Design Report
Engine: Intake and Exhaust. Engine Used The selection of Engines were governed by the following Formula SAE rules: 1. Engine Type: The engine(s) must be a Piston-Engine, Petrol fuelled and operate on a 4-stroke cycle. 2. Engine capacity: Maximum engine displacement must not exceed 610cc. Considering the familiarity with the engine, the team budget and the time limitation, a Honda CBR 600RR engine, which satisfied the above rules, was c hosen.
Fig 1 Honda CBR 600RR Engine
Fig 2 CATIA model of HONDA CBR 600RR
Intake The following general rules govern the design of Intake System: 1. Throttle Body: The car must be equipped with a throttle body or a carburetor of any size or design. The throttle must be actuated mechanically m echanically and the actuation must use at least two return springs located on the throttle body. 2. Restriction on Air Intake: In order to limit the Power output of the Engine, a single-circular Restrictor must be placed in the intake system between the throttle and the engine. All engine air flow must pass through the restrictor. restr ictor. The maximum restrictor diameter for a gasoline fuelled car is 20mm. The Intake system is designed considering the above mentioned factors.
Components of Intake System The primary function of an intake system is to deliver a charge of air/fuel to the cylinders in correct proportions. The different components of intake system which work together in achieving the correct air/fuel supply to the cylinders are as follows: 1. Air Filter Air filter, a piece of equipment made of fibrous material, prevents t he atmospheric dust entering
into the cylinders and ensures clean air to the cylinders. 2. Throttle Body Throttle is the mechanism by which the fluid flow is controlled. The output characters of an engine
can be controlled by controlling the amount of air supply to the cylinders. A throttle body typically consists of a butterfly valve which regulates the supply of air to the cylinders. 3. Restrictor As per the Formula SAE rules, the intake system must have a 20mm diameter circular restrictor.
The restrictor limits the output power of the Engine by restricting the air supply. 4. Plenum A plenum is an anti-pulsation chamber which dampens the pressure pulsations from the r unners
reaching the restrictor. It simulates ambient conditions for the runners. The magnitude of dampening is determined by the shape and volume of the plenum. 5. Runners Runners are ducts connecting the plenum to the cylinders. The length, diameter, inner surface
characteristics and orientation of the runners affect the performance output of the engine. 6. Injector Sleeves Injector sleeves are the mounting for the fuel injectors. Fuel injectors are usually mounted close
to the intake valves. Since runner e nds are the closest position for mounting any component close to the intake valve, the injector sleeves are attached to the end of runners which facilitates fuel injector mounting.
Intake The following general rules govern the design of Intake System: 1. Throttle Body: The car must be equipped with a throttle body or a carburetor of any size or design. The throttle must be actuated mechanically m echanically and the actuation must use at least two return springs located on the throttle body. 2. Restriction on Air Intake: In order to limit the Power output of the Engine, a single-circular Restrictor must be placed in the intake system between the throttle and the engine. All engine air flow must pass through the restrictor. restr ictor. The maximum restrictor diameter for a gasoline fuelled car is 20mm. The Intake system is designed considering the above mentioned factors.
Components of Intake System The primary function of an intake system is to deliver a charge of air/fuel to the cylinders in correct proportions. The different components of intake system which work together in achieving the correct air/fuel supply to the cylinders are as follows: 1. Air Filter Air filter, a piece of equipment made of fibrous material, prevents t he atmospheric dust entering
into the cylinders and ensures clean air to the cylinders. 2. Throttle Body Throttle is the mechanism by which the fluid flow is controlled. The output characters of an engine
can be controlled by controlling the amount of air supply to the cylinders. A throttle body typically consists of a butterfly valve which regulates the supply of air to the cylinders. 3. Restrictor As per the Formula SAE rules, the intake system must have a 20mm diameter circular restrictor.
