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An Investigation into Formula SAE Performance

Exhaust Design and Analysis

Anthony I. McLeod1

University of New South Wales at the Australian Defence Force Academy

The Formula SAE competition demands that teams pursue synergistic designs in creating a competitive high performance vehicle. As a result, the design of each component fitted to a vehicle, including that of the exhaust, must be undertaken with a sound foundation of technical understanding in conjunction with creativity and innovation. Exhaust design is shown to make a significant contribution to engine performance, economy and noise attenuation. Hence, this work aims to assist the ADFA Formula SAE team of 2012 develop an understanding of current exhaust analysis and tuning techniques such that they may be innovatively applied to the design of a high performing exhaust system as a part of a holistic engine tuning approach. Extensive research has been conducted into the mechanisms by which an exhaust design may enhance engine performance and attenuate noise. In particular, an exhaust design is understood to effect engine performance via influences upon engine scavenging. Furthermore, the action of automotive silencers was identified to be governed by their ability to manage the mass flow rate from the exhaust outlet as opposed to that of acoustic theory. In addition, research has identified methods such mechanisms may be analysed and predicted. Engine simulation software Ricardo WAVE was used to demonstrate and analyse the performance and noise attenuation implications of exhaust system componentry and their design parameters. A volume restricted silencer design proposed by Professor Blair of the University of Belfast formed the basis of further experimental and theoretical analysis of the governing principles of silencer operation. Specifically, a derivative of this design concept was manufactured with in-built variability to enable an experimental investigation of the design and to also help validate data obtained using WAVE. Finally, WAVE was used to enable a theoretical analysis which underpinned a design proposal for a high performing silencer.

Contents

I. Introduction ... 3 A. Motivation ... 3 B. Project Aims ... 3 C. Project Methodology ... 3

II. Part A - Literature Review ... 3

A. Exhaust design for engine scavenging performance ... 4

B. Acoustics, Vehicle Noise and Exhaust Silencing ... 5

C. Exhaust Silencing ... 6

D. Design and Modelling of Exhaust Systems ... 9

E. Conclusion ... 9

III. Part B –Concept Development and Investigation ... 10

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A. WAVE Model Development ... 10

B. Experimental Silencer Parameter Study ... 10

C. WAVE Investigation – Exhaust Design Parameters for Engine Scavenging ... 13

IV. Part C – Preliminary Design and Design Proposal ... 15

A. Design Requirements... 15

B. Silencing Strategy ... 15

C. Design Investigation and Definition ... 16

D. Design Proposal ... Error! Bookmark not defined. E. Vehicle Integration of Exhaust System ... 18

V. Limitations of Ricardo WAVE ... 19

VI. Conclusion ... 19

VII. Recommendations and Future Work ... 19

Acknowledgements ... 20

References ... 21

APPENDICES

Appendix A. Combined theoretical and experimental investigation into exhuast pipe geometry A1 Appendix B. Exhaust pipe optimisation using NSAGA2 and ANSYS Fluent A2

Nomenclature

ADFA = Australian Defence Force Academy

CFD = Computational Fluid Dynamics SPL = sound pressure level (dB) Q = volume flow rate(m3/s) D = pipe diameter (m) ̇ = flow velocity(m/s)

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I.

Introduction

A. Motivation

The Formula SAE® competition constitutes a variety of rules and regulations that aim to challenge design teams whilst maintaining fairness and safety. A number of pertinent rules to this study [1] include:

 The vehicle‘s engine must be a four cycle piston engine with a maximum swept displacement of 610cc,  An intake restrictor must be fitted with a maximum diameter of 20.0 mm for vehicles operating with

gasoline and 19.0 mm for vehicles operating with E85, and  A vehicle‘s measured noise level must be less than 110 decibels.

It is obvious from these rules in particular that teams are challenged to form a competitive advantage via [2] synergistic vehicle designs by applying technical knowledge innovatively as well as through the application of advanced performance tuning techniques. In this way, teams may attain the necessary combination of power and efficiency to be competitive throughout a series of trying auto-cross events.

The concept of ―exhaust tuning‖ has been under development for over 60 years [3]. In this time, exhaust design has been proven to have a marked influence upon the performance and efficiency of an engine by way of power output, specific fuel consumption, heat production and radiated noise level. It is as a consequence of the flow on effects of such factors that the implementation of a sound understanding in the design of an exhaust is crucial in order to obtain a high performing racing vehicle. For the benefit of competitiveness, it is therefore important for the Formula SAE® team representing ADFA to learn to approach the design of the exhaust in such a way that maximizes performance of the competition vehicle and ensures compliancy.

B. Project Aims

The ADFA Formula SAE® team of 2012 has purchased a Yamaha WR450 single cylinder engine to be integrated into a new competition vehicle. It is therefore the intent of this project to assist the team to understand the potential benefits of exhaust tuning as well as the methods that are available in the analysis and design of an exhaust. This project will consist of the validation of theories currently used to enhance engine output. Furthermore this investigation will be extended to noise generating phenomenon and associated analytical and prediction techniques. The project will employ a one-dimensional engine simulation software, Ricardo WAVE to then undertake analysis to be validated experimentally, culminating in a final design proposal for a new high performing silencer.

C. Project Methodology

This project utilised a series of methods to carry out an investigation into exhaust systems and their design. Initially, extensive research constituting a literature review was undertaken to build a knowledge base requisite of applied analysis and design. The complex trade-offs found to characterise silencer design then motivated an experimental investigation using a promising silencer design concept that was developed and proven upon a similar engine as the Yamaha WR450, by Professor Blair of the University of Belfast. An experimental muffler was manufactured based upon this design concept which incorporated in-built variability to enable an experimental parameter study of silencer attenuation. DOE methodology was utilised to conduct this experimental study which employed an available and operable WR250 motorbike in lieu of the engine testing rig still under development by the FSAE team. This experiment obtained insertion loss for the silencer within the frequency domain to such that the governing principles of silencer operation could be identified. An engine simulation model was then developed using the one-dimensional engine simulation software Ricardo WAVE. This software then underpinned the demonstration and theoretical analysis of performance exhaust tuning and silencer theories. Exhaust performance aspects investigated include the concept of ‗tuned length‘, the effect of stepped pipes and diffuser components as well as the nature of ‗inertial scavenging‘ phenomena. Performance data obtained is discussed such that these methods become yet another tool for the ADFA FSAE team to utilise within an integrated engine tuning process. The silencer experiment was duplicated within WAVE to provide a level of validation of the developed model. Continued silencer analysis employed the WAVE transmission loss work bench. The conclusions drawn from these analyses then facilitated the development of a design proposal for a high performance silencer.

II.

