F
ACULDADE DEE
NGENHARIA DAU
NIVERSIDADE DOP
ORTOTheoretical and Practical Analysis of
the Exhaust System in a 2-Stroke
Engine
Performance Analysis of Different Fuels
Performance Analysis of the 2019 and
2020 Rotax Max Challenge Setups
Diogo Neto Moreira da Silva
Mestrado Integrado em Engenharia Mecânica Orientador na FEUP: José Manuel Ferreira Duarte
Orientador na Empresa: Gonçalo Ferreira da Silva Gomes Duarte
c
Theoretical and Practical Analysis of the Exhaust System
in a 2-Stroke Engine
Performance Analysis of Different Fuels
Performance Analysis of the 2019 and 2020 Rotax Max
Challenge Setups
Diogo Neto Moreira da Silva
Mestrado Integrado em Engenharia Mecânica
Abstract
The company Korridas & Kompanhia, founded in 2000 and based in Matosinhos, Portugal, is the official distributor of Rotax in Portugal. They also organise two sporting events, Rotax Trophy, in Portugal and Rotax Series, in Spain, and are also responsible for organizing the world finals named Rotax MAX Challenge Grand Finals. The main purpose of this thesis was to verify if bringing changes to the Rotax exhaust system would influence the engine performance. Addition-ally, we checked how the octane index of gasoline (95, 98 and 102 octane) influenced the engine’s performance; the same tests were made after adding nitromethane in 95 octane gasoline (1, 2 and 5% nitromethane). Lastly, we compared the changes in the engine configurations according to the 2019 and 2020 Rotax Trophy regulations.
Acknowledgements
I would like to thank to my coordinator Professor José Manuel Ferreira Duarte for the guidance and help throughout the execution of this project. Without his help I would not have found this internship opportunity, which helped me grow professionally in the past six months. I also want to thank to Gonçalo Duarte, who was responsible for my orientation in the company and who was always available to help me.
I also want to thank to Arnaldo Frias, who worked closely with my professor to decide the area of my research, offering me the company’s resources to conduct my studies. Then, I would like to thank to everyone at Korridas & Kompanhia for their support.
I want to express my gratitude to the team at Racing4You for offering me 102 octane petrol and to the President of Clube de Aeromodelismo do Norte, Manuel Cardoso for providing me with nitromethane, both materials being essential for this research.
I also want to thank to Elisabeta Nicolae for helping me with the editing and proofreading of this paper.
Lastly, I would like to thank to my family for supporting me throughout my academic journey.
Diogo Neto Moreira da Silva
“Once something is a passion The motivation is there.”
Michael Schumacher
Contents
Abbreviations xiii
1 Introduction 1
1.1 Project Framework and Motivation . . . 1
1.2 Objectives . . . 1 1.3 Structure . . . 2 2 2-Stroke Engine 3 2.1 Principle of Operation . . . 3 2.2 Characteristics . . . 4 2.3 Engine Components . . . 5 2.3.1 Cylinder Head . . . 5 2.3.2 Carburettor . . . 6 2.3.3 Cylinder . . . 8 2.4 Engine Characteristics . . . 10 2.4.1 Torque Curve . . . 10 2.4.2 Power Curve . . . 10 2.4.3 Air-Fuel Ratio . . . 11
2.4.4 Power Correction Factor . . . 12
2.4.5 Compression Ratio . . . 12
2.5 Ignition Advance . . . 13
2.6 Engine Test Bench . . . 14
2.6.1 Inertia Dynamometer . . . 14
2.6.2 Hydraulic Dynamometer . . . 15
2.6.3 Electric Dynamometer . . . 16
2.6.4 Korridas Test Bench . . . 17
2.7 Fuel Characteristics . . . 18
2.7.1 Gasoline . . . 19
2.7.2 Nitromethane . . . 20
2.7.3 Ether . . . 20
3 Exhaust System 21 3.1 Sonic waves’ properties . . . 21
3.1.1 Reflection of Pressure Wave at a Closed end in a Pipe . . . 22
3.1.2 Reflection of Pressure Wave at an Open end in a Pipe . . . 22
3.1.3 Reflection of Pressure Waves at Sudden Area Changes . . . 22
3.1.4 Reflection of Pressure Waves in Tapered Pipes . . . 23
3.2 Design & Pressure Waves . . . 23
viii CONTENTS
3.3 Bell’s Method . . . 23
4 Methodology 31 4.1 Test Bench Trials . . . 31
5 Experimental Procedures 35 5.1 Exhaust Design (Rotax MAX Engine) . . . 35
5.2 Engines’ Preparation . . . 37
5.3 Fuel Tests . . . 38
5.3.1 Octane Rating Influence . . . 38
5.3.2 Nitromethane’s Influence . . . 41
5.3.3 Ether Influence [Micro] . . . 45
5.3.4 Korridas Checkup . . . 46
5.4 Exhaust Tests . . . 48
5.4.1 Muffler w/ and w/o Wool . . . 49
5.4.2 Used wool vs new wool . . . 52
5.5 2019 & 2020 setup Comparison . . . 53
5.5.1 Micro . . . 53
5.5.2 Mini . . . 54
5.6 Micro Main Jet Comparison (2020) . . . 54
5.7 Micro Main Jet Comparison (2019) . . . 55
5.8 Conclusions . . . 57
List of Figures
2.1 2-stroke engine phases. . . 4
2.2 Main components of 2-Stroke Engines. 1) spark plug; 2) cylinder head; 3) piston; 4) intake manifold; 5) carburettor; 6) fuel pump; 7) clutch drum; 8) centrifugal clutch; 9) connecting rod; 10) exhaust manifold; 11) ignition coil 12) cylinder; 13) exhaust valve. . . 5
2.3 Squish Chamber With Central Spark Plug (Bell 1999: 15). . . 6
2.4 Rotax Cylinder Head (RMC 2019: 8) . . . 6
2.5 Interior scheme of a carburettor (Bell 1999: 97) . . . 7
2.6 Pilot air screw, adapted from (Bell 1999: 95). . . 8
2.7 How to Draw the cylinder ports design (MacDizzy 2012: 1). . . 9
2.8 Crankshaft and cylinder ports sketch (MacDizzy 2012) & Port timing diagram (The Engineers Post 2020). . . 9
2.9 Typical Torque and Power Curve. . . 11
2.10 Geometric and trapped compression ratio. (Martins 1996: ...) . . . 13
2.11 A) Load cell, B) Dynamometric break. (Duarte 2018: 42) . . . 14
2.12 Inertial dynamometer used to measure a Motorbike output (Metro Bike Perfor-mance 2017). . . 15
2.13 Hydraulic dynamometer. . . 16
2.14 Dynoteg KED-5 (Duarte 2018: 44). . . 18
2.15 A)Temperature sensor, B) Radiator, C) Fan. (Duarte 2018: 44) . . . 18
3.1 Basic two stroke expansion chamber (Bell 1999: 73). . . 24
3.2 Diffuser taper influence on the duration and intensity of the return wave (Bell 1999: 77). . . 25
3.3 Rotax two staged diffuser for a liquid cooled 125cc kart (Bell 1999: 79). . . 26
3.4 Different output of two baffle angles over the rpm range (Bell 1999: 80). . . 27
4.1 carburettor restrictor (RMC 2020: 16) . . . 32
4.2 Throttle restrictor 2019 regulation . . . 33
5.1 Proposed exhaust design for a Rotax MAX engine configuration (2019). . . 37
5.2 Micro Engine Power Curve (2019): P95 vs P98 and P102. . . 39
5.3 Micro octane rating: Standardisation to P95 fuel. . . 39
5.4 Lambda probe: P95 vs P98 and P102. . . 39
5.5 Junior Engine Power Curve: P95 vs P98 and P102. . . 40
5.6 Junior octane rating: Standardisation to P95 fuel. . . 41
5.7 Micro Engine Power Curve with main jet 105: P95 vs P95N1, P95N2 and P95N5. 42 5.8 Micro, nitromethane influence: Standardisation to P95 fuel, main jet 105. . . 42
5.9 Micro Engine Power Curve with main jet 108: P95 vs P95N1, P95N2 and P95N5. 43
x LIST OF FIGURES
5.10 Micro, nitromethane influence, main jet 108: Standardisation to P95 fuel. . . 43
5.11 Lambda probe: G95 vs G95N5. . . 44
5.12 Junior, Engine Power Curve: P95 vs P95N1, P95N2 and P95N5. . . 44