The restrictor limits the output power of the Engine by restricting the air supply. 4. Plenum A plenum is an anti-pulsation chamber which dampens the pressure pulsations from the r unners
reaching the restrictor. It simulates ambient conditions for the runners. The magnitude of dampening is determined by the shape and volume of the plenum. 5. Runners Runners are ducts connecting the plenum to the cylinders. The length, diameter, inner surface
characteristics and orientation of the runners affect the performance output of the engine. 6. Injector Sleeves Injector sleeves are the mounting for the fuel injectors. Fuel injectors are usually mounted close
to the intake valves. Since runner e nds are the closest position for mounting any component close to the intake valve, the injector sleeves are attached to the end of runners which facilitates fuel injector mounting.
Exhaust The following general rules govern the design of the Exhaust: 1. Exhaust outlet: The exhaust must be routed so that the driver is not subjected to fumes at any speed considering the draft of air. 2. Noise: The maximum noise produced must be limited to 110 dB. 3. Exhaust runners must fit around the frame members of the car. 4. All components must be no more than 58 4.2 mm off the ground.
Components of the Exhaust System The main function of the exhaust system is to push the exhaust gases out of the cylinder with least resistance and use the pressure pulsations to increase the performance of engine during valve overlap. The exhaust system is primarily divided into the following components: 1. Exhaust Port These form the path for the exhaust gases from the exhaust valve leading to the primaries. The
exhaust ports are an integral part of the engine. 2. Exhaust Primaries These are the tubular structures leading from the exhaust port ends, which helps in moving the
exhaust gases out of the cylinder with least back pressure. The length and diameter of the primaries determines the performance of the engine in the required rpm range. 3. Collector Collector is a component which merges all the primaries into one tube leading to the muffler. The
shape and dimensions of the collector are determined considering the mass flow and acoustic phenomenon. 4. Exhaust Secondary
It is a single tube which emerges from the collector outlet and connects to the muffler. 5. Muffler Muffler is a component which functions in reducing the exhaust noise level by using a sound
absorptive material and friction phenomenon.
Previous Year’s Design A thorough study of the previous year’s design was made to understand the merits and demerits in the design. Intake :
A conical spline intake design concept was followed in the previous year’s design. This system was designed to attain equal mass air flow distribution to each o f the cylinders. To facilitate the mounting of intake system on the inline engine, the runners were made bent. The design was mainly based on acoustic supercharging and the runner lengths were unequal which effectively widened the power and torque band in the required range. The intake system was designed to achieve peak torque at 8000rpm and peak power at 10500rpm.
Fig 3 Intake System of Previous Year Car
The intake system was modelled in CATIA considering the packaging constraint and the analysis was made mainly in Ricardo Wave, a 1-D simulation software. Boundary conditions were obtained from Ricardo Wave for optimizing the restrictor design in ANSYS Fluent.
Exhaust :
A 4-1 configuration of exhaust system was chosen over 4-2-1 configuration owing to the space limitation and performance goals. The primaries were made with stepped diameter to use wave reflection phenomenon and obtain different resonant frequencies in a very close rpm range thus widening the power and torque band. A conical collector was designed to merge the primaries into a single duct. The design was analyzed and optimized using Ricardo Wave software.
Fig 4 Exhaust Design of Previous Year Car
Observations 1. The design was made giving more preference to performance. But the manufacturing feasibility was ignored. Finally, the manufactured product failed to produce the expected output due to the errors made during the manufacturing.
2. A great difficulty was experienced in manufacturing the bent runners which caused the final product to vary from the actual design. The deviation caused by this error made it very difficult to mount the intake system on to the engine. 3. The restrictor was designed considering the space constraint, which was a compromise on performance. Thus there was a turbulence caused in the plenum which affected the performance output. 4. Stepped primaries in the exhaust system increased the back pressure which effectively reduced the performance output. 5. A 900 bend was made in the exhaust secondary due to packaging issue which caused excessive back pressure. 6. The noise level was very close to 110dB which is the limit set by Formula SAE.
