The silencing of a high performance motorcycle

Journal of Sound and Vibratiou (1974) 33(1), 29-39

THE SILENCING OF A HIGH PERFORMANCE MOTORCYCLE G. E. ROE

Simon Engineering Laboratories, Unirersityof ~r AIanchester hi 13 9PL, England (Receired 20 Jttne 1973, and in revisedform 25 September 1973) The noise output of a 750 cc motorcycle was reduced from 98 dB(A) to 86 dB(A) under current European test conditions, without significant power loss. Principal noise sources were exhaust, induction, and mechanical noise, with induction noise being higher than expected, and the untreated mechanical noise mainly responsible for the final level settling no lower than 86 dB(A). Induction noise was reduced by 12 dB(A) by using a damped cavity side-resonator, and exhaust noise by 20 dB(A) (compared with standard silencer equipment) by using a new silencing principle. Both intake and exhaust silencers were inexpensive to produce, of compact dimensions, and presented no special manufacturing problems.

1. INTRODUCTION Motorcycles, particularly those of large capacity, are traditionally noisy, and this has contributed significantly to their poor public image. This situation is changing rapidly with the legal enforcement of noise limits in those countries with important retail outlets, notably the U.S.A. and Europe. Of these the European standard is the more difficult to achieve, and is currently set at 86 dB, A-weighted, for motorcycles over 125 co. The test procedure is to accelerate the machine at full throttle from 50 km/h in second gear for 20 metres, and then shut the throttle quickly. The microphone is 7.5 m away from the mid-point ofthe acceleration run, measured at right angles, and is 1.2 m above the ground. The meter is set at fast-response, A-weighted. The current West German limit is 84 dB(A) under the same test procedure. A 750 cc Norton " C o m m a n d o " motorcycle was supplied by the manufacturer, with a view to reducing its noise output to the above 86 dB(A). An initial test-run gave 98 dB(A), and a recording was made of this for subsequent analysis. A further constraint on the project was that there should be little power loss, as past attempts at motorcycle silencing have resulted in loss of performance. Motorcycles are sold t o a large extent because of their high power/weight ratio, and power loss is more serious than in the ease of motorcars. An early attempt at motorcycle silencing was made by Cave-Brown-Cave [I]. Since then, most work has used the now-classic methods of Davis [2] for exhaust silencing, with more recent investigations by Blair and Spechko [3]. Davies and Alfredson [4] have produced a computer programme for predicting exhaust silencer performance in the ease of expansion box silencers, and this has been tried for motorcycles. For intake noise, motorcycles have been virtually all unsilenced, until very recently when simple low-pass expansion box silencers have been used, but again with power loss. The silencing of an internal combustion engine is particularly difficult in the case of a motorcycle, because of the lack of available space and the need to have a final styling which is acceptable to current vogue. Exhaust silencers must be less than 1 m long and 0.I m 29

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G.E. ROE

diameter, and provide cornering clearance under high angles of lean. There is insufficient available length for multiple silencer boxes with long coupling pipes as for motorcar practice. For conventional engines, any intake silencer must fit under the seat, a space already fully occupied. Moreover, there are limits on machine width at this point. The engine does not benefit from the enclosure of the motorcar, and air-cooled engines do not-have a water jacket to suppress mechanical noise. Another problem is the higher frequencies of motorcycle 9 intake noise due to smaller cylinder size. Low frequencies benefit from A-weighting. In the face of these difficulties, the further silencing of an existing machine offers limited scope, and one really needs a frame and engine configuration designed specially to provide the necessary space. Nevertheless, in this case the limitations of an existing machine had to be accepted. The investigation was entirely empirical, but the successful result now prompts more fundamental work into the precise silencing mechanism used. 2. ANALYSIS OF THE INITIAL TEST-RUN The recording was scanned with a 21 ~o bandwidth filter (at - 3 dB) from 50 Hz to 10 kHz at overlapping intervals (an 8-5 ~o filter was tried but it provided no significant new information in the finer structure of the spectrum). Peak levels during the run within each band were then plotted (Figure 1). This spectrum is characterized by a large broad peak at 300-500 Hz, and lesser peaks at 2000 and 5500 Hz, although the fall-off at low frequencies is due largely

