if this is an enduro type of engine, or about 8 bar BMEP peak.
The latter will have a comprehensive silencer following the tuned section.
Motocross and road racing engines have rudimentary silencers, and the design philosophy being presented is not at all affected by the attachment of a simple absorption silencer section in conjunction with the tailpipe. That, as with all such debate on silencers, is to be discussed in Chapter 8.
The next set of data is required to calculate lengths, all of which will be
proportioned from the tuned length, LT, to be calculated: engine speed for peak
power, RPM, in rev/min units; exhaust gas temperature, TexC, as estimated in the pipe mid-section, in SC units.
For those who may not have immediate access to temperature data for expansion chambers, the potential values have a loose relationship with BMEP. Therefore, if the reader expects to have a design approach a Grand Prix racing engine power level, then an exhaust temperature of 6502C can be anticipated. If the engine is a lesser rated device, such as an enduro motorcycle, then a temperature of 5002C could be safely used in the calculation. As the speed of sound is the real criterion, then as it is proportional to the square root of the Kelvin temperature, the selection of exhaust pipe temperature is not quite so critical as it might appear to be. The speed of sound, Ao, is given by Eq. 2.1.1:
Ao=V[g*R*(TexC+273)]
The calculation of LT is based on the return of the plugging pulse within the exhaust period. The most effective calculation criterion, based on many years of empirical observation, is the simplest. The reflection period is decreed to be the exhaust period and the speed of wave propagation is the local speed of sound in the pipe mid-section. Much more complex solutions have been used from time to time for this empirical calculation, but this approach gives the most reliable answers in terms of experimental verification from engine testing.
As with Eq. 6.2.1, the time, t, taken for this double length travel along the tuned length, LT, must equal the reflection period, where:
t=(EP/360)*(60/RPM)=2*(LT/1000)/Ao
Rearranging for the pipe length, LT, which is in mm units:
LT=83.3*EP*Ao/RPM mm (6.2.2) The remainder of the solution is straightforward, following the nomenclature of
Fig. 5.2(b). The end of the diffuser is positioned as in Fig. 6.10, about two-thirds of the distance between the exhaust port and the tail-pipe entry.
LP01=0.10*LT LP 12=0.41*LT LP23=0.14*LT LP34=0.11*LT LP45=0.24*LT LP56=LP45
The preferred value of DEP1 =K 1 *EXD. The value for K1 ranges from 1.125 for broadly tuned enduro type engines to 1.05 for road-racing engines of the highest specific power output.
The preferred value of DEP3=K2*EXD. The normal design criterion is for the value of DEP4 to be the same as DEP3. The value for K2 ranges from 2.25 for
Chapter 6 - Empirical Assistance for the Designer broadly tuned enduro type engines to 3.25 for road-racing engines of the highest specific power output.
The remaining diameter to be determined is that for DEP2 and it is estimated by the following expression:
DEP2=DEP3*[LP12/(LP12+LP23)]1 (6.2.3)
It is self-evident that all diameters in the previous discussion are the internal diameters of the exhaust pipe, and that all lengths are distances along the center-line of a pipe which may be curved in any of its segments.
Some years ago, the author presented a paper concerning the empirical design of expansion chambers(2.17). It is instructive to compare the above discussion with that paper and to note that the empiricism has actually become simpler with the passage of both time and experience.
For those who wish to have these calculations performed automatically, the Computer Program Appendix lists Prog.6.2, "EXPANSION CHAMBER." This carries out the useful dual function of deducing all of the above dimensions and data and the computer screen and line printer will show a dimensioned sketch of the exhaust system. An example of that is shown for a prediction of an expansion chamber for the 125 Grand Prix engine used as one of the working examples in these chapters. The screen/line-print output is shown in Fig. 6.11. If the data predicted by the empirical design program is compared with those listed in Fig. 5.4(b), a considerable level of close correspondence will be observed. It will be recalled that the engine modelling program predicted a high specific power performance for this engine and exhaust system in Fig. 5.23. This should give the reader some confidence that this simple program, Prog.6.2, albeit overlaid with some considerable practical experience, enables the rapid and efficient preparation of the geometry of an expansion chamber for further complete analysis by the engine modelling program, Prog.5.2.