Improvements aimed at in the new design 1. Considering the difficulty in manufacturing, it was decided to design simple, equal length runners with zero degree bend angle, which ensures easy manufacturing and accurate analysis. 2. To design a restrictor with less than 70 taper angle to obtain maximum mass flow through the restrictor with least turbulence. The last year’s design had a 100 taper angle at the diffuser. 3. The exhaust port diameter of the engine is designed for a non-restricted intake system. Since the intake system for Formula SAE is restricted, the port diameters prove to be larger for the exhaust gas mass flow and thus increases back pressure. Therefore the primaries have to be designed with diameters not more than the port diameters. 4. The intake and the exhaust system must be designed to reduce the overall noise level below 105dB.
Intake Design This section deals with the detailed procedure and theory behind the intake design. Different types of intakes are explained and a comparison is made between them to select the best intake suitable for our application. Calculations involved in the design of each component in the intake system are described.
Different types of Intake system
The different types of intakes commonly used in Formula SAE are as follows:
1. Side Entry Intake:
In this type of intake system the restrictor is placed at the side of engine and the plenum. The restrictor feeds into the side of the plenum. It is often described as a non-symmetrical intake since the flow axis of restrictor is not symmetric with respect to any of the ports.
Fig 5 Side Entry Intake System
2. Conical Spline Intake:
In this type of intake system, the runners are placed in radial symmetric fashion at the plenum base plate. The restrictor feeds into the plenum from the top and is symmetric
with respect to all the intake ports. It is characterised by bends in the runners. Equal mass flow distribution to each of the cylinders is one of the main advantages of this design but the difficulty in manufacturing the bent runners shields the advanta ge.
Fig 6 Conical Spline Intake
3. Top/Centre feed intake design In centre feed intake design, the restrictor is placed above and in the centre of linearly arranged runners. Thus the flow axis of the restrictor is symmetric with respect to two centre ports. Majority of the teams competing in Formula SAE prefer centre feed intake design owing to its simplicity in manufacturing.
Fig 7 Top/Centre feed Intake Design
Table below shows the statistics of intake design preferences in FSAE: Top/Centre
feed Side entry intake Conical
intake design
design
intake design
42%
36%
14%
spline
Other intake design 8%
Following table shows the Comparison between the three different intake designs mentioned above: Top/Centre `
feed
intake
design Manufacturing Feasibility
Mass flow distribution
Easy Almost Equal
Side entry intake Conical design
intake design
Easy
Difficult
Unequal
Equal
Performance
Good
Average
Best
Ease of assembly
Easy
Easy
Difficult
spline
From the above table it can be seen that the conical spline intake is best for obtaining maximum performance. But from the previous experience, the difficulties in manufacturing and assembly of the conical spline intake were known. Hence, the next best option was to be considered. Clearly from the table, the Centre feed intake system provides almost similar performance as that of the conical spline intake and is easier to manufacture and assemble. Hence a centre feed intake system was chosen.
Design of Different components of the Intake system
After deciding the type of intake system, different components of the intake system were designed. The design of each of the components is explained in the following paragraphs.
Air Filter
Air filter is a non-critical component in the intake system. Hence the selection of the air filter mainly depends on the availability and the outer diameter of the throttle body. First the calculations for throttle body selection was made and a throttle body was selected. Based on the outer diameter of the throttle body, the air filter was selected. K&N air filters are one of the most preferred air filters. So it was decided to use the air filter from K&N.
Throttle Body
The throttle body bore was calculated using the following equation which relates the engine specifications and the peak power rpm to the throttle body bore.
