CYLINDER, VOLUME V
Chapter 3 Scavenging the Two-Stroke Engine design direction acquired by Smyth(3.17) and Kenny(3.31) was applied to the CFD
3.5 Scavenge port design
3.5.4 Loop scavenging
The design process for a loop scavenged two-stroke engine to ensure good scavenging characteristics is much more difficult than either the uniflow or the cross scavenged engine. In the first place, the number of potential porting layouts is much greater, as inspection of Figs. 1.2 and 3.35 will prove, and these are far from an exhaustive list of the geometrical combinations which have been tried in practice.
In the second place, the difference between a successful transfer port geometry and an unsuccessful one is barely discernible by eye. If the reader cares to peruse the actual port plan layouts of Yamaha DT250 enduro motorcycle engine cylinders 1, 2, 3,4, 5, 6, 7, 8, 9,10,12,14,15, presented in papers from QUB (1.23), (3.13) and (3.23), it is doubtful if the best scavenging cylinders could have been predicted in advance of conducting either the firing tests or the single cycle experimental assessment of their SE-SR behavior.
In a loop scavenge design, it is the main port which controls the scavenge process and it is necessary to orient that port correctly. The factors which influence that
The Basic Design of Two-Stroke Engines
:« MT1 »
(A) PLAN SECTION THROUGH TRANSFER PORTS
Fig. 3.35 Nomenclature for design for Prog.3.4, LOOP SCAVENGE DESIGN.
control are illustrated in Fig. 3.35. These geometrical values are the angles AMI and AM2 and their "target" points on the plane of symmetry, MT1 and MT2, respec- tively. A further factor is the upsweep angle of the main port, UPM. Until recent times and the publication of the papers from QUB by Smyth et al(3.17) and Kenny et al(3.31), it was suspected that the exiting gas flow from the main ports did not follow the design direction of the port, but deviated from it in some manner. In particular, it was speculated that the plan angle of the exiting fresh charge would be contorted by an angle El from the design direction, due to the entrainment or short- circuiting of the flow with the exiting exhaust gas flow. That the flow might have a deviation of angle E2 in the vertical direction was regarded as more problematic.
Smyth(3.17) showed that the value of El varied from 24.5
sto 10.4
sfrom the port opening point to full port opening at bdc, and that the value of E2 varied from 34
sto 14
sover the same range. As these measurements were taken in a particular engine geometry, the answer from Smyth's experiments cannot be regarded as universal.
Nevertheless, as his engine had rather good scavenging for a simple two-port design, it is of the type sketched in Fig. 3.35, with AMI and AM2 values of 50
Q, and as the values of gas flow deviation quoted are considerable in magnitude, it must be assumed that most engines have somewhat similar behavior. The actual experimen- tal data presented by Kenny et al(3.31), and measured by laser doppler velocimetry in an optical access cylinder, are shown in Fig. 3.36.
As no designer can predict such gas flow deviation behavior, the recommenda- tions for angles and targets for the main port, which have been demonstrated to
100 TRANSFER PORT OPENING, *
Fig. 3.36 Deviation angles from the design direction for loop scavenge flow.
provide successful scavenging characteristics, should be followed. In the not too distant future, accurate CFD calculations will replace much of the empirical advice in the succeeding sections. Nevertheless, as CFD calculations are expensive in terms of computer time, designers will always have a need for some better starting point which is based more on experimental evidence than intuition.
Finally, for those who wish to examine the full extent of porting design seen in two-stroke engines, albeit mostly loop scavenged two-stroke motorcycle engines, the comprehensive design compendia(3.32) emanating from the Technical Univer- sity of Graz deserve study as a source of reference on what has been produced.
3.5.4.1 The main transfer port
The orientation of the main port, which is the scavenge port immediately beside the exhaust port, is already stated as the designer's first priority and this has some potential for empirical guidance. If one examines the port plan layouts of all of the cylinders known to provide successful scavenging, several factors stand out quite, noticeably:
(a) The upsweep angle of the main port, UPM, is rarely larger than 10
s. (b) The value of AM2 is usually between 50
sand 55
a.
(c) The target point for MT2 is usually between 10% and 15% of the cylinder
bore dimension, BO.
(d) The target point for MT1 is approximately on the edge of the cylinder bore.
