How To Improve Diesel Sound Quality

Transcription

1 Copyright 7 SAE International Improving Diesel Sound Quality on Engine Level and Vehicle Level a Holistic Approach Philipp Sellerbeck, Christian Nettelbeck HEAD acoustics GmbH, Herzogenrath, Germany Ralf Heinrichs, Thomas Abels Ford Werke GmbH, Cologne, Germany ABSTRACT Diesel impulsiveness (so called Diesel knocking) present in the cabin of diesel vehicles is perceived as unpleasant because of its impulsive time structure. JD Power data clearly show the customers preference of vehicles with little Diesel knocking over those with severe knocking. Corresponding objective descriptors that reflect the customers perception are introduced. The occurrence of such noise patterns is influenced by the combustion process itself as well as by all excited mechanical components within the power train. Further the transfer characteristics of the engine structure and various vehicle noise paths do contribute to a poor Diesel Sound Quality. It is essential that all these factors have to be considered in combination. This paper provides an overview about suitable methods and technologies, including Binaural Transfer Path Analysis and Synthesis. The potential of the approach is demonstrated by an example. Investigations are carried out on the complete vehicle and on the engine test bench, and principle improvements on engine level and on vehicle level are derived. INTRODUCTION Turbocharged direct-injection (DI) Diesel engines in passenger cars are gaining popularity due to their good fuel economy and high output torque. Ambitious CO emission reduction targets are supported by growing market shares of Diesel-powered vehicles especially in Europe. However, usually both interior and exterior sound quality of Diesel cars doesn t reach levels of refinement competitive with their gasoline-powered counterparts. A dominating factor is the mid- and high-frequency noise generated by the DI combustion process. The excitation is caused by a steep rise of cylinder pressure after selfignition of the mixture. In combination with the inherent modulation by cylinder firing this leads to an impulsive noise character. The so called Diesel knocking is mainly covering the frequency range of 5 Hz to 6 khz. Human hearing is highly sensitive to broad-band impulsive noise, thus annoyance is caused even if the absolute level is comparably low. Beside the impulsiveness itself the cylinder-to-cylinder variation is perceived as unpleasant too. Therefore it is important to apply proper analysis methods to first get sufficient insights into the physical properties of a Diesel power train and second to be able to reflect the customer perception regarding Diesel Sound Quality. Previous investigations [] show that the dominating factors 'Pleasantness' and 'Powerfulness' do describe the Diesel Sound Quality. Further it is shown that Diesel Knocking is the major driver for a poor Diesel Sound Quality. In particular the operating condition Idle did turn out to be important for the Customer. JD Power is the standard customer questionnaire in US and Japan and is becoming more and more popular in Europe too. The 5 JD Power APEAL question 'Sound of engine while idling' is chosen to verify the Diesel Knocking importance for Sound Quality and to define proper customer driven objective targets. Modern common-rail Diesel injection systems offer the flexibility for NVH tuning, but this is often antagonized by emission and performance calibration. Especially the trade-off between emission and noise targets is tightened with EU5 and US bin5 regulations coming. Thus possibly even higher combustion excitation levels are expected. With the combustion process offering less acoustic optimization potential, the transfer characteristics of the engine structure and the vehicle noise paths become more and more important factors for sound quality improvements. Especially airborne and structure borne transfer paths including engine mounts and sound package offer various parameters for noise control and sound design. Matching the acoustic properties of the

