Design and Simulation of Two Stroke Engines

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Design and Simulation of Two-Stroke Engines > DC Z LU o D_ I- W 1.0 n 0.8 0.6 0.4 0.2 - 0.0 •O— YAM14 1 SCAVENGE RATIO, SRv
Chapter 3 - Scavenging the Two-Stroke Engine vides vital information without which simulation of scavenging within a computer model could not be conducted. 3.3.3 Connecting a volumetric scavenging model with engine simulation The engine simulation by computer, as presented in Chapters 2 and 5, traces the thermodynamics and gas dynamics of flow in, out and through the engine cylinder. The previous section, and in particular Eq. 3.3.12, provided the vital piece of information regarding the value of the purity of the charge leaving the cylinder. It is not an unreasonable assumption that the gas leaving the cylinder is sufficiently well mixed that the exiting gas has a common temperature, Te. It is also a very reasonable assumption that the pressure throughout the cylinder is uniform and that the exiting charge has a pressure which is the same as the mean value for all of the cylinder contents. However, the temperature of the exiting charge of a stratified process must be composed of exhaust gas and fresh charge elements which are a function of their physical position within the cylinder. There are two ways of dealing with this problem of the determination of the exiting charge temperature, one simple and one more complex. The most complex method is actually the easiest to debate, for it consists of conducting a CFD computation using a proprietary computer code, such as Phoenics or StarCD, and obtaining the information very accurately and very completely for the entire scavenge operation. The effectiveness of such a code is described in Sec. 3.4. At the same time, it must be stated that CFD computations are slow, even on the fastest of computers, and the incorporation of a CFD computation of scavenging into the ID engine simulation as described in Chapters 2 and 5 would change its computation time from several minutes into many days. Thus, until the great day comes when CFD computations can be accomplished in minutes and not days within an engine simulation model a simpler criterion is required to estimate the temperatures of the air and exhaust components of the charge lost from the cylinder during scavenging. I have presented such a simple criterion [3.35] consisting of the realistic assumption that the temperature differential AT.^ at any instant between the air and the exhaust is linked to the mean temperature in the cylinder. In other words, as the scavenge flow rate rises, this reduces both the mean temperature within the cylinder, Tcy, and the temperature of the exhaust gas, Tex; but also there is an increase of the temperature of the trapped air, Tta. The arithmetic value of the temperature differential, ATax which links these literal statements together is defined by a factor, Ctemp, thus: Tex = Tta + ATax = Tta + QempTCy (3.3.13) The temperature differential factor, Ctemp> is clearly a complex function of time, i.e., the engine speed, the displacement/mixing ratio of the fluid mechanics of the particular scavenge process, the heat transfer characteristics of the geometry under investigation, etc. Thus no simple empirical criterion is ever going to satisfy the needs of the simulation for accuracy. My best estimation for the arithmetic value of Ctemp is given by the following function based on its relationship to (a) the attained value of cylinder scavenging efficiency, SEcy, computed as being trapped at the conclusion of a scavenging process, and (b) the mixing caused by time and piston motion, the effects of which are expressed by the mean piston speed, cp. The 241
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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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Page 302 and 303: Chapter 4 Combustion in Two-Stroke
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Chapter 4 - Combustion in Two-Strok
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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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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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§ 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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• • / . • 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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