Emission from Engines: Introduction, Classification, Pollutants from SI Engines and Measuring Equipment!
Introduction to Emission from Engines:
The atmosphere which is the largest fraction of biosphere continuously absorbs a wide range of solids, liquids and gases from both man-made and natural sources. These substances travel through air, disperse and react with each other both physically and chemically. The portion of these substances which interacts with the environment to cause toxicity, disease, aesthetic distress, physiological effects or environmental decay, has been labelled by man as a “pollutant”.
Air pollution is basically the presence of foreign substances in air. Thus it is defined as the presence of one or more contaminants such as dust, fumes, gas, mist, odour, smoke or vapour which is injurious to human, plant or animal life or property. And the day man invented fire air pollution started.
Air pollutants can be classified as follows:
(1) Natural contaminants
e.g., pollen grains, natural fog, bacteria etc.
e.g.- dust, smoke, fog and fumes, mists, carbon particles etc.
(3) Gases and vapours
(i) Oxides of Nitrogen (NOx)
(ii) Oxides of sulphur (SOx)
(iii) Oxygen compounds O3, CO, CO2,
(iv) Vapours from various chemical reactions e.g., paraffin, olefins, acetylenes, chlorinated hydrocarbons etc.
(v) Radioactive compounds.
The main pollutants contributed by IC engines are CO, unburnt hydrocarbons (UBHC), oxides of nitrogen (NOx), lead and other particulate emissions. Apart from IC engines, other sources of air pollution are electric power generating stations, industrial and domestic fuel consumption, refuse burning (incinerators), industrial processes etc. also contribute heavily to contamination of environment.
The SI engine exhaust gases contain:
(i) Oxides of nitrogen e.g., NO, NO2 etc. collectively called as NOx
(ii) Carbon Monoxide (CO)
(iii) Organic compounds—unburned or partially burned hydrocarbons (UBHC).
The relative amounts depend on the engine design and operating conditions. Normally their presence is as follows:
(i) NOx – 500 to 1000 ppm or 20 gm/kg of fuel.
(ii) CO – 1% to 2% or 200 gm/kg of fuel.
(iii) HC – 3000 ppm or 25 gm/kg of fuel.
(1 ml/m3 = 1 ppm)
In gasolene engines other sources of unburned hydrocarbons (UBHC) are:
(a) Piston blow by gases. (Exhaust gases and vapours of fuel which leaks into the crank case are called blow by gases).
(b) Fuel evaporation from carburetor and fuel tank. These are called as non-exhaust sources.
The four possible sources of atmospheric pollution from a petrol engine are as follows:
These losses are the direct losses of raw gasolene from the engine fuel system. These emissions amount to 15 to 20% of total HC emission and two main sources are (i) fuel tank, (ii) the carburetor.
(i) Fuel Tank Losses:
While the petrol is being filled into the tank these losses occur. The vapours in the fuel tank are released to the atmosphere through breather vent. The tank temperature increases if the vehicle is parked in bright sun light or due to vehicle operation.
Parameters affecting the fuel tank losses are:
(a) Volatility of fuel
(b) Location, capacity and design of fuel tank
(c) Amount of fuel in the tank
(d) Ambient temperature
(e) Mode of vehicle operation etc.
(ii) Carburettor Losses:
These losses are because of two things (a) float chamber being vented to atmosphere in order to relieve the internal pressure as the carburettor gets heated up and (b) “hot soak” losses which occur when the engine is stopped after the vehicle is run for some time. The loss from the carburettor takes place due to boiling of fuel in it. During hot soak the carburettor temperature rises from 16°C to 48° C above the ambient.
(iii) Crank Case Blow By:
The leakage of flue gases past the piston and piston rings from the cylinder to the crank case, is called as blow by. The blow by loss is about 18 to 20% of total HC loss from the engine. This loss may go upto 32% if the piston rings get worn out.
During compression and combustion, the increased cylinder pressure forces some of the gas, in the cylinder, in to crevices viz. volume between piston, rings and cylinder wall. Most of this gas is unburned fuel air mixture and much of the gas escapes into the atmosphere as unburned HC, because these crevices are too narrow for the flame to enter.
