Design and Simulation of Two Stroke Engines

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H Chapter 8 - Reduction of Noise Emission from Two~Stroke Engines The messages for the designer are obvious; to silence an engine, the frequency at which maximum noise is being created is that which must be tackled as a first priority and, for the silencing of the exhaust of the small industrial engine every extra cubic centimeter of exhaust box volume is well worth the ingenuity spent in its acquisition. 8.5.7 Shaping the ports to reduce high-frequency noise The profiling of the opening edge of a port changes the mass flow-time history at that port and has an influence on the noise that gas flow creates. The intake ports If the intake system of the engine is controlled by a port and piston skirt or is a disc valve intake engine as introduced initially in Sees. 1.0 and 1.3, the profile of the opening of that port affects the noise level. An expanded discussion on this matter is found below for exhaust ports, where the topic is of even greater importance, so it will be sufficient at this stage to draw your attention to the upper half of Fig. 8.24. In this diagram, the designer is presented with the option of shaping the opening or timing edge of the intake port so that the flow areatime characteristics of the port have a shallower profile. This provides an induction pulse pressure-time and mass flow-time behavior which is less steep, and this reduces the highfrequency content of the intake noise spectrum [8.15]. By definition, this implies that, for equality of intake port time-areas, a shallow profile designed on the timing edge can require a longer port timing duration, otherwise there may be a reduction of the delivery ratio. Nevertheless, this is a viable design option, particularly for such engine applications where there is little room available for a conventional low-pass intake silencer; as usual, the handheld power tool is the archetypical example of such an application. The exhaust ports The effect of profiling the exhaust port in a chainsaw, and its ensuing behavior in both noise and power terms, is discussed in some detail in a paper by Johnston [8.14]. It is impor- NORMAL A PISTON I MOTION STEEP FRONTED PULSE Po SHALLOWER PROFILE Fig. 8.24 Intake port profiling affects the pulse shape. 577 Po
Design and Simulation ofTwo~Stroke Engines tant to emphasize that slowing down the exit rate of exhaust gas flow during the blowdown phase inevitably leads to longer port timing durations and some reduction of trapping efficiency. This can be alleviated to some extent by making the exhaust port wider without unduly increasing the total timing duration, as the shallower area-time profile necessary for noise reduction also permits a wider port without endangering the mechanical trapping of the piston ring by the timing edge. Even with a suitably profiled exhaust port timing edge, the pressure-time rise characteristics of an exhaust pulse in a two-stroke power unit is much faster than that in a four-stroke cycle engine where a poppet valve is employed to release exhaust gas from the cylinder. As already remarked, the noise spectrum from a rapidly rising exhaust pressure wave is "rich" in high-frequency content, irritating to the human ear. In Fig. 8.25 it can be seen that the shaping of the exhaust port can be conducted in the same fashion as for the bottom of the inlet port. However, Johnston [8.14] shows more extreme profiles than the simple radiusing and proposed the "Q" port shape shown on the same figure, with a view to enhancing the effects already discussed. Simulation of the effect by engine modeling In the input data to the chainsaw engine simulation, both the exhaust and intake ports have a maximum width of 28 mm. The simulation result, described above as F, is compared with heavily radiused intake and exhaust ports, labeled as R. The original top and bottom radii for the intake and exhaust ports is 2 and 3 mm, respectively. For the simulation of R, this radius is increased to 10 mm in both cases. However, this reduces the port area and so it is necessary to open the exhaust port 1 ° earlier, at 107° atdc, and to open the inlet port 3° earlier, at 78° btdc, so as to provide for the same delivery ratio to pass through the engine. The result of the simulation of design R is shown in Table 8.5, where it is compared with design F. The reduction of intake and exhaust noise as forecast by the theory is achieved, even though the modification of changing the port radii from 2 or 3 mm to 10 mm is a relatively modest change, by comparison with the "Q port" shown in Fig. 8.25. The equality of delivery i NORMAL §1 l-l- E5 RADIUSED STEEP FRONTED PULSE t SHALLOWER PROFILE Fig. 8.25 Exhaust port profiling to reduce noise. 578
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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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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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CURRENT INPUT DATA FOR 2-STROKE 'SP
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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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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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Page 608 and 609: Active radical (AR) combustion and
Page 610 and 611: initial conditions, assumptions reg
Page 612 and 613: in two-stroke engine (schematic dia
Page 614 and 615: exhaust timing control valve, effec
Page 616 and 617: I Exhaust emissions/exhaust gas ana
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Page 622 and 623: in crankcase heat transfer analysis
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Page 626 and 627: I PISTON POSITION (computer program
Page 628 and 629: Pressure wave reflection (in pipes)
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Page 632 and 633: Roots blower in turbocharged/superc
Page 634 and 635: Scavenging (continued) scavenging e
Page 636 and 637: Simulation, engine See Computer mod
Page 638 and 639: temperature vs. crankshaft angle (c
Page 640 and 641: work per thermodynamic (Otto) cycle
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Design and Simulation of Two Stroke Engines

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