Oxford Octane Formula Student Report

Department of Engineering Science

Third Year Engineering Design Project 2014/2015

Design of a Car for the Formula

Student Competition

Team Oxford Octane

Alastair Adams-Cairns (St. Edmund Hall)

Robert Bainbridge (St. Catherine’s College)

Justin Hubbard (Hertford College)

Edmund Moss (Somerville College)

Yuntao Zhu (Lincoln College

Talbot Kingsbury (Keble College)

1 Introduction...1

2 Engine...2

2.1 Introduction... 2

2.1.1 Objectives & Specification...2

2.2 Engine Selection... 4

2.2.1 FSAE Rules...4

2.2.2 Available Engines...4

2.2.3 Decision... 5

2.3 Engine Modelling... 6

2.4 Analysis and Optimisation...8

2.4.1 Air Intake... 8

2.4.2 Throttle... 9

2.4.3 Venturi... 10

2.4.4 Plenum... 13

2.4.5 Forced Induction and Natural Aspiration...15

2.4.6 Cylinders... 17

2.4.7 Valves... 19

2.4.8 Fuel Injection... 22

2.4.9 Fuel... 23

2.4.10 Air/Fuel and Equivalence Ratio...24

2.4.11 Fuel Consumption...25 2.4.12 Fuel Tank... 27 2.5 Fine Tuning... 28 2.6 Conclusion... 30 2.7 References... 31 3 Thermal Management...32 3.1 Introduction... 32

3.2 Basics of the cooling system...32

3.3 Cooling system components...33

3.3.1 Coolant pump selection...34

3.3.2 Cooling fan selection...35

3.4 Radiator analysis...35

3.5 Conclusion... 38

3.6 References... 39

4. Aerodynamics...40

4.1 Introduction... 40

4.2 Material selection analysis for the aerodynamic package...40

4.2.1 Introduction... 40

4.2.2 Material properties...41

4.2.3 Manufacture... 41

4.2.4 Multi-criteria decision analysis...43

4.3 Undertray... 44

4.3.1 Introduction... 44

4.3.5 Ground clearance analysis...50

4.3.6 Conclusion... 51

4.4 Wings... 51

4.4.1 Introduction... 52

4.4.2 Top speed performance...52

4.4.3 Aerofoil selection...54

4.4.4 Rear wing... 56

4.4.5 Front wing... 56

4.4.6 Cornering performance...57

4.4.7 Conclusion... 59

4.5 The complete aerodynamic package...60

4.6 Conclusion... 61

4.7 References... 61

5 Chassis and Packaging...62

5.1 Introduction... 62

5.2 Design Procedure...63

5.2.1 Decision Making (Space frame vs. Monocoque)...63

5.2.2 Regulations and Dimension Specifications...64

5.3 Materials... 67

5.3.1 Comparison of Materials...67

5.3.2 Composite Materials...68

5.3.3 Material Selection...74

5.3.4 Composite Sandwich Structures...77

5.3.5 Methods for Manufacturing Composite Materials...79

5.4 Analysis of the Chassis...81

5.4.1 Finite Element Analysis (FEA)...81

5.4.2 Impact Testing the Chassis...82

5.4.3 Torsion Test... 83

5.4.4 Rollover stability test...84

5.5 Packaging... 85

5.6 Conclusion... 88

5.7 References... 90

6 Suspension, Steering, Tyres and Brakes...91

6.1 Introduction... 91

6.2 Suspension... 91

6.2.1 Suspension type...91

6.2.2 Setting Suspension Geometry...92

6.2.3 Dynamic Suspension Simulation...94

6.2.4 Actuation methods...95

6.2.6 Spring-Damper Calculations...96

6.2.7 Suspension System Fabrication...98

6.2.8 Anti-roll bar... 99

6.3 Steering... 99

6.3.1 Ackerman Steering Principle...100

6.3.2 Steering Properties...101

6.3.3 Upright Fabrication...102

6.4 Tyres... 104

6.4.1 Tyre Options... 104

6.4.5 Tyre Simulation...109

3.4.6 Tyre Friction Ellipse...111

6.5 Brakes... 112

6.5.1 Brake Discs... 113

6.5.2 Brake Callipers...114

6.5.3 Pedal Box... 115

6.5.4 Braking Force Calculations...116

6.5.5 Heat Simulation...118

6.6 Conclusion... 120

7. Electrical and Control System (Yuntao Zhu)...121

7.1 Introduction... 121

7.2 Engine Control Unit (ECU)...121

7.2.1 Introduction... 121

7.2.2 ECU... 122

7.2.3 Control of Air/Fuel ratio...123

7.2.4 Ignition timing control...125

7.2.5 Cooling control...125

7.2.6 Electronic throttle control (ETC)...126

7.2.7 Sensors... 127

7.3 Braking Control System (Stability Control)...129

7.3.1 Electronic Stability Program (ESP)...129

7.3.2 Anti-lock Braking System...131

7.3.3 Traction Control System (TCS)...135

7.4 Battery System...135 7.4.1 Introduction... 135 7.4.2 Battery... 136 7.4.3 Battery Management...137 7.4.5 Alternator... 139 7.4.6 Shutdown system...140 7.4.7 Brake Light... 141

7.4.8 Overall system circuit diagram...141

7.5 Conclusion... 142 7.6 Reference... 144 8. Drivetrain...145 8.1. Introduction... 145 8.2. Gearbox... 145 8.2.1 Gearbox Comparison...145 8.2.3 CVT Selection... 148 8.2.4 CVT Guard... 149 8.3. Differential... 150

8.3.1 Necessity and dual CVTs...150

8.3.2 Differential type comparison...150

8.3.3 Limited Slip Differential Selection...151

8.4. Chain Drives... 151

8.4.1. Chain drive, belt drive and driveshaft...151

8.4.2 Ratio calculation...152

8.4.3 Chain and sprocket selection...153

8.5. Half Shafts and Constant Velocity Joints...154

8.5.1. Half shaft material selection...154

8.6 Conclusion... 156

8.7 References... 157

9. Simulation...158

9.1. Introduction... 158

9.2. Software... 159

9.3. Driving Force Simulation...159

9.3.1. Overview... 159

9.3.2. Drivetrain... 159

9.3.3. CVT... 160

9.3.4. Engine and Clutch...161

9.4. Driver... 162

9.4.1. Requirements... 162

9.4.2. Velocity look up...162

9.4.3. Braking point calculator...163

9.4.4. Velocity Control...165

9.5. Brakes... 165

9.5.1. Brakes subsystem...165

9.6. Drag and rolling resistance...166

9.6.1. Drag... 166 9.6.2. Rolling resistance...166 9.7. Rotational inertia...166 9.7.1. Introduction... 166 9.7.2. Effective masses...167 9.8. Results... 168 9.8.1. Acceleration event...168

9.8.2. Autocross event (single lap)...169

9.8.3. Endurance event...170

9.9. Conclusion... 170

9.10. References... 171

10. Project Management and Finance...172

10.1. Introduction... 172 10.2. Project planning... 172 10.3. Finance... 172 10.4. Conclusion... 173 10.5. Appendices... 174 11 Conclusion...176

1

Introduction

Oxford Octane’s 3rd _{Year Project is a design project based on the Formula Student competition run} by SAE International, previously known as the Society of Automotive Engineers, where the aim is to produce a Formula-style racing car to compete in an event run annually. It is a globally

recognised competition with 12 different events running in different regions across the globe. Each student team designs, builds and tests a prototype based on a series of rules, which ensure on-track safety and encourage clever problem solving. The points for the Formula Student Event, obtained from the 2015 SAE Rules are outlined in Table 1.1. However, this project was set out as a design project, with no final construction necessary.

