Design and Simulation of Two Stroke Engines

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Design and Simulation of Two-Stroke Engines the combination of incoming air at 40°C flowing into an expanding crankcase volume containing air already at about 65°C, decreases the crankcase temperature somewhat to about 60°C. This seems curiously insufficient as a drop in temperature; however, in terms of the mass of air already within the crankcase, the entering quantity is quite small. Put crudely, the DR value is 0.5, or about 32.5 cm 3 in volume terms for a 65 cm 3 engine. The crankcase compression ratio is 1.5, so its maximum volume is 195 cm 3 , therefore the entering quantity is only some 17% of the total in residence. As the air is gulped into the intake duct from the airbox muffler, the temperature around the tdc point briefly drops down close to the atmospheric value. The peak temperature of the crankcase air rises to 120°C and, as shown in Fig. 5.21, can be heated within the cylinder up to 600°C during the early stages of scavenging. Not surprisingly, the majority of fuel vaporization occurs within the cylinder. The cylinder pressure and temperature, Figs. 5.26 and 5.27 The design objective for the engine, in terms of the torque and power it will produce, or the NOx emission it may create, is summed up in Fig. 5.26. It is, after all, cylinder pressure which pushes on the piston area to create that torque and power. It is the level of peak cylinder temperature that influences directly the amount of the emissions of oxides of nitrogen it will produce; this statement, although true, is too naive and requires the amplification supplied in Appendices A4.1 and A4.2. The open cycle period is indicated in the figure and the pressure events during it are barely visible on a diagram drawn to this scale. They would appear to have no influence on the behavioral outcome of the engine in terms of its performance characteristics. From the discussion above that is known to be incorrect, for it is the state conditions of the cylinder at trapping, dictated by the gas-dynamic behavior of the breathing and scavenging system, which results in the pressure and temperature created within the cylinder during the closed cycle period. The temperature profile during the open cycle period is demonstrably much more dramatic during the open cycle period. The point being made is better illustrated in Fig. 5.27. In the next section, the discussion will focus on high-performance engines, with a 125 cm 3 motorcycle racing engine used as the design and simulation example. In Fig. 5.27, there is drawn the calculated cylinder pressure diagrams of the chainsaw engine and the racing motorcycle engine. They are both high-speed engines, but the disparity in the attained cylinder pressures and the attained bmep and specific power output could not be greater. One outperforms the other by a factor approaching three. That means that the specific trapped cylinder mass of air and fuel must be three or more times greater for the motorcycle engine than the chainsaw engine. This disparity of cylinder filling and emptying must occur during the events of the open cycle. On this diagram, drawn to this scale, there is little hint from the two pressure traces during the open cycle that anything untoward, or even different, is occurring. The next section explains how it is possible, and how a simulation procedure by computer can incorporate it by design. Further simulations involving this chainsaw engine In Chapter 6, where the focus is on design assistance for the selection of engine dimensions for either simulation or experimental development, the chainsaw engine discussed here 392
DC LU DC D CO CO LU DC 0_ 30 20 10 - PRESSURE \ TEMPERATURE 0 100 200 300 400 CRANKSHAFT ANGLE, deg. atdc Chapter 5 - Computer Modeling of Engines 2000 - 1000 Fig. 5.26 Cylinder pressure and temperature in a chainsaw at 9600 rpm. 100 -, 125 cc motorcycle racing engine bmep 10.5 bar 25.5 kW 11740 rpm 65 cc chainsaw engine bmep 3.7 bar 3.9 kW 9600 rpm open cycles 0 100 200 300 400 CRANKSHAFT ANGLE, deg. atdc Fig. 5.27 Comparison of cylinder pressures in a chainsaw and a racing engine. is employed as a working example. The effects on the performance characteristics of relatively minor changes of exhaust port timing, or for transfer port timing, are shown in Figs. 6.8-6.11, and in Figs. 6.12-6.15, respectively. In Chapter 7, where the focus is on fuel economy and emissions, the chainsaw engine is again employed as one of the working examples. The results of the simulation, shown here to 393 O Ol LU DC 3 I
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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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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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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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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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