Dilute and Raw Sampling
Dilute and raw sampling are two methods of obtaining exhaust gas for analysis. In dilute sampling, the blower draws air into the sampling system (Figure 7.30). The air
Exhaust Gas Emissions and Analysis 139 then is filtered and mixed with the exhaust gas. Particles are separated from the air and
exhaust gas by the cyclone. The flow rate of the mixed air and exhaust gas is regulated to a constant value by the critical flow venturi. A sample of the dilutant air is put into the ambient air bag. When the vehicle is operated over a drive cycle, a sample of the mixed air and exhaust gas is put into the sample bag. The flow rate of the samples is regulated to ensure that the samples are representative of the emissions produced dur- ing the drive cycle. The drive cycle may be split into several phases; there will be one ambient bag and one sample bag for each phase. At the end of the test, the contents of both bags are analyzed. The level of each pollutant is determined by subtracting the measured value in the ambient bag from the measured value in the sample bag.
Ambient air from test cell
"
displacement pump Measurement (Tedlar),e bag
Particulate Measurement
Before continuing, it is useful to give a definition of particulate. In the context of auto- motive engineering, it refers to the matter collected on a pair of fluorocarbon-coated glass fiber filter papers, or fluorocarbon-based membrane filter papers with a minimum stain diameter of 60 mm. The diluted exhaust gas temperature must be below 5 1.7"C (measured in the sampling zone), per U.S. EPA Regulation 40. Particulates consist of several materials. The main constituents are carbon, condensed and absorbed hydro- carbons, and sulfates. The particulate analyzer cannot determine the relative quantities of each constituent; it can only measure the combined mass of all of the constituents.
Figure 7.30 Basic CVS system.
8 I
Positive
L
Principles and Operation of Flow Tunnels
To measure the particulate matter in the exhaust gas, all particulate matter must be removed from the dilutant air. The filtered dilutant air then is passed down a tunnel, where it is mixed with exhaust gas. To achieve good mixing, the flow rate must be sufficient to
Silenclx r
Next Page
140 An Introduction to Enaine Testina and Develo~ment
ensure that the air is turbulent. The exhaust gas is cooled by the dilutant air to simulate exhaust gas leaving the exhaust pipe of a vehicle. A sample of the cooled exhaust gas and dilutant air is drawn from the tunnel and is filtered. The particulates are trapped on the filter papers. The flow rate through the filter papers and the mass of particulate on the filter paper are measured. This gives the concentration of particulate matter per unit volume of diluted exhaust gas sampled (in grams per cubic meter). For example, if x grams of particulate are collected on a filter paper during a sample period of y seconds, and the flow rate of the sample is in cubic meters per second, then the concentration of particulate is
low rate y
I
x z)[
g x sx
m-3x
s-'I = [gn"]where
x = grams of particulate on filter paper y = sample time
s = sample volume
(gs) = total volume of diluted gas z = concentration pf particulate
The total particulate emission for the engine is calculated as follows. First, find the concentration of particulate in the sampled volume of gas, and then take the ratio of the sampled volume and the total volume of the diluted exhaust gas. Finally, multiply the two together.
Several features of the design of a particulate tunnel are important to achieve accurate and repeatable results. If a new tunnel is inspected after a few hours of operation, a significant amount of particulate will have been deposited on the walls of the tunnel and in the exhaust inlet or transfer tube. Any measurements taken during this period therefore will be light because some of the particulate that should have been deposited on the filter paper now is on the walls of the pipe-work and so forth. Therefore, the tunnel must be conditioned before accurate measurements can be taken.
If the tunnel is disturbed, some of the particulate deposits may be transported from the pipe-work and onto the filter paper. This will make a measurement heavy. To test the stability of a tunnel, it is useful to run some dummy tests with no exhaust gas flow before the tunnel is used. If the mass of the filter papers does not change during the dummy test, then the tunnel can be assumed to be stable.
The temperature of the tunnel and the pipe-work should be kept as constant as possible.
Changes in temperature can affect the rate at which particulate is deposited onto the walls and the pipe-work.
The temperature of the dilution air should be maintained to 25 f 1°C. Although this recommendation is much more stringent than most regulations, it is important to maintain accurate control of air temperature if good repeatability is to be achieved.
The dilution ratio is the mass flow rate of the exhaust gas entering the tunnel divided by the mass flow rate of the dilutant air. A target dilution ratio of approximately 6: 1 to 10:l should be set for the tunnel. If high dilution ratios are used, then the sensitivity Previous Page
Exhaust Gas Emissions and Analysis 141
of the measurement is reduced. If low dilution ratios are used, then the nature of the particulates formed in the tunnel may become unstable. At all times, the dilution ratio should never be allowed to fall below 4: 1. Figures 7.3 1 and 7.32 show examples of two commonly used types of flow tunnels.
Exhaust
1_11
Diluted exhaust gas
, , - -
Filter
7 --
Flow meterPump
I I
Filter Flow meter
L b q
L l l
\
Flow meter
Air
Figure 7.31 Full-jlow particulate tunnel.
Figure 7.32 Simplijied mini-particulate tunnel.
