Design and Simulation of Two Stroke Engines

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Design and Simulation of Two-Stroke Engines time from zero to a maximum value and then returns to zero at transfer port closure. If one attempts to simulate the scavenging flow by holding the piston stationary, and subjects the cylinder to steady flow of either gas or liquid, then that flow will form attachments to either the piston crown or the cylinder walls in a manner which could not take place in the real velocity-time situation. This effect has been investigated by many researchers, Rizk [3.11], Percival [3.16], Sammons [3.15], Kenny [3.18] and Smyth [3.17], and they have concluded that, unless the test is being conducted in steady flow for some specific reason, this is not a realistic simulation of the scavenging flow. As more recent publications show Jante test results conducted in steady flow it would appear that not all are yet convinced of that conclusion [3.50]. 3.2.3 Absolute test methods for the determination of scavenging efficiency To overcome the disadvantages posed by the Jante test method, it is preferable to use a method of assessment of scavenging flow that will provide measurements of scavenging, trapping and charging efficiency as a function of scavenge ratio. It would also be preferable to conduct this test isolated from the confusing effects of combustion and exhaust pipe tuning characteristics. This implies some form of model test, but this immediately raises all of the issues regarding similarity discussed in Sec. 3.2.2. A test method which does not satisfy these criteria, or at least all of the vitally important dimensionless criteria, is unacceptable. It was Sammons [3.15] who first postulated the use of a single-cycle apparatus for this purpose. Because of the experimental difficulty of accurately measuring oxygen concentrations by gas analysis in the late 1940s (a vital element of his test method), his proposal tended to be forgotten. It was revived in the 1970s by Sanborn [3.13], who, together with researchers at QUB, investigated the use of a single-cycle apparatus using liquids to simulate the flow of fresh charge and exhaust gas. Sanborn continued this work [3.21] and other researchers investigated scavenging flow with this experimental methodology [3.22]. At QUB there was a growing realization that the occasional confusing result from the liquid-filled apparatus was due to the low Reynolds numbers found during the experiment, and that a considerable period of the simulated flow occurred during laminar or laminarturbulent transition conditions. Consequently, the QUB researchers reverted to the idea postulated by Sammons [3.15], for in the intervening years the invention of the paramagnetic analyzer, developed for the accurate determination of oxygen proportions in gas analysis, had realized the experimental potential inherent in Sammons' original technique. (As a historical note, for it illustrates the frailty of the human memory, I had forgotten about, or had relegated to the subconscious, the Sammons paper and so reinvented his experimental method.) However, as will become more evident later, the QUB apparatus incorporates a highly significant difference from the Sammons experimental method, i.e., the use of a constant volume cylinder during the scavenge process. The QUB apparatus sketched in Fig. 3.6, and appearing in Plate 3.3, is very thoroughly described and discussed by Sweeney [3.20]. The salient features of its operation are a constant volume crankcase and a constant volume cylinder during the single cycle of operation from tdc to tdc at a known turning rate. The equivalent rotational rate is 700 rev/min. The cylinder is filled with air to represent exhaust gas, and the crankcase is filled with carbon dioxide to represent the fresh air charge. Here, one similarity law is being well satisfied, in 224
Chapter 3 - Scavenging the Two-Stroke Engine that the typical density ratio of crankcase air charge to cylinder exhaust gas is about 1.6, which corresponds almost exactly with the density ratio of 1.6 for the carbon dioxide to air used in the experiment. The Reynolds numbers are well into the turbulent zone at the midflow position, so yet another similarity law is satisfied. Test Engine Cylinder To i micromonometer ^^ C r o S S h " d Crankshaft Fig. 3.6 QUB single-cycle gas scavenging apparatus. Plate 3.3 The single-cycle gas scavenging experimental apparatus at QUB. 225
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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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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 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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^ s C£ Q 2^ E Q. Q_ z' o ISSI HCEM
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o > z~ Q w CO UJ w g x O z o z o m
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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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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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