Part A - Literature Review

Quality design of an exhaust system requires a sound understanding of its contribution to both the overall power output of an engine and to noise attenuation. Furthermore it is important to understand the mechanisms that enable these contributions as well as their significance. A wide variety of sources were studied to determine current exhaust theories, design and analysis methods as well as to better understand the restrictions imposed by Formula SAE noise regulations.

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A. Exhaust design for engine scavenging performance

Performance considerations of exhaust design are a result of the nature of the gas exchange process in a four stroke engine. This process includes a period of valve overlap where both the intake and exhaust valves are open simultaneously as seen in Fig.(1). Without due regard by the designer this period could see the induction of exhaust gases into the cylinder as shown in Fig.(2), effectively reducing the amount of fresh combustibles ingested and therefore overall power.

Performance aspects of exhaust design are concerned with minimising such residual quantities or otherwise stated as maximising scavenging efficiency of the engine. This is achieved, one way or another by reducing the exhaust valve pressure during valve overlap such as to bias this exchange process to achieve this scavenge.

Exhaust scavenging is achieved via two methods. This is because the exhaust phase of the four stroke cycle consists of not only the expulsion of a high speed column of exhaust gases but also a pressure wave. Consequently, scavenging is achieved through techniques known as ‗wave tuning‘ and ‗inertial scavenging‘ depending on which of these mechanisms we utilise.

The aim of wave tuning is described by Professor Blair of the University of Belfast who states that ―the tuned exhaust pipe harnesses the pressure wave motion of the exhaust process to extract a greater mass of exhaust gas from the cylinder during the exhaust stroke and initiate the induction process during the valve overlap period.‖ This scavenging effect is possible if a pressure wave originating from the exhaust valve travel at the local acoustic velocity, over a tuned length such that it is reflected back to the valve face as a rarefaction wave, as seen in Fig.(3), in time to assist the gas exchange process during valve overlap.

The phasing of the exhaust valve and the pressure wave is dependent upon the length over which the waves travel. Commonly known as the ‗tuned length‘, it is defined by the length of pipe bounded by the exhaust valve and a discontinuity in the pipe of an area ratio of 6. This being a point that significant wave action can operate from.

In addition, scavenging may be achieved via inertial scavenging. This is a scavenging effect achieved as a result of the inertia of a high velocity column of gas. It functions under the principle that a fixed volume flow rate is achieved at a certain engine speed and for a fixed volume flow rate, gas velocity varies inversely with pipe diameter. There then exists a pipe diameter where the scavenging effect produces a more than proportionate amount of power than pumping work required to achieve an effective gas velocity.

In addition to the stated performance exhaust theory there exists a number of complicating factors for the realistic exhaust system designer. Firstly, the periodic nature of wave phenomenon in the exhaust suggests that whilst tuning may be carried out for the benefit of power at one engine speed this will inevitably lead to poorer performance in another [4, 6]. Furthermore, tuning of the exhaust without due regard of the interactions taking place with other mechanisms such as similar wave action occurring in the intake, has the potential to produce irregular shapes including troughs and peaks within the power curve. As a consequence the drivability characteristics of the vehicle could diminish as a result of unpredictable power output behaviour.

Figure 1. Valve timing events showing valve overlap

Figure 2. Poorly tuned engine ingests exhaust gas into the cylinder during valce overlap

Figure 3. Reflection of rarefaction wave at exhaust pipe end which returns to the exhaust valve

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Another perspective of the priorities of exhaust system design is provided by a parameter study conducted by Sammut and Alkidas [11]. This study utilizes the engine simulation software Ricardo WAVE to quantify the effects of and interactions between exhaust, intake and valve timing parameters. For a constant valve timing and engine speed, Fig.(4) shows a comparison of the scavenging effect of the intake and exhaust measured in volumetric efficiency. The data presented firstly shows that the individual contributions of the intake and exhaust are independent as the contribution made by the exhaust is relatively constant for any intake length. However, it is important to note that such independence should not be assumed between all parameters. All data presented herein illustrating variation in scavenging as a function of tuned length is obtained for constant valve timing. Variation in valve timing would inevitably change the characteristics of the overlap period and therefore the action of exhaust scavenging. The effects of this are well documented throughout literature but assumed

constant for the purpose of this investigation of exhaust design. Secondly, data presented in Fig.(4) also concludes that the effect of exhaust tuning is relatively small compared to the benefits of intake tuning. As a consequence of the diminishing significance of exhaust scavenging benefits, minimizing the losses conceded to increased pumping losses whilst achieving sufficient noise attenuation becomes of relatively high importance if a maximum amount of power is to be derived from the engine. With the realization that there as much potential for an exhaust system design to reduce performance as to improve it, the design of an efficient silencer becomes crucial to the competitiveness of the vehicle. Moreover, it needs to be integrated within a system with minimal prejudice towards efforts to attain an effective scavenge by providing low exhaust valve pressure at valve overlap [6].

B. Acoustics, Vehicle Noise and Exhaust Silencing

Literature was consulted in order to define the problem of vehicle noise as well as to gain an appreciation of current vehicle noise attenuation techniques such that this design issue could be effectively addressed. A noise measurement of sound radiated from a vehicle is subject to a variety of sources including mechanical noise, shell vibration radiated noise and duct noise where duct noise then consists of intake and exhaust tail pipe noise [13]. Fig.(5) is provided to illustrate the prevalence of intake duct noise, being a source not considered here. The sound pressure measured at any point in space is

relative to the radius defining the distance between the source and the point of measurement as well as the directivity of the source with respect to this radius vector. Furthermore, an important consequence of the logarithmic scale of sound measurement is that the sum of SPL from multiple sources varies little from the maximum SPL [4] as seen in Fig.(5). Consequently, a vehicle silencing strategy formulated to control sound pressure at a specified location relative to the vehicle, needs to acknowledge the most significant source at that location in order to effectively control the final measurement. Therefore, the following discussions detailing exhaust tail pipe noise attenuation can only be effective within spatial regions where this is the dominant noise source and sound pressure contributions of

Figure 5. Noise level of intake and exhaust duct noise

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other sources such as intake duct noise and engine noise become negligible.

Such conclusions then underpin the next priority in forming an efficient vehicle noise attenuation strategy being to recognize the dominant components of exhaust tail pipe noise. The design of an efficient muffler should then target these most significant noise components in order to attain the required attenuation level whilst maintaining low resistance to flow. Tail pipe noise component of duct noise and consists of [8]:

1) Pulse/Engine noise which describes sound of frequencies corresponding to harmonics of the engine firing rate (EFR), as seen in Fig.(6). The EFR is the rate at which the exhaust valve releases combustion gases from the cylinder, and

2) Gas flow noise which consists of high frequency broadband noise resulting from pressure fluctuations inherent to the turbulent mean gas flow in the exhaust duct.

