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Module 4: Vehicle Emission Standards and Measurement
Lecture18:Emission Measurement
The Lecture Contains:
EMISSION MEASUREMENT
CO and CO 2 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
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.
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 CO 2 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,
where
I = Radiation energy absorbed
I 0 = Incident radiation energy
k = characteristic absorption constant for the gas, m 2 /gmol
c = concentration of the gas, gmol/m 3
d = length of the gas column, m
Module 4: Vehicle Emission Standards and Measurement
Lecture18:Emission Measurement
contd....
NDIR analyzers enable accurate measurements of CO and CO 2 in the exhaust gases.
Figure 4.
Schematic of an NDIR analyzer for measurement of
CO and CO 2 concentration.
NDIR instruments are seldom used for measurement of hydrocarbons except in the garage type analyzers as the IR absorbance to different hydrocarbons varies substantially. The unsaturated hydrocarbons are primarily responsible for photochemical smog but they do not have an adequate absorption in the IR wavelength range that is specific to the saturated hydrocarbons and vice versa. Sensitivity and response of NDIR to the exhaust HC is typically only half of the probable true value. NO absorbs only weekly in the infrared region. Moreover, CO, CO 2 and water vapours interfere seriously;
hence NDIR analyzers are also not used for NO measurement.
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 (C 3 ) 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.
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) NMHCâCO 2 âH 2 O.
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 CH 4 are oxidized to CO^2 and water on a catalyst, so that
when the gas sample is passed through NMC only CH 4 is detected by HFID. The NMC cutter is
calibrated for catalytic effect on CH 4 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.
Module 4: Vehicle Emission Standards and Measurement
Lecture18:Emission Measurement
Chemiluminescence Analyzer (CLA)
When NO and ozone (O 3 ) react a small fraction (about 10% at 26.7° C) of excited NO 2 * molecules is produced as per the following reactions:
As the excited molecules of NO 2 * decay to ground state, light in the wavelength region 0.6-3.0 ÎŒm is
emitted. The quantity of excited NO 2 produced is fixed at a given reaction temperature and the intensity
of light produced during decay of excited NO 2 is proportional to the concentration of NO in the sample.
Figure
Schematic and flow diagram of a c hemiluminescence NO x
analyzer.
A schematic diagram of the chemiluminescence NO (^) x analyzer is shown in Fig. 4.12.
The sample containing NO flows to a reactor where it reacts with ozone produced from oxygen in âozonator' .In the reactor NO is converted to NO 2. A photomultiplier tube detects the light emitted by the excited NO 2. The signal is then amplified and fed to recorder or indicating equipment. For the measurement of nitrogen oxides (NO (^) x ), NO 2 in the sample is first converted to NO by heating in a NO 2 - to-NO converter prior to its introduction into the reactor. At 315Âș C, about 90 percent of NO 2 is converted to NO 2. The total concentration of NO (^) x in the sample is thus, measured as NO. When the sample is introduced in the reactor bypassing the NO 2 - to- NO converter, concentration of NO alone is determined. The difference between the two measurements provides the concentration of NO 2 in the sample. The response of the instrument is linear with NO concentration. The technique is very sensitive and can detect up to 10 -3^ ppm of NO (^) x. The output signal is proportional to the product of sample flow rate and NO concentration. As the
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 âopacimeters' 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 light
extinction 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, k (^) s which has units
of m -1 and is given by:
where L is length of smoke column in meter through which light from the source is made to pass, I 0 is
the intensity of incident light, I is the transmitted light falling on the smokemeter receiver.
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, k (^) s for different exhaust pipe diameter or path lengths
are made available for the full flow smoke meters.
bag to correct for any background concentration of pollutant present in the dilution air. The sample bags are analyzed after the test is completed. The mass of individual pollutants is determined from its measured concentration in the sample bag, its density and the total volume flow rate of the diluted exhaust during the test through CVS.
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Module 4: Vehicle Emission Standards and Measurement
Lecture18:Emission Measurement
s
Particulate Emission Measurement
For measurement of particulate emissions, the gas is diluted with air in a dilution tunnel and, a sample is continuously collected from the diluted gas and filtered to collect particulate matter. The mass of the collected PM is measured to determine specific PM emissions in terms of g/km or g/kWh.
The dilution tunnels are of two types
Partial flow, and Full flow dilution tunnel
Partial Flow Dilution Tunnel
In the partial-flow system, only a small part of the exhaust stream is diluted. To withdraw a true representative of the exhaust gas the following systems have been developed;
Isokinetic sampling systems Flow controlled systems with concentration measurement, and Flow controlled systems with flow measurement
In an isokinetic system, the gas velocity in the sampling tube which leads the sampled exhaust gas to dilution tunnel is kept same in magnitude as the velocity of the bulk exhaust gas stream. In this way, an undisturbed and uniform exhaust gas sample flow at the inlet of sampling probe is obtained.
An isokinetic system is shown schematically in Fig. 4.15. Raw exhaust gas is transferred from the exhaust pipe to the dilution tunnel (DT) through isokinetic sampling probe (ISP) and the transfer tube. The differential pressure of the exhaust gas between exhaust pipe and inlet to the probe is measured with a pressure transducer. The signal is fed to a flow controller that controls the suction blower and, a differential pressure of zero at the tip of probe is maintained. Under these conditions, exhaust gas velocities in the exhaust pipe and probe are identical and the flow through isokinetic probe is a constant fraction of the exhaust flow. The sampling rate can be obtained by the ratio of cross sectional areas of probe and the exhaust pipe. The dilution airflow rate is measured with the flow meter. The dilution ratio is calculated from the dilution air flow rate and exhaust sample to total exhaust flow ratio.
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Figure
Full flow dilution tunnels for measurement of
particulate emissions
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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.
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.
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.