2 vehicle to the noise characteristics of the engine requires mutual support and holistic efforts of involved departments. modulations can be detected. These patterns are created by a single cylinder and cause a disturbing noise. Thus an effective NVH optimization of the complete vehicle is appropriate in order to achieve high levels of refinement and good customer acceptance. COMBUSTION NOISE ANALYSIS AND PERCEPTION SUITABLE ANALYSIS METHODS Engine Orders.5.5 Engine Orders.5.5 The noticeable time structure of Diesel combustion noise and the human ear s sensitivity to modulation call for dedicated time-based analysis methods suitable for the detection of combustion noise patterns. The common methods for target setting based on frequency spectra turned out to be insufficient. As directivity plays an important role in case of high-frequency noise binaural recording and signal processing technology is used throughout. High-Resolution Spectrogram In contrast to standard third-octave or FFT analyses, a high-resolution spectrogram based on 48 th octave filtering provides both the necessary time and frequency resolution. Figure shows the analysis of a binaural interior noise recording of a passenger car equipped with a four cylinder DI Diesel engine (idle condition). Diesel knocking patterns are clearly visible at.-. khz,.4- khz and 3-4 khz. Differences between left and right channel can also be seen. Figure : Modulation spectrogram of interior noise at idle condition, binaural recording (left and right channel) In particular a high frequency resolution modulation analysis such as NBMA (Narrow Band Modulation Analysis) is capable to differentiate between different types of knocking contribution e.g. combustion noise and injector ticking []. In Figure 3 the modulation patterns from the abovementioned recording can be recognized as peaks at respective frequencies. Figure 3: NBMA of interior noise at idle condition, binaural recording (left channel). The dotted black line shows the average spectrum and the colored lines show the high resolution engine order modulation cuts. Relative Approach Figure : High-resolution spectrogram of interior noise at idle condition, binaural recording (left and right channel) Modulation Spectrum A modulation analysis reveals details about the composition of modulations and thus about the perceived sound quality. Moreover the Diesel Knocking severity and characteristic can be depicted with a modulation analysis since the modulations carry all impulsiveness information. Figure shows a modulation spectrogram of the same recording as in Figure. Besides nd order modulations inherently generated by the four cylinder engine also strong.5 th order Based on a model of human hearing, Relative Approach detects signal changes in the time domain and in the frequency domain and thus highlights disturbing noise patterns [3]. The analysis proves especially useful for transient conditions like run-up or tip-in. For such operating conditions, in a spectrogram combustion noise may be masked by other power train noises. The left diagram in Figure 4 presents the Relative Approach analysis of the interior noise recorded in a different vehicle during a run-up from 5 to 5 rpm at low load, a transient condition during which severe Diesel noise was found. Obviously several modulated noise patterns can be detected at. khz,.4 khz, 3.4 khz and 5.8 khz while other noises are hidden. The right

3 diagram shows a high-resolution spectrogram for comparison. Figure 4: Relative Approach Analysis (left) and highresolution spectrogram (right) of interior noise at run-up condition, respectively left channel of binaural recording DIESEL KNOCKING INDEX DKI Based upon NBMA a single number has been developed in order to quantify the Diesel Knocking performance of a vehicle and to support objective target setting. Figure 5 shows the DKI equation which comprises a weighted summation of all relevant modulations and level in db(a). Carrier frequency f C Modulation frequency f M.5.3 no DKI= O( step= i=.5 c.5) weo Mi, o L N/ db Figure 5: Single number DKI composition for interior noise at idle condition Figure 6 shows that the Diesel Knocking Index DKI delivers an excellent correlation to the JD Power question 'Engine sound while idling'. Further it turns out that meanwhile the customer's expectation with regard to absence of Diesel Knocking is very high for CD class vehicles. Moreover one may assume that this trend will also become true for C and B class vehicles within the next couple of years. With that knowledge the automotive OEM is now able to define targets according to his marketing strategy. Developing a best in class vehicle in terms of Diesel Knocking can be achieved with either having a perfectly running power train without any knocking or with a perfectly isolating power train installation, means very well isolating transfer path. An ideal method/tool for supporting the Power train <> Installation Trade-Off discussion is the Binaural Transfer Path Analysis and Synthesis. Herewith all imaginable Trade-Off scenarios can be played through without any hardware changes i.e. in the CAE world. fu fl APEAL Score CD Segment B, C Segment Vehicle 7 Vehicle 3 Vehicle 5 Vehicle 8 Correlation : APEAL Score <> DKI (Diesel : B, C & CD Segment) Vehicle 5 Vehicle 8 Vehicle 6 Vehicle Vehicle Vehicle 4 Vehicle Vehicle Vehicle 6 Vehicle Vehicle 7 Vehicle Vehicle 4 R =.89 Vehicle 9 Note: DKI only applicable for vehicle interior R =.8 Vehicle DKI Figure 6: JD Power APEAL score correlation with Diesel Knocking Index DKI for the question 'Engine sound while idling' INVESTIGATIONS ON VEHICLE LEVEL Once Diesel knocking patterns have been defined in the interior noise, the transfer of the noise from engine to the ears is investigated. BINAURAL TRANSFER PATH ANALYSIS AND SYNTHESIS Binaural Transfer Path Analysis and Synthesis (BTPA and BTPS) technology has been developed in order to find the origins of unwanted noise patterns and to predict sound quality in vehicles, not only in terms of numbers and graphs, but also for binaural auralization. The results can be used to create an accurate model for path-related noise heard anywhere inside the car, thus yielding possibilities for troubleshooting and sound design. Each individual path contribution or combination of such may be listened to independently, in order to assess their respective impact on the overall sound quality. Paths can be modified synthetically to simulate countermeasures and their effect on the interior noise The method makes use of FIR-filtering of measured excitation signals in order to create synthesized sound contributions. The filters are based on measured transfer functions including the phase - between source and receiver locations, taking into account both air-borne and structure-borne noise paths. Binaural recording and playback technology is used in order to achieve a realistic sound reproduction. Time domain signal processing is mandatory for capturing transient noise patterns like Diesel knocking. More details about the method and related technologies can be found in [4, 5]. INTERIOR NOISE CONTRIBUTIONS For the operating condition described in Figure and Figure (idle) a binaural transfer path synthesis has been carried out at which 39 structure borne paths and

