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O C T O B E R 2 0 1 7

AUTHORS

Sven HolzbächerProduct Manager for Gas Flow Measuring TechnologyEkkehard RiedelProject Manager for Research & DevelopmentSebastian StoofHead of Global Product Management

SUMMARY

Flow measurement using ultrasound is ideally suited to meeting the diverse requirements of exhaust gas flow measurement in test bench applications. The measuring results produced by the FLOWSIC150 Carflow demonstrate a very level of high measurement accuracy with a very short response time.

WHITE PAPER | SICKDIRECT VOLUME FLOW MEASUREMENT OF EXHAUST GASES USING ULTRASOUND

8022030/2017-10Subject to change without notice

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TABLE OF CONTENTS

Introduction .........................................................................................................3

Measurement techniques .................................................................................. 4

Requirements ..................................................................................................... 4

Technological approach ..................................................................................... 5Flow conditioning ............................................................................................ 6Sensor cooling ................................................................................................ 6Calibration ....................................................................................................... 7Zero-point stability .......................................................................................... 7

Applications......................................................................................................... 8

Discrete emissions curves ............................................................................. 8

Bag Mini-Diluter .............................................................................................. 8

Summary ............................................................................................................ 9

Bibliography.........................................................................................................11

DIRECT VOLUME FLOW MEASUREMENT OF EXHAUST GASES USING ULTRASOUND



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With vehicle development cycles becoming ever shorter, and given the need to tackle the enormous challenge of meeting future emission limits, direct volume measurement of exhaust gases is crucial. To comply with emission legislation, the use of engines with state-of-the-art combustion processes and complex exhaust gas treatment systems is on the rise. There are significant costs associated with assessing the potential of these concepts, and how they can be coordinated with regard to emissions performance and on-board diagnostics. For this reason, studies on these subjects begin at a development stage where, in some cases, it is not possible to determine any emission results on the exhaust gas roller dynamometer test bench. Engineers are searching for ever smaller areas of potential optimization, which rely on simple and accurate time-resolved pollutant processes. State-of-the-art measurement techniques such as direct ultrasonic flow rate measurement can generate the required data and are an important contribution to the development and optimization of new generations of engines.




4

Measurement techniques

With the FLOWSIC150 Carflow, SICK has been offering an ultrasound exhaust gas flow meter for test bench applications for more than 15 years now. The instrument was developed in collaboration with leading automobile manufacturers and has been continu-ously optimized since the time of its launch. The focus has been on improving its metrological properties while expanding its field of application.

The current instrument generation of the FLOWSIC150 Carflow takes advantage of the most recent SICK ultrasonic technology, which the sensor manufacturer – one of the world’s leading vendors of industrial exhaust gas flow technology – has been using with great success.

Piezoelectric ultrasonic sensors are used for flow measurement at SICK, on the basis of the ultrasonic transit time difference method. In this, ultrasonic signals are transmitted alternately through the exhaust gas flow at an angle. Driving and braking effects due to the exhaust gas flow lead to different transit times of the signals through the flow of exhaust gas (Fig. 1). This difference in transit time is analyzed by the integrated electronics and converted into a flow velocity along the ultrasonic measuring path. A representative area velocity can be determined using 4 ultrasonic measuring paths arranged across the flow cross-sec-tion (equation 1). The exhaust gas volume flow results from offsetting against the pipe cross-section at the measuring point.

Requirements

In recent years, the requirements for exhaust gas flow meters have increased in many ways, They involve real-time capability, a wide measuring range at high resolution, high temperature resistance, low pressure loss, low-maintenance operation and maxi-mum measurement accuracy even under dynamic flow conditions, and very low flow rates. The flow condition in the exhaust gas of internal combustion engines greatly depends on operating conditions and is subject to high dynamics. Pulsations arise during idling, as do highly turbulent flows and very low flow velocities.


Fig. 1: Measuring principle of ultrasonic flow measurement

∑=

⋅=N

iP,iiA vw

Nv

1

1

Equation 1



5

Technological approach

The exhaust gas velocity is measured in the measurement chamber of the instrument on four independent ultrasonic measuring paths, whose individual results are factored into the measurement result on a weighted basis. By using this 4-path arrangement, even disrupted flow profiles that result from less-than-ideal flow from the exhaust pipe can be accurately measured. The measuring range, measurement accuracy, and repeatability can be seen in the table below (Fig. 2).