5.13 Junior, nitromethane influence: Standardisation to P95 fuel. . . 45
5.14 Micro, Engine Power Curve: P95 vs P95E5 . . . 45
5.15 Micro, Ether influence: Standardisation to P95 fuel. . . 46
5.16 P95, P98 and P102 colour test. . . 47
5.17 P95, P95N1, P95N2, P95N5 colour test. . . 47
5.18 P95, P95E5 colour test. . . 47
5.19 Fuel test device . . . 48
5.20 Micro, engine power curve (2019): w/ wool and w/o wool . . . 49
5.21 Micro, wool influence (2019): Standardisation to w/ wool. . . 50
5.22 Micro, engine power curve (2020): w/ wool and w/o wool . . . 51
5.23 Micro, wool influence (2020): Standardisation to w/ wool. . . 51
5.24 Micro, engine power curve (2019): burnt wool and new wool . . . 52
5.25 Micro, wool influence (2019): Standardisation to burned wool. . . 52
5.26 Micro, main jet 100: 2019 vs 2020 . . . 53
5.27 Mini, main jet 105: 2019 vs 2020 . . . 54
5.28 Micro, main jet influence (2020): 105, 108, 112, 115 main jets. . . 55
5.29 Micro, main jet influence (2020): Standardisation to 105 main jet. . . 55
5.30 Micro, main jet influence (2019): 105, 108, 112 main jets. . . 56
List of Tables
2.1 Carburettor Running Conditions . . . 7
3.1 Header Coefficients . . . 24
3.2 Standard Exhaust Port Diameter . . . 25
3.3 Diffuser Stage Angles . . . 26
3.4 Baffle Angles . . . 28
3.5 Stinger Size . . . 29
4.1 Technical Regulations . . . 32
4.2 Tests Criteria . . . 32
5.1 Designed Exhaust Diameters . . . 37
5.2 Designed Exhaust Lengths . . . 37
5.3 Engine Measurements [mm] . . . 37
5.4 Octane Rating Batches . . . 38
5.5 Nitromethane Batches . . . 38
5.6 Fuel Korridas checkup results. . . 49
Abbreviations and Symbols
Cylinder, Port Timing
BDC Bottom Dead Centre
EPC Exhaust Port Closes
EPO Exhaust Port Opens
IPC Inlet Port Closes
IPO Inlet Port Opens
TDC Top Dead Centre
TPC Transfer Port Closes
TPO Transfer Port Opens
Compression Ratio
CRcc Crankcase Compression Ratio
CRg Geometric Compression Ratio
CRt Trapped Compression Ratio or Compression Ratio
Vcc Crankcase Clearance Volume
Vcv Clearance Volume
Vsv Swept Volume
Vts Swept Volume Trapped
Bell’s Method
Cot(D) Cotangent of the Diffuser’s Angle of Taper Cot(B) Cotangent of the Baffle’s Angle of Taper Cot(H) Cotangent of Header Pipe’s Angle of Taper
D1 Header Pipe Inside Minor Diameter
D2 Header Pipe Inside Major Diameter
xiv Abbreviations and Symbols
D3 Both Diffuser Major Inside Diameter & Baffle Major Inside Diameter
D4 Stinger Inside Diameter
H Header Pipe Angle in Degrees
L Exhaust Length
LH header pipe length minus the length of the exhaust port and flange
OLB Overall Length of the Baffle
Power Correction Factor
CF Power Correction Factor
Ppd Standard Atmospheric Pressure [kPa]
Patm Measured Atmospheric Pressure [kPa]
Pvap Measured Atmospheric Partial Vapor Pressure [kPa]
Tpd Standard Temperature [K]
Tatm Measured Atmospheric Temperature [K]
Pe,s Standard Effective Power
Pe,m Mean Effective Power
Others
C Breaking Torque of the Dynamometer Break [N.m]
ECU Electronic Control Unite
I Dynamometer Inertia [kg.m2]
RMC Rotax MAX Challenge
rpm Revolutions per Minute
t Time [s]
θ Crankshaft Angle [rad]
w Angular Acceleration [rad/s]
Chapter 1
Introduction
This dissertation was written within the scope of the study program Production, Conception and Manufacturingat the University of Porto and it analyses the performance of 2-stroke Rotax engines, with or without modifying factors. In this section, we will introduce the motivation behind this thesis, as well as the main goals and how it was structured.
1.1
Project Framework and Motivation
The field of mechanical engineering is very vast and complex, but one thing that I have always been interested in was automotive racing. This is why, when the opportunity for an internship in this field came across, and within it writing this thesis, it was an immediate choice. As part of this dissertation, I did a six-months internship at Korridas & Kompanhia, the exclusive representative of Rotax karting engines, Mojo tires and XPS lubricants in Portugal, Spain, Andorra and Angola. The company’s main activity is the import and commercialization of karting products, as well as the organization of national and international automotive events, mainly karting related (Rotax Trophy, Rotax Series, and Rotax Max Challenge Grand Finals).
During the internship, I took part in various activities, such as two karting races. My main responsibility there was to check the participants’ engines, but I also helped with the preparation of the engines and the organisation of races. Moreover, as part of my intern job, I helped with the after-sale maintenance of Rotax engines. So all the tests and the analysis that will be presented in this dissertation were made in the environment of Korridas & Kompanhia.
1.2
Objectives
This section describes the goals that were set prior and during the internship, that will also be reflected in this thesis. Initially, the main goal was to analyse the engine’s performance when it suffers exhaust alterations, and how race participants could take advantage of such alterations. Besides this, we established secondary objectives that were relevant to this dissertation, namely (1) theoretical and practical study of 2-stroke exhaust systems, (2) performance analysis of fuel
2 Introduction
with different octane rating available in the market, (3) performance analysis of 95 octane rating fuel when adding nitromethane, and (4) comparison between the engine configurations of 2019 and 2020 Rotax Trophy. Each of them will be presented in this thesis, taking into consideration theoretical principles, methodology and results, in this very order.
1.3
Structure
This dissertation contains five chapters, namely (1) Introduction, (2) 2-stroke engine, (3) Ex-haust system, (4) Methodology, and (5) Experimental procedures. However, we consider that the text content is divided into state-of-the-art and practical application. The first part contains the theories and information needed to understand the functioning of a 2-stroke engine and of an ex-haust system, which will later be applied and verified in the second part, the analysis. For the first part, we used the work of different authors in the field, such as Jennings (1973), Blair (1996), Bell (1999), Martins (2006), and Duarte (2018).
Chapters 4 and 5 correspond to the practical application. In chapter 4, we explain the param-eters of this study, as well as the test bench’s functioning in this company, the same that was used for our tests. This chapter also contains the demonstration and the analysis of all the experimental results obtained for each of the parameters that were tested. Finally, chapter 5 presents the final conclusions, as well as some suggestions for future work related to this subject.
Chapter 2
2-Stroke Engine
A 2-stroke engine is a type of internal combustion engine, that produces thermal energy which is then converted into work inside the cylinder. This type of engines completes one power cycle with two strokes of the piston during one crankshaft revolution (Martins 2006: 12).