Throttle body size in mm =
ℎ)( maℎ)( ℎ) (14.)( )( 64 It was found that there were many equations to calculate the bore of the throttle body. The above equation was selected as it related the throttle bore to the engine specifications and the p eak power rpm. The above equation seemed realistic. Hence the throttle bore obtained from this equation was used as a starting point for the analysis. The initial calculation for the throttle bore was made with the following inputs: Number of cylinders =
4
Strokes in inches
=
1.673 inches
RPM at max HP
=
10000
Bore in inches
=
2.638
Therefore,
The throttle body diameter in mm
=
(14. )(4)(1. 6 )(1)(. 6 ) 64
=
32.6 mm
≈
32 mm
A thorough research was done on the availability of 32mm diameter throttle body. It was found that Bajaj Pulsar 220cc bike uses a 32mm diameter throttle body, with 55mm length. In the further analysis of the intake system it was found that increasing the throttle body diameter decreased the performance of the engine, also a bigger diameter throttle body affects the throttle response which is undesirable. Hence it was finally decided to go with 32mm diameter throttle body.
Restrictor Design According to the rules of Formula SAE a 20mm circular restrictor has to be placed in between the throttle and the engine. The throttle bore diameter was calculated to be 32mm. Hence there is an area change from the end of the throttle body to the restrictor. From the fundamentals of fluid dynamics a sudden change in area of cross section is accompanied with high losses in pressure. Hence to minimise the pressure loss, a tapered section has to be constructed between the throttle and the 20mm restrictor. Therefore the first part of the restrictor should be a convergent nozzle. In a convergent nozzle pressure decreases to its minimum at the exit of the nozzle. But the restrictor must be connected to the plenum, which has to simulate the atmospheric conditions. Therefore to regain the pressure there should be an increase in the cross section area. But a sudden increase in the cross section area results in turbulence due to flow separation. Therefore the area must change gradually which results in a divergent section with constant change in area from the 20mm throat to the plenum entry. This helps in regaining the p ressure and simultaneously avoiding turbulence.
Following factors play a major role in decidin g the final design of the restrictor: 1. Convergent region 2. Divergent region 3. Throat shape
Convergent Region:
Deciding the length of the convergent region is a compromise between performance and packaging constraint. As the length of the convergent region is increased, the pressure loss is minimised. But the length of the convergent region is limited by the space constraints. Based on literature study it was found the maximum taper angle for the convergent region should not exceed 70.
Divergent Region:
Similar to the convergent section, the length of the divergent section is limited by space constraints. From literature study we came to know that the maximum divergent angle should not exceed 70 else flow separation from the wall occurs resulting in a turbulent flow which is undesirable in the plenum region.
Throat Shape:
The orifice connecting the convergent and the divergent regions should offer a smooth transition. If the transition causes a sudden area change, then it leads to a change in the flow direction which causes turbulence. To avoid this, a fillet is incorporated at the orifice region.
Restrictor Optimization The restrictor design was optimized in ANSYS Fluent to obtain maximum possible mass flow rate through the throat. The diameter of the throat section was fixed to 19.96mm by the Formula SAE rules and the total length of the restrictor was fixed to 175mm due to space constraints. The inlet area of the nozzle was also fixed to be 32mm in diameter by the throttle bore diameter. So the only changes possible were the length of the nozzle, length of diffuser and the diffuser angle. Different models of the restrictor were made in CATIA and analyzed in ANSYS Fluent. The CAD models were imported into ANSYS geometric modeler for meshing. The mesh file was then imported into ANSYS Fluent for analysis. The pressure difference across the restrictor was used as the boundary conditions for analysis in ANSYS Fluent. These boundary conditions were obtained from the analysis in Ricardo Wave. Initially the diffuser angle is kept at 7 0 and nozzle angle less than 70. The nozzle length is kept at 50mm. In the second model, the nozzle length is increased to 55mm and the diffuser angle is reduced to 5.90. The models and the results from analysis are as shown below:
Fig 8 Restrictor Model 1
Fig 9 Restrictor Model 2
Observations:
Properties
Model 1
Model 2
1.
Diffuser Angle (degrees)
7
5.9
2.