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to A-weighting. The inflexion at about 80 Hz is the mean engine fundamental during the run. A recording was also taken when "mutes" were inserted in the ends of the standard manufacturer's exhaust silencers. These are simply orifice plates reducing the exit diameter from I] in (35 mm) to 88in (19 ram). Overall noise level was 94 dB(A) (instead of 98). The main noise reduction was at high frequencies, but this small loss in overall noise was more than offset by a serious loss in engine power (itself contributing to the noise reduction). The upper trace of Figure 2 shows a typical filter output from the test-run recording, this one at 500 Hz. There is a slight throttle lag followed by a broad peak as the engine runs through a resonance, and a second resonance which is interrupted by the closing of the throttle. The lower trace is the output from a magnetic pick-up attached to the rear wheel,

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MOTORCYCLE SILENCING

subsequently converted into engine rev/min. One thus deduces that the engine accelerates from 3200 to 5000 rev/min, in approximately 1-3 seconds. It was then attempted to simulate these conditions in an acoustically lined test-cell.

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Figure 2. Initial test run--filter output at 500 Hz. 3. THE TEST-CELL INSTALLATION The test-cell had reverberation times of 0.07 s for octave band noise at 250 Hz, 0.06 at 500 Hz, and less than 0.06 for 1000 Hz and over. The gearbox output was connected by chain to a dynamometer with constant load pre-set to give an "acceleration" rate of 3200 to 5000 rev/miri, in 1-3 seconds. For the "Commando" motorcycle, the engine and rear fork form a separate unit coupled to the main frame by rubber mountings, with sideplay limited by shims. To take power from the engine thus required clamping the rear fork to the ground. This effectively locked-out the rubber mounting, and the severe vibration from the twin-cylinder engine (360~ cranks) caused fatigue failure in several components of the installation. The test-cell was fitted with extraction fans, and engine cooling was by a centrifugal compressor outside the cell blowing through flexible ducting to a sheet-metal cowl surrounding the engine. Engine temperature was monitored by a thermocouple under one sparking plug, with normal running temperature 200-220~ Ear protectors and breathing apparatus were worn. Background noise was 35 dB(A). Noise from the extraction fans was 55 dB(A), and from the cooling duct 75 dB(A), so engine cooling was switched off briefly during recordings. 4. INITIAL TEST-CELL RECORDINGS, AND THE STANDARD EQUIPMENT Four recordings were taken and analysed; (a) microphone 6 in (152 mm) from end of one standard exhaust silencer, on axis; overall 130 dB(A), (b) microphone 6 in (152 mm) from end of one open exhaust pipe, on axis; overall 133 dB(A), (c) microphone 2 in (51 mm) from side of intake air-filter box; overall 127 dB(A) and (d) microphone 4 in (102 mm) from open carburettor intake; overall 128 dB(A). Lead shielding was used to isolate intake from exhaust noise, but it was still necessary to place the microphone in the near field, and accept any resulting error, because of the limited size of the cell, and also the nearby intense sources of engine mechanical noise. In practice the error was not large, with the main features of these measured spectra appearing also in the total machine noise spectrum. Note the magnitude of induction noise, little short of exhaust noise. Note also the small

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G.E. ROE

overall silencing effect of the standard exhaust silencer, and of the intake air-filter box. Both caused reduction at high frequency only (over 1000 Hz), giving a subjective impression of silencing without affecting the overall sound pressure level appreciably. The intake air-filter box was a folded paper element in a perforated metal box, and the standard exhaust silencer was nominally of "absorption" type, with annular cavity packed with damping material. However, this cavity was connected to the central pipe by louvres sufficiently restricted for the cavity to be regarded as a tuned cavity of natural frequency [5] c

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summation. Here l = 0, and using an equivalent diameter for the louvres gave f = 1500 H z at room temperature for waves of acoustic strength. One might expect, therefore, that for finite amplitude waves these silencers would absorb predominantly a frequency band centred on approximately 1500 Hz, rising with the increasing temperature of the exhaust gas. The spectral analysis of exhaust noise with silencers is shown as the dashed curve in Figure 3, and intake noise with air-filter box as the dashed curve in Figure 4. Both these spectra are very peaked, the exhaust case with a broad peak from 300-700 Hz, and the 140