An engine is designated as a 2-stroke if one engine cycle is done in two movements of the piston or one crankshaft revolution. As the mixture starts combusting inside the cylinder and the piston descends, work is produced and as it reaches the BDC, (bottom dead centre), the admis-sion and exhaust start occurring. And then, as it goes towards the TDC, (top dead centre), the compression phase begins, and the cycle repeats.
2.1
Principle of Operation
In comparison with a 4-stroke engine, where the four phases (admission, compression, ex-pansion, and exhaust) are all done in separate piston strokes, in a 2-stroke engine the admission and compression phases are done in one stroke and the expansion and exhaust phases are done in another stroke (Martins 2006: 12).
The first stroke starts after the air-fuel mixture is compressed inside the cylinder, as shown in figure 2.1, the spark plug ignites and the combustion starts; consequently raising both the tem-perature and pressure. At this point, the piston is driven from the TDC downwards, transferring energy to the crankshaft. Before reaching the BDC the exhaust port is uncovered, letting the burn gases flow through the exhaust system.
During the downward movement of the piston, the mixture in the carter is compressed and as the piston uncovers the inlet port the pre-mixed gases in the carter enter the cylinder. As they enter the cylinder they force the burnt gases out, cleaning the cylinder. When the piston reached the BDC, the first stroke ends. (Martins 2006: 12)
During the second stroke, the piston goes up from the BDC, closing the inlet port and after that the exhaust port. This motion ends up compressing the mixture inside the cylinder, while simultaneously the piston uncovers the carter inlet port causing the pre-mixture to enter. The second stroke ends the moment the piston reaches the TDC (Martins 2006: 12-13).
4 2-Stroke Engine
Figure 2.1: 2-stroke engine phases.
2.2
Characteristics
2-stroke engines are preferred when mechanical simplicity is required and according to Bell (1999: 9), the 2-stroke engine design is very simple. Martins (2006: 12) goes a step further explaining that these engines do not need a valve distribution system but instead, there are ports in the cylinder that are opened and closed by the piston, which act similarly as a valve would. These engines are also known for being lightweight and used for lightweight power units (Blair 1996: 1).
2.3 Engine Components 5
2.3
Engine Components
In this section, we will enumerate the main components of a 2-stroke engine and further ex-plain the function of some of them, which we believe are important to understand the experimental procedure. Figure2.2shows a Rotax engine, to which some cuts were made, this way gaining vi-sual access to its interior.
Figure 2.2: Main components of 2-Stroke Engines. 1) spark plug; 2) cylinder head; 3) piston; 4) intake manifold; 5) carburettor; 6) fuel pump; 7) clutch drum; 8) centrifugal clutch; 9) connecting rod; 10) exhaust manifold; 11) ignition coil 12) cylinder; 13) exhaust valve.
2.3.1 Cylinder Head
A 2-stroke cylinder head design has a significant influence on how well the engine will run. The shape of the combustion chamber and the position of the spark plug are two important char-acteristics of this component. A poorly designed cylinder head may result in an unreliable and/or low power (not power optimised) engine, in more extreme cases even promoting detonation 1. According to Bell (1999: 14), a good design ideally keeps the end gases2 cool and reduces the time required for the combustion flame to reach the end gases. He states that this can be achieved by reducing the expansion chamber as much as possible, positioning the spark plug in the centre of the chamber, and by moving the combustion chamber as close as possible to the piston. This describes a squish-type combustion chamber shown in figure2.3, which is also the design used by Rotax in their engines. (Bell 1999: 13-14)
1"Detonation occurs when a portion of the fuel/air change begins to burn spontaneously before normal ignition takes place" (Bell 1999: 13).
6 2-Stroke Engine
Figure 2.3: Squish Chamber With Central Spark Plug (Bell 1999: 15).
The Rotax cylinder head design is shown in figure2.4and according to the Rotax MAX Chal-lenge Technical Regulation 2019 version 2, it must comply with the following parameters: (1) the height of the combustion chamber insert has to be 28.80mm +/- 0.2mm(H), (2) the expansion
chamber design must be the same as the template (Rotax 277390), and (3) the cast identification numbers must coincide with the ones provided by Rotax.
Figure 2.4: Rotax Cylinder Head (RMC 2019: 8) .
2.3.2 Carburettor
A carburettor is a device used to atomize the fuel with air in a suitable proportion to be easily combustible, enabling the engine to produce good power over a wide rpm range. Table 2.1 ex-presses the ideal fuel/air ratio (by weight) under different conditions, which must be guaranteed by the carburettor. It is significant to point out that the carburettor has to "sense" the engine oper-ating conditions and to adjust the fuel/air mixture accordingly; failing in doing so results in poor performance. For these reasons, choosing a carburettor for a specific engine requires selectiveness and fully understanding of how a carburettor works (Bell 1999: 93-94).
2.3 Engine Components 7
Table 2.1: Carburettor Running Conditions Running Conditions Fuel/Air Ratio
Starting 1: [1; 3]
Idling 1: [8; 10]
Low speed running 1: [10; 13] Light load ordinary running 1: [14; 16] Heavy load running 1: [12; 14]
We will not look extensively to the way a carburettor works or to its components. However, we must emphasize on the parameters that were altered in some of our experimental tests, namely the main jet and the pilot air screw.
2.3.2.1 Main Jet
The main fuel system consists of three components, the main jet, the needle jet, and the needle, as shown at the left of figure2.5. The main jet controls the fuel flow from the carburettor into the needle jet. By increasing the main jet diameter, more fuel is pushed into the engine, consequently enriching the mixture. At the right of figure2.5are shown different main jet sizes (Bell 1999: 96).
Figure 2.5: Interior scheme of a carburettor (Bell 1999: 97)
For most of the throttle openings, one to three quarters throttle, the amount of fuel introduced into the air stream is mainly controlled by the needle and needle jet sizes, despite the main jet also having a small influence. The fuel flow is mainly controlled by the main jet only above three quarter throttle (Bell 1999: 96).
2.3.2.2 Pilot Air Screw
When the throttle is closed or nearly closed, there is not enough air flow going through the main jet, which creates a high vacuum in the piston side of the engine. By incorporating an idle
8 2-Stroke Engine
system, capable of taking advantage of this vacuum, we are able to feed some air and fuel into the engine. This idle system is composed by the pilot hole, bypass hole, pilot jet, and pilot screw. The pilot screw is responsible for providing some control over the fuel/air ratio. Tightening the screw enriches the mixture by reducing the amount of air passing through the bleed hole. On the other hand, loosening the screw allows more air to go through the bleed hole, therefore lowering the fuel/air ratio (Bell 1999: 94-95). Figure2.6helps to visualise these concepts.
Figure 2.6: Pilot air screw, adapted from (Bell 1999: 95).
2.3.3 Cylinder
In a piston engine, the cylinder is the space in which the piston travels. Although it seems like a simple concept, over the years it evolved significantly, and nowadays the walls of a cylinder are filled with ports to handle the induction, the transfer and the exhaust phases of gas to flow through the engine (Bell 1999: 27).
To understand how a 2-stroke engine work, one should know how to map a cylinder. When an engine is mapped correctly, it will reveal both its port timing and area of the windows. According to MacDizzy3, The Enthusiast, mapping the engine is easy to do but it can be time-consuming. The method suggested by MacDizzy consists in imprinting the cylinder ports against a piece of paper, using a pencil and tape as shown in figure2.7.
After all the ports are imprinted into the paper, we need to measure the area of the imprinted sections. The last step is to make the drawing represented in figure (3), which is done following these steps:
(1)-Draw a circle the size of the crankshaft;
(2)-Draw a vertical line indicating the centre of the bore;
(3)-Locate both the TDC and BDC (this is the space swept by the piston);
3This method is extensively explained in this website (http://www.macdizz.com/cyl
2.3 Engine Components 9
(4)-Finally, draw the ports to better understand the cylinder timings.
Figure 2.7: How to Draw the cylinder ports design (MacDizzy 2012: 1).
