Exploration of Homogeneous Charge Compression. Ignition in a 100 cc 2-Stroke Motorcycle Engine

Exploration of Homogeneous Charge Compression Ignition in a 100 cc 2-Stroke Motorcycle Engine by Yi-Hann Chen B.S. (National Chung-Hsing University, Taiwan) 2001 A thesis submitted in partial satisfaction of the requirements for the degree of Master of Science in Engineering Mechanical Engineering in the GRADUATE DIVISION of the UNIVERSTY OF CALIFORNIA, BERKELEY Committee in Charge: Professor Jyh-Yuah Chen, Chair Professor Robert W. Dibble Professor Catherine P. Koshland Spring 2003

Table of Contents Chapter 1: Scope of Thesis 1.1 Motivation 1.2 Abstract 1.3 Organization 9 9 10 Chapter 2: Fundamentals of Two Stroke Engines 2.1 Introduction 2.2 Components 2.3 Interaction 2.4 Advantages and Disadvantages 11 12 13 13 Chapter 3: Fundamentals of HCCI Engines 3.1 Introduction 3.2 Kinetics 3.3 Advantages 3.4 Disadvantages and Limitations 14 14 15 17 Chapter 4: Test Engine / Experimental Equipments 4.1 Test Engine 4.2 Experimental Equipments A. Electric Motor B. Butterfly Valve C. Safety Devices D. Thermocouples E. Fuel System 19 20 20 20 22 23 24 2

Chapter 5: Engine Operation 5.1 Normal Gasoline as Fuel 5.2 Ethyl Ether-Gasoline as Fuel 5.3 Nitromethane-Gasoline as Fuel 26 27 28 Chapter 6: Engine Performance 6.1 Discussion of Testing Results 29 6.2 Comparisons between HCCI and SI ---Power output and temperatures 30 6.3 Comparisons between HCCI and SI ---Fuel Consumption 6.4 Discussion of Comparisons between HCCI and SI 35 38 Chapter 7: Emission Analysis 7.1 Introduction 7.2 Emission Data 7.3 Discussion of Emission Results 39 41 41 Chapter 8: Numerical Simulation 8.1 Introduction 8.2 Well-Mixed Reactor (WMR) 8.3 Discussion of Results of WMR 8.4 KIVA3V 8.5 Discussion of Results of KIVA 47 47 49 55 56 Chapter 9: Conclusions / Future Work 9.1 Conclusions 9.2 Future Work 61 62 3

9.3 Acknowledgement 62 References 63 Appendices 1. Schematics of EGR valve (a) and (b) 2. Tables of comparisons between HCCI and SI modes 3. Input file of the fuel mixture for WMR code 4. Input file of the engine information for WMR code 65 66 71 71 5. Chemical mechanisms of nitromethane and iso-octane for WMR code 6. Input file of 0% butterfly valve blockage of KIVA3V 7. Input file of 50% butterfly valve blockage of KIVA3V 72 100 104 8. Input file for modeling temperature distribution of KIVA3V 108 4

List of Figures Figure 2.1: Fuel intake phase of a 2-stroke engine Figure 2.2: Compression phase of a 2-stroke engine Figure 2.3: Basic components of a two-stroke engine Figure 3.1: HCCI Simulation Figure 4.1: Electric motor and its pulley connected to the engine Figure 4.2: Control panel of electric motor Figure 4.3: Butterfly valve fully opened Figure 4.4: Butterfly valve fully closed Figure 4.5: Control box controlling the turning of EGR valve Figure 4.6: Gears, motor and valve Figure 4.7: Control box with switches (opened) Figure 4.8: Control box (closed) Figure 4.9: Air supply valve Figure 4.10: Wired thermocouple on the cooling fin of cylinder head Figure 4.11: Thermocouple behind the butterfly valve Figure 4.12(a): Switch for different fuel tanks Figure 4.12(b): Fuel tanks Figure 4.12(c): Fuel supply system Figure 5.1: Platform of engine 11 11 12 15 20 20 21 21 22 22 23 23 23 24 24 24 24 25 26 Figure 6.1: Engine cylinder head temperature vs. butterfly valve blockage under different nitromethane concentration 28 5

Figure 6.2: Comparison of HCCI and SI combustions (same throttle) Figure 6.3: Comparison of HCCI and SI combustions (same power) Figure 6.4: Ratio of power output Figure 6.5: Comparison of HCCI and SI combustion (same throttle) Figure 6.6: Comparison of HCCI and SI combustion (same power) Figure 6.7: Ratio of power output Figure 6.8: Weight scale for fuel consumption Figure 6.9: Fuel consumption under HCCI and SI combustion 32 32 33 33 34 34 35 36 Figure 6.10: Relationship between specific fuel consumption and engine speed Figure 6.11: Fuel consumption under HCCI and SI combustion 36 37 Figure 6.12: Relationship between specific fuel consumption and engine speed Figure 7.1: Horiba gas analyzer Figure 7.2: Cold-ice bath Figure 7.3: Emission test tube Figure 7.4: Filter for taking off dirty oil and particles Figure 7.5: Entire set of emission test Figure 7.6(a): Total unburned hydrocarbon in the exhaust stream Figure 7.6(b): Ratio of oxygen in the exhaust stream (SI/HCCI) Figure 7.6(c): CO in the exhaust stream Figure 7.6(d): CO2 in the exhaust stream Figure 7.6(e): NOx in the exhaust stream 37 40 40 41 41 41 43 43 44 44 45 6

Figure 7.7: Difference of the sum of T.HC, CO and CO2 between HCCI and SI combustions Figure 8.1: Well-mixed reactor of 2-stroke engine Figure 8.2: Temperature vs. crank angle degrees by WMR Figure 8.3: Pressure vs. crank angle degrees by WMR 46 48 49 49 Figure 8.4 to 8.10: Start of combustion vs. nitromethane concentration under different EGR by WMR in various engine speeds Figure 8.11: Evolution of CH3NO2 vs. CAD during HCCI Figure 8.12: Predicted evolution of HO2 vs. CAD during HCCI Figure 8.13(a): Meshes of engine under 0% butterfly valve blockage Figure 8.13(b): Meshes of engine under 50% butterfly valve blockage Figure 8.14: Temperature Distribution by KIVA (Piston is located at BDC) 50~53 54 54 57 57 58 Figure 8.15: Relationship between butterfly valve blockage and temperature predicted by KIVA3V 60 7

List of Tables Table 4.1: Specifications of the Kymco 2-stroke engine Table 6.1: Test result with 7.5% nitromethane in gasoline Table 6.2: Test result with 9% nitromethane in gasoline Table 7.1: Sum of CO, CO2 and total unburned hydrocarbon (T.HC) 19 29 30 46 8

CHAPTER 1 Scope of Thesis 1.1 Motivation Small two-stroke engines are widely used in Asia and some parts of Europe for transportation due to low cost, simple construction, good mobility, and little parking space required. Hazardous pollutants, such as unburned hydrocarbon, CO, NOx, and soot are the main concerns in the increasing use of small 2-stroke engines. It is imperative for combustion engineers to lower the emissions and at the same time to improve the efficiency of small 2-stroke engines. Homogeneous Charge Compression Ignition (HCCI) [1, 2, 3] is an emerging technology in this area because the homogeneous fuel mixture results in less soot and lower temperature and the highly diluted fuel mixtures leads to lower NOx. The Honda Motorcycle Company has pioneered HCCI technology in small 2-stroke engines [4]. However, diluted fuel mixture results in lower power density. More importantly, it is difficult to control the timing of combustion timing to stabilize HCCI. It s desirable to investigate if a stock small 2-stroke Spark-Ignition (SI) scooter engine can sustain HCCI to achieve lower emissions while maintaining good efficiency. It s also interesting to compare the performances with different combustion modes. 1.2 Abstract This thesis describes research efforts in modifying a small 2-stroke SI gasoline engine to operate under HCCI mode. The main objective is to determine the potential in 9

reducing pollution from small 2-stroke engines by operating them under HCCI. Comparisons of engine performances between HCCI and SI, including exhaust emissions and power output, are conducted to illustrate the potential benefits of HCCI. Numerical simulations with a Well-Mixed Reactor (WMR) [5] and the KIVA3V code [6] are performed to gain further understanding of HCCI in small 2-stroke engines. 1.3 Organization This thesis is organized into eight main sections. Chapters 2 and 3 introduce the fundamentals of SI and HCCI engines respectively. Chapter 4 contains details of the test engine and experimental equipments. Chapter 5 describes the testing procedures and the approach to achieve HCCI for the small 2-stroke engine, followed by Chapter 6, which discusses the engine performances and presents comparisons between HCCI and SI mode. Chapter 7 presents the emission results. Chapter 8 discusses two numerical simulations that provide a deeper understanding of the combustion process under different situations. The last chapter contains conclusions and future work to be explored on the 2-stroke motorcycle engine. 10