Nozzle Angle (degrees)
6.865
6.246
3.
Nozzle Length (mm)
50
55
4.
Total Length (mm)
175
175
5.
Max. Mach number
0.709
0.856
6.
Avg. Mach number
0.60
0.75
7.
Mass Flow through restrictor (kg/s)
0.064
0.0678
The Mach number, velocity and pressure distribution along the length of the intake for the 1st model are as shown.
Fig 10 Mach number Distribution
Fig 11 Velocity Distribution
Fig 12 Pressure Distribution
The Mach number, velocity and pressure distribution along the length of the intake for the 2nd model are as shown.
Fig 13 Mach number Distribution for 2nd model
Fig 14 velocity Distribution for the 2nd Model
Fig 15 Pressure Distribution of 2nd Model
The 2nd model was found to be better than the first model as the mass flow rate is increased by 6%. But it is found that the velocity varies suddenly at the edges of the throat. This is due to the sudden change in shape from convergent to divergent. So a fillet is introduced at the throat such that the throat diameter is still 19.96mm.
The Mach number, velocity and Pressure distribution for the filleted restrictor is as shown:
Fig 16 Mach number Distribution for the Final Model
Fig 17 Velocity Distribution for the final model
Fig 18 Pressure Distribution for the Final Model
It can be seen that the velocity and Mach number changes are not sudden at the edge of the throat section. The maximum Mach number is 0.78 and the average Mach number is 0.72. The mass flow rate has also increased to 0.0682kg/s. So the final dimensions of the restrictor are as shown in the figure.
Fig 19 Dimensions of the final model
Final Values: Properties
Final Model
1.
Diffuser Angle (degrees)
6.0
2.
Nozzle Angle (degrees)
6.492
3.
Nozzle Length (mm)
55.0
4.
Total Length (mm)
175
5.
Max. Mach number
0.780
6.
Avg. Mach number
0.72
7.
Mass Flow through the restrictor
0.0682
Plenum Design
The two important factors which are critical in designing a plenum are: 1. Volume of the plenum 2. Shape of the plenum
The volume of the plenum affects the throttle response. If the volume of the plenum is too big then it negatively affects the throttle response. Whereas if the volume is too small then it may lead to engine starving for air. Based on the researches made, a plenu m volume in the range 2 to 3 times of the total volume of the engine is found to be optimum. The shape of the plenum decides the type of flow. Any sharp bends in the plenum results in the formation of turbulent region. A key point in designing any component of intake system is to have a laminar flow. Thus the shape of the plenum should have smooth curves. Depending on the different types of intakes discussed above the shapes of the plenum can be as below: 1. Conical Plenum:
This type of plenum is used with the conical spline intake design. The shape of the plenum is conical facilitating the accommodation of radially arranged runners.
2. Log type Plenum:
This type of plenum design is used with side entry intake system. It accommodates the linearly arranged runners.
3. Top feed Plenum:
This type of plenum has a transition cross section and is used with top feed intake system. It is characterized by a wide base to accommodate linear runners and a smaller cross section at the top which forms the inlet to the plenum.
4. Cylindrical Plenum:
This plenum is of cylindrical shape and is used with top feed intake system. This is a special case of top feed plenum with constant cross sections over all the runners.
Keeping in mind the above factors, it was decided to go with a Centre feed plenum. Height of the plenum was the major constraint in the design. After fixing the lengths of runners and restrictor, the plenum was to be fitted in the remaining length. Also smooth flow in the plenum without turbulence was desired. Within the given space constraint, different shapes of the plenu m were analyzed for better air flow. Finally a design was chosen which satisfied all the criteria. The volume of the plenum was measured to be 3 times the total engine displacement. Plenum Analysis
3-D model was built in CATIA. 3-D analysis of plenum was carried out in ANSYS Fluent under transient conditions. Pressure conditions at each crank angle were obtained from Ricardo Wave and pressure curves were fitted.