Table 1.1 The scoring system for Formula Student competition

Static Events Technical Inspection No Points

Design 150

Presentation 100

Cost and Manufacturing Analysis 75

Dynamic Events Acceleration 75

Skid Pad 50

Autocross 150

Efficiency 100

Endurance 300

Total 1000

The group was subdivided into more specific roles to try and give the chance for more in depth research into how to optimise the performance of the car. This resulted with: Alastair taking control of the engine design, Robert with the aerodynamics and thermal management design, Justin with the chassis and packaging design, Edmund with the suspension, steering, tyres and brakes design, Yuntao with the electrical and control systems design and Talbot with the transmission and simulation design along with the financial aspects of the project. Once everyone established what their roles entailed, a decision was made about the key aim of the group. It was decided that the car would consist of an internal combustion engine (ICE) only, without a hybrid or electric drive system. A theoretical budget of £40 000 was set to construct a theoretical model of the car, with the main aim of the team being for it to compete with the top end competitors at the 2015 Formula Student event.

2 Engine

2.1 Introduction

The Formula Student car will be designed to partake in both static and dynamic events in order to gain points and compete with other teams whilst keeping within the constraints of the rules set out by FSAE (Formula Society of Automotive Engineers). [1] _{The sole source of power to the car is an} internal combustion engine so the design is crucial to the success of the car on race day. In this chapter, the selection, modelling and optimisation of the engine will be covered as well as the fuel to power the engine. The engine of choice is a Honda CBR600RR from a motorcycle.

2.1.1 Objectives & Specification

In order to create a useful method for selecting the engine, it is important to specify which parameters are the most important for the engine. The main objective will be maximising power available to the car; the reason for this decision will be addressed below. Whilst there are points available for efficiency of the car, the dynamic event with most points available is the endurance event where a score is calculated based on the time taken for the car to complete the event. As per the FSAE rules [1]_{, three times the number of points available for efficiency are available for the} endurance event so maximising the power of the engine will be the number one priority. There is also an acceleration event for which points are available. An engine with high torque is desirable for this event as torque is a better measure for determining the acceleration of a car than power, so selecting an engine with a high torque output as well as high power output will be a priority.

Another objective will be to minimise weight. As described above, performance of the car is very important and adding any unnecessary weight to the car will only reduce its performance.

As the efficiency of the car carries with it some points, efficiency will be another priority when it comes to modelling and optimisation, although it is rather less important than engine power output.

Other important considerations are cost, availability and packaging. The project has a budget of £40,000 and large though that might seem, most of it will be spent on the design and manufacture of a carbon monocoque chassis, so keeping the cost of the engine down will be important as this is

a key component of the car. The availability of the engine will have to be taken into account too; it will need to be sourced within a few weeks so that modifications can be made to it. This means any rare engines are out of the question. Packaging is a factor that is sometimes overlooked. Fitting the engine and transmission behind the driver as dictated by the FSAE rules [1] _{will be a challenge as} well so a bulky engine may not fit.

Taking the above into account and looking at cars from previous years that have performed well, the following engine specification should be adhered to without fail:

 Minimum Peak Power of no less than 65 hp (48.5 kW)

 Minimum Torque of no less than 50 Nm

 Weight of no more than 60 kg

 Cost of no more than £2000

Efficiency will be considered in the design of the engine along with availability and packaging considerations but these are much more difficult to specify so no figures will be assigned to them in this specification; however, discretion will be observed when selecting a suitable engine.

2.2 Engine Selection

2.2.1 FSAE Rules

The FSAE rules [1] _{aim to ensure that every team will be able to compete together with maximum} flexibility of design whilst at the same time trying to keep the competition fair. [1] _{In particular, the} engine selected must comply with the rules in order for the car to be allowed to enter the Formula Student competition. The set of rules is extensive but a summary of the key engine rules is discussed below. The maximum allowable engine displacement is 610 cc. Engines may be

naturally aspirated, turbocharged, supercharged or a combination of the two. All engines must use a restrictor plate in the intake system with a diameter of 20 mm. The location of the restrictor plate varies depending on whether the engine is naturally aspirated or whether it makes use of forced induction. Engines must be four-stroke and can make use of carburetion, low pressure injection (port injection) or high pressure injection (direct injection). Fuel additives are prohibited and the maximum allowable sound level from the engine is 110 dB when racing and 100 dB when idling. [1] The above rules eliminate large, high power engines, such as those you might normally see in road or track cars, so attention will be focussed elsewhere.

2.2.2 Available Engines

Short of manufacturing an engine entirely from scratch, something rather complicated and too difficult to achieve within the time constraints of this project, the maximum allowable engine displacement of 610 cc limits the available engines largely to those from motorcycles or similar small vehicles. In order to keep within the specification, an engine would need to be purchased in a used condition as new ones are too expensive.

Looking at previous competitors’ selections, the two types of engines used come from either motorcycles or snowmobiles, excluding hybrid and electric power units. This is because more often than not, these engines tend to be in the 450-600 cc range, which is ideal for meeting the rules. One such snowmobile engine is the engine from a Yamaha Phazer M-TX which is 499 cc and produces 80 hp at 12,000 rpm. [2] _{After further research, it was decided that snowmobile engines} would not be considered for a number of reasons. First, whilst some have a displacement within

the desired range, many of the engines are two-stroke and most of the four-stroke engines tend to be a lot larger than 610 cc. [3] _{Furthermore, whilst researching the engines online, the technical} specifications are rather hard to come by, perhaps due to the fact that fewer units are produced than motorcycle engines. Not only are snowmobile engines difficult to research but second-hand snowmobile engines for sale are more difficult to source than their motorcycle counterparts. Although a big advantage of snowmobile engines is that they often come with a Continuously Variable Transmission (CVT), which we shall be using as will be explained in a later chapter of this project, this is not enough to outweigh the disadvantages.

This leaves the decision of which motorcycle engine to choose. The Honda CBR600RR is the most popular choice amongst Formula Student teams, but a comparison of other similar engines is worthwhile. Table (1) below is a tabulated comparison of the Honda CBR600RR [4]_{, the Suzuki} GSX-R600 [5] _{and the Yamaha WR 450F} [6]_:

Table (1) Comparison of Honda CBR600RR, Suzuki GSX-R600 and Yamaha WR 450F

Engine

Honda CBR600RR Suzuki GSX-R600 Yamaha WR 450F

Displacement (cc) 599 599 449

Arrangement 4 Cylinder Inline 4 Cylinder Inline Single Cylinder

Bore x Stroke (mm) 67 x 42.5 67 x 42.5 95 x 63.4

Peak Power (hp) 118.1 @ 13,500 rpm 103 @ 13,550 rpm 58 @ 9,000 rpm Peak Torque (Nm) 66 @ 11,250 rpm 59.1 @ 11,140 rpm 49 @ 7,000 rpm

2.2.3 Decision

The single cylinder engine was included in Table (1) above because single cylinder engines provide the greatest thermal efficiency per unit displacement. [7] _{However, whilst there are a few} engines available with a displacement of around 450 cc, the next largest engines have a

displacement larger than the maximum allowable 610 cc. The Yamaha WR 450F simply does not produce enough power for the outlined specification. The Honda and Suzuki engines have the same displacement but the Honda produces more power and more torque than the Suzuki so this will be the engine used in the project for modelling and optimisation. Furthermore, used Honda CBR600RR engines can be sourced very easily from the online marketplace, ebay, for well under £2000. [8]