To reliably capture the dynamics of the exhaust gas flow and to achieve very high measurement accuracy in all areas, 50 time-of-flight measurements are taken per second on each of the four measuring paths. This guarantees a solid base of data for signal evaluation without extending the response time of the instrument. Fig. 3 shows a typical step response. The increase here is 30 m/s². The T90 time is approx. 0.9 s.


Fig. 2: Technical data (excerpt)

Fig. 3: Flow measurement step response



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The individual ultrasonic signals are sampled by the electronics in the MHz range, where a reliable transit time determination is achieved in the ± 5 ns range. This accuracy is particularly critical in the range of low flow velocities – when the engine is idling, for example – because the physical measuring effect is very small in this case. In addition, adaptive path compensation adjusts faulty measurements to individual paths under dynamic flow conditions and guarantees uninterrupted measurement even when the exhaust gas is strongly pulsating. To achieve this, the ratio of the time-of-flight measurements between the individual measuring paths is acquired during uninterrupted measurement and compensated in the event of short-term signal interference. Another advantage of the measurement principle is its independence from the state-dependent speed of sound in the exhaust gas, enabling it to be largely independent of the composition of the exhaust gas, the temperature and the pressure. In addition to the operating volume flow, the measuring instrument also calculates the normalized flow rate through the integrated pressure and temperature measurement (equation 2).

Flow conditioning To further improve the metrological properties, innovative flow conditioning is used in the FLOWSIC150 Carflow to compensate for interrupted flow conditions. Based on experience, ideal inlet conditions are often difficult to achieve when installing the measuring instrument due to the confined spaces. In addition, variable flow pattern controls lead to different flow profiles. From the user’s perspective, the immunity of the instrument to flow perturbation is therefore of great importance – thus, the actual measurement of the flow velocity becomes independent of the specific flow conditions. Copper plates in the preheating section of the instru-ment cause flow rectification due to their location and support the occurrence of a flow profile that is as rotationally symmetrical as possible, with minimum pressure loss. In addition, they are welded to the heated pipe wall, whereby heat transfer is optimized for the inflowing exhaust gas. To keep the temperature gradients between the pipe wall and exhaust gas as small as possible and to prevent condensation, the entire measurement section of the instrument is heated throughout. This solution ensures uniform temperature control of the exhaust gas over the entire pipe cross-section, with the result that the temperature measurement and subsequent normalization of the volume flow are carried out with considerably higher accuracy.

Sensor cooling The piezoelectric materials in the ultrasonic sensors lose their piezoelectric properties above the Curie temperature (approx. 280 °C). Alternating thermal stress loads near the Curie temperature also lead to an increased aging effect on the materials. To enable the instrument to be applied at exhaust gas temperatures of up to 600 °C, patented sensor cooling with ambient air is used (Fig. 4). Thermal management of the ultrasonic sensors allows the sensor temperature to remain safely below the Curie temperature in the range of up to 600 °C, even when used continuously. In addition, the temperature fluctuations of the probes can be significantly reduced by cooling, which leads to significantly lower aging behavior in the probes.

Equation 2

BN VTT

ppV && ⋅⋅= 0

0

Pipe wall

Sensor membrane

Cool air outlet

Cool air inlet

Ultrasonic sensor

Fig. 4: Patented sensor cooling principle



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Calibration Each instrument is also calibrated to achieve the highest possible accuracy. To achieve this, the manufacturer uses certified test benches in accordance with the Measuring Instruments Directive, 2004/22/EC. Calibration is performed on the basis of the Reynolds number, using equation 3 . For this purpose, the measuring instrument is installed on the test bench with a series con-nection to a calibrated ultrasonic gas flow meter with eight measuring paths. By testing, the remaining variance from the measuring instrument and reference count is determined as an error curve and recorded using the Reynolds number. Subsequently, the error is corrected using a suitable polynomial, whose coefficients can be configured as parameters in the measuring instrument (equa-tion 4). The correction factor is multiplied by the measured area velocity and thus leads to the corrected gas velocity (equation 5). Figure 5a shows the results of this kind of calibration. In addition to the low residual error of max. ±0.3%, the reproducibility at <0.2% is assessed as very good.

Zero-point stability The accuracy of flow meters is primarily affected by the zero-point stability at low flow velocities. The minimum quantity range is also of particular interest for the exhaust gas measurement; for example, in the range near idling. For this reason, the gas veloc-ity of a sealed measuring instrument was recorded over several hours. The results in Figure 5b show that stability is better than 5 mm/s. The results of zero-point testing with hot gas are in the same range.


