Design and Simulation of Two Stroke Engines

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Design and Simulation of Two-Stroke Engines is Te which is found from the following approximate consideration of the enthalpy of the exiting mixture of trapped air, Tta, and the exhaust gas, Tex. T - em P ta ta v emJ^pex^ex /o o i o-v * ^em^p ta "*" U — * *em A'p ex Further properties of the gas mixture are required and are determined using the theory for such mixtures, which is found in Sec. 2.1.6. Incorporation of the theory into an engine simulation This entire procedure for the simulation of scavenging is carried out at each step in the computation by a ID computer model of the type discussed in Chapter 2. In Chapter 5, Sec. 5.5.1, is a detailed examination of every aspect of the theoretical scavenging model on the in-cylinder behavior of a chainsaw engine. In particular, Figs. 5.16 to 5.21 give numerical relevance to the theory in the above discussion. Further discussion on the scavenge model, within a simulation of the same chainsaw engine, is given in Chapter 7. Examples are provided of the effect of widely differing scavenging characteristics on the ensuing performance characteristics of power, fuel economy and exhaust emissions; see Figs. 7.19 and 7.20 in Sec. 7.3.1. 3.4 Computational fluid dynamics In recent years a considerable volume of work has been published on the use of Computational Fluid Dynamics, or CFD, for the prediction of in-cylinder and duct flows in (fourstroke cycle) IC engines. Typical of such publications are those by Brandstatter [3.26], Gosman [3.27] and Diwakar [3.28]. At The Queen's University of Belfast, much experience has been gained from the use of general-purpose CFD codes called PHOENICS and StarCD. The structure and guiding philosophy behind this theoretical package has been described by their originators, Spalding [3.29] or Gosman [3.27]. These CFD codes are developed for the simulation of a wide variety of fluid flow processes. They can analyze steady or unsteady flow, laminar or turbulent flow, flow in one, two, or three dimensions, and single- or two-phase flow. The program divides the control volume of the calculated region into a large number of cells, and then applies the conservation laws of mass, momentum and energy, i.e., the Navier-Stokes equations, over each of these regions. Additional conservation equations are solved to model various flow features such as turbulent fluctuations. One approach to the solution of the turbulent flow is often referred to as a k-epsilon model to estimate the effective viscosity of the fluid. The mathematical intricacies of the calculation have no place in this book, so the interested reader is referred to the publications of Spalding [3.29] or Gosman [3.27], and the references those papers contain. A typical computational grid structure employed for an analysis of scavenging flow in a two-stroke engine is shown in Fig. 3.20. Sweeney [3.23] describes the operation of the program in some detail, so only those matters relevant to the current discussion will be dealt with here. 244
Chapter 3 - Scavenging the Two-Stroke Engine Fig. 3.20 Computational grid structure for scavenging calculation. In the preceding sections, there has been a considerable volume of information presented regarding experimentally determined scavenging characteristics of engine cylinders on the single-cycle gas scavenging rig. Consequently, at QUB it was considered important to use the PHOENICS CFD code to simulate those experiments and thereby determine the level of accuracy of such CFD calculations. Further gas-dynamics software was written at QUB to inform the PHOENICS code as to the velocity and state conditions of the entering and exiting scavenged charge at all cylinder port boundaries, or duct boundaries if the grid in Fig. 3.20 were to be employed. The flow entering the cylinder was assumed by Sweeney [3.23] to be "plug flow," i.e., the direction of flow of the scavenge air at any port through any calculation cell at the cylinder boundary was 245
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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 216 and 217: Design and Simulation of Two-Stroke
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CD 30 -, W 20 - LU CO iS _J LU OC 1
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Chapter 4 - Combustion in Two-Strok
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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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§ 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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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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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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