Modelling, Simulation and Validation of a Top/Central-Fed ...-Simulation-and... · Modelling,...

International Journal of Scientific & Engineering Research, Volume 7, Issue 12, December-2016 406 ISSN 2229-5518

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Modelling, Simulation and Validation of a Top/Central-Fed Air-Intake System for a FSAE Restricted 600cc Four-Stroke

EngineFawwaaz Hosein, City University London

This paper sets out to investigate what are the ideal geometric parameters and ergo design of an air intake, specifically for a restricted, 600cc Four-Stroke Engine. 1D engine modelling using Ricardo WAVE is used to determine the plenum volume, runner length and diameter and injector positioning, producing the best engine performance at a primary RPM range of 7000 to 9000.

Two design concepts were then produced using the parameters determined through engine modelling in addition to profiles like bellmouths and diffuser profiles known to reduce pumping losses. The CAD geometry is then simulated using 3D CFD to analyse the flow inside of the air intake system. Intake performance is determined using: restrictor choked flow analysis, diffuser lag, runner flow uniformity and swirl, volumetric efficiency and general packaging. In addition FEA Static Structural analysis is carried out at the maximum suction pressure on the inner walls of both designs made of Duraform GFin order to prove that the plenum will not implode at these conditions.

The results show that the plenum volume should be between 5L and 6L to reduce the probability of choking and the runner length for the primary RPM range is 180mm at 38.1mm diameter for the maximum performance. Additionally simple radius bellmouths are employed to reduce spitbackand 140° runners aid in air-fuel mixing and packaging.A multi-cylinder air intake system incorporating all the features outlined is the best design of the two systems evaluated in the study, resulting in 64 Nm Peak Brake Torque and 70 kW Peak Brake Power.

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INTRODUCTION Restricting the airflow to the engine is a

convenient, and therefore common, method of regulating engine performance in many forms of motor sport. Formula Student imposes such restrictions on the engine configuration. The capacity of the engines must not exceed 610cc but, more specifically, the air-intake system must be fitted with a 20mm diameter restrictor through which all the air must pass for Gasoline fueled cars[1]. There are, however, a number of geometrical parameters (Plenum Volume, Intake Runner Length and Intake Runner Inner Diameter) which can be adjusted to positively influence the performance of the restricted engine.

The governing rules contain a limited number of technical regulations that constrain the students’ design decisions, allowing for almost unimpeded design exploration prospects. With four-cylinder FSAE engines, the geometry, layout, volume and packaging of air intake systems vary. Direct comparisons with each team is made difficult as they all use different runner lengths, plenum volumes, throttle body designs, etc. Additionally as the air-intake system is designed in tandem with the exhaust system, each team again having different exhaust designs, further complicates

direct comparison. For this reason teams develop their air-intake system based on performance targets, expertise and resources. For pragmatic purposes, this study was carried out as part of the powertrain development for City Racing, the Formula Student Team of City University London. The choice of the air-intake system layout was therefore constrained by the chassis, available engine and drivetrain designs. Ergo, only Top/Central Fed air-intake systems were considered.

Before designing the air-intake system, the plenum volume, intake runner lengths and diameters has to be determined as they are critical areas. Following this, the geometry of the plenum can be adjusted so as to increase the volumetric efficiency where possible. Furthermore, structural integrity and mass of the air-intake system has to be taken into consideration for the manufacture and operation of the system. Ricardo 1D WAVE Engine Simulation was used to simulate a number of scenarios, particularly different plenum volumes, runner lengths and diameters, and fuel injector positioning in tandem with the exhaust system design, across the entire RPM range. To verify the intake design, the internal flow has to be evaluated through Computational Fluid

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Dynamics modelling (ANSYS FLUENT and BETA CAE Systems), using the results from the WAVE simulations as the inputs. To verify the intake design was structurally reliable, ANSYS Static Structural FEA was performed on the desired intake model to be manufactured using SLS-Duraform GF, in order to determine if the deformation and equivalent stress developed are tolerable. From the performance results produced from the WAVE simulations, experimental evaluation of the previouslymanufactured air-intake will help predict how the newly designed intake systems will perform.

ENGINE MODELLING In order to maximise the performance we

must first understand the basic operating principles of an intake system and further the influence of the restrictor on the airflow. In order to validate ideas and to optimize engine parameters, a means by which to measure any effects that changes had on the engine was required. For this reason, Ricardo WAVE 1D Engine Simulation was used to model the engine being used along with the air-intake and exhaust systems for the current (CR14) and previous (CR13) powertrain designs.

The engine

modelled is a 2004 Yamaha YZF-R6, with Piper Cams PY6 Camshaft. This is the engine used by the team, with all parameters specified in ‘Table 1’ were taken

from the service manual [2]and previous modelling of engine conducted[3].

CONSTANTS CONSTANTS Engine Configuration – Yamaha YZF-R6 2004 Valves Configuration

Nomenclature Value Units Inlet Valve Ø 25 mm No. of Cylinders 4 # Outlet Valve Ø 22 mm Strokes per Cycle 4 # Butterfly Valve Bore Ø 28 mm Bore 65.5 mm Valve Lift Profile See ‘Figure 2’ Stroke 44.5 mm Fuel and Injectors Configuration Connecting Rod Length 270 mm Fuel Type Indolene N/A Wrist Pin Offset 0.5 mm Fuel to Air Ratio 1/14.7 N/A Compression Ratio 12.4 N/A VARIABLES Clearance Height 2 mm Intake System Engine Type Spark Ig. N/A Plenum Volume 4, 5.5 and6(x106) mm3 ACF 0.3 bar Runner Length 100, 180, 200, 300 mm BCF 0.005 N/A Exhaust System CCF 325 Pa.min/m Primary Pipe Length 550 and 645 mm QCF 0.2 Pa.min2/m2 Secondary Piper Length 300 mm