� � = � ∗ � ��

uncorr � η

Equation 3

)4..0(Re, ccccfk =Equation 4

Acorr vkv ⋅=

Equation 5Fig. 5a: Calibration curve and zero flow stability

Fig. 5b: Calibration curve and zero flow stability



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Applications

Direct exhaust gas flow measurement has significant advantages over the alternative calculation of the exhaust gas volume flow from other measurands or model approaches. The measurement error increases when calculating from various measurands on the one hand – and on the other, model approaches are often limited to specific system environments. Therefore, measurement of the exhaust gas flow rate represents the simplest, most accurate method and is used in various applications.

Discrete emissions curves Direct exhaust gas flow measurement allows allocation of pollutant concentrations from exhaust gas analyses to the actual exhaust gas volume flow with no time-consuming dead time correction. If the exhaust gas volume flow corrected to standard conditions is used, the emission densities known under standard conditions are used to calculate mass-related emissions (equation 6). The result is the time curve of the quantity of pollutants. Representative pollutant emissions corresponding to the bag results from CVS systems on chassis dynamometer test benches result from the accumulation of the measured values (equation 7). This method can also be used on engine test benches to determine discrete pollutant curves and thus allows early and very accurate determina-tion of the anticipated CVS measurement results.

Bag Mini-Diluter In addition to the CVS methodology, the Bag Mini-Diluter has been certified in the USA by the EPA since 2002. [1] With this mea-surement method, vehicle emissions are diluted with synthetic air and directed to the exhaust gas bag proportionately in relation to the exhaust gas volume flow. The exhaust gas volume flow of the vehicle is required in real time as a performance indicator for the mass flow controller for sampling. Various test results confirm that the volume measurement of exhaust gases by ultrasound is very well suited to this method. [2], [3]

[ ]

⋅

⋅=

³3600

3

mg

hmV

ppmKsgE pollutant,N

Npollutantpollutant ρ

&

Equation 6

[ ]∫

=

cyclet

t

pollutant

pollutant, kum kmdistancesgE

kmgE

0

Equation 7



9

Summary

Flow measurement using ultrasound is ideally suited to meeting the diverse requirements of exhaust gas flow measurement in test bench applications. The measurement results produced by the FLOWSIC150 Carflow demonstrate a very level of high measure-ment accuracy with a very short response time. Measures in the area of flow conditioning and the use of state-of-the-art sensor technology in the 4-path design are making it possible to optimize independence from the flow pattern control, zero-point stability, and small quantity measurement. The patented sensor cooling enables the instrument to be used at exhaust gas temperatures of up to 600 °C, and at the same time ensures the ultrasonic sensors achieve a long service life through optimum thermal management.

The FLOWSIC150 Carflow is available as a mobile dynamometer wagon in a compact design and is ideally suited to flexible exhaust gas flow measurement on exhaust gas and chassis dynamometer test benches.


[1], [2], [3] Bibliography page 11



10

Equations vA Mean area velocity vP Mean path velocity N Number of measuring paths wi Weighting factor of a measuring path VN Standard flow VB Operating flow p Pressure T Temperature p0 Standard pressure T0 Standard temperature Re Reynolds number k Correction factor cc0–cc4 Polynomial coefficients ρ Exhaust gas density η Dynamic viscosity


Equation 1

Equation 2

Equation 3

Equation 4

Equation5

Equation 6

Equation 7

∑=

⋅=N

iP,iiA vw

Nv

1

1

BN VTT

ppV && ⋅⋅= 0

0

� � = � ∗ � ��

uncorr � η

)4..0(Re, ccccfk =

Acorr vkv ⋅=

[ ]

⋅

⋅=

³3600

3

mg

hmV

ppmKsgE pollutant,N

Npollutantpollutant ρ

&

[ ]∫

=

cyclet

t

pollutant

pollutant, kum kmdistancesgE

kmgE

0



11


Bibliography:

[1] United States Environmental Protection Agency, “ Dear Manufacturer letter CCD-01-23”, December 6, 2001.

[2] Guenther, M.; Vaillancourt, M.; Polster, M.: Advancements in Exhaust Flow Measurement Technology. SAE Technical Paper 2003-01-0780, 2003

[3] Yassine, M., Kirchoff, C., Laymac, T., Berndt, R. et al., „Improving Direct Vehicle Exhaust Flow Measurement“, SAE Technical Paper 2005-01-0686, 2005, doi:10.4271/2005-01-0686.

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