GRAND PRIX PIPE
DESIGN LENGTHS AND DIAMETERS OF AN EXPANSION CHAMBER FOR A TWO-STROKE B0RE= 54 STROKE; 5 4 C0N-R0D= 105 EXHAUST 0PENS,2atdc= 61 EXHAUST P0RTS=1 WIDTH EACH PORT; 41 EXHAUST PORT TOP RADIUS; 12 AND BOTTOM RADIUS; 12 ENGINE SPEED; 11500 EXHAUST TEMPERATURE.2C; 6 0 0
AREA RATIOS FOR PORT TO PIPE=1.117 AND PORT TO MID-SECTION; 9 70
Fig. 6.11 Example of design of expansion chamber for a 125 Grand Prix engine from Prog.6.2.
i e Basic Design of Two-Stroke Engines
P \nother useful feature of this empirical program, Prog.6.2, is that there is
m lilable a second page of line-print output. This gives the manufacturing data fortk various cones of the expansion chamber so that they may be marked out and
d led from flat sheet metal, then subsequently seam welded into the several cones.I e cones and parallel sections are butt welded to form a pipe for either competition
* dynamometer testing. An example of the actual line printer output for the Grand HP x engine design of Fig. 6.11 is shown in Fig. 6.12. In that figure, RX and RY are fr radii from the center of a circle marked on the metal sheet, and 0 is the angle of fc flat sheet segment. This information alone is quite sufficient to permit the
m nplete marking of the sheet metal. The extra information for the chords, CHX and(S IY, allows an alternative method of marking out the flat sheet metal segment, or
p vides a cross-check on the accuracy of setting out the angle 0.CHV FLAT SHEET TO
ROLL INTO CONE
DX DV LC RX RY CHX CHV
02
DIFFUSER CONES C0NE1
38 76 348 350 699
1 19
237
19.5
C0NE2
76
1 12 1 19
249 368 229 339 55
TAIL PIPE CONE
22
1 12
204 49 253 62 324 80
CONE DIMENSIONS
FLAT PLATE DIMENSIONS
Fig. 6.12 Example of manufacturing data available as computer output from Prog.6.2.
i 6.2.6 Concluding remarks on data selection
I By the methods discussed above the information is selected for the data bank for
fcje main engine geometry and porting, as typified by the set in Figs. 5.3 and 5.4.I ose who pursue this empirical approach by using the several computer packages
k i the creation of the data bank necessary for the operation of an engine modellingprogram will discover that with a little practice the data bank can be assembled in a remarkably short time. This approach reduces the number of times which the engine modelling programs need to be run to create an optimized design to meet the perceived requirement for the performance characteristics. As the engine modelling programs take the longest time to run on the computer, indexing in the first instance the virtually instantaneous response of the empirical design programs is an effective use of the designer's time. Perhaps more important, it tends to produce porting and exhaust systems which are well matched. An example of that is the design for the chainsaw, where the transfer and inlet ports would have been better matched by an initial application of the empirical approach.
There will be the natural tendency, about which the reader is cautioned earlier, to regard the empirical deductions as a final design for either the porting or the exhaust system. That warning is repeated here, for the author has made that mistake sufficiently often to realize its validity!
6.3 The design of alternative induction systems for the two-stroke engine
In Sect. 1.3, there is an introduction to the use of mechanical devices which permit asymmetrical timing of the exhaust or induction process. The three types discussed are poppet valves, disc valves and reed valves. Needless to add, the inventors of this world have produced other ingenious devices for the same purpose, but the three listed have withstood the test of time and application. It is proposed to discuss the design of disc valves and reed valves in this section, but not poppet valves for they are covered in the literature of four-stroke cycle engines to an extent which would make repetition here just that. As the design of reed valves and disc valves is not covered extensively in the literature, it is more logical that the appropriate space within this book should be devoted to the "unknown."
