Design and Simulation of Two Stroke Engines

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Chapter 8 - Reduction of Noise Emission from Two-Stroke Engines splits the total box volume into two parts, labeled as Vi and V2, and the simulation models the entire system as two complete boxes separated by the filter element in precisely the same fashion that it does for the exhaust system. The geometry is detailed in Table. 8.1. Name S E F G df (mm) 30 30 30 30 Table 8.1 Intake silencer geometry for the chainsaw d2 (mm) 22 22 22 22 dp (mm) 22 17 15 19 LP (mm) 50 160 125 200 V1 (cm 3 ) 200 200 200 200 v2 (cm 3 ) 100 300 300 300 fi (Hz) 240 88.9 88.3 80.1 It will be seen that those characterized in Table 8.1 above as E, F, and G are the various alternatives as designed by the acoustic criteria, whereas that called S is the original "standard" system. The unsteady gas-dynamic simulation prepares an output file of the mass flow-time history at the intake from the atmosphere. For the four systems analyzed, this is shown in Fig. 8.19 as the results of simulation at 9600 rpm, at a slightly different, i.e., leaner, air-to-fuel ratio than that employed for the more complete data map in Fig. 5.9. The theory of Sec. 8.4.1 conducts a Fourier analysis of the data in Fig. 8.19 and produces the noise spectra shown in Fig. 8.20, at a distance, rm, of 1.0 m from the intake and exhaust exit points, at an orientation angle, 0, of zero. Not surprisingly, the more open silencer, S, has the greater mass flow fluctuations and has a higher noise spectra profile than any of the more optimized silencers, E, F or G. They are all reasonably similar, but the silencer, F, has the smoothest mass flow-time history and comes out with the lowest peak noise level at 640 Hz, the fourth harmonic. Thus the design, F, one of those determined above by the acoustic criteria, emerges as being superior from a noise standpoint to the standard silencer, S. The overall noise levels, the summations of these spectra, are given in Table 8.2. Name S E F G Table 8.2 Performance and intake silencing for the chainsaw Intake (dBlin) 103.6 92.2 90.1 93.8 Intake (dBA) 98.5 87.2 82.9 89.5 Exhaust (dBA) 89.0 88.8 88.8 88.8 Total (dBA) 99.0 91.1 89.8 92.2 bmep (bar) 3.65 3.82 3.79 3.79 DR 0.525 0.546 0.541 0.550 The intake geometry of F is overall quieter than E and G by 4 dB or more. That is very much quieter, for, recall from Eq. 8.1.6, two equal sound sources give an increase of approximately 3 dB. The acoustic criteria gave no guidance on this matter. The standard silencer, S, is actually noisier than the exhaust system. 571
Design and Simulation of Two-Stroke Engines •5 UJ fee cr o _i LL CO < 0.02 -n 0.01 - 0.00 -0.01 - -0.02 -0.03 CHAINSAW ENGINE, 9600 rpm STANDARD INTAKE, S MODIFIED INTAKES 0 100 200 300 400 CRANKSHAFT ANGLE, deg. atdc Fig. 8.19 Effect of modified intake system on the intake mass flow rate. 100 a> 90 - JC | 80 - < CO T3 70 - UJ 60 - > UJ m 50 " co O 40 - 30 0 CHAINSAW ENGINE, 9600 rpm MODIFIED INTAKES STANDARD INTAKE, S —r 500 1000 1500 FREQUENCY, Hz / 2000 Fig. 8.20 Effect of modified intake system on the (dBA) noise spectra. The power of the simulation technique is that it gives the designer the trade-off in terms of noise reduced by comparison with power or torque, lost or gained. Here, the standard and noisiest silencer, S, actually gave less torque, as bmep, by comparison with all of the modified silencers, simply because it breathed less air, as shown by the delivery ratio, DR, results. The best silencer, F, breathed in marginally less air than E or G, and lost a little torque as a consequence, but at a very considerable noise reduction. The overall noise level for F, however, is only 1.2 dB less than E, due to the exhaust silencer being the single biggest source of noise output. To silence the chainsaw engine fitted with intake system F any further, the exhaust 572
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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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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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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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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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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 620 and 621: gas constant/universal gas constant
Page 622 and 623: in crankcase heat transfer analysis
Page 624 and 625: Mercury Marine air-assisted fuel in
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
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work per thermodynamic (Otto) cycle
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Design and Simulation of Two Stroke Engines

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