Design and Simulation of Two Stroke Engines

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O 01 III cr cc LU Q. LU h- LU Z o N Q LU Z cc 3 900 -, 880 - 860 840 • 820 - 800 - 780 760 Chapter 7 - Reduction of Fuel Consumption and Exhaust Emissions CHAINSAW ENGINE, 9600 rpm 75% MASS FRACTION BURNED ~ —i - 10 TCR 7.5 20 —i - 30 CRANK ANGLE, °atdc —i 40 Fig. A7.4 Effect of compression ratio on unburned zone. at the same instant in each simulation. It is observed that the end zone temperature at this instant, at a trapped compression ratio of 8.0, is above the peak value for a compression ratio of 6.5. The end zone contains air and vaporized fuel at these state conditions of temperature and pressure. If the temperature in this end zone, or in smaller localized pockets of this end zone, is above the auto-ignition temperature of any fuel in that pocket, then auto-ignition can occur and detonation, or "knocking," will be experienced. It is reported by Bosch [7.56] that detonation can occur by auto-ignition in gasoline-fuel vapor mixtures at temperatures at and above 880°C. It is observed that this condition is met by the chainsaw simulation for the entire unburned zone at 12° atdc at a trapped compression ratio of 8.0, where it is 882°C. Detonation will assuredly occur at this point. This opinion is reinforced by the data from Kalghatgi [7.57] who shows measured temperature data for the combustion of iso-octane in a four-stroke cycle engine at 11° atdc. At a temperature of 830°C, knocking or detonation was not present. At a temperature of 887°C at the same crankshaft position, knocking was present. The fine limits between the presence of detonation, or its absence, are clearly to be found within this publication. The peak temperature in the unburned zone at a CRt of 7.5 is 873°C. Detonation may not occur at this point, but my more cynical view, already mooted above, is that Murphy, whose Law applies always and everywhere, will have arranged already to have localized pockets of air and fuel meet the auto-ignition criterion at a trapped compression ratio of less than 8.0, but maybe not as low as 7.5. In terms of the GPB simulation techniques used here, it can be reasonably safely assumed that detonation will not occur if the unburned zone temperatures are detected not to exceed 870°C, at any simulated speed or load within the operating range of the engine. 539
Design and Simulation of Two-Stroke Engines General The onset of detonation, as can be seen from the above, and even with the extended information available from the above, is still not totally straightforward to predict. It is not simply a matter of using the highest possible detonation-free compression ratio so as to obtain the best power and fuel consumption, as the accompanying NO emission levels may well be above that required by legislation. Nevertheless, this simulation clearly provides the type of in-depth information upon which the design decision regarding the maximum, or the optimum, compression ratio for a given engine can be made. 540
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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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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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S.S.F.C. , C./SHP-HS Chapter 7 - Re
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Chapter 7 • Reduction of Fuel Con
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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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