Analysis and Control of Noise Emissions of a Small Single Cylinder D.I. Diesel Engine

Transcription

1 NVC23 3NVC-5 Analysis and Control of Noise Emissions of a Small Single Cylinder D.I. Diesel Engine Felice E. Corcione, Daniela Siano, Bianca M. Vaglieco, Istituto Motori, CNR Napoli (Italy) Giuseppe E. Corcione and M. Lavorgna ST Microelectronics, Arzano, Italy Massimo Viscardi, Michele Iadevaia and Leonardo Lecce DPA, University of Naples, Italy Copyright 22 Society of Automotive Engineers, Inc. ABSTRACT Comfort requirements, government regulations as well as consumer action groups are pressing the automotive industry to produce less noisy vehicles than in the past. These circumstances become more and more important for off-road and human operating machines forcing engine developers to investigate new and more effective control strategies of noise emissions. This paper concerns with the experimental vibroacoustic analysis of a small (224 cc) single-cylinder direct-injection diesel engine used for agricultural and industrial applications as well as off road small vehicles. In order to evaluate the engine acoustic behaviour, experimental identification and localization of noise sources were performed at different speed and load engine conditions by several investigating tools. Within them, the intensity technique was chosen because of its peculiarities to be performed in situ without a specific anechoic test environment. Vibration measurements were also performed by accelerometer sensors. It is an alternative investigating tools for both source location and global sound power determination. To improve the spatial matching with the acoustical data and for better data interpolation, during these tests a 55-measurement points grid was used. A good correlation of data was determined and very useful information about noise sources location and relative transmission path were defined. INTRODUCTION Requirement in designing a quiet, smooth running engine is minimization of the engine vibration and noise caused by combustion and mechanical process. The sources of noise are the forces generated within an engine. These forces can best be described as having their origin with the engine combustion process or with the mechanical action of the engine [1]. Irrespective of source, the forces that are generated are transmitted through the engine structure to the outer surfaces or to the mount locations. Surface vibrations, from either the engine or vehicle body structure, give rise to microscopic air motion; which is perceived as noise. That part of the engine noise that can be directly attributed to the combustion process is generally defined as Combustion Noise ; the remainder is termed Mechanical Noise due to the interaction of the moving components (Motoring Noise) and a load additional aspect (Load Induced Mechanical Noise), [2]. During the last years, requirements for the minimization of gaseous and particulate emissions have lead to the use of higher fuel injection pressures in recent light duty diesel engine designs. The use of higher injection pressure has resulted in higher fuel pump instantaneous torques and, as a direct consequence, higher noise from the (usual) gear timing drive, [8]. In direct contrast, vehicle noise emission legislation and customer preferences are demanding lower engine radiated noise levels. It can be shown theoretically that when a force is quickly applied to an elastic structure, vibrations will be set up in the structure with an amplitude depending on the time of force application and on the natural periods of structure vibration. It is evident that engine vibration and consequent noise, due to a high rate of combustion pressure rise, must be due to natural periods of vibration. They are consequently longer than the period of rapid pressure rise in the cylinder. Moreover, noise due to combustion can be reduced either by reducing the rate of pressure rise or by increasing the natural frequencies of the vibrations involved (i.e. stiffening the structure). The rate of pressure rise with detonation is so high that only the elimination of that form of combustion is effective in eliminating this source noise. Control of the rate of pressure rise is an important objective of diesel engine. A right response to market demand could come by the use of high pressure multiple injection system (common

