EXHAUST
Chapter 3 Scavenging the Two-Stroke Engine
excellent correlation with experiments conducted under firing conditions. Some of those results are worth repeating here, for that point cannot be emphasized too strongly.
The experimental performance characteristics, conducted at full throttle for a series of modified Yamaha DT250 engine cylinders, are shown in Fig. 3.7. Each cylinder has identical engine geometry so that the only modifications made were to the directioning of the transfer ports and the shape of the transfer duct. Neither port timings nor port areas were affected so that each cylinder had almost identical SR characteristics at any given rotational speed. Thus, the only factor influencing engine performance was the scavenge process. That this is significant is clearly evident from that figure, as the BMEP and BSFC behavior is affected by as much as 15%. When these same cylinders were tested on the single cycle gas scavenging rig, the SE-SR characteristics were found to be as shown in Fig. 3.8. The figure needs closer examination, so a magnified region centered on a SR value of 0.9 is shown in Fig. 3.9. Here, it can be observed that the ranking order of those same cylinders is exactly as in Fig. 3.7, and so too is their relative positions. In other words, Cylinders 14 and 15 are the best and almost as effective as each other, so too are cylinders 9 and 7 but at a lower level of scavenging efficiency and power. The worst cylinder of the group is cylinder 12 on all counts. The "double indemnity"
nature of a loss of 8% SE at a SR level of 0.9, or a TE drop of 9%, is translated into the BMEP and BSFC shifts already detailed above at 15%.
A sustained research and development effort has taken place at QUB in the experimental and theoretical aspects of scavenging flow. For the serious student of the subject, the papers published from QUB form a series, each using information and thought processes from the preceding publication. That reading list, in consecu- tive order, is (3.6), (1.23), (3.13), (3.20), (1.10), (3.23), (1.11), and (3.17).
3.2.4 Comparison of loop, cross and uniflow scavenging
The QUB single cycle gas scavenge permits the accurate and relevant compari- son of SE-SR and TE-SR characteristics of different types of scavenging. From sane of those previous papers, and from other experimental work at QUB hitherto u^mblished, test results for uniflow, loop and cross scavenged engine cylinders are pitsented to illustrate the several points being made. At this stage the most inportant issue is the use of the experimental apparatus to compare the various methods of scavenging, in order to derive some fundamental understanding of the efectiveness of the scavenging process conducted by these several methodologies.
In Sect. 3.3 the information gained will be used to determine the theoretical refevance of this experimental data in the formulation of a model of scavenging to reincorporated within a complete theoretical model of the firing engine.
Figs. 3.10 and 3.11 give the scavenging and trapping characteristics for six
engine cylinders, as measured on the single cycle gas scavenging rig. It will be
ote-erved that the test results fall between the perfect displacement line and perfect
mking line from the theories of Hopkinson(3.1). By contrast, as will be discussed
inSect. 3.3, some of the data presented by others(3.3)(3.4) lie below the perfect
raking line.
~ Scovenging Choroc*eristi Cylinder 7 — Cylinder 9 Cylinder 12 Cylinder H Cylinder 15
Scovenge rotiolVolui
0-90 092 0-94 0-96 0 9 8 V0
Fig. 3.9 Magnified region ofSE-SR characteristics in Fig. 3.8.
1.2
SE-SR CHARACTERISTICS
PERFECT DISPLACEMENT SCAVENGING
SE=1-e"S R PERFECT MIXING LINE
SE UNIFLOV SE SCRE SE GUBCR SECD SE YAM1 SE YAM6 SE PERFECT DISP SE PERFECT MIX SCAVENGE RATIO
Fig. 3.10 SE-SR characteristics for the test cylinders.
Chapter 3 - Scavenging the Two-Stroke Engine
1 . 1 -
1 . 0 -
£ 0 . 9 -
± u u.
u 0 . 8 - (9 Z
Z
% 0 . 7 - at
0 . 6 -
0 . 5 -
TE--SR CHARACTERISTICS
• i i
-+- TE UNIFLOV -*- TESCRE -er TEQUBCR -*- TE CD
•*• TEYAM1 -D- TEYAM6
l ~"
SCAVENGE RATIO 1
Fig. 3.11 TE-SR characteristics for the test cylinders.