Figure 2.8 4 represents the port timing diagram, which can be calculated using MacDizzy
method. Abbreviations used in figure2.8: EPC (Exhaust Port Closes), EPO (Exhaust Port Opens), IPC (Inlet Port Closes), IPO (Inlet Port Opens), TPC (Transfer Port Closes), TPO (Transfer Port Opens), IGN (Ignition).
Figure 2.8: Crankshaft and cylinder ports sketch (MacDizzy 2012) & Port timing diagram (The Engineers Post 2020).
4Two-Stroke Engine: Parts, Types, Working Principle with Diagram [Petrol and Diesel Engines], The Engineers Post, https://www.theengineerspost.com/two-stroke-cycle-engine/, (accessed 5 February 2020).
10 2-Stroke Engine
2.4
Engine Characteristics
The engine characteristics are usually given by its maximum torque (N/m) and effective power (J). According to Martins (2006: 62), torque can be measured by a torque brake, further explained in section4.1. He also establishes a connection between the torque and the effective power output of an engine. This relation is demonstrated in the formula below:
˙
We= 2π × N × T (2.1)
As explained by Martins (2006: 62), the torque represents the amount of work produced while the engine’s power shows the rate at which that work is produced.
Understanding these two principles, torque and effective power, provides the reader with enough knowledge to analyse the data collected and explained in chapter 5. In the following sections, we will provide a more detailed explanation about this and how these concepts are going to be represented in graphics.
2.4.1 Torque Curve
According to Martins (2006: 77), the torque is approximately proportional to the air intake per cycle. At low revolutions per minute, the pressure drop created inside the carter is not as accentu-ated when compared to the pressure drop creaccentu-ated at high revolutions per minute. This results in a lower air intake, which itself results in a lower torque. As the speed increases, the dynamic effects of the air columns gain relevance allowing more air intake per cycle and consequently increasing the torque output until it reaches the maximum torque value (Martins 2006, cit in Duarte 2018: 37). From this maximum torque point, the increase in speed only decreases the torque output due to the losses in the intake and exhaust manifold, as well as to the friction between the components inside the engine (Martins 2006: 77).
In figure2.95, there is an example of a typical torque curve represented in green. The X-axis represents the engine speed and the Y-axis represents the torque value.
2.4.2 Power Curve
The power curve, as mentioned in section2.4, is a function of torque and speed (in revolutions per minute), therefore both the torque and power curves are related.
As the torque increases with speed so does the power curve. When the maximum torque is reached, the power curve keeps increasing but only due to the increase in speed. The maximum power occurs after the maximum torque. After the maximum power point, the reduction in the torque curve can no longer compensate the increase in the engine speed resulting in a drop-in of the power curve (Martins 2006: 77-78). This phenomenon is also easily visualized in the
5Cédric, H., UDK Motorsport: Torque curves and engine sound, Technical Game Design, 2011, http://technicalgamedesign.blogspot.com/2011/02/udk-motorsport-torque-curves-and-engine.html, (accessed 3 Jan-uary 2020).
2.4 Engine Characteristics 11
figure2.9where the power curve is represented in red and the torque curve, as mentioned in2.4.1, is represented in green.
Figure 2.9: Typical Torque and Power Curve.
2.4.3 Air-Fuel Ratio
The air intake per cycle in an internal combustion engine is limited to the volume displaced by its pistons, in our case piston. Depending on the amount of air present in the piston, a percentage of fuel must be added. The factor between the quantity of air by the amount of fuel is known as Air-Fuel Ratio and is defined by the following formula:
A F = ˙ mair ˙ mf uel (2.2) There are three ways to classify an air-fuel mixture depending on their quantities. If all the air is used to burn all the fuel, then the mixture is considered stoichiometric. On the other hand, if there is more air than needed to burn all the fuel, the mixture is considered poor and if there is not enough air to burn all the fuel, the mixture is considered rich. The formulas below will help visualise this concept:
RichMixture: φ = A F stoichiometric A F (2.3) PoorMixture: λ = A F A F stoichiometric (2.4)
12 2-Stroke Engine
To sum up, a stoichiometric mixture has φ =1 and λ =1, if the mixture is rich φ >1 and λ <1 and if the mixture is poor φ <1 and λ >1 (Martins 2006: 64-65).
2.4.4 Power Correction Factor
The power output of one engine is dependant on several factors including pressure, tempera-ture and humidity of the air. For this reason, a correction factor is used in order to compare tests done under different conditions, making them comparable (Martins 2006: 80).
With the aim of standardizing, all the experimental results must be expressed under three well defined air properties:
(1)- Atmospheric Pressure = 98.2kPa;
(2)- Vapor Pressure = 1287Pa;
(3)- Temperature = 29.4oC;
Martins (2006: 80) refers that the correction factor is based on the single-dimensional equa-tion of compressible flow through a hole. The factor is then calculated using equaequa-tion2.5. Duarte (2018: 38) also mentions in his thesis several methods that make use of this factor, such as DIN 70020, SAE J 1349, JIS D 1001 and ISO 1585. These methods are all explained by Sodré & Soares (2003)6.
Korridas & Kompanhia test bench in configured using the DIN 70020 correction factor. This factor does not account for changes in air humidity. According to this norm overall efficiency, fuel specific heat and air/fuel ratio can be considered constants, regarded changes in atmospheric conditions are small to the standard condition. Equation2.5 shows how this factor is calculated (Sodré & Soares 2003).
CF = Ppd Patm− Pvap × s Tatm Tpd (2.5) In the end, the standard effective power is calculated using the mean effective power with the following formula:
Pe,s= Pe,m×CF (2.6)
2.4.5 Compression Ratio
Another important concept to understand is the compression ratio. The compression ratio value is the ratio of the maximum volume in any chamber of an engine by the minimum volume in that chamber. Blair (1996: 22) explains that we can distinguish three different compression ratio values, such as (1) Crankcase compression ratio (CRcc), (2) Geometric compression ratio (CRg),
(3) Trapped compression ratio or simply compression ratio (CRt).
6Sodré. J.R. Soares. S.M.C. 2003. “Comparison of engine power correction factors for varying atmospheric conditions”. J. Braz. Soc. Mech. Sci. Eng 25, no. 3: 279-284. ISSN 1678-5878. https://doi.org/10.1590/S1678-58782003000300010
2.5 Ignition Advance 13
Equation 2.7 is used to calculate the crankcase compression ratio. The Vcc represents the
crankcase clearance volume or the crankcase volume at the BDC. We will not look much further into this subject, but Blair (1996: 22) explains it in more details in his book.
CRcc=
Vcc+Vsv
Vcc
(2.7) Equations 2.8and2.9account for the compression that actually happens inside the cylinder. For this reason, it measures something we can directly correlate, for example, the octane rating of a fuel explained in subsection2.7.1.1.
The difference between the geometric and trapped compression ratio lies at the moment at which we start measuring. According to Martins (2006: 85), different manufacturers might decide to express the engine compression by one or the other ratio, and it is up to the reader to interpret which one they are referring to.
The geometric ratio compares the volume inside the cylinder, when the piston is at the BDC, to the volume inside the cylinder, when the piston is at the TDC. In comparison, the trapped ratio, instead of measuring from the BDC to the TDC, starts measuring from the moment the piston closes the exhaust port (Blair 1996: 22).
CRg= Vsv+Vcv Vcv (2.8) CRt = Vts+Vcv Vcv (2.9)
Figure 2.10: Geometric and trapped compression ratio. (Martins 1996: ...)
2.5
Ignition Advance
As mention in section2.3.3, the ignition must occur close to the TDC, ensuring that the max-imum torque and power are reached. During a cycle, deciding the instant at which the spark plug ignites is extremely important. The ignition must be performed so that the cycle benefits from the
14 2-Stroke Engine
high pressure of the mixture right after the TDC. As the ignition is advanced, the work done by the piston during its upward movement will increase. But since the work done by the downward movement is much higher, the ultimate result is more beneficial (up until a certain limit). It is not advantageous to keep advancing the ignition when the pressure increases excessively during the upward movement, which reduces the time the piston is producing work (Martins 2006: 252-253).