2.3 Engine Modelling

There are two main reasons for engine modelling. The first reason is that by modelling the engine, we can predict performance without having to carry out actual tests. Testing a full scale model engine would be costly as well as time-consuming and out of the question for this project. The second reason for modelling the engine is that we can determine engine parameters that in practice would be nearly impossible to measure. Furthermore, in the case of an engine, many of the processes are too complicated to be modelled conventionally. After spending some time experimenting with a single cylinder model of an engine on a free version of the software, Lotus Engine Simulation software was acquired from Lotus Engineering Software. [10] _{The Lotus Engine} Simulation software is very powerful and with the use of the handbook, it was possible to begin analysing the engine of choice. [11]

A model of a stock Honda CBR600RR was constructed with the addition of a 20 mm restrictor in the intake system. A schematic of the engine taken from the software can be seen below in Figure 2.2:

Figure 2.2 Schematic of Honda CBR600RR in Lotus Engine Simulation Software

After running a simulation with the engine as above, including the 20 mm restrictor, initially with a few data points, the software was able to give figures for power, torque, brake specific fuel consumption (BSFC) and brake mean effective pressure (BMEP), along with many other useful parameters at different engine speeds. The stock engine with restrictor produced a peak power of 50.4 hp @ 11,400 rpm and a peak torque of 48.9 Nm @ 3,600 rpm. These figures were an early indication that there was much work to be done to the engine to meet the specification of a

minimum peak power of 65 hp and a minimum peak torque of 50 Nm, whilst also raising questions about whether to run the engine at peak power or peak torque, bearing in mind that a CVT will be used. A summary of the performance of this engine simulation can be seen below in Figure 2.3 with Power, Torque, BSFC and BMEP vs engine rpm:

2.4 Analysis and Optimisation

2.4.1 Air Intake

The first component of the intake system is the air intake. The engine draws in stationary air at atmospheric pressure through the air intake due to a pressure difference. We can analyse the flow of air by assuming pipe flow initially in an infinite section before a sudden decrease in cross-section (to a diameter of 40 mm which is the stock size for the Honda CBR600RR). [9] _{The air} intake can be modelled as a sudden change in cross-sectional area which will result in head loss. The equation for head loss is shown below in Equation (1):

Head Loss=K

_L

× Local Dynamic Head∨h

_L

=

K

_L

u

´

2

2 g

(1)

[12]

where KL is an empirical loss coefficient. For the case described above, the highest value of loss coefficient is 0.5 and this occurs when the intake is sharp-edged. We can minimise the loss coefficient by rounding the edges of the intake to achieve a loss coefficient of 0.04 which is small enough to neglect. A round-edged inlet, as described above, is known as a bell mouth [13] _{and this} minimises losses; this will be the shape of the air intake which will be used. A diagram of a sharp-edged and round-sharp-edged or bell mouth inlet can be seen below in Figure 2.4:

Figure 2.4 Comparison of rounded and sharp-edged inlets [12]

2.4.2 Throttle

The next component of the intake system is the throttle. There are a number of throttle valve types available but the two most popular types will be considered here. The first type of valve is a

butterfly valve which consists of a circular disc on a spindle, housed in pipework. When the throttle valve is closed, the disc is perpendicular to the direction of flow and blocks the flow through the

pipe. When the throttle valve is fully open, the disc is orientated parallel to the direction of flow. An example of a butterfly throttle can be seen below in Figure 2.5:

Figure 2.5 Partially open butterfly throttle [14]

Furthermore, there are losses associated with a butterfly valve; even when open, a butterfly valve reduces the effective cross-sectional area at the throttle and separation may occur at any of its edges. [7] _{An example of how a butterfly valve disturbs the flow can be seen in Figure 2.6 below:}

Figure 2.6 Streamlines showing butterfly valve impeding flow when partially open (left) and fully

open (right) [16]

The second type of throttle which will be considered is a barrel valve. A barrel valve consists of a barrel shaped object housed within pipework. When the barrel valve is closed, it restricts the flow through the pipe entirely. When the barrel valve is fully open, it does not impede the flow through the pipe in any way, giving it a great advantage over the butterfly valve. A barrel valve will therefore be used to minimise losses. An example of a barrel valve fully open, with no obstruction to flow can be seen below in Figure 2.7:

Figure 2.7 Fully open barrel valve [15]

2.4.3 Venturi

The next component of the intake system is the restrictor of diameter 20 mm, as specified by the FSAE Rules. [1] _{As a restrictor plate of diameter 20 mm inserted into a pipe of a larger diameter (in} this case 40 mm), also called an orifice plate, would result in large losses from separation, a more efficient method is needed. A simple but effective way to implement a restrictor is to weld together two pipes, connected in series between the throttle and the plenum (the next component of the intake system). The first pipe takes the form of a contractor while the second pipe is a diffuser. Where the two pipes meet, their diameters will be 20 mm, creating a venturi effect. The design of the contractor is less important than that of the diffuser because a contractor will not cause any flow separation. Although there will be no separation in the contractor, there will still be losses; the equation for head loss is the same as Equation (1), but now we have an expression for KL as per Equation (2) below:

K

L

=0.8 sin θ

(

1−

(

D

₁

D

₂

)

2

)

∧

θ=tan

−1

D

2

−

D

1

2 L

(2) [17]

Figure 2.8 below shows the parameters for Equation (2) above. Taking L = 100 mm, a number which does not result in too sudden a contraction, [18] _{we get a value of K}

L = 0.06 which is similar to the value for the bell mouth air inlet and is small enough to be neglected. A schematic for the contractor is shown below in Figure 2.8 and Figure 2.9:

Figure 2.8 (Left) Schematic diagram of setup described above with parameters corresponding to

Equation (2) [19]

Figure 2.9 (Right) Schematic of contractor pipe taken from software model

The rate at which the diameter of the diffuser pipe increases requires careful attention during its design. In order to avoid separation, the angle between a line on the inside surface of the pipe and an imaginary line running parallel to the pipe, of radius equal to the narrower end of the pipe from the centreline, should be 5-10°_. [18] _{A value between the two, of 8}° _{will be used.The loss coefficient,} KL, has a different expression as shown below in Equation (3):

K

L

=2.6 sin θ

(

1−

(

D

₁

D

₂

)

2

)

2

θ=tan

−1

D

2

−

D

1

2 L

(3) [17]

Figure 2.10 below shows the parameters for Equation (3) above. Taking L = 250 mm, we find that D2 = 90 mm and KL = 0.036 which is again small enough to neglect. There will also be no

separation in the diffuser. A schematic of the diffuser can be seen below in Figure 2.10 and Figure 2.11:

Figure 2.10 (Left) Schematic diagram of setup described above with parameters corresponding to

Equation (3) [19]

Figure 2.11 (Right) Schematic of diffuser pipe taken from software model

By modelling the engine with and without the inclusion of the restrictor, it is clear that the restrictor limits the power by a considerable amount. A comparison of peak power output from the stock

engine without the restrictor and the engine with the designed intake system and restrictor can be seen below in Figure 2.12:

Figure 2.12 Graph showing peak power output from engine with (designed intake) and without

(stock) 20 mm restrictor.

