Emission Measurement - Engine Combustion - Lecture Notes, Study notes of Sustainability Management

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Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture18/18_1.htm[6/15/2012 3:03:15 PM] Module 4: Vehicle Emission Standards and Measurement Lecture18:Emission Measurement The Lecture Contains: EMISSION MEASUREMENT CO and CO2 NDIR Analyzers Flame Ionization Detector (FID) Chemiluminescence Analyzer (CLA) Smokemeters Constant Volume Sampler (CVS) Particulate Emission Measurement Partial Flow Dilution Tunnel Full Flow Dilution Tunnel Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture18/18_2.htm[6/15/2012 3:03:15 PM] Module 4: Vehicle Emission Standards and Measurement Lecture18:Emission Measurement EMISSION MEASUREMENT The emission regulations specify the type, principle of operation used and generic construction of the exhaust gas analyzers which can be employed for emission certification of vehicles and engines. Table 4.9 gives the type of analyzers used for measurement of different exhaust gas constituents. Table 4.9 Measurement Principles of Exhaust Gas Analyzers Gas component Measurement Principle CO NDIR (Non-dispersive infrared) HC FID (Flame Ionization detector) NO x CLD (Chemiluminescence detector) CO and CO2 NDIR Analyzers Beer-Lambert's Law is used for operation of NDIR analyzers by measuring the degree of absorption of infrared (IR) radiations when they pass through a column of gas. The fraction of incident radiations absorbed is given by, (4.1) where I = Radiation energy absorbed I0 = Incident radiation energy k = characteristic absorption constant for the gas, m2 /gmol c = concentration of the gas, gmol/m 3 d = length of the gas column, m Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture18/18_5.htm[6/15/2012 3:03:16 PM] Module 4: Vehicle Emission Standards and Measurement Lecture18:Emission Measurement Flame Ionization Detector (FID) Pure hydrogen-air flames are practically ion-free but on introduction of even little amount of hydrocarbons the flame causes considerable ionization and becomes electrically conducting. The ionization current is proportional to the number of carbon atoms present in the hydrocarbon molecules. Thus, FID is effectively a carbon atom counter e.g., one molecule of propane generates three times the response generated by one molecule of methane. The measurement of HC by FID is expressed as parts per million of methane i.e. as ppmC 1 i.e., ppm of hydrocarbon containing equivalent of one carbon atom. The HC concentration is commonly written as ppmC. HC concentration measured as ppm propane (C3 ) is to be multiplied by a factor of 3 to convert it to ppmC. All classes of hydrocarbons i.e., paraffin, olefins, aromatics, etc. show practically the same response to FID. Oxygenates, e.g. aldehydes and alcohols however, have a somewhat lower response. FID essentially consists of a hydrogen-air burner and an ion collector assembly as shown in Fig. 4.11. Sample gas is introduced with hydrogen in the burner assembly and the mixture is burned in a diffusion flame. An electric potential is applied between the collector plates that makes the ionization current to flow and generate signal proportional to HC concentration in the sample gas. This current is amplified and the output signal is measured. A well-designed burner will generate ionization current that is linearly proportion to hydrocarbon content over a dynamic range of almost 1 to 10 6 . The commercial FID analyzers have the most sensitive range set at about 0-50 ppmC and the maximum range reaching 0-100,000 ppmC. Hydrogen is mixed with helium in ratio of 40:60 to decrease flame temperature that increases flame stability. The FID analyzer is calibrated with propane or methane mixtures in nitrogen. For the measurement of hydrocarbons in diesel exhaust, sampling line and FID are heated to a temperature of 191± 11°C to minimize condensation of heavy hydrocarbons present in the diesel exhaust in the sampling system. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture18/18_5.htm[6/15/2012 3:03:16 PM] Figure 4.11 FID for HC measurement. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture18/18_6.htm[6/15/2012 3:03:16 PM] Module 4: Vehicle Emission Standards and Measurement Lecture18:Emission Measurement contd... Measurement OF Non-Methane Hydrocarbons (NMHC) Presently, the emission standards are specified in terms of non-methane hydrocarbons. Methane content of HC emissions is determined by one of the following methods: Gas chromatographic (GC) method or Non-methane cutter (NMC) method In the GC method, sample is injected into GC column which separates the sample into two parts: (i) CH 4 -air-CO, and (ii) NMHC–CO2 –H2O. A molecular sieve column separates methane from air and CO before passing it to FID. Thus methane content is measured that is deducted from the total hydrocarbon content. In the NMC method, all hydrocarbons except CH4 are oxidized to CO2and water on a catalyst, so that when the gas sample is passed through NMC only CH4 is detected by HFID. The NMC cutter is calibrated for catalytic effect on CH4 and higher hydrocarbon (ethane) mixtures in presence of water vapours with values typical of exhaust gas at or above 600 K. The sample can be alternatively passed through NMC or bypasses the NMC. In this manner, the total HC and methane alone present in the exhaust gas sample are determined. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture18/18_8.htm[6/15/2012 3:03:16 PM] Module 4: Vehicle Emission Standards and Measurement Lecture18:Emission Measurement s Smokemeters In the filtration type smokemeters like Bosch smokemeter a fixed volume of the exhaust gas is drawn through a white filter paper of specified quality. The density of smoke stain obtained on the filter paper is evaluated using a reflectance meter which gives the measure of smoke density of diesel exhaust gas. Now, mostly light extinction/absorption smokemeters based on Beer-Lambert Law are used. The light extinction type smokemeters are more commonly called as ‘opacimeters' as these provide a more realistic measurement of the visible smoke emissions from diesel engines. . Both the sampling type and full flow type opacimeters are in use. The construction requirements, installation and operational details of opacimeters are described in the relevant international standards. A sampling type smokemeter is shown schematically in Fig. 4.13. Figure 4.13 Schematic of a sampling type lightextinction smokemeter. An incandescent lamp with a colour temperature in the range of 2 800 K to 3 250 K or a green light emitting diode (LED) with a spectral peak between 550 nm and 570 nm is used as light source. The transmitted light is received on a photocell or a photo diode (with filter if necessary). When the light source is an incandescent lamp, the receiver should have maximum response in the range 550 nm to 570 nm wavelength as is for the human eye. When light from a source is transmitted through a certain path length of the exhaust gas, smoke opacity is the fraction of light that is absorbed in the exhaust gas column and does not reach the light detector of smoke meter. The absolute smoke density is given by the absorption coefficient, ks which has units of m -1 and is given by: (4.4) where L is length of smoke column in meter through which light from the source is made to pass, I0 is the intensity of incident light, I is the transmitted light falling on the smokemeter receiver. Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture18/18_8.htm[6/15/2012 3:03:16 PM] In the full flow type smokemetersm, the light source and detector are placed directly across the exhaust gas stream usually at the end of exhaust pipe. In this case, path length of smoke measurement varies with the cross sectional size of the exhaust gas stream or tail pipe. Hence, conversion charts of the measured value to the absolute smoke density, ks for different exhaust pipe diameter or path lengths are made available for the full flow smoke meters. ; Objectives_template file:///C|/...%20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture18/18_9.htm[6/15/2012 3:03:16 PM] Module 4: Vehicle Emission Standards and Measurement Lecture18:Emission Measurement Constant Volume Sampler (CVS) When emissions are to be measured from a vehicle being run on a driving cycle sampling of the representative gas is very critical. Constant Volume Sampling (CVS) is used in European, US and other tests to make it possible that a representative sample of the exhaust gas is withdrawn for measurement of the gaseous emissions. Figure 4.14 C onstant volume sampling (CVS) unit using critical flow venturi (CFV-CVS ) for measurement of mass of the exhaust emission. A Constant Volume Sampling (CVS) system is shown schematically in Fig. 4.14 . In the CVS system; The entire exhaust gas from the vehicle is diluted with the filtered room air. An air to exhaust gas dilution ratio of about 10:1 is used. The dilution with air lowers partial pressure of unburned hydrocarbons and water, and prevents their condensation in the sampling line. The diluted exhaust gas is drawn by a constant volume pump system employing either a positive displacement pump (PDP) or a critical flow venturi (CFV) and a blower. A PDP capacity of about 10 to 12 m3/min of air flow provides sufficient dilution for most passenger cars The volume flow rate of the diluted exhaust (exhaust gas + air) is maintained constant throughout the test . Before the diluted exhaust gas enters the CFV or PDP, its temperature is controlled within the ± 5 ºC of the average gas temperature during the test by a heat exchanger. From the diluted gas a small sample is continuously withdrawn and collected in evacuated Teflon bags. This process integrates the concentration of the pollutants