4 airborne paths have been used. Figure 7 shows the spectrograms of the synthesized interior noise along with the structure borne and airborne contributions. Several knocking patterns can be clearly identified. While combustion noise at. and.6 khz is transferred mainly structure borne, patterns at.,.4,.8- and 3-4 khz are transferred mainly airborne. respective interior noise contributions are shown in Figure 9. While the. khz noise is radiated from all engine sides, the other noise patterns can be assigned to certain engine surfaces. right up/dn left front top rear up/dn bottom Figure 7: High-resolution spectrogram of BTPS results for idle condition, top: left channel, bottom: right channel Consecutively the interior noise contributions are broken down into single transfer paths. Figure 8 contains BTPS results of transfer paths related to the left and right engine mount featuring two attachment points each (front, rear) analyzed in 3 directions (x, y, z). The. khz pattern originates mainly from the front attachment of the left engine mount, x-direction and the.6 khz noise is transferred via the right engine mount, y- direction. Figure 9: Interior noise contributions of engine s airborne radiation, top: left channel, bottom: right channel Also DKI values can be calculated from the synthesized time signals, supporting the subjective listening and making an easy ranking of transfer paths possible, e.g. Figure shows the air borne noise contributions. Airborne noise DKI contributions right engine mount front y rear y front x left engine mount,,,9 DKI Left Ear DKI Right Ear,8 DKI,7,6,5,4,3 Bin_IG_LS right rear right front left front top bottom rear up rear down Transfer Path Figure : DKI of interior noise contributions Figure 8: Interior noise contributions of left and right engine mount paths, top: left channel, bottom: right channel The airborne noise paths are based on eight microphones positioned in the engine compartment. The Once the significant noise paths contributing to the combustion noise heard inside the vehicle are identified, more detailed information about the transfer characteristics on the one hand and about the relevant sources on the other hand can be made available from the measured data. TRANSFER PATH INFORMATION

5 Structure borne Paths As shown in Figure 8 a strong knocking pattern at.6-.7 khz is transferred via the right engine mount in y- direction. The effective mount transfer functions related to the two attachment points show no specific weakness at the frequency of interest, neither do the vibroacoustic transfer functions from the body attachment points to the ears (Figure and Figure ). Thus the knocking pattern must be mainly due to strong excitation from the engine. right up right dn left front top rear up rear dn right up right dn left front top rear up rear dn Figure : Effective mount transfer function (F/a) of right engine mount (y), front (red) and rear (green) attachment point Figure 3: Binaural airborne transfer functions of the engine compartment, top: left channel, bottom: right channel REFINEMENTS In order to develop refinements in principle, noise reduction can be applied at the source and at the transfer path: Reduction at the source o Use of absorbers o Change of excitation (combustion calibration, piston clearance etc.) Reduction at the transfer path o Change of mount o Improved body stiffness o Modified sound insulation package o mending of leaks BTPS results can directly lead to solutions for improvements and the method can be used to predict potential for optimization. Figure : Vibro-acoustic transfer function (p/f) of right engine mount (y), front (red) and rear (green) attachment point, top: left channel, bottom: right channel Airborne Paths Listening to the airborne interior noise contributions reveals that the 3-4 khz combustion noise transferred from the upper rear side of the engine is most disturbing. This problem can be directly related to insufficient attenuation of the respective transfer path, as shown in Figure 3. Further the.8- khz pattern detected in the interior noise can be related to a peak in the lower right side transfer function. Experimental approach As an example, an absorber was used on the engine side of the right mount in order to reduce excitation levels. Figure 4 shows that the measured acceleration (y-dir) at the engine side of the mount carries mainly the.5-.8 khz pattern. With the absorber attached, the acceleration is significantly reduced at the frequency of interest. Interior noise measurements reveal the effect on the overall sound quality, Figure 5. This result validates the significance of the transfer path predicted by the BTPS.

