Exhaust-Gas Analyzer

The present invention relates to an exhaust-gas analyzer, particularly for emissions testing of motor-vehicle engines. More specifically, the present invention is related to an analyzer having a sampling tube into which a mixture of exhaust gas and ambient air is fed through gas lines, a gas feed pump being disposed downstream of the sampling tube.

Many countries require the certification of motor vehicles, especially with respect to engine emissions. Many of the prescribed certification processes require a so-called CVS (constant-volume sampling) dilution system. In this CVS certification procedure, a sample is taken from a predetermined quantity of gas, composed of engine exhaust gas and ambient air. The ratio between exhaust gas and air changes continually because a driving cycle involves different operation modes such as acceleration, deceleration, etc., of the vehicle. Each mode results in different exhaust-gas/air ratios. In the known exhaust-gas analyzers for the CVS certification procedure, an average dilution rate has been used to determine the pollutant concentration. An average dilution rate will necessarily yield only an integrated value. An instantaneous result (in acceleration phases, for example) cannot be ascertained with such a procedure. Erroneous conclusions concerning the actual relationships therefore are not precluded.

The present invention substitutes direct evaluation for the integrated evaluation employed in the known exhaust-gas analysis. Thus, it is possible to measure the relationship between concentration and quantity of the exhaust gas unambiguously, during each operation mode, that is, in predetermined sampled portions. The present invention further seeks to achieve a simplified and lower-cost design of the exhaust-gas analyzer.

The present invention achieves these results by inserting a flowmeter into the air supply line. The flowmeter delivers electrical signals to a computing unit which computes the instantaneous standard total flow rate and the standardized exhaust gas value. The computing unit then computes a dilution factor based on the instantaneous standard flow rate and the standardized exhaust value. The computing unit compensates for gas pressure and temperature in the gas mixture. This dispenses with the need for an expensive heat exchanger to maintain temperature equalization. Thus, there is no continuous energy consumption for cooling water or heat. A flowmeter in the air supply line, from whose values the standard flow rate is determined, allowing for gas pressure and temperature, provides a surprisingly simple and accurately operating CVS diluting system. For a mass flowmeter, the type of flowmeter used to great advantage, the condition of the air is unimportant. The air must merely be filtered to prevent fouling of the flowmeter based on the mass-flow principle.

The flowmeter is advantageously designed as a vortex-shedding flowmeter and operates on the Karman vortex street principle. Vortices form at an impingement body with trapezoidal cross section and are alternately shed. The frequency f with which the vortices are shed is directly proportional to the fluid velocity, and hence proportional to the flow rate (volumetric flow rate Q). The measurement obtained is independent of pressure, temperature, density and viscosity of the measured medium, provided that a critical Reynolds number Re is observed. Such vortex flowmeters are known and are mass-produced. Surprisingly, their use yields synergistic advantages with respect to the makeup of the entire exhaust-gas analyzer.

FIG. 1 illustrates a diluting and sampling system in accordance with the present invention.

FIG. 2 illustrates an alternative embodiment of the dilution and sampling system of the present invention.

In FIG. 1, the analyzer includes a flowmeter 1, such as a vortex-breaking flowmeter. The pressure and temperature prevailing in it are continuously measured and electronically compensated. This may be done in the manner shown or in a decentralized manner.

