVAPORATIVE EMISSIONS
There are two main sources of evaporative emissions: fuel tank and carburettor.
Principal factors governing fuel tank emissions are fuel volatility and ambient temperature. Insulation of the fuel tank to reduce temperature, sealed and pressurised fuel systems, and vapour collection systems have all been explored to reduce 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. On account of internal venting of carburettor the running losses are insignificant. Carburettor losses are substantial only during hot soak following a period of vehicle operation. The fuel voltality and carburettor design also greatly affect the carburettor emissions.
CONTROL OF CRANK-CASE EMISSIONS
These consists of engine blowby gases, ventilation air and crank-case lubricant fumes. New engines equipped with Positive Crank-case Ventilation System (PVC) return crank-case vapours through a vaccum valve, back to downstream side of the carburettor. Recycling burns hydrocarbons in the cylinders, dropping overall population by 25%.
1. Electric cars
Even though the electric car promises a great future of pollution-free cars, its wide use for vehicular application may accentuate and aggravate the problem. Power generating steps have to be stepped up which is difficult. Furthermore more fuel has to be burnt at power plant on an equivalent basis to supply power to these cars.
2. Natural gas
Compressed natural gas can essentially eliminate the pollutants but supply and proved reserves are limited.
3. Wankel engine
This engine being compact has more space available for emission-control-equipment, can operate on fuel of low octane rating and what is more important, NOx emissions are 30% of those of piston engines. However hydrocarbon emissions are decidedly higher and CO emissions equal.
4. Gas turbine
A properly designed automative gas turbine offers significant potential for alleviating air pollution caused by conventional otto-cycle engines. Because of high air-fuel ratios associated with gas turbines, CO and HCx emissions are usually negligible. NOx concentration is 100ppm with 200% excess air in a gas turbine as compared to 1200 ppm in as SI engine. However NOx emissions are comparable in magnitude. \
5. Ammonia-fuelled SI engine
The use of ammonia will reduce two main pollutants: CO and HCx. The emission of NH3 in engine exhaust is to be avoided because of its irritating odour and toxic effect. This can be minimized by adding hydrogen in small quantities (2%) which will act as a combustion promoter in accelerating the burning of ammonia.
6. Unleaded-gasoline powered SI engine
Lead compounds are toxic and conductive to more exhaust HC emissions. The most immediate benefit accruing from the removal of lead from gasoline is a 20ppm reduction in HC emissions in new cars. This reduction is effected by the process of natural oxidation of the hydrocarbons in the exhaust gas which is apparently inhibited by the presence of lead. The lead deposit is also responsible for spark plug fouling which increases HC emissions.