CHAPTER 2 Fundamentals of 2-Stroke Engines 2.1 Introduction As shown in Figures 2.1 and 2.2, two-stroke engines are internal combustion engines with two separate operating strokes: (a) Power stroke (intake) and (b) compression stroke. In contrast to 4-stroke engines which fire once every other revolution, 2-stroke engines fire once each revolution giving 2-stroke engines a significant power boost. [7] Figure 2.1 Fuel intake phase of a 2-stroke engine Figure 2.2 Compression phase of a 2-stroke engine 11

2.2 Components As illustrated in Figure 2.3, there are eight basic components of a 2-stroke engine identified as follows: 1) Combustion chamber, an enclosure in which combustion is initiated and controlled; 2) Spark plug, a device that ignites the fuel mixture by means of an electric spark; 3) Piston, solid cylinder or disk that fits snugly into a larger cylinder and moves under fluid pressure; 4) Reed Valve, a valve that allows fluid to flow in one direction; 5) Fuel intake, a port in which the fuel mixture is introduced to the combustion chamber; 6) Fuel, usually gasoline; 7) Crack case, the enclosure of the crank shaft and fuel; and 8) the Exhaust outlet, the port in which the byproducts of combustion are removed from the combustion chamber [8]. Figure 2.3 Basic components of a two-stroke engine 12

2.3 Interaction As shown in Figures 2.1 and 2.2, the crankcase of a 2-stroke engine is sealed, and the downward motion of the piston is used to slightly pressurize the air-fuel mixture in the crankcase. During the latter part of the power stroke, the piston uncovers first the exhaust port, allowing the exhaust gases to be partially expelled, and then the intake port, allowing the fresh air-fuel mixture to rush in and drive most of the remaining gases out of the cylinder. This mixture is then compressed as the piston moves upward during the compression stroke and is subsequently ignited by a spark plug [9]. 2.4 Advantages and Disadvantages In comparison with 4-stroke engines, simpler construction, cheaper cost, lower weight, higher power-to-weight and power-to-volume ratios are several advantages of 2-stroke engines. Excessive pollution is the main disadvantage that restricts the use of 2-stroke engines in mainstream applications, such as in a car. Two-stroke engines are generally less efficient than their four-stroke counterparts because of the incomplete expulsion of the exhaust gases. Another area of concern is the lubricant oil which is mixed with fuel. The oil makes all 2-stroke engines smoky to some extent, and a badly worn 2-stroke engine can emit huge clouds of oily smoke. Also, the partial expulsion of the fresh air-fuel mixture along with the exhaust gas causes an even great environmental concern [10]. 13

CHAPTER 3 Fundamentals of HCCI Engines 3.1 Introduction HCCI engines can be regarded as a combination of SI and Compression-Ignition (CI) engines. As in a SI engine, the fuel and air are mixed homogeneously before entering into the combustion chamber. As in a CI engine, the air-fuel mixture is compressed to high pressure and temperature leading to auto-ignition without a spark plug. Unlike CI engines, HCCI engines use premixed air-fuel mixtures rather than injection of fuel into compressed air. Thus, HCCI engines have less tendency to form soot. 3.2 Kinetics Figure 3.1 presents a visual summarization of the different phases of HCCI: (a) the intake contains a premixed charge; (b) the charge is compressed and in-homogeneities in the air-fuel ratio will lead to temperature difference due to the difference in the heat capacity; (c) the charge is further compressed leading to auto-ignition; and (d) the combustion continues very rapidly throughout the chamber [11]. 14

(a) Intake of the premixed charge (b) The charge is being compressed (c) Further compression leads to auto-ignition Figure 3.1 (a) to (d): HCCI Simulation (d) Combustion continues 3.3 Advantages In some regards, HCCI incorporates the best features of both SI and CI engines. Like a SI engine, the charge of HCCI engines is well mixed which minimizes particulate emissions. Similar to a diesel engine, the HCCI engine is ignited by compression, with a high compression ratio leading to high thermal efficiency. However, unlike either conventional SI or CI engines, combustion occurs simultaneously throughout the cylinder volume rather than in the flame front. HCCI engines have the potential to be lower cost 15

than diesel engines because a HCCI engine can be run with a lower pressure fuel-injection system [12]. It is now generally agreed that HCCI combustion is dominated by local chemical reaction rates, with no requirement for flame propagation. If a truly homogenous mixture exists at the time of combustion, turbulence has little direct effect on HCCI combustion, but it may have an indirect effect by altering the temperature distribution and the boundary layer thickness within the cylinder [13]. Small temperature differences inside the cylinder can have a considerable effect on the combustion because combustion chemistry is very sensitive to temperature. As a result, heat transfer and mixing are important in forming the right condition for the charge prior to ignition. However, these two factors play a secondary role during the HCCI combustion process itself because HCCI combustion is very rapid [14]. Turbulence introduces great complexity to the analysis of spark-plug ignited and diesel engines. The fact that HCCI combustion is not very sensitive to turbulence makes it possible to develop a thorough, accurate numerical method of HCCI combustion. The insensitivity to turbulence constitutes another great advantage for HCCI engines, since a numerical analysis then becomes a very powerful tool to advance the technology. HCCI combustion can be analyzed with better accuracy than ever achieved for spark-ignited or diesel engines. Furthermore, numerical analyses can be used as an important tool in the design of control strategies for HCCI engines [15]. 16

3.4 Disadvantages and Limitations One of the most difficult problems associated with practical implementation of HCCI is the fact that it lacks a direct control of combustion timing. Both SI and CI engines have direct control of combustion timing through spark timing and fuel injection timing, respectively. However, for HCCI engines, combustion timing is a complicated function of equivalence ratio, fuel octane number, residual gas fraction, intake temperature, compression ratio, valve timing, and intake pressure. At some operating points, a HCCI engine can even become unstable, in the sense that with constant operating parameters the combustion event drifts towards progressively earlier combustion causing hotter combustion chamber walls, which in turn makes combustion occur even earlier [16]. HCCI engines rely on indirect control methods, such as intake mixture properties, including temperature, pressure, fuel equivalence ratio, and the amount of Exhaust Gas Recirculation (EGR). The mixture properties are adjusted such that auto-ignition occurs near the Top Dead Center (TDC) by controlling the amount of EGR or preheating intake mixtures [17]. For small 2-stroke engines, EGR is the preferred choice due to minor modifications required. Two kinds of EGR, external and internal, are used for getting a sufficiently high temperature inside the combustion chamber. External EGR uses a pipe to re-circulate part of the exhaust gases to the intake and the amount of EGR is controlled by an EGR valve inside the pipe. Internal EGR can be achieved by using a butterfly valve inserted inside the exhaust pipe directly to control the amount of EGR. Other methods may include controlling of valve timing which is much more complicated. How to control the EGR valve or butterfly valve to make auto-ignition occur near the TDC is a main issue in optimizing HCCI engines. 17