142 An Introduction t o Engine Testing and Development
Filter Handling and Weighing
The amount of particulate deposited on the filter paper is small when compared to the weight of the paper. To achieve accurate results, it is important to handle the filter papers carefully. If any dust or dirt is allowed to get onto the paper, this will increase the mass of the paper and give rise to a heavy result. If any damage is caused to the paper (e.g., too high a clamping pressure in the filter holder), then the mass of the paper will be reduced, and a light result will occur. Figure 7.33 shows an example of a typical filter paper. The balance used to weigh the filter papers must be of sufficient accuracy to detect changes in the microgram range. Also, it must be fitted within a closed weighing chamber to reduce the effects of air currents. Calibration of the balance must be done before use and checked regularly with weights that are traceable to national standards.
Two clean reference papers should be weighed before each batch of test filter papers.
If there is any change in the mass of the filter papers, the precision of the balance and the humidity of the conditioning environment must be checked. The amount of water absorbed by the paper must be consistent during the pre- and post-test measurements.
To achieve this, the paper must be conditioned in a clean humidity-controlled cabinet for several hours before weighing.
Figure 7.33 A 72-mm- diameter standard reference filter paper
In conclusion, the major advantages of a full-flow tunnel are as follows:
1. The flow rate of the engine exhaust does not have to be measured.
2. The mixed gas flow rite is constant, even for transient test applications.
3. The constant volume control system is simple, relatively accurate, and generally reliable.
Principles and Operation of Micro- Tunnels
In a full-flow tunnel, only a tiny proportion of the mixed gas is drawn through the filter paper. If a fixed proportion of exhaust gas is diverted into the dilution tunnel (rather than taking all of the exhaust gas), then a much smaller tunnel and dilution system can be employed. The amount of particulate emission measured in the sampled gas then can be multiplied by the proportion of sampled gas to total exhaust gas to deter- mine the total particulate emission for the engine. The micro-tunnel operates by first measuring or estimating the exhaust gas flow rate. A controlled quantity of dilution air is mixed with the exhaust gas to create a similar dilution ratio to that which would have occurred within a full-flow tunnel. The whole of the mixed gas then is passed through the filter papers where the particulate is extracted. The flow rate of the dilution air is adjusted rapidly to follow the changes in exhaust gas flow caused by changes to engine operating conditions. The quantity of exhaust gas sampled is calculated by
Exhaust Gas Emissions and Analysis 143
subtracting the measured amount of dilution air added to the sampled exhaust gas from the measured total flow of mixed gas.
Principles and Operation of Mini-Tunnels
Mini-tunnels normally are suitable only for steady-state testing. This is true because the gas splitting system is not able to respond in sufficient time to follow the rapid changes in exhaust gas flow rate encountered when running transient tests. Several types of gas splitting systems are in use; a common one uses an isokinetic probe to maintain a fixed ratio of sampled gas flow to exhaust gas flow. The isokinetic probe functions by keeping the average velocity of the sampled gas in the probe equal to the average velocity of the gas in the exhaust pipe. If equal velocities are maintained, then the sampled gas flow rate is proportional to the total gas flow rate. The ratio of the amount of gas sampled to the total exhaust gas flow then is equal to the ratio of the cross-sectional area of the probe to the cross-sectional area of the exhaust pipe. The velocity of the gas in the probe is controlled by equalizing the static pressure of the sampled gas in the mouth of the probe, with the static pressure of the gas in the exhaust pipe (measured in align- ment with the tip of the probe). Mini-tunnels can function adequately if the isokinetic probe is kept clean and the temperature changes of the surface of the transfer tube can be controlled or at least made to be repeatable from test to test.
The two major advantages of mini-tunnels and micro-tunnels are as follows:
1. They require less space.
2. The dilution air can easily be filtered and conditioned; this will become a major advantage as the amount of emissions from engines continues to fall to very low levels.
Principle of Continuous Particulate Analyzers
If a continuous measurement of engine particulate emission is required, then the filter paper technique is not suitable. Several devices can be attached to a conventional full or partial flow tunnel to measure continuous particulate emission. One technique now in common use is the tapered element oscillating microbalance (TEOM), as shown in Figure 7.34.
Flow
Filter
10-rnrn diameter
Tapered element glass tube oscillates at its own frequency.
Vibrating Particulate matter
collects on filter.
Frequency decreases with the accumulation of particulate mass.
Direct relationship
between mass and Figure 7.34 Tapered ele- frequency change. ment oscillating microbal-
ance (TEOM).
1 44 An Introduction t o Engine Testing and Development
In this device, the particulate filter paper is attached to a hollow tube. A vacuum pump draws a controlled amount of diluted exhaust gas through the filter paper. A constant force is applied to the tube, which causes it to vibrate. As particulate builds on the filter paper, the increased mass causes the frequency of the vibration to change. This change in frequency is detected and converted into a measurement of continuous particulate mass emission.
The bag method of sampling returns one value for each pollutant for each pair of bags.
If the level of pollutants for the transient parts of a drive cycle is required, then the sample must be measured continuously. If catalyst performance is to be measured, then two continuous raw samples must be taken: one upstream of the catalyst, and one downstream of the catalyst (or three if mid-bed values are required). If measurement of EGR is required, then a fourth raw sample must be taken from the intake manifold.
Hot and Cold Sampling
To prevent hydrocarbons from condensing on the walls of the cold raw sample lines, the lines that feed the hydrocarbon (and often the NOx) analyzer are heated to 190°C.
Therefore, the total number of raw sample lines on a typical system is five (hot and cold pre-catalysts, hot and cold post-catalysts, and EGR) but could be seven if mid-bed catalyst measurements are required.