Fig. (7) [14] indicates that the pulse noise is dominant at low engine speeds and is superseded by flow noise as it increases in magnitude with volume flow rate and engine speed. A silencer design must therefore incorporate elements that can target the dominant noise source at the engine speed of interest. Specifically, silencing at low engine speeds must be concerned with discrete harmonics of the engine firing rate while silencing at high engine speeds is more concerned with high frequency flow noise.

Pang et al [13] shows a direct proportional correlation between flow noise and flow velocity and therefore exhaust pipe diameter given by Eq (1), where the volume flow rate is a function of engine speed. The relationship between flow noise and diameter is seen in Fig.(8). This shows that at high engine speeds where flow noise is dominant, a fixed volume flow tranlsates to greater flow velocity for a smaller diameter and therefore increased noise emissions.

.

̇ ̇ ̇ (1)

The conversion of flow power to sound power is identified by Wiemeler, Jauer and Brand [14] to be relative to an efficiency factor that is proportional to the flow mach number. They show that a critical flow

velocity mach number of 0.25 represents a transition between flow noise generation mechanisms leading to an icreased efficiency and increased flow noise sound pressure level (SPL).

C. Exhaust Silencing

In order to moderate exhaust tail pipe noise there exists a variety of muffler designs that are commonly employed. The performance of a silencer may be characterized by its insertion loss defined as the difference in measured SPL with and without the muffler fitted; its transmission loss which is defined by the difference in SPL at the inlet and outlet of the muffler; or its effect upon the brake mean effective pressure. An efficient silencer is defined here by a design that achieves a relatively high ratio of attenuation achieved to reduction in engine power output. Silencers may consist of a single or a combination of standard silencing components which include reactive, absorptive and resonator types. These components vary in the manner and efficiency

Figure 6. Frequency analysis of radiated vehicle noise showing noise corresponding to EFR frequencies

Figure 7. Sound pressure with engine speed showing increasing dominance of flow noise with engine speed

Figure 8. Variation in flow noise with flow velocity caused by pipe diameter

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with which they enable the viscous dissipation of acoustic energy. Their unique action often makes them highly effective attenuators in discrete frequency ranges or otherwise less effective over a more general range of frequencies. As a result hybrid silencers aim to utilise a combination of such components in order to form an effective broadband attenuator. A summary of the acoustic theory for these common silencer types is attached in Annex A.

Blair quotes the work of Coates [15] who shows that the sound pressure level at any point in space beyond the termination of an exhaust system to the atmosphere, is a direct function of the instantaneous mass flow rate from the end of the exhaust pipe, the relative distance between source and microphone and the directivity of the pipe end. The instantaneous mass flow rate was calculated using the Eq.(2) [4].

̇ ̇ ( )( ) ( ) (2)

This expression states that the radiated noise is a function of gas temperature, the discharge coefficient of the pipe end, the outlet diameter as well as pressure wave amplitude ratio travelling in the left and rightward direction. As a result of this direct relationship with the mass flow rate Blair states that silencing is easily achieved given an unlimited volume able to dampen the pressure and mass flow oscillations. However, when subject to space restrictions the design of a silencer must conform to the following empirical design guidelines:

 A silencer should have a minimum silencer-cylinder volume ratio of ten.

 If this cannot be achieved the silencer must choke the exhaust system via a restrictive muffler in order to sufficiently damp the mass flow rate for effective noise attenuation. (However increased back pressure will result from increased restriction, therefore a silencer with minimal choke would represent the most efficient attenuator).

Blair [4] uses this theory to conduct a study into the effectiveness of motorcycle silencers via experimental and numerical methods. This study tests a plenum, absorption, diffusing and side-resonant type mufflers all with a constant silencer-cylinder volume ratio of ten. Data shown in Fig.(9) and Fig.(10) illustrates that individually these mufflers either offer excessive reductions in the BMEP of up to 30% whilst being unable to attenuate noise sufficiently or offer negligible effect to power and noise.

A novel ‗two-box‘ hybrid silencer design seen in Fig.(11), comprising an absorption and diffusing silencer component, is then verfied to result in an average reduction in BMEP of only 7% whilst notably attenuating noise. This is seen to be a result of the effectiveness with which the mass flow rate at the outlet is reduced as seen in Fig.(11). Comparisons are shown in Fig.(13) and Fig.(14), of achieved engine performance and measured noise emission data for this design as well as its individual constituent components.

Figure 11. Schematic of tow-box silencer

Figure 10. Noise characteristics of muffler varieties determined by Blair

Figure 9. Torque characteristics of muffler varieties determined by Blair

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Data illustrate the potential of the volume restricted two-box silencer design as an effective and efficient attenuator of exhaust tail pipe noise. Noise spectra in Fig.(15) shows the attenuation achieved by the two-box silencer as well as by individual absorption and diffusing silencer components. This demonstrates the highly non-linear interaction between the absorption and resonant/diffusing components.Acoustic theory would suggest that the effectiveness of this particular hybrid silencer represents a combination of the attenuation of the diffusing silencer at low frequency and the attenuation of the absorption silencer at high frequency which is to a limited extent demonstrated within Fig.(15). However, acoustic theory is experimentally shown by Blair to be highly ineffective in accurately predicting the achieved attenuation from a silencing element. Data in Fig.(16) and Fig.(17) compares the experimentally obtained attenuation with that predicted by acoustic theory which

Figure 13. BMEP with silencer component

Figure 14. Noise attenuation performance of silencer components

Figure 12. Exhaust outlet mass flow rate for silencer varieties

Figure 15. Noise spectra at 7500 rpm with two-box silencer

Figure 16. Noise attenuation of plenum and

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indeed shows poor correlation. The inaccuracy of acoustic theory is understood to stem from its specific relevance to waves of infinitesimal amplitude which differ fundamentally from wave phenomenon experienced in exhaust flows being waves are of finite amplitude. Methods of accurately appreciating the true nature of exhaust waves must therefore employ an appreciation of the instantaneous mass flow rate emenating from the outlet which is proven by Coates [15] to an accurate approach.

The design of an efficient and effective hybrid silencer is understood to be a highly complex task that may employ acoustic theories as merely the basis of an informed estimate for attenuation in order to commence a design process characterised primarily by experimental trial and error. Luckily, one-dimensional gas dynamic computational programs have the capability of providing accurate estimates of the time-varying mass flow rate from the exhuast outlet and may therefore be used to supplement the experimental silencer design process.