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MOTORCYCLESILENCING

33

intake with narrow peaks at 400, 850 and 2100 Hz. It is not surprising, therefore, that the overall machine noise of Figure I shows excessive noise at 300-500 Hz, and a lesser peak at 2000 Hz. The 80 Hz fundamental is apparent in both of these curves. Spectral peaks have a disproportionate effect in lifting the overall sound pressure level, and obtaining satisfactory overall noise levels is very much a case of attempting to remove these peaks from the spectra: i.e., of aiming at a fiat spectrum. 5. TRANSMISSION LOSS EXPERIMENTS FOR EXHAUST NOISE In an attempt to explain the performance of tile standard exhaust silencers, and to assist in the development of new units, transmission loss tests were made after the fashion of Davis [2]. Sound of acoustic wave strength from a 25 VA speaker horn drive unit was passed directly down the exhaust pipe and silencer combination, with mi.crophone readings at input and output. The result for the standard pipe and silencer, total length 65 in (1.65 m), is shown in Figure 5. Five harmonics of half-wavelength resonance on a length of 68 in (1-73 m) (pipe with both ends open) areclearly visible with low loss (poor silencing) at these frequencies. I00

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Figure 5. Transmission loss of standard pipe and silencer. The other dominant feature is the high loss in a broad band centred on about 1600 Hz. This was anticipated above on the basis of concentric cavity resonance, with the damping material in the cavity giving the bandwidth. The net effect is a broad trough in the transmission loss centred on about 250 Hz: i.e., 500 Hz for two alternately firing cylinders. The broad peak in the exhaust noise output spectrum is centred on 500 Hz (see Figure 3) so there is obvious correspondence here. There is also a trough in the transmission loss spectrum at about 8000 Hz, i.e., 16 000 Hz on two cylinders, but the noise output from the engine into the pipe and silencer is low at this high frequency. Nevertheless, the silencer does seem to behave better at high frequencies under the large amplitude waves of engine running conditions than the acoustic wave transmission loss curve suggests. Perhaps the damping material and louvre area act to some extent as a simple absorption silencer at these frequencies. 6. THE NATURE OF INDUCTION NOISE The exhaust noise spectrum (standard silencers) of Figure 3 having been explained to some extent, it remained to explain the corresponding spectrum for induction noise (Figure 4). This has two main peaks, at 400 and 850 Hz, with a lesser peak at 2100 Hz, or, in terms of a single cylinder, 200, 425 and 1050 Hz, respectively, This motorcycle has a slightly curved inlet pipe of total length approximately 9in (230 ram). A quarter-wavelength resonance on

34

G . E . ROE

this length at room temperature (appropriate to the inlet system) corresponds to a fundamental frequency of 370 Hz, with the next harmonic at 3 x 370 = 1110 Hz. These frequencies are close to two of the observed peaks. But does an inlet pipe act as a quarter-wavelength resonator: i.e., a pipe with one end closed (the valve end) and the other open ? A hot-wire anemometer in the inlet pipe of a gas engine produced the signals of Figure 6 (approximately proportional in this case to the square root of the total fluid velocity, with both +ve and -ve

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velocities shown as +ve). The large humps correspond to the rush of gas through the open inlet valve. Note, however, the large amplitude oscillations in the pipe after valve closure-precisely those suggested above. The 9 in (230 ram) inlet pipe oscillated at 370 Hz, and the longer 11 in (280 ram) pipe at 300 Hz, with greater damping. The very large peak at 200 Hz is more difficult to explain, for both half and quarter wavelength resonances at this frequency require lengths much greater than any physical dimension inthe engine or inlet system. There is the possibility, however, of the cylinder and inlet pipe resonating as a cavity (valve open) according to the natural frequency relation quoted in section 4. Taking the inlet pipe as 9 in (230 ram) long • 1.2 in (32 ram) diameter, and a cylinder volume of 12 in3 (200 co) (approximately at maximum inlet valve opening) gives a frequency of 200 Hz. This therefore offers a probable explanation. Again, acoustic wave strength formulae have been used, but to obtain frequency estimates only. Accordingly, no end corrections have been applied. In practice, the waves in the intake system are much nearer to the acoustic wave approximation than for the exhaust system output. 7. THE SUPPRESSION OF INDUCTION NOISE Attempts to suppress induction noise inevitably have a more serious effect on performance than for exhaust silencing. The slightest pressure loss reduces volumetric efficiency significantly, For this reason, expansion box silencers, including the low-pass type of large box breathing through a short tuned pipe often used for motorcars, were not considered. All current motorcycle induction silencers are of this type, and carry a performance penalty. Some simple attempts at induction silencing were made first, and these proved surprisingly