Fig 20 Plenum Model in Wave 3-D Build
Fig 21 Stream Lines during the Suction Stroke of Cylinder 1
Fig 22Air velocity vectors during Suction Stroke of Cylinder 1
Runner Design Design of the length and diameter of the intake runners are mainly dependent on the rpm at which maximum torque is desired. Based on the previous year experience and a thorough literature study, it was found that 5000 to 9000 rpm is the most drivable rpm range. Therefore it was decided to achieve the peak torque at 7500rpm. Having set the target peak torque, the length of the runners were decided using a pressure wave phenomenon called acoustic supercharging. According to the concept of acoustic supercharging, the pressure waves of air are assumed to be similar to the sound waves. When the intake valve opens a negative pressure wave is sent back along the length of the runners. This negative pressure wave gets reflected back as a high pressure wave when it meets a change in cross section area. Determining the optimum length of intake runners involves utilising this high pressure wave just before the intake valve closes. When the piston is moving downwards in the intake stroke, the air is sucked into the cylinder due to the low pressure created by the downward movement of the piston. The air flows into the cylinder due to momentum. But when the intake valve is just about to close, the piston moves up and there is a chance of back flow of charge from the intake valve. So this high pressure wave is used to retain the charge inside the engine. This concept is called acoustic supercharging. The diameter of the runners were fixed by the intake port diameter and a starting point in optimizing the runner lengths was obtained from the following Formula: L = ((EVCD X 0.25 X V X 2) ∕ (RPM X RV)) – 0.5D X D
Where, L
– Length of runners (in inches)
V
– Velocity of sound at expected pressure and temperature (in ft. /sec)
RPM – Rpm at which maximum torque is desired RV
– Reflective value
D
– Diameter of intake runners (in inches)
Velocity of sound can be calculated using the below Formula:
√ .
V = =
= 347.18 m/s
≈ 1157 ft. /s
0 EVCD = (720 – 245) = 475
D = 38/25.4 = 1.5 inch. Desir ed max. Tor que rpm, RPM =7500
Therefore, L = ((475 X 0.25 X 1157 X 2) ∕ (7500 X 8)) – 0.5 X 1.5 L = 3.743 inches
L ≈ 95 mm
Fig 23 Mass Flow rate vs. Time Steps for the 4 runners
Exhaust Design The details about the exhaust design are explained in this chapter. Firstly, different configurations of exhaust systems and the criteria for the selection of an exhaust configuration to be used on the engine are described. Then, the calculations for the exhaust primaries, the collector and the secondary and the selection of muffler are described. Exhaust System Configuration
Based on the way the primaries are merged at the ends, there are two types of exhaust system configurations: 1. 4-1 Configuration
In this type of configuration, the four primaries of each of the cylinders are merged into one secondary through a collector junction. This configuration of exhaust system is mainly preferred because of the simplicity in design and manufacturing. This configuration is used when peak torque is required in a narrow range of rpm. The length of the primaries determines this rpm range.
2. 4-2-1 Configuration
In this type of configuration, two primaries each merge to form two secondaries and these two secondary’s merge through the collector to form a single secondary. This configuration is preferred when a wider torque band width is desired across a wide rpm range. Here pressure wave reflection phenomena can be used to obtain multiple resonant frequencies, thus widening the drivable resonant frequency range.
The most usable rpm range on FSAE event track is in the range 5 000 to 8000 rpm. Therefore better torque is desired only in this range. Since 4-1 configuration offers better torque in a small rpm range and also considering the difficulties in manufacturing a 4-2-1 configuration exhau st system, a 4-1 conf igur ati on exhau st system is selected.