D. Design and Modelling of Exhaust Systems

Literature was also consulted in order to determine the capability and implementation of current numerical techniques for the design and modelling of exhaust systems. This review identified that as unsteady gas dynamics within an exhaust system are predominantly one–dimensional in nature, a great deal of research, design and optimization is carried out with one-dimensional engine simulation software. These analyses also achieve excellent agreement with experimental data providing the exhaust geometries of concern are free of excessive curvatures or complex silencing components characterized by strong three dimensional turbulence[4]. Additional advantages of employing this simplification include the ability to simultaneously calculate the effect of duct and silencer geometry upon engine performance and noise spectra, as well as to do so across a variety of designs within a reasonable timeframe. Consequently, one-dimensional codes have demonstrated an ability to enhance the efficiency of the design process [16]. Enhanced accuracy is accomplished with the use 3D CFD and coupled 1D/3D analyses however these generally have a far greater computational cost.

E. Conclusion

This literature review summarises current theory and practices relating to exhaust system design, performance analysis and optimisation. Specifically, the mechanisms by which an exhaust design contributes to engine performance via scavenging were detailed. The nature of noise measurement, vehicle noise generation and attenuation were described. Acoustic theory of common silencing elements was detailed, and the deficiency of this theory in providing accurate predictions of silencer performance was addressed. The complexity of the silencer design process was established and studies demonstrating successful design methods have been described.

Importantly, the outcomes of this literature review enable a more informed requirement definition for a new high performing exhaust system for the Yamaha WR450 engine. The aims of the exhaust design are summarised as an ability to assist in engine scavenging at the desired engine speeds and also to incorporate an efficient silencer design so as to minimise engine pumping work whilst achieving the specified noise target. To demonstrate the implementation of the presented theories within an exhaust design, this project undertook to varying degrees, the stages of concept development and preliminary design of the exhaust system.

Activities under the concept development stage stemmed from the manufacture of a ‗two-box‘ hybrid silencer as well as the development of a WR450 engine simulation model using Ricardo WAVE. The conduct of this stage consisted of an experimental investigation of achieved noise attenuation with parameter variations in the manufactured design. Furthermore, the simultaneous conduct of this investigation with a physical engine as well as within WAVE provided a means for comment upon the effectiveness of the design, as well as the quality of the simulation model developed. This experimentation thereby performed both the roles of a parameter study as well as an experimental validation of the developed WAVE model.

In addition, the concept development stage included the investigation of each of the scavenging mechanisms detailed, using the WAVE model. (These investigations were also intended to encompass an experimental component utilising an in-house developed engine testing rig, however this could not be accomplished due to technical difficulties). This analysis considered all possible design parameters within a reasonable range such that the ADFA FSAE team would be enabled to make an informed design selection as a part of an integrated engine tuning strategy.

Finally, the preliminary design stage was conducted to provide the team with a refined design concept for a high performance silencer based upon that which was manufactured. The proposed prototype design was formulated using engine simulation results as well as data obtained from a silencer parameter analysis assisted by Ricardo WAVE transmission loss work bench.

These processes thereby underpinned the aims of this project being to provide the ADFA FSAE team with the means of designing, analysing and implementing a high performance exhaust system.

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III.

Part B –Concept Development and Investigation

A. WAVE Model Development

A Yamaha WR450 engine simulation model was developed to underpin the continued concept development and analysis of the engine and associated componentry. A view of the WAVE model is provided in Fig.(18). The model employed a host of user defined inputs available from tabulated data or otherwise from physical measurements. Due to time and resource constraints an experimental validation could not be undertaken. However, the validity of the model utilised within this study is supported by agreement found between the generated engine power output prediction and data generated independently by the Cal Poly FSAE team who managed to conduct an experimental validation of their WR450 WAVE engine model. Power curves obtained from the WAVE model as well as by data from Cal Poly FSAE team are provided in Annex (B).

Figure 18. WR450 Ricardo WAVE model

B. Experimental Silencer Parameter Study

Having developed an engine simulation model of the Yamaha WR450, an orthogonal experiment was designed to effect a parameter study of the ‗two-box‘ silencer design. The experiment would be implemented within WAVE as well as upon an experimental WR250 engine using a manufactured experimental silencer. The purpose of this experiment was to investigate the achieved attenuation of the muffler concept, in addition to the variation in this attenuation as a function of parameter modification. Furthermore, this experiment will be used to quantify the effectiveness of silencer design theories including that of acoustic theory and of Blair‘s mass flow rate theory. Upon comparison of theoretical and experimental data, comment will then be made as to the validity of the WAVE model (Only partial validation could be obtained as the WAVE model is a simulation of the team‘s Yamaha WR450 whereas the physical experiment could only be conducted using a Yamaha WR250). 1. Experimental Silencer Design

An experimental silencer was manufactured as per the CAD model in Fig.(19), based upon the ‗two-box‘ silencer design proposed by Blair.

Figure 19. CAD model of manufactured silencer showing initial discontinuity, absorption component and expansion/resonator chamber

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In addition to the original concept, the manufactured design incorporates an initial discontinuity of area ratio equal to six which is conformant to the findings of Blair. This was included to provide a decisive location that may be used to define the tuned length as well as to decouple design parameters concerned with silencing from performance aspects of the exhaust. (Blair‘s original design would instead tune from the end of the perforated pipe. However, an industry SME advises that wave propagation through the perforated pipe enhances wave degradation and therefore wave tuning effectiveness). This design was subject to some variation from the original design as it was constrained by the availability of off-the-shelf (OTS) components which were preferred in order to simplify the manufacturing process. The design may be fully disassembled as per Fig.(19), such that parameters including the choke size, the resonator length and packing density could be varied in accordance with the experimental intent. Table 1 shows a comparison of the non-dimensional parameters of the Blair design and the current experimental design.

Table 1. Non-dimensional parameter of Blair silencer and mancufactured silencer

Blair Experimental Design Comments

Inlet Diameter (D) 46.6 mm 51 mm OTS component and recommended

by industry SME

Major Diameter 2.58D 2.49D OTS

Absorption Length 8.58D 9.0D OTS

Resonator Length 4.29D Variable up to 5.88D Custom telescoping component

Perforated Area 19% 25% OTS

Silencer- Cylinder Volume Ratio

15 14.5 - 19.5

2. Orthogonal Experiment Design

Acoustic theory predicts that the manufactured two-box silencer will achieve broadband attenuation as a result of the combination of resonant effects of the expansion chamber and viscous dissipation of the absorption silencer. However, as shown in experimental data obtained by Blair in Fig.(15), the operation of this silencer does not explicitly conform to predictions underpinned by acoustic theory, nor does data show that attenuation achieved is linear addition of the attenuation achieved by its constituent components. In contrast, Blair‘s mass flow rate theory hypothesises that broadband attenuation achieved by this silencer is a direct consequence of the manner with which it damps the magnitude of the mass flow rate from the exhaust outlet. Consequently, an experiment was conducted to identify the true manner of operation of this silencer so as to provide the means for the design of an efficient silencer for the ADFA FSAE team.

