Emission Control

ABSTRACT

Emission is nothing but exhaust from I.C engines which contains harmful compounds like unburned hydrocarbon, NOx, CO, particulates etc. which has harmful effect not only on human body but also on environment. Thus a immensive research is going on, on emission control from past few decades. Emission can be controlled to a great extent simply by varying engine parameters like air fuel ratio, engine speed, spark timing etc. now a days most commonly used emission control devices in most of the vehicles are catalytic converter, EGR etc. NOx emission can be controlled effectively by De (NOx) catalyst, electrically heated catalyst. In order to meet future coming strict norms for emission from vehicles we can adopt advanced technologies such as plasma exhaust treatment, electronic injection and ammonia injection etc. Alternative fuel is also one of the effective and economical way of emission control 

INTRODUCTION

 The rise in civilization is closely related to improvements in transportation. In the development of transport the internal combustion engines, both petrol & diesel engines occupy a very important position. The petrol engine has provided reliable small power units for personalized transport & in this way revolutionized the Living habits of people to a great extent. The diesel engine has provided the power units for transportation system, i.e. buses & goods transportation system ,i.e trucks. Indeed the petrol engines powered automobile & diesel engine powered buses & trucks are the symbols of our modern technological society.

 However, in recent times the internal combustion engine powered vehicles have come under heavy attack due to various problems created by them. The most serious of these problems is air pollution .Whereas the main problem facing the developing countries is pollution. India, however, faces the same severe problem of pollution in her metropolitan cities like Delhi, Mumbai, Calcutta , Chennai , Pune etc., as in developed countries.

 It is true that the emission from one car do not amount much, say half kg of pollutants for a single days driving.But if we consider the very large number of cars & this number is rising very rapidly . The pollutants amount becomes millions of tones. Thus it is necessary to control emission.

 NEED

 Boon of the one time now has become one of the worst curses of present time. The same engine which helped to achieve tremendous progress in transportation are now causing one of the worst calamities, a human society has ever faced, "air pollution"! The major cause of air pollution is the emission from I.C.engine.

At the turn of 20^th century man has realized the need to protect environment.Thus he has made serious attempts to conserve the earths environment from degradation . By developing several emission control methods.

 Emission Control Technologies for Off-Road Diesel Equipment

·         Catalytic Converters

·         Particulate Filters

·         Oxygen, NOx, and Temperature Sensors

·         Thermal Management Strategies

·         Engine/Fuel Management Strategies

·         Evaporative Emission Control Technologies

·         Enhanced Combustion Technologies

·         Plasma-Based Technologies

·         Crankcase Emission Control Technologies

 Emission Control Technologies for Off-Road SI Equipment

·         Catalytic Converters

·         Particulate Filters

·         Oxygen, NOx, and Temperature Sensors

·         Thermal Management Strategies

·         Engine/Fuel Management Strategies

·         Evaporative Emission Control Technologies

·         Enhanced Combustion Technologies

 EMISSION CONTROL IN S.I.ENGINE

1.S.I.ENGINE EMISSION:

Ø CO

Ø Oxides of nitrogen

Ø HC

2.METHODS TO CONTROL EMISSION :

I. CONTROL ON UNBURNED HYDROCARBONS AND CARBON MONOXIDE :-

a).AIR-FUEL RATIO :

 Hydrocarbon emissions are high at rich air-fuel ratio and decrease as the mixture is leaned upto about 17:1. When operation leaner than 17 or 18:1 is attempted emission increases because of incomplete flame propagation and the engine begins to misfire.

 The basic factor contributing to HC emission are the effect of mixture ratio on quench layer thickness and on fuel concentration within that quenched layer, and the effect of mixture ratio on the availability of excess oxygen in the exhaust to complete the combustion and on the exhaust system temperature . When the temperature is over 650ºc and with oxygen available appreciable exhaust after reaction does occur.

b). POWER OUTPUT:

 Hydrocarbon concentration does not change as load is increased while speed and mixture ratio are held constant and spark is adjusted to MBT. This result is to be viewed as arising from effect of several factors some of which tend reduce HC while other tend to increase them, apparently counter balancing one another.