The top land clearance and position of the top ring greatly affects the blow by losses, because some of the quenched gas is re-cycled in the combustion chamber. The nearness of this un-burned charge to spark plug, flame speed, local temperature etc. decide the ability to burn, otherwise it goes as UBHC.
(iv) Exhaust Pipe:
Mixture of various hydrocarbons is present in petrol fuel. Thus if the combustion is perfect, then exhaust emissions will have CO2, water vapours and the air that did not take part in combustion process. However because of numerous reasons, the combustion is not complete and hence CO and UBHCs are also present in exhaust gases. The smog (the mixture of smoke and gas) is formed mainly because of presence of HCs in air.
Apart from CO and HCs, exhaust also contains the third main pollutant NOx. The air in the combustion chamber used for combustion contains 78% of N2. At low temperature N2 is not so active but at about 1100 to 1150° C, the reactivity of N2 with O2 is very high and various oxides of nitrogen, collectively called as NOx are formed.
In addition to the pollutants, the exhaust also contains organic compounds namely ketones, aldehydes which form smog. To increase the antiknock quality of petrol, it is added with TEL (Tetra- Ethyl-lead). Thus because of TEL the exhaust emission also contains poisonous lead compounds.
Organic and inorganic compounds of higher molecular weights and lead compounds are exhausted in the form of very small size particles of the order of 0.02 to 0.06 µ.
Table 27.1 shows effect of engine operating conditions on exhaust (car)-
In diesel engine, emissions can be classified in the same categories as for the gasoline engines. In diesel exhaust concentrations of NOx are comparable to those from SI engines. Diesel hydrocarbon emissions are significant though exhaust concentrations are lower by about a factor of 5 than typical SI engine levels.
Smoke, odour and particulate emission are the other important factors of concern, which cause air pollution. Hydrocarbons in diesel exhaust may also condense to form white smoke during engine starting and warm-up. The particulate emissions are of the order of 0.2 to 0.5% of the fuel mass and diameter of the particles is as small as 0.1 µm. However diesel engines are not a significant source of CO.
Gasolene and diesel contain sulphur. In gasolene it is in small amounts ≤ 600 ppm by weight and in diesel fuel ≤ 0.5 P.C.
Sulphur is oxidised to produce SO2 of which a fraction can be oxidised to SO3 which combines with water to form a sulphuric acid-aerosol.
Table 27.2 shows approximately the possible variations in concentration of different constituents of diesel exhaust.
Emission levels of different engines at full load and rated speed are given in Table 27.3.
Thus with reference to the above table the conclusions can be drawn as under:
1. The two stroke air scavenged engine produces high UBHC and intermediate NOx emissions. The smoke level remains low.
2. The four stroke medium speed normally aspirated engine has lowest emissions in all categories except for a very high smoke intensity.
3. The four stroke high speed normally aspirated engine has high HC. The odour intensity is also high.
4. The turbocharged four stroke engine is notably low in UBHC and high in NOx. Generally very low smoke levels are recorded.
(ii) Effect of Mode of Operation on Engine Exhaust:
The various modes of operation like idle, full load at rated speed and acceleration at full rack and its effect on emission levels in diesel exhaust are shown in Table 27.4.
During idle mode, concentration of HC, NOx emissions are lower than other modes. Emissions at idle are less significant than during any other mode. Smoke and odour are highest during acceleration. Emissions at full load relative to emissions at other modes vary significantly with engine type. Two stroke and 4 stroke turbocharged engines smoke lightly at load, while 4 stroke normally aspirated engines smoke very much at rated full load.
(iii) NOx in Diesel Exhaust:
The quantity of NOx varies from few hundred to well over 1000 ppm. The highest local peak temperature and presence of sufficient O2, causes highest NOx concentration in diesel exhaust.
A pre-combustion engine gives rise to less NOx level than a direct injection (DI) engine. For high fuel-air ratio the additional fuel tends to cool the charge, so the localised peak temperatures are lowered resulting in drop in NOx level.
The NOx concentration is also significantly affected by injection system and time. Also the fuel characteristics such as Cetane Number (CN), viscosity, rate of burning etc. all contribute to differences in NO levels obtained from different levels.
The visible products of combustion are called as smoke. It originates early in the combustion cycle in a localised volume of rich fuel-air (F/A) mixture. If the fuel is burnt in a volume where F/A ratio is greater than 1.5, soot is produced.