In order to attempt to validate acoustic theory, noise measurements recorded the noise spectra such that the insertion loss of the muffler could be calculated. Hence, validation of acoustic theory could be obtained if this data was to show agreement with transmission loss data calculated.

The experimental plan was based upon Taguchi Design of Experiment methods [30] for orthogonal experiments. This experiment was then implemented upon a WR250 engine as well as implemented within WAVE. Unfortunately, as the WAVE model has been developed to simulate a Yamaha WR450 engine and the physical experiments were conducted upon a WR250 engine the comparison of these results were not able to provide conclusive validation of the developed engine model. Instead these two sources of data were simply used to comment on the nature of operation of the muffler as well as to confirm similarity of trending.

The chosen independent variables include the silencer parameters of resonator length, choke diameter and packing density. Using the Taguchi L4 orthogonal array the experiment seen in Table 2 was formulated.

Table 2. Orthogonal experimental plan

Experiment Choke Diameter (mm) Resonator Length (mm) Packing Density (g/L)

1 20 100 200

2 20 300 100

3 30 100 100

4 30 300 200

3. Parameter Study Results and Discussion

Noise measurements taken at position A and position B for experimental silencer variations as well as for the standard WR450 muffler is shown in Fig.(C1) and Fig.(C2) in Annex C. WAVE data is also provided in Fig.(C3) which shows the predicted sound pressure at position A for each of the experimental test silencers. As expected, comparison with experimental data at position A does not show agreement of sound pressure magnitude due the difference in engine displacement. However, good agreement is found as to the relative variation between designs. Measurements obtained of SPL with frequency are provided in Fig.(C4) and

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Fig.(C5). This data shows variation in emitted noise from the unsilenced pipe with engine speed as well as the broadband attenuation achieved by the manufactured silencer which is seen to be equivalent to a standard WR450 muffler. Calculated insertion loss is provided in Fig.(C6), Fig.(C7) and Fig.(C8). Data provided is limited to a frequency of 1000 Hz as consistently high levels of attenuation are achieved for all designs at frequencies beyond this point. Transmission loss for the each of the silencer configurations was also calculated using WAVE for comparison, and is seen in Fig.(C9) and Fig.(C10).

The data obtained is generally supportive of the theory of silencer design concerned with the management of the mass flow rate from the exhaust outlet as opposed to acoustic theory. For instance, within Fig.(C1), Fig.(C2) and Fig.(C3) the most choked designs 1 & 3 record significantly lower sound pressure level recordings compared to the less choked designs. Furthermore, both experimental and WAVE data agree that those designs with a larger volume will attenuate noise to a greater extent. Plots in Fig.(C11), Fig.(C12), Fig.(C13) and Fig.(C14) of outlet mass flow rate, generated with WAVE, illustrate the silencing action of the muffler variations. These plots show data for all tested engine speeds. Prominent features include the higher peaks recorded for the less choked designs as well as the higher steady mass flow rate recorded for the more choked designs. This steady flow rate leading up to the peak is much more constant for choked designs, which increases in magnitude at high engine speeds in comparison to the relatively less choked designs. This behaviour suggests that designs utilising a 20mm orifice are likely to become aerodynamically choked leading to a rapid increase in back pressure, but also attests to the effectiveness of a choke for the purpose of exhaust tail pipe silencing under Blair‘s theory concerned with the outlet mass flow rate. Furthermore, fluctuations within this mass flow rate are seen to be damped by silencers employing a larger expansion chamber volume.

However, with reference to Fig.(C1), Fig.(C2) and Fig.(C3) it could also be argued that higher attenuation is achieved by those designs with larger expansion chambers due to an increased ability to attenuate low frequency noise as per acoustic theory for an expansion chamber. This low frequency noise is recorded in Fig.(C4) and Fig(F5) as a source that is relatively constant as well as relatively elevated in comparison to other regions of the noise spectra emanating from the unsilenced pipe. To investigate this possibility, the transmission loss for each of the silencer configurations was attained from WAVE and is shown in Fig.(C9) and Fig.(C10). (This was conducted within the WAVE transmission loss workbench which employs the well documented two source method to compare sound power at the inlet and outlet of the silencer). As seen in Fig.(C10), the predicted transmission loss below 500Hz for both designs 1 and 4 is seen to be consistently up to 5dB greater than for designs 2 and 3 which employ a smaller chamber. However, it is questionable that this extra achieved attenuation could be the main reason for these larger designs consistently out performing corresponding designs with an equal choke diameter and smaller chambers. This doubt is particularly pertinent as transmission loss data predicts generally lower attenuation achieved by larger designs 1 and 4 at higher frequencies, yet experimental data states that these larger designs record lower SPL even at high engine speeds where high frequency flow noise becomes more dominant. Furthermore, calculated insertion loss data does not show any significant agreement with theoretical attenuation for the silencer represented by the transmission loss data. Subsequently, the achieved attenuation of a silencer in practice is seen to be more dependent upon the manner in which the design manages the exhaust outlet mass flow rate to atmosphere than the attenuation predicted by acoustic theory. Results leading to this conclusion are in line with published theory of Blair described previously.

The value of acoustic theory is not, however, totally diminished as some agreement is found between transmission loss data and calculated insertion loss data by way of comparative performance. Discrepancies between these sources may also be exaggerated by inaccurate assumptions and experimental error. This would include the assumption of nil mean flow during the transmission loss analysis and aliasing within experimental measurements. Furthermore, the consistently high attenuation achieved at high frequency by the manufactured design agrees with the acoustic theory of absorption silencer. (The effectiveness of an absorption silencer was also experimentally determined by Blair and is shown in Fig.(17)). Therefore whilst acoustic theory and its implementation may not take into account all non-idealities it may provide a good initial estimate of silencer performance.

Further comment can be made as to the effectiveness of the manufactured design with reference to Fig.(C4) and Fig.(C5). These show measured sound pressure with frequency at position B for engine speeds of 3000 and 7000rpm. These plots show that the test silencer offers significant attenuation averaged between 20dB and 30dB which is also recorded for the standard WR450 muffler. The WR muffler has a silencer-cylinder volume ratio of 5 in its intended role upon a WR450 and a ratio of 7.5 for the test engine and as a result it employs a 10mm choke in order to meet road authority noise regulations. The trade-off between silencer volume and choke is made clear as experimental data concludes that the designed silencer of a volume ratio of 4 to 5 times greater than the standard WR muffler, yet far less choked is able to achieve equivalent attenuation. This data thereby emphasises the importance of exploiting available vehicle space for a silencer in order to minimise the level of choke required and therefore minimise power losses. A comparison of these figures also illustrates the

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increasing prevalence of high frequency flow noise at high engine speed, a trend which is also shown by Honda et al in Fig.(7).