Today in Asia, the hazardous emissions of 2-stroke engines are the issues of most concern. For HCCI engines, the lower combustion temperature can reduce the NOx emission below that of SI engines, as observed from previous research. The high levels of unburned hydrocarbon emission from HCCI engines remain a major problem. The experimental results from this research of a small 2-stroke motorcycle engine are intended to demonstrate whether or not HCCI combustion has lower emissions than SI and to help us get deeper understanding about the difference between SI and HCCI combustion in small 2-stroke engines. 18

CHAPTER 4 Test Engine / Experimental Equipments 4.1 Test Engine The test engine for this research is a Kymco 2-stroke 97.4 cc scooter SI engine imported from Taiwan. The engine specifications are listed in Table 4.1 [18]. A whole summer was spent setting up the engine on a platform and several months were taken to prepare the engine to operate properly with safety devices in the SI combustion configuration. A control panel was built to enable the operator to manage the throttle and safety devices at a safe distance from the engine. Table 4.1 Specifications of the Kymco 2-stroke engine Swept Volume 97.4cc Bore 50mm Stroke 49.6mm Compression ratio 7.3 Idle speed 1800±100 rpm* Emission (Particles) 15% Emission (CO) Less than 4.5% Emission (HC) Less than 7000ppm** *rpm: revolutions per minute **ppm: particles per million 19

4.2 Experimental Equipments A. Electric Motor As displayed in Figure 4.1, an electric motor served as a load to the engine. To get the desired speed, the frequency of the electric motor can be adjusted from its control panel shown in Figure 4.2. When a higher gas mixture temperature inside the combustion chamber is needed to get auto-ignition, the throttle of the engine must be opened wider. At the same time the engine speed would increase if the engine load is kept the same. The speed control will automatically increase the load of the motor such that the speed is kept constant. Figure 4.1 Electric motor and its pulley connected to the engine Figure 4.2 Control panel of electric motor B. Butterfly Valve Based on previous research and experiments for HCCI combustion, EGR can be used to increase the intake charge temperature and at the same time reduce the amount of NOx created by the engine [19]. EGR dilutes the air-fuel mixture with a small amount of residual gases. Since there is no pipe for external EGR in the Kymco 2-stroke engine, a butterfly valve is used to block the exhaust pipe to create internal EGR to get auto-ignition. For 20

this research, the exhaust pipe of the engine was cut and a butterfly valve was inserted near the outlet of combustion chamber. As shown in Figures 4.3 and 4.4, a stainless butterfly valve was machined to fit the modified exhaust pipe. The butterfly valve is attached to a shaft which has a gear set driven by a 12 volts DC motor. The motor is connected to a box controlling the turning of the valve as displayed in Figures 4.5 and 4.6. The butterfly valve (Schematics of the butterfly valve are presented in Appendix 1) is used to control the amount of exhaust gases trapped inside the combustion chamber. The trapped exhaust gases mix with fresh charge and raise the temperature to get auto-ignition. Some exhaust gas leakage is visible from outside at places where the butterfly valve was set. To reduce the leakage, a high temperature sealant was applied to the joint. As demonstrated in Figures 4.3 and 4.4, the exhaust pipe itself was insulated by a combination of fiberglass insulation and aluminum tape. This insulation was intended to minimize the heat losses from the hot exhaust gases to the outside environment, thus keeping the combustion chamber gases at a sufficiently high temperature for triggering auto-ignition. Figure 4.3 Butterfly valve fully opened Figure 4.4 Butterfly valve fully closed (Around the pipe are fiberglass and aluminum tape for insulation) 21

Figure 4.5 Control box controlling the turning of butterfly valve Figure 4.6 Gears, motor and valve (Outside the valve is high temperature sealant) C. Safety Devices In the process of switching from SI mode to HCCI for the small 2-stroke engine, safety is a major concern since this engine was designed for SI combustion only. To address safety concerns, a couple of relays and several switches were used to control the electric motor, the spark plug, and the main power supply. An emergency stop button was installed to shut off the entire electrical system immediately. The switch for the spark plug was originally used to cut off the current going through the plug when the operator desires to change the engine from SI to HCCI. While the engine is operated in SI, this switch can be used as a safety device to shut down the engine. As shown in Figures 4.7 and 4.8, the relays and switches were enclosed in a control box so that the operator can set controls easily. As displayed in Figure 4.9, an air supply valve could be used to cut off the air flowing into the carburetor. Because the fuel and air are mixed in the carburetor for this particular engine, a valve could be used manually to control the air flow to the carburetor. When the valve is closed, the gas mixture inside the cylinder would become too rich and the engine would stop gradually. The disadvantage of using this device is that the 22

engine would be flooded when the air supply was cut off. It would take a while to purge out the excessive fuel before the engine can be run again. Figure 4.7 Control box with switches (opened) Figure 4.8 Control box (closed) Figure 4.9 Air supply valve D. Thermocouples Two OMEGA K-Type thermocouples were mounted at two different locations to monitor the temperature correlations between HCCI and SI mode. As shown in Figure 4.10, one thermocouple is mounted on the fin of cylinder head. From this thermocouple, the impact of changing from SI to HCCI on cylinder head temperature can be determined. As the Kymco engine is air-cooled, the cylinder head temperature is directly correlated with the combustion temperature. The other thermocouple was mounted inside the exhaust pipe behind the butterfly valve as displayed in Figure 4.11. This thermocouple monitors the exhaust gases temperature right behind the butterfly valve and provides an estimated temperature of internal EGR used to promote auto-ignition. Although the 23

exhaust pipe is insulated, there are still heat losses to the ambient. Therefore, one needs to consider this factor when correlating the estimated internal EGR temperature difference between HCCI and SI mode. Figure 4.10 Wired thermocouple on the cooling fin of cylinder head Figure 4.11 Thermocouple behind the butterfly valve E. Fuel System Due to the low compression ratio and the specific design of the Kymco 2-stroke engine, this engine could not be run under HCCI with normal gasoline even with internal EGR. Thus explosive additives were mixed with gasoline to explore if HCCI combustion could occur. As presented in Figures 4.12(a), (b), and (c), two fuel tanks and a 2-way switch were used for preparing different fuel mixtures. Figure 4.12(a) Two-way switch for different fuel tanks Figure 4.12(b) Fuel tanks 24

Figure 4.12(c) Fuel supply system 25

CHAPTER 5 Engine Operation 5.1 Normal Gasoline as Fuel As illustrated in Figure 5.1, after the Kymco 2-stroke engine had been operated properly on the platform with all control switches and safety devices, attempts at getting HCCI combustion were conducted. The temperature behind the butterfly valve was used to determine if HCCI occurred or not since the existence of combustion would lead to high exhaust gases temperature. During the engine testing, the SI combustion was operated first to achieve sufficiently high temperature in the exhaust gases. Next the spark is switched off and the butterfly valve is activated to get exhaust gases trapped inside the cylinder. If auto-ignition does not occur, the temperature of exhaust gases behind the butterfly valve will drop rapidly. This event can also be recognized by the sound of engine. Figure 5.1 Platform of engine 26