(e) The port is tapered to provide an accelerating flow though the port, i.e., AM1>AM2, and AMI is rarely larger than 70B.
(f) The larger the angle, AMI, the more the target point, MT1, is inside the cylinder bore. The narrower the angle, AM 1, the farther outside the cylinder bore is the target point, MT1. The target length, MT1, is a function of the angle, AMI, and a useful empirical relationship is:
MTl=BO*([(50-AMl)/275]+0.55) (3.5.7) To formalize some of the criteria stated above, the range of values for AMI is
usually in the band 50<AM1<70.
Eq. 3.5.7 shows a bore edge intersection for MT1 at a value for AMI of 63.75s. (g) In multiple transfer port layouts, i.e., greater than three, it is not uncommon to find the main port with parallel sides.
3.5.4.2 Rear ports and radial side ports
Such ports are sketched in Fig. 3.37, and the main design feature is that they should have upsweep angles between 50- and 60fi to ensure attachment of the flow to the cylinder wall. Jante(3.5) gives a good discussion of such an attachment phenomenon, often known as the Coanda effect.
3.5.4.3 Side ports
Such ports are sketched in the bottom half of Figs. 1.2 and 3.37. They can have straight sides or, more usually, the side nearest to the main port has a similar slope to the main port, AMI. The objective is to have the opposing flow paths meet at the back wall, attach to it, and flow up to the cylinder head in a smooth manner.
Consequently, an upsweep angle of between 15° and 25s helps that process to occur.
Such ports, in conjunction with a rear port, help the rear port flow to attach to the back wall by providing a strong pressure differential across the face of the rear port.
3.5.4.4 The use of Prog.3.4, LOOP SCAVENGE DESIGN
The reader is provided with a useful design program for all of the porting layouts illustrated in Figs. 1.2 and 3.37. The program is self-explanatory and the typical outputs shown in Figs. 3.38 and 3.39 are exactly what the operator sees on the computer screen. The opening screen display asks the operator to decide on the number of transfer ports in the design, i.e., two, three, four or five, and from then on the design procedure will take the form of changing a particular data value until the operator is satisfied that the aims and objectives have been achieved. Such objectives would typically be the satisfaction of the criteria in Sects. 3.5.4.1 to 3.5.4.3 and to achieve the desired value of port width ratio, WR.
Upon completion of the design process, the printer will provide hard copy of the screen and, as can be seen from Fig. 3.39, give some useful manufacturing data for
Chapter 3 - Scavenging the Two-Stroke Engine
Fig. 3.37 Together with Fig. 1.2 this shows the port layouts for design by Prog.3.4. LOOP SCAVENGE DESIGN.
the cylinder liner. This becomes even more pertinent for four and five port layouts, where the structural strength of some of the port bars can become a major factor.
It is interesting to note that in the actual design shown for a 70 mm bore cylinder in this three transfer port layout, the WR value of 1.06 is about the same as for the QUB deflector piston engine; it was 1.03. It is clear that it would be possible to design a four or five port layout to increase the WR value to about 1.3. It will be remembered that the uniflow engine had an equivalent value of 2, although if thai uniflow engine is of the long stroke type, or is an opposed piston engine, it will require a WR value at that level to achieve equality of flow area with the other more conventional bore-stroke ratio power units.
:URRENT INPUT DATA FOR BOOK FIG.3 38 SORE DIAMETER= 70
LINER THICKNESS= 5
MAXIMUM EXHAUST WIDTH= 40 MAIN PORT, UPe AT
TARGET MT1 = 33 5 TARGET MT2= 14.0 ANGLE AM1= 65 ANGLE AM2= 50
REAR PORT, UPR2 AT 55 4 0
REAR PORT PLAN WIDTH=30
OUTPUT DATA
MAIN PORT PLAN WIDTH= 28.6 TOTAL EFFECTIVE PORT WIDTH= 74.2 TOTAL EFFECTIVE WIDTH/BORE RATI0=1.06
Fig. 3.38 First page of output from Prog.3.4, giving the scavenge geometry of the ports.