The air measured in the flowmeter 1 is first filtered by means of an air filter 6. The latter may be a cartridge filter or a bag filter. The exhaust gases from the internal-combustion engine being tested arriving through line 8 are combined with the air flowing through the line 7. A pipe 11 withdraws a desired sample portion from a stabilizing section 5. The stabilizing portion 5, at which measuring instruments P and T measure pressure and temperature, is advantageously disposed between flanges 9 and 10. On the inlet side, it may be provided with equalizing elements (not shown), for example, a baffle grid, for the two partial streams of exhaust gas and air brought together. Both the measured value from the flowmeter 1 and the pressure and temperature values are fed to a computing unit 2, which may be a Siemens 16-bit SMP computer, for example, the computing unit determines the instantaneous total flow rate (e.g., in m.sup.2 /min) through the venturi nozzle 4 in accordance with equation (1) where K is a correction factor {m.sup.2 * sqrt (K.degree.) / (min * kPa)}, P is the instantaneous absolute pressure {kPa}, and T is the instantaneous temperature {K.degree.}. The instantaneous standard flow rate (e.g., in m.sup.2 /min) is then determined in accordance with equation (2) where P.sub.o is a calibration pressure {kPa}, P.sub.n is a standardizing pressure based on environmental regulations {kPa}, T.sub.o is a calibration temperature {K.degree.}, and T.sub.n is a standardizing temperature based on environmental regulations {K.degree.}. The instantaneous standard flow rate through an ambient air probe (e.g., in m.sup.2 /min) is determined in accordance with equation (3) where L is the instantaneous flow rate through the ambient air probe {m.sup.2 /min}. The standardized exhaust gas volume can then be determined in accordance with equations (4). Finally, the dilution factor is determined in accordance with equation (5). The computer 2 then sends that information to the test-stand master computer. ##EQU1##

The stabilizing length or steady flow zone 5, sampled through the pipe 11, is followed by a venturi nozzle 4. That nozzle is a critical nozzle and allows only a constant, maximum flow rate. A gas feed pump 3 is downstream of the nozzle 4. The rated suction capacity of the pump 3 is greater than the maximum mass rate of flow through the exhaust-gas analyzer. This assures constant flow through the exhaust-gas analyzer. Pulsations in the exhaust-gas flow are advantageously dampened, and an accurate instantaneous value can be determined in standard quantities. Because of the smooth flow through the dilution system and the elimination of the heat exchanger with its flow-stabilizing properties, this behavior of the suction side of the dilution system is of particular importance.

FIG. 2 illustrates an alternative embodiment of the present invention in which a constant flow rate is produced by a displacement pump 13 having a pressure difference 14 across its intake and outlet in place of the venturi nozzle 4 and the gas feed pump 3 of the first embodiment. The displacement pump 13 is preferably a "Root's" blower or "Root's-type" compressor. A means for measuring 14 the pressure difference across the displacement pump 13 is also included.

In this alternative embodiment the dilution factor is computed as follows. The instantaneous flow rate through the displacement pump (e.g., in m.sup.2 /min) is determined in accordance with equation (6) where D is the rated suction volume per revolution {m.sup.2 /revolution}, M is a suction volume reduction {m.sup.2 /min}, A is the RPM of the flow stage {1/min}, B is a factor of the RPM reduction {1/(min * kPa)}, T is the instantaneous temperature {K.degree.}, P is the instantaneous absolute pressure {kPa}, .DELTA.P is the pressure difference across the displacement pump {kPa}, N is a counter reading {pulses/sec}, Z is the number of pulses per revolution, T.sub.b is a standardizing temperature {K.degree.}, and P.sub.b is a standardizing pressure {kPa}. The instantaneous standard flow rate through the displacement pump 13 (e.g., in m.sup.2 /min) is then determined in accordance with equation (7) where T.sub.o is a calibration temperature {K.degree.}, T.sub.n is a standardizing temperature based on environmental regulations {K.degree.}, P.sub.o is a calibration pressure {kPa}, and P.sub.n is a standardizing pressure based on environmental regulations {kPa}. The instantaneous standard flow rate through the ambient air probe (e.g., in m.sup.2 /min) is determined in accordance with equation (8) where L is the instantaneous flow rate through the ambient air probe {m.sup.2 /min}. The standard exhaust gas volume is then determined in accordance with equations (9). Lastly, the dilution factor is determined in accordance with equation (10). ##EQU2##

While the individual components of this analyzer are known per se, one skilled in the art could not have expected the combination arrangement in accordance with the invention, as shown in the drawing, to be serviceable in the absence of invention since its basic design differs in principle from the prior-art CVS analyzers.