Calculation of primary dimensions for preliminary design: Primary diameter for preliminary design was taken to be the exhaust port diameter in the engine. Exhaust port diameter of Honda CBR 600RR engine = 31.5mm Primary lengths were obtained from the following Formula
Where, P
- Primary length in inches
ED
– 1800 + number of degrees exhaust valve opens before BDC
rpm
– engine speed being tuned to
In the above equation, ED = 1800 + 40 = 2200 rpm = 7500 Substituting these values in the above equation,
= 850x220 7500 3 = 557 mm
Collector design: A key point in designing any component of exhaust sytem is to provide smooth outflow of exhaust gases, without causing any back pressure. A collector’s function is to merge all the primaries into a single exhaust pipe called the secondary. In doing so, the collector must have optimum volume for mass flow of exhaust gases and at the same time it should not drastically decrease the velocity of exhaust gases. Any region where the exhaust gases lose velocity result in back pressure build up. Back pressure is very undesirable as the preceding gas has to spend part of its energy in pushing the gas infront of it. Hence designing a collector plays a critical role in achieving better performance output.
Calculations:
= Where, ID2 – Diameter of collector inlet ID3 – Diameter of collector outlet Cot A – cotangent of angle of taper. ID2 was found to be 100mm for the collector inlet to accommodate all the primaries. Bell recommends a maximum of 100 as the taper angle and
Graham
the diameter of
the collector outlet is calculated using the following Formula
Where , D2
– Collector outlet diameter (in mm)
D1
– Primary diameter (in mm)
Since we have considered the primary pipe diameter to be same as the exhaust port diameter, the collector outlet diameter is
= . . ≈ 42mm
Therefore , the collector length is given by, Coll ector length =
−
≈ 164 mm
Secondary Design Secondary of the exhaust system has very less effect on the performance output. Our target rpm range was 5000 – 8000 rpm. Since our primaries were tuned to 7500 rpm, we decided to tune the secondary for 5000 rpm which would effectively widen the usable torque curve bandwidth.
Where, P2
- secondary length (in mm)
rpmP1 - tuned rpm of the primaries rpmP2 -tuned rpm of secondary therefore,
= ≈ 185mm Secondary diameter, D2 = 42mm
Muffler design: Following factors have to be taken into consideration before designing a muffler for FSAE car application: 1. Offer less restriction to the flow of exhaust gases 2. Limit the exhaust sound level to below 110db 3. Should not extend more than 45cm behind the rear axle.
Types of Mufflers:
1. Reactive mufflers Reactive mufflers use ducts with perforations, baffles and a resonant chamber to reduce the sound level in the exhaust gases. This type of mufflers offer more restriction to exhaust gas flow but reduce the sound level by a decent margin. The sound levels can be reduced over a wide range of frequencies in this type of mufflers. Commercial vehicles use this type of mufflers. 2. Dissipative mufflers Dissipative muffleers use a straigt perforated pipe and a sound absorptive material to reduce the noise level. It offers least resistance to exhaust gas flow but the sound level is reduced by a low margin. Unlike reactive mufflers, these mufflers reduce sound level only in a limited frequecy range. The sound level reduction can be increased by increasing the length or the diameter of the muffler. The perforated pipe functions as a supporting structure for the sound absorptive material behind it. Sound waves which travel into the muffler pass through the absorptive material and the sound energy is lost as heat due to friction. High performance cars prefer using this type of mufflers. The only major Formula SAE constraint on sound level is not to exceed 110db. Engines usaully produce such high sound level only at higher rpm’s. Our goal was to reduce the sound level at these higher engine rpm. Therfore a straight through muffler or the dissipative muffler which offers sound reduction at a selected frequency was chosen to be the best option. The previous year car used a custom made straight through muffler, which was compact, reduced the noise level below 110db and also did not offer much restriction to exhaust gas flow. Hence, the same muffler was used.