Normal gasoline (Octane number is 91) was tested as the fuel with different percentages of butterfly valve blockages and engine speeds. Although a high internal EGR temperature (around 623K) was obtained under certain conditions, the 2-stroke engine was unable to sustain HCCI combustion. This is probably due to the low compression ratio and specific design of the Kymco 2-stroke engine. Given time constraints, it was decided not to change the compression ratio by making different kinds of cylinder or changing the shape of combustion chamber. Also, there are concerns about the capacity of the electric motor. If the engine were run at a higher compression ratio, a higher load could be placed on the motor. If the motor is subjected to a too high torque, it would stop automatically for protection. To reduce the torque to the electric motor, the original pulley was replaced by a smaller one. With this modification, the throttle of the engine could be opened wider to get higher engine speeds and internal EGR temperatures. Although a higher internal EGR temperature (around 673K) could be obtained, the Kymco 2-stroke engine still would not sustain HCCI with normal gasoline as fuel. 5.2 Ethyl Ether-Gasoline as Fuel Changing fuel type was another option to achieve HCCI based on the consideration of chemical mechanisms and combustion reactions, two fuel blends were tried. First, ethyl ether (C2H5OC2H5) was mixed with gasoline with different proportions (10% and 15%). The monatomic oxygen (O) in the ethyl ether was thought to promote auto-ignition. Several tests for 10% and 15% ethyl ether-gasoline fuel mixtures were tried but failed to achieve HCCI. Higher concentrations of ethyl ether were not attempted due to safety concerns. 27

5.3 Nitromethane-Gasoline as Fuel Nitromethane (CH3NO2) was tried next because nitromethane is commonly used as racing fuel. The first attempt was conducted with a fuel mixture of 9% nitromethane in gasoline. A successful HCCI combustion was demonstrated. However when the engine cylinder heated up to a sufficiently high temperature, the engine achieved HCCI even without the assistance of internal EGR. It was determined that a fuel mixture with 9% nitromethane is too explosive for the 2-stroke engine to achieve controllable HCCI. Lower nitromethane concentrations of 7.5% and 5 % were tested subsequently. From several trial runs, a fuel mixture of 5% nitromethane was found too low to sustain steady HCCI. It was determined that 7.5% nitromethane concentration was needed to achieve HCCI with the assistance of internal EGR. 28

CHAPTER 6 Engine Performance 6.1 Discussion of Testing Results Test results are summarized in Tables 6.1 and 6.2 with Figure 6.1 for fuel mixtures with 7.5% and 9% nitromethane in gasoline. As revealed in Figure 6.1, the cylinder head temperature increases rapidly when the butterfly valve is set more than 60%. As the Kymco 2-stroke engine is cooled by air, the engine cylinder head temperature is directly correlated to the combustion temperature. This observed trend is expected because more exhaust gases are trapped inside the combustion chamber when the butterfly valve blockage was set higher. It is interesting to notice in Figure 6.1 that with the 7.5% nitromethane-gasoline fuel mixture, a steady HCCI combustion couldn t be sustained with butterfly valve blockage less than 60% as indicated by a dashed line. It is believed that the temperature inside the combustion chamber did not get high enough for steady auto-ignition. A. 7.5% Nitromethane Table 6.1 Test results with 7.5% nitromethane in gasoline Butterfly valve Engine cylinder head Observation blockage (%) temperature(ºc) 50 105 Ran HCCI only seconds 60 116 Ran HCCI 5 minutes 70 120 Steady HCCI 80 124 Steady HCCI 90 127 Steady HCCI Condition: 1. Frequency of electric motor: 25.95Hz 2. Engine speed: 2281RPM 29

B. 9% Nitromethane Table 6.2 Test results with 9% nitromethane in gasoline Butterfly valve Engine cylinder head Observation blockage (%) temperature(ºc) 0 113 Steady HCCI 20 112 Steady HCCI 40 111 Steady HCCI 50 112 Steady HCCI 60 114 Steady HCCI 70 116 Steady HCCI 80 118 Steady HCCI 90 122 Steady HCCI Condition: 1. Frequency of electric motor: 25.95Hz 2. Engine speed: 2290RPM Test results of different nitromethane concentration Engine cylinder head temperature (ºC) 130 125 120 115 110 105 100 0 20 40 60 80 100 Butterfly Valve Blockage (%) 7.5% 9% Figure 6.1 Engine cylinder head temperature vs. butterfly valve blockage under different nitromethane concentration 6.2 Comparisons between HCCI and SI---Power Output and Temperatures Because the dynamics of exhaust pipe were slightly modified when samples of emissions were taken, HCCI and SI combustion are compared both before and after the 30

sampling system was included. The load of the electric motor is correlated with the power output of HCCI and SI since no suitable dynamometer can be set in the small 2-stroke engine. For each test, HCCI and SI are compared by setting the same throttle level. HCCI is also compared with SI under the same power output. Details of how and where the dynamics of exhaust pipe were changed are discussed thoroughly in Chapter 7. For the purpose of comparison, all tests were based on 7.5% nitromethane-gasoline fuel mixture. The butterfly valve blockage was set at 85% closure for HCCI combustion and all the temperatures were taken at steady state. Graphs of comparisons are presented in Figure 6.2 to 6.7. (Detailed data are listed in Appendix 2) As seen from the figures, the differences between HCCI and SI under varied engine speeds can be noted for several parameters. Under the same throttle setting, Figures 6.2 and 6.5 compare power output and temperatures at the cylinder head as well as in the exhaust. First, the cylinder head temperatures under HCCI are seen higher than those under SI for all engine speeds tested here, indicating higher combustion temperatures under HCCI than SI. Second, the exhaust temperatures under HCCI are lower than SI, attributed to the combined effects of earlier combustion and more work under HCCI than SI. Third, HCCI combustion has a higher power output than SI. At first this seems inconsistent with intuition because the internal EGR dilutes the fuel mixture while the piston is at the Bottom Dead Center (BDC) lowering the power output. Figures 6.4 and 6.7 present the ratio of power output between HCCI and SI before and after the exhaust pipe is modified for sampling emissions. It is clearly shown in the figures that more power output (5~20%) is produced under HCCI than SI. Under the same power output, Figures 6.3 and 6.6 demonstrate that in comparison with SI, HCCI has a higher engine cylinder head temperature but a lower exhaust gas temperature behind the butterfly valve. This is consistent with the comparisons obtained under the same throttle setting. 31

A. Before setting up emission test system Comparison of HCCI and SI combustions (same throttle) Temperature (ºC) 330 280 230 180 130 80 2250 2350 2450 2550 2650 Engine Speed (RPM ) 60 55 50 45 40 35 30 25 20 Electric Motor Load (%) Temperature Behind Butterfly V alve ( C ) - HCCI mode Temperature Behind Butterfly Valve ( C ) - SI mode Cylinder Head Temperature ( C ) - HCCI mode Cylinder Head Temperature ( C ) - SI mode Pow er Output (%) - HCCI mode Pow er Output (%) -SI mode Figure 6.2 Comparison of HCCI and SI combustion (same throttle) Comparison of HCCI and SI combustions (same power) Temperature (ºC) 280 240 200 160 120 80 2250 2350 2450 2550 2650 Engine Speed (RPM) Temperature Behind Butterfly Valve ( C ) - HCCI mode Temperature Behind Butterfly Valve ( C ) - SI mode Cylinder Head Temperature ( C ) - HCCI mode Cylinder Head Temperature ( C ) - SI mode Figure 6.3 Comparison of HCCI and SI combustion (same power) 32

Ratio of Motor Load (HCCI / SI) 1.25 1.2 Ratio 1.15 1.1 1.05 1 2250 2300 2350 2400 2450 2500 2550 2600 Engine Speed (RPM) Figure 6.4 Ratio of power output B. After setting up emission test system Comparison of HCCI and SI combustions (same throttle) 330 60 Temperature (ºC) 280 230 180 130 80 2250 2350 2450 2550 2650 Engine Speed (RPM) 55 50 45 40 35 30 25 20 Electric Motor Load (%) Temperature Behind Butterfly Valve ( C ) - HCCI mode Temperature Behind Butterfly Valve ( C ) - SI mode Cylinder Head Temperature ( C ) - HCCI mode Cylinder Head Temperature ( C ) - SI mode Pow er Output (%) - HCCI mode Pow er Output (%) -SI mode Figure 6.5 Comparison of HCCI and SI combustion (same throttle) 33

