Gas flow analyzer for measuring automobile emission transient operating conditions

1 Introduction

Automobile pollution is one of the most important issues that people are most concerned about and need to solve urgently. As an important method for the detection of automobile exhaust pollutants, the simple transient operating method (VMAS method, IG method) can count the total quality of emissions, monitor the true emission of vehicles, the equipment cost is not high, the measurement is more accurate, and it is certified with new vehicles The test results are correlated, and can detect a series of advantages such as NO related factors, which has become a hot research topic. The VMAS (Vehicle Mass Analysis System) detection method has a good application foundation in the United States and Europe. At present, the flow analyzer of the VMAS test system used in China is basically imported, especially the most products of the American sensors company. The product is not only expensive but also inconvenient to maintain and repair. It is not conducive to complete the secondary development with a complete test system composed of a chassis dynamometer and a gas analyzer. To this end, we conducted a research on the flow analyzer to reduce the cost of equipment and promote the localization of products.

2. Working principle of flow analyzer

The structure of the VMAS test system is shown in Figure 1.

VMAS test system composition

Figure 1 VMAS test system composition

The vehicle runs on the chassis dynamometer according to the specific working conditions required by the national standard. The chassis dynamometer simulates the acceleration inertia of the vehicle under different working conditions and the transient operating load on the road. A part of the original exhaust gas of the car directly enters the high Accurate five-gas analyzer analyzes the concentration values ​​of CO, CO2, HC, NOX and O2 in exhaust emissions. At the same time, the rest of the exhaust gas passes through the collection device, is diluted by the surrounding air, and is sent to the flow analyzer under the action of the fan to measure the volume flow of the dilution gas, and the volume flow measurement value is sent to the main control computer. The gas concentration analysis results and the driving speed and mileage of the chassis dynamometer are also sent to the main control computer. The computer calculates the total amount of harmful gases CO, CO2, HC, NOX in the automobile exhaust according to the gas concentration, volume, dilution ratio, speed, and time Emissions (g / km). The dilution ratio Dil is calculated according to Equation 1 based on the oxygen concentration in the air, the oxygen concentration in the mixed gas, and the oxygen concentration in the original exhaust gas of the car.

Oxygen concentration (1)

3. Structural design of the flow analyzer

The general vortex flow meter cannot be installed on the gas flow analyzer due to its large body and complicated structure. To meet the requirements of use, a new design idea is adopted to combine the triangular column vortex generator and piezoelectric vortex detection probe. It is directly installed on the sampling tube of the gas flow analyzer, and is designed into an integrated structure, so the structure is more compact.

The overall structure of the flow analyzer is shown in Figure 2. The total length of the sampling tube is 800mm. According to the gas flow range during operation, the inner diameter D of the pipe of the flow analyzer is determined to be 100mm. The installation position of each sensor is marked in the figure.
The overall structure of the flow analyzer

1- Oxygen concentration sensor 2- Pressure sampling hole 3- Triangular column vortex generator
4- Piezoelectric vortex detection probe 5-Temperature sensor Figure 2 Overall structure of flow analyzer

Select the triangular column vortex generator. The vortex signal generated by this generator is strong. Because the separation point of the vortex at its boundary layer is fixed, the Strouhal number is relatively constant, about S1 = 0.16. The linear relationship between frequency and flow rate is good. The triangular column is installed vertically in the gas pipeline. When the gas bypasses the spoiler column, there is a separation of the boundary layer. The vortex will alternately be generated behind the left and right sides of the spoiler column. Is, where: d-the width of the bottom edge of the triangular column, taken as 28mm; u-fluid flow rate. According to the D and d values, determine the parameters L1, L2 and θ of the triangular column, as shown in Figure 3.

The structure of the triangular column

Figure 3 The structure of the triangular column (unit: mm)

According to the Reynolds number calculation formula, in the formula: ν-the kinematic viscosity of air, the flow rate range of 140m3 / h ~ 930m3 / h at a standard pressure of 20 ℃, the Reynolds number corresponding to 3.3 ~ 21.9 × 104, which meets The lower limit is required, and has an ideal meter factor. The theoretical calculation instrument factor K is:
Theoretical calculation instrument factor K (3)

S1 and S2 are the cross-sectional area of ​​the pipeline and the windward area of ​​the triangular column, respectively. The final meter coefficient should be based on the results of the calibration experiment.

4. Circuit design of flow analyzer

The circuit design of the flow analyzer includes the selection and measurement circuit design of four sensors: flow sensor, oxygen concentration sensor, temperature sensor, and pressure sensor. The block diagram of the circuit structure is shown in Figure 4.

Circuit block diagram of flow analyzer

Figure 4 Circuit block diagram of the flow analyzer

4.1 Flow sensor

The piezoelectric vortex detection method is adopted. When the gas flows through the pipe, the vortex generated by the triangular column periodically exerts a force on the piezoelectric ceramic of the probe, so that it generates a periodically changing charge, and then passes through the charge amplifier circuit The filtering and shaping step by step are converted into corresponding square wave voltage pulses with a frequency of 44Hz to 297Hz and an amplitude of about 8.8V. This signal is isolated by photoelectricity, on the one hand, it can suppress the interference from the test site, on the one hand, the amplitude will be below 5V, and sent to the single-chip microcomputer for periodic measurement. Since the frequency of the vortex street is proportional to the gas flow, the gas volume flow is measured by measuring the frequency (period) of the voltage pulse.
The circuit structure of the vortex flow sensor is shown in Figure 5.

