Design and Simulation of Two Stroke Engines

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Design and Simulation of Two-Stroke Engines The left-hand side of Eq. 6.1.6 or Eq. 6.1.7 is known as the "time-integral of area per unit swept volume" for the port of a two-stroke engine. This expression is often shortened colloquially to specific time area, Asv. The units of "time-integral of area per unit swept volume," or specific time area, can be seen to be: m s —s- = s/m m The evaluation of the area term in Eq. 6.1.6 or 6.1.7 has been discussed in Sec. 5.2.1. The summation of the term Aed9 is the port area shown in Fig. 6.1 for each of the ports on that diagram. The term dt/d0 is a fixed relationship between crankshaft angle and time given by: dt time per revolution 60 + rpm _ 1 d0 degrees per revolution 360 6 x rpm On the assumption that the mass of gas which flows into an engine on each cycle must also flow through it and ultimately out of it, the evaluation of specific time area, Asv, for inlet ports, transfer ports and exhaust ports can be carried out and an attempt made to relate that data to the bmep produced by an engine of a known swept volume, Vsv, at a known engine speed, rpm. Another important flow parameter can be evaluted by calculating the Asv for the blowdown period between the exhaust port opening and the transfer port opening. For the cylinder pressure to fall from the release point to a value approaching that in the scavenge ducts in order to avoid excessive backflow into the transfer ports, a certain proportion of the cylinder mass must pass out of the exhaust port during that period. The higher the bmep in a given engine, the greater will be the trapped charge mass and pressure, and therefore, so will be the need for a larger specific time area to let the cylinder pressure fall appropriately during the blowdown phase of the exhaust process. The blowdown value of Asv is indicated in Fig. 6.1. At QUB over the years a considerable volume of data has been collected from engine design and testing, not only from in-house designs but from production engines as part of R&D programs. A small fraction of this "expert system" data is illustrated in Figs. 6.3-6.6 for the specific time areas related to brake mean effective pressure. On those diagrams, the engine cylinder sizes range from 50 cm 3 to 500 cm 3 , the speeds at which the performance data were recorded range from 3000 to 12,000 rpm, and the bmep spread is from 3.5 bar to 11 bar. The graphical scatter is, not unexpectedly, considerable. The reasons are fairly self-evident, ranging from engines which have over- or under-designed ports, to engines with exhaust systems attached in varying degrees of tuning level from the Grand Prix racing type to that for an outboard motor or a chainsaw. Further complications arise, such as the use of differing crankcase compression ratio, or the employment of supercharging; these factors do not emerge in any simplistic analysis of the specific time area of engines. Despite such logic, in each of these figures, trends for a bmep-Asv relationship can be seen to exist and a straight line opinion as to the best fit is drawn on each diagram; in the case of the transfer ports a band of a 420
12 -, TOTAL TIME AREA OF INLET PORTING 10 - «5 8 .O a." LU I « Chapter 6 - Empirical Assistance for the Designer bmep=774 Asv -1.528 0 100 200 SPECIFIC TIME AREA, Asv, s/m x 10000 Fig. 6.3 Specific time areas for inlet ports from measured data. 12 -, 10 c5 8 jQ a; LU 1• - TOTAL TIME AREA OF TRANSFER PORTING — bmep=2400 Asv - 9.66 a bmep=587 Asv+ 0.128 - i — | — i 1 1 — | — • 1 1—|—i 1 1 — | 50 60 70 80 90 100 110 120 SPECIFIC TIME AREA, Asv, s/m x 10000 Fig. 6.4 Specific time areas for transfer ports from measured data. 421
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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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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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0.35 • 0.34 LU ° 0.33 - c 03 DC
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§ 2000
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Chapter 8 Reduction of Noise Emissi
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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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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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