ABSTRACT
Many relief scenarios involve the discharge of a two-phase fluid mixture, and the proper method for sizing the relief valve for these conditions is the subject of considerable discussion. Sizing a valve is based on the flow through an isentropic nozzle, the pressure’density relation for the fluid properties, and a discharge coefficient (Kd) to match the calculated mass flux to that measured for the flow of air or water in the actual valve. For single-phase flow, this is straightforward, since the fluid properties are simple and measured values of Kd are available. For two-phase flow, the density’pressure relation is complex and no values of Kd are available, so a variety of ”models” have been proposed in the literature to address this problem. Since the various models produce various results, the appropriate value of Kd required to match the model to the actual valve will depend on the model. This paper utilizes a simple, rigorous method for sizing relief valves for two-phase flow that utilizes the fluid properties directly and hence does not require a ”model” for these properties. It is shown how this method can be applied to two-phase frozen or flashing (equilibrium or non-equilibrium) nozzle flows, and how the available values for Kd for single-phase flow can be used directly with this method, depending on the critical state of flow in the nozzle, to accurately predict two-phase flow in any valve. The calculations are compared with data from the literature for frozen air/water and flashing steam/water flows in actual safety relief valves.
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LIST OF TABLES
TABLE TITLE PAGE
Table 4.1: Processing Time (in hours) of Bread for Different Production Line 24
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LIST OF FIGURES
FIGURE TITLE PAGE
Figure 2.1: Two Stroke Engine (Anon, n.d.) 3
Figure 2.2 : Uniflow Scavenging (Anon, 2010) 5
Figure 2.3 : Spark Plug (Ofria, n.d.) 11
Figure 2.4 : Fouling of Spark Plug (Spamsafe, 2010) 12
Figure 4.1: UTAR Logo 24
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LIST OF SYMBOLS / ABBREVIATIONS
cp specific heat capacity, J/(kg’K)
h height, m
Kd discharge coefficient
M mass flow rate, kg/s
P pressure, kPa
Pb back pressure, kPa
R mass flow rate ratio
T temperature, K
v specific volume, m3
‘ homogeneous void fraction
‘ pressure ratio
‘ density, kg/m3
‘ compressible flow parameter
ID inner diameter, m
MAP maximum allowable pressure, kPa
MAWP maximum allowable working pressure, kPa
OD outer diameter, m
RV relief valve
LIST OF APPENDICES
APPENDIX TITLE PAGE
A Graphs 14
B Computer Programme Listing 15
INTRODUCTION
Background
Internal combustion engine is important to our daily lives. It plays the major role in simpler machinery like grass cutter and lawnmower, as well as complex like automobiles and motor boats. Furthermore, it had always been the major course of study in engineering field for achieving better perfomance. Generally, internal combustion engine had mainly categorized into two stroke engine and four stroke engine. Both engine is in fact having the similar working principle; however, as two stroke engine fire once every revolution, it offered twice the power given both are same in sizing. As the title suggested, only two stroke engine will be discussed henceforth.
Scavenging is the process of removal of exhaust gas which results from the combustion from the combustion chamber to the exhaust channel. Efficient scavenging means that all exhaust gas to be remove but not remain in the cylinder and interfere with the subsequent combustion. As such, it is one of the essential process in the engine as it intermediate between both up and down stroke. Factors which could induced good scavenging is mainly the exhaust valve. This could be further analysed for parameters like valve placement, valve size and valve type. Nevertheless, there is still numerous issue which indirectly promise good scavenging like cylinder head design and spark plug location.
Problem Statement
Previous attempt to develop a cylinder head for a small uniflow scavenged two stroke engine had ended up with many compromise and non-reliability. The ignition spark plug location was not ideally and practically suited. With so many compromises, the exhaust system was not reliable as it develop major problem soon after the testing on the engine was conducted.
Objectives
To design and develop a new cylinder head for optimum operation of a two stroke engine.
Project Schedule
LITERATURE REVIEW
Introduction to Two Stroke Engine
Two stroke engine had its working process distributed into two stroke, which is the up stroke and down stroke. Figure 2.1 below had better illustrate the basic component as well as the operation of the engine.