Comparison of HCCI and SI combustions (same power) 280 Temperature (ºC) 240 200 160 120 Temperature Behind Butterfly Valve ( C ) - HCCI mode Temperature Behind Butterfly Valve ( C ) - SI mode Cylinder Head Temperature ( C ) - HCCI mode Cylinder Head Temperature ( C ) - SI mode 80 2250 2300 2350 2400 2450 2500 2550 2600 Engine Speed (RPM) Figure 6.6 Comparison of HCCI and SI combustion (same power) Ratio of Motor Load (HCCI / SI) Ratio 1.2 1.18 1.16 1.14 1.12 1.1 1.08 1.06 1.04 1.02 1 2250 2300 2350 2400 2450 2500 2550 2600 Engine Speed (RPM) Figure 6.7 Ratio of power output 34

6.3 Comparisons between HCCI and SI---Fuel Consumption As shown in Figure 6.8, a mechanical weight scale was used to measure the fuel consumption for a fixed period of 5 minutes so that the fuel consumption between HCCI and SI can be compared. HCCI and SI were operated under the same throttle at various speeds. Comparisons of fuel consumptions between HCCI and SI were done before and after the dynamics of exhaust pipe were modified. The power output of HCCI and SI is recorded by the load of electric motor and then used for calculating the Specific Fuel Consumption (SFC) using the following equation: (SFC) SI (SFC) HCCI Fuel consumption Power output Fuel consumption Power output SI HCCI Fuel consumption SI Fuel consumption HCCI Power output HCCI Power output SI (1) All the tests for fuel consumption were conducted with a fuel mixture of 7.5% nitromethane in gasoline and the butterfly valve was set at 85% closure for HCCI. The throttle of the engine was kept at the same position as that used in the previous comparisons between HCCI and SI mode for power output and temperatures. The results of fuel consumption versus engine speed are graphed in Figure 6.9. Corresponding ratios of SFC versus engine speed are plotted in Figure 6.10. The results after the exhaust pipe is modified are presented in Figures 6.11 and 6.12. Figure 6.8 Weight scale for fuel consumption 35

A. Before setting up emission test system Fuel Consumption vs. Engine Speed Fuel Consumption (Kg/min) 0.008 0.0075 0.007 0.0065 0.006 0.0055 0.005 0.0045 0.004 2250 2300 2350 2400 2450 2500 2550 2600 Engine Speed (RPM) HCCI SI Figure 6.9 Fuel consumption under HCCI and SI combustion Specific Fuel Consumption Ratio (SI / HCCI) 1.4 1.35 1.3 Ratio 1.25 1.2 1.15 1.1 1.05 1 2250 2300 2350 2400 2450 2500 2550 2600 Engine Speed (RPM) Figure 6.10 Relationship between specific fuel consumption and engine speed 36

B. After setting up emission test system Fuel Consumption vs. Engine Speed Fuel Consumption (Kg/min) 0.008 0.0075 0.007 0.0065 0.006 0.0055 0.005 0.0045 0.004 2250 2300 2350 2400 2450 2500 2550 2600 Engine Speed (RPM) HCCI SI Figure 6.11 Fuel consumption under HCCI and SI combustion Specific Fuel Consumption Ratio (SI / HCCI) 1.35 1.3 1.25 Ratio 1.2 1.15 1.1 1.05 1 2250 2300 2350 2400 2450 2500 2550 2600 Engine Speed (RPM) Figure 6.12 Relationship between specific fuel consumption and engine speed 37

6.4 Discussion of Comparisons between HCCI and SI From the previous comparison results (Figure 6.2 to 6.7), the power output of HCCI combustion was found higher than SI under the same condition whether or not the exhaust pipe was modified for emission analysis. This observed trend is somewhat surprising and appears inconsistent with intuition. As the internal EGR displaces the volume that would be occupied by the fresh charge, one anticipates that the power output should decrease under HCCI mode [20]. From the results of fuel consumption (Figure 6.9 to 6.12), the HCCI combustion has less specific fuel consumption whether or not the dynamics of exhaust pipe were changed for emission tests. It s believed that the internal EGR inside the combustion chamber causes more complete combustion under HCCI than SI. The observed lower specific fuel consumption under HCCI is a strong evidence explaining why more power output is obtained under HCCI than SI. This point will be further examined in Chapter 7 as emission data are analyzed. 38

CHAPTER 7 Emission Analysis 7.1 Introduction To further understand the performances of the Kymco engine under HCCI and SI mode, measurements of exhaust gases were conducted. By knowing the components in the exhaust gases, one can gain some insight into the combustion process inside the cylinder. Also from the point of view of environmental protection, the exhaust gas analysis is an important factor to determine if an engine can be applied practically to the transportation sector. Five different exhaust gas components including oxygen (O2), carbon monoxide (CO), carbon dioxide (CO2), nitric oxides (NOx), and total unburned hydrocarbon (T.HC) were measured in the exhaust from the 2-stroke motorcycle engine. As shown in Figure 7.1, a Horiba gas analyzer was used for the emission measurements. Hot exhaust gases were routed from the exhaust pipe and immediately brought to a cold-ice bath to condense the water, as displayed in Figure 7.2. The water will absorb a small amount of water-soluble species, such as NO2 but the impact is negligible. This condensation step is necessary to prevent the fouling of the gas analyzers by water vapor. The cooled exhaust gas was then routed to a heated line (kept at 70ºC). The stream is then brought into gas analyzer system by a heated pump (also kept at 70ºC) and through a heated Nafion dryer/particulate matter (PM) filter. The Nafion dryer will further remove moisture from the exhaust stream. The presence of particulate matter in the system will cause blockage to the system s fine capillary tubes, resulting in costly repairs [21]. (More details of Horiba gas analyzer can be also checked in the reference above) 39

Figure 7.1 Horiba gas analyzer Figure 7.2 Cold-ice bath Samples of exhaust gases were taken from the exhaust pipe before the catalyst. There is an existing tube connecting the exhaust pipe to the air-cleaner, as shown in Figure 7.3. This tube was used to get samples of exhaust gases. As mentioned on the previous page, particulate matter in the exhaust stream will cause blockage to the fine capillary tubes, resulting in costly repairs. For the 2-stroke motorcycle engine, dirty oils and particles in the exhaust must be removed. As demonstrated in Figure 7.4, the exhaust gases passed through a fuel filter prior to entering into the Horiba gas analyzer. During the testing, this filter worked well in blocking the particles and dirty oils from going through the Horiba gas analyzer. A picture of the entire emission test system is shown in Figure 7.5. 40

Figure 7.3 Emission test tube Figure 7.4 Filter for taking off dirty oils and particles 7.2 Emission Data Figure 7.5 Entire set of emission test The emission results were taken with the engine running with a fuel mixture of 7.5% nitromethane in gasoline under various engine speeds. For HCCI combustion, the butterfly valve closure was set at 85%. Also, the throttle was kept at the same position as it was in the comparison study between HCCI and SI mode in Chapter 6. 7.3 Discussion of Emission Results The emission results for HCCI and SI of the Kymco 2-stroke engine are displayed in Figure 7.6(a) to (e). Figure 7.6(a) shows that the amount of total unburned hydrocarbon was about 25% lower in HCCI than SI. This result may be attributed to two factors. Because residual gas was trapped inside the cylinder, the unburned hydrocarbon has 41

another chance to be burned again under HCCI. Another possibility is that more complete combustion occurs under HCCI than SI. This result is consistent with the greater power output observed under HCCI mode. Figure 7.6(b) presents the ratio of oxygen levels between SI and HCCI mode. The ratio is greater than unity under all engine speeds inducting that more oxygen was found within the exhaust stream under SI than HCCI. It is also interesting to note that HCCI consumes about 20~25% more oxygen than SI. This result is consistent with the more complete combustion observed under the HCCI mode. In comparison with SI, Figures 7.6(c) and (d) show that HCCI has higher CO2 and lower CO concentration within the exhaust stream. These two figures again support the idea that more complete combustion happens under HCCI combustion. Therefore one expects that more power output can be obtained by HCCI than SI. As presented in Figure 7.6(e), more NOx was found in the exhaust stream of HCCI than SI. This is because higher temperatures were found inside the combustion chamber under HCCI than SI as shown in Figures 6.2 and 6.5. This phenomenon is contradictory to the theory because diluted mixtures ignite later in the combustion cycle giving a lower peak cylinder pressure as well as a lower maximum combustion temperature [22]. Lower combustion temperature leads to lower NOx emissions. By the numerical analysis in Chapter 8, the additive nitromethane makes auto-ignition earlier. The butterfly valve is at more than 80% closure. Both these factors mean a higher combustion temperature in HCCI, thus leading to higher NOx emissions. Incomplete combustion leads to lower combustion temperature [23]; however, as seen in Figure 7.6 (a) to (d), more complete combustion in HCCI was observed. This is a further proof that higher combustion temperature under HCCI in the Kymco 2-stroke engine leads to more NOx emissions. 42