Figure 1: Ricardo Wave Engine Model for CR13 and CR14 – Yamaha YZF-R6 2004 with Piper Cams PY6 Camshaft IJSER




Table 1: Major Inputs for Ricardo Wave Engine Simulation

It was found that one of the most significant factors to affect the torque output was the duration of the intake camshaft. As performance cam replaced the stock cam of the engine, it was crucial include this profile in the engine model (figure 2). The result of reducing the duration of the camshaft while retaining the standard lift means that the valves will open significantly faster compared to the standard camshaft. This provides good mid-range torque, without sacrificing it at the top end of the speed range [4]. Air-Intake Plenum Volume

The plenum volume was determined to be 6L for the new system, as it provided a sufficient air volume buffer for restricted flow intake.The effect that volume of the plenum has on engine power, in terms of restricted inlet manifolds, is described quite sufficiently here. In this case, volume of the plenum needs to be increased.Inlet manifold comprising of a large volume plenum, incorporates lower velocity values at the restrictor during a full engine cycle. The latter, is translated into less pressure losses inside the inlet manifold. Instead, when the volume of plenum is small, during engine operation, air flow into cylinders is not sufficient; therefore there is constant need for extra inducted air. Ergo, resulting in continuously high velocity values at the restrictor, increasing pressure losses and the possibility of choking[5]. Air-Intake Runners Length

The purpose of an intake manifold is to evenly distribute the combustion mixturebetween engine cylinders, to improve the performance and efficiency.In order to maximise the performance of the engine through modification of the intake we must combust a greater mass of fuel in a given period of time. In orderto combust a greater mass of fuel, more oxygen is required in the combustionchamber. To get a greater amount of air [and hence oxygen] into the

combustionchamber we must increase the momentum of the air travelling into thecombustion chamber. During the valve overlap period, this increase inmomentum will aid in the removal of combustion products from the combustionchamber, and upon conclusion of the intake stroke, a higher air density.Before we can comment on methods by which the momentum of the air will beincreased, we must understand the pressure fluctuations occurring within the intake. Every time the Inlet Valve Opens (IVO), the reduction in cylinder pressurecreates a negative pressure-wave pulse. This pressure pulse propagates up theintake runners until it reaches the boundary between the intake runners andplenum, rarefaction occurs. [Rarefaction – the density of the air at the entranceof an intake runner suddenly decreases causing the formation of a wake. Thesurrounding air rushes into the runner to fill this wake causing the formation ofa positive pressure-wave pulse (propagating down the intake runners towardsthe valves). Thus the pressure has been reflected upon contact with a largechange in cross sectional area. With each reflection, the amplitude decays.] Thereflected positive pressure pulse can be tuned to increase the momentum of theair at a given engine speed. This phenomenon is known as wave ram charging(WRC), and can increase volumetric efficiency to a peak of 125% [6].

Figure 2: Yamaha YZF-R6- Piper Cams PY6 Valve Lift Profile

a b c d e

Figure 3: Rarefaction (a) Negative pressure-wave pulse propagating through intake runner towards plenum, (b) Negative pressure-wave pulse reaches intake-plenum boundary, (c) Higher density air in plenum fills the wake caused by the negative pressure-wave pulse (d) Smaller positive pressure-wave pulse is formed (e) Positive pressure wave pulse travels towards intake

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We can calculate the period of time required for

the reflection of a pressure pulse, as we know a pressure pulse will propagate at the speed of sound. However the time period is not a very useful; knowledge of the position in the engine cycle that the time period corresponds to is required knowledge of the crankshaft displacement, θ t, to find the position in

the cycle. Studies have shown that the optimal θ tis between 80 and 90. Taking θ tas 85, the optimal runner length for a given engine speed can be determined as follows[7]:

𝐿𝐿𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 =𝜃𝜃𝑡𝑡 . 𝑐𝑐

12.𝑁𝑁 (𝑚𝑚𝑚𝑚)

From the intake runner relations to engine speed graph ‘Figure 4’, four runner lengths of interest were used in the Ricardo simulation; 100mm, 180mm, 200mm and 300mm. As the maximum RPM of the Engine is 13000 RPM and the main running RPM range is between 8000 to 9000 RPM, it is expected that the runner length should be between 150mm and 300.

Air-Intake Runners Inner Diameter

The velocity of the flow and therefore momentum of the air entering the combustion chamber at different engine speeds can be adjusted by varying the diameter of the intake runners. A small diameter runner will be beneficial at low engine speeds, as it will increase the velocity of the flow when the mass flow rate remains the same. This higher velocity will increase the momentum of the air, thus increasing the density of the air in the combustion chamber. The mass flow rate will remain relatively constant until the point where the flow becomes chocked, that is the mass flow is limited by sonic flow occurring at some point in the runner network. In a restricted system this may occur at higher engine speeds.

𝐷𝐷𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅 = �2 × �𝐴𝐴𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝑅𝜋𝜋

� ∗ 1000 = 44.7 (𝑚𝑚𝑚𝑚)

Where: ARunner= Inlet Valve Area = 0.00157 (m2) To achieve optimal performance, a diameter

variation between 30mm and 40mm will suffice. Although at very low, and very high engine speeds, a runner diameter outside this range would be beneficial, the magnitude of the benefit is, at most, 2kW for a very short period, thus the additional complexity of a system with 100% greater variation range cannot be justified. For this reason the inner diameter was set at the same diameter as the engine intake port (38.1mm).