As can be seen in Figure 2.12 above, the value for peak power without the restrictor agrees well with the value given in Table (1) for the Honda CBR600RR engine. This agreement is reassuring as it means that engine modelling software is producing realistic values that can be used

elsewhere in the project. Moreover, Figure 2.12 above also makes it clear by how much the restrictor decreases the peak power of the engine. Further optimisation is necessary to meet the specification. Whereas without the restrictor, the peak power is achieved at maximum rpm, the addition of the restrictor results in peak power being achieved at just over 11,000 rpm. This behaviour will be discussed later in the chapter.

2.4.4 Plenum

The next component of the intake system is the plenum. The plenum, combined with pipes at its exit leading to the cylinders (called runners), completes the intake system. A plenum is a

pressurised chamber containing a fluid (in this case air) at higher pressure than its surroundings. [20] _{In the case of an engine, which has an irregular demand from the cylinders, the plenum works}

to even the distribution of pressure. In the case of the exhaust, a plenum has the added function of acting as a silencer. The size of the plenum will have an effect on the power output of the engine but the relationship between the two is not linear. Within a reasonable range, an increase in plenum size will increase the power output. However, outside this range, any further increase in size will have little effect on power output. Furthermore, a large plenum will have a negative effect on throttle response, as a large plenum will take a longer time to fill than a smaller one. In addition, the pressure inside a larger plenum will decrease resulting in decreased volumetric efficiency, which is a measure of the effectiveness of the induction and exhaust processes. [23] _{As well as} power output, a very important consideration regarding plenum sizing is that of packaging. A large plenum could take up a volume similar to that of the engine itself, meaning that a balance between power output and size will be needed.

By making use of the Parametric/ Optimiser Tool in the software, it is possible to determine how power output changes with plenum size. Plenum volumes ranging from 1-12 L were chosen, with increments of 1 L between each, for use in the simulation. After running a number of

simulations with plenum sizes of less than 5 L, it was clear that power output drops considerably so any smaller sizes than this were not considered for use with the engine. The results of the

simulation are shown below in Figure 2.13 and Figure 2.14:

Figure 2.14 As Figure 2.13 above with focus on the peak power output

In Figure 2.13 and Figure 2.14 above, the red line displays the power output vs engine speed for a plenum volume of 10 L. When running simulations prior to the use of the Parametric/ Optimiser Tool, a plenum volume of 10 L appeared to yield the highest peak power output for the engine, so this volume was used for the software baseline score. However, it is clear from Figure 2.14 that there are plenum sizes which produce peak power output values above and below that of a 10 L plenum. An important point to note is that the peak power output in each case is achieved at around 11,500 rpm and this peak is more prominent than that shown in Figure 2.12. The results displayed in Figure 2.14 are presented in a more useful format below, in Figure 2.15:

Figure 2.15 Peak power output vs plenum volume @ 11,500 rpm

It can be seen in Figure 2.15 above that the peak power output is achieved with a plenum volume of 7 L. Below this volume, the power output decreases rapidly and above it, there is a decreasing oscillatory behaviour. Taking everything into account, a plenum volume of 7 L will be used to obtain the maximum power possible; the size of the plenum will not cause a problem and can be

accommodated.

2.4.5 Forced Induction and Natural Aspiration

The final decision with regard to the intake system is whether to make use of forced induction or whether to have the engine naturally aspirated. We shall analyse both cases below.

Forced induction works on a simple principle whereby a greater mass of air is forced into the cylinders via some sort of compressor than would otherwise be drawn into the cylinders without a compressor (natural aspiration) and this results in a charge (air and fuel mixture in a cylinder) with more potential energy for combustion. One useful application of this is in propeller driven aircraft that would otherwise lose power at altitude due to a decrease in atmospheric pressure whereas the forced induction largely negates this. Forced induction can also be very useful for cars with internal combustion engines as is the case here. There are two common types of compressors:

superchargers and turbochargers. The former are driven directly from the crankshaft of the engine via a belt which means there is no delayed response, otherwise known as lag. The downside to superchargers is that they are less efficient than turbochargers. Turbochargers work on the same principle as superchargers but are driven by exhaust gases in the exhaust manifold. Turbochargers suffer from lag because there is a delay between throttle depression and the compressor spooling up from the increased exhaust gas mass flow rate. As the engine will be running at a constant rpm, the reasons for which will be explained in the CVT section later in the report, it is not a problem that turbochargers suffer from lag. In addition, turbochargers tend to be more efficient than

superchargers and also offer greater maximum boost pressure. Therefore, the proposal of using a supercharger can be discarded in favour of analysing turbochargers. A schematic diagram of a turbocharger implementation is shown below in Figure 2.16:

Figure 2.16 Schematic diagram of turbocharger implementation in an engine [21]

The limiting factor on the maximum mass flow rate of air being drawn into the engine is given by conditions of choked flow. In other words, the maximum mass flow rate of air drawn into the engine is achieved when the flow through the venturi reaches a Mach number of M = 1. [7] [18] _{If this choked} condition is reached without the use of a turbocharger, then the addition of one will simply add weight to the engine and have no effect on the peak power produced. Before analysing the

turbocharger any further, it is important to check if the choked condition has already been reached. It is possible to work out the mass flow rate of air through the venturi under choked conditions (M = 1) using Equation (4) below:

´

m= Ap

√

γ

RT

(

γ+1

2

)

−(γ +1) 2(γ−1) ₍₄₎ [22]

Where cross-sectional area of venturi A = 3.14e-4 m2_{, pressure p = 1e5 Pa, heat capacity ratio}

γ = 1.4, gas constant R = 287 J/kg/K and temperature T = 293 K. Solving Equation (4) above gives a mass flow rate of air, m´ = 0.0927 kg/s under choked conditions. Using this value for mass flow rate along with Equation (5) below, it is possible to determine the engine rpm at which choked conditions will occur:

N=

120 ´m

ρ V

_s

η

_vol (5) [23]

Where N is engine rpm, swept volume

V

s = 0.599e-3 m3, volumetric efficiency

η

vol = 0.88, density of air ρ = 1.90 kg/m3 [24] _{and m´ is as above. Solving Equation (5) above gives a value}

of N = 11,300 rpm for choked conditions. This value agrees with the behaviour seen in Figure 2.13. Figure 2.13 shows that peak power is reached at a similar engine rpm as calculated above using Equation (5); the flow is choked at this rpm and running the engine at a greater rpm than this will yield no benefit. It is for this reason that there is no benefit to be obtained by using a turbocharger. Therefore, the engine will be naturally aspirated and will run at choked conditions to produce maximum possible power output.