2.6
Engine Test Bench
The engine is tested in a test bench and is connected to a dynamometric brake, as illustrated in figure2.11. The brake is represented in image B and consists of a rotor connected to the engine and a stator supported by roll bearings and braked by a load cell. The load cell is connected at a length b resulting in a force F measured by the load cell. Where a torque of value b times F, which times the engine RPM equals the engine’s power (Martins 2006: 406).
Figure 2.11: A) Load cell, B) Dynamometric break. (Duarte 2018: 42)
According to Martins (2006: 400), there are two types of dynamometers used to measure the characteristics of an engine, namely the dynamometric brakes and dynamometers of inertia. In the next sections, we will explain in detail the differences between them and their different applications.
2.6.1 Inertia Dynamometer
The use of a dynamometer of inertia is more appropriate when measuring out the power output of vehicles such as motorcycles or race cars (fast acceleration). This is because the weight of both vehicles is insignificant when compared with the power of their engines.
The engines are tested by connecting them to an installation that simulates the inertia of the vehicle and records its velocity through time (Martins 2006: 407). This way, it is guaranteed that the speed and acceleration that the engine reaches is close to the track performance. As Martins (2006: 407) explains, the acceleration, which is equal to the second derivative of the position of
2.6 Engine Test Bench 15
the crankshaft, times the inertia of the dynamometer and equals the torque output of the engine. In equation 2.10it is possible to visualize this concept.
w=d
2θ
dt2 =
Z−C
I (2.10)
An inertia dynamometer consists of masses (inertia cylinders) that the engine forces to rotate, as demonstrated in image 2.12 7. There are two different engine assembly configurations on these test benches, either the engine is directly connected to the dynamometer or the wheels of the vehicle are connected to the inertia cylinders. For both cases, the torque output readings are different, since the transmission system adds extra inertia to the measurement. This difference increases accordingly to how much lighter the inertia cylinders are (Martins 2006: 407).
Figure2.12presents from left to right an inertia cylinder, its mount, and in the third image the way the motorbike connects to the cylinder.
Figure 2.12: Inertial dynamometer used to measure a Motorbike output (Metro Bike Performance 2017).
The use of these test benches is more advantageous in some cases because they are easy to use, do not need control and do not require a cooling system. However, even though they provide the torque and power output of the engine in a matter of seconds, they are not suitable to map the engine. Also, they do not allow to choose and record data collected by the ECU about the injection and the ignition and to measure the fuel consumption, but there are some exceptions (Martins 2006: 408).
2.6.2 Hydraulic Dynamometer
Although there are several types of hydraulic dynamometers, they all work with the same principle. An axle forces a cylindrical rotor to move water to compartments in the stator. The transference of water transmits movement to the stator. This process achieves a dissipation of the energy in the axle, and since the stator remains still, the energy ends up being transmitted as heat (Martins 2006: 408). Figure2.138illustrates the interior of a hydraulic dynamometer.
7Allister from Metro Bike Performance and his quality Motorbike Dyno, Metro Bike Performance, 2017, http://www.metrobikeperformance.com.au/, (accessed 7 January 2020).
8Pandey, R., Hydraulic dynamometer, Mechanical Engineering, 2017, https://mechanical-engg.com/gallery/image/2651-hydraulic-dynamometer/, (accessed 7 January 2020).
16 2-Stroke Engine
Figure 2.13: Hydraulic dynamometer.
According to Martins (2006: 408), there are three types of hydraulic dynamometers, classified in accordance with the way they control the torque. The most common types are the Froude dynamometer and the hydraulic dynamometer with variable filling, and the third type identified is the disk Dynamometer.
The Froude dynamometers are able to control the extension of their connection and, con-sequently, the torque by partially blocking the rotor and the stator. Although robust and able to sustain overloads, they show a slow response and a deficient load control when operating (Martins 2006: 408-410).
The hydraulics dynamometer with variable filling controls the flow of the hydraulic fluid with a valve, the fluid then acts on a break, controlling the torque (Martins 2006: 408). This type of dynamometer, just like the Froud, is not only robust and sustains overloads, but also responds fast to variations and allows control automation (Martins 2006: 410).
The third type of dynamometer, disk dynamometer, consists of one or more plain disks con-nected to the drive shaft and separated by a small space between them and the stator. The power is absorbed by the shear stress generated by the film of water between the water and the disks. This type of dynamometer does not work well at a regime of low revolutions per minute; instead, it is most commonly used at higher speeds, for instance, testing turbines. The control of its load is done by the water flow (Martins 2006: 408).
2.6.3 Electric Dynamometer
As explained by Martins (2006: 408), there are several types of electric dynamometers but in all of them, the absorbed energy is transformed in electric energy, which in turn can be picked up from the break either as electricity or as heat generated by electromagnetic losses (Foucault’s currents).
2.6 Engine Test Bench 17
A direct current dynamometer is a direct current motor connected to a generator, where its load is controlled electronically. They have characteristics such as fast response time and easy electronic control and also, it works as a motor and does not require a cooling system. On the other hand, they are expensive to manufacture and have high inertia (Martins 2006: 408-410).
Another type of dynamometer similar to the DC one is the alternated current dynamometer but instead of using a DC motor, it uses an induction or an asynchronous motor. The control is achieved by the variation of the activating currents which are received and translated by a variable-frequency drive. In contrast with a DC, an AC dynamometer has the same advantages but it also has low inertia; unfortunately, they are also expensive (Martins 2006: 409-410).
Another type, as mentioned at the beginning of this section, are the Foucault’s currents dy-namometers, also known as Eddy current dynamometer, and probably the most commonly used according to Martins (2016: 409-410). The revolution of the rotor, electrically excited, pro-duces an electromagnetic induction which in turn creates Foucault’s currents, then dissipated as resistive losses in the stator. The variation of the exciting current produces variations in the load of the break. These dynamometers are normally water- or air-cooled depending on the application. The inconvenience in their use is their sensibility to bad refrigeration, not suitable for uses under overload and also, they do not work as a motor. On the other hand, they are robust, respond fast to variations, are easy to control electronically and have low inertia (Martins 2006: 409-410).
2.6.4 Korridas Test Bench
Korridas test bench is based on the working principals of an electric dynamometer. The bench is equipped with both a load cell and a dynamometer break, both showed previously in figure2.11. The load cell has a capacity of 150kg and measures force which multiplied by the distance equals the torque (Duarte 2018: 41).
The test bench model is "Dynoteg KED-5" from Roteg Racing and it is shown in figure2.14. This bench is appropriate for kart engines up to 60hp. It Includes a water cooling system that pre-heats the water to a chosen temperature and maintains the engine cooling water temperature constant. The bench dimensions are 116 x 75 x 76 cm (Duarte 2018: 43).
As explained in subsection2.6.3, this type of test bench produces Foucault currents, generated from the movement of the rotor via electromagnetic induction, and dissipated as resistive losses in the stator. The heat generated in the stator is drawn to the atmosphere, through a fan exclusive to that effect (Duarte 2018: 42).
The stator is not the only source of heat in the bench, since the working engine generates a fair amount of heat, also the engine water cooling and losses in the exhaust system. The generated heat is extracted by a tube, located just outside of the exhaust, connected to a three-phase fan capable of removing air at a rate of 5 050mh3. The cabin fresh air renewal is accomplished using another three-phase fan with 1.10kW (Duarte 2018: 42).
The water temperature control is done using a heat exchanger as illustrated in figure 2.15. While the engine is working, the water is actively cooling the engine, then it goes through a radiator paired with a fan, that dissipates the generated heat by convection . The temperature is
18 2-Stroke Engine
Figure 2.14: Dynoteg KED-5 (Duarte 2018: 44).
measured using a thermistor, NTC temperature sensor, which has a maximum temperature reading of 150o(Duarte 2018: 44).