NOTATION FOR CHAPTER 3
NAME SYMBOL SI UNIT OTHER UNITS
Charging Efficiency CE Grankcase Compression Ratio CCR
Deflection Ratio KD Geometric Compression Ratio GCR
iPort Width Ratio WR Squish Area Ratio SAR Scavenging Efficiency SE Scavenge Ratio SR Trapped Compression Ratio TCR
Trapping Efficiency TE
Brake Mean Effective Pressure BMEP Pa bar, atm, psi
162
Chapter 3 - Scavenging the Two-Stroke Engine
PORT SECTION THROUGH INCLINED TRANSFER PORT
•UP' AT UP2
^L
UP2
PORT WIDTH IS THE CHORD
AT 902 TO THE GAS FLOW DIRECTION THE EFFECTIVE WIDTH IS THE PLAN CHORDAL WIDTH*COSINE(UP).
TRANSFER PORT WIDTH RATIOS ARE TOTAL EFFECTIVE WIDTH-CVLINDER BORE
Fig. 3.39 Second page of output from Prog.3.4, giving port edge machining locations.
NAME SYMBOL SI UNIT OTHER UNITS
Brake Specific Fuel Consumption Clearance Volume
Cylinder Bore Cylinder Stroke Density
Engine Rotation Rate Pressure
Swept Volume Temperature
Trapped Swept Volume Volume
BSFC CV BO ST D RPS P T
sv
TSV V
kg/Ws m m m kg/m3
rev/s Pa m3
K m3
m3
kg/kWh,lb/hp.hr mm, in
mm, in mm, in rev/min bar, atm, psi cm3, cc, in3 SC,2F cm3, cc, in3
cm3, cc, in3
163
REFERENCES FOR CHAPTER 3
3.1 B. Hopkinson, "The Charging of Two-Cycle Internal Combustion Engines,"
Trans. NE Coast Instn. Engrs. Shipbuilders, Vol. 30, 1914, p. 433.
3.2 R.S. Benson, P.J. Brandham, "A Method for Obtaining a Quantitative Assessment of the Influence of Charge Efficiency on Two-Stroke Engine Performance," IntJ.Mech.Sci., Vol.11, p.303, 1969.
3.3 F. Baudequin, P. Rochelle, "Some Scavenging Models for Two-Stroke Engines," Proc.I.Mech.E., Vol.194, 1980, pp.203-210.
3.4 C. Changyou, F.J.Wallace, "A Generalized Isobaric and Isochoric Thermo- dynamic Scavenging Model," SAE Intnl. Off-Highway Vehicle Meeting, Milwau- kee, Wisconsin, September 1987, SAE Paper No.871657.
3.5 A. Jante, "Scavenging and Other Problems of Two-Stroke Spark-Ignition Engines," SAE Mid-Year Meeting, Detroit, Michigan, May, 1968, SAE Paper No.
680468.
3.6 G.P. Blair, "Studying Scavenge Flow in a Two-Stroke Cycle Engine," SAE Farm, Construction and Industrial Machinery Meeting, Milwaukee, Wisconsin, September, 1975, SAE Paper No.750752.
3.7 G.P. Blair, M.C. Ashe, "The Unsteady Gas Exchange Characteristics of a Two-Cycle Engine," SAE Farm, Construction and Industrial Machinery Meeting, Milwaukee, Wisconsin, September, 1976, SAE Paper No.760644.
3.8 T. Asanuma, S. Yanigahara, "Gas Sampling Valve for Measuring Scaveng- ing Efficiency in High Speed Two-Stroke Engines," SAE Transactions, Vol.70, p.420, Paper T47,1962.
3.9 R.R. Booy, "Evaluating Scavenging Efficiency of Two-Cycle Gasoline Engines," SAE Paper No. 670029, 1967.
3.10 N. Dedeoglu, "Model Investigations on Scavenging and Mixture Forma- tion in the Dual-Fuel or Gas Engine," Suher Tech. Review, Vol.51, no.3, p.133.
1969.
3.11 W. Rizk, "Experimental Studies of the Mixing Processes and Flow Configurations in Two-Cycle Engine Scavenging," Proc.I.Mech.E., Vol.172, p.417,
1958.
3.12 S. Ohigashi, Y. Kashiwada, "A Study on the Scavenging Air Flow through the Scavenging Ports," BulU.S.M.E., Vol.9, No.36, 1966.
3.13 D.S. Sanborn, G.P. Blair, R.G. Kenny, A.H. Kingsbury, "Experimental Assessment of Scavenging Efficiency of Two-Stroke Cycle Engines," SAE Intnl.
