Design and Simulation of Two Stroke Engines

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Design and Simulation of Two-Stroke Engines ¥ 1 1 \ , A (a) (b) (c) (d) Fig. 3.28 Yamaha cylinder No. 12 charge purity plots at 29° ABDC. > LU h- > UJ CO >•" o UJ o UL u_ LU 1.0 n 0.8 - 0.6 0.4 - 0.2 YAMAHA CYLINDER 14 SCAVENGE RATIO, SRv SEv EXP TEv EXP SEv THEORY TEv THEORY Fig. 3.29 Comparison of experiment and CFD computation. Comparisons of the measured and calculated SEV-SRV and TEV-SRV profiles are given in Figs. 3.29 and 3.30. It will be observed that the order of accuracy of the calculation, over the entire scavenging flow regime, is very high indeed. For those who may be familiar with the findings of Sweeney et al. [3.23], observe that the order of accuracy of correlation of the theoretical predictions with the experimental data is considerably better in Figs. 3.29 and 30 than it was in the original paper. The reason for this is that the measured data for the variance of the flow from the port design direction acquired by Smyth [3.17] and Kenny [3.31] was applied to the CFD calculations for those same modified Yamaha cylinders to replace the plug flow assumption originally used. It is highly significant that the accuracy is improved, for this moves the CFD calculation even closer to becoming a proven design technique. 248
LU hoQ ft CO 1.0 -i 0.8 g0.6 LU o LL 0.4 LU 0.2 Chapter 3 - Scavenging the Two-Stroke Engine YAMAHA CYLINDER 12 1 SCAVENGE RATIO, SRv SEv THEORY TEv THEORY SEv EXP TEv EXP Fig. 3.30 Comparison of experiment and CFD computation. In terms of insight, Figs. 3.21-3.24 and 3.25-3.28 show the in-cylinder charge purity contours for cylinders 14 and 12, respectively. In each figure are four separate cylinder sections culled from the cells in the calculation. The cylinder section (a) is one along the plane of symmetry with the exhaust port to the right. The cylinder section (b) is at right angles to section (a). The cylinder section (c) is on the surface of the piston with the exhaust port to the right. The cylinder section (d) is above section (c) and halfway between the piston crown and the cylinder head. In Fig. 3.21(a) and Fig. 3.25(a) it can be seen from the flow at 39° bbdc that the short-circuiting flow is more fully developed in cylinder 12 than in cylinder 14. The SEV level at the exhaust port for cylinder 14 is 0.01 whereas it is between 0.1 and 0.2 for cylinder 12. This situation gets worse by 29° bbdc, when the SEV value for cylinder 14 is no worse than 0.1, while for cylinder 12 it is between 0.1 and 0.4. This flow characteristic persists at 9° bbdc, where the SEV value for cylinder 14 is between 0.1 and 0.2 and the equivalent value for cylinder 12 is as high as 0.6. By 29° abdc, the situation has stabilized with both cylinders having SEV values at the exhaust port of about 0.6. For cylinder 12, the damage has already been done to its scavenging efficiency before the bdc piston position, with the higher rate of fresh charge flow to exhaust by short-circuiting. This ties in precisely with the views of Sher [3.24] and also with my simple scavenge model in Sec. 3.3.2, regarding the ne-SRv profile at the exhaust port, as shown in Fig. 3.18. Typical of a good scavenging cylinder such as cylinder 14 by comparison with bad scavenging as in cylinder 12 is the sharp boundary between the "up" and the "down" parts of the looping flow. This is easily seen in Figs. 3.23 and 3.27. It is a recurring feature of all loopscavenged cylinders with good scavenging behavior, and this has been observed many times by the research team at QUB. 249
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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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Page 220 and 221: Design and Simulation ofTwo-Stroke
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Chapter 4 - Combustion in Two-Strok
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181 also x3 = — = 0.905 x6 = 2.17
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to be cur .3.20) .3.21) .3.22) for
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3.23) com- :on is hhas has a /eng-
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stroke .3.29) mean me to 3land ssio
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CD 1.2 -i 1.0 - di „„ LU 0.8 cc
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CO —> 111" cc LU < LU _J LU CC £
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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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Chapter 7 Reduction of Fuel Consump
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Chapter 7 • Reduction of Fuel Con
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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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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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