1. T.HC (Total unburned hydrocarbon) Emission Result (T.HC) PPM 15000 14000 13000 12000 11000 10000 9000 8000 7000 6000 5000 2200 2300 2400 2500 2600 HCCI SI Engine Speed (RPM) Figure 7.6(a) Total unburned hydrocarbon in the exhaust stream 2. O2 1.4 Emission Result (O2) (SI / HCCI) 1.35 1.3 Ratio 1.25 1.2 1.15 1.1 1.05 1 2250 2300 2350 2400 2450 2500 2550 2600 Engine Speed (RPM) Figure 7.6(b) Ratio of oxygen in the exhaust stream (SI/HCCI) 43

3. CO Emission Result (CO) PPM 8200 8000 7800 7600 7400 7200 7000 6800 6600 2200 2300 2400 2500 2600 Engine Speed (RPM) HCCI SI Figure 7.6(c) CO in the exhaust stream 4. CO2 Emission Result (CO2) Percentage (%) 8 7.5 7 6.5 6 5.5 5 4.5 4 2200 2300 2400 2500 2600 Engine Speed (RPM) HCCI SI Figure 7.6(d) CO2 in the exhaust stream 44

5. NOx Emission Result (NOx) ppm 510 490 470 450 430 410 390 370 350 2200 2300 2400 2500 2600 Engine Speed (RPM) HCCI SI Figure 7.6(e) NOx in the exhaust stream The sum of CO, CO2 and total unburned hydrocarbon (T. HC) indicate the amount of fuel used by the engine. Comparisons between HCCI and SI are presented in Table 7.1 and Figure 7.7 showing the difference was within ±5%. This difference could be due to both physical changes and measurement uncertainty. From the instruction manual of Horiba gas analyzer, the repeatability for CO and CO2 are both ±1% of full scale. The full scale for CO and CO2 are 3000 ppm and 20% (200,000 ppm), respectively. Hence the repeatability for CO and CO2 are ±30 and ±2000ppm, respectively. As seen in Table 7.1 and Figure 7.7, most points fall within the accuracy bound. This means that the amount of energy carried by the C atom in the fuel was about the same for SI and HCCI in the emission tests. Thus it was meaningful to compare the power output between HCCI and SI mode under the same throttle as done in Chapter 6. 45

Table 7.1 Sum of CO, CO2, and total unburned hydrocarbon (T.HC) Engine Speed (RPM) HCCI (ppm) SI (ppm) Difference (%) 2283 91115 86287 5.298798 2343 87058 85660 1.605826 2385 83394 81395 2.397055 2444 81628 82050-0.51698 2501 80798 82530-2.14362 2540 79748 80309-0.70347 2580 78381 78325 0.071446 Difference between HCCI and SI (%) 6 5 4 3 2 1 0-1 -2-3 T.HC+CO+CO2 2283 2343 2385 2444 2501 2540 2580 Engine Speed (RPM) Figure 7.7 Difference of the sum of T.HC, CO and CO2 between HCCI and SI 46

CHAPTER 8 Numerical Simulation 8.1 Introduction During the course of the investigation, numerical models were used to help us understand the effects of nitromethane and internal EGR on auto-ignition delay. As HCCI is controlled largely by chemical kinetics, a Well-Mixed Reactor (WMR) model is used to simulate detailed chemical kinetics inside the cylinder. As described in Chapter 4, internal EGR is achieved by using a butterfly valve in the exhaust pipe. The fluid dynamics impact of the butterfly valve on the overall temperature at the BDC is an important parameter in setting the experiments. The KIVA3V engine code developed by the Los Alamos Labs was used to model the blockage effect of the butterfly valve on the temperature distribution at the BDC. 8.2 Well-Mixed Reactor (WMR) The well-mixed reactor is used to model Region A in the 2-stroke motorcycle engine as shown in the Figure 8.1. 47

Region A Figure 8.1 Well-mixed reactor of a 2-stroke engine (Region A) To study the effect of nitromethane on the Start Of Combustion (SOC), two chemical mechanisms of nitromethane and gasoline (iso-octane) were combined [24]. The input files of fuel mixture, engine information, and chemical mechanism for WMR are attached in Appendices 3 to 5. The total number of species in the combined mechanism is 298 and the total number of chemical reactions is 912. The compression ratio (CR) set in the WMR was 7.3 and the equivalence ratio was 1.0. The WMR simulations were conducted with engine speeds ranging between 1800 and 3000 rpm. From the WMR simulation results, we can better understand the relationship between temperature, pressure, and the SOC for the HCCI with different nitromethane concentrations and various amount of EGR. Figures 8.2 and 8.3 present typical results showing the relationship between temperature, pressure and Crank Angle Degrees (CAD) under 3000rpm, 0% nitromethane concentration and 47.5% EGR. As seen in the figures, a sharp rise in either temperature or pressure profile signals the auto-ignition event. The SOC is defined arbitrarily at the time when heat release reaches 50% of its peak value. 48

3000 Temperature vs. Crank Angle Degree by WMR Temperature (K) 2500 2000 1500 1000 500 SOC 0-200 -100 0 100 200 Crank Angle Degrees (CAD) Figure 8.2 Temperature vs. crank angle degrees by WMR Pressure vs. Crank Angle Degrees by WMR Pressure (atm) 40 35 30 25 SOC 20 15 10 5 0-200 -150-100 -50 0 50 100 150 200 Crank Angle Degrees (CAD) Figure 8.3 Pressure vs. crank angle degrees by WMR 8.3 Discussion of Results of WMR The predicted SOC versus nitromethane concentration with various amount of EGR under different engine speeds at 1800, 2000, 2200, 2300, 2500, 2750, and 3000 rpm are summarized in Figure 8.4 to 8.10. Several trends are observed from the WMR results. 49

When the amount of EGR inside the combustion chamber is fixed, the higher nitromethane concentration is set, the earlier start of combustion occurs. Ignition Delay (CAD) vs Nitromethane Concentration at 1800RPM (*CAD: Crank Angle Degrees) Ignition Delay (CAD) 12 7 2-30% 5% 10% 15% 20% -8-13 -18 55%EGR 50%EGR 47.5%EGR 45%EGR -23 Nitromethane Concentration (%) Ignition delay (CAD) vs. Nitromethane Concentration at 2000RPM (*CAD: Crank Angle Degrees) Ignition Delay (CAD) 12 7 2-30% 5% 10% 15% 20% -8-13 -18 55%EGR 50%EGR 47.5%EGR 45%EGR -23 Nitromethane Concentration (%) 50

