Design and Simulation of Two Stroke Engines

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Design and Simulation of Two-Stroke Engines 2-STROKE ENGINE CYLINDER _/ ////;;;;;////_ Jffl/Zv/77777, PORTS hrr - OCA I ci B~BS1042 ORIFICE T Fig. A2.3 Experimental apparatus for Cj measurement. either Fig. 2.16 or Fig. 2.18. The theory is normally modified to incorporate the possibility of sonic flow at the throat, and the careful researcher [2.7] ensures that the critical pressure ratio is not applied to the inflow condition for flow from pipe to cylinder. Some [2.61] employ overall pressure ratios sufficiently low so as to not encounter this theoretical problem. The coefficient of discharge, Cdis, is then determined as: r< _ m ex v-Hic 'dis — m IS (A2.3.2) The extra appellation of "is" to the subscript for Cd refers to the fact that the measured mass flow rate is compared with that calculated isentropically. If the objective of measuring the coefficient of discharge, Cd, is simply as a comparator process for the experimental improvement of the flow in two-stroke engine porting, or of poppet valving in the cylinder heads of two- or four-stroke engines, then this classic method is completely adequate for the purpose. The only caveats offered here for the enhancement of that process is that the correct valve curtain area for poppet valves should be employed (see 206
Chapter 2 - Gas Flow through Two-Stroke Engines Appendix A5.1), and that the range of pressure ratios used experimentally should be much greater than the traditional levels [2.61]. The determination of Q for accurate application within an engine simulation However, if the object of the exercise is the very necessary preparation of Cd maps of the type displayed by Kirkpatrick [2.41], for employment within engine simulation and modeling, then the procedure described above for the deduction of the coefficient of discharge, Cd, is totally inaccurate. The objective is to measure and deduce Cd in such a manner that when it is "replayed" in a computer simulation of the engine, at identical values of pressure, temperature and area ratio, the calculation would predict exactly the mass flow rate that was measured. At other thermodynamic state conditions, the discharge coefficient is characterized by the pressure ratio and the area ratio and fluid mechanic similarity is assumed. There may be those who will feel that they could postulate and apply more sophisticated similarity laws, and that is a legitimate aspiration. For the above scenario to occur, it means that the theoretical assessment of the ideal mass flow rate during the experimental deduction of Cd must not be conducted by a simple isentropic analysis, but with exactly the same set of thermodynamic software as is employed within the actual computer simulation. In short, the bottom line of Eq. A2.3.2 must be acquired using the non-isentropic theory, described earlier, applied to the geometry and thermodynamics of the steady flow experiment which mimics the outflow or inflow at a cylinder, or some similar plenum to duct boundary. This gives a significantly different answer for Cd by comparison with that which would be acquired by an isentropic analysis of the ideal mass flow rate. To make this point completely, this gives an "ideal" discharge coefficient, Cdi, and is found by: where mnis is the theoretical non-isentropic mass flow rate described above. However, even with this modification to the classic analytical method, the value of "ideal" discharge coefficient, Cdi, is still not the correct value for accurate use within a computer engine simulation. Taking cylinder outflow as the example, if Eqs. 2.16.8 and 2.17.8 are examined it can be seen that the actual coefficient of discharge arises from the need for the prediction of an effective area of the throat of the restriction, formed by the reality of gas flow through the aperture of the cylinder port. Hence, the value of port area, Aeff, is that value which, when presented into the relevant thermodynamic software for the analysis of a particular flow regime at the measured values of upstream and downstream pressure and temperature, will calculate the measured value of mass flow rate, rhex. This involves an iterative process within the theory for flow to or from the measured value of pipe area, A2, until the experimentally measured values of pi, Ti, P2, T2, and mex coincide for a unique numerical value of effective throat to pipe area ratio, kgff. The relevant value of Cd to be employed with 207
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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 178 and 179: Design and Simulation of Two-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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o > z~ Q w CO UJ w g x O z o z o m
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S.S.F.C. , C./SHP-HS Chapter 7 - Re
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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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o Q_ Ld < CD 15-, 10- Chapter 7 - R
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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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