Figure 2.1: Two Stroke Engine (Anon, n.d.)
From figure 2.1, it shown the operation of up stroke and down stroke. During the down stroke, piston travel downward and uncovered the transfer port and exhaust port. Exhaust gas remove via exhaust port, and air fuel mixture enter into swept volume via transfer port. That will prepare the needs for next cycle of compression and combustion. It is then the start of up stroke.
During the up stroke, piston move upward to top dead center for compression of air fuel mixture and later combustion. Since most air had consume up in the compression, pressure reduce in the volume beneath the piston. Such pressure difference makes the intake valve open and feed in the air fuel mixture to crankcase. The mixture feed in are one of the force in pushing the piston upward. Another force that pushing piston upward is combustion of mixure which comes after the downstroke cycle.
Also, exhaust port covered up by the piston instantaneously when it travel up then mixture would not leak out through the exhaust port. After the ignition of spark plug, compressed air fuel mixture begin the combustion as the fire travelled across them. Next, the outcomes is rising pressure and temperature in the top dead center. This pressure will push the piston downward to bottom dead center. That is then the down stroke start.
While the piston is reciprocating back and forth, the connecting rod beneath will transfer the linear motion into rotational motion via the crankshaft. The force is then used to move the vehicles as well as other applications.
Uniflow Scavenging
Scavenging is the process of removal of exhaust gas which results from the combustion from the combustion chamber to the exhaust channel by fresh charge from intake. Importance of scavenging had been discussed in previous section. Scavenging in two stroke engine mainly categorized into three types, that is cross scavenging, loop scavenging and uniflow scavenging. Uniflow scavenging are the most efficient among the three.
Figure 2.2 : Uniflow Scavenging (Anon, 2010)
As the name implies, there is only one direction for flow of air fuel mixture and exhaust gas in uniflow scavenging. Air fuel mixture enter the cylinder at the bottom dead center. During compression, air fuel mixture travel vertically upward. After combustion, exhaust gas instantly leaves the cylinder via top valve. It gives best scavenging efficiency mainly due to its simplicity, direct exhaust and design of exhaust channel which did not hold up exhaust gas inside the cylinder.
As compare with other method, its main disadvantage is the added in exhaust valve. Such addition inevitably increasing the complexity, cost of manufacturing, size and weight. These in fact minus out the real advantages of two stroke engine, which are simplicity and lower cost. Therefore, it is not much seen in the conventional two stroke engine unless there are substantial needs.
Combustion
Combustion is the main purpose for engine to be designed as it is the only way to extract the energy effectively from air fuel mixture for further application. If scavenging is the breathing process, compression is the chewing then combustion is like the digestion. An efficient combustion will ensure most energy from air fuel mixture is extracted and give minimum waste. For effective combustion, correct ratio of air fuel mixture known as stoichiometric is important for quality of combustion and energy produced.
In spark ignition engine, fire start from the ignition of spark on the air fuel mixture. On the other hand, compression ignition engine rely on high pressure result from compression to make the air fuel mixture combusted automatically. Two stroke engine had mainly practice the spark ignition.
First, the spark plug would produce the spark, and energy in this spark will raise ambient temperature up to several thousand Kelvin. This value is far greater than air fuel mixture auto ignition temperature. Such hot environment will raise the internal energy of the air fuel mixture, followed by breaking of bonding between the carbon and hydrogen and oxidized into carbon dioxide and hot steam. It is then the fire or combustion start.
Compression beforehand purposed for feeding the mixture near the spark plug. Other than that, rising pressure makes the mixture vaporized but not combust yet. Then, it will travel faster in upward and helps in establish the initial flame front. In fact, combustion happens very fast which could be seen as instantly. Once the flame front setup, it will propagate to the nearest flammable material, which is unburned mixture. Furthermore, heat may also transfer via radiation and convection. Layer by layer, unburned mixture will rapidly burned by the fire throughout whole combustion chamber. Velocity of the flame front are reported between 20 m/s to 50 m/s. (Blair 1996)
In fact, combustion start on vaporize air fuel mixture before piston reach top dead center. Therefore, most combustion would happened when the piston reach top dead center, in other words, piston virtually stop and mixture is compacted to minimum volume. The following combustion will hence force the piston travel downward, and crankshaft will rotated in the next cycle of 360 degree. Now, combustion is fully completed.