Ignition Delay (CAD) vs. Nitromethane Concentration at 2200RPM (*CAD: Crank Angle Degrees) Ignition Delay (CAD) 12 7 2-30% 5% 10% 15% 20% -8-13 -18-23 Nitromethane Concentration (%) 55%EGR 50%EGR 47.5%EGR 45%EGR Ignition delay (CAD) vs. Nitromethane Concentration at 2300RPM (*CAD: Crank Angle Degrees) Ignition Delay (CAD) 12 7 2-30% 5% 10% 15% 20% -8-13 -18-23 Nitromethane Concentration (%) 55%EGR 50%EGR 47.5%EGR 45%EGR 51

Ignition Delay (CAD) vs. Nitromethane Concentration at 2500RPM (*CAD: Crank Angle Degrees) Ignition Delay (CAD) 12 7 2-30% 5% 10% 15% 20% -8-13 -18-23 Nitromethane Concentration (%) 55%EGR 50%EGR 47.5%EGR 45%EGR Ignition delay (CAD) vs. Nitromethane Concentration at 2750RPM (*CAD: Crank Angle Degrees) Ignition Delay (CAD) 12 7 2-30% 5% 10% 15% 20% -8-13 -18-23 Nitromethane Concentration (%) 55%EGR 50%EGR 47.5%EGR 45%EGR 52

Ignition delay (CAD) vs. Nitromethane Concentration at 3000RPM (*CAD: Crank Angle Degrees) Ignition Delay (CAD) 12 7 2-30% 5% 10% 15% 20% -8-13 -18 55%EGR 50%EGR 47.5%EGR 45%EGR -23 Nitromethane Concentration (%) Figure 8.4 to 8.10: Start of combustion vs. nitromethane concentration under different EGR by WMR in various engine speeds The effects of CH3NO2 on combustion chemical kinetics are illustrated in Figures 8.11 and 8.12 under a typical condition. These two figures show that CH3NO2 decomposes at high pressure and temperature just before the TDC leading to high HO2 concentration prior to auto-ignition. A detailed analysis of chemical path ways led to the following picture. First, CH3NO2 decomposes at high pressure and temperature due to third-body collision via CH3NO2 (+M) CH3 + NO2 (+M) (R894) immediately followed by two reactions CH3 + NO2 CH3O + NO (R840) and NO + HO2 NO2 + OH (R672) Reaction (R840) oxidizes CH3 and Reaction (R672) produces OH radical. Second, the following reactions occur subsequently leading to an overall increase of HO2 and radical pools. 53

CH3O (+M) CH2O + H (+M) CH2O + H HCO + H2 CH2O + OH HCO + H2O HCO + OH H2O + HO2 0.0016 0.0014 CH3NO2 Mole Fraction 0.0012 0.001 0.0008 0.0006 0.0004 0.0002 0-50 -40-30 -20-10 0 10 20 CAD Figure 8.11 Evolution of CH3NO2 vs. CAD during HCCI (x: 10% nitromethane concentration, 47.5% EGR, and 1800rpm) Figure 8.12 Predicted evolution of HO2 vs. CAD during HCCI (+: 0% nitromethane concentration, 47.5% EGR, and 1800rpm) (x: 10% nitromethane concentration, 47.5% EGR, and 1800rpm) 54

When the nitromethane concentration is fixed, the more EGR within the cylinder, the earlier start of combustion happens. This results from the increased temperature effect by EGR. The above results simulated by WMR show us the nitromethane concentration and the amount of EGR affect strongly the combustion timing and chemical reactions inside the combustion chamber. A point which should be noted is that the concentrations of nitromethane used in the numerical simulations were set by the volume ratio in the gas phase. In the experiments, the nitromethane volume ratio was set in the liquid phase. The differences are minor and the qualitative effects of nitromethane and EGR upon auto-ignition are still retained in the simulations. 8.4 KIVA3V KIVA is the major computer code used at ERC (Engine Research Center). Over the years, the researchers at ERC have developed many new physical and chemistry sub-models based on KIVA for engine simulations. These sub-models include the RNG k-ε and LES turbulence model, the fuel injection model, the fuel spray atomization model, the multi-component fuel vaporization model, the spray/wall interaction model, the spark-ignition model, the auto-ignition model, the combustion model, detailed chemistry combustion model, and the soot and NOx emission model, etc. [25] KIVA s pre-processing code is used to generate the meshes of intakes, exhaust port of the cylinder (about 33,000 grids). Here the meshes of half cylinder were generated since the combustion chamber of this engine is symmetrical. KIVA is then used to predict the temperature distributions inside the combustion chamber under the effects of different butterfly valve blockages when the piston is at BDC. 55

8.5 Discussion of Results of KIVA Figures 8.13(a) and (b) show the geometry and meshes of the Kymco 2-stroke single cylinder engine. Details of intake and exhaust ports were obtained from Kymco manufactory. The meshes were generated to match as closely as possible the physical dimensions of the engine. The effect of blockage by the butterfly valve was simulated using different exhaust geometry shapes. For example, two different butterfly blockages (0% and 50%) in the exhaust pipe are shown in these two figures. The input files of KIVA3V for generating different butterfly valve blockages (0% and 50%) are attached in Appendices 6 and 7. In the KIVA simulations, gasoline was used as fuel and a one-step simple chemistry was used due to the complexity of calculation. Since how the butterfly valve blockage affects the temperature distributions inside the combustion chamber was the major objective of the simulation, the combustion was initiated by a spark rather than HCCI. By specifying different percentages of butterfly valve blockage, the temperature distributions can be modeled by KIVA and the file for modeling the temperature distribution is attached in Appendix 8. Figure 8.14(b) to (h) present the predicted temperature distributions in the entire engine including the combustion chamber, air-fuel mixture inlets and exhaust port under different percentages of butterfly valve blockage. As revealed in the figures, the temperature inside the cylinder increases with the percentage of blockage. This computed trend is consistent with our test results (Tables 6.1 and 6.2, Figure 6.1). 56

The meshes of engine under different butterfly valve blockages (A)---Exhaust pipe (B),(C),(D)---Fuel/Air inlets (A) (D) (C) (B) Figure 8.13(a) Meshes of engine under 0% butterfly valve blockage Figure 8.13(b) Meshes of engine under 50% butterfly valve blockage 57

(A)-Exhaust pipe (B),(C),(D)-Fuel/Air inlets TDC-Top Dead Center BDC-Bottom Dead Center TDC (E) (D) 4 Z Z Temp 600 570.909 541.818 512.727 483.636 454.545 425.455 396.364 367.273 338.182 309.091 280 6 (A) 2 Unit of temperature from (b) to (h) is Kelvin (C) (B) BDC 0-4 -2 0 0 X 1 Y Y 3 4 Temp 600 570.909 541.818 512.727 483.636 454.545 425.455 396.364 367.273 338.182 309.091 280 4 0 Temp 600 570.909 541.818 512.727 483.636 454.545 425.455 396.364 367.273 338.182 309.091 280 6 4 Z Z 2 4 (d) The temperature distribution under 40% internal EGR (a) The engine cylinder 6 X 2 2 2 0-4 -4-2 -2 0 0 1 Y 3 4 0 0 X 2 2 1 Y 3 4 4 Temp 600 570.909 541.818 512.727 483.636 454.545 425.455 396.364 367.273 338.182 309.091 280 4 0 Temp 600 570.909 541.818 512.727 483.636 454.545 425.455 396.364 367.273 338.182 309.091 280 6 4 Z Z 2 4 (e) The temperature distribution under 50% internal EGR (b) The temperature distribution under 0% internal EGR 6 X 2 2 2 0-4 -4-2 -2 0 0 1 Y 2 2 3 4 0 0 X 1 Y 4 2 2 3 4 X 4 (f) The temperature distribution under 70% internal EGR (c) The temperature distribution under 20% internal EGR 58

Temp 600 570.909 541.818 512.727 483.636 454.545 425.455 396.364 367.273 338.182 309.091 280 6 4 0 4 Z Z 2 Temp 600 570.909 541.818 512.727 483.636 454.545 425.455 396.364 367.273 338.182 309.091 280 6 2 0-4 -4-2 -2 0 0 1 Y 2 2 3 4 0 0 X 1 Y 4 2 2 3 4 (g) The temperature distribution under 80% internal EGR X 4 (h) The temperature distribution under 90% internal EGR Figure 8.14((a) to (h): Temperature distributions simulated by KIVA (Piston is at BDC) 59

