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Analysis of the Turbocharger Compressor Surge Margin Using a Hurst-Exponent-based Criterion
KTH, School of Information and Communication Technology (ICT), Materials- and Nano Physics, Optics and Photonics, OFO.ORCID iD: 0000-0002-4937-8915
KTH.
KTH, School of Industrial Engineering and Management (ITM), Machine Design (Dept.), Internal Combustion Engines.ORCID iD: 0000-0001-9483-7992
KTH, School of Engineering Sciences (SCI), Mechanics. KTH, School of Industrial Engineering and Management (ITM), Centres, Competence Center for Gas Exchange (CCGEx). KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.ORCID iD: 0000-0001-7330-6965
2016 (English)In: SAE International Journal of Engines, ISSN 1946-3936, E-ISSN 1946-3944, Vol. 9, no 3Article in journal (Refereed) Published
Abstract [en]

Turbocharger compressors are limited in their operating range at low mass flows by compressor surge, thus restricting internal combustion engine operation at low engine speeds and high mean effective pressures. Since the exact location of the surge line in the compressor map depends on the whole gas exchange system, a safety margin towards surge must be provided. Accurate early surge detection could reduce this margin. During surge, the compressor outlet pressure fluctuates periodically. The Hurst exponent of the compressor outlet pressure is applied in this paper as an indicator to evaluate how close to the surge limit the compressor operates. It is a measure of the time-series memory that approaches zero for anti-persistence of the time series. That is, a Hurst exponent close to zero means a high statistical preference that a high value is followed by a low value, as during surge. Maps of a passenger-car sized turbocharger compressor with inlet geometries that result in different surge lines are measured on a cold gas stand. It is demonstrated that the Hurst exponent in fact decreases as the compressor moves towards surge, and that a constant value of the Hurst exponent can be used as a threshold for stable operation. Transient pressure signals of the compressor entering surge are analyzed in order to evaluate the time lag until surge can be detected using the Hurst exponent. Two surge cycles are usually needed to detect unstable operation. However, since the amplitude of these oscillations is relatively small for the first cycles, detection is possible before the oscillations grow into deep surge.

Place, publisher, year, edition, pages
SAE International , 2016. Vol. 9, no 3
Keyword [en]
Centrifugal compressor, surge analysis, surge detection, surge margins, Turbocharging, Hurst exponent
National Category
Mechanical Engineering
Research subject
Engineering Mechanics
Identifiers
URN: urn:nbn:se:kth:diva-185757DOI: 10.4271/2016-01-1027Scopus ID: 2-s2.0-84975230385OAI: oai:DiVA.org:kth-185757DiVA: diva2:923430
Projects
Compressor Off-design Operation- CCGEx
Funder
Swedish Energy Agency
Note

QC 20160429

Available from: 2016-04-26 Created: 2016-04-26 Last updated: 2017-05-08Bibliographically approved
In thesis
1. On Stability and Surge in Turbocharger Compressors
Open this publication in new window or tab >>On Stability and Surge in Turbocharger Compressors
2017 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Turbochargers are used on many automotive internal combustion engines to increase power density. The broad operating range of the engine also requires a wide range of the turbocharger compressor. At low mass flows, however, turbo compressor operation becomes unstable and eventually enters surge. Surge is characterized by large oscillations in mass flow and pressure. Due to the associated noise, control problems, and possibility of mechanical component damage, this has to be avoided.

Different indicators exist to classify compressor operation as stable or unstable on a gas stand. They are based on pressure oscillations, speed oscillations, or inlet temperature increase. In this thesis, a new stability indicator is proposed based on the Hurst exponent of the pressure signal. The Hurst exponent is a number between zero and one that describes what kind of long-term correlations are present in a time series.

Data from three cold gas stand experiments are analyzed using this criterion. Results show that the Hurst exponent of the compressor outlet pressure signal has good characteristics. Stable operation is being indicated by values larger than 0.5. As compressor operation moves towards the surge line, the Hurst exponent decreases towards zero. An additional distinction between the long-term correlations of small and large amplitude fluctuations by means of higher order Hurst exponents can be used as an early warning indicator.

Further tests using compressor housing accelerometers show that the Hurst exponent is not a good choice for real-time surge detection on the engine. Reasons are the long required sampling time compared to competing methods, and the fact that other periodically repeating oscillations lead to Hurst exponents close to zero independent of compressor operation.

Abstract [sv]

Turboladdare används ofta på förbränningsmotorer för att öka motorns effekttäthet. Motorns breda driftområde ställer krav på ett brett driftområde för turboladdarens kompressor. Vid låga massflöden blir kompressordriften dock mindre stabil, och surge kan uppträda. Surge innebär stora oscillationer i tryck och massflöde genom kompressorn. På grund av oljud, reglerproblem och risken för mekaniska skador vill man undvika surge.

Det finns indikatorer för att bedöma kompressorns stabilitet på ett gas stand. Indikatorerna är baserade på tryckoscillationer, varvtalsoscillationer, eller temperaturökning i gasen i kompressorinloppet. I denna avhandling presenteras en ny indikator baserad på Hurst-exponenten, beräknad på trycksignalen. Hurst-exponenten är ett tal mellan noll och ett som beskriver vilka typer av långtidskorrelationer det finns i signalen.

Mätningar från tre gas-stand-experiment har analyserats på detta sätt. Analyserna visar att Hurst-exponenten baserad på kompressorutloppstrycket fungerar bra som som surgeindikator. Stabil drift av kompressorn indikeras av att Hurst-exponenten är större än 0.5. När kompressordriftpunkten närmar sig surgelinjen faller Hurst-exponenten mot noll. En distinktion mellan oscillationer med små och stora amplituder kan används för att få en tidig varning.

Analyser av vibrationsmätningar på kompressorhuset vid motorapplikation visar att Hurst-exponenten inte är lämplig som realtidsindikator på en motor. Detta kommer sig dels av att data behöver samlas in under en längre tid än med andra tänkbara indikatorer, dels av att andra periodiska oscillationer i signalen kopplade till motorns naturliga beteende leder till Hurst-exponenter nära noll även vid stabil kompressordrift.

Place, publisher, year, edition, pages
Stockholm, Sweden: KTH Royal Institute of Technology, 2017. 109 p.
Series
TRITA-MMK, ISSN 1400-1179
Keyword
Turbocharger, Radial Compressor, Stability, Surge, Hurst exponent, Fractals
National Category
Mechanical Engineering
Research subject
Machine Design
Identifiers
urn:nbn:se:kth:diva-206737 (URN)978-91-7729-378-1 (ISBN)
Public defence
2017-06-02, D1, Lindstedtsvägen 17, Stockholm, 10:00 (English)
Opponent
Supervisors
Projects
CCGEx - Compressor off-Design
Note

QC 20170510

Available from: 2017-05-10 Created: 2017-05-08 Last updated: 2017-05-10Bibliographically approved

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