Influential Factors Runner length, diameter and taper have varying degrees of influence upon the engine

performance. The runner length and diameter are the most influential factors and their influence on. A small angle of convergence applied to the intake runners can help accelerate the flow, thus increasing the momentum of the air entering the combustion chamber. A greater taper angle results in greater performance around the ideal WRC conditions. The effect is at its largest where WRC occurs and some effect is seen at high engine speeds. At the lower engine speeds, variation of the diameter is more influential, whilst at higher engine speeds the runner length is more influential. Clearly, variation of both the diameter and runner length lead to a significant increase in engine performance over the control. At low engine speeds, a variation of diameter is more influential than variation of length. When a large length is used, a high Reynolds number will cause a pressure drop of up to 10 mbars, diminishing the effects of WRC, (WAVE uses tabulated values of pressure drop due to Reynolds number and pipe length); conversely when the diameter is varied, the increase of flow velocity at low engine speeds will lead to an increase in flow momentum thus increasing the density of the air within the combustion chamber. Flow with a greater velocity, as a consequence of smaller diameter intake runners will promote superior mixing between the air and the fuel, thus producing more complete combustion and therefore greater engine power. This simulation, completed with WAVE cannot

Figure 4: Relationship between Runner Length and Engine Speed IJSER




model turbulence, thus does not account for this phenomenon.

Optimal Conditions Plenum Volume

Above 3 liters, an increase in plenum volume does not lead to an increase inpower output. However, as previously discussed 6 liters is required to prevent choking and provide a sufficient air volume buffer for restricted flow intake.

Runner Length

From ‘Figures 6 and 7’ the runner length of 180mm was selected as Wave Ram Charging (WRC) occurs, however the previous cycle rather than the current one generates the pressure pulse. The performance is better than with the longer runners because the pressure wave is weaker as a results of the greater frictional effects associated with the longer runners.

The optimal length of runners will change at different engine speeds. These lengths will be influenced by factors external to the intake such as cam timings, the valve overlap period and exhaust geometry. To determine optimal conditions for a given

system, the entire system must be simulated; it is not sufficient to merely state shorter runners will benefit performance at high engine speeds and longer runners at low engine speeds as the optimal conditions at any given engine speed fall within a relatively small window. A runner length of 180mm allows for 36% increase in peak brake power over the CR13 (shown later on) and an 18% increase in brake torque with steep torque curve between 6000 and 8000 RPM (primary RPM range). AIR INTAKE DESIGN

From the Ricardo WAVE results presented prior, two different intake manifold systems were designed. However due to the lack of computational power and time, only one of the design was simulated via CFD. Both Air Intake models were to be manufactured using SLS-Duraform GF and therefore modelled taken the material properties into consideration. Duraform GF, offers elevated stiffness and heat resistance. Additionally, it is typically used for air intake system prototyping providing further confidence in its use[8].

MEASUREMENT UNITS Density 1490kg/m3 Moisture Absorption 0.22% over 24 hours Tensile Strength, Yield 140 MPa Elongation at Yield 1.4% Hardness, Shore D 77 Heat Deflection Temperature

179 °C

Table 2: Properties of Duraform GF Dome-Shaped Plenum Air Intake

The dome-shaped intake incorporates an ovoid plenum, 140° runners at 180mm length and a continuation of the divergent portion restrictor nozzle as the runner to the plenum. The upper portion of the

Figure 5: Power Produced (Unoptimised) for a

Figure 7: The Effect of Various Runner Lengths on Brake Engine Torque

Figure 6: The Effect of Various Runner Lengths on Brake Engine Power

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plenum is ribbed to add stiffness and to reduce deformation through high suctions pressures (suction on the plenum walls only occurs when choking occurs, therefore the likelihood of the plenum being deformed through suction is highly unlikely).The fluid volume of the plenum is 6.2L, with a total mass of 2.6kg inclusive of throttle body and air filter. The runner thickness was set to 8mm as the air intake is essentially a cantilever. The plenum wall at the unribbed areas was set at 5mm.

ISOMETRIC VIEW

Figure 8: Isometric View of Dome-Shaped Air Intake

INTAKE RUNNER X-SECTION

Figure 9: Cross Sectional View of Dome-Shaped Air Intake

FULL SYSTEM ASSEMBLY

Figure 10: Full System Assembly of Dome-Shaped Air Intake

Multi-Cylinder Plenum Air Intake The multi-cylinder intake incorporates multi-

cylindrical plenum, 140° runners at 180mm length and a continuation of the divergent portion restrictor nozzle as the runner to the plenum. The plenum itself consists of multiple chambers designed in such a way that they act as ribs to dissipate stress and increase stiffness. The fluid volume of the plenum is 5.5L to increase throttle response, whilst preventing choking. As with the previous intake, the minimum plenum wall thickness was set to 5mm and the intake runners set to 8mm.The mass of the intake system is 2.57kg inclusive of throttle body and air filter.