2 rail) where injection parameters could be opportunely modulated in order to minimize the unwanted pollution aspects (gaseous, particulate and noise) [8]. As a part of a more diffuse research program, the aim of this work was to better understand the governing rule of these phenomena, in order to assess a dedicated injection strategy for noise emission requirements. A set of numerical and experimental investigation activities were so planned in order to evaluate the engine acoustic behaviour, experimental identification and localization of noise sources, trying to best correlate cause and effects [7]. In future, experimental measurements will be so performed with and without the high pressure multiple injection system (common rail) and for different operative conditions in order to evaluate the effect of this new injection approach on the noise emission. TEST ENGINE AND TEST SETUP COMBUSTION NOISE GENESIS AND PREDICTIVE FORMULATIONS To understand the mechanisms of noise generation within the engine is to first establish the noise associated with combustion (i.e. the in-cylinder bang or pressure change characteristics). This phenomenon is evident from figure 2 where the pressure inside the combustion chamber is reported. It was measured by a pressure transducer for the motored and firing conditions. Belonging to the main function of an engine (i.e., the conversion of fuel into the mechanical forces which power the vehicle), the vibration and noise levels related to the engine structure arises from the mechanical events within the engine and the direct excitation from the fuel combustion process. Pressure increment was more than 4 bar and was correlated to the combustion process. In order to investigate ways in which the increase in engine combustion noise could be minimized, a 224 cc two stroke single cylinder diesel engine was characterized. Engine characteristics are reported in tab.1. The engine is an air cooled, direct injection 69 mm bore and mm stroke representative of a light diesel engine family mainly used for agricultural and industrial applications as well as off road small vehicles. During the experimental investigations it was mounted on a stationary test bench in Istituto Motori (CNR) laboratory. In-cylinder pressure (bar) Motored Firing Crank angle (degree) FIGURE 2 : In-Cylinder pressure vs crank angle for motored and firing condition at 2 rpm FIGURE 1 Engine Layout Cylinder N. 1 Bore mm 69 Stroke mm Displacement cm Dry weight Kg 28 Dimension mm 265x158x417 Tab. 1 Engine Characteristics To better understand in which frequency band it arose its level, a signal pressure frequency analysis was performed. figure 3 shows the energy value of the incylinder pressure level reported in narrow frequency band. It presents peaks in low frequency band in correspondence of which the subsequently combustion, the amplitude determined principally of the maximum pick in-cylinder i.e. fundamental resonance. it could be seen that the fundamental resonance of the detonation process dominates at very low frequency An understanding of the engine s actual structure attenuation and combustion noise signatures is very useful. To establish these parameters the aims were to make changes to the in-cylinder noise, while having only minimal effect on the mechanical noise sources, including the effects of peak cylinder pressure. Incylinder pressure noise can be changed by varying the induction air temperature, by fuels with a range of cetane number, and/or by changing the fuel injection timing. The two assumptions made are first that mechanical noise remains constant and secondly that the structural response to in-cylinder noise changing, is linear [1].

3 When these two assumptions hold, the following equation can be written: SP = m CP + a (1) where : SP is the energy related value of the sound pressure level; either for a particular microphone or, more commonly, for the average level from around the engine, CP is the energy related value of the in-cylinder pressure level, a is the energy related value of the mechanical noise (i.e. when there is no combustion noise), m; is the inverse of m represents the engine s structure attenuation. So, the total mechanical noise, the engine s structure attenuation and the combustion noise heard outside the engine can be then calculated for each one-third octave frequency band as follows: Mechanical noise = log ( a ) (2) Sound pressure level incylinder (db) FIGURE 4 : Sound pressure level in-cylinder Sound pressure level in-cylinder in one-third octave frequency band frequency (Hz) 14 rpm 2 rpm 3 rpm Struct. Attenuation = log ( 1 ) (3) m Comb. noise = log ( b a) (4) Comb. noise = cylinder pres. Struct. Attenuation (5) where b is a value at normal operating conditions [2]. Based upon the CP frequency diagram and according to classical numerical formulation [1] the one-third octave band in-cylinder pressure level has next been evaluated for three main engine speed conditions (figure 4). Structure attenuation (db) x 15 Energy value of the in-cylinder pressure level FIGURE 5 A typical structure attenuation curve vs Hz CP=(prms/prif)^2 5 Still referring to Figure 4, it can be stated that the 2 to 2 Hz frequency band represents the most important band from an acoustical point of view [2]. For this reason, the curve s trend (table 2) is generally referred as indicative parameter for alternative semi-empirical formulations frequency [H ] FIGURE 3: The evaluated Cp in frequency band According to the equation 1, the in-cylinder pressure levels may be directly evaluated if the structural attenuation curve is known (an example is next reported in figure 5); otherwise if the external noise spectra is known, the same formulation may be used in a reverse sense as an experimental way for the structural attenuation curve detection. Engine speed Medium trend Dp/dθ (rpm) (db/decade) Tab.2: Medium decrease spectra (2 2 Hz) It can be adopted a semi-empiric formula for a prevision of the engine radiated noise that links the intensity to the engine speed express in rpm, through the spectrum trend of the pressure level in-cylinder (Figure 4), to the