The use of reed valves for the induction system has always been common for outboard motors and increasingly so for motorcycle and other types of two-stroke engines. The incorporation of a reed valve into an engine is shown in Fig. 1.7(b) and the details of a reed block assembly are sketched in Fig. 6.13, with a photograph giving further illustration in Plate 6.1. The technology of the design has improved greatly in recent times with the use of new materials and theoretical design procedures. The new materials, such as plastics reinforced by either glass-fiber or carbon-fiber, are effective replacements for the conventional use of spring steel for the reed petal. In particular, any failure of a plastic reed petal in service does not damage the engine internally, whereas it would in the case of a steel reed.
The use of disc valves for the induction system has been confined mostly to high-
performance racing engines although there are some production examples of lesser
specific power output. The initial discussion in Chapter 1 gives a sketch of the
installation in Fig. 1.7(a) and Plate 1.8. A more detailed drawing of the disc and the
inlet port which it uncovers is shown in Fig. 6.14. There is not a large body of
technical information available on the design and development of disc valves, but
the papers by Naitoh et al(6.4)(6.5) and the book by Bossaglia(6.3) contain valuable
insights to the design and development process.
SECTION OH ft
TOP HALF SHOVS REED PETAL EXPOSED AS STOP PLATE IS REMOVED
BOTTOM HALF SHOVS THE PORT EXPOSED AS REED PETAL IS REMOVED
ELEVATION ON R
Fig. 6.13 Significant dimensions of reed valves and block required for design purposes.
Plate 6.1 A reed valve block with six steel reeds and stop-plate. The block is rubber coated to reduce both noise and damage to bouncing reeds.
Chapter 6 - Empirical Assistance for the Designer
Fig. 6.14 Controlling dimensions of disc valve timings and inlet port area.
The incorporation of these induction systems into an engine modelling program is straightforward in the case of the disc valve and complex for a reed valve design.
That it has been successfully achieved for the reed valve case is evident from the technical papers presented(1.13)(4.10)(5.9). Actually there is a considerable body of literature on reed valves when used in air and refrigeration compressors as automatic valves; the technical paper by MacClaren(6.6) will open the door to further references from that source. The modelling of the disc valve case within an engine design program, such as Prog.5.1 or 5.2, is regarded as requiring a simple extension to the program to accommodate the inlet port area geometry as uncovered by the disc. The discussion in Sect. 6.4 clarifies this statement.
However, just as the parameters for the engine in Figs. 5.3 and 5.4 have to be assembled in some logical manner, it is necessary to be able to arrive at an initial decision on the dimensions of the reed or disc valve in advance of either the use of a computer modelling package or the experimental testing of prototype devices. The following discussion should be of practical assistance within that context.
6.3.1 The empirical design of reed valve induction systems
The reed valve induction system is installed in the engine between the atmos- phere and the crankcase of the engine, as shown in Figs. 1.7(b) and 6.13. If the carburetor is the fueling device, it is placed between the reed valve and the atmosphere. If low-pressure fuel injection is used, the injector nozzle can be situated in the same position as a carburetor, but it is also possible to inject the fuel directly into the crankcase. A photograph of a cylinder of a motorcycle engine with a reed induction system appears in Plates 1.9 and 4.1 and a closer view of the block, petals and stop plate is shown in Plate 6.1. The typical opening and closing characteristics of this automatic valve are illustrated in Fig. 1.8(c). To expand this information, and to illustrate the point made above regarding the incorporation of a reed valve model into an engine simulation program, a few of the results from the experimental and theoretical paper by Fleck et al( 1.13) are presented here as Figs. 6.15-6.17. The engine used as the research tool in this paper is the high-performance YPVS RD350LC twin-cylinder Yamaha motorcycle engine, with a peak BMEP of nearly 8 bar at 9000 rpm. Each cylinder has a block holding four reed petals and ports. In this case, designated as RV1, they are steel reed petals of 0.20 mm thickness, i.e., from Fig. 6.13 the dimension RDTH=0.20.
