Advanced Ic engines unit 3

ME2041 Advanced Internal Combustion Engines

Unit II Department of Mechanical Engineering, St. Joseph’s College of Engineering

FORMATION OF POLLUTANTS

There are some unburned or partially burned hydrocarbons in the exhaust. The amount is insignificant from an energy standpoint, but it is objectionable from the viewpoint of its odour, its photochemical smog, and from the standpoint of its having a carcinogenic effect. The products of photochemical smog cause watering and burning of the eyes, and affect the respiratory system, especially when the respiratory system is marginal for other reasons.

HYDROCARBON EMISSIONS FROM SI ENGINES The most widely accepted causes for hydrocarbon emissions in exhaust gases of spark ignition engines are:1. Flame quenching at the combustion chamber walls, leaving a layer of unburned fuel-air mixture adjacent to the walls.2. Crevices in the combustion chamber, small volumes with narrow entrances, which are filled with the unburned mixture during compression, and remains unburned after flame passages, since the flame cannot propagate into the crevices. The main crevice regions are the spaces between the piston, the piston rings and the cylinder walls. The other crevice regions are the threads around the spark plug, the space around the plug centre electrode, crevices around the intake and exhaust valve heads, and the head gasket crevice.3. The oil film and deposits on the cylinder walls absorb fuel during intake and compression, and the fuel vapour is desorbed into the cylinder during expansion and exhaust.




4. Incomplete combustion, either partial burning or complete misfire, occurring when the combustion quality is poor, e.g. during engine transients when air-fuel, exhaust gas recirculation, and spark timing may not be adequately controlled.

•All these processes, except misfire, result in unburned hydrocarbons close to the combustion chamber walls. Mixing of unburned hydrocarbons with the bulk cylinder gases occurs during expansion and the exhaust blowdown processes. During the blowdown process a high concentration of hydrocarbons is released from the cylinder through the exhaust valve. •During the exhaust stroke the piston pushes most of the remaining fraction of the cylinder mass with its high hydrocarbon concentration into the exhaust. •The residual gases in the cylinder thus contain a high concentration of hydrocarbons. •Unburned hydrocarbons are thus exhausted in two pulses, the first peak is obtained with the exhaust blowdown and the second occurs towards the end of the exhaust stroke.




Hydrocarbon emissions from CI enginesThe CI engines operate with an overall fuel-lean equivalence ratio, therefore they emit only about one-fifth of the hydrocarbon emissions of an SI engine. The following are the major causes for hydrocarbon emissions in the exhaust of CI engines:

1.The diesel fuel contains components of higher molecular weights on average than those in a gasoline fuel, resulting in higher boiling and condensing temperatures.

This causes some hydrocarbon particles to condense on the surface of the solid carbon soot generated during combustion.

Most of this is burned as mixing continues and the combustion process proceeds but a small amount is exhausted out of the cylinder.

2. The air-fuel mixture in a CI engine is heterogeneous with fuel still being added during combustion. It causes local spots to range from very rich to very lean and many flame fronts exist at the same time unlike the homogeneous air-fuel mixture of an SI engine that essentially has one flame front.




Hydrocarbon emissions from CI engines

Incomplete combustion may be caused by undermixing or overmixing.

With undermixing, in fuel-rich zones some fuel particles do not find enough oxygen to react with, and in the fuel-lean zones some local spots will be too lean for combustion to take place properly.

With overmixing, some fuel particles may be mixed with burned gases and it will therefore lead to incomplete combustion.

3. A small amount of liquid fuel is often trapped on the tip of the injector nozzle even when injection stops. This small volume of fuel is called sac volume. This sac volume of liquid fuel is surrounded by a fuel-rich environment and therefore it evaporates very slowly causing hydrocarbon emissions in the exhaust.

4. CI engines also have hydrocarbon emissions for some of the same reasons as SI engines do, e.g. flame quenching, crevice volume, oil-film and deposits on the cylinder wall, misfiring, etc.




Carbon Monoxide (CO)

Carbon monoxide is toxic. The haemoglobin in the blood, which carries oxygen to the different parts of the body, has a higher affinity for carbon monoxide than for oxygen. The percent carboxy haemoglobin gradually increases with time to an equilibrium value which depends upon the carbon monoxide concentrations.

Carbon monoxide is generated in an engine when it is operated with a fuel-rich equivalence ratio as there is not enough oxygen to convert all carbon to carbon dioxide. For fuel-rich mixtures, CO concentrations in the exhaust increase steadily with the increasing equivalence ratio. The engine runs rich when it is started or when it is accelerated under load. For fuel-lean mixtures, CO concentrations in the exhaust are very low and are of the order 10-3 mole fraction.