4 engine bore and to the engine structural characteristics. It may be express by the relation: L pa = Alog N + B log D C (6) where: L pa is the medium pressure level in db(a) around the engine to a distance of 1 meter; C is the structural characteristics [1], that assuming a typical characteristic of an engine family (in this case C=31); N is the engine speed in rev/min; A is the trend of the pressure level spectra in cylinder (see tab.1); D is the bore of combustion chamber in cm, B is a reference bore related parameter, in this case it is assumed to be 5 [1] By solving this formula for different engine speeds it can be estimated the pressure level in db(a) at 1 meter around the source (see figure 6). Extimated pressure level (db(a) coming from the semi-empirical formulation (eq.6); such a circumstance may be probably due to a different structural attenuation curve for the investigated engine. The experimental SPL data, have later been also used for a sound power evaluation, [6]; main results will be later on reported and compared with other investigating tools. Vibro-acoustical analysis Noise and vibrations were measured using a set of complementary instrumentation including sound analyser, sound intensity probe and accelerometers transducers. The final target of this multi analysis approach was the identification of both total and local noise sources, as well as the assessment of a test procedure to be performed without the specifically acoustic requirements for the test chamber (i.e. anechoic characteristics) that cannot be generally satisfied in operational environments. 2 Pres. level at 1 m db(a) Pressure level (db(a) engine speed (rev/min) FIGURE 6 Estimated pressure level at 1 meter of distance by the source This predicted value was compared with those evaluated at 1 meter of distance from the source by an experimental Sound Pressure Level (SPL) measurement. This device was both a first class sound meter and spectrum analyser and the experimental measurements were performed according to the Standard ISO 3746 [6]. Figure 7 shows data obtained at 14 rpm in firing condition. FIGURE 8: Test bench, sensor position and experimental chain The experimental vibration tests were performed through a 55 accelerometer grid in order to have a fine surface mesh for vibration detection. db p ( ) q Hz Vibration 3 rpm in an exhaust point of engine acceleration in-cylinder pressure (bar) Hz K 2K 4K 8K 16K -4 - FIGURE 7 Sound Pressure Level at 14 rpm; 1m from the source The comparison between SPL data from analytical and experimental evaluation showed a little over estimation Crank angle(degree) FIGURE 9 : Typical accelerometer signal Vs. crank angle

5 Five operating engine speeds condition ( rpm) were chosen as most significant reference condition and a full load of the combustion pressure was maintained. Figure 9 shows a typical vibration amplitude shape Vs. crank angle. It appears clear that the strong level increment in correspondence of combustion detonation; this evident circumstance reinforces some of the theoretical assumption illustrated within the previous paragraph. Figure shows as the overall rms (root mean square) level increase with the engine rpm speed. FIGURE Typical accelerometer signal Vs. engine speed The vibration data may be used for successive acoustic consideration because of the strict correlation of the metallic engine body vibration levels and the associated acoustic emission, (tab.2). As a general rule, the correlation between vibration level (L w ) of a metallic structure and its emitted noise power level, may be written as, [2]: L W RMS (m/sec^2) = log + log RMS (root mean square level) σ rad ( ρ c / W rif + log engine speed (rpm) ) + log v 2 T RMS firing (m/sec^2) RMS motored (m/sec^2) A + (7) where: ρ c = characteristic impedance of the air around the engine, Kg/m 2 s; A = surface area of engine single panel, m 2, σ rad = radiation efficiency, [ v 2 ] T = space and time averaged squared surface velocity, (m/s) 2, and W ref = the standard reference power of 1x -12 watts (1pW). Average mean square velocity (db) rpm 1/3 octave frequency band Motored Firing Frequency (Hz) FIGURE 11 Engine surface- average vibration velocity (db) at 3 rpm On the basis of experimental vibration data and adopting such a formulation, acoustic power level for each component of the engine may be so calculated; in fact the combination of the averaged mean square velocity (see an example in next figure 11) with the radiating factor and the dimension of single vibrating surface contribute to the overall noise sound power definition. By the use of this formulation the single and global sound power level has so been calculated for the aforementioned typical engine speed. As an alternative and powerful mean of investigation for both noise source identification and sound power level definition, the acoustic intensity technique has been also used. INTENSITY MEASUREMENTS Any piece of machinery that vibrates radiates acoustical energy. Sound Power is the rate at which energy is radiated [energy per unit time]. Sound intensity describes the rate of energy flow through a unit area (S=1 m 2 ), the units for sound intensity are Watts per square meter. Sound intensity gives also a measurement of direction as there will be energy flow in some directions but not in others. During the measurement and successive computation, no assumptions need to be made. The use of sound intensity rather than sound pressure to determine sound power means that measurements can be made in situ, with steady background noise and in the near field of machines in order to identify it. During the tests, noise level was quantified by the A- weighted intensity level measured near the engine and tests were conducted at sites surfaced in accordance with ISO 9614/1 standard, [7]. Above a sketch of the virtual surface utilized for the calculation of the sound pressure level according to the standard ISO, is reported.