 A factor which increase the HC formation as load increase is the reduced time within the exhaust system. The residence time of the exhaust gas in the very hot section of the exhaust system is very important for increased exhaust after reaction. Factors tending to reduce HC concentration include decreased quench thickness and increased exhaust temperature. Quench layer thickness decreases inversely as pressure increases and the mean cylinder pressure increases linearly with increase in load. Increased temperature with increasing load tends to increase exhaust after reaction.

 However, an almost linear increase in HC mass emission is observed as load is increased . A light car with a small frontal area and low pressure train losses has the advantage on mass emission basis. At a fixed air-fuel ratio there is no effect of power output on CO emission concentration . However, as in case of HC emission , CO emission on mass basis will increase directly with increasing output , giving advantage to a small , light and efficient car .

c). ENGINE SPEED:

 Emission concentration is markedly reduced at higher engine speeds. Primarily the increase in engine speed improves the combustion process within the cylinder by increasing turbulent mixing and eddy diffusion . This promotes after oxidation of the quenched layer. In addition,increased exhaust port turbulence at higher speeds promotes exhaust system oxidation reactions through better mixing .

 Speed has no effect on CO concentration because oxidation of CO in the exhaust is kinetically limited rather than mixing limited to normal exhaust temperature.

d).SPARK TIMING:

 The effect of spark timing on HC emissions is studied at constant power output and constant speed. A retard of 10 deg. From the manufactures recommended timing of about 30º BTDC reduced hydrocarbon by 100 ppm but increased fuel consumption by 10%. The importance of the precise spark timing and distributor tolerances are stressed by the fact of 100 ppm reduction for 10º retard. The more timing is retarded, the lower are the emissions .

 The effect of spark retard on HC emission reduction arises primarily from an increase in exhaust temperature, which promotes CO and HC oxidation . This advantages is gained by compromising the fuel economy . Spark advance has very little effect on CO concentration except at very retarded timing where the lack of time to complete CO oxidation leads to increased CO emission.

e).VALVE OVERLAP:

 Increasing valve overlap has an effect similar to increasing the back pressure. The charge is further diluted with residual gases. A slight 2º overlap gives minimum emission due to reburning of increased tail and exhaust residual which is rich in HC.

 Combustion detoriates with lean mixtures as residual is increased (increased overlap). If the mixture ratio is richened to provide stable idle and off-idle performance, then HC advantage will be lost and CO will be increased and . In general, minimum HC emissions are obtained with moderate or low back pressure and minimum overlap. There is no effect of overlap on CO concentration at a constant mixture ratio. However, any increase in the richness of mixture for smooth idle or off idle operation will increase the CO directly .

f).INTAKE MANIFOLD PRESSURE:

 As was already discussed the engine horsepower has no effect on HC or CO emission (at a fixed mixture ratio) on a concentration basis . The intake manifold pressure variation reflects the variation of output from the engine. Between about 22 cm and 60 cm of Hg manifold pressure the mixture is lean which minimizes HC and CO emission. Above 60 cm, the carburettor power valve may richen the mixture increasing the HC concentration and limited by higher exhaust temperature. At light loads and low manifold pressure, additional HC emission result from increased wall quenching accompanying the rich mixture delivered by the carburettor and incomplete combustion at manifold pressure bellow 15 cm of Hg . carbon monoxide emission concentration is similar to that of HC emission. The enrichment at light and heavy loads evidences itself in the higher levels of CO at these points. Mass emission of CO is particularly high at WOT because of the rich mixture needed for maximum power. It may be noted that CO is an intermediate compound in HC oxidation. In this test relatively good HC oxidation in the exhaust at WOT produced some CO which was not completely burnt.     

g).COMPRESSION RATIO :

 Decreasing the compression ratio is a way of decreasing the surface-to-volume ratio and, as expected this decreases HC. Another reason for the HC emission reduction with compression ratio reduction is the increased exhaust system oxidation due to the exhaust temperature of the less efficient cycle. This variable acts in very much the same way as spark retard.