In case of spark timing, it happens just before piston reach top dead center. Spark ignition timing is commonly advance before top dead center (BTDC). That is, several crankcase rotational degree before reach top dead center and before the combustion chamber achieve the minimum size. For ignition is carried out to expand combustion chamber, in other words, force the piston travel downward and transmit the force to crankshaft. Thus, it is meaningless for spark to happen after top dead center (ATDC), as that will cause performance reduce and engine knock. In short, fire needs times to grow as well as propagate although the time is short. Such time maybe short compare to naked eyes but consider engine runnning thousand revolution per minute, several second seem relatively longer. In practice, advancing of spark timing is more common in larger cylinder engine. Since cylinder is bigger, air fuel mixture is greater in volume and thus fire needs more time to travel and complete the combustion.
When engine revolution per minute increase, piston reciprocate faster and lesser time for spark to respond, fire to grow and start. However, air fuel mixture needs the same time for burning except beyond 7000 rpm which turbulence further added from squish band. Therefore, spark timing advance is practical for increment of engine speed.
Air fuel Ratio for Combustion
It is important for maintain the ratio in stoichiometric which is 14.7:1. However, there is various consideration for engine to run in different ratio. In combustion of excessive air is known as lean burning. Such burning allows lesser emission of hydrocarbons since it combust more completely of the fuel. In stoichiometric condition, some fuel may not have meet up with the air properly for the combustion and thus waste generated. Excessive air of lean condition will hinder this problem from happen. Also, lean burn could reduce throttling loss.
There is engine which designed for utilize lean burn effectively, and it provide certain advantages like higher compression ratio, efficient fuel use and lower emission of hydrocarbon. In this engine, direct fuel injection is usually employed.
However, there is also disadvatanges when air fuel ratio becomes over lean. Value like 16.7:1 could posed some problem as well. For spark in lean mixture, fuel vapour burned by spark is too little for heat to propagate and transmit the flame. Subsequently, layers of mixture remain unburned. That is the condition known as misfiring.
It is advisable for running mixture in rich, for value like 13:1 especially in turbocharged engine. Turbocharger increases the density of air and thus making denser air fuel mixture. Since it is denser, peak cylinder pressure is raised in compression and chance for engine knocking is boosted. If lean mixture is used instead, temperature of exhaust gas will increase furthermore. This would raise again the possibility of knocking as heat need to dissipates faster for hot spot not forming.
In turbocharged engine, there are ways to minus out the knocking under full load. These are reduce the turbocharged boost, running richer mixture and retard ignition timing.
On the other hand, there is also disadvantage posed from running mixture over rich like 12.5:1. Since air is lesser, fire may not grow on the time of ignition. Then, it makes the problem of misfiring again.
Knocking
Knocking or better known as detonation is the phenomenon where unburned layer of mixture had reach its auto ignition temperature and thus combust even before the flame front from the upper portion of cylinder reach them. Such combustion had raise temperature and pressure of the region to very high with non-uniformly direction. As such, sound of knocking is heard from the engine and there goes the name knocking. Perfomance dropping is the most direct experience.
Countless reason may account for knocking. One of it is high compression ratio. Reducing the clearance volume, will increase the compression ratio. Pressure is increase further and thus when the spark ignites, flame front will travel faster. Then heat will propagate faster through both convection and radiation to the unburned layer of mixture. Mixture which accumulates the heat may reach its auto ignition temperature before flame front above reach them.
Since it is an unexpected detonation, and heat release is not achieved as designed. Then, it leave locally hot spot on piston and cylinder wall. Subsequently, this provide great chance for next knocking and the cycle goes on more severely until mechanical failure like piston wear or cylinder wall damaged.