A comparison of all SPL spectra data obtained experimentally is compared with corresponding data attained from the WAVE model in Fig.(C15), Fig.(C16), Fig.(C17) and Fig.(C18). Obvious discrepancy should be expected as the data sources are generated by different engines. Despite this, however, fair agreement is found highlighting the capability of the developed WAVE engine model.

Finally, noise measurement data obtained from position C is given in Fig.(C19) and Fig.(C20). This data shows no clear indication of any significant resonances that are absent from unsilenced data.

4. Conclusion

The base silencer design as proposed by Blair is here shown to have high potential as an effective silencer. With further design refinement and testing, the efficiency of this concept may also be appreciated. The expansion chamber has shown enhanced sensitivity to the non-idealities present within an exhaust silencing application. As a result, more significant correlation is found between the volume of this component than with its length as per acoustic theory. However, acoustic theory was demonstrated to generate predictions of absorption silencer performance with relatively high accuracy.

This experiment established the priority for silencer design as to control the mass flow rate from the exhaust outlet. Acoustic theory was determined to have limited effectiveness in predicting silencer performance. The usefulness of acoustic theory within silencer concept development is recognised.

Fair agreement was found with WAVE data obtained despite variation in engine displacement used to generate the data sets. This agreement was represented mainly by similar trending. As per theory detailed in the literature review, the value of a one-dimensional software for concept development is shown to be founded in its ability to quickly describe unsteady gas flow throughout an engine.

C. WAVE Investigation – Exhaust Design Parameters for Engine Scavenging

A literature review identified that exhaust performance considerations are resultant of the nature of the gas exchange process in a four stroke engine. The following investigation was undertaken using WAVE to demonstrate the potential of the identified exhaust tuning strategies. This included the variation in ‗wave tuning‘ and ‗inertial scavenging‘ with exhaust pipe geometry. Specifically, this study was concerned with identifying the extent of variation in engine performance possibly accomplished via the design of an exhaust pipe for a single cylinder engine assuming a constant intake length and valve timing. The purpose of this investigation is therefore to inform the ADFA FSAE team of the methods commonly incorporated within the design of an exhaust system, that aim to enhance or merely shape an engine‘s performance characteristic. The following findings should therefore act as a tool to be used in conjunction with many other powertrain parameters to obtain a desired engine performance target.

1. Exhaust Wave Tuning

As stated by Professor Blair of the University of Belfast ―the tuned exhaust pipe harnesses the pressure wave motion of the exhaust process to extract a greater mass of exhaust gas from the cylinder during the exhaust stroke and initiate the induction process during the valve overlap period.‖ This scavenging effect is possible if a pressure wave originating from the exhaust valve travels over a tuned length such that it is reflected back to the valve face as a rarefaction wave in time to assist the gas exchange process during valve overlap. The coincident phasing of valve overlap and the arrival of pressure waves, seen in Fig.(D1), is dependent upon the length over which the waves travel known as the ‗tuned length‘. Resultant valve mass flows and pressure differentials are provided in Fig.(D2) and Fig.(D3).

As per Fig.(D4), Fig.(D5) and Fig.(D6), this scavenging effect is characteristic of a certain engine speed where the correct phasing occurs. These figures shows the variation of residual gas fraction with tuned length and the resulting effect upon torque and power output of the engine as a result of an increased delivery of combustibles. Here relatively small variation in residual gas fraction is seen to have a marked effect upon torque. Furthermore, when this effect is achieved at high engine speeds a highly significant influence is exercised over the shape of the power curve. This data therefore demonstrates the significance of the exhaust wave tuning and resultant scavenging effect achieved. Finally, Fig.(D7) is provided to demonstrate the possible effect of interaction between intake and exhaust tuning. As seen, the scavenging ratio (a measure of the quantity of fresh combustible mixture ingested to the engine) is greater than unity at 3000rpm for a 1450 mm tuned length. This suggests that at this point wave tuning is interacting with other tuning effects to achieve a greater scavenge than could be achieved alone. Such interactions are also important when defining the design target for exhaust tuned length such that these interactions are used to their full potential.

To summarise, a contour plot of residual gas fraction with tuned length and engine speed is provide provided in Fig.(D8). The region representing the most effective scavenging effect is seen in blue. An inverse relationship is seen to exist between exhaust length and tuned engine speed. Since pressure waves travel within the exhaust

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system at the acoustic speed which is a function of temperature, the hyperbolic nature of this relationship is therefore present due to the asymptotic nature of heat transfer from the exhaust pipe.

Stepped Pipe Tuning

As a part of an investigation into exhaust wave tuning techniques, the industry practice of utilising stepped pipes was considered. A schematic of a stepped exhaust pipe is shown in Fig.(20). A stepped pipe offers extra degrees of freedom in wave tuning practices as there exists more discontinuities able to create rarefaction wave reflections. Furthermore this characteristic can also lead to varied heat transfer properties. To illustrate, Fig.(D9) is provided. This plot is the product of a 1500mm pipe with a fixed expansion at 500mm and

another expansion whose location in the pipe varies between 510mm to 1490 mm. Wave action from the pipe end at a tuned length of 1500mm is tuning at 4000 rpm and secondly at 7500rpm shown by two regions of relatively low residual gas fraction. In addition, wave action from the first expansion occurs at 500mm enhancing the region of low residual at 7500 rpm. Of note, data shows a noticeable change within each of these regions as a result of the location of the second expansion. To illustrate Fig.(D10) is provided which shows variation in exhaust gas temperature (in blue) as well as acoustic velocity (in green) with position for two stepped pipes with location of the steps indicated. Fig.(D10) specifies that these shifts in the tuning behaviour of the pipe can be attributed to the effect stepping behaviour of the pipe has on heat transfer, average gas temperature and the average acoustic speed which are indicated to vary slightly. In addition to this effect, Fig.(D11) and Fig.(D12) show that for these same two stepped pipe designs, the intermediate expansions are in fact also able to reflect rarefaction waves for the purpose of scavenging, despite being of a lesser magnitude. Specifically, these figures show that for two different stepped pipes (parameters indicated in plot caption), a variation in the arrival of the first smaller wave is recorded, giving rise to corresponding change in the recorded valve mass flow rate purely as a consequence of the unique positioning of the intermediate discontinuity.

Diffusers

The most effective exhaust component design for wave scavenging is that of the diffuser as seen in the schematic provided in Fig.(21). This type of exhaust component is known to be able to tune over a wider range of engine speeds offering superior scavenging and engine performance. For comparison however, a generally accepted rule-of-thumb states that a third of the length of the diffuser is used in the calculation of the total effective tuned length.

