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.
Exhaust Gas Emissions and Analysis 145
Flow Rate
The time taken for the sample gas to pass from the vehicle exhaust to the analyzer must be kept constant to time-align the continuous results to the drive cycle. The high bypass flow rate helps to shorten the response time and to lessen the time-alignment error due to partial filter blockage.
Dryness
Water vapor can cause measurement errors by condensing on optical lenses, absorbing infrared radiation, and reducing the concentration of some gases by absorption. Water vapor in the sample normally is reduced to a low level by passing the sample gas through a coil cooled to 4OC. The water usually is collected in a reservoir at the base of the coil and then is blown out periodically.
Solid Particles
Solid particles can cause measurement errors by blocking capillaries and absorbing infrared radiation. A filter usually removes the solid particles. The filter normally is the first component in the sampling system after the connecting line in order to protect as much of the sampling system as possible.
Sample Pump
The sample pump must be able to generate a sufficient and stable flow rate, the wetted parts must be resistant to attack from the sample gas, and it must not add to, or lose, any of the components being measured.
Leak Checking
Sampling additional air from the surroundings probably is the most common form of measurement error. Small leaks are difficult to detect because the exhaust gas usually contains some oxygen. Leaks are common because the joints in the sample line often are disturbed (i.e., when connecting a sample line to a new vehicle or when changing a filter). Leaks can be found by overflow checking the sampling system with nitrogen or a cocktail mixture of several reference gases (e.g., NOx, HC, and CO).
Back Flushing
To prevent the sampling system from becoming contaminated too quickly, it is important to back flush the system between readings. The back flush blows contaminants back down the sample line and into the engine exhaust pipe. It also drains any water that has been trapped below the cooling coil.
Non-Dispersive Infrared Analyzer
A gas will absorb light energy of a frequency band that generally is peculiar to the gas.
If this frequency band is narrow and is not shared by any of the other gases that are likely to be present in the sample, then the amount of energy within a certain frequency band that is absorbed by a sample of exhaust gas can be determined. The amount of energy absorbed is proportional to the concentration of the gas to be measured. The non-dispersive infrared (NDIR) analyzer works by passing a pulsed beam of infrared light through a chamber containing the sampled gas. The amount of light absorbed by
146 An Introduction to Engine Testing and Development
the measured gas is detected and is converted into an electrical signal. The electrical signal then is scaled to reflect the concentration of the measured gas. The light is pulsed by a chopper, which is a rotating disc with openings that alternately stop and allow the light to pass. The light normally is passed through a chamber containing nitrogen, called the reference cell, and the chamber containing the gas to be measured is called the sample cell. The detector contains the same gas as the gas to be measured. The detector measures the difference in the amount of light energy absorbed between the gas in the sample cell and the gas in the reference cell. Figures 7.35 and 7.36 show the basic principle of an NDIR analyzer and infrared absorption wavelengths.
Figure 7.35 Basic prin- ciple of an NDIR analyzel:
Figure 7.36 Horiba cell;
typical reference cell NDIR CO and C02
Optical filter Optical chopper
I
Main Compensate(
detector detectordetector
+
interference) SubtractionY
signal (- interference) Output (target compounded)Exhaust Gas Emissions and Analvsis 147
Flame Ionization Detector
In the flame ionization detector (FID), a sample of exhaust gas is passed through a flame. The flame bums any hydrocarbons present in the sample gas. When an electri- cal current is passed across the flame, the current changes in proportion to the amount of hydrocarbons contained in the sample gas. The electrical signal from the FID then is amplified and is passed to a display and logging device. The flame is sustained by mixing a gaseous fuel and air within a burner. The sample gas is introduced to the flame by mixing it with the fuel. Maintaining stable and precise flow rates of the sample gas, burner fuel, and burner air is vital if good accuracy is to be achieved. The flow rate of each gas is set by controlling the pressure drop across a capillary. Therefore, maintain- ing a constant pressure (especially between the sample and span gases) and a clean capillary are vital to achieve good performance. Care also must be taken to prevent any unwanted hydrocarbons from entering the burner, sampling system, or fuel and air lines.
Contamination will show up as a high background level reading, and a true zero will be impossible to achieve. For this reason, high-purity gases must be used for the burner fuel and air. The basic principle of an FID analyzer is demonstrated in Figure 7.37, and an FID is shown in Figure 7.38.