The cylinders used in this study, in order of their listing on Figs. 3.10 and 3.11, are:
(a) A ported uniflow scavenged cylinder of 302 cc swept volume, called UNIFLOW. It has a bore stroke ratio of 0.6, and the porting configuration is not dissimilar to that found in the book by Benson(1.4, Vol.2, Fig. 7.7, p.213). The engine details are proprietary and further technical description is not possible.
(b) A loop scavenged cylinder of 375 cc swept volume, called SCRE. This engine has three transfer ports, after the fashion of Fig. 3.38.
(c) A 250 cc QUB cross scavenged cylinder, called QUBCR, and previously described in (1.10) in considerable detail. The detailed porting geometry is drawn in that paper. The general layout is almost exactly as illustrated in Fig. 1.4.
(d) A 250 cc classic cross scavenged cylinder, called CD, and previously described in (1.10) as the classic deflector. The detailed porting geometry is drawn in that paper. The general layout is almost exactly as illustrated in Fig. 1.3.
(e) A 250 cc loop scavenged cylinder, modified Yamaha DT 250 cylinder no. 1, and called here YAM1, but previously discussed in (3.20). The detailed porting geometry is drawn in that paper.
(f) A 250 cc loop scavenged cylinder, modified Yamaha DT 250 cylinder no.6, and called here YAM6, but previously discussed in (3.20). The detailed porting geometry is drawn in that paper.
The disparate nature of the scavenging characteristics of these test cylinders is clearly evident. As might be expected, the uniflow scavenged cylinder is very good, indeed it is the best, but not by the margin suggested by Changyou(3.4). The behavior of the two types of cross scavenged cylinders is interesting, ranging from an equal-best situation at SR=0.4 to near "perfect mixing" mediocrity for the CD cylinder when SR=1.4. The QUBCR cylinder has a scavenging behavior which is quite similar to the best loop scavenged cylinder, SCRE. This SCRE cylinder is seen to have very superior scavenging to YAM1, even though the internal porting configuration is not too dissimilar.
When assessing the effectiveness of scavenging at light load, i.e., a low BMEP condition, it is worth remembering that the scavenge ratio axis is directly related to throttle opening. By definition, a small throttle opening will permit a reduced air flow to be induced into the engine; a low SE gives a low charging efficiency, CE, and hence a low torque output under firing conditions. Some of the points made in connection with the good low-speed and low-power characteristics for conven- tional cross scavenged engines in Sect. 1.2.2 can now be seen to be accurate. Only the CD, QUBCR and UNIFLOW engines have high trapping efficiencies at low scavenge ratios, whereas the best of loop scavenged engines, SCRE, is some 6%
inferior in this SR region. In Sect. 3.1.5, this matter was discussed in terms of the ability of the engine to have the best possible power, fuel economy and emission characteristics at idle speed or at light load. There are measured performance characteristics which substantiate the statements above: a QUBCR engine and a loop scavenged engine with scavenging behavior at least equal to the SCRE cylinder, each of 400cc swept volume and both carburetted with identical exhaust systems, have idle fuel flow rates of 150 and 275 g/h, respectively.
The inferior nature of the scavenging of the cross scavenged engine, CD, at full throttle or a high SR value, is easily seen from the diagrams. It is for this reason that such power units have generally fallen from favor as outboard motors. However, in Sect. 3.5.2 it is shown that classical cross scavenging can be optimized in a superior manner to that already reported in the literature( 1.10) and to a level which is rather higher than a mediocre loop scavenged design. It is probably true to say that it is easier to develop a satisfactory level of scavenging from a cross scavenged design, by the application of the relatively simple empirical design recommendations of Sect. 3.5, than it is for a loop scavenged engine.
The modified Yamaha cylinder, YAM6, has the worst scavenging overall. Yet, if one merely physically examined the cylinders YAM1, YAM6 and SCRE, it is doubtful if the opinion of any panel of "experts" would be any more unanimous on the subject of their scavenging ability than they would be on the quality of the wine being consumed with their dinner! This serves to underline the important function served by an absolute test for scavenging and trapping efficiencies.
3.3 Comparison of experiment and theory of scavenging flow
3.3.1 Analysis of experiments on the QUB single cycle gas scavenging rig As has already been pointed out, the QUB single cycle gas scavenging rig is a classic experiment conducted in an isovolumic, isothermal and isobaric fashion.
