Exhaust Gas Emissions and Analvsis 151
Maintenance
The secret to successful emission measurements is good housekeeping and attention to detail (e.g., keeping the sample lines clean and undamaged, changing the filters regularly, leak checking, calibrating before each measurement). Daily servicing and operating procedures should be established, together with regular routine maintenance and a system calibration plan (normally performed every three months).
Calibration of Analyzers Using a Gas Divider-NOx Efficiency Checks
The linearity of analyzers should be checked after any routine maintenance or major repair. The linearity is checked by accurately blending zero and span gases using a gas divider (Figure 7.43).
Zero gas
s~an
gasoutput
A NOx efficiency check is performed by diluting span gas with NO2 and 02. The amount of NO2 in the span gas is measured by putting the NOx analyzer in NO mode and measuring the reduction in level of NO as NO2 is introduced. The analyzer then is operated in NOx mode by passing the diluted span gas through the converter. If the converter is working correctly, then the NO reading should recover to its undiluted level as the converter changes the generated NO2 into NO.
152 An Introduction to Engine Testing and Development
3. Blue smoke, which is caused by lubricating oil that has passed from the engine and into the exhaust pipe
The level of smoke emission from engines is controlled by legislation in most devel- oped countries. The level of smoke emission from vehicles in the United Kingdom is controlled in three ways:
1. At the vehicle or engine manufacture
2. During the annual vehicle Ministry of Transport (MOT) check 3. By observations or checks performed by the police
Two techniques for the measurement of smoke are in common use. The first technique uses a pump (Figure 7.44) to draw exhaust gas through a filter paper. The blackness of the paper then is given a value by measuring the amount of light that is reflected back from the blackened area. In the second technique, light is passed through the exhaust gas, and the amount of light that is able to reach a detector is measured. In the latter (light extinction technique), light can be passed either across the exhaust pipe (across the duct method) or along the exhaust pipe. The filter paper technique measures only the carbon particles that are deposited on the paper; it cannot detect blue or white smoke.
The light extinction technique measures all smoke but cannot determine the color of the smoke. The main advantage of the light extinction technique is that the smoke emis- sion can be measured continuously. With the filter paper technique, only snapshot or steady-state measurements are possible.
When taking exhaust gas samples for analysis, it is recommended that a special probe design (Figure 7.45) is adopted to ensure that the relative gas velocity in the main exhaust pipe does not impinge on the sample rate of the relative analyzer.
Figure 7.44 Bosch hand Pump.
Exhaust gas flow
2 ' )
{
as sampleFigure 7.45 Preferred
I
exhaust gas sample probe
design. ______)
/
Exhaust Gas Emissions and Analysis 153
Operation of Smoke Meters Using the Filter Paper Technique
The simplest smoke meters consist of a calibrated chamber, a spring-loaded piston assembly, a trigger mechanism (pneumatic), and a filter holder. The chamber is evacu- ated by displacing the piston against the resistance of the spring. The piston is held in the evacuated position by the trigger mechanism. A filter paper is inserted in the holder and clamped using the knurled nut. When the engine is running at the required condition, applying a pneumatic signal to the trigger mechanism triggers a reading. The piston is displaced by the spring, and a calibrated volume of exhaust gas is drawn through the filter paper. The smoke level in the exhaust gas is determined by measuring the light that is reflected from the blackened filter paper.
The following are the main problems encountered with these types of smoke meters:
1. Premature blackening of the filter paper fiom deposits remaining on the holder 2. Condensation on the filter paper
3. Blocking of the sample probe 4. Heat damage to the sample line
5. Inaccurate calibration of the sample volume 6. Inaccurate calibration of the evaluation unit
The AVL smoke meter shown in Figure 7.46 is, in effect, an automated hand pump in its operation.
Light source Filter paper
'u"
/Sampling cylinder
Operation of Smoke Meters Using the Opacity Technique
The smoke meters that use the opacity technique can be divided into two main types:
1. Those that take a sample from the exhaust system (i.e., along the duct) (Fig- ure 7.47)
Figure 7.46 A VL smoke meter:
2. Those that measure across the exhaust pipe (i.e., across the duct) (Figure 7.48)
1 54 An Introduction to Enaine Testina and Develo~ment
Figure 7.47 Along-the- duct smoke meter
A
Eshaust gas
-
I
Smoke tube -t i I
I
1
Detector -,
A
t- Reference
I~lk
i
L
Air- air
Figure 7.48 Across-the- duct smoke meter
Detector I
Air
1
The amount of light that fails to reach the detector depends on the following:
1. The number of smoke particles 2. The size of the smoke particles
3. The light absorption characteristic of the particles 4. The length of the path of the light beam
For a particular smoke meter, the length of the path of the light beam is constant. There- fore, changes to the number, size, or absorption characteristics of the smoke particles
Exhaust Gas Emissions and Analysis 155
contained in the exhaust gas will alter the amount of light that reaches the detector. The change in the amount of light that reaches the detector will cause a change in the electri- cal output of the detector. To achieve accurate measurements, it is important to keep the lenses of the light emitter and detector clean. This can be difficult when dirty exhaust gas is being passed continually over the lenses. A curtain of filtered air often is blown across the lenses to prevent the exhaust gas from coming into contact with the lenses and depositing particles on them. If an air curtain is used, care must be taken to ensure that only the minimum amount of air is introduced to the exhaust gas. Otherwise, the exhaust gas will be diluted by the air and will cause an inaccurate measurement. This is especially important at low exhaust gas flow rates.
The following are the main sources of problems with these smoke meters:
1. Contamination of the lenses due to loss of purge air
2. Contamination of the lenses due to water or oil in the purge air 3. Poor optical alignment of the emitter and detector
4. Mechanical or heat damage to the emitter, the detector, and the electrical leads and connections
The main advantage of the across-the-duct smoke meter is the simplicity of the installa- tion. The absence of the need to install an exhaust gas sampling system greatly reduces the chances of errors due to hang-up, condensation, and sample system blockage. Care must be taken to present the exhaust gas to the meter correctly. The meter should not be installed close to any bends or sudden changes in exhaust pipe cross-sectional areas.
One possible disadvantage of across-the-duct smoke meters is that the relatively short length of the path can cause reduced sensitivity to exhaust gas with low smoke content.