IBC BDC IB0
T28~ 1"B0 208 210 280 320 3B0 CRANKANGLE
Fig. 6.15 The crankcase and inlet port pressure and reed lift behavior at a low engine speed.
Chapter 6 - Empirical Assistance for the Designer
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CD
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1 .50 1 .10 o i ,3a
t\ 1.20 5 1.10
<~ z a 1 .00 z <
w 0 . 9 0
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CRANKANGLE
6.76 The crankcase and inlet port pressure and reed lift behavior at a high engine speed.
RV1
Predicted Measured
4000 5000 500C 7000 8000 9000 Engine speed (rev/min)
10000
Fig. 6.17 The comparison between measured and calculated delivery ratio from an engine model incorporating a reed valve simulation.
The Basic Design of Two-Stroke Engines
The upper portion of Fig. 6.15 shows the crankcase and inlet port pressure at a
"low" engine speed of 5430 rev/min. Those pressure diagrams are predicted by the engine simulation program and one can see that they are very similar in profile to those observed for piston ported engines in Chapter 5. In short, there is nothing unusual about the pressure difference across the reed valve by comparison with that observed for a piston controlled induction system. The solid line in the lower half of that figure shows the measured reed tip lift in mm, and that predicted by the reed valve simulation within the computer program is the dashed line. The close correspondence between the calculation and measurement of reed lift is evident, as is the resulting calculation and measurement of delivery ratio over the entire speed range of the engine in Fig. 6.17. The timing of opening and closing of the reed petal at the low speed of 5430 rpm shows the reed opening at 160
sbtdc and closing at 86
satdc, which is an asymmetrical characteristic about tdc.
At the "high" engine speed of 9150 rpm, the reed petal opens at 140° btdc and closes at 122° atdc. This confirms the initial view, expressed in Sect. 1.3.4, that the reed petal times the inlet flow behavior like a disc valve at low engine speeds and as a piston controlled intake port at high engine speeds. In other words, it is asymmetrically timed at low speeds and symmetrically timed at high speeds.
Within this same paper by Fleck et al (1.13), there is given a considerable body of experimental and theoretical evidence on reed valve characteristics for steel, glass- and carbon-fiber reinforced composites when used as reed petal materials.
To help with the correlation of the discussion in this book and that paper, the nomenclature for the reed block and petals shown in Fig. 6.13 is identical in both instances. The paper(1.13) also gives the dimensions of the engine with its tuned expansion chamber exhaust system and associated measured power data, which the reader will find instructive as another working example for the comparison of Prog.6.2 with experiment.
6.3.2 The use of specific time area information in reed valve design
From the experimental and theoretical work at QUB on the behavior of reed valves, it has been found possible to model in a satisfactory fashion the reed valve in conjunction with an engine modelling program. This implies that all of the data listed as parameters for the reed valve block and petal in Fig. 6.13 has to be assembled as input data to run that engine model. Naturally, this data would replace the block of inlet port data for a piston controlled port in Fig. 5.3. The data set for a reed valve is even more extensive than that for a piston controlled intake port, adding to the complexity of the data selection task by the designer before running the engine model. This places further emphasis on the use of some empirical design approach to obtain a first estimate of the design parameters for the reed block and petals, before the insertion of that data set into an engine modelling calculation. It is proposed to pass on to the reader the data selection experience of the author, in the form of an empirical design for reed valves as a pre-modelling exercise.
In Sect. 6.1 there is a discussion on specific time area and its relevance for exhaust, transfer and intake systems. The flow through a reed valve has to conform to the same logical approach. In particular, the value of specific time area for the reed
Chapter 6 - Empirical Assistance for the Designer