6 Predictive Approach The transfer path model has also been used to predict the overall effect of specific path modifications as presented in Figure 7. Figure 4: Vibration at right engine mount bracket (y), left: original state, right: absorber attached Figure 7: Interior noise synthesis, left original, right: synthetically improved airborne attenuation of upper rear engine side, top: left channel, bottom: right channel Figure 5: Interior noise measurement, left: original, right: with absorber at right engine mount bracket, top: left channel, bottom: right channel Figure 6 shows the absorber effect on the critical modulations for the artificial heads right hand side ear. The NBMA plots display a significant reduction in the.5 to.8 khz region. Further the 3 to 3.5 khz region is improved with regard to nd E.O. modulation; thereby the JD Power score can be enhanced by. scale points FFT: 5, Mod-FFT: 5, Overlay:.5 4. db(a) mdki:.494 FFT: 5, Mod-FFT: 5, Overlay: db(a) mdki:.57.5 EO 3.5 EO 3 Figure 6: NBMAs for interior noise measurements, left: original, right: with absorber at right engine mount bracket, top: left channel, bottom: right channel FFT: 5, Mod-FFT: 5, Overlay: db(a) mdki:.49 FFT: 5, Mod-FFT: 5, Overlay: db(a) mdki: EO 3.5 EO 3 The idle interior noise synthesis of the original state is compared to a synthesis based on a virtual improvement of the airborne noise DKI contributors (compare Figure ) upper rear side and bottom. The most disturbing combustion noise pattern at 3-4 khz could be reduced significantly, leading to a JD Power score improvement of.35 scale points FFT: 5, Mod-FFT: 5, Overlay:.5 5. db(a) mdki:.53 FFT: 5, Mod-FFT: 5, Overlay: db(a) mdki: EO 4.5 EO FFT: 5, Mod-FFT: 5, Overlay:.5 Figure 8: NBMAs for interior noise synthesis left: original, right: synthetically improved airborne attenuation of top and upper rear engine side, top: left channel, bottom: right channel FFT: 5, Mod-FFT: 5, Overlay: db(a) mdki: db(a) mdki: EO 4.5 EO 4 3 3

7 INVESTIGATIONS ON ENGINE LEVEL A most effective noise refinement is achieved if vehicle transfer path optimizations are complemented by acoustic improvement of the sources. Several investigations can be accomplished at engine level in order to deduce modifications. If a certain noise problem may be addressed both at the engine and at the transfer path, interior noise simulations enable the engineer to select the most effective and/or most economical solution. Furthermore, high-resolution time and frequency-based analyses of cylinder pressures reveals characteristic properties of the excitation, namely pressure gradient and oscillations due to cylinder cavity resonances. Both are important factors influencing Diesel sound quality [6]. Figure illustrates that a Wavelet analysis can be used to generate a spectrogram of the excitation frequencies due to a single firing event. The noticeable spot at 3-4 khz is caused by combustion bowl resonances and corresponds to the knocking pattern described above. With the results of vehicle level investigations already present, engine measurements can be focused on certain source locations or noise patterns. If applicable, engine test bench measurement points (i.e. sensor positions) should include but not be limited to the points used in the vehicle investigations. ENGINE TEST BENCH MEASUREMENTS Data from standard measurement procedures can be combined and used with advanced analysis methods. Usually available data include: far field and/or near field microphone signals cylinder pressures acceleration at engine mounts crank angle As an example the correlation of cylinder pressures and microphone signals is demonstrated in Figure 9. Due to the high resolution of the analysis the modulated noise pattern can be assigned to a certain cylinder if the distance to the microphone is taken into account. Figure : Wavelet analysis of cylinder pressure during firing event showing cavity resonance Additional measurements provide helpful information regarding the engine operation and transfer of impulsiveness: acceleration at cylinder block and head surfaces acceleration at main bearings injectors current COMBUSTION NOISE SEPARATION Figure 9: Cylinder pressure traces and high-resolution spectrogram of test bench microphone signal, noise pattern can be associated to specific cylinder Diesel engine sound quality greatly depends on combustion noise characteristics. Thus separating the noise contributions that are originating by combustion excitation from the total engine noise is crucial for NVH optimization. Several methods exist for this task and related applications [6, 7, 8, 9, ]. An easy procedure that proved sufficient for most applications is based on measuring engine noise during motored (dragged) operation and during fired operation at various load points. The total combustion-related noise may further be split up into direct combustion noise (due to cylinder pressure alone) and indirect combustion noise (due to piston slap and other mechanical effects that are exited by cylinder pressure).