MOTORCYCLE SILENCING

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effective. For instance, diffusing the length of the inlet pipe by adding a straight 4 in (100 mm) extension from the carburettor, drilled with twelve 88in (6-3 mm) diameter holes gave a 5 dB(A) reduction. A similar result was obtained by adding two concentric tubes, an outer one 4 in (100 mm) long, and a smaller diameter inner tube 11 in (280 mm) long. 2 dB(A) of this reduction was due to the lowering of resonance frequency by increase of pipe length, shifting the spectral peaks down the A-weighting curve, and the remainder to the length diffusing effect: i.e., removal of a precise resonance. Another surprising result was that adding a cross-pipe between the two exhaust pipes (as the manufacturer had chosen to do for future production anyway) gave a small drop in hltake noise, presumably effective during valve overlap. The main effort against induction noise was concentrated on the use of tune d side cavities, in View of the very peaked nature of the unsilenced spectrum. It seemed appropriate to attempt the "tuning out" of these dominant frequencies. Moreover, a side cavity leaves the central flow pipe virtually unimpeded, giving low pressure loss. The final design is shown in Figure 7, and several features need particular comment. (a) A separate test with a cavity fitted to a quarter-wave resonator pipe showed cavity absorption efficiency to increase markedly with the ratio cavity dimension/wavelength absorbed. It was therefore logical to join the two inlet tracts to double the frequency (halve the wavelength) and use one large cavity instead of two smaller ones. Indeed, an attempt to use a small cavity on each pipe had minimal success.

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Figure 7. Inductionsilencer. (b) Packing the test cavity with absorptive material broadened the absorption peak significantly (at some expense to height) and packing was used to great effect in the final design. Fibreglass was tried initially, but small particles of glass were ingested by the engine over a period of time, and nylon wool, with continuous strands, was subsequently used successfully. It was also 1 dB(A) more effective than the fibreglass. Damping material has also been used by Chen [6] to minimize turbulence at entry to a side cavity. (c) This cavity is tuned to a theoretical frequency of 520 Hz (see section 4). It was in practice given final optimization by drilling holes in the central pipe. This frequency lies between the two original spectral peaks at 400 and 850 Hz, but it must be remembered that in order to fit the cavity the inlet pipe was extended (and diffused) so that the appropriate frequencies are lower. Indeed, some reduction in noise is again due to a shifting of the peaks down the A-weighting curve. A larger capacity engine has recently been introduced by the

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G.E. ROE

manufacturer, and the lower frequency of cylinder cavity resonance has expectedly given lower induction noise, A-weighted. Some further improvement might be expected if two cavities were used, tuned to different frequencies. (d) A sudden rise in induction noise (about 1 dB(A)) was observed on throttle closure. The bell-mouth at entry removed this. (e) The simple air-filter of fine nylon mesh sandwiched between stainless steel fine-mesh gauze gave 1 dB(A) reduction, predominantly at high frequencies. (f) The final spectrum is shown in Figure 4, and is much flatter than the original, although obviously with room for improvement. The total reduction over original equipment was 12 dB(A). 8. THE SUPPRESSION OF EXHAUST NOISE As with intake noise, the aim here was to provide noise reduction with the minimum of flow restriction. For this reason, the traditional multiple expansion box type of silencer, with or without tuned coupling pipes, was not considered. Also, the reduction was needed over a wide frequency range, so the tuned side cavity type was initially rejected. Direct exhaust noise without silencers had a very flat spectrum (+3 dB from 100 Hz-10 kHz linear, from 500 Hz-10 kHz A-weighted). Auxiliary side cavities were later used, but only to "clean up" the final spectrum. The time available for the project was limited, and it was decided to try a new working principle, admitting that there would be insufficient time for proper optimization, or investigation of the precise physical mechanism. It was Considered that some noise reduction by viscous dissipation might be achieved by passing the exhaust gas down, effectively, a channel with high width/height ratio, and with the height very small compared with the wavelength of sound to be suppressed. With sufficient width the flow area could still be maintained at least equal to that of the feedpipe from the engine. In practice, the annular gap between two cylinders was used (Figure 8). This peripheral volume would also act as a quarter-wavelength resonator (closed end/open end), 4mm