ISOMETRIC VIEW

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Figure 11: Isometric View of Multi-Cylinder Air Intake

INTAKE RUNNER X-SECTION

Figure 12: Cross Sectional View of Multi-Cylinder Air Intake

FULL SYSTEM ASSEMBLY

Figure 13: Full System Assembly of Multi-Cylinder Air Intake

Intake Design Similarities From the results of the engine modelling

detailed above, both air intake designs share a number of design similarities in addition to the runner length and diameter. The engine intake port gasket profile, intake runner angle,intake runner bellmouth, injector port location and angle and throttle body with restrictor are the same in both designs. Engine Intake Port Gasket Profile

As seen in figures 9 and 12 respectively, the intake port gasket profile allows for the intake to be slotted into the engine intake port gaskets. Although this profile does not influence the performance directly, it is necessary to get this profile accurate so that the intake is held in its position appropriately and is sealed in such a way to prevent air flow leakage into the engine. To ensure the designed gasket profile was accurate, it was manufactured and slotted into an engine port gasket as shown in figure 14. MANUFACTURE

D GASKET PROFILE

Figure 14: Manufactured Gasket Profile Slotted into Engine Intake Port Gasket

Intake Runner Angle Unlike previous air intake designs, the intake

runners were angled rather than the diffuser, as shown in figures 9 and 12 respectively. Unlike the gasket

Figure 16: Coefficient of Discharge Through Intake Runner

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profile, the angle of the runners does have an influence on the engine performance. Sharp bends (≤90°), negatively affect volumetric efficiency of the air intake system with efficiency losses of up to 4%. However the air intake suction runner were set at 140°, resulting in the losses being negligible (< 0.5%) as seen in figure 15. This bend was required to allow for the air intake to encompass the constraints of the chassis design.

Figure 15: Effect of Runner Angle on Intake Volumetric Efficiency

Intake Runner Bellmouth[9] Entries and exits of pipes within the intake

system can produce significant headloss; the use of bellmouthswas investigated to minimize this loss. The effectivenessof flow transfer (through the end of a pipe) can be expressed numerically as the Coefficient of Discharge, (Cd).When flow travels through the pipe of area (Ap), a “vena contracta” of area Ac will form and Cd can be found as Cd = Ac/Ap as shown in figure 16. Three systems were considered for the design: 1. Plain-ended pipe (control) 2. Simple radius 3. Elliptical profile

The performance advantage of the considered elliptical profile over the simple radius is shown in figure 17. The performance advantage of using a simpleradius over a plain-ended pipe is 27%, and the advantage of the elliptical profileis 4% over the simple radius. To determine the rationale behind the bellmouth results highlighted below, the Mach number was observed for the three considered systems at a constant pressure ratio.

Figure 17: CdPlotted Against Pressure Ratio for the Three Considered Bellmouths

Figure 18: Velocity Flow Profiles into a Plain Pipe End

Figure 19:

Velocity Flow Profiles into a Simple Radius End Pipe Figure 21: Corresponding Maximum Spitback for Maximum Flow of the Three Considered Bellmouths

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Figure 20: Velocity Flow Profiles into aEllipticalPipe End

Looking at figure 18, a very strong vena contracta is clearly evident. In figure 19, the vena contracta is significantly reduced, with the vena contracta further reduced in figure 20. The type of bellmouth was found to dramatically affect Cd as shown in table 3 below.

SYSTEM Cd Plain Ended 0.57 Simple Radius Ended 0.72 Elliptical Radius 0.74 Table 3: Cd of the Three Considered Systems

Coefficient of discharge (Cd) in inversely proportional to reverse flow or spitback. This is normally observed at particular crank angles during regular flow. However, the actualbellmouth is placed on an engine which breathes most unsteadilyand so the pressure ratio across the bellmouth varies with crankangleand the air particles will not only enter that intake pipe from theatmosphere but will also reverse (spitback) during various periods.

The spitback is extremely prominent in the case of the plain ended pipe that a toroidalvortex has formed. The strength of spitback reduces with a simple radius and isalmost negligible when using an elliptical profile.The elliptical profile produces the greatest bellmouth performance, a plainendedpipe the worst. The addition of a simple radius produces a significantadvantage of a plain-ended pipe. Where the elliptical profile cannot be used dueto design complexity, the use of a simple radius is acceptable as thedifference in effectiveness is 4%; a plain-ended pipe should be avoided (as it is27% less effective than a simple radius). Ergo the simple radius bellmouth design was selected as previously shown in figures 9 and 12 respectively.

Injector Port Location and Angle Where one injector is to be used per cylinder

the best compromise position is immediately downstream of the butterfly valve. This gains maximum advantage from local turbulence and gives results surprisingly close to the optimum at both ends of the rev-range. This is the recommended position for most applications. For performance at low RPM, economy and low emissions the injector needs to be close to the valve and firing at the back of the valve head. This is the favoured position for production vehicles.For higher RPM (7000-9000) the injector needs to be near the intake end of the induction tract to give adequate mixing time and opportunity[10]. The higher the RPM, the further upstream the injector needs to be. The injector angle should prevent the spray profile from being directed onto the intake port walls and be directed as unimpededly as possible into the combustion chamber. The position and angle of the injector port shown in figure 22, outlines the geometric profiles that met with the ideal conditions outlined above for the intake systems under consideration.

Figure 22: Injector Port Positioning

Throttle Body with Restrictor As part of the CR series continuing

development, the decision was taken to purchase a throttle body with required restrictor. The previous design incorporated a clunky throttle body, with the restrictor having to be remanufactured each year. A 28mm single bore ‘shaftless’ throttle body with the required 20mm restrictor was sourced and purchased. A shaftless blade throttle body allows for up to a 10% increase in air flow.Additionally the restrictor angles and ram trumpet have be carefully design to decrease the pumping losses as much as possible. Furthermore with the entire part being manufactured out of aluminium 6082 T6, its overall mass has been reduced by 90% as compared to the previous version.