over the entire driving schedule. A small part of the dilution air is sampled simultaneously and collected in a separate Objectives_template file:///C|/...20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture18/18_11.htm[6/15/2012 3:03:17 PM] Module 4: Vehicle Emission Standards and Measurement Lecture18:Emission Measurement Figure 4.15 Partial flow dilution systems withisokinetic sampling probe. Full Flow Dilution Tunnel In the full flow system, entire exhaust of the engine/vehicle is diluted with the filtered room air. The full flow system is quite large in size and expensive. A full-flow double dilution tunnel is shown schematically in Fig. 4.16 . For small engines/vehicles only the primary dilution tunnel is used. For the large engines to provide the desired dilution ratio, the gas is diluted again in the secondary dilution tunnel. The sample is withdrawn for measurements from the secondary dilution tunnel in the large engines. The dilution ratio is maintained around 10:1. The temperature of the diluted exhaust gases at the primary filter is maintained at 325º K or less by a heat exchanger. The flow rate of diluted gas is kept constant during the test by a CVS system. After thorough mixing of exhaust and air in the dilution tunnel, a constant flow rate sample is extracted that is filtered through a Teflon coated glass fibre filter. The mass of particulate is determined by weighing the particulate mass collected on the filter. A reference filter is used to determine the particulate mass in the dilution air for correction of PM measurement for the background PM. The filter papers are conditioned before and after filtration to prevent condensation of any moisture or deposition of foreign particulate matter from atmosphere. With the full flow dilution tunnel and CVS system, gaseous pollutants are also measured simultaneously with particulates Objectives_template file:///C|/...20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture18/18_11.htm[6/15/2012 3:03:17 PM] Figure 4.16 Full flow dilution tunnels for measurement of particulate emissions Objectives_template file:///C|/...20and%20Settings/iitkrana1/My%20Documents/Google%20Talk%20Received%20Files/engine_combustion/lecture18/18_12.htm[6/15/2012 3:03:17 PM] Module 4: Vehicle Emission Standards and Measurement Questions (4.1) Average engine-out emissions before catalytic treatment from a SI engine car operating on stoichiometric gasoline-air mixture are CO = 12 g/km, HC = 1.5 g/km and NOx (as NO) = 1.0 g/km. The fuel consumption of car is 80 g/km. Estimate the average concentration of pollutants CO (as % volume), NO (ppm), HC (ppmc1). Take gasoline as isooctane, universal gas constant = 8314.3 kJ/kmol.K. (4.2) The above car meets the Euro 3 standards CO = 2.3 g/km, HC = 0.2 g/km and NOX = 0.15 g/km. What would be the concentration of each pollutant in the exhaust gas at exit of the tail pipe. (4.3) In the US FTP cycle (Fig 4.3) the first 505 seconds of driving schedule is repeated after 10 minutes of hot engine soak. How would the average emissions of CO, HC and NOX differ during the initial and the last 505 seconds of the driving cycle? (4.4) Discuss why NDIR does not measure the unburned hydrocarbons correctly even though it is calibrated with standard mixtures of n-hexane in nitrogen gas. (4.5) A light extinction type smoke meter has smoke column length, L= 430 mm and it shows smoke density of 60% (60% light is absorbed in the smoke column). Determine absolute smoke density of the exhaust gas in terms of m-1. The full flow type of smoke meters are used by fitting them at the end the exhaust pipe. The smoke column length for the full flow smoke meter may be taken equal to the exit diameter of the exhaust pipe. Make a table for the absolute smoke density varying from 0.25 m-1 to 2.5 m-1 in steps of 0.25 m-1 correlating the smoke opacity readings observed with the sampling smoke meter having L = 430 mm and full flow smoke meter fitted to the exhaust pipe of 100 mm dia. (4.6) Discuss the advantages and disadvantages of the full and partial flow dilution tunnels for measurement of PM emissions. (4.7) To measure emissions under fluctuating and transient operating conditions of a vehicle during the driving cycle operation, the emission analyzers having very fast response are required so that instantaneous concentration measurement for the pollutants can be done and the concentrations are then integrated over the driving cycle to determine emissions in terms of g/km. Such emission analyzers are not yet available and there is also delay in the sampling line from the exhaust to analyzer. Discuss how a constant volume sampling (CVS) system functions and this problem is taken care of. In case, the entire exhaust gas during the driving cycle is collected in a big balloon and then the average pollutant concentration is determined, what may be the disadvantages compared to the CVS system.