Figure 8.15 presents the relationship between the average temperature inside the combustion chamber at BDC and the butterfly valve blockages simulated by KIVA. This figure shows that the temperature distribution is affected strongly only when the butterfly valve blockage is more than about 80%. Again, the computed trend is consistent with our test results. In our tests, 85% blockage was set. 600 Average temperature vs. Butterfly Valve Blockage predicted by KIVA3V Average temperature (K) 550 500 450 400 350 300 250 200 0 10 20 30 40 50 60 70 80 90 100 Butterfly Valve Blockage (%) Figure 8.15 Relationship between butterfly valve blockage and temperature predicted by KIVA3V 60

CHAPTER 9 Conclusions / Future Work 9.1 Conclusions For the Kymco 2-stroke motorcycle engine, HCCI combustion was achieved with the help of explosive additive, nitromethane. Experiments reveal that 5 to 20% more power output was obtained under HCCI than SI. This phenomenon can be explained with the help of exhaust gas analysis and specific fuel consumption. It was concluded that more complete combustion occurred when the engine was operated under HCCI. In principle, HCCI should have lower NOx emission based on previous HCCI research since EGR lowers the temperature inside the cylinder. However, 1 to 5% more NOx was measured since higher combustion temperatures were recorded. Overall, the present investigation concludes that HCCI is beneficial for the Kymco engine both in terms of power output and emissions. Numerical simulations with WMR helped us understand not only how nitromethane but also EGR affects the start of combustion. When a higher nitromethane concentration was set, the start of combustion occurs earlier due to the chemical effects of CH3NO2. When more EGR was set, the start of combustion also advances simply due to the temperature effect. KIVA helped us realize how the butterfly valve affects the temperature distribution inside the combustion chamber. The temperature distributions simulated by KIVA exhibit higher values when the exhaust pipe is blocked as expected. However, a very high blockage was predicted to be effective as found in the tests. 61

9.2 Future Work It is desirable to measure in-cylinder pressure versus CAD under HCCI and SI. With this information, one can determine SOC and deduce heat release rate. Since it s impossible to add explosive additives into gasoline for practical transportation purposes, research needs to be done in the future to modify the Kymco engine to a higher compression ratio such that it can auto-ignite using normal gasoline as the fuel. Also setting a suitable dynamometer to compare the actual power output between HCCI and SI mode is another area needing work in the future. Because the air pollution from 2-stroke engines is the most important concern for every engine research, more efforts still need to be made for lowering emissions while increasing thermal efficiency. HCCI shows a promising future in realizing this goal. 9.3 Acknowledgement The Kymco engine and many assistances were provided through Mr. Chung-Ying Chen and Mr. Sane-Chen Tseng at Kwang Yang Motor CO. LTD, Taiwan. Their technical assistances are greatly appreciated. 62

References 1. Thring, R., Homogeneous-Charge Compression Ignition Engines, SAE paper 892068 2. Najt, P., Foster, D., Compression-Ignited Homogeneous Charge Combustion, SAE paper 830264 3. Flowers, D., Aceves, S., Smith, R., Torres, J., Girad, J., Dibble, R., HCCI In a CFR Engine: Experiments and Detailed Kinetic Modeling, SAE paper 2000-01-0328 4. Göran Haraldsson, Mapping of a Honda ARC engine, Department of Heat and Power Engineering, Lund Institute of Technology 5. Aceves, S.M., Flower, D.L., Westbrook, C.K., Smith, J.R., Pitz, W.J., Dibble, R.W., Christensen, M., and Johansson, B. (2000) A Multi-Zone Model for Prediction of HCCI Combustion and Emissions. SAE paper 2000-01-0327 6. Engine Research Center (ERC) University of Wisconsin-Madison, http://www.erc.wisc.edu/modeling/modeling_index.htm 7. HowStuffWorks.com Website: http://www.howstuffworks.com/two-stroke1.htm 8. HowStuffWorks.com Website: http://www.howstuffworks.com/two-stroke1.htm 9. Yunus A. Cengel, Micael A. Boles, Thermodynamics: an engineering approach, 4 th Ed. New York: McGraw-Hill Comp, Inc, 2002, pp. 458-459 10. Yunus A. Cengel, Micael A. Boles, [9] above, p.459 11. HCCI Engines Simulations Website: http://www.forbrf.lth.se/~per/per_1.htm 12. Martinez-Farias, J.,Aceves, SM., Flowers, D. Smith, J.R., Au, M., Girard, J. and Dibble, R.,2001, HCCI Combustion: Analysis and Experiments, (SAE Paper 2001-01-2077) p.1, World Wide Web: http://www.osti.gov/hvt/2001-01-2077.pdf 13. Martinez-Farias, J.,Aceves, SM., Flowers, D. Smith, J.R., Au, M., Girard, J. and Dibble, R., [12] above p.2 14. Martinez-Farias, J.,Aceves, SM., Flowers, D. Smith, J.R., Au, M., Girard, J. and Dibble, R., [12] above p.2 15. Martinez-Farias, J.,Aceves, SM., Flowers, D. Smith, J.R., Au, M., Girard, J. and Dibble, R., [12] above p.2 63

16. Per Tunestål, Jan-Ola Olsson, Bengt Johansson, HCCI Operation of a Multi-Cylinder Engine, Lund Institute of Technology, p.1 World Wide Web: http://www.vok.lth.se/ce/personnel/per/publications/ci2001.pdf 17. J.-Y. Chen and R. W. Dibble, Optimization of Homogeneous Charge Compression Ignition wit Genetic Algorithms, Combust. Sci. and Tech., 175: 373-392, 2003 18. Instruction manual of Taiwan Kymco Motorcycle Company 19. Ramadan, Bassem. A Study of EGR Stratification in an Engine Cylinder Kettering University p. 1, Website: http://www.erc.wisc.edu/multi_dimensional/modelingmtng2002/written-7- Ramadan.pdf 20. Homogeneous Charge Compression Ignition The Holy Grail of Internal Combustion Engines but Can we Tame the Beast? Presentation at Windsor Workshop 2000 Transportation Fuels ATF Engine Management Systems Session Toronto, ON June 6,2000 by Jan-Rogen Linna, Richard Stobart, Robert P Wilson, Arthur D. Little Inc http://www.windsorworkshop.ca/downloads/roger.pdf 21. Michael Y. Au, fall 2001, Operation of a 1.9-Liter 4-Cylinder homogeneous charge compression ignition (HCC) engine by means of thermal and exhaust gas recirculation control p.21 22. Michael Y. Au, fall 2001, Operation of a 1.9-Liter 4-Cylinder homogeneous charge compression ignition (HCC) engine by means of thermal and exhaust gas recirculation control p.58 23. Michael Y. Au, fall 2001, Operation of a 1.9-Liter 4-Cylinder homogeneous charge compression ignition (HCC) engine by means of thermal and exhaust gas recirculation control p.58 24. Iso-Octane: Curran, H. J., Gaffuri, P., Pitz, W. J., and Westbrook, C. K. "A Comprehensive Modeling Study of iso-octane Oxidation", Combustion and Flame, volume 129, pp. 253-280 (2002). Nitromethane: Bendtsen AB. Glarborg P. Dam-Johansen K. Low temperature oxidation of methane: the influence of nitrogen oxides. [Article] Combustion Science & Technology. 151:31-71, 2000. 25. Engine Research Center (ERC) University of Wisconsin-Madison, http://www.erc.wisc.edu/modeling/modeling_index.htm 64

Appendices 1. Schematic of EGR valve (a) Schematic of EGR valve (b) 65