To demonstrate the action of the diffuser, Fig.(D13) shows residual quantity achieved for a diffuser 400mm long and with a taper angle of 6.34 degrees attached to a

variable length of pipe. What is instantly noticeable is the greater dominance of the scavenging region compared to that of the straight pipe. Again Fig.(D14), shows residual quantity for a larger diffuser of 600mm in length with a taper angle of 6.65 deg. Once more this shows a vastly greater scavenging ability than that shown by a single straight pipe. Finally, Fig.(D15), is showing residual gas fraction for a diffuser larger still, of 900mm in length with a taper angle of 6.65 degrees, however no significant benefit is seen to be gained by the extra length. Through the course of this study a diffuser was manufactured in order to obtain experimental validation of this theory and to promote the diffuser as an innovative technique to enhance overall engine power. However, as a result of the continuing inoperability of the team‘s engine testing rig this validation could not be undertaken. 2. Inertial Scavenging

Inertial scavenging describes a scavenging effect that is enabled by the inertia of a high velocity column of exhaust gas escaping from the cylinder. As seen in the simplified schematic in Fig.(22), the interaction between this column of gas and the gas exhange process during valve overlap may see a build up of pressure energy at the exhaust valve that acts to assist in engine breathing. This mechanism functions under the principle that a fixed volume flow rate is achieved at a certain engine speed and for a fixed volume flow rate through a pipe, gas

Figure 20. Simplified schematic of stepped pipe

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velocity varies inversely with pipe diameter. Consequently, there then exists a pipe diameter where the scavenging effect produces a more than proportionate amount of power than pumping work required to achieve an effective gas velocity. Under these circumstances an increase in engine power will be realised.

To demonstrate Fig.(D16) is provided, which depicts the variation of scavenging effect measured in residual gas fraction with differing pipe diameters for a constant tuned length of 1200mm and for a silenced exhaust system. A

constant diameter 35 mm pipe is seen to scavenge well between 4000 and 5000 rpm. The addition of a 42mm step sees a shift in this scavenging effect to 5500rpm and similarly if this step is increased to 48mm in diameter, a further increase in the scavenged engine speed is observed. A pipe consisting of multiple steps and therefore including pipe diameters of 35mm, 42mm and 48mm achieves a scavenging effect which almost represents the addition of scavenging effects for both single step pipes. A consideration of the corrresponding pumping torque data in Fig.(D17), may be used to explain the transfer of energy from the piston, to pressure energy within the exhaust gas and then to its kinetic energy which eventually promises a scavenging effect capable of making proportionately more power than that lost in pumping. A comparison of trending in residual gas fraction and pumping torque for the constant 35mm pipe shows that within the range of engine speed of 4000rpm to 5000rpm, where most significant scavenging is achieved, pumping torque data is recorded as the greatest of all data sets. This increase in pumping torque acknowledges the transfer of energy from the piston to exhaust gas which in turn assists in cylinder scavenge. A similar phenomenon is observed for all other data sets such that a relative increase in pumping torque corresponds to the range of engine speeds where inertial scavenging is achieved. Valve mass flow rate data provided in Fig.(D18) and Fig.(D19), illuminates this theory further. For data at 4000rpm, the highest mass flow rates are attributed to the 35mm pipe. However, for data at 7500 rpm the stepped pipes are seen to record the highest valve mass flow rates. Finally, the subsequent torque and power curves are provided in Fig.(D20) and Fig.(D21).

Contours of residual gas fraction for exhaust tuned length and pipe diameter were generated to explore this concept further. Three plots are provided in Fig.(D22), Fig.(D23) and Fig.(D24), one each for the engine speeds of 6000rpm, 7000rpm and 8000rpm. Data is summarised in Table (3).

Table 3. Pipe parameters for optimal scavenging

6000rpm 7000rpm 8000rpm

Pipe Diameter (mm) 724 559 586

Tuned Length (mm) 31 31 34

These plots draw attention to the minimum residaul gas fraction which is achieved with a specific combination of pipe length and diameter. Comparison of data at 6000rpm and 7000rpm suggests that wave tuning is the dominant mechanism at these engine speeds. This can be concluded as a constant diameter has been maintained, suggesting no significant variation in inertial scavenging achieved between these two engine speeds. Furthermore, a longer pipe is seen to contribute to scavenging at lower engine speed which is in line with previously discussed wave tuning concepts. However, a comparison of data at 7000rpm and 8000rpm shows that inertial scavenging is seen to gain significane again due to devaition from this logic. Now at 8000rpm, peak scavenging is achieved with a longer tuned length than at 7000rpm, which is contrary to wave tuning trends, as well as a larger diameter. This larger diameter can then be understood to underpin a required flow velocity such that inertial scavenging is maximised.

The reciprocal exchange of energy from exhaust gas inertia to pressure energy which works to assist in pumping is seen in Fig.(D25) which shows variation in pumping torque with the length of constant 35mm diameter pipe. Here, at low engine speeds longer pipes offering back pressure record higher relative pumping losses. However, at higher engine speeds the maintenance of flow enery within longer pipes works to assist the piston during the exhaust stroke leading to a reduction in pumping losses relative to shorter pipes.

IV.

Part C – Preliminary Design and Design Proposal

A. Design Requirements

As per the design of any engineering product, the design of a performance exhaust system must be conducted relative to specified requirements. Therefore before implementing knowledge gathered as a part of the concept development stage, customer requirements were explicitly stated. For a performance exhaust for the WR450 engine, they are stated in order:

Figure 22. Simplified schematic of inertial scavenging

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1. Adequate insertion loss—in accordance with FSAE regulations the noise measurement taken near the exhaust must not exceed 110dB;

2. Back pressure minimal—to maximize vehicle competitiveness throughout the competition the implemented silencer should introduce minimal engine power losses by way of back pressure and be integrated within an exhaust system that provides a desired torque and power output characteristic. 3. Size—the silencer must be able to be easily integrated within the vehicle;

4. Cost—low cost desirable;

5. Durability—high durability desirable.

B. Silencing Strategy

In accordance with findings of the literature review, the formulation of a preliminary design proposal was to be carried out relative to the identified ‗noise problem‘. In this case ‗noise problem‘ was established from an estimate of the SPL spectra, measured under FSAE conditions, emanating from a 1000mm stepped pipe, which was obtained from WAVE. This data is provided in Fig.(E1) in Annex I. It shows the predicted sound pressure relative to a ceiling of 108dB. This target was calculated in accordance with theory detailed in Annex B. This calculation recognises that the intake is the second most dominant source of noise upon a vehicle, as well as that any other sources of noise varying from the maximum of more than 15dB offers a negligible addition to the total SPL measurement. A pessimistic estimation of the intake noise is taken as 100dB, and therefore with the addition of a 108dB exhaust noise contribution, a total measurement of 109dB would be achieved which is in accordance with noise level design requirement.

