Signal out

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.

1 48 An Introduction t o Engine Testing and Development

NOx Analyzers

NOx (chemiluminescence) analyzers measure NO by detecting the light emitted when NO is reacted with ozone (03). The intensity of the light is proportional to the amount of NO in the sample gas. The intensity of the light is measured and converted to an electrical signal, mixed together in a reaction chamber. This chamber normally is evacu- ated by a vacuum pump to improve the sensitivity and stability of the analyzer. Other oxides of nitrogen are measured by passing them through a converter located before the reaction chamber. The converter changes any NO2, NO3, and so forth present in the sample gas into NO. The converter can be bypassed so that either the total NOx or NO alone can be measured. The analyzer relies on the precise control of the flow rate of the sample gas and ozone. The flow rates are maintained by controlling the pressure drop across the sample and ozone capillaries. The vacuum pump must give a stable vacuum to maintain a constant pressure drop across the capillaries. All of the ozone must be removed from the outlet of the reaction chamber to prevent a health hazard.

The basic principle of a chemiluminescence NO/NOx analyzer is given in Figure 7.39, and a chemiluminescence detector is shown in Figure 7.40.

Ozone In

Flow control valves

Figure 7.39 Operating principle of the chemi-

luminescence NOhVOx analyzer:

Photo cell

'M

Oxygen Analyzers

At least three types of oxygen analyzers are in common use, as follows:

1. Those that work using the property of paramagnetism

2. Those that use oxygen as part of the electrolyte in an electrical cell

3. Those where the output of a fuel cell will vary, depending on the level of oxygen that enters the analyzer.

Paramagnetic analyzers have an advantage over the other two types because the sens- ing element does not have to be replaced regularly (as in the case of the fuel cell type)

Exhaust Gas Emissions and Analysis 1 49

100

I

Wavelength (nm)

Luminescence

intensity distribution

Figure 7.40 Chemilumi- nescence detector:

or serviced (as in the case of the electrolyte type). Figure 7.41 gives an example of a paramagnetic O2 analyzer.

Oxygen is strongly paramagnetic, which means that the oxygen molecules will tend to align themselves in a way that adds to the strength of a magnetic field. In the design of one popular instrument, the exhaust gas is passed through a chamber shaped similarly to a dumbbell. The aligning force within the oxygen molecules is measured, and this force therefore is proportional to the amount of oxygen in the exhaust gas. Another popular method is to create a magnetic field around the outlet of a small pipe. As oxy- gen collects around the end of the pipe, it restricts the amount of nitrogen that will flow through the pipe. The change in flow of the nitrogen therefore is proportional to the amount of oxygen in the exhaust gas.

Exhaust gas

Nitrogen

Magnets

Figure 7.41 Paramag- netic

O2

analyzex

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1 50 An Introduction to Enaine Testina and Develo~ment

Figure 7.42 Calibra- tion example showing y = mx

+

^c relationship for an analyzel; where

c = zero error and m = gain.

Most gas analyzers measure the concentration of a gas by comparing the gas with two known reference points. The reference points are known as the zero and span values.

The analyzer is calibrated before each measurement by passing a zero gas (usually nitrogen or air) through the analyzer and setting the response of the analyzer to zero.

A gas containing a known concentration of the gas to be measured (span gas) then is passed through the analyzer, and the sensitivity or gain of the analyzer is adjusted to match the value of the span gas (Figure 7.42).

Offset

0

Input

The concentration of the zero and span gases must bracket the expected concentration of the sample gas. To choose the correct span gas, a small sample of exhaust gas should be taken before the analyzer finally is calibrated. The process of pre-sampling before calibrating is called sniffing.

Time Alignment

When continuous gaseous emission measurements are taken during transient engine or vehicle operation, it is important to be able to identify the correct time at which a par- ticular level of emission was produced by the engine. Being able to correctly identify the time when a particular emission was produced is important for two reasons:

1. To be able to link the emission level with an event in the vehicle drive cycle or engine test program

2. To be able to calculate the real-time AFR

To time-align all of the gaseous emission analyzers, the time taken for the exhaust gas sample to travel to each analyzer and the concentration determined must be measured.

When all data from the test run are processed, the emission data points must be repo- sitioned versus test time by an amount equal to the measured time delay because each type of analyzer has different response times.

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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

gas

output

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.