2.4.6 Cylinders

The key component of any engine is its cylinders, where the useful power is produced. The Honda CBR600RR engine chosen has four cylinders in line with one another. In order to try and smooth the torque at the crankshaft resulting from combustion and to help balance the engine, the two outer cylinders move out of phase with the inner two cylinders. The bore (or diameter) of the cylinders is 67 mm while the stroke (the distance each piston moves within a cylinder) is 42.5 mm. The bore/stroke ratio is a useful parameter when considering the design of an engine. The

bore/stroke ratio above is 1.576 which is typical of an engine designed for racing; the Renault engine for the 2014 Formula 1 season, for example, has a bore/stroke ratio of 1.509. [25] _The reason for the comparatively high bore/stroke ratio is to reduce the frictional losses in the cylinders. The frictional losses increase with increasing piston speed, and thus engine rpm, as can be seen in Equation (6) below:

FMEP=0.97+0.15

(

N

1000

)

+0.05

(

N

1000

)

2 (6) [23]

With frictional mean effective pressure FMEP (bar) and engine rpm N. It is possible to achieve a given engine displacement by varying either bore, stroke, or a combination of the two; by

increasing the bore size and keeping the stroke small, even at high engine rpm, the piston speed will be lower and thus frictional losses will also be reduced. Solving Equation (6) above with N = 11,500 gives FMEP = 9.31 bar. Considering the BMEP = 12.39 bar at this speed (taken from the engine simulation), the engine is losing almost as much power to friction as it usefully produces. It is for this reason that it is important to keep the piston speed low. Having a large bore has the

added benefit that there is a larger area available for valves, allowing them to be larger or greater in number, and a comparatively short stroke reduces crank stresses. [26] _{A schematic diagram of a} cylinder from the chosen engine can be seen in Figure 2.17 below:

Figure 2.17 Schematic diagram from the engine simulation software showing a cylinder in the

chosen engine with intake valve shown in blue and exhaust valve shown in orange

Another useful parameter to consider is the compression ratio rv. The idealised cycle of an internal combustion engine with spark ignition is the Otto cycle. The efficiency of the Otto cycle ηOtto

increases with increasing rv as can be seen in Equation (7) below:

η

_Otto

=1−

1

r

v

γ −1 (7) [23]

The compression ratio for the chosen engine is 12.2:1 [27] _{which gives an Otto cycle efficiency of} 0.63. Increasing the compression ratio will achieve greater efficiency because more mechanical work can be extracted from a given charge [28]_{. Unfortunately, the compression ratio cannot be} increased to achieve greater efficiency. This is because increasing the compression ratio could lead to engine knock, a phenomenon whereby the combustion of a charge in a cylinder does not coincide with the spark from the spark plug and one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front. [29] _{Engine knock can destroy an engine in} severe cases. Engine knock could be avoided by using fuel with a higher octane rating; the FSAE

Rules [1] _{state that E85 is the only other option and its use will be discussed later. If the fuel octane} rating and compression ratio were to be increased, the limiting factor would be high stresses on mechanical components in the engine, such as the crank and connecting rod. Due to the factors mentioned above, [7] _{the compression ratio of 12.2:1 will be retained.}

2.4.7 Valves

The valves in an engine allow gas flow into and out of its cylinders. The Honda CBR600RR has four valves per cylinder, two inlet and two exhaust valves, which are controlled by a double overhead camshaft or DOHC. A diagram of DOHC implementation is shown in Figure 2.18 below:

Figure 2.18 DOHC arrangement in an engine [30]

A DOHC arrangement gives greater scope for tuning and thus increasing performance. There are two important things to consider with the design of valve gear, assuming the size and shape of each valve does not change: valve lift and valve timing. Valve lift refers to the distance a valve moves between fully closed and fully open positions. Valve timing refers to the angle of the crankshaft at which the inlet and exhaust valves open and close with respect to top dead centre and bottom dead centre.

The valve movements are as follows: the exhaust valves open shortly before top dead centre (BTDC) to allow air to flow into the cylinders on the induction stroke. Next, on the compression stroke, the inlet valves close after bottom dead centre (ABDC). The exhaust valves then open towards the end of the combustion stroke, before bottom dead centre (BBDC), which results in a

loss on the combustion stroke but ensures all combustion products have time to escape. Finally, the exhaust valves close, at the end of the exhaust stroke, just after top dead centre (ATDC).

The process described above makes up the four strokes of a four-stroke engine cycle. It should be noted that there is some intake and exhaust valve overlap at top dead centre and this is usually limited by the clearance between the piston and the cylinder-head. [23] _{Engines designed for high} performance have a large valve overlap and both inlet and exhaust valves open earlier and close later than standard engines. [18] [23] _{After optimisation in the engine modelling software, the valve} timing was determined for the engine of choice. The valve timing diagrams before and after optimisation can be seen in Figure 2.19 below:

Figure 2.19 Valve timing diagrams before (left) and after (right) optimisation

Whilst valve timing determines when each event takes place, valve lift determines the effective area for gas flow into and out of the cylinders. Increasing the valve lift above that originally intended for the engine without changing the piston design and/or connecting rod length could result in the engine being destroyed if the piston hits a valve and fractures; modifying the valve lift will not, therefore, be considered. Furthermore, there is a value of valve lift at which the effective area for gas flow through the valves reaches a maximum; any increase in valve lift beyond this point will not produce any increase in effective area for gas flow. The valve lift vs crank angle diagrams for the inlet and exhaust valves can be seen in Figure 2.20 below:

Figure 2.20 Valve lift vs crank angle for intake (left) and exhaust (right) valves

The effect of the optimised valve timing on the engine power output is considerable and is graphically represented in Figure 2.21 below:

Figure 2.21 Engine peak power output vs engine rpm for standard and optimised valve timing

2.4.8 Fuel Injection

There are three ways in which to add fuel to air in the engine intake system to create a charge; carburetion, low pressure injection (port injection) and high pressure injection (direct injection).

A carburettor is a mechanical device used more commonly in older engines to regulate the amount of fuel going to the cylinders by making use of Bernoulli’s principle which states that for a fluid flowing in a pipe, the sum of the static and dynamic pressures is constant. If the air intake is reduced to a smaller diameter at a section, the velocity of the air will increase, while the static pressure will decrease. A carburettor makes use of this by having a petrol outlet orifice inserted into

a reduced diameter section of the air intake and tiny petrol droplets are sucked from this orifice by the low static pressure described above. The droplets are carried along by the flow of air and ideally, if the air intake is placed close to the exhaust, the heat given off by the exhaust will help to vaporise the droplets. A diagram of a carburettor can be seen in Figure 2.22 below:

Figure 2.22 Diagram of a Carburettor [31]

As the amount of fuel is controlled by the rate of air flow, the engine will respond slowly if the throttle is opened suddenly at low engine rpm. The placement of the intake next to the exhaust to help vaporise the fuel droplets also results in a lower intake air density and thus reduced

performance. [32] _{Furthermore, the need for another venturi upstream of the throttle results in} losses. For these reasons, a carburettor will not be considered.

Fuel injection is more fuel efficient than carburetion because the fuel flow can be altered according to the downstream and user-input conditions to produce maximum power. Fuel injectors are solenoid operated plungers that squeeze fuel through a nozzle to atomise it and are controlled by engine electronic control units (ECU). Sensors measure engine rpm, intake manifold pressure, air temperature, coolant temperature and throttle position and the information is fed to the ECU which then determines how much fuel is needed. The required amount of fuel is injected at the right time

either into the inlet ports (port or low pressure injection) or into the cylinders directly (direct or high pressure injection).

Port injection uses injectors at the inlet ports to each cylinder, just upstream of the inlet valves, whereas direct injection uses injectors located in the cylinder head to inject fuel directly into the cylinders. Direct injection achieves greater efficiency than port injection because there are no throttling losses but direct injection requires much more sophisticated and expensive hardware and software; as the injectors are exposed to higher pressures and temperatures, higher quality

materials have to be used as well as higher precision electronic systems. [23] [33] [34]

The Honda CBR600RR comes with a dual stage fuel injection system which uses low pressure injection. Fuel injectors are placed at the inlet ports and at the upstream end of the intake runners. The injectors at the upstream end of the intake runners operate at high rpm and give greater vaporisation time as well as greater exposure to the turbulent mixing of the flow; this cools the flow of air and ultimately results in a denser charge, giving more power. [35] _{As direct injection would} require expensive equipment and significant alterations to the engine, the stock dual stage low pressure fuel injection system on the engine will be used.