The direct method is use low compression ratio, as it decrease pre combustion temperature and pressure. But that would be undesirable as performance dropping due to lower thermal efficiency. Other ways are increase octane rating of fuel, stratified charged, squish effect and improve of scavenging.
Feed of Air Fuel Mixture
In common two stroke engine, air fuel mixture feed into cylinder has undergone vaporization via the hot crankcase. Remaining of mixture vaporize again during compression. During combustion, components in combustion chamber are vapour, air and exhaust gas residual from previous combustion. This is homogenous combustion.
In homogeneous combustion, air fuel ratio is maintained to stoichiometric for stable combustion. However, it limits the engine efficiency. If lean burn adopt, it cause unstable combustion and emission of nitrogen oxides.
In case of direct fuel injection engine, air fuel mixture is feed into cylinder directly and no contact point with hot crankcase. Then, all vaporization would happen in compression. This is stratified combustion. It gives different air fuel ratio regionally in the mixture. When air fuel mixture feed into cylinder in such way, it provides higher compression ratio but no knocking happen. That is because air fuel mixture is not injected until the point of combustion, therefore knocking will not possible.
Spark Plug
Spark plug is an electrical device found in spark ignition engine. It create electric spark which ignite the compressed air fuel mixture for combustion. The operation is based on the open circuit principle, where it is well known that no current will flow in the open circuit. This condition will break under certain condition, that is when relatively smaller gap existed and higher voltage presented in the connecting point. Then, higher voltage will jump across this small gap and complete the circuit.
Figure 2.3 : Spark Plug (Ofria, n.d.)
Inside spark plug, there are two electrode where separated with a small gap as discussed and figure shown. Center electrode connected to high voltage source through ignition wires. The other electrode will ground to the engine. Ignition wires had designed to handle high voltage and high temperature without fail. During the time high voltage flow through spark plug and later center electrode and then jump across the gap to ground electrode, electrical spark created and ignite the compressed air fuel mixture within the cylinder.
Gap of electrode is important as too short will cause weak spark and even lead to misfiring. That means combustion not carried out. Gap too big may misfire as there may not enough voltage to jump and create the spark. Modern spark plug has gap value of 0.7 to 1.7 mm. Bottom of spark plug is threaded for screw and mount to the engine tightly. The part span threaded part until the grounded electrode known as reach. Reach with over length will cause spark plug hit by piston. While too short cause the spark plug to run relatively lower temperature than usual and later fouling of spark plug.
Fouling is build up of impurities include fuel or carbon at the insulator nose that holds the center electrode. Fouling will get worst as times goes. That is because impurities will accumulate when no proper cleaning or replacement of spark plug. Initially, engine may show misfiring and difficult acceleration and later ceases to fire. For carbon build up, as it is conductive, current will eventually flow through it and the external shell and later to the ground electrode. Thus, there will be no electric current for the creation of spark and dead cylinder as not functioning piston.
Figure 2.4 : Fouling of Spark Plug (Spamsafe, 2010)
Spark start the flame on vaporize fuel. However, in winter climate or lower temperature region, compression might not raise the temperature high enough for vaporize. In order to troubleshoot this, operating value for capacitor discharge ignition system typically set at 20 kV and 4 ??s. The high voltage and short responsive time will give chance to spark plug to produce the spark even when electrodes of spark plug are covered in liquid fuel.
Spark Plug Location
Location of spark plug directly affects engine performance. It is well known that spark plug should place as close to center of mixture compressed as possible. Then, spark would create fire which need shortest time and distance to propagate regarding all direction.
However, if spark plug located in centre of cylinder bore, there need greater surface to volume ratio. Also, there is risk of spark plug overheating and caused knocking problem if it is too near to piston. Nevertheless, there are designs permit location of spark plug to not locate in center of cylinder bore.
Figure 2. , shown an example of spark plug locate in side of cylinder head. It is known as wedge type combustion chamber. Exhaust valve is slanting from vertical. In this design, surface area is smaller, thus it make waste of mixture lesser. Therefore, emission of hydrocarbon will reduced.