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CFD SIMULATION Modelling using regarding Ricardo WAVE

results are useful but not entirely satisfying. WAVE does not provide a clear consideration of the efficiency of designs themselves, only specific geometric parameters. Since, it is important for modelling and simulation to depict reality as closely as possible, it is necessary to perform three dimensional simulations. For this reason CFD was used in order determine and analyse the air flow inside the air intake systems. This will provide improved results, if exported data regarding one dimensional simulation (Ricardo WAVE) are applied. Therefore, in order to describe comprehensively air flow distribution inside both inlet manifolds, a combination of both methods is required. Finally, before CFD simulation begins another issue must be emphasized. It is possible regarding both inlet manifold designs, not to consolidate ideal air flow. The latter, is another consideration why CFD simulation was performed.

The following steps below, highlight CFD procedure used for this research.

Figure 24: CFD Procedure

As there were a number of issues with time and computational power, only one of the intake designs was able to have its flow analysed. As the Dome-Shaped Plenum Air Intake was the first to be designed out of the two systems under consideration, it followed that it was the first and only one to be simulated. The rationale for this is explained in the forthcoming sections.

Boundary Conditions

As mentioned previously, exported data from WAVE was used in the CFD simulation. This exported data forms the inlet and outlet boundary conditions of the designs in question. All exported results from WAVE were taken at 8000 RPM, being the upper limit of the primary RPM range. Ricardo WAVE Inlet and Outlet Conditions

Although for transient analysis or any analysis via CFD, pressure values are sufficient for inlet and outlet boundary conditions, mass flow rate should also be incorporated where possible. WAVE allows for both these conditions to be gathered.

Figure 25: Air Intake Inlet Pressure at 8000RPM

Figure 23: Formula Student 28mm Single Bore 'Shaftless' Throttle Body with 20mm Restrictor

Repeat for Differen

t Models

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The pressure inlet graph above (figure 25)was measured at thebellmouth of the throttle. For a full combustion cycle the pressure changes are plotted and are observed to be identical between 180° crank angles. Moreover, the pressure is relatively lower at the IVO (intakevalve opening) period, which is ideal. The total pressure difference is approximately 0.001barand the pressure is maintained below 1bar, keeping the supply of air constant into the intake.

Figure 26: Air Intake Inlet Mass Flow Rate at 8000RPM

Looking at the mass flow rate in figure 26, the graph shows that the mass flow rate remains relatively high during IVO, aiding engine performance. Additionally, as with the pressure the mass flow rates are identical between 180° crank angles.

Figure 27 Air Intake Outlet Pressure at 8000RPM

As with the inlet boundary conditions outlined above, the outlet conditions were acquired using the same procedure, with the exception being the location of the results taken. Each intake runner requires pressure and mass flow rate results at 8000RPM. Figure 27 shows the outlet pressure plots for all four runners. Pressure fluctuations are observed due to the

Helmholtz effect coupled with the air rushing into the cylinders on the suction stroke (IVO) resulting in a localised pressure drop. The pressure then peaks (positive) as the intake valve closes (IVC). As with the inlet pressure, the runner pressures are identical between 180° crank angles. This is also true for the mass flow rates, as shown in figure 28 below. Further analysing figure 28 shows that the mass flow rate of the second runner reached maximum at IVO, normally correlating with positive engine performance characteristics.

Figure 28 Air Intake Outlet Mass Flow Rate at 8000RPM

It should be noted that one complete combustion cycle involves two crank angle revolutions. Furthermore all figures 26 to 28 were all exported into (.txt) to be used when setting the boundary conditions on the CFD model. Pre- Processing

For CFD simulation in particular, the speed and accuracy of results depending fundamentally on meshing and the ability to prepare model geometry correctly. This is arguable the most difficult and time consuming step in the CFD process, normally only second to the solver run time. Mesh Generation

The grid quality is a complex characteristic that determines theoverall accuracy of solution at a given number of grid points and agiven discretisation scheme. In the case of unstructured grids the grid is a complex characteristic, newaspects appear because of larger diversity and flexibility of cellshapes. Typically, an unstructured grid needs quality control andoptimisation after it has been built.

Three assessment criteria needed to be considered when generating the mesh for the model[11]: 1. Distortion: This term refers to the deviation from

orthogonality between the intersecting lines of thenon-Cartesian coordinate system. The best results are provided by orthogonal grids. If thegrid

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needs to be non-orthogonal, the distortion angles should be possibly close to 90°.There are no universal limits, but grids with angles smaller than 45° or larger than 135° are usually considered dangerously distorted.

2. Ratio of Adjacent Cell Sizes: We have to avoid strong variations of grid steps. As a general guideline, it is usuallyaccepted that the ratio between the typical sizes of any two adjacent grid cells should not exceed two. This requirement often needs enforcing when we construct stretched grids for flows with boundary layers and other zones of strong gradient. The transition from the fine grid within such a zone to a crude grid in the outer flow should be gradual.

3. Cell Aspect Ratio: We have to avoid strongly anisotropic grids. In general, it is recommended that the ratiobetween the grid steps in different directions is neither large nor small. The specific limits on the aspect ratio vary with the nature of the flow and the type of the computational scheme. Aspect ratios approaching or exceeding 5 should be avoided.

From the three criteria outlined above, the

dome-shaped air intake system was meshed taking about three weeks to complete. Generally the mesh quality was acceptable. However due to the lack of computational power the control volume sizes could not be made as small as one would prefer. The multi-cylinder air intake geometry was orders of magnitude more complicated and would have taken more than a month simply to mesh within an acceptable quality.

Figure 29: Inlet Mesh (Air Filter) Cross-Section

The inlet mesh profile displayed in figure 29,

shows that at the boundary conditions where there is expected to have strong gradients has smaller cells. Additionally to reduce computational effort, a stretching or stepping factor is used to transition from strong to weak gradients. IJSER




Figure 30: Restrictor Mesh Cross-Section

The meshing for the restrictor shown above, follows in the manner as the inlet. The only difference is the stepping factor and cell size. Both have been reduced to increase the quality of the results in this area as it is one of the primary areas of interest.