II. CONTROL ON NOx :

a).MANIFOLD PRESSURE :

 An increase in manifold vaccum decreases load and temperature and increases the mass of residual gas. As a result the ignition delay is increased and the flame speed is reduced. Both these factors increase the time of combustion. This would reduce the maximum cycle temperature reducing the NO concentration in the exhaust.

b).COOLANT TEMPERATURE :

 As increase in the coolant temperature results in a reduction of heat losses to the cylinder walls and an increase in the maximum gas temperature. This results in an increase in NO concentration. An increase in deposit thickness causes an increase in CR, reduction in heat losses to the coolant and an increase in NO concentration.

c).HUMIDITY :

 The reduction in NO formation caused by an increase in mixture humidity is mainly due to the drop in maximum flame temperature. Test on hydrogen -air and ethylene -air mixtures indicated that 1% of water vapour reduced the flame temperature by 20ºc. This reduces the initial rate of NO production by about 25%.

EMISSION CONTROL IN C.I.ENGINE :

1.   C.I.ENGINE EMISSION :

·      Visible emission :

Ø Smoke

Ø Metallic particulates

·      Invisible emission :

Ø CO

Ø Unburnt HC

Ø Oxides of nitrogen

Ø Sulphur dioxide

2.  METHODS TO CONTROL EMISSION :

i). Water in Diesel Combustion :

Addition of water to the diesel process decreases combustion temperatures and lowers NO_x emissions. The most common methods of introducing water are direct injection into the cylinder, a process commercialized in certain marine and stationary diesel engines, and water-in-fuel emulsions. Emulsified fuels, due to increased mixing in the diesel diffusion flame, can be also effective in simultaneous reduction of PM and NO_x emissions.

ii).Ceramic In-Cylinder Coatings :

Zirconia based ceramic combustion chamber coatings originally developed for adiabatic or low heat rejection engines have been shown to reduce diesel emissions. Reported results indicate that in-cylinder zirconia coatings are capable of reducing the carbonaceous fraction of diesel particulates without increasing NO_x or other regulated emissions. Reductions in total PM emissions may be achieved by combining zirconia coatings with diesel oxidation catalysts. In-cylinder coatings are most effective in reducing emissions from older technology engines of relatively low thermal efficiency.

 iii). Engine Design for Low Emissions :

 Changes in diesel engine design contributed to some 10-fold decrease in emissions over the period from the late 1980’s to early 2000’s. The most important of these engine technologies are advanced fuel injection systems, air intake improvements, combustion chamber modifications, and electronic engine control. Additionally, exhaust gas recirculation (EGR) was introduced on both light- and heavy-duty diesel engines to control NO_x emissions. Low emission engine design—combined with increased exhaust gas after treatment—will continue to play important role in future diesel engines.

a).Advanced Technologies: Fuel Injection & Combustion

Diesel fuel injection systems for meeting future emission standards require very flexible rate shaping capacity and capability for pilot- and post-injections with controllable parameters. Combustion systems for future engines, designed using computerized tools, provide optimized swirl conditions for efficient air/fuel mixture preparation.

b).Advanced Technologies: Air Induction

Emerging air induction technology options for meeting future emission standards include improved air charging strategies, through the use of electric superchargers, charge air cooling, optimized intake manifolds and intake ports, and variable valve actuation

    EXHAUST TREATMENT DEVICES :

 1.CATALYTIC CONVERTER :

     What is a catalytic converter?

 The term covers the stainless steel box mounted in the exhaust system. Inside is the autocatalyst, a ceramic or metallic substrate with an active coating incorporating alumina, ceria and other oxides and combinations of the precious metals - platinum, palladium and rhodium. The substrate can be protected from vibration and shock by a resilient ceramic or metallic mat'.