Preliminary Intake and Exhaust simulation in Ricardo Wave Following table shows the dimensions of the intake and exhaust system for preliminary design: Parameters
Dimensions (in mm)
Throttle body diameter
32 mm
Throttle body length
55 mm
Restrictor inlet diameter
32 mm
Restrictor throat diameter
19.96 mm
Restrictor outlet diameter
44.744 mm
Restrictor length
175 mm
Runner length
90 mm
Runner diameter
38 mm
Plenum volume
1800000 mm3
Primary length
557 mm
Primary diameter
31.5 mm
Collector inlet diameter
100 mm
Collector outlet diameter
42 mm
Collector taper angle
100
Secondary diameter
42 mm
Secondary length
185 mm
Muffler length
457 mm
Using the values from the above table, a preliminary model was made in CATIA. 1-D simulation was run in Ricardo WAVE to optimise the design. The following assumptions were made: 1. Flow in intake and exhaust is 1-D. 2. The flow is steady. 3. Constant air-fuel ratio of 14:7. 4. Approximate values, in the acceptable range, wer e used for parameters which couldn’t be measured.
1-D Simulation in Ricardo WAVE: WaveBuild, Wave3Dbuild, and Wave Mesher are the three pre processors of Ricardo Wave. Network model was built on the wavebuild canvas. Any complex geometry, like plenum, can be meshed in wave3dbuild or wave mesher and imported to the wavebuild canvas. Procedure followed in building a model is explained in the following paragraphs:
1. Setting general parameters
General parameters like number of cycles, auto convergence condition, number of time steps per uni degree of crank anfle etc. were set in the general parameters table.
2. Building the network model on the canvas
A network model was built on WAVE canvas by dragging and dropping the flow elements. All the elements were connected to each other from left to right on the canvas using ducts.
3. Defining the dimensions and operating conditions
Once the network model has been created, the dimensions of each of the flow elements and the operating conditions were input into the model. Ambient temperature and pressure were assumed to be 300K and 1.013bar respectively.
4. Modelling the engine and valves.
In this step, various engine parameters like stroke length, bore, connecting rod length, firing order etc were input to model the engine. Intake and exhaust valves were modelled as lift valves. Valve profile was obtained using maximum valve lift value, valve opening and closing duration provided in the Honda CBR600 RR manual.
5.
Using Wave Mesher/ Wave3Dbuild to mesh the plenum.
Wave mesher is used to manually mesh any component unlike Wave3Dbuild which automatically meshes the component and outputs the component as flow elements, which can directly be used in Wavebuild canvas. Output of Wave3Dbuild is more accurate than wavemesher. Therefore the plenum was meshed in Wave3Dbuild. Wave3Dbuild also has default models of a few commonly used components like air box, throttle, silencer etc. . A straight through muffler for our simulation was modelled in Wave3Dbuild and imported into the Wavebuild canvas.
6. Setting up of combustion model:
To check the correctness of a combustion model, the peak pressure of each cylinder in every case has to be noted. Peak pressure in a cylinder of a correctly built model should occur in the range 120-160 ATDC. Cylinder pressure plots were obtained and checked for this criteria. If the criteria is not satisfied then the combustion parameters have to be varied to get the required result.
Fig 24 Transient analysis set up of Intake and Exhaust System in Ricardo Wave
Although the preliminary design did not provide satisfactory results, it provided a base for improvement and optimisation of the design. For further iterations, the maximum available length for straight runners in the chassis was found to be 120mm. Considering this as the upper limit for runner length, various other possible parameters like runner diameter, primary length and diameter, plenum volume, primary length and diameter etc. were varied to get desired torque output. FINAL DESIGN:
Finally a design was obtained which provided a good torque in the most usable rpm range. Final design specification is as follow:
Fig 25 Final Intake and Exhaust Assembly
Results The following Plots show the Performance Output for the final Intake Exhaust Design.
Fig 26 Variation of Brake Engine Torque with Engine RPM
Fig 27 Variation of Brake Horse Power with Engine RPM
Fig 28 Variation of Brake Engine Torque f or different Throttle diameter
Fig 29 Mass flow through restrictor inlet and intake runners