Figure 2. shown pentroof hemi engine, that because it is hemisphere shaped. Again surface area is smaller compare to flat cylinder head and it gives less heat to remove and pressure will raised higher in compression phase. Valves are located at the side and incline to vertical as well. This gives more space for valves and achieves better scavenging.
Squish
From figure 2. , squish is shown for direct the mixture towards the central and main ignition point. This reduces mixture from contained far from ignition point which induce pre ignition from radiated heat wave. As mention earlier, one way to reduce knocking in two stroke engine is addition of squish. Other than that, squish also help combustion by increase turbulence speed for air fuel mixture to violently mix up. Turbulence increase due to rapid strike on mixture from piston travel upward and it squeeze the mixture out rapidly from the small area. Thus, more portion of fuel would meet up with air and combust effectively.
Small portion of mixture conceal in area between squish band and piston was exerted the compression heating. However, as concealed, it is secured from radiation heat. Even then, heat would transmit toward the relatively cooler piston. Subsequently, the mixture will still burned when the fire propagate as piston travel down. (Jennings 1973)
In practice squish band is made about 50 % of the cylinder bore area. Area between piston and cylinder head must be remain in order for avoid collision and contact between them. Still, such area has to be small enough as to avoid heat radiated to it during combustion and value practically used is less than 0.06 inch. In small engine, there is also reported to be 0.015 inch.
Surface to Volume Ratio
Surface to volume ratio is important in combustion chamber design. There are still deposits of mixture which stick to the metal surfaces and not combust even with combustion chamber combustion chamber completely exposed to flame front. Their temperature is cooled by the relatively cooler cylinder head or piston. Thus, they never ignite. Subsequently, they will escape from combustion chamber via exhaust port with fresh charge push to them. In terms of surface to volume ratio, best combustion chamber would be simple hemisphere shape without squish band or contour changes. Therefore, it is advisable to kept as small as possible. (Jennings 1973)
METHODOLOGY
Introduction
This chapter center on overall methodology taken during design of cylinder head. In this project, main objective is to redesign the cylinder head. Relevant literature and documents have been revised regarding this issue and presented in previous section. Such information and knowledge had been engaged in order to minimize the possible encountered error and mistake. Other than that, it gives useful guidelines to the solution of the project.
In order to better understanding of the issue arise from the senior work, project team had taken thorough study on their report as well. That is, performance of engine had drop and significant power reduced as according to them. From their report, there are numerous reasons proposed, however significant reason may come from the improper valve lash as also stated in their report.
Also, project team carried out on field study on subject engine. Crack had found in exhaust valve holder sidewall which can be refers from senior report. Previous work stated that it is due to overturning moment of cam follower which results in external pushing force in the exhaust port. While such force remains unsolved, eventually exhaust holder sidewall is cracked. That lies down another problem for the project.
Design Process
Several attempt on redesign of cylinder head and exhaust system began since project took. However, there are various issue that need solved in order to proceed.
It is difficult to abandon the use of cam after serious study. There is no better alternative than cam to control opening and closing of valve. Electrical actuator and motor are considered as well. Such ideas abolish thinking that electrical or electronic component are not strength of project team and time is very limited in this project.
Spark plug position is deemed to locate in center because it gives better combustion and that is fundamental for scavenging. Bad combustion make bad scavenging. Then, exhaust valve can only put in side of cylinder head. Several restraints included, like size of valve, port timing and location of valve. This forced the team to make completely new design.
There is no practical testing on the design, therefore the design must show its effect in software simulation. Theoretical further addition may or may not work. This means there is no significant meaning for further added in since it can never be tested by current testing method. Since so many restraint, safer bet is follow senior work. Finalised design is slot type combustion chamber. It is considered applicable with no problem as senior done the testing for it. In regarding other element that can added, squish band was considered.
Piston angle measured and reported to be 2 degree. Refer Two-Stroke Tuner’s handbook by Gordon Jennings, around 50% of squish band is advisable. Consider the purpose of engine is grass cutting and value suggested by the author may refer to motorbike racing engine. Further online study shown street motorbike may use around 30%. Moreover, subject engine shown is slot type design around 30% squish band. Then, dimension of piston and cylinder head was measured. Next, 30% of squish band decided. Squish band distributed to two sides of slot type combustion chamber. The area is get by trial and error in Solidworks to save some time on mathematics.