6 reported in figure 9. The acoustic intensity was measured in the frequency range 63-3 Hz, and the total measured sound power is equal to 94.3 db(a), (tab.3). The experimental results have shown that the main noise sources involved in the total noise can be recognized in 4 components: injection pump, valve, intake system, exhaust system. In tab.3, main results of the whole work are reported as referring to the sound pressure level measured at 1 m distance from the engine, at the 3 rpm engine speed at full load. empirica formula ISO 3746 ISO 9614 vibration formulation 97 db(a) 95 db(a) 94.3 db(a) 92 db(a) Table 3 Sound Pressure Level at 1mt-at 3 rpm Acoustic intensity measurements were mapped the whole engine, in order to identify the each component of total noise level. An example of intensity level express in db(a) for the front engine view is reported below. 7 Fig. 12 Sound intensity positions p p A little over estimation of the empirical approach is probably due to the peculiar structural characteristics of the engine; anyway this represent a very useful approach to be easily implemented into control development system for preliminary on-line prediction. The tab.3, also shows the under estimation coming from the vibration formulation; this is very probably to be related to the basic principle of the approach that only evaluate the structure-borne noise, without take into account the air-borne noise coming in example from the intake, exhaust and so on CONCLUSIONS FIGURE 13 : A-weighted intensity level db(a) 3 rpm- firing condition 92 db The engine sound intensity level measurements were performed at over discrete points on the 5 surfaces enclosing the source (exhaust, intake, front, top and back sides) using the sound intensity probe, according to the standard ISO 9614/1. The total radiated sound power from the engine was evaluated through the ISO 9614/1 calculation. This technique provides a measure of the sound power radiated into the air by all sources located within the enclosing surface, and excludes sound power generated by sources located outside the surface, []. The figure 13 shows the A-weighted intensity level for the front side measurement where the acoustic intensity probe was positioned parallel to the ground. High intensity level in correspondence of the axis of rotation of the wheel have been measured. From the 5 sides experimental data the overall sound power level has been calculated (see tab.3). Experimental results has shown that the predominant excitation is caused by the combustion process as In this work a vibro-acoustic characterization of the engine source was performed. Different experimental and analytical techniques have been implemented to better study the correlation between them in order to identify the combustion noise. The measurement approaches may be summarized : Intensity measurements, Vibration analysis and Sound Pressure Level measurement. The value comparison shows a good correlation of the data even if derived through different measurement and/or theoretical approaches. Obviously, this contribution of the air-borne noise is tacking into account with the two experimental approach and this contribution increase the overall sound level of the engine. All the aroused consideration regarding the combustion noise, as well as all the investigating tools will represent very useful approach during the multi injection based optimization process today under investigation. REFERENCES 1. R. Hickling and M. M. Kamal, eds., Engine Noise: Excitation, Vibration and Radiation, Plenum Press, New York-London, T. Priede, Noise and Vibration Control of the Internal Combustion Reciprocating Engine Chapter 19, Noise and Vibration Control Engineering, L. L. Beranek and I. L. Ver, eds., John Wiley & Sons, New York, 1992.

7 3. Taylor, C. F., The Internal Combustion Engine in Theory and Practice, Vol.2: Combustion, Fuels, Materials, Design, MIT Press, M.Mattia, F. E. Corcione, R. Paciucci, T. Manna, Analisi delle sorgenti di rumore di un motore diesel ad iniezione diretta turboalimentato, Atti di convegno Internazionale per la Sperimentazione nel Settore Automobilistico, Firenze F.E. Corcione, M.G. Mattia, R. Paciucci, Acoustic Intensity measurements of Noise Emission From a Light Duty T.C.D.I. Diesel Engine, Noise and Vibration Conference, Traverse City, ISO 3746, Acoustics-Determination of sound power levels of noise sources using sound pressure- Survey method using an enveloping measurement surface over a reflecting plane, 1995(E). 7. ISO 9614/1, Acoustics-Determination of sound power levels of noise sources using sound intensity- Part 1:Measurement at discrete points ; Beranek L.L.: Acoustics, Crocker M.J.: Comparison between Surface Intensity, Acoustic Intensity and Selective Wrapping Noise Measurements on a Diesel Engine, International; Symposium on Engine Noise: Excitation, Vibration and Radiation, 1981 CONTACT corresponding author: Dr. Ing. Daniela Siano tel () fax +39 () d.siano@im.na.cnr.it