Design and Simulation of Two Stroke Engines

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Design and Simulation of Two-Stroke Engines < CC HI DC => CO CO LU DC Q_ 3 n CYLINDER 100 200 300 CRANKSHAFT ANGLE, deg. atdc EXHAUST Fig. 6.20 Cylinder and exhaust pipe pressures at 12,300 rpm 3 E 2 - < CC LU DC Z> CO CO LU DC a. EXHAUST CONTROL VALVE LOWERED 0 100 200 300 CRANKSHAFT ANGLE, deg. atdc EXHAUST Fig. 6.21 Cylinder and exhaust pipe pressures at 8500 rpm. An alternative design approach is to be able to vary the tuned length, LT, as a function of engine speed and this has been done in some go-kart racing machines with trombone-like sliding sections within a tuned exhaust pipe. Yet another method is to inject water into the exhaust pipe at the lower engine speeds of the power band to lower its temperature [6.7]. All of these approaches to broadening the effective power band of the engine rely on some linear relationship between the exhaust period, 9ep, the tuned length, LT, and the reference speed for 440 400 400
Chapter 6 - Empirical Assistance for the Designer pressure wave propagation, ao- That this is the case, empirically speaking only, is seen from Eq. 6.2.3 and Fig. 6.17. The principal aim of the empirical design process is to phase the plugging reflection correctly for the engine speed desired for peak power, which means calculating the tuning length from piston face to tail-pipe entry, Lj. The next important criterion is to proportion the tail-pipe exit area as a function of the exhaust port area so that the pipe will empty in a satisfactory manner before the arrival of the next exhaust pulse. The third factor is to locate the end of the diffuser and the beginning of the tail nozzle so that the spread of the suction and plugging effects are correctly phased. The fourth function is to locate the second half of the diffuser so that the reverse reflection of the plugging pulse from it, and the primary reflection of it from the exhaust port, can recombine with the next outgoing exhaust pulse, thereby producing the resonance effect demonstrated in Fig. 5.32 or Fig. 6.20. This will happen only in racing engines with very long exhaust port periods, typically in excess of 190°. A multi-stage diffuser is an efficient method of reflecting the exhaust pulse and the shallower first stages reduce the potential for energy losses from shock formation at the entrance to that diffuser. The shocks form as high-amplitude compression exhaust waves meet strong suction waves which are progressing back to the engine from the diffuser. As seen in Sec. 2.15, this raises the local gas particle velocity, and in the diffuser entrance area this can attempt to exceed a Mach number of unity. The result is a shock which effectively "destroys" the wave energy and, although the energy reappears as heat, this does nothing for the retention of the strength of the all-important pressure wave action. The slower rate of area expansion of the first stage of the diffuser (see also Sec. 2.15) smears the reflection process more widely and efficiently over the entire length of the diffuser. The entirety of the empirical design process is founded in very simple equations which have been tried and tested for many years in the theoretical and experimental development of expansion chambers for racing engines at QUB. As remarked earlier, remember that this is an empirical approach, not a precise calculation, and is the starting point for optimization by simulation, as seen in Chapter 5, preferably in combination with the experimental testing of the firing engine [4.35]. As this is an empirical calculation for the lengths and diameters shown in Fig. 5.7, and are conventionally discussed in mm units, the data are inserted and produced by the calculation in those units. The data required for the calculation are in blocks, the first of which is: engine bore, stroke, connecting rod length, exhaust port total opening period, 9ep, in units of crankshaft degrees, number of exhaust ports, maximum width of each exhaust port, and the radii in the corners of each exhaust port. If the exhaust port shape is complex, as in Fig. 5.1(b), some preliminary arithmetic must be carried out prior to computation to find the "mean" exhaust port width, so that the data are inserted accurately into the calculations in the above format. The above set of data permits the calculation of the exact exhaust port area. Incidentally, it is presumed that the designer has followed some design process to this point and the data being used are matched to the required performance of the engine. With this knowledge of the exhaust port flow area, equivalent to a diameter, do, the empirical calculation describes this as the ideal exhaust pipe initial diameter. In a racing pipe, where maximizing the plugging pressure gives the highest trapping efficiency, there seems little point in having the first downpipe diameter, di, much more than 1.05 times larger than do, for in unsteady gas flow, the larger the 441
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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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LLI •z. O N -z. 0.21 -, 0.20 DC 0
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Chapter 7 • Reduction of Fuel Con
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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 Consu
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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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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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