Poor mixing, local rich regions, and incomplete combustion create some CO. The SI engines often operate close to stoichiometric at part load and operate fuel rich at full load. Under these conditions, the CO emissions are significant. However, CI engines operate well on the lean side of stoichiometric and therefore produce very little CO emissions.




Oxides of Nitrogen (NOx)• The oxides of nitrogen tend to settle on the haemoglobin in the blood. The most undesirable toxic effect of oxides of nitrogen is their tendency to join with the moisture in the lungs to form dilute nitric acid. NOx is one of the primary causes of photochemical smog (smoke + fog). Smog is formed by the photochemical reaction as follows: NO2 + energy from sunlight NO + 0 + Smog

Monoatomic oxygen reacts with O2 to form ozone (03) as follows:O + O2 O3

• Ozone is harmful to lungs and other biological tissues. It is harmful to crops and trees. It reacts with rubber, plastics and other materials causing damage. • Most of the oxides of nitrogen comprise nitric oxide (NO), a small amount of nitrogen dioxide (NO2) and traces of other nitrogen oxides. These are all grouped together and the group is called NOx.• NOx is mostly formed from atmospheric nitrogen. There are a number of possible reactions that form NO. NO forms in both the flame front and the post flame gases.




Some of the NO forming reactions are:

• N, O, OH are formed from the dissociation of N2, O2 and H2O vapour at high temperatures that exist in the combustion chamber (2500-3000 K). • The higher the combustion reaction temperature, the more diatomic nitrogen (N2) will dissociate to monatomic nitrogen (N) and more NOx will be formed. • At low temperatures, a very small quantity of NOx is created. The flame temperature is maximum at the stoichiometric equivalence ratio (0 = 1.0) but maximum NOx, is formed slightly at a lean equivalence ratio (0 = 0.95). At this condition the flame temperature remains very high but excess oxygen helps in the formation of more NOx. The most important engine variables that affect NOx emission are the fuel/air equivalence ratio, the burned gas fraction (EGR and residual gas fractions) and combustion duration within the cylinder. NOx is reduced in modem engines with fast-bum combustion chambers. • If ignition spark is advanced, the cylinder temperature will be increased and more NOx will be produced. CI engines with divided combustion chambers and indirect injection (DI) tend to generate higher levels of NOx.




PARTICULATES• The particulates from SI engines are lead, organic particulates including soot and sulphates.

Gasoline may contain some sulphur, which is oxidized within the engine cylinder to form SO2. • is oxidized to SO3 which combines with water to form a sulphuric acid aerosol. • Leaded gasolines emit lead compounds. Soot emissions (black smoke) can result from

combustion of overly rich mixtures. In properly adjusted spark-ignition engines, soot in the exhaust is not a significant problem.

• Diesel particulates consist mainly of combustion generated carbonaceous material (soot) on which some organic compounds have been absorbed.

• Most particulates are generated in the fuel rich zones within the cylinder during combustion due to incomplete combustion of fuel hydrocarbons; some particulate matter is contributed by the lubricating oil.

• These are undesirable odorous pollutants. Maximum particulate emissions occur when the engine is under load. At this condition, maximum amount of fuel is injected to obtain maximum power from the engine. It results in a rich mixture and poor fuel economy.

• At temperatures above 500°C, soot particulates appear as clusters of a large number of solid carbon spheres with individual diameters of about 15 to 30 nm.




PARTICULATES

• As the temperature decreases below 500°C during expulsion, the particles become coated with HC and with traces of other components.

• The words particulates and soot are often used synonymously, but there is a difference in nature between these two emissions.

• Dry soot is usually the carbon that is collected on a filter paper in the exhaust of an engine.

• The unit of measurement of soot is usually the Bosch Smoke Number, which is assessed by the reflectance of a filter paper on which the soot has been collected.

• Particulates contain more than simply the dry soot; they are the soot particles on which the other compounds, often the polycyclic aromatic hydrocarbons (PAH), have condensed.

• The PAH compounds have a tendency to be carcinogenic.

• The level of particulates increases with the sulphur content in the fuel. Particulates are measured by trapping the particles on glass-fibre filter papers placed in a dilution tunnel, and then weighing the quantity.



MEASUREMENT OF POLLUTANTS

• The measurement of exhaust emissions is very important for the control of air pollution from IC engines.

• CO concentrations are measured by infrared absorption,

• NO concentrations are measured by chemi-luminescence and

• Unburned HC are measured by flame ionization detector.

Non-dispersive Infra-red (NDIR) Analyzer

• The NDIR analyzers are used for measuring the concentrations of carbon monoxide and carbon dioxide. This device is based on the principle that the infrared energy of a particular wavelength, peculiar to a certain gas, will be absorbed by that gas. The infrared energy of other wavelengths will be transmitted by that gas.

• Carbon dioxide absorbs infrared energy in the wavelength band of 4 to 4.5 microns (m) and transmits the energy of the surrounding wavelengths. The carbon monoxide absorption band is between 4.5 and 5 microns (gm).