2.4.9 Fuel

The FSAE Rules stipulate that there are two types of fuel available for use: gasoline and E85. If using E85, the air intake restrictor has to have a 19 mm diameter. [1] _{E85 is a mix of 85% denatured} ethanol fuel and 15% gasoline, giving it a higher octane rating than standard gasoline. An engine using a high octane fuel such as E85 needs to have a very high compression ratio. Use of E85 in an engine designed for gasoline achieves lower fuel economy as more fuel is needed per unit of air than when using gasoline. E85 is corrosive to the rubber seals often found in fuel systems

designed for use with gasoline so these components may fail prematurely. [36] _{The Honda}

CBR600RR engine is designed for use with gasoline. If E85 was to be used, seals would need to be changed, the compression ratio would have to be increased (which would increase engine stresses), a lower fuel economy would be achieved (resulting in the need for a greater amount of fuel and this would decrease the number of points scored for efficiency) and a smaller diameter air

restrictor would have to be used (which would make a 10% difference to restrictor area and would reduce performance). For these reasons, gasoline will be used rather than E85.

2.4.10 Air/Fuel and Equivalence Ratio

The air to fuel ratio (AFR) is the ratio of the mass of air to the mass of fuel in a combustion

process. The condition when there is exactly the right amount of air present to burn all of the fuel is called stoichiometric combustion. The combustion of gasoline in air produces CO2 and H2O as can be seen in Equation (8) below:

2C

₈

H

₁₈

+25 O

₂

→16 C O

₂

+18 H

₂

O

(8) [37]

Calculating the AFR using the respective molecular masses of air and gasoline, together with the equation above, it can be shown that the stoichiometric AFR = 14.7. This means that for total combustion of 1 kg of gasoline, 14.7 kg of air are needed. [38] _{Unfortunately, this theory does not} hold in the case of internal combustion engines. The charge of air and fuel in each cylinder will not be perfectly mixed and the time available at each combustion stroke is short (in the region of a few milliseconds), especially at high engine rpm. In order to compensate for this, the equivalence ratio, the ratio of the stoichiometric AFR to the actual AFR, needs to be greater than one. [23] _The

equivalence ratio is shown below in Equation (9):

ϕ=

AFR

stoichiometric

AFR

(9)

[38]

If a charge has a greater AFR than stoichiometric, the charge is called lean and the equivalence ratio is less than one. Conversely, if the AFR is less than stoichiometric, the charge is considered rich and the equivalence ratio is greater than one. In the case of a lean mixture, all the fuel in the charge will be burned, leaving some air unburned, resulting in reduced performance. In the case of a rich mixture, the maximum amount of fuel will be burned in the available air, resulting in lower fuel efficiency (although a rich mixture helps to cool the engine). This behaviour is shown in Figure 2.23 below:

Figure 2.23 Response of specific fuel consumption and engine power output with change in

equivalence ratio. [23]

If the mixture of fuel and air becomes too rich, the power output drops. Ideally, the engine should be run at the highest point on the top curve in Figure 2.23 to produce maximum power, even though the specific fuel consumption will not be at a minimum. Using the engine modelling software, this optimum equivalence ratio is Φ = 1.05, providing maximum power.

2.4.11 Fuel Consumption

An important consideration of the engine is the fuel consumption because the fuel and fuel tank will add weight to the car, decreasing its performance. The brake specific fuel consumption is given for the engine by the engine modelling software and can be seen in Figure 2.24 below:

Figure 2.24 Graph of BSFC against engine rpm

The graph in Figure 2.24 above shows that the specific fuel consumption does not increase dramatically until about 11,500 rpm, the point at which the engine will be operating. At this point, BSFC = 250.7 g/kW/hr. It is possible to find the amount of fuel needed in the endurance race by using Equation (10) below:

V =P × BSFC ×t ×

1

ρ

_f (10)

With the volume V in litres, power P in kW, brake specific fuel consumption BSFC in g/kW/hr, time t in hours and the density of fuel ρf = 0.71e3 kg/ m^3. [37] In order to determine the value for t (the time the car will take to complete the race), the lap time will need to be taken into account along with a weighting for throttle position within each part of the track. A safety factor will be used so the discrepancy in lap times between the first and subsequent laps can be ignored. In a lap completed in 36 seconds, as will be shown later in the report, roughly 16 seconds are spent at full throttle, 8 seconds at mid throttle, 6 seconds at low throttle and 6 seconds are spent braking. Working on the assumption that under braking and at low throttle, the BSFC goes down to 10% of the value at full throttle and when at mid throttle, BSFC goes down to 40% of the value at full throttle, the

equivalent time spent at full throttle can be worked out. [18] _{For the 27 lap endurance race, the} equivalent time t in hours can be calculated using Equation (11) below:

t=

27

3600

(

16+0.4 (8)+0.1(6+6)

)

=0.153 hours

(11)

Using the value of t calculated in Equation (11), and using the values given by the engine modelling software, it can be shown that the required amount of fuel for the endurance race V = 3.91 litres; this is the worst case as the density of fuel used was a minimum value. Using a safety factor of 1.3, to account for any waiting at the start of the race and driving to and from the pits, a fuel tank of 5 litres will be necessary.

2.4.12 Fuel Tank

In order to comply with the FSAE Rules, [1] _{a fuel tank made of rigid material must not carry any} structural loads but it can be any size and can be made of any material. The fuel tank should lie entirely within the monocoque chassis and a firewall must separate the tank from the driver to reduce the risk of burns. A suitable material is carbon fibre as this is lightweight and provides rigidity. Carbon fibre is an expensive material to buy and to work with but there are products available that meet the requirements. The carbon fibre fuel tank used in Formula Seven is suitable for Formula Student and this will be the tank used for the car. At a cost of £400, the tank is not cheap but comes equipped with a fuel pump. [39] _{A picture of the fuel tank can be seen in Figure} 2.25 below:

2.5 Fine Tuning

At this stage of the design considerations, the engine is giving maximum power output; the flow is choked at the venturi so no more air is able to get into the cylinders during each cycle. This is a good result considering the specification. However, it should be possible to optimise the intake and exhaust runner lengths, making use of resonance tuning. Resonance tuning would not increase the power output of the engine but would reduce the engine rpm at which the maximum power is produced, thus lowering fuel consumption. This would make the car lighter as it would need to carry less fuel, which in turn would increase its performance. Resonance tuning works as follows: as valves open and close, compression and rarefaction waves are induced in the compressible fuel and air mixture in the intake and exhaust runners. When the waves reach the end of the runners they are reflected. If the length of the runners is just right, the waves act to increase volumetric efficiency. The available benefits are very sensitive to engine rpm. A diagram showing the operation of resonance tuning can be seen in Figure 2.26 below:

Figure 2.26 Diagram displaying the operation of resonance tuning [40]

Figure 2.26 above shows an inlet valve open at stage 1. At stage 2, the inlet valve is snapped shut and a compression wave forms. This compression wave travels along the pipe at stage 3. At stage 4, the compression wave is reflected at the end of the pipe and travels back down the pipe at stage 5. At stage 6, the compression wave arrives at the open inlet valve at just the right time to force more air and fuel mixture into the cylinder than would otherwise be the case, increasing volumetric efficiency. The same principle applies to the exhaust side of the engine, but instead a rarefaction wave arrives at the exhaust valve at just the right time to help draw the burnt mixture from the cylinder, also known as scavenging.