The amount of soot formed depends upon type of fuel, pressure and local F/A ratio. The soot formed if it finds sufficient Ox, it will burn completely and will become visible. The size of soot particles affects the appearance of smoke.
The diesel engine smoke is basically of two types:
(a) Blue white smoke and
(b) Black smoke.
(a) Blue White Smoke:
While engine is started from cold, this smoke is caused by liquid droplets of lubricating oil or fuel oil, due to lower surrounding temperature and the intermediate products of combustion do not burn. If the piston rings are worn out and when the lubricating oil leaks past the rings, it gives rise to this type of smoke.
(b) Black Smoke:
It is nothing but carbon particles suspended in the exhaust gas. It largely depends upon air-fuel ratio and increases rapidly as load is increased and available air is depleted.
Till date no equipment is developed to measure the intensity of odour. But it is a general practice to have a human panel, who will decide the intensity of odour by comparison—which is an inaccurate method.
Note that aldehydes give a pungent odour for diesel engines, which also causes irritation to nose and eyes.
The main approaches to minimise exhaust emissions are:
1. Modifications in the engine design and operating variables.
2. Treatment of exhaust gases.
1. Modifications in the Engine Design:
The following modifications may help in cleaner exhaust:
(a) Use of lean mixtures and maximum spark retard compatible with good power output and drivability. It is a well-known fact that burning leaner mixtures greatly reduces HC and CO emissions. This reduction will result only if there is good mixture formation and distribution.
One of the methods of producing a uniform mixture and refined fuel metering is the adoption of triple venturi carburettor.
(b) Use of minimum valve over-lap.
(c) Pretreatment of the mixture to improve vaporisation and mixing of fuel with air. To achieve this narrow venturies are used, to produce higher air speeds and better fuel automisation. The exhaust heat can be used to pre-heat the mixture at part loads. Also to minimise the “Crevice volume” between topland and bore and to block the leakage path through the gas into the crank case, a system known as “Sealed ring orifice system” is adopted.
2. Exhaust Treatment Devices:
Here the basic technique is to promote oxidation of HC and CO after emission from the engine.
Exhaust oxidation devices fall into two categories:
(i) Promotion of after burning of the pollutants by exhaust heat conservation, introduction of supplementary, air and providing sufficient volume to ensure adequate reaction time.
(ii) Use of catalytic converters.
Catalytic convenors depend on the action of a catalyst containing certain exotic chemicals to convert HC and CO emissions to their oxidised products.
(i) Blow by Control:
Here the blow by vapours is re-circulated back to the intake manifold via air cleaner or inlet of the carburettor. This system is called as Positive Crank Case Ventilation (PCV). Thus the blows by gases are consequently re-introduced into the combustion chamber where they are burned along with fresh in coming air and fuel.
(ii) Evaporation Loss Control Device (ELCD):
In this case the device collects the evaporative emissions, they are absorbed in an absorbent chamber and are re-circulated. The activated charcoal or foamed polyurethane is used as an adsorbent to hold the HC vapours before they escape to the atmosphere. The two main sources of HC emissions viz. fuel tank and the carburettor bowl are directly connected to the adsorbent chamber when the engine is turned off. i.e., under hot soak. Apart from it the diurnal cycle loss from the tank and carburettor is also taken care of. Diurnal cycle is the daily cyclic variation in the temperature which causes tank breathing or forcing the gasolene out of the tank.
Adsorbent bed when saturated is relieved of the vapours, by purging action i.e., by purging the air from air cleaner to the intake manifold via the adsorbent bed.
(a) Petrol Injection:
This system provides the flexibility to meet the engine requirements and eliminates the problem of mixture distribution completely and hence the engine will have low levels and exhaust emissions and better fuel economy. Thus HC emissions are controlled by making use of optimum lean A/F ratios at all operating conditions. NOx level is also controlled as maximum temperature reached in the combustion chamber is low.
(b) Stratified Charge Engine:
It operates on very lean air-gasolene mixtures, depending on a localised rich mixture region near the spark plug to initiate combustion. The emission levels of all three pollutants CO, HC and NOx are very much lower with this engine at part throttle due to the leanness of the mixture. Mixture as lean as 30 : 1 can be successfully used.