• Nitric oxide (NO) has also a weak absorption band, allowing it to be analyzed by NDIR, but lack of sensitivity and interference by water vapour do not give high accuracy with low concentrations.

• A schematic arrangement of the IR analyzer is shown in Figure.

• A wideband infrared radiation source consists of a heated wire, which is placed in a quartz tube mounted in the source block.

• Radiation from the source is reflected within the mounting block and passes out of a symmetrical pair of rectangular apertures as two parallel beams into the two separate cells a sample cell and a reference cell.

• These cells are internally highly polished and gold plated to ensure high transmission of radiation.

• After passing through these cells the infrared radiation is received in two separate detector cells, which are full of the gas whose concentration is to be measured.




• The two detector cells contain equal amounts of this gas and are separated by a flexible diaphragm.

• The sample cell is a flow-through tube that receives a continuous stream of the mixture of gases to be analyzed.

• When the particular gas to be measured is present in the sample, it absorbs the infrared radiation at its characteristic wavelengths. The percent of radiation absorbed is proportional to the molecular concentration of the component of interest in the sample.

• The sample cells may be divided by quartz windows into various lengths to give different ranges of sensitivity.

• The quartz windows do not absorb infrared energy in the region of interest. Low concentrations are best measured by longer cells so that more molecules of interest are present.

• The unused sample cells are generally flushed with a non-infrared absorbing gas such as oxygen or nitrogen, or with a gas free of the components being measured, e.g. fresh air for carbon monoxide analyzers.




• The reference cell is sealed and is physically identical to the sample cell. It is filled with an inert gas (usually nitrogen) which does not absorb the infrared energy of the characteristic wavelength of the species of interest.

• The radiant energy, after passing through the cells, heats the gas in the corresponding chamber of the detector. Since no radiant energy is absorbed in the reference cell, the corresponding chamber in the detector is heated more and its pressure becomes higher than that in the other chamber.

• This pressure differential causes the diaphragm to move and vary the capacitance. Therefore, the variation in the capacitance is proportional to the concentrations of the species of interest in the exhaust sample.

• The radiation from the source is interrupted by a rotating two-bladed shutter driven by a synchronous motor.

• The shutter is placed between the infrared source and the cells.




• When the shutter blocks the radiation, the pressure in the two compartments of the detector is equal because there is no energy entering either of the chambers of the detector.

• This allows the diaphragm to return to its neutral position. As the shutter alternatively blocks and unblocks the radiation, the diaphragm fluctuates causing the capacitance to charge cyclically.

• This sets up an ac signal, which is impressed on a carrier wave provided by a radio-frequency oscillator (amplifications of ac signals have better drift-free characteristics than the amplifications of dc signals). Additional electronic circuitry in the oscillator unit demodulates and filters the resultant signal.

• This signal is then amplified and rectified to a de signal which is measured by a meter or recorder. The final dc signal is a function of the concentration of the species of interest in the exhaust sample.




• To set the zero point, a non-infrared-absorbing gas, e.g. dry air, is passed through the instrument. For the other points on the scale, calibrating gases with known concentrations are passed through the analyzer.

• An error in the NDIR readings may arise if the exhaust sample contains other species that can absorb radiation at the same frequencies that the gas in the detector will absorb.

• In order to minimize this interference, a large concentration of the interfering gas is placed in the filter cells.

• The analyzer zero is then set with this large concentration of the interfering gas.




FLAME-IONIZATION DETECTOR (FID)

• Some hydrocarbons have an infrared absorption at 3.4 microns, but some others, notably aromatics, have almost none. Only about 50 % of exhaust hydrocarbons is measured by NDIR, therefore, this method is not suitable for the measurement of HC concentrations.

• The flame ionization detector is mainly used to measure the unburned hydrocarbon concentrations in the exhaust gases. It is based on the principle that pure hydrogen-air flames produce very little ionization, but if a few hydrocarbon molecules are introduced the flames produce a large amount of ionization. The ionization is proportional to the number of carbon atoms present in the hydrocarbon molecules.

• A schematic arrangement of the instrument is shown in Figure. It consists of a burner assembly, an ignitor, an ion collector and electric circuitry. The burner consists of a central capillary tube.




• Hydrogen, or a mixture of hydrogen and nitrogen, enters one leg of the capillary tube and the sample enters through another leg. The length and bore of the capillary tubes are selected to control the flow rates. The mixture of H2 - N2 - C„H,„ then flows up the burner tube.

• The air required for combustion is introduced from around the capillary tube.

• The combustible mixture formed in the mixing chamber is ignited by a hot wire at the top of the burner assembly and a diffusion flame stands at the exit to the burner tube.

• An electrostatic field is produced in the vicinity of the flame by an electric polarizing battery.

