Design and Simulation of Two Stroke Engines

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Design and Simulation of Two-Stroke Engines (a) Plan section through transfer ports (b) Elevation section on transfer ports Fig. 3.35 Nomenclature for design for Prog.3.4, LOOP SCAVENGE DESIGN. on the plane of symmetry defined by the distances, MT1 and MT2, respectively. A further factor is the upsweep angle of the main port, UPM. Until the publication of the papers from QUB by Smyth et al. [3.17] and Kenny etal. [3.31], it was suspected that the exiting gas flow from the main ports did not follow the design direction of the port, but deviated from it in some manner. In particular, it was speculated that the plan angle of the exiting fresh charge would be deviated by an angle, El, from the design direction, due to the entrainment or shortcircuiting of the flow with the exiting exhaust gas flow. That the flow might have a deviation of angle, E2, in the vertical direction was regarded as more problematic. Smyth [3.17] showed that the value of El varied from 24.5° to 10.4° from the port opening point to full port opening at bdc, and that the value of E2 varied from 34° to 14° over the same range. As these measurements were taken in a particular engine geometry, the answer from Smyth's experiments cannot be regarded as universal. Nevertheless, as his engine had rather good scavenging for a simple two-port design, it is of the type sketched in Fig. 3.35, with AMI and AM2 values of 50°, and as the values of gas flow deviation quoted are considerable in magnitude, it must be assumed that most engines have somewhat similar behavior. The actual experimental data presented by Kenny et al. [3.31], which was measured by laser doppler velocimetry in an optical access cylinder, are shown in Fig. 3.36. A more extensive discussion of research into this same topic, and for this same engine cylinder, can be found in the paper by McKinley et al. [3.45]. As no designer can predict such gas flow deviation behavior, the recommendations for angles and targets for the main port, which have been demonstrated to provide successful scavenging characteristics, should be followed. In the not too distant future, accurate CFD calculations will replace much of the empirical advice in the succeeding sections. Neverthe- 264
Chapter 3 - Scavenging the Two-Stroke Engine CYLINDER BORE=70 mm OPTICAL ACCESS CYLINDER AM1=50... AM2=50... MT1 =37.5 MT2=11.4 UPM=0 40 60 80 TRANSFER PORT OPENING, % (SEE Fig.3.35 FOR SYMBOLISM) ELEVATION ANGLE E2 Fig. 3.36 Deviation angles from the design direction for loop scavenging. less, as CFD calculations are expensive in terms of computer time, designers will always have a need for some better starting point based more on experimental evidence than intuition. Finally, for those who wish to examine the full extent of porting design seen in two-stroke engines, albeit mostly loop-scavenged two-stroke motorcycle engines, the comprehensive design compendia [3.32] emanating from the Technical University of Graz deserve study as a source of reference on what has been produced. 3.5.5.1 The main transfer port The orientation of the main port, which is the scavenge port immediately beside the exhaust port, is already stated as the designer's first priority and this has some potential for empirical guidance. If one examines the port plan layouts of all of the cylinders known to provide successful scavenging, several factors stand out quite noticeably: (a) The upsweep angle of the main port, UPM, is rarely larger than 10°. (b) The value of AM2 is usually between 50° and 55°. (c) The target point for MT2 is usually between 10% and 15% of the cylinder bore dimension, BO. (d) The target point for MT1 is approximately on the edge of the cylinder bore. (e) The port is tapered to provide an accelerating flow though the port, i.e., AMI is greater than AM2, and AMI is rarely larger than 70°. (f) The larger the angle, AMI, the more the target point, MT1, is inside the cylinder bore. The narrower the angle, AMI, the farther outside the cylinder bore is the target 265 100
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Design and Simulation of Two-Stroke
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A Second Mulled Toast When as a stu
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Acknowledgments As explained in the
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Table of Contents Nomenclature xv C
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Table of Contents 2.10.1 Flow at pi
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Table of Contents 3.5.3.1 The use o
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Table of Contents 6.1.3 The effect
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Nomenclature NAME SYMBOL UNIT (SI)
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speed of rotation speed of rotation
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attenuation or transmission loss wa
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Design and Simulation ofTwo-Stroke
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Page 236 and 237: Design and Simulation of Two-Stroke
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Chapter 4 - Combustion in Two-Strok
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Combustion in compression-ignition
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stroke ari more often lseful lental
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lat the ssi ;tribu- ELEVATION VIEWS
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CYLINDER HEAD TYPE IS CENTRAL BORE,
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CO E 50 40 - O O _i Hi > I CO 30 Z)
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CURRENT INPUT DATA FOR 2-STROKE 'SP
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Cylin- PP £ Pa- tics of Paper Chap
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vfitro- ;titi >tants 1970. i(In- Ch
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£2 = Pi P2 I Pi J Chapter 4 - Comb
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CHAINSAW AT 9600 rpm. Chapter 4- Co
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LLI •z. O N -z. 0.21 -, 0.20 DC 0
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Chapter 7 Reduction of Fuel Consump
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Chapter 7 • Reduction of Fuel Con
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Chapter 7 - Reduction of Fuel Consu
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§ 2000
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y, ,6 Chapter 7 - Reduction of Fuel
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Chapter 7- Reduction of Fuel Consum
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Chapter 8 Reduction of Noise Emissi
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Chapter 8 • Reduction of Noise Em
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Chapter 8 - Reduction of Noise Emis
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• • / . • Appendix Listing of
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Active radical (AR) combustion and
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initial conditions, assumptions reg
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in two-stroke engine (schematic dia
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exhaust timing control valve, effec
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I Exhaust emissions/exhaust gas ana
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Exhaust systems (continued) untuned
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gas constant/universal gas constant
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in crankcase heat transfer analysis
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Mercury Marine air-assisted fuel in
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I PISTON POSITION (computer program
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Pressure wave reflection (in pipes)
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local acoustic velocity, 60 local M
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Roots blower in turbocharged/superc
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Scavenging (continued) scavenging e
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Simulation, engine See Computer mod
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temperature vs. crankshaft angle (c
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work per thermodynamic (Otto) cycle
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Design and Simulation of Two Stroke Engines

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