Autocatalysts can be oxidation or three-way types. Oxidation catalysts convert carbon monoxide (CO) and hydrocarbons (HC) to carbon dioxide (CO2) and water and decrease the mass of diesel particulate emissions, but have little effect on nitrogen oxides (NOx) and particulate number. Three-way catalysts operate in a closed-loop system including a lambda, or oxygen, sensor to regulate the air:fuel ratio on petrol engines. The catalyst can then simultaneously oxidise CO and HC to CO2 and water while reducing NOx to nitrogen.

Fig.1

Autocatalysts in a protective 'mat' inside a strong steel catalytic converter.

Fig2 Three way catalytic converters work with the vehicle's engine management system.

a).Fast light off catalysts allow the catalytic converter to work sooner by decreasing the exhaust temperature required for operation. Untreated exhaust emitted at the start of the legislated emissions test and on short journeys in the real world is curtailed. Changes to the thermal capacity of substrates and type and composition of the active precious metal catalyst have together affected big improvements.

b).More thermally durable catalysts with increased stability at high temperature allow the catalytic converter to be mounted closer to the engine and increase the life of the catalyst, particularly during demanding driving. Precious metal catalysts with stabilized crystallites and washcoat materials that maintain high surface area at temperatures around 1000°C are needed. Improved oxygen storage components stabilize the surface area of the washcoat, maximize the air:fuel 'window' for three-way operation and indicate the 'health' of the catalyst for On Board Diagnostic (OBD) systems.

Fig.3

Evolution ceramic and metallic substrates with thinner walls and increased catalyst surface

c).Hydrocarbon Adsorber Systems incorporate special materials, such as zeolites, into or ahead of the catalyst. Hydrocarbon emissions are collected when exhaust temperatures are too low for effective catalyst operation. The hydrocarbons are then desorbed at higher temperatures when the catalyst has reached its operating temperature and is ready to receive and destroy the hydrocarbons. This technology has the potential to reduce hydrocarbons to less than half the levels emitted from a three-way catalytic converter.

d).Electrically Heated Catalyst Systems use a small catalyst ahead of the main catalyst. The substrate, onto which the catalyst is deposited, is made from metal so that, when an electric current is passed, it will heat up quickly. This brings the catalyst to its full operating temperature in a few seconds.

Fig.4

Electrically heated catalysts work in seconds.

e).Lean combution :With the development of lean burning direct injection gasoline engines and increased use of diesel engines, lean combustion is the big challenge for automotive catalysis. Lean combustion is essential to limit carbon dioxide emissions and to reduce fuel consumption. New diesel technologies with greater use of electronic management and direct injection with unit injectors or common rail injection, can achieve further fuel consumption improvements. The conventional three-way catalyst technology used on petrol engines needs a richer environment with lower air:fuel ratios to reduce NOx, so a radical new approach is required.

f).DeNOx (or Lean NOx) Catalysts use advanced structural properties in the catalytic coating to create a rich 'microclimate' where hydrocarbons from the exhaust can reduce the nitrogen oxides to nitrogen, while the overall exhaust remains lean. Further developments focus on increasing the operating temperature range and conversion efficiency.

g).NOx adsorbers (NOx traps) are a promising development as results show that NOx adsorber systems are less constrained by operational temperatures than DeNOx catalysts. NOx traps adsorb and store NOx under lean conditions. A typical approach is to speed up the conversion of nitric oxide (NO) to nitrogen dioxide (NO₂) using an oxidation catalyst so that NO₂ can be rapidly stored as nitrate on alkaline earth oxides. A brief return to stoichiometric or rich operation for one or two seconds is enough to desorb the stored NOx and provide the conditions for a conventional three-way catalyst mounted downstream to destroy NOx.