• This causes the electrons to go to the burner jet and the positive ions go to the collector.



• The collector and the capillary tube form part of an electric circuit.

• The flow of ions to the collector and the flow of electrons to the burner complete the electrical circuit.

• The dc signal produced is proportional to the number of ions formed and the number of ions is proportional to the number of carbon atoms in the flame.

• The dc signal generated is attenuated by a modulator and then fed to an ac amplifier and a demodulator.

• The signal is then recorded on a meter. The meter is calibrated directly in amount of hydrocarbon concentrations.

• To calibrate, the samples of known concentration of hydrocarbons are fed to the instrument and the meter readings are adjusted accordingly.




CHEMILUMINESCENCE ANALYZERS (CLA)

• The chemilutilinescent analyzer measures the nitric oxide (NO) concentrations. This technique is based on the principle that NO reacts with ozone (03) to give some NO2 in an electronically excited state. These excited molecules on decaying to the ground state emit red light (photons) in the wavelength region from 0.6 gm to 3 gm, i.e.

• NO + 03 NO2* + 02

• NO2* --> NO2 + hv

• where h is Planck's constant and v represents a photon of light.

• The oxides of nitrogen (NO„) from the engine exhaust comprise mainly a combination of nitric oxide (NO) and nitrous oxide (NO2).

• By converting any exhaust NO2 to NO in a thermo-catalytic converter before supplying the exhaust gas to the analyzer, the value of total nitrogen oxides (N0x) can be obtained.



• A schematic arrangement of the chemiluminescent instrument is shown in Figure. The vacuum pump controls the pressure in the reaction chamber and draws ozone and the exhaust sample. The ozone is produced by an electric discharge in oxygen at low pressure.

• An NO2-to- NO converter is also shown in the diagram. An arrangement is made by using a bypass line, so that it may be possible to measure only the NO concentrations or NO + NO2, i.e. NOx concentrations in the combustion engine exhaust.

• A mixture of a gas sample and ozone enters a reaction chamber (reactor) which is maintained at a very low absolute pressure. The reaction of the ozone and nitric oxide when heated under vacuum at 600°C produces some electronically excited molecules of NO2.

• The electronically excited molecules on decaying, emit light. The light can readily be detected accurately by a photo- multiplier.

• The signal is then amplified and fed to a recorder. Many parameters affect light emission in the reactor, it is therefore essential to calibrate the analyzer regularly.

• Pure nitrogen may be used for zero setting. The zero control is adjusted until the digital voltmeter reads zero, the nitrogen gas is then disconnected and a standard NO / N2, mixture is connected.

• The NO/NOx, switch is set to 'NO' mode and the span control is used to adjust the NO reading to correspond with the standard. For the NOx reading the NO/NOx, function switch is pressed to initiate the NOx mode.




A dilution tunnel is used to measure the amount of particulate present in the exhaust gas from the diesel engine. In the dilution tunnel, the exhaust gases are diluted with ambient air to a temperature of 52°C or less, and a sample stream from the diluted exhaust is filtered to remove the particulate material. The particulate is trapped after dilution because the particulate gets condensed over the filter at this temperature. The amount of particulate trapped is obtained by weighing the filter before and after the experiment.

MEASUREMENT OF PARTICULATES

MEASUREMENT OF EXHAUST SMOKE

Smoke-meters are used to measure the intensity of exhaust smoke. Smoke-meters may measure either the relative quantity of light that passes through the exhaust gas (Hartridge smoke-meter), or the relative smudge left on a filter paper (Bosch smoke-meter).



HARTRIDGE SMOKE-METER

It is based on the principle that the intensity of a light beam is reduced by smoke which is a measure of smoke intensity. A schematic diagram to illustrate the principle of this smoke-meter is shown in Figure. Light from a source is passed through a standard length of a tube where the exhaust gas sample is continuously supplied from the engine and at the other end of the tube the transmitted light is measured by a photo-electric cell. The photoelectric cell converts the light intensity to an electric signal, which is amplified and recorded on a meter. The intensity of smoke is expressed in terms of smoke density. It is defined as the ratio of electric output from the photoelectric cell when an exhaust sample is passed through the tube to the electric output when clean air is supplied.



• It is based on the principle that when a certain quantity of exhaust gas passes through a fixed filter paper, some smoke smudge is obtained on it, which is a measure of smoke intensity.

• A schematic diagram to illustrate the principle of this instrument is shown in Figure.

• A fixed quantity of the exhaust gas from the engine is introduced into a tube, where it passes through a fixed filter paper. Depending upon the smoke density, some quantity of smudge is deposited on the filter paper, which can be evaluated optically.

• A pneumatically-operated sampling pump and a photoelectric unit are used for the measurement of the intensity of smoke smudge on the filter paper.

BOSCH SMOKE-METER

Advanced Ic engines unit 3

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