A diagram showing the effect of the timing of the arrival of pressure waves on volumetric efficiency for the intake valve can be seen in Figure 2.27 below:

Figure 2.27 Diagram displaying the effect of timing on volumetric efficiency [23]

Figure 2.27 shows that the pressure waves must arrive at just the right time to maximise volumetric efficiency, so precise calculation of the timing is needed. [41] _{The same considerations apply to} silencers, where the geometry of the silencer can be made such that pressure waves cancel each other out, reducing the noise level whilst also minimising the restriction to flow. [23]

A further consideration for manifolds is the layout of the intake and exhaust runners. In order to maximise the effects of resonance tuning, the system should avoid sending pressure waves from separate cylinders into the same pipe at the same time as this will lead to increased losses. Two obvious options for the arrangement of the exhaust manifold are a four-to-two-to-one (4-2-1) or a four-to-one (4-1) connection. Only the latter reaps the benefits of resonance tuning at high engine rpm so the 4-1 arrangement will be used. The layout of the intake system is less problematic and will make use of a one-to-four (1-4) system. [23] _{An example of the intake layout with the inclusion of} a plenum (‘intake chamber’) and a throttle can be seen in Figure 2.28 below:

2.6 Conclusion

After all optimisation, the final engine power output is shown in Figure 2.29 below:

Figure 2.29 Final peak power output vs engine rpm

As can be seen in Figure 2.29 above, the engine gives a peak power output of 96.9 hp (72.2 kW) at 11,500 rpm; this is well above the specified 65 hp. The engine also exceeds the other criteria set out in the specification. The torque achieved at 11,500 rpm is 59.1 Nm which is greater than the 50 Nm specified. Despite the requirement of a 20 mm restrictor, the power produced after optimisation is close to the stock engine power output compared with the restricted engine with no optimisation. At 32 kg, it is well within the 60kg weight specification. [42] _{The Honda CBR600RR} engine is easily sourced from ebay for less than £2000. [8] _{Efficiency has been taken into account} throughout, including during the selection of a 5 L fuel tank. Packaging is also important and where possible, the size of components has been kept to a minimum whilst maximising power. The fact that the car will use a CVT means the engine is able to run constantly at 11,500 rpm, producing 96.9 hp at all times; this will make the car highly competitive. In practice, the actual values for power and torque may vary from the theoretical values given in the engine modelling software; if the project included the actual manufacture of the car and its components, testing on track could be done and the performance optimised using telemetry. The Honda CBR600RR appears to be a good choice of engine.

2.7 References

[1] http://www.fsaeonline.com/content/2015-16%20FSAE%20Rules%20revision %2091714%20kz.pdf [2] http://www.topspeed.com/motorcycles/motorcycle-reviews/yamaha/2013-yamaha-phazer-m-tx-ar131346.html [3] http://en.wikipedia.org/wiki/Snowmobile [4] http://en.wikipedia.org/wiki/Honda_CBR600RR [5] http://en.wikipedia.org/wiki/Suzuki_GSX-R600 [6] http://www.motorcyclespecs.co.za/model/yamaha/yamaha_wr450f%2003.htm [7] Private communication with Professor Richard Stone

[8] http://www.ebay.com/itm/Honda-CBR600RR-CBR-600RR-Engine-Motor-2008-07-09-Guaranteed-Low-miles-/121594369273 [9] http://www.motorcycle-usa.com/115/396/Motorcycle-Article/2003-Honda-CBR600RR-First-Ride.aspx [10] http://www.lotuscars.com/engineering/engineering-software [11] https://lotusproactive.files.wordpress.com/2013/08/getting-started-with-lotus-engine-simulation.pdf [12] P4 Fluid Mechanics Lectures: Dr. P McFadden, HT2012

[13] http://en.wikipedia.org/wiki/Bell_mouth

[14] http://www.bosch-mobility-solutions.de/de/de/_technik/component/PT_PC_CNG_Air-Management_PT_PC_Compressed-Natural-Gas-02_12737.html?compId=980

[15] http://sohc.nl/pictures/engine/%5B2010-03-25%5D_parts/roller_barrels_01.jpg

[16] Flow Characteristics and Performance Evaluation of Butterfly Valves Using Numerical Analysis: IOP Publishing, doi:10.1088/1755-1315/12/1/012099

[17] Flow of Fluids Through Valves, Fittings and Pipe: Crane 1982 [18] Private communication with Professor Ray Lohr

[19] Contraction, Expansion, Pressure Drop: Saeid Rahimi, 2011 [20] http://en.wikipedia.org/wiki/Plenum_chamber

[21] http://auto.howstuffworks.com/turbo2.htm

[22] http://www.grc.nasa.gov/WWW/k-12/airplane/mflchk.html

[23] Introduction to Internal Combustion Engines: R Stone, Macmillan 1992

[24] Engineering Tables and Data: A Howatson, P Lund, J Todd, Department of Engineering Science, 2009 [25] http://www.f1fanatic.co.uk/2013/06/21/renault-reveals-2014-f1-engine/ [26] http://en.wikipedia.org/wiki/Stroke_ratio [27] http://www.aperaceparts.com/tech/2009hondacbr600rr.html [28] http://en.wikipedia.org/wiki/Compression_ratio [29] http://en.wikipedia.org/wiki/Engine_knocking [30] http://commons.wikimedia.org/wiki/File:Four_stroke_cycle_power.png [31] http://journeytoforever.org/biofuel_library/z-image/dranefig5-1.jpg

[32] How Things Work, The Universal Encyclopedia of Machines: Paladin, Granada Publishing Limited, 1972 [33] http://en.wikipedia.org/wiki/Fuel_injection [34] http://en.wikipedia.org/wiki/Indirect_injection [35] http://www.honda.com/newsandviews/article.aspx?id=1775-en [36] http://en.wikipedia.org/wiki/E85 [37] http://en.wikipedia.org/wiki/Gasoline [38] http://en.wikipedia.org/wiki/Air%E2%80%93fuel_ratio [39] http://www.formula-seven.com/shop-products/carbon-fiber-fuel-tank/ [40] http://rennlist.com/forums/944-turbo-and-turbo-s-forum/784852-ultra-high-flow-low-cost-8v-head-project-12.html [41] http://www.autozine.org/technical_school/engine/Intake_exhaust.html [42] http://cbrforum.com/forum/cbr-600rr-12/enigne-weight-cbr600rr-25219/

Management

3

Thermal Management

3.1

Introduction

The thermal management of a formula student race car is a vital component in the performance of the car. The car needs to have an adequate cooling system, which has a capacity proportional to the power output of its engine. The primary function of the cooling system is to maintain proper and reliable engine performance within an optimal temperature range. In order to design the cooling system the relevant 2015 Formula SAE competition rules must be reviewed. The main limitation states that any “water-cooled engine must only use plain water” [1]_.

Operating below the minimum engine temperature can cause excess use of fuel and corrosion of the engine and cooling system. Operation above the maximum engine temperature decreases the oil viscosity causing wear, excessive oil consumption and decreased mechanical power output. It is therefore vital to maintain control over the engine temperature. The first variable to consider when designing an engine cooling system is the required heat loss. A commonly used rule is shown in the equation below [2]_:

Fuel Energy=

1

3

HP+

1

3

Heat +

1

3

Cooling System Load

Effectively one third of the heat produced by the engine is converted into mechanical work; one third is lost to ambient air as exhaust and frictional heat. The final third is required to be removed by the cooling system. This is a reasonable estimate; internal combustion engines are inefficient, usual efficiency of about 25 % to 30 % [3] _{and the waste heat removed from the engine is fairly} evenly split between the hot exhaust gases and the cooling system. For this car the power output is 72 kW leading to a cooling load of 24 kW.

