Design and Simulation of Two Stroke Engines

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Design and Simulation of Two-Stroke Engines mometer, and the performance characteristics of power, torque, fuel consumption rate, and air consumption rate, at various engine speeds, are recorded. Many texts and papers describe this process and the Society of Automotive Engineers provides a Test Code J1349 for this very purpose [1.14]. There is an equivalent test code from the International Organization for Standardization in ISO 3046 [1.16]. For the measurement of exhaust emissions there is a SAE Code J1088 [1.15] which deals with exhaust emission measurements for small utility engines, and many two-stroke engines fall into this category. The measurement of air flow rate into the engine is often best conducted using meters designed to British Standard BS 1042 [1.17]. Several interesting technical papers have been published in recent times questioning some of the correction factors used within such test codes for the prevailing atmospheric conditions. One of these by Sher [1.18] deserves further study. There is little point in writing at length on the subject of engine testing and of the correction of the measured performance characteristics to standard reference pressure and temperature conditions, for these are covered in the many standards and codes already referenced. However, some basic facts are relevant to the further discussion and, as the testing of exhaust emissions is a relatively new subject, a simple analytical computer program on that subject, presented in Sec. 1.6.2, should prove to be useful to quite a few readers. A laboratory engine testing facility is diagrammatically presented in Fig. 1.17. The engine power output is absorbed in the dynamometer, for which the slang word is a "dyno" or a "brake." The latter word is particularly apt as the original dynamometers were, literally, friction brakes. The principle of any dynamometer operation is to allow the casing to swing freely. The reaction torque on the casing, which is exactly equal to the engine torque, is measured on a lever of length, L, from the centerline of the dynamometer as a force, F. This restrains the outside casing from revolving, or the torque and power would not be absorbed. Consequently, the reaction torque measured is the brake torque, Zb, and is calculated by: Zh=FxL (1-6.1) Therefore, the work output from the engine per engine revolution is the distance "traveled" by the force, F, on a circle of radius, L: Work per revolution = 2TUFL = 27tZb (1.6.2) The measured power output, the brake power, Wb, is the work rate, and at rps rotational speed, is clearly: Wb = (Work per rev) x (rev/s) „ „ „ rpm (1.6.3) = 27tZhrps = nZh 30 To some, this equation may clear up the apparent mystery of the use of the operator K in the similar theoretical equation, Eq. 1.5.26, when considering the indicated power output, Wj. 36
AIR FLOW ma—> imep mf FUEL FLOW ENGINE Chapter 1 - Introduction to the Two-Stroke Engine EXHAUST FLOW -^•ex CO, CO2, O2 (% vol) HC, NOx (ppm) TORQUE ARM Fig. 1.17 Dynamometer test stand recording of performance parameters. The brake thermal efficiency, r\\j, is then given by the corresponding equation to Eq. 1.5.23: Tib power output Wu rate of heat input rhf Cfl A similar situation holds for brake specific fuel consumption, bsfc, and Eq. 1.5.28: fuel consumption rate rhf power output Wb (1.6.4) (1.6.5) However, it is also possible to compute a mean effective pressure corresponding to the measured power output. This is called the brake mean effective pressure, bmep, and is calculated from a manipulation of Eq. 1.5.25 in terms of measured values: bmep = Wu Vsv x rps (1.6.6) It is obvious that the brake power output and the brake mean effective pressure are the residue of the indicated power output and the indicated mean effective pressure, after the engine has lost power to internal friction and air pumping effects. These friction and pumping losses deteriorate the indicated performance characteristics by what is known as the engine's mechanical efficiency, T|m. Friction and pumping losses are related simply by: Wj = Wb + friction and pumping power 37 (1.6.7)
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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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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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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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CURRENT INPUT DATA FOR 2-STROKE 'SP
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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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Chapter 7 Reduction of Fuel Consump
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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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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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