Figure 2.15: A)Temperature sensor, B) Radiator, C) Fan. (Duarte 2018: 44)
For safety reasons, the engine is enclosed in a cabinet prepared for that purpose and the control is performed outside, in the control table. All the sensors such as water temperature controls and its display, the throttle calibrator, both the throttle and the break pedals, and the computer where all the data is received and displayed (Duarte 2018: 43).
This test bench uses a PID controller, collecting data unite SP-5, which allows the miniaturiza-tion and control of the several systems during the trial (SportsDevices 2017), using the SportDyno software. The Foucault’s current dynamometer is powered by the module PW1.5.
2.7
Fuel Characteristics
In this section, we will describe the characteristics of the petrol used for the tests, such as oc-tane rating, calorific power, flame front, and air/fuel ratio. We will also describe the nitromethane and the ether characteristics, because they were used as additives in the tests.
2.7 Fuel Characteristics 19
2.7.1 Gasoline
2.7.1.1 Octane Rating
The anti-knock rating is measured by the octane rating which determines how compressed a fuel can be without self-igniting. According to Martins (2006: 212-213), an engine running on a type of fuel with a high octane rating is able to run more compressed. On the other hand, he states that a fuel with higher octane rating (in this case higher than needed) will not make an engine more efficient and the emissions will be the same, with some exceptions.
2.7.1.2 Calorific Power
Calorific value refers to the total energy available when a substance undergoes complete com-bustion with oxygen, under certain specific conditions. This property can be measured, depending on the user’s convenience, in kgJ or kmolJ or mJ3, these units representing an amount of energy by a
quantity of fuel (Pinho 2018: 43).
2.7.1.3 Volatility
Volatility is a material quality which describes how readily a substance vaporizes. At a given temperature and pressure, a substance with high volatility is more likely to exist as a vapor, while a substance with low volatility is more likely to be a liquid or solid (The Encyclopedia of The Earth).
2.7.1.4 Flame Front
Flame front is a region where a rapid change in the chemical composition of the fluid occurs, consequently releasing chemical energy in the form of heat (Tsien 2012).
Flame front speed is a linear displacement velocity measured from a fixed referential. It is necessary to define this referential since the flame front speed is not the speed at which the flame travels through the gas; it would be the same if the gases did not expand with the temperature (Martins 2006: 247).
Martins (2006: 247-248) explains how this property is measured in the pages mentioned. We will not look further into this subject since it is not related to the thesis, but the read is advisable to better understand this concept.
2.7.1.5 Flash Point
The flash point of a volatile material is the lowest temperature at which its vapours ignite if given an ignition source (BRITANNICA).
20 2-Stroke Engine
2.7.1.6 Air-Fuel Ratio
The biggest difference between oxygenated and conventional fuels lies in the stoichiometric air-fuel ratio. Oxygenated fuels, like nitromethane, described in subsection2.7.2, have a stoichio-metric air-fuel ratio significantly lower than petrol and require less air to burn all the fuel.
The phenomenon described above is relevant because, when oxygenated fuels are added to petrol, the previously referred stoichiometric air-fuel ratio changes. As a consequence, carburet-tors and injection systems tuned to run on petrol will not run optimally but instead, the result will be poor combustion with an excess of air (Martins 2006: 227).
2.7.2 Nitromethane
Nitromethane is a compound known for increasing the power output of an engine and by its explosive properties. When compared to petrol, its chemical formula not only consists of carbon and hydrogen atoms but also oxygen and nitrogen CH3NO2. The nitromethane low volatility and
its high latent heat make it a safe fuel (Martins 2006: 234-235).
The properties of nitromethane are better understood when compared with other, more com-mon fuels. For example, when compared with petrol, the use of alcohols as fuels increases the fuel consumption significantly due to a notably FA ratio. To have another perspective, when ethanol is used it almost increases consumption by 70%; with methanol, it can go over a 130% increase. These increases are mitigated when compared with nitromethane, whose consumption reaches 850%.
Petrol’s calorific power is four times higher than nitromethane, but in comparison the ni-tromethane’s FA ratio is eight and a half times higher than petrol’s. For this reason, the amount of energy in the mixture air/nitromethane is more than double compared to air/petrol. According to Martins (2006: 235), in some competitions nitromethane is added to other fuels in proportions of approximately 5 to 30%. One example of this practice is a model aeroplane, where nitromethane is added to methanol.
2.7.3 Ether
Ether is a colorless, highly volatile, flammable organic compound. According to Encyclopae-dia Britannica, it is commonly used as a solvent and as a starting fluid for some engines, due to its high volatility and low flash point. Also, it is used as a component of fuel mixture for carburetted compression ignition model engines (BRITANNICA).
Chapter 3
Exhaust System
According to Bell (1999: 70), the exhaust system in 2-stroke engines was at first only designed to get the burnt gases out of the engine as fast as possible. With time, designers learned more about pressure waves and how to take advantage of this phenomenon, granting the engines higher power levels. Although easy to understand, the practical application of these pressure waves is very difficult to formulate. As the author explains, understanding exhaust designs begins with an appreciation of the behaviour of sonic waves travelling through a pipe. Pressure waves or sonic waves travel through a fluid at the speed of sound and the speed of sound itself is a function of temperature and pressure. The speed of sound in hot exhaust gas averages 1675 feet per second or 510.54 meters per second (Bell 1999: 70).
3.1
Sonic waves’ properties
For this part of the study, we based our research on Blair’s work about the two-stroke engine (1996). As he explains, the exhaust gases flowing throughout an engine is considered an unsteady flow (1996: 49). This means that the pressure, temperature, and gas-particle velocity vary with time. So, the exhaust flow behaves as an unsteady flow because of the rapid pressure changes in the cylinder. This gives an exhaust pipe pressure that changes with time (Blair 1996:49).
We already know about the existence of one kind of pressure wave of low amplitude, precisely the acoustic or sound waves. Understanding the fundamental principles and motion of these waves is key to understanding much larger amplitude waves. As mentioned by Blair (1996:52), there are two types of waves: compression and expansion.
A compression wave, of pressure pe> p0, moving at a certain velocity also moves gas
parti-cles in the same direction it is being propagated. On the other hand, expansion waves, of pressure pi < p0, moving at a certain velocity moves gas particles in the opposite direction than the one
towards it is being propagated. It is important to mention that the velocity at which the gas parti-cles move is not the same as the velocity at which the compression or expansion waves propagate. (Blair 1996:52)
22 Exhaust System
Moreover, we have to understand not only the propagation of pressure waves but also the way they reflect when they encounter obstacles. As Blair (1996) explains, reflected pressure waves can arise from several sources, but we will focus our study on what happens inside the exhaust duct. These obstacles include junctions at the open or closed end of a pipe and also from changes in area (gradual or sudden), within a pipe. The following subchapters present various properties and behaviours of pressure waves from Blair’s perspective (1996).
3.1.1 Reflection of Pressure Wave at a Closed end in a Pipe
According to Blair (1996:91), when a pressure wave arrives at the closed end of a pipe, it is re-flected as an exact image of itself moving in the opposite direction. As he recalls, this phenomenon is a "classic echo situation".
3.1.2 Reflection of Pressure Wave at an Open end in a Pipe
When a pressure wave encounters an open-ended pipe, it reacts differently with a compression wave or with an expansion wave, as explained below.
The reflection of a compression wave is an expansion wave which impels particles opposite to its direction of propagation. This means that the exhaust pulse arriving at an open end pipe sends a suction reflection back towards the engine. Later, this phenomenon will help extract the gas particles further down the cylinder and away from the engine (Blair 1996:92-93).