8 STRUCTURE ATTENUATION Structure attenuation (SA) is a useful measure for quantifying acoustic properties of the engine block and head regarding airborne radiation of impulsiveness. SA is defined by the relation of radiated combustion noise sound pressure level to cylinder pressure level: SA [ db] = Cylinder pressure level Combustion noise level Several methods exist for the determination of SA or, alternatively, the combustion noise transfer function [9, ]. With combustion excitation being a broad band source, weaknesses in structure attenuation almost always lead to specific patterns in radiated noise. If such frequencies coincide with weaknesses in the vehicle sound package (airborne transfer functions) this will likely result in noticeable combustion noise patterns in the interior. Thus with both transfer functions and structure attenuation data available, countermeasures can be designed either in the car or in the engine. Although SA is usually applied to radiated noise, a similar investigation can be carried out for the structure borne combustion noise. INTERIOR NOISE SYNTHESIS OF ENGINE MODIFICATIONS Synthesis of interior noise (BTPS) is possible based also on test bench signals if respective transfer path input points are used in the vehicle and on the test bench. Differences in fixture impedance and room acoustics compared to the engine compartment have to be compensated for given operating conditions. Thus it is much easier to assess the customer-relevant effects of engine modifications, namely in the interior noise, while comparing test bench signals alone may lead to wrong conclusions. customer i.e. in the interior noise. If all these factors and presented methods are considered in combination, competitive solutions for sound quality improvements can be found in a shorter timeframe and require less hardware changes. REFERENCES. R. Heinrichs, U. Groemping: Customer Driven diesel Vehicle Sound Quality. Inter-Noise 4.. T. Leonhard, R. Heinrichs, K.-H. Bürger: Diesel Powertrain Sound Quality. Aachener Kolloquium K. Genuit: A New Approach to Objective Determination of Noise Quality Based on Relative Parameters. Inter-Noise K. Genuit, J. Poggenburg: The Design of Vehicle Interior Noise Using Binaural Transfer Path Analysis. SAE NVC9, R. Sottek, D. Riemann, P. Sellerbeck: Virtual Binaural Auralization of Vehicle Interior Sounds. DAGA 4 6. G. Shu, H. Wei, X. Liang: The Relationship between the Combustion Noise and the High-Frequency Oscillation Pressure of Internal Combustion Engine. Inter-Noise 4 7. J, Antoni, R. Boustany, F. Gautier, S. Wang: Source Separation in Diesel Engines with the Cyclic Wiener Filter. EuroNoise 6 8. C. Renard, L. Polac: Combustion Noise and Piston Slap Noise: Identification of Two Sources Responsible for Diesel Engine s Sound Signature. Daga 4 9. K. Nakashima, Y. Yajima, M. Yamamoto: Measurement of Structural Attenuation of a Diesel Engine and its Applications for Reduction of Noise and Vibration. SAE 97, 99.. S. Ge-qun, W. Hai-qiao, H. Rui: The Transfer Function of Combustion Noise in DI-Diesel Engine. SAE , 5. CONCLUSION Impulsive noise caused by Diesel combustion is strongly influencing the perceived vehicle interior noise sound quality. Impulsiveness requires time-based analysis methods some of which have been demonstrated in this paper. A sound quality metric for Diesel knocking has been developed that correlates well with subjective evaluation including JD Power APEAL score. Time-based Binaural Transfer Path Analysis and Synthesis allow tracking of impulsiveness along the vehicle noise paths and identifying acoustic weaknesses in the power train installation. If the results are combined with acoustic engine test bench data, sources for different transfer paths can be assessed and noise patterns can be related to properties of engine components or the combustion process itself. Effects of source modifications are evaluated considering the