Off-Highway Vehicle Meeting, Milwaukee, Wisconsin, September, 1980, SAE Paper No.800975.
3.14 V.L. Streeter, Fluid Mechanics. McGraw-Hill, 3rd Edition, Tokyo, 1962.
3.15 H. Sammons, "A Single-Cycle Test Apparatus for Studying 'Loop- Scavenging' in a Two-Stroke Engine," Proc.I.Mech.E., Vol.160, 1949, p.233.
3.16 W.H. Percival, "Method of Scavenging Analysis for Two-Stroke Cycle Diesel Cylinders," Transactions SAE, Vol.62, p.737, 1954.
Chapter 3 - Scavenging the Two-Stroke Engine 3.17 J.G. Smyth, R.G. Kenny, G.P. Blair,"Steady Flow Analysis of the Scaveng- ing Process in a Loop Scavenged Two-Stroke Cycle Engine-a Theoretical and Experimental Study," SAE International Off-Highway & Powerplant Congress, Milwaukee, Wisconsin, September 12-15, 1988, SAE Paper No.881267.
3.18 R.G. Kenny, "Scavenging Flow in Small Two-Stroke Cycle Engines,"
Doctoral Thesis, The Queen's University of Belfast, May 1980.
3.19 R.G. Phatak, "A New Method of Analyzing Two-Stroke Cycle Engine Gas Flow Patterns," SAE International Congress, Detroit, Michigan, February, 1979, SAE Paper No.790487.
3.20. M.E.G. Sweeney, R.G. Kenny, G.B. Swann, G.P. Blair, "Single Cycle Gas Testing Method for Two-Stroke Engine Scavenging," SAE International Congress, Detroit, Michigan, February, 1985, SAE Paper No.850178.
3.21 D.S. Sanborn, W.M. Roeder, "Single Cycle Simulation Simplifies Scav- enging Study," SAE International Congress, Detroit, Michigan, February, 1985, SAE Paper No.850175.
3.22 C. Mirko, R. Pavletic, "A Model Method for the Evaluation of the Scavenging System in aTwo-Stroke Engine," SAE International Congress, Detroit, Michigan, February, 1985, SAE Paper No.850176.
3.23 M.E.G. Sweeney, R.G. Kenny, G.B. Swann, G.P. Blair, "Computational Fluid Dynamics Applied to Two-Stroke Engine Scavenging," SAE Intnl. Off- Highway Vehicle Meeting, Milwaukee, Wisconsin, September, 1985, SAE Paper No.851519.
3.24 E. Sher, "An Improved Gas Dynamic Model Simulating the Scavenging Process in a Two-Stroke Cycle Engine," SAE International Congress, Detroit, Michigan, February, 1980, SAE Paper No.800037.
3.25 E. Sher, "Prediction of the Gas Exchange Performance in a Two-Stroke Cycle Engine," SAE International Congress, Detroit, Michigan, February, 1985, SAE Paper No.850086.
3.26 W. Brandstatter, R.J.R. Johns, G. Wrigley, "The Effect of Inlet Port Geometry on In-Cylinder Flow Structure," SAE International Congress, Detroit, Michigan, February, 1985, SAE Paper No.850499.
3.27 A.D. Gosman, Y.Y. Tsui, C. Vafidis, "Flow in a Model Engine with a Shrouded Valve—A Combined Experimental and Computational Study," SAE International Congress, Detroit, Michigan, February, 1985, SAE Paper No.850498.
3.28 R. Diwakar, "Three-Dimensional Modelling of the In-Cylinder Gas Ex- change Processes in a Uniflow-Scavenged Two-Stroke Engine," SAE International Congress, Detroit, Michigan, February, 1987, SAE Paper No.850596.
3.29 D.B. Spalding, "A General Purpose Computer Program for Multi-Dimen- sional One and Two-Phase Flow," Mathematics and Computers in Simulation, Vol.23, 1981,p.267-276.
3.30 B. Ahmadi-Befrui, W. Brandstatter, H. Kratochwill, "Multidimensional Calculation of the Flow Processes in a Loop-Scavenged Two-Stroke Cycle En- gine," SAE International Congress and Exposition, Detroit, Michigan, February, 1989, SAE Paper No. 890841.