2.EXHAUST GAS RECIRCULATION :

Exhaust gas recirculation (EGR) is an effective strategy to control NO_x emissions from diesel engines. The EGR reduces NO_x through lowering the oxygen concentration in the combustion chamber, as well as through heat absorption. Several configurations have been proposed, including high- and low-pressure loop EGR, as well as hybrid systems. NO_x emissions may be further reduced by cooled EGR, in which recirculated exhaust gas is cooled in an EGR cooler using jacket water.

Addition of exhaust gasses to the inlet charge increases dilution. This reduces both the flame speed and maximum temperature reached in the cycle  Increase in dilution decreases NOx emission . According to Zeldowich mechanism, the chain reactions for NO formation are initiated by oxygen atoms . The oxygen atoms are produced from the dissociation of oxygen molecules at the maximum cycle temperature. About 15% recycle will reduce NOx emission by about 80%.

OTHER EMISSION CONTROL DEVICES

 1.Plasma Exhaust Treatment:

Non-thermal plasma technologies are being developed to reduce NO_x emissions from gasoline and diesel exhaust. Since oxidation reactions dominate during plasma discharges in lean exhaust, the plasma alone is ineffective in reducing NO_x. Combined plasma-catalyst systems, however, have been shown to enhance the catalyst selectivity and NO_x removal efficiency. Non-thermal plasma reactors can be also designed as diesel particulate matter reducing devices. Plasma technologies still require a significant improvement in their consumption of electrical energy and in other areas.

 2.AMMONIA INJECTION :

 As a fuel, ammonia does not hold much promise, but if used as an exhaust additives it can give excellent control for NOx emission . Ammonia and nitric oxide interact to form nitrogen and water. Ford motor co. has been doing investigation with injecting ammonia water in the exhaust manifold, downstream from the port.

 For an effective utilization of ammonia injection, the exhaust gas temperature has to be kept within strict limits and the injecting device has to be put sufficiently down to bring the gas temperature to 165ºc.This also demands a very close tolerance in air fuel ratio supplied by the carburettor . The present carburettors are incapable of this and it might be necessary to adopt electronic injection system to keep close control over fuel-air ratios.

 3.ELECTRONIC INJECTION :

 It is possible to develop an electronic injection system with sensors for air temperature,manifold pressure and speed which will precisely regulate the fuel supply giving only such air-fuel ratio as will give no HC or CO emissions .

The emissions on deceleration can be completely removed by shutting off the fuel supply when the throttle is closed.But this system will still not be able to control the NOx emission .Combination of electronic injection and ammonia as an exhaust additives has an attractive future .

EFFECT OF ALTERNATIVE FUELS ON EMISSION :

All available alternative fuels reduce some emissions. However, each fuel has its own characteristics, as does each vehicle type. For example,Biodiesel is an oxygenated fuel, so it contributes to a more complete fuel burn and a greatly improved emissions profile. The more biodiesel used in a blend, the higher the emission reductions. One of the unique benefits of biodiesel is that it significantly reduces air toxics that are associated with petroleum diesel exhaust and are suspected of causing cancer and other human health problems. NOx emissions are an exception to the rule, since biodiesel tends to increase NOx emissions. Recent research has shown a number of ways to mitigate this problem. No tailpipe emissions! This is the number one benefit of owning an EV. Emissions that can be attributed to EV, would be the emissions that are generated in the electricity production process at the power plant.

Some types of CNG vehicles may reduce CO and NOx compared to some conventional fuels but may increase HC emissions.

Conclusion :

 The change in the design of engines (use of unleaded petrol with high octane no.), alternate fuels like alcohol blended petrol, CNG etc., electricity driven vehicle and phasing out of old vehicles could be the answer to the ion problem due to automobile vehicular emission.

 BIBLIOGRAPHY

Ø  Internal Combustion Engines By M.L. Mathur, R.P.Sharma

Ø  www.diselnet.com

Ø  www.aecc.be

Ø  www.emissioncontrol.com

Ø  www.meca.org

Ø  www.corning.com