Design and Simulation of Two Stroke Engines

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Design and Simulation of Two-Stroke Engines I LU DC CO CO LU DC CL 2 n 1 - 3 CYLINDER SUPERCHARGED ENGINE AT 3500 RPM 0 100 200 300 CYLINDER CRANKSHAFT ANGLE, deg. atdc on cylinder no.1 Fig. 5, 36(a) Open cycle pressures and charging for cylinder No. 1. < DC LU DC Z> CO CO LU CC 2 -i 1 - 400 3 CYLINDER SUPERCHARGED ENGINE AT 3500 RPM CYLINDER 0 100 200 300 400 CRANKSHAFT ANGLE, deg. atdc on cylinder no.2 Fig. 5. 36(b) Open cycle pressures and charging for cylinder No. 2. The peak pressure supplied by the blower, to flow a delivery ratio of 0.8, is just 1.20 atm. The crankcase compression engine reached a peak pressure in the crankcase of 1.5 atm to flow a delivery ratio of 0.5. However, the lower peak pressure in the blower-scavenged engine has one drawback: It is essential to arrange for a longer blowdown period so that the cylinder pressure can drain off to a level approaching that in the supply plenum. Even though the blowdown period is long for what is a relatively low-speed engine, it is 25° crankshaft, and the exhaust port is apparently of an adequate width, the cylinder and scavenge line pressures do not equalize until 30° after the scavenge ports have opened. This is seen by the 404
DC UJ DC Z> CO CO LU DC Q_ 2 n 1 - Chapter 5 - Computer Modeling of Engines 3 CYLINDER SUPERCHARGED ENGINE AT 3500 RPM EXHAUST \ CHARGING EFFICIENCY CYLINDER 0 100 200 300 400 CRANKSHAFT ANGLE, deg. atdc on cylinder no.3 Fig. 5.36(c) Open cycle pressures and charging for cylinder No. 3. charging efficiency curve commencing at 150° atdc. Further information on this topic is given in Fig. 5.37, for the purity and temperature in the scavenge ports at cylinder 1, an end cylinder. The blowback into the exit from the scavenge ports is considerable, with the purity dipping to 0.4 and the temperature rising to 600°C. It is almost bottom dead center before the situation has been recouped and the purity has risen again to unity. It is a much more extreme characteristic than that seen for a crankcase compression engine in Figs. 5.17 and 5.18. It also illustrates a common failing of supercharged designs, in that they are rarely endowed with adequate blowdown characteristics, for the cylinder backflow into the scavenge ports backs up the compressor, increasing the pumping losses significantly. Designers of such engines should re-examine Figs. 3.40 and 3.41, for it is possible you may have initially considered the exhaust port width shown there as excessive. In the light of the evidence given here, you may wish to reconsider that opinion. The exhaust tuning of a three-cylinder engine The pressure characteristics within the exhaust pipe at the exhaust port are also shown in Figs. 5.36(a)-(c) for each of the three cylinders. Just prior to the closing of the exhaust port, there is a "plugging" pulse reminiscent of that seen for the racing engine in Fig. 5.34. While this engine does not have a tuned expansion chamber, the effect of employing the compact manifold is almost the same. The "plugging" pulse effect in this case is caused by another cylinder blowing down into the manifold. In the case of Fig. 5.36(a) for cylinder 1, the "plugging" pulse emanates from the opening of the exhaust port of cylinder 3. In Fig. 5.32(b) it is from cylinder 1; in Fig. 5.32(c) it is from cylinder 2. This information is gleaned from the firing order being 1-3-2. The effect is to hold the charging efficiency at its plateau level, in this case about 0.61. This behavior in the multi-cylinder engine is known as "cross-charging." 405
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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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Design and Simulation ofTwo'Stroke
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Chapter 4 Combustion in Two-Stroke
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Chapter 4 - Combustion in Two-Strok
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a stratilpl o tions to ivior. If m
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CD 30 -, W 20 - LU CO iS _J LU OC 1
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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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a LU z: en 3 CO a LU z EC D m O ho
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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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Chapter 7 - Reduction of Fuel Consu
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^ s C£ Q 2^ E Q. Q_ z' o ISSI HCEM
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o > z~ Q w CO UJ w g x O z o z o m
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S.S.F.C. , C./SHP-HS Chapter 7 - Re
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0.35 • 0.34 LU ° 0.33 - c 03 DC
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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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INLET FOR r AIR AND FUEL Chapter 7
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o Q_ Ld < CD 15-, 10- Chapter 7 - R
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O 01 III cr cc LU Q. LU h- LU Z o N
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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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H Chapter 8 - Reduction of Noise Em
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Name F R Chapter 8 • Reduction of
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1 Chapter 8 - Reduction of Noise Em
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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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