CAD design
Before proceed to simulation, distribution of forces in cylinder head had to be understand. Combustion pressure raised from combustion of mixture. It exerted on whole bottom surface of combustion chamber in all direction. Cylinder head is clamped to cylinder body with four allen screw. Next, exhaust channel clamped on top surface is exerting the clamping force downward. These forces will act as reaction forces to support the cylinder head from combustion pressure.
Next, it is calculation of combustion pressure. Indicated mean effective pressure is considered for this. Quality of combustion like amount of pressure rise due to combustion is well reflected through this number. It is possible to obtained via direct measure on the engine. Unfortunately, no engine testing is carried out. Therefore, only way is estimate using formula. (Blair 1996)
imep=bmep+fmep (3. )
Where:
Imep = indicated mean effective pressure, Pa
Bmep = brake mean effective pressure, Pa
Fmep = friction mean effective pressure, Pa
From the equation 3. , it is shown that brake mean effective pressure is the remainder from indicated mean effective pressure (imep) after piston had done the work against friction mean effective pressure (fmep). Friction mean effective pressure include factors like effective sliding velocity of piston, oil viscosity and pumping loss from crankcase. This value can be obtained from book named The Two-Stroke Cycle Engine – Its development, design and characteristics by Eran Sher and John Heywood.
fmep=a+b (N/1000)+ c (‘N/1000)’^2 (3. )
Where:
A = Friction constant between piston rings and cylinder walls
B = Shear which subject to engine speed
C = Loss from turbulent dissipation which subject to square of engine speed.
In journal named Theoretical Limits of Scaling-Down Internal Combustion Engines by Eran Sher and Ilai Sher, a is 190 kPa , b is -20 kPa min and c is 3.6 kPa min2.Then, refer to senior report, N is recorded as 3500 rpm as their peak brake mean effective pressure (bmep). Calculation of fmep is shown.
fmep=190+(-20 ??3500/1000 )+ (3.6 ??3500/1000 )
=190+(-20 ??3500/1000 )+ (3.6 ??3500/1000 )^2
=164.1 kPa
From senior report, bmep is 103.55 kPa at 3500 rpm. Thus imep can be identified as:
imep=103.55+164.1=267.65 kPa=2.6765 bar
Material for cylinder head and exhaust design mainly follow senior guidance. That is plain carbon steel except the exhaust valve and valve seat is titanium.
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RESULTS AND DISCUSSION
Simulation result
Since the main concern lies on the cylinder head, it suggest the needs for simulation merely on cylinder head. As such, exhaust system on top surface has to represent in some ways. Clamping force against the cylinder head and gravitational force is considered for it.
Clamping force= (Torque )/(friction constant of screw ??nominal diameter) (4. )
Where:
Clamping force = force applied on both connected surface, N
Torque = Tightening torque comes with respective screw, N m
Nominal diameter = major diameter of screw, m
Friction constant of screw = friction from geometry, material and condition of screw
Equation 4. obtained via engineersedge.com and other values can be obtain from company catalogue, torque 7.23 Nm, diameter 4.83 mm comes from calcuttayellowpages.com and friction constant 0.15 as condition ‘lubricated screw’ may obtain from catalogue of Fastenal.com. Calculation is shown.
clamping force= 7.23/(0.15 ??4.83 ‘??10’^(-3) )
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=9980 N
However, value showed in catalogue is 7962 N. While compare other company catalogue, given data usually lower than calculated value. It suggests company might have conduct further practical experiment or other tolerance to obtain the more accurate value. Therefore, value provided from catalogue is used instead.
Clamping force for side plate is calculated used same equation as well. Value obtain from catalogue of spaenaur.com.
clamping force= 34/(0.15 ??9.53 ?? ’10’^(-3) )
= 23785 N
Figure 4. show the pressure applied to all the region where combustion would take place. Then, von mises stress with value 0.237 MPa of respective region displayed.