Figure 31: Diffuser and Plenum Mesh Cross-Section

The meshing of the diffuser and plenum shown above, follows in same manner as the others. As the diffuser experiences strong gradients, its cell size and stepping factor are smaller than that of the plenum, which, has relatively weak gradients in this case.

Figure 32: Bellmouth and Runner Mesh Cross-Section

As the bellmouths and runners are areas of interest and also experiencing fairly strong gradients, the cell size and stepping factor are smaller and shown above.

Generally, distortion, cell skewness and stepping factors were kept within the acceptable levels as outlined in the steps for all areas of the air intake system. The total control volume was over 3300000, a moderate amount to produce acceptable results given the computational power available. Model Boundary Condition In addition to the inlet and outlet boundary conditions, the geometric boundary conditions need to be set for the model (walls, internal, fluid, etc.). The boundary conditions of the model are shown in figure 33 below. Simulation Solver

The Fluent solver was used for this simulation

based on the model prepared on ANSA and the Ricardo WAVE exported boundary conditions. As the intake manifold is relatively large with respect to what is normally simulated on the available CFD cluster and the mesh created very dense, the minimum solver time per model was three weeks. This, coupled with meshing time resulted in only this design being able simulated given the project timeframe. The solver ran two crank revolutions, the first being a stabilisation run and the second refining the results. Additionally figure

Figure 33: Air Intake CFD Model Boundary Conditions

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34 below verifies the mesh quality via Fluent mesh overview.

Figure 34: Fluent Mesh Quality Overview of Dome Shaped Air Intake CFD Model

Post- Processing From the results obtained from the Fluent

solver (650GB), visual representations of specific fluid flow features were generated to better understand the air flow performance of the air intake design. META was chosen as the post-processor as it was not only very powerful with respect to flow representations, but also simple to use when large result files are being used. Flow Through Restrictor

As previously mentioned, a primary concern was choking of the air intake system. The restrictor was the focus as it will influence the performance of the engine throughout the rev range by increasing pumping losses through choking (sonic flow, Mach number (M≥1 [340.29 ms-1])[12]. This occurs as a result of overcoming the pressure difference between the air intake and atmosphere.

Figure 35: Maximum Velocity Magnitude at Restrictor for Each Runner

Figure 36: Maximum Velocity Magnitude through Restrictor with Flow Through Runner 3 (714° Crank Angle)

From figures 35 and 36 respectively, at no point does the restrictor choke during a full cycle with the maximum velocity magnitude experienced was Mach 0.52, far less than sonic flow at 714° crank angle. Flow at Diffuser It is important to observe how well the fluid flow recovers after the restrictor. This can be done by observing how quickly the pressure recovers after the maximum pressure drop at the restrictor. This is positively correlated to throttle response.

Figure 37: Pressure Recovery After Maximum Pressure Drop at Restrictor with Flow Through Runner 3 (714° Crank Angle) From figure 37, there is noticeable lag with respect to pressure recovery after the restrictor into the diffuser. However this initial lag is recovered over the course of the diffuser length. Shorter and wider diffusers are favourable for pressure recovery and improved throttle response.

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Flow Through Runners Optimal intake performance involves having runners which allow for each combustion chamber to experience exactly the same fluid flow characteristics and not have leading and/or lagging cylinders.

Figure 38: Velocity Vector Plots for Each Runner at Maximum Velocity Values at 8000RPM

From figure 38, at each maximum valve lift position, velocities inside runners 2 and 3 at the centre are marginally higher than runners 1 and 4 at the outside. It is clear that the closer the runners are downstream of the restrictor the better the performance however small. The plenum volume coupled with the runner length may be responsible for the little variation in velocity across all four runners through resonance and the plenum being able to normalise the velocity prior to entrance into the runners.

Figure 39: Rotational Flow for Each Runner at Maximum Valve Lift at 8000RPM

Figure 39, shows that each runner experiences varying swirl frequencies. Swirl helps to promote rapid combustion by aiding fuel mixing in the combustion chamber in fuel injected engines[13]. The level of swirl

observed may be a result of tangentially directing the flow into the cylinder. Overall System Flow Having looked at the specific areas of interest, the system as a whole is now analysed. The plenum geometry usage is an important factor needed to determine is the volume is being used efficiently. Ideally, the entire plenum volume should be used so as to not have wasted or unused sections not adding to the system.

Figure 40: Maximum Velocity Magnitude through System with Flow Through Runner 3 (714° Crank Angle) Flow circulation is observed over the majority of the plenum geometry as shown in figure 40, with the majority of the airflow beinguniform through the inlet. This is intrinsic of air intakes that are not undergoing choked flow, as the plenum volume is able to compensate for any air flow rate drop through the restrictor.

Figure 41: Maximum VorticityValues at Runner 1 (534° Crank Angle)

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Localised fluid rotation is observed where there are transitional edges as seen at the throttle body to the diffuser and the plenum to runner bellmouths (figure 41). These areas experience higher turbulent flows and as a result, noticeable flow separation, with recirculation tending to occur in these areas. FEA SIMULATION

FEA (Static Structural) was completed using the maximum suction pressure from Ricardo WAVE. When a restricted intake becomes choked, the plenum volume can no longer act as a buffer for the restricted air flow. Extremely low pressure levels are deployed internally during particular loads; large pressure differences are set up between the environment pressure externally and pressure inside the inlet manifold at that point and can lead to implosion (by suction) of inlet manifold. As the possibility of choking is unlikely, it is highly improbable that the maximum suction pressure (0.116 MPa) will act on the system. However, as it is very difficult to determine the actual suction pressure on the intake, the maximum was used to ensure that the system will work even in the worst case scenario. Pre-Processing

As with CFD, FEA simulation requires a high quality meshed model in order to attain results as close as possible to reality. However FEA meshing is considerably easier and less time consuming than with meshing models for CFD simulations.

