Design and Simulation of Two Stroke Engines

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stroke .3.29) mean me to 3land ssion stant, nula- ature, of air, id the 3.30) i, can 3.31) to be may .32) may Chapter 4 - Combustion in Two-Stroke Engines expression is the average temperature of the cylinder wall, the piston crown and the cylinder head surfaces. The heat transfer, 5QL, over a crankshaft angle interval, d0, and a time interval, dt, can be deduced for the mean value of that transmitted to the total surface area exposed to the cylinder gases: as then dG 60 dt = x 360 rpm 5QL=(Ch(TCy-Tcw) + Cr)Acwdt The surface area of the cylinder, Acw, is composed of: (4.3.33) (4.3.34) Acw — ACyijn(jer liner + Apjst0n crown + A.Cy]jncjer neaci (4.3.35) It is straightforward to expand the heat transfer equation in Eq. 4.3.34 to deal with the individual components of it to the head or crown by assigning a wall temperature to those specific areas. It should also be noted that Eq. 4.3.34 produces a "positive" number for the "loss" of heat from the cylinder, aligning it with the sign convention assigned in Eq. 4.2.2 above. The typical values obtained from the use of the above theory are illustrated in Table 4.2. The example employed is for a two-stroke engine of 86 mm bore, 86 mm stroke, running at 4000 rpm with a cylinder surface mean temperature, Tcw, of 220°C. Various timing positions throughout the cycle are selected and the potential state conditions of pressure (in atm units) and temperature (in °C units) are estimated to arrive at the tabulated values for a two-stroke engine, based on the solution of the above equations. The timing positions are in the middle of scavenging, at the point of ignition, at the peak of combustion and at exhaust port opening (release), respectively. It will be observed that the heat transfer coefficients as predicted for the radiation component, Cr, are indeed very much less than that for the convection component, Cn, and could well be neglected, or indeed incorporated by a minor change to the constant, a, in the Annand model in Eq. 4.3.27. Timing Position scavenge ignition burning release Table 4.2 Heat tranfer coefficients using the Annand model Pcy (atm) 1.2 10.0 50.0 8.0 Tcy(°C) 250 450 2250 1200 Nu 349 1058 860 402 Re 29433 143376 106688 36066 ch 168 652 1130 411 Cr 2.2 4.0 84.7 20.2 It can also be seen that the heat transfer coefficients increase dramatically during combustion, but of course that is also the position of minimum surface area and maximum gas tem- 307
Design and Simulation of Two-Stroke Engines perature during the heat transfer process and will have a direct influence on the total heat transferred through Eq. 4.3.34. 4.3.5 Internal heat loss by fuel vaporization Fuel is vaporized during the closed cycle. For the spark-ignition engine it normally occurs during compression and prior to combustion. For the compression-ignition it occurs during combustion. Fuel vaporization for the spark-ignition engine Let it be assumed that the cylinder mass trapped is mt and the scavenging efficiency is SE, with an air-fuel ratio, AFR. The masses of trapped air, mta, and fuel, mtf, are given by: mta = mtSE and mtf = —-&- (4.3.36) AFR If the crankshaft interval between trapping at exhaust port closing and the ignition point is declared as Gvap, and is the crankshaft interval over which fuel vaporization is assumed to occur linearly, then the rate of fuel vaporization with respect to crankshaft angle, rhvap, is given by: mVap=^ iL kg/deg (4337) D vap Consequently, the loss of heat from the cylinder contents, §Qvap, f° r an y given crankshaft interval, d9, is found by the employment of the latent heat of vaporization of the fuel, hvap. Numerical values of latent heat of vaporization of various fuels are to be found in Table 4.1. S Qvap = m vap h vap d0 (4-3.38) It should be noted that this equation provides a "positive" number for this heat loss, in similar fashion to the application of Eq. 4.3.34. Fuel vaporization for the compression-ignition engine Let it be assumed that the cylinder mass trapped is mt and the charge purity is II, with an overall air-fuel ratio, AFR. The masses of trapped air, mta, and fuel, mtf, are given by: ,-r , m ta mta = nmt and mtf = —^ (4.4.39) ArK The crankshaft angle interval over which combustion occurs is defined as b°. Fuel vaporization is assumed to occur as each packet of fuel, dmbe, is burned over a time interval and is related to the mass fraction burned at that juncture, Be , at a crankshaft angle, 0b, from the onset of the combustion process. Let it be assumed that an interval of combustion is occurring 308
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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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Page 278 and 279: Design and Simulation of Two-Stroke
Page 302 and 303: Chapter 4 Combustion in Two-Stroke
Page 304 and 305: Chapter 4 - Combustion in Two-Strok
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Page 342 and 343: Combustion in compression-ignition
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Page 350 and 351: lat the ssi ;tribu- ELEVATION VIEWS
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Page 356 and 357: CO E 50 40 - O O _i Hi > I CO 30 Z)
Page 358 and 359: CURRENT INPUT DATA FOR 2-STROKE 'SP
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Page 364 and 365: vfitro- ;titi >tants 1970. i(In- Ch
Page 370 and 371: £2 = Pi P2 I Pi J Chapter 4 - Comb
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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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§ 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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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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