First simulation is done on assembly file without cylinder body and other component like cam shaft to simplify the simulation. Simulation based on closing of exhaust valve. It is done to understand the behaviour of cylinder head while interact with exhaust system during combustion. Exhaust channel show minimal changes under such pressure except region near the exhaust valve. However, in order to fully understand cylinder head respond, simulation is done on merely cylinder head also.
From figure 4. , maximum von mises stress 0.949 MPa at exhaust opening in cylinder head. Bottom surface had experience zero von mises stress. Compare to yield strength of the material 220.59 MPa, this show signs of safe for pressure exerted on the cylinder head. Higher value of stress is located against exhaust valve and wall of second exhaust channel as expected. This pressure is concentrated on that since exhaust valve is closed while combustion take place.
Maximum value of displacement is only 0.000 02314 mm. Practically, there is no displacement since such value could never seen in naked eyes. It show consistent result in both displacement and von mises stress. Connection point of exhaust valve with second channel show concentration of the stress, then displacement is present as well.
Performance of the design
Cylinder head is redesign for optimum operation of the engine as the title suggested. However, there is no practical testing on the design. Therefore, performance data is not possible acquire. In such case, has to attempt with theoretical equation. Compression ratio is the foremost value in concerning the performance since it is a simple engine without turbocharged or supercharged.
compressio ratio= V_(C + V_S )/V_C (4. )
Where:
VC = clearance volume, mm3
VS = swept volume, mm3
swept volume= ?? (‘bore radius )’^2 ??length of stroke (4. )
= ?? ?? ’18’^2 ??30
=30536.28 mm3
compressio ratio= (20789.57+30536.28)/20789.57
=2.5
Clearance volume is get by added up all exhaust channel volume after cylinder head together with original combustion chamber in cylinder head. That is because exhaust valve has place very far from cylinder head. The increase in clearance volume directly lowered the compression ratio. Under such value, it is definite performance is very low.
Other than assumption from compression value, combustion quality may derive from the design. First, mixture enters the long exhaust channel in compression phase via opening in cylinder head. It suggests fire has to propagate on longer path. Fire from spark plug propagates follow mixture into exhaust channel and return down again to piston since exhaust valve is closed. When there is a need to reciprocate like this, it delay time for combustion on reaching piston and driven force on piston also lesser. Next, pressure raised not that much in rather high clearance volume that also said value of combustion force is not that high.
Somehow, regarding combustion on mixture in slot of cylinder head, it gives better quality. That is because spark plug focus in center of mixture. Fire can propagate evenly to all direction. Also, squish band is added in. Mixture compress against squish band will produce turbulent form into center of bore. This turbulent drives air and fuel to mix up thoroughly and more fuel would burn. That effectively reduces fuel waste.
During design process, compromises always exist between area like space consumed, location of exhaust valve and location of spark plug. Why the compromise important? That is because there is a desperate need to strike a balance between performance and scavenging. Generally, they affect each other. Good performance must be comes from good scavenging. Under such idea, exhaust system has top priority. If exhaust valve was put in middle, there will be no possible place for spark plug. Because spark plug reached its minimum size follow senior guidance after their conduct on field testing. Again, exhaust valve cannot be smaller since there is also limitation on port size for exhaust gas to remove completely. This suggest the solution of two exhaust valve.
Additional channel designed might be possible solution although it seems there are still rooms for improvement. That means it still has problems on performance and reliability on installing such channel on top of cylinder head. Despite unexpected result obtained, problems had further identified and it lays foundation for possible future solution.
CONCLUSION AND RECOMMENDATIONS
Subsection Title 1
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REFERENCES
Ahmad, F.B. and William, P., 1998. Rheological properties of sago starch. Journal of Agriculture and Food Chemistry, 46 (1), pp. 4060 ‘ 4065.
Bakshi, A., Patnaik, P.R. and Gupta, J.K., 1992a. Pullulanase and ‘-amylase production by a Bacillus cereus isolate. Letters in Applied Microbiology, 14, pp. 210 ‘ 213.
APPENDICES
APPENDIX A: Graphs
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APPENDIX B: Computer Programme Listing
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