Mesh Generation

A similar assessment criteria was used to generate the required mesh as used for the CFD analysis[14]: 1. Element size should be uniform wherever possible

and maintain orthogonality. 2. Maintain a smooth transition from coarse to fine

mesh (sizing factor). 3. Warpage should be less than 15° 4. Aspect ratio should be less than 5:1 5. Skewness should not exceed 60°

Figure 42: Dome-Shaped Air Intake Mesh

Figure 43: Multi-Cylinder Air Intake Mesh

As the standard ANSYS pre-processor was used to mesh both air intake geometries as shown in figured 42 and 43 respectively, the control granted to the user was to the same level as with a dedicated pre-processor. However as only the plenum geometry was really of interest in terms of structural analysis, it was sufficient to produce a suitable mesh. The material properties of Polyethylene was modified to match those of Duraform GF, however as none of the fatigue curves were available to be imported into ANSYS, it was not possible to produce life or safety factor results.

Table 4: General Model Mesh Statistics for Skewness

Table 4 highlights how dense the mesh for both models are. Each model is seen to have approximately 1.5 million elements, mostly on the plenum. The mesh on the runners are of very low quality as it is not actually being considered.

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Boundary Conditions Both air intake systems were subjected to the same boundary conditions. The intake port gasket profiles and bracing tabs were both fixed and the maximum suction pressure set to the inner walls of the models as seen in figures 44 and 45 below.

INITIAL CONDITIONS

Figure 44: Dome-Shaped Air Intake Initial Conditions

INITIAL CONDITIONS

Figure 45: Multi-Cylinder Air Intake Initial Conditions

Post-Processing For both air intake system designs, only equivalent stress and total deformation were the quantities of interest in determining the structural integrity. For the reasons stated previously, it was not possible to acquire life cycle or safety factor results unfortunately as the system does undergo a high order of cyclic loading. From figures46 and 47 above it is clear that both designs are structurally sound even under the worst case scenario. Both equivalent stresses are well below the yield strength of Duraform GF (140 MPa) and the total deformation again in both cases are well within safe operational parameters.

Figure 46: Dome-Shaped and Multi-Cylinder Air Intake Equivalent Stress

Figure 47: Dome-Shaped and Multi-Cylinder Air Intake Total Deformation

PERFORMANCE VALIDATION One of the primary targets for the CR14

powertrain was to get the engine performance on par with other Universities running the same engine (Peak Power of 65 kW and Peak Torque of 60 Nm). In order to realise this, it was imperative to understand where performance was being impaired and what changes within the time and monetary constraints can be made

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to increase engine performance. Looking again at figure 1, the CR13 engine setup was modelled, simulated and verified. The dynamometer results were compared to the simulated results and this difference was used to predict the performance of the CR14 using the determined air intake geometric parameters. Air Intake Volumetric Efficiency Air intake volumetric efficiency (VE) from cylinder to cylinder in primarily influenced by the distance between the individual runners and the restrictor. From figure 48, cylinders 1 and 4 are grouped together and 2 and 3 are grouped together. Up to 4000RPM all four cylinders experience the same VE. However after this clear transition point, they swap positions in terms of higher VE every 2000RPM. Notably from 7000RPM up to 9000RPM the VE is fairly linearly increasing, surpassing 100% at 7800RPM up to 118% at 9000RPM. This result shows that the air intake is efficient at supplying each cylinder with air at the required RPM range.

Figure 48: Air Intake VE for Each Cylinder

Engine Performance From figures 49 ad 50 below, the CR14 will produce 18% (64 Nm) more Peak Brake Torque than the CR13 (54 Nm) and 36% (70 kW) more Peak Brake Power than the CR13 (51 kW). Brake engine torque for the predicted CR14 results is observed to be precisely what is required with a steep gradient from 7000 RPM up to 8500 (main running range). This linear, steep torque distribution is aided by the uniformity in the length of the exhaust pipes and the

exhaust configuration 4-2-1. Although these values can only be sufficiently verified from the Dynamometer testing, the simulated engine performance is very close to what the experimental results were for the CR13, therefore there is high confidence in the results. CONCLUSION Formula Student teams on a whole have yet to

find the most competitive air intake design as the spread in intake types is still very large. It is however evident that the top/central-fed is the most preferred, with over 45% of the teams using this design. This is not evidence however to classify this design as the best type even though it was the design type used in this study. This study sought to develop the best possible top/central-fed air intake design using modelling, simulation and validation through experimental results. Using 1D engine modelling, set geometrical parameters of the air intake were determined, following which two design concepts were developed to be used as competing ideas. CFD and FEA simulation were then carried out to analyse and evaluate the fluid flow (only one system considered) and structural integrity respectively for each system. The air intake efficiency and effect on engine performance was then used to determine how effective the designs are. Concerning the geometric parameters of the air intake system, one of the primary areas is the plenum volume. Large plenum volumes (>3L) result in lower Mach numbers being observed at the restrictor during a full engine cycle, thereby reducing pressure losses and the probability of choking. Another primary area of concern is the runner dimensions. There is no perfect length for runners, however there are ranges in which they are optimal. Given a plenum volume of 6L the optimal runner length range based on engine performance is 180mm to 200mm. This allows for the engine to perform better at the primary RPM range (7000RPM to 9000RPM). Furthermore it is better to have bent runners than a bent diffuser, as it facilitates improved fuel injection and overall packaging.