(c) Proper Maintenance of an Engine:
The pollution from a poorly maintained engine is more. Because of mis-fire the entire A/F charge gets exhausted without combustion. If the air cleaner is not periodically cleaned, then its element gets choked and can reduce A/F ratio, generally giving rise to increased emissions of HC and CO. Similar problem is encountered if the choke gets automatically sticked.
(d) Fuel Variation:
Methane and propane could burn at leaner ratios than gasoline at a given throttle. Also at leaner mixtures, lower throttle position can be used. Thus methane and propane will produce much lower CO as their A/F mix is lean i.e., it contains more O2.
Hence if we change from the gasoline to propane as an engine fuel, CO emissions can be substantially reduced with reduced HC and NOx and if we change propane by methane the CO as well as HC touch zero emission level and only NOx remains as a significant factor. From point of view of emissions, these fuels are attractive but because of technological progress, we are unable to use these fuels.
The various methods like Catalytic converters, Exhaust gas Recirculation, Passive regeneration. Trap systems are used to control pollution.
In order to control the air pollution from IC engines (or automobiles with diesel or gasolene fuel), the necessity arises in after treatment of exhaust gases. Thus it is required to reduce or oxidise HC, CO, NOx to H2O, CO2, and N2. The Figure 27.2 shows the anatomy of a catalytic converter, it starts with a ceramic or a metallic monolithic substrate. This substrate is coated with a wash coat which serves as a primer for the purpose of holding noble metal (Pt, Rh or Pd) and that noble metal molecules dispersed to allow maximum surface exposure to the exhaust gases.
After the washcoat substrate is catalysed, it is canned with a heat-expanding sealing mat between the catalysed substrate and its can. The substrate material is selected to provide adequate mechanical strength, surface area and low flow resistance. Careful consideration has to be given to the type and amount of wash coat used. Alumina (Al-oxide) is very popular because of its surface area and other favourable physical properties.
However silica (silicon oxide) oxides are also used as wash coat. The container design, lub-oil additives, fuel sulphur level, application and particulate composition are other important design parameters. The performance of three catalyst viz. Pt, Pd and Rh to convert to SO3 relative to exhaust temperature.
The most accepted technology for NOx reduction is exhaust gas recirculation. It is commonly being used in light duty applications but is not accepted in heavy duty diesel engines because of its negative impact on engine wear. EGR is defined by following formula-
In this method the peak cylinder combustion temperature is reduced by circulating a portion of exhaust gas thereby reducing the quantity of O2, required for combustion. Nevertheless the re-circulated exhaust gas serves as heat sink. The exhaust gas from the exhaust manifold goes through an EGR control valve, then a cooler and finally into intake manifold by way of air-to-air intercooler or a bypass system.
With 10% EGR, there is substantial reduction in NOx level. The EGR inter cooler is used in order to reduce the temperature of exhaust gases so that the temperature in the combustion chamber does not reach its peak value and there by reactivity of N2 with O2 is reduced.
However the disadvantage of the system is that there is increase in smoke as well as particulates.
I. For Petrol Engines (2-Stroke and 4-Stroke Engines) – Idling CO-Meter:
Keeping the engine running at idle conditions, when the probe is inserted into the exhaust gases i.e., in the silencer, it analyses and displays the constituents i.e., it shows the % of CO and HC ppm directly. Note that these are harmful emissions and hence these are to be kept under controlled limits.
Pollution Under Control (PUC) contemporary norms are as follows:
For petrol vehicles vide rule 115 of Central Motor Vehicle (CMV) Rules 1989 emission norms are as follows:
(i) Idling CO emission limit for all four wheeler petrol driven vehicle shall not exceed 3% by volume.
(ii) For 2 and 3 wheeled petrol driven vehicle shall not exceed 4.5% by volume.
II. For Diesel Engines—Hartridge Smoke-Meter (Exhaust Gas Analyser):
Figure 27.7 shows the exhaust gas analyser, when the probe is inserted into the exhaust gases, it analyses and displays smoke density directly.
Pollution Under Control contemporary norms are as follows:
For diesel vehicles vide rule 115 of CMV Rules 1989 is as follows:
Smoke density on full load at 60% to 70% of Maximum engine rated RPM declared by manufacturer- 75 Hartridge Units OR Free acceleration- 65 Hartridge Units.