Predicted noise spectra for the WR450 at its test engine speed of 7000rpm shows that the noise measured from the outlet of the stepped pipe constitutes a number of significant contributions from a range of frequencies corresponding to flow noise as well as to the engine firing rate. Therefore the proposed silencer is required to offer broadband attenuation of up to 20 dB to satisfy noise the level design requirement.

C. Design Investigation and Definition

The definition of a final design proposal comprised a process of systematic analysis and selection, of silencer components. Each of the significant silencer components including the absorption silencer, expansion chamber and the choke were investigated individually such that the final silencer assemblage would represent the option best able to satisfy the design requirements.

In line with findings of experiments conducted, the analysis of these individual components was conducted relative to their governing principles. Since good correlation was found between acoustic theory and achieved attenuation for an absorption silencer, the analysis of this component was based upon the predicted acoustic transmission loss, which ignores the non-idealities of an exhaust silencing application. In contrast, the analysis of the choke and expansion chamber volume was conducted upon the developed WAVE engine model such that variation in outlet mass flow rate and consequent attenuation could be appreciated.

In accordance with findings of the WAVE enabled investigation into exhaust scavenging mechanisms, the final silencer design proposal will be implemented upon a 1000mm stepped pipe as this design offers the highest level of inertial scavenging for a fixed tuned length. Tuning of this integrated system will then be undertaken in order to demonstrate the process of exhaust tuning relative to a performance target. In this case, the performance target will be the unsilenced performance trend such that the attainment of this target will help demonstrate the efficiency of the silencer design proposed by way of minimal back pressure.

1. Absorption Silencer Component

Acoustic theory of an absorption silencer was seen to hold true during experiments. Therefore an analysis of transmission loss is recognised to represent a reasonable prediction of achieved performance if not merely relative performance. Fig.(E2) shows variation in attenuation with increasing diameter assuming the current 51mm diameter perforated pipe is used and holding the length of the component and packing density as constant. Attenuation is seen to increase linearly with diameter. This result agrees with acoustic theory such that the increased depth of sound absorbing material increases the viscous dissipation of sound energy via interaction with particle oscillations. Fig.(E3) shows variation in attenuation with length of the absorption silencer component. Attenuation is seen to asymptote such that large increases in length are required for only a fractional increase in attenuation. Acknowledging that the absorption silencer component will constitute the majority of the weight of the overall silencer, the attained data was analysed in terms of attenuation achieved per unit mass. Fig.(E4) shows that for a specified increase in attenuation, an increase in diameter represents a more efficient means than increasing the length. As a result, the proposed silencer design should incorporate as large a diameter as possible that still remains conformant to the size constraints specified in the design requirements.

Fig.(E5) shows variation in attenuation with sound absorbing material density. This plot shows the convergence of attenuation to an asymptote. Consequently, this data suggests that a density greater than 150g/L offers a negligible increase in attenuation for the extra weight. Finally, Fig.(E6) shows variation in attenuation

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with perforated area of the inner pipe used. Negligible variation is illustrated therefore the selection of perforated tube used will be constrained the requirement for durability of the silencer, as too high a perforated area will allow violent exhaust gas flow to degrade or remove sound absorbing material.

2. Choke Diameter

The choke was proven to practice significant control over emitted noise within experiments conducted which is demonstrated further in Fig.(E7). This shows the ability of the choke to scale the emitted noise levels via practicing direct control over the outlet mass flow rate. Fig.(E8) is also provided in order to emphasise the silencing action of the choke. This data is generated within the WAVE Transmission Loss Workbench, which conducts a comparison of sound power at the inlet and outlet of a silencer under nil mean flow conditions. As a result, the choking of an expansion chamber element is seen to have minimal effect in the absence of mean flow.

In accordance with the design requirement for engine power losses, Fig.(E9) and Fig.(E10) are provided. These plots show that for a choke diameter greater than 26mm a minimal effect upon engine scavenging, measured in total residual quantity, and brake torque is predicted. Meanwhile a choke of 26mm also achieves an increase in overall attenuation of up to 5dB making this a highly efficient component for silencing.

3. Expansion Chamber

Having acknowledged the trends in attenuation achieved via absorpion parameters as well as choke, the volume of the expansion chamber was varied to gain a similar appreciation. Variation in predicted outlet mass flow rate with expansion chamber length with fixed diameter is provided in Fig.(E11). The maximum mass flow rate recorded is seen to decrease consistently until a length of 150 mm is reached. A negligible change in peak mass flow rate is found beyond this length and instead a phase shift is noted. Similar behaviour is seen in Fig.(E12) which illustrates the resultant SPL measurement taken under SAE noise test conditions. A consistent reduction in SPL is recorded up to a length of 150mm at which point the negligible change in the magnitude of the mass flow rate results in no further reduction in SPL.

B. Design Proposal

The conducted parameter study was used to inform the formulation of a final design concept. Data generated justified the selection of silencer parameters such that concepts could be verified using the developed engine model. So as to minimise size and weight of the silencer, a conservative choke diameter of 30mm was selected to exercise meaningful control over the outlet mass flow rate without deliberately increasing engine pumping losses. The diameter of the silencer was identified to represent the most significant factor per unit mass, for increasing absorption attenuation and silencer volume. As a result, in order to minimise weight of the silencer and with a consideration of space constraints relevant to the current vehicle‘s side pod arrangement, a diameter of 175mm was selected. Again to minimise weight, a minimal length was sought for the absorption silencer component. Data suggested that lengths beyond 300 mm were subject to diminishing returns in terms of attenuation and so this was selected as the final absorption length. By doing so the requirement for extra chamber volume would also be minimised. The packing density was selected to be 150g/L as acquired data suggested diminishing returns beyond this value. Finally, expansion chamber length was increased until a satisfactorily low outlet mass flow rate was obtained giving a prediction of under 108 dB as per design goal.

The ability of WAVE to accurately calculate the instantaneous mass flow rate from the exhaust outlet, being the definitive measure of silencer performance, underpins confidence within this design proposal. The characteristics of the proposed silencer design are provided in Table (4).

Table 4. Design Proposal

Component Parameters Attenuation Characteristics

Resonator Chamber Length: 150 mm

Diameter: 175 mm

Dampen outlet mass flow rate thereby providing broadband attenuation.

Absorption Component

Length: 300 mm Diameter: 175mm Packing Density: 150 g/L

Target high frequency flow noise

Choke Diameter: 30 mm Scale outlet mass flow rate thereby

providing broadband attenuation.

Silencer-Cylinder Volume Ratio

References

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