Figure 49: Brake Engine Torque Comparison Between CR13 and CR14

Figure 50: Brake Engine Power Comparison Between CR13 and CR14

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The use of 1D/3D modelling and simulation methods respectively provided additional benefits and insights into the flow through the intake systems and the effect of geometric parameters on its efficiency and engine performance. Additionally as high suction pressures are applied to intake manifolds, the structural integrity of the system is paramount as the possibility of plenum implosion is very likely. Low pressure levels are experience inside the air intake on the suction stroke of the combustion cycle setting up a large pressure difference between the inside of the intake and the outside environment, which can result in the walls of the plenum imploding as the system experiences choking. Both designs take this into consideration and are designed to mitigate against this problem. The engine performance targets set out for the CR14 are also expected to be met. As experimental results from the CR13 were highly correlated to the engine model results using the intake and exhaust system used, there is high confidence that the same will be the case for the CR14. Concept Selection

Although the multi-cylinder air intake system was not simulated using 3D CFD, it was chosen as the best overall design (figure 51). It incorporates the best parts of the dome-shaped air intake system, however its plenum geometry aids in uniform air flow to each cylinder with no protruding runners, alleviating turbulent recirculating flow. Additionally, the design of the plenum itself increases the stiffness and acts as stress dissipation faces. Additionally it comprises of a shorter and wider diffuser reducing the lag in pressure recovery after the restrictor.

Figure 51: Manufacture Multi-Cylinder Air Intake System

ACKNOWLEDGMENTS Special thanks to Prof. Nouri and Prof. Pullen for their interest and support in this research. Thank you to Dr. Mitroglou and CFD department for continuing assistance in all CFD analysis conducted. Ricardo and BETA CAE Systems software support to the Formula Student Team is appreciated and thoroughly acknowledged, without these software, this paper would not have been possible. Thank you to the City Racing Team and Mr. Valsler, the team principal for the opportunity to conduct this research so that it may benefit the team as a whole. REFERENCES

1. Formula Student SAE International (2013) 2014 Formula SAE Rules, Available at: http://students.sae.org/ (Accessed: 25th November 2013).

2. Yamaha (2014) Yamaha YZF-R6-2004 Service Manual, Available at: http://www.yamahapubs.com (Accessed: 25th November 2013).

3. Sarwar, W (2011) Design and Development of a Variable Geometry Intake for a 4-Stroke 4 Cylinder Engine, Manchester, United Kingdom: The University of Manchester.

4. Taylor,R, McKee,R, McCullough,G, Cunningham,G and McCartan,C (2006) 'Computer

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Simulation and Optimisation of an Intake Camshaft for a Restricted 600cc Four-Stroke Engine', SAE Technical Paper Series, JSAE20066571(SAE 2006-32-0071), pp.

5. Mitroglou, A (2010) Fuel Injection Mapping of a Restricted 600cc 4 Cylinder Engine Using Two Different Inlet Manifold Configurations, London, United Kingdom: City University London.

6. Harrison,M.F and Dunkley, A (2003) 'The acoustics of racing engine intake systems', Journal of Sound and Vibration, 271(271 (2004) 959-984), pp. .

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8. MATBASE (2014) PA6 GF30, Available at: http://www.matbase.com/ (Accessed: 1st December 2013).

9. Blair,G.P and Cahoon,W.M (2014) Design of an Air Intake Bellmouth, Available at: http://www.profblairandassociates.com/ (Accessed: 1st December 2013).

10. Jenvey (2014) Where is the best place for the injectors? Available at: http://www.jenvey.co.uk/ (Accessed: 3rd December 2013).

11. Pinelli,A (2014) Grid Generation, Available at: http://www.moodle.ac.uk (Accessed: 10th May 2014).

12. Singhal,A and Parveen,M (2013) 'Air Flow Optimization via a Venturi Type Air Restrictor', World Congress on Engineering, 3(978-988-19252-9-9 ), pp. .

13. Queens University Canada (2014) Intake, Exhaust, and In-cylinder Flow, Available at: http://me.queensu.ca/ (Accessed: 15th May 2014).

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DEFINITIONS, ACRONYMS,ABBREVIATIONS AFR: Air to Fuel Ratio

CA: Crank Angle

CAD: Computer Aided Design

CFD: Computational Fluid Dynamics

CR: City Racing

Concept: Engineering idea that could conform to some if not all of the engineering specifications for a given application.

Design: A manifestation of the concept that conforms to all engineering specifications for a given application.

FEA: Finite Element Analysis

GF: Glass Fibre

IVC: Intake Valve Closes

IVO: Intake Valve Opens

M: Mach Number

Model:A physical or digital representation that is intended to stand for a system of interest. Their parts closely resembles a full sized working production of the target object or system.

RPM: Revolutions Per Minute

Simulation:the use of a model to study the behaviour or performance of the model in real situations.

SLS: Selective Laser Sintering

VE: Volumetric Efficiency

WRC: Wave Ram Charging

NOMENCLATURE Ac: Vena Contracta Area (m2)

Ap: Pipe Area (m2)

Arunner: Inlet Valve Area (m2)

C: Speed of Sound (ms-1)

Cd: Coefficient of Discharge

Drunner: Runner Inner Diameter (mm)

Lrunner: Runner Length (mm)

N: Engine Speed (RPM)

θt: Crankshaft Displacement (degrees)

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