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Figure 8. Experimentalexhaustsilencer. and it was hoped to balance this to some extent against the observed half-wavelength resonance of the feedpipe. For perfect mutual cancellation of resonance, the peripheral volume would have needed effectively the same length as the feedpipe (approximately 39 in (1 m)), but a silencer of this length is not a practical fitment on a motorcycle. For convenient installation on the machine in question, a length of 26 in (0.66 m) was chosen, with outside diameter 388 in (82 mm). The feedpipe and peripheral volume were connected by a short expansion chamber of low expansion ratio and adjustable length, although further development of this silencer will aim at removing this. The transmission loss of this silencer under acoustic wave input is shown in Figure 9. Performance up to 1000 Hz (usually poor with compact silencers) was excellent, but there were two troughs in the curve, one at 1300 Hz

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Figure 10. Prototypeexhaust silencer. (4.8 mm). This silencer gave a 16 dB(A) reduction, but this was influenced by background noise from the noisy exhaust of the other cylinder (fitted with a standard silencer) in spite of lead shielding. When two of these prototype silencers were fitted, a reduction of 20 dB(A) was achieved, and a very fiat spectrum (Figure 3). 9. POWER TESTS To find the effect on engine power of these intake and exhaust silencer fitments, brake measurements were made, with results as shown in Figure 11. The intake fitment improves mid-range torque at the expense of some low and high speed torque, most probably the result of a longer inlet tract. The new exhaust system gives a large boost to low speed torque, at the expense of higher speeds. The net effect is an increase in engine power from 2000--4000 rev/min., a slight loss from 4000--5000 rev/min., and a gain

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Figure 11. Power curves. , Prototype silencers and induction silencer; - - - - , standard silencers and induction silencer; - - - , standard equipment. above 50130 rev[min., but no change amounted to more than 3 hp compared with the original machine. The two most significant features of these power curves, apart from the fact that power levels are little changed, are that the new induction system seems to match well the new exhaust system, and the increase in low speed torque. This latter effect permits a higher gear ratio for the machine to reduce mechanical noise via lower engine speed during the acceleration run. Lower engine speed also improves intake and exhaust noise to some extent. 10. FINAL FIELD TESTS The motorcycle fully equipped with silencing equipment was then returned to the test track for acceleration runs under European conditions, and the spectra are shown in Figures 12 and 13. The target of 86 dB(A) mean was just achieved, but exhaust and intake were i

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Figure 13. Overall machine noise--final test run--right-hand side. quieter (in a stationary test) than the mechanical noise. The left-hand side of the machine was slightly noisier than the right-hand side, probably corresponding to the intake release on that side of the machine. Further improvement in overall machine noise is thus dependent largely on reducing mechanical noise, with some room for improvement in induction noise. The manufacturer has subsequently fitted higher gearing (see previous section) and the reduction in mechanical noise has produced a further small reduction in overall noise. 11. CONCLUSIONS The required reduction of the noise output from 98 to 86 dB(A) under European test conditions of a 750 cc motorcycle was achieved by suitable intake and exhaust silencing, and without serious effect on power output. The improvement in low speed torque has permitted higher gearing with consequent reduction in mechanical noise. Further work is now needed to investigate the precise damping mechanism of passing finite amplitude sound along narrow apertures. ACKNOWLEDGMENTS Acknowledgments are due to Norton-Villiers Limited for their full co-operation and loan of the machine, and to Loughborough University of Technology for permission to print Figure 6. The exhaust silencer and its principle are the subject of Norton-Villiers patents.

REFERENCES 1. T. R. CAVE-BROWN-CAvE 1934 Engineering 138, 316-318. The reduction of exhaust noise of motorcycles. 2. D. D. DAvis 1954 NACA Report No. 1192. Theoretical and experimental investigation of mufflers with comments on engine-exhaust muffler design. 3. G. P. BLAIRand J. A. SPECHKO1972 Society of Automotive Enghzeers Paper No. 720155. Sound pressure levels generated by internal combustion engine exhaust systems. 4. P. O. A. L. DAVIESand R. J. ALrREDSON1971 bzstitution ofAIechanical EnghzeersConference on Vibration and Noise in Alotor Vehicles, 17-23. Design of silencers for internal combustion engine exhaust systems. 5. P. O. A. L. DAVIES1964 Journal of Sound and Vibration 1, 185-201. The design of silencers for internal combustion engines. 6. Y. N. CHEN 1968 Proceedings of the bzstitution of Alechanical Enghreers 1967-68 182, Part 1, No. 3. Lateral Helmholtz resonator silencer with turbulence absorption.