The reflection of an expansion wave is, as predicted by Blair, a compression wave. This phenomenon, if timed correctly to arrive at the intake port while the intake is still in progress, will push air back into the cylinder (Blair 1996:95).
3.1.3 Reflection of Pressure Waves at Sudden Area Changes
The behaviour of a flow approaching a sudden area change resembles, very simplistically, the two cases mentioned before.
A sudden enlargement works just like a slightly less-effective "open-end" where an incident compression wave is reflected as an expansion wave with a higher pressure ratio.The onward moving pressure wave is also as a compression wave but with reduced pressure ratio. For an incident expansion wave, the behaviour resembles a less-effective "open-end", which, compared to a "perfect" bellmouth open end, produces a weaker reflected pressure ratio; the onward moving wave is an expansion wave with diminished pressure ratio.
On the other hand, the sudden contraction works like a partially "closed-end", where the reflected wave maintains its type, partial "echo" of the incident wave. The onward moving wave is also the same type as the incident wave and in either case a higher pressure ratio is registered (Blair 1996:100-101).
3.2 Design & Pressure Waves 23
3.1.4 Reflection of Pressure Waves in Tapered Pipes
Comparable to what we studied in 3.1.3, a tapered pipe works similarly to a sudden change in the area. The difference is exactly how the area change is carried. Instead of a sudden change, there is a gradual one, which is more efficient as a reflector of a wave. Besides more efficient, the reflection of the wave is also spread out in terms of both length and time, which allows engine tuning to be more pronounced and over a wider speed range (Blair 1996: 124-125).
3.2
Design & Pressure Waves
Once the physics behind pressure waves was understood, designers could create more efficient exhausts. Regarding what was mentioned earlier in Blair’s reference (1996: 124-126), Bell (1999: 71) also affirms that a diffuser is a relatively efficient energy inverter. The pressure wave coming from the engine is a compression wave which is reflected as an expansion wave. This reflected wave creates a vacuum that can reach pressures as low as 6 psi. Again, this wave is effective in removing gas out of the cylinder and pulling fresh air from the crankshaft (Bell 1999: 71).
Keeping the fuel and air mixture from going straight from the crankshaft and cylinder to the exhaust without burning is as important as removing the exhaust gases from the cylinder. The idea behind the prevention of this problem was to add a nozzle. According to the author, this turned out to be the "real breakthrough" in two-stroke exhaust design. These exhausts are now commonly called expansion chambers. The nozzle will work exactly like a closed-end pipe, and as studied by Blair, the reflected wave retains its type and, if timed correctly, helps to push the fuel and air mixture back from the exhaust to the cylinder (Bell 1999: 71). On the next chapter, we will follow the suggested procedure to design an expansion chamber for different engines.
3.3
Bell’s Method
All the following calculations were based on the work developed by Bell (1999: 64-87). Firstly, the author calculates the length of the exhaust. The length is the distance that goes from the end of the cylinder to the middle point of the nozzle. Since the nozzle does not have a visible "tip" the calculation is done mathematically, as demonstrated in figure3.1.
24 Exhaust System
Figure 3.1: Basic two stroke expansion chamber (Bell 1999: 73).
According to Bell (1999: 72), the length, L, is a function of the exhaust duration and the engine speed at which we want to maximise the power. This calculation is assuming an average exhaust gas speed of 510.54 meters per second, as mentioned in the introduction of Chapter 4.
L=ED× 42545
r pm (3.1)
The next step is to determine the dimensions of the header. Bell (1999: 72) explains that it can only be determined accurately by testing, or as he puts it, "I have never found a formula that works too well, it is much quicker to make an educated guess and work from there". The rule he formulated was to start with the engine size and exhaust port diameter and from there, to calculate the header size. He also made the distinction between single- and multi-stage diffuser, which we will discuss a little further.
Table 3.1: Header Coefficients Engine Size [cc] Single-stage Multi-stage
50-80 8.5-9.5 8-9
100-125 7.8-8.5 6.5-7.5 175-250 7.3-8.3 6.5-7.5
To calculate the header length, we have to multiply the chosen factor and the exhaust port diameter. When choosing the appropriate factor, we should understand the consequence they have on the power curve of the engine. If we want to increase maximum power of the engine we should shorten the header and use the lower coefficient value. On the other hand, if we want more mid range power, we should lengthen the header using a higher coefficient value (Bell 1999: 72-73).
LH= Headercoe f f icient× ExhaustPortdiameter (3.2)
The author warns us that this formulation is only valid for some standard exhaust port diam-eters, as shown in table3.2. When the ports are out of that interval, the header length has to be determined based on the standard port diameter (Bell 1999: 72-74).
3.3 Bell’s Method 25
Table 3.2: Standard Exhaust Port Diameter Cylinder Size [cc] Port Inside Diameter [mm]
62-80 30-32
100 34-37
125 37-40
175 42
250 44-48
One of Bell’s conclusions was that over the years manufacturers realised the advantages of using tapered wall headers. Some of the reasons for the use of this kind of design are the decrease in flow resistance and the increase in the size of the chamber volume, which broadens the power range. Also, and most important of all, according to Bell, this allows the exhaust gases to expand and cool more gently, resulting in a less kinetic pulse energy loss when compared with a straight header (Bell 1999:74).
To calculate the header pipe major inside diameter we used the following rule:
D2 = LH× 2
Cot(H)+ D1 (3.3)
Once the header is dimensioned, the next step is to design the diffuser, which works as a wave inverter, as mentioned in3.1.2. As stated by Bell, the diffuser taper has a considerable influence on the engine’s behaviour.
A shallow taper returns a long duration, low-intensity wave. This translates into a cut of the maximum power output, but it positively boosts mid-range power, allowing the engine to stay in tune over a larger rpm band. On the other hand, a steeply tapered diffuser returns a short duration, high-intensity wave. It increases maximum power output at the expense of narrowing the rpm range.
These two phenomena explained above are easily visualised in figure3.2, which graphically represents the behaviour of two reflected pressure waves over time.
Figure 3.2: Diffuser taper influence on the duration and intensity of the return wave (Bell 1999: 77).
26 Exhaust System
Regarding this matter, Bell proposes the use of certain angles, represented in table3.3. As he writes, "some people build expansion chambers with larger diffuser tapers, but I tend to value good mid-range power and a wide easily managed power band much more than all-out power". (Bell 1999: 76)
Table 3.3: Diffuser Stage Angles Cylinder Displacement [cc] Diffuser Angle [
o]
Single Stage Two Stage Three Stage 50-80 [6.5; 7] 4.5 & 7 4 & 6 & 8 100-125 [6.5; 7.5] 4.5 & 7.5 4.5 & 7 & 9
175 [6.5; 7.5] 4.5 & 7 4.5 & 7 & 10 250 [7; 7.5] 4.5 & 7 4.5 & 7 & 10
Once the diffuser’s angle or angles are decided, we need to define its length. The diffuser length is determined by the diameter to which it expands. Bell suggests it should normally be 2.5 times the exhaust port diameter (Bell 1999: 76). The Diffuser length is calculated accordingly:
LD=
D3− D2
2
×Cot(D) (3.4)
If we decide to build a diffuser with two stages it is important to understand how the length of each stage affects the engine’s performance. Increasing the length of the diffuser’s first stage will broaden the power band, which is used when the engine is too peaky. Contrarily, if the engine lacks peak power decreasing the first diffuser stage length, and consequently lengthening the second, it will be helpful to adjust the engine in this way. (Bell 1999: 78)
Figure3.3is an example of a Rotax exhaust pipe used in a liquid cooled 125ccengine.
Figure 3.3: Rotax two staged diffuser for a liquid cooled 125cc kart (Bell 1999: 79).