3.2

Basics of the cooling system

In general two basic types of cooling systems exist. They both transfer heat from the engine block to the ambient air. The two methods for removing heat are air or liquid cooling. Air cooling works by relying on air flowing directly over hot parts of the engine to dissipate heat. Cooling fins can be

Management

used to increase the effective cooling area that heat transfer can occur across. This method is limited to low power engines as they can only dissipate small amounts of heat. Liquid cooling relies on parts of the air cooling method in combination with the use of a coolant, usually water, flowing through the engine as an intermediate medium to remove heat and achieve more effective cooling. The limitations of air cooling can be made evident by comparing air and water, air has a lower heat capacity and less than a tenth the conductivity [4]_{. Thus air cooling requires unrealistically high air} flow velocities and an extremely large heat exchanger surface area to achieve the same level of cooling. Liquid cooling is therefore used in formula student race cars.

Figure 3.1 Engine cooling system [5]

Following the SAE rules the coolant used here is plain water. A thermostat or the engine control unit, ECU, is used to maintain the engine temperature. It controls the power to the coolant pump; if the engine is overheating the thermostat will increase the power and thus increase the flow rate of the coolant through the engine and vice versa. When designing a cooling system there are three different variables to work with: the coolant flow rate through the engine block; the airflow rate through the radiator and the heat transfer capability of the radiator.

3.3 Cooling system components

There are a number of different components within a cooling system which can be chosen and designed to ensure the desired values of the variables above. The coolant flow rate is set by a

Management

range of different pumps and is controlled by either a thermostat or an ECU, which vary the flow rate depending on demand. The airflow rate through the radiator will vary depending on the overall shape of the body work, side pods, and positioning of the radiator. It can also be increased by attaching a fan to the radiator. The radiators heat transfer capability relies on a number of different variables which will be looked at in more detail later.

3.3.1 Coolant pump selection

The initial decision regarding the coolant pump is whether to use a mechanical or an electrical pump. A mechanical pump is traditionally used in cars. It works by taking mechanical energy from the engine, in the form of a spinning rubber belt, and uses it to drive an internal pump mechanism. This results in a decrease in either the cars power output or fuel economy or both as it is running off energy directly from the engine. The mechanical pump spins all the time at a speed proportional to the engine speed. As a result coolant is sometimes being pumped when the engine is not at a temperature which requires cooling. Conversely an electrical pump runs on the battery power and can be controlled through an ECU .This is significantly more accurate as coolant flow rate through the engine is set depending on a given temperature range. The drawbacks of electric pumps are that they are more expensive and typically are not as powerful.

The electrical system has been chosen, as is the case for most formula student teams, mainly due to the higher level of control, ECU – section 7.2.5, typically smaller size of the pump, and

improvements of around 3.7 kW over a mechanical system. The chosen pump is the Davies Craig EWP80 [6]_{, costing £135 and weighting 0.9 kg. This pump has a maximum flowrate of 80 L/min.}

Management

3.3.2 Cooling fan selection

Cooling fans are used to control the airflow velocity over the radiator. The airflow velocity affects the ability to transfer heat; slower airflow has a higher exit temperature hence a higher average cooling temperature because more heat transfers by convection. A higher average cooling

temperature means that less heat is transferred from the coolant to the air. Faster airflow velocities will increase the differential between the coolant and the cooling air; it will have a lower average cooling temperature, and therefore increase the capability to transfer heat.

For this formula student race car the Jegster 40533 – Jegster fan and shroud kit [7] _{has been} chosen. This kit contains a fan with a 12” diameter with the ability to produce an airflow rate of 1200 CFM. The shroud reduces losses in thrust from the tips of the fan blades. The kit costs £98 and has a weight of 1.5 kg.

3.4

Radiator analysis

Nomenclature:

cp, water – water specific heat cp, air – air specific heat ṁwater –water mass flow rate ṁair – air mass flow rate CW – heat capacity of water

C

W

= ´

m

water

c

p , water CA – heat capacity of air

C

A

= ´

m

air

c

p ,air Cmin – minimum heat capacity Cmax – maximum heat capacity hwater – heat transfer coefficient for the water hair – heat transfer coefficient for the air q – heat transfer rate qmax – maximum possible heat transfer rate ε – effectiveness NTU – number of transfer units Th, inlet – inlet temperature of the hot fluid Tc, inlet – inlet temperature of the cold fluid Aex – external surface area Ain – internal surface area

UA – overall heat transfer coefficient Cr – heat capacity ratio

C

r

=

C

_min

C

max

The radiator is a heat exchanger used for cooling the internal combustion engine. Heat exchangers are implemented to allow the process of heat exchange between two fluids that are at different

Management

temperatures and separated by a solid wall; in this case the two fluids are the coolant (water) and the ambient air. The analysis applied here was first carried out on a stock aluminium radiator [8] where every dimension was known. This was extended to design a new radiator which fits the needs of the formula student race car being designed. In order to save both time and money the designed radiator was then compared to radiators available on the market and the most similar one was chosen. The method of analysis used is based on the effectiveness – number of transfer units, NTU, method described in “Fundamentals of Heat and Mass Transfer” [9] _{and has been} summarised below.

In order to define the effectiveness one must first determine the maximum possible heat transfer rate [9]_.

q

_max

=

C

_min

₍

T

_{h ,inlet}

−

T

_{c ,inlet}

₎

ε=

q

q

_max

The number of transfer units is a dimensionless parameter that is widely used for heat exchanger analysis. The overall heat transfer coefficient is needed to be known in order to find the NTU [9]_.

1

UA

=

1

h

_air

A

_ex

+

1

h

_water

A

¿

NTU =

UA

C

_min

For each heat exchanger there is a specific form of the effectiveness – NTU relation. In this case a cross flow (single pass) heat exchanger is being used with both fluids unmixed. The relationship between effectiveness and NTU is shown below [9]_.

ε=1−exp ⁡[

(

1

C

_r

)

(

NTU )

0.22

_{

exp

[

−

C

r

(

NTU )

0.78

]

−1

}

]

Management

The dimensions of the stock aluminium radiator initially analysed are given in table 3.1. The values of both the external and internal surface areas can be calculated and were found to be 34.2 m2 _and 3.14 m2 _{respectively.}

Table 3.1 Radiator dimensions [8]

Lradiator (m) Hradiator (m) Wradiator (m) Wtube (m) Htube (m) Lfin (m) Wfin (m) Hfin (m) Ntube Nfin 0.6635 0.4572 0.0603 0.0254 0.0021 0.0040 0.0584 0.0001 86 780

A simulation of the cooling system was made in Matlab, in which the mass flow rates of both the water and the air could be varied to show how they affect the heat rejected by the cooling system. This cooling system with the stock aluminium radiator could only achieve the required heat loss, 24 kW, with a mass flow rate of water equal to or greater than 1 kg/s and a mass flow rate of air equal to or greater than1.3 kg/s. The chosen pump has a maximum flow rate of 1.33 kg/s which is over that required, however this is a simplified model and in practise it would be advised to be further away from the pumps working limit. The mass flow rate of air cannot be achieved by the chosen fan. Therefore the radiator needs to be redesigned.