Block diagram of vortex flowmeter circuit

Figure 5 Block diagram of vortex flowmeter circuit

4.2 Oxygen sensor

A-02 / T type oxygen concentration sensor is adopted. The product is designed according to PTB 18.10, which is mainly used for automobile exhaust gas measurement. In a dry environment, when the oxygen concentration changes in the range of 0 ~ 100%, the output analog voltage signal is 7 ~ 13.5 mV, the response time is not more than 5 seconds, the linear error is <0.5%, the working pressure range is 75 ~ 125kPa, NTC temperature compensation . The signal is amplified through the measuring circuit, and the zero point adjustment and gain adjustment are performed to make the circuit output voltage 0 ~ 2.40V, and the corresponding oxygen concentration is 0 ~ 22.5%. 4.3 Temperature sensor

Pt100 platinum resistance is adopted as the temperature-sensing element, and it is integrated with metal protection tube and insulation material to reduce the harmful gas and impurities in the mixed gas and affect the performance of platinum resistance. In order to improve the accuracy of temperature measurement, multi-point calibration was carried out, and the method of piecewise linearization was used in the software. At the same time, in order to improve the anti-interference ability of the sensor, a three-wire circuit structure is adopted. The actual temperature measurement accuracy of the sensor is better than 1.0 ℃ in the range of (5 ~ 150) ℃.

4.4 Pressure sensor

The pressure measurement adopts MPXA6115A series on-chip integrated silicon pressure sensor. The sensor integrates a bipolar operational amplifier circuit and a thin film resistor network circuit. The internal structure is shown in Figure 6.
MPXA6115A internal structure

Figure 6 MPXA6115A internal structure

After testing, the output voltage signal of the sensor has a very good linear relationship with the input gas pressure. When the absolute pressure of the input gas changes within the range of 15kPa ~ 115kPa, its maximum relative error is not greater than 1%.

4.5 Data processing system

Using the C8051F021 single-chip microcomputer as the core of data processing, the acquisition and processing of temperature, pressure, flow, and oxygen concentration signals are completed. The C8051F021 MCU integrates a multi-channel 12-bit successive approximation ADC converter with a conversion rate of 100kps. An internal voltage reference generator is used to generate a 2.4V high-precision reference voltage with a temperature coefficient of 15 × 10-6 / ℃. The output of the temperature, pressure and oxygen concentration sensors, after zero point adjustment and gain adjustment, is adjusted to a voltage signal of 0 ~ 2.40V, and enters the single-chip microcomputer for AD conversion. The accuracy of AD conversion is 0.025%.

The output of the flow sensor is a square wave voltage pulse of 44Hz to 297Hz. For the signal of this frequency range, it is more convenient and more accurate to adopt the method of measuring period. Select the counter TIME0 to count and perform period measurement. Using the twelfth of the 22.1184MHz main frequency as the counting pulse, the 16-bit counter has a counting error of ± 1, and the uncertainty of the period measurement is about two ten thousandths.

The single-chip microcomputer calibrates the data and cycle measurement values ​​after the three-way AD conversion, and converts them into the corresponding temperature, pressure, oxygen concentration and flow values, and then passes them to the main control computer through the serial port. On the control computer, the communication program and the man-machine interface program of the computer and the single chip are written in VB language, the measurement parameters are displayed and saved, and an interface is left to perform data with the control program of the five gas analyzer and the chassis dynamometer Exchange to complete the calculation of the final emission of the car

5. Experiment and conclusion

In order to verify the measurement accuracy of the flow analyzer, the flow rate, temperature, pressure, and oxygen concentration of the flow analyzer are calibrated separately, and the comprehensive accuracy evaluation is performed. Among them, the temperature measurement accuracy is ± 1 ℃, the pressure measurement accuracy is better than 1%, and the oxygen concentration measurement accuracy is better than 0.5%, which fully meets the requirements of national standards for testing instruments.

Due to the large number of experimental projects, this paper only gives the calibration experiment methods and data of the main sensor vortex flow sensor.

The gas flow analyzer was installed on a pipe with a diameter of 100mm and the calibration experiment was carried out. Using the LXH series critical flow sonic nozzle gas flow standard device as the flow standard, the gas flow in the air passage was adjusted by selecting different combinations of 12 sonic nozzles In the measuring range of 140m3 / h to 930m3 / h, select 5 calibration points, the experimental data is shown in Table 1.

1 Flow calibration experiment data

Experimental data of flow calibration

It can be seen from Table 1 that the average instrument system of the flow analyzer is 1159.91, and the basic error is 0.93%. Through the calibration experiment of the flow sensor in the flow analyzer, we can see that the deviation of the flow measurement does not exceed 1%, compared with 5% of the similar products produced by the US sensors company, the accuracy is greatly improved, and the cost is greatly Reduced, suitable for large-scale domestic use.

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