After the diffuser is dimensioned, the next step would be to calculate the belly section of the expansion chamber. Naturally, its diameter is the same as the outlet of the diffuser, but we cannot predict its length before designing the baffle cone (reverse cone). After that, the midsection fills the gap to give the chamber its correct length. (Bell 1999: 78)
3.3 Bell’s Method 27
As explained in section3.1.4, the baffle cone reflects a wave of the same sign which pushed the fresh air-fuel mixture back from the exhaust to the cylinder. Bell also establishes a comparison between a flat plate and a cone. He explains that on one hand the rpm range in a flat plate is very narrow and on the other hand a cone extends the duration of the pressure pulse which serves to broaden the engine’s useful power band (Bell 1999: 78). This phenomenon is also explained in section3.1.3.
At this point in the designing process, we have to define the baffle taper. This affects the time duration and the intensity of the reflected pulse, just like it did on the diffuser. On one hand, a short and sharp baffle increases the maximum power output but the engine will lose out mid range performance; also, it is important to note that the engine will tend to cut dead after a couple of hundred revs past the maximum power rpm. On the other hand, a shallow taper reduces the engine’s maximum power output, but it will develop more power at a lower down rpm range and it will rev on well past the maximum power engine speed. This will broaden the engine’s effective power range (Bell 1999: 78-79).
On figure3.4it is possible to visualise this phenomenon. For two different baffle angles, 10 and 12 degrees, and keeping everything else unchanged, the engine power output shows different behaviours. The baffle with 12 degrees, although with a lower power output over a larger rpm range, is able to peak higher compared with the 10 degrees baffle (Bell 1999: 80).
28 Exhaust System
In Bell’s experience (1999: 80), the baffle angles with which he prefers to work are the ones in table3.4. He also mentions they proved to work not only by providing the engine with a good power range but also by not suppressing its maximum power output excessively.
Table 3.4: Baffle Angles Cylinder Sizecc Baffle Angle[o]
50-80 [10.5; 12]
100 [10.5; 12]
125 [9.5; 12]
175 [10; 12]
250 [10; 12]
Once decided the baffle angle, we can proceed to calculate the baffle overall length. This is the function of its major inside diameter and the chosen angle, as demonstrated in equation 3.5.
OLB=
D3
2 ×Cot(B) (3.5)
As mentioned above, equation3.5determines the overall length of the cone which is not that useful when it is directly applied.
The next step is to determine the reflection point of the baffle and work "backwards". Going back to equation3.1, we calculate the length from the cylinder to the baffle reflection point (mid-point). At this point, by adding half the cone’s overall length (which coincides with the mid-point) to the previously calculated sections, we are able to define the parallel section of the chamber.
Equation3.6is used to calculate the length of the parallel section of the chamber (Bell 1999: 81). LP= L − LH+ LD+ OLB 2 (3.6) Bell points out the effort done by manufacturers and tuners trying to broaden the power band on motorcycles and small displacement road race bikes by experimenting with two-stage baffles, unfortunately with no success. Although the results were as promising as expected, small gains were recorded when working with 62cc and 125cc size cylinders (Bell 1999:81). However, we consider that this is still an interesting area to perform some testing since Bell reported some gains in testing with small cylinders.
The last section to be designed is called the stinger. The stinger works as a bleed pipe, as Bell mentions (1999: 81), providing the exhaust with a restriction to the outflow of gases and generating backpressure. Table3.5includes the dimensions that Bell found to be the most appropriate for the different cylinder sizes.
Some manipulation is also possible. For instance, if we want to raise the power output, then a minor reduction in the pipe diameter should do it, although it is advised to be careful. A stinger pipe too small in diameter may result in different problems, such as engine overheating1 and
3.3 Bell’s Method 29
Table 3.5: Stinger Size
Cylinder Size [cc] Stinger Length [mm] Inside Diameter [mm]
[50; 80] [205; 230] [17; 19]
100 [230; 250] [19; 21]
125 [265; 290] [22; 24]
175 [270; 295] [25; 27]
250 [280; 305] [26; 28]
seizure. Bell suggests to only make small changes, and then test the pipe thoroughly before going any smaller. Finally, when deciding the stinger’s size, we can calculate the entire geometry of the exhaust system. Using equation 3.7we are able to calculate the true length of the baffle cone (Bell 1999: 83). T LB= D3− D4 2 ×Cot(B) (3.7)
Chapter 4
Methodology
This chapter presents the steps of our analysis. We will explain the preparation that was done prior to the tests, the variables taken into consideration, as well as the Rotax Trophy regulations.
4.1
Test Bench Trials
The first step was to define the order that was going to be followed for the tests. Many vari-ables were taken into consideration, like the fidelity of the results, proper isolation of the changed variables, but the most restrictive were time and materials available. Since we were dealing with confidential data and difficulty to find material (nitromethane, G102), time was of most impor-tance.
We intended to evaluate the influence of the fuel and of the exhaust system in the engine’s per-formance. These were then defined as the two main categories. Also, the Rotax Max Challenge went through some rule changes due to the feedback received about the engine’s performance. To test out if the goals were achieved, we compared the differences between the 2019 and 2020 engines’ setups.
Regarding the petrol, we tested both the influence of the octane rating and the addition of nitromethane and ether in the mixture, dilutions of 1, 2 and 5 % of nitromethane and 5% of ether.
To test the performance of the exhausts, we did a controlled test with the standard exhaust and compared it with changes that the participants could try to do, with the goal of alerting in case it gave them any kind of advantage in the competition. Also, giving some continuity and verification to chaper3, section3.3, we established a comparison between the Rotax exhaust and the one presented by Bell (1999).
The Rotax Trophy is currently divided into six different categories, namely (1) Micro Academy, (2) Micro, (3) Mini, (4) Junior, (5) Max, (6) and DD2. In this thesis we are only going to focus on three categories: Micro, Mini, and Junior. This decision was reached because low power en-gines or restricted enen-gines have more room and are more sensible to small tweaks than the ones with a higher performance. This is easily visualised in the studies conducted by Duarte (2018) and
32 Methodology
published in his thesis. It is also important to mention that the engines were prepared in accor-dance with the technical regulations of the Rotax Trophy of 2019 and 2020, thus comparing the differences between the two setups as shown in table4.1.
Table 4.1: Technical Regulations
Squish[mm] ECU Silencer Exhaust Manifold[mm] Expansion Chamber
Micro 2019 2.40 666815 260657 18 -2020 273136 Mini 2019 1.50 666815 260658 20 273200 2020 1.20 666818 273137 22 -Junior 2019 1.20 666813 273210 20 273200 2020
To proceed to our tests, we had to prepare the three engines (Junior, Mini, and Micro) and to measure two variables in each one, the engine squish and piston clearance. By doing this, we made sure that other researchers could replicate these tests in the future, using the same parameters, as illustrated in table5.3.
The tests were performed using the software SportsDyno. For each category we followed different configurations because each engine has a different power output and not all of them operate at the same rpm range. For more clarity, we explained the configurations in table4.2.
Table 4.2: Tests Criteria
Range [rpm] Acceleration[r pms ] Test Mode Junior 5000 to 13000
600 Ramp & Full Throttle Mini
5000 to 11000 Micro
Another setup change predicted in the regulations was the substitution of a throttle restrictor with a carburettor restrictor, as demonstrated in both figure4.11and figure4.22.
Figure 4.1: carburettor restrictor (RMC 2020: 16)
1regulamento da rotax 2019
2https://www.spellfame.co.uk/acatalog/Dellorto
4.1 Test Bench Trials 33
Figure 4.2: Throttle restrictor 2019 regulation
To ensure the fidelity of the results, we defined a minimal number of five tests for each ele-ment of our analysis, precisely fuel, exhaust system, and setup configurations. The data was then exported to Excel, where we analysed the average result of the two most recent tests. However, only the power curve was considered for our study, since it is directly related to the torque curve.
In this way, using the mentioned method and the existing test bench at Korridas, several tests were carried out to assess the influence of various parameters on the functioning of a two-stroke engine, which will be explained in the following chapter.