Therefore, one is entitled to compare the measurements from that apparatus with the theoretical models of Hopkinson(3.1), Benson and Brandham(3.2), and others(3.3), to determine how accurate they may be for the modelling of two-stroke engine scavenging.
Eq. 3.1.19 for perfect mixing scavenging is repeated here:
SE=l-e(SR) (3.1.19)
Manipulation of this equation shows:
loge(l-SE)=-SR (3.3.1) Consideration of this equation for the analysis of any experimental SE and SR
data should reveal a straight line of equation form y=mx+c, with a slope of value - 1 and an intercept at y=0 and x=0.
The Benson-Brandham model contains a perfect scavenging period, PS, before total mixing occurs, and a short-circuited proportion, SC. This resulted in Eqs.
3.1.23 and 24, repeated here:
If (1-SC)*SR<PS then SE=(1-SC)*SR (3.1.23) If (1-SC)*SR>PS then SE=l-(l-PS)*e(PS(lsC)*SR) (3.1.24)
Manipulation of this latter equation reveals:
loge(l-SE)=(SC-l)*SR+logc(l-PS) +PS (3.3.2)
Again, further consideration of this equation on the straight line theory that y=mx+c shows that test data of this type should give a slope of (SC-1) and an intercept of loge(l-PS)+PS. Any value of short-circuiting other than 0 and the maximum possible value of 1 would result in a line of slope m, where 0>m>-l. The slope of such a line could not be less than -1, as that would produce a negative value of the short-circuiting component, SC, which is clearly theoretically impossible.
Therefore it is interesting to examine the experimentally determined data presented in Sect. 3.2.4. The analysis is based on plotting log(l-SE) as a function of SR from the experimental data for two of the cylinders, SCRE and YAM6, as examples of "good" and "bad" loop scavenging, and this is shown in Figs. 3.12 and 3.13. It is gratifying that the experimental points fall on a straight line. In reference (1.11), the entire set of cylinders shown in Figs. 3.10 and 3.11 are analyzed in this manner and are shown to have a similar quality of fit to a straight line. What is less gratifying, in terms of an attempt at correlation with a Benson and Brandham type
The Basic Design of Two-Stroke Engines
of theoretical model, is the value of the slope of any of the lines. All of the slopes lie in the region between -1 and -2. A summary of the findings is shown in Fig. 3.14 for selected cylinders, representing (a) the uniflow cylinder as an example of the best scavenging yet observed, (b) the SCRE cylinder as an example of very good loop scavenging, (c) the YAM1 cylinder as an example of quite good loop scavenging, (d) the YAM6 cylinder as an example of bad loop scavenging, (e) the CD cylinder as a somewhat mediocre example of conventional cross scavenging, and (e) the QUBCR cylinder as an example of QUB cross scavenging with radial ports. The slope which is nearest to correlation with a Benson-Brandham type of theoretical model is the CD cylinder with a slope of -1.0104, yet it is still on the wrong side of the -1 boundary value. All the rest have values closer to -2 and, clearly, the better the scavenging the closer the value is to -2. Therefore, there is no correlation possible with any of the "traditional" models of scavenging flow, as all of those models would seriously underestimate the quality of the scavenging in the experimental case.
It is possible to take the experimental results from the single cycle rig, plot them in the logarithmic manner illustrated and derive mathematical expressions for SE- SR and TE-SR characteristics representing the scavenging flow of all significant design types, most of which are noted in Fig. 3.14. This is particularly important for the theoretical modeller of scavenging flow who has previously been relying on models of the Benson-Brandham type.
The straight line equations, as seen in Figs. 3.12 and 3.13, and in reference (1.11), are of the form:
log
e(l-SE)=M*SR+C
(3.3.3)-3
-o- LOG 1-SE,SCRE 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.
SCAVENGE RATIO
Fig. 3.12 Log J1-SE)-SR plot for the SCRE cylinder.
Chapter 3 - Scavenging the Two-Stroke Engine
-2.0
y = 0 . 1 4 3 5 - 1 . 3 5 1 6 x R = 1.00
YAMAHA CYLINDER 6
-Q- LOG 1-SE,YAM6
'" I ' I ' I ' I ""'' " " I ' ""'
0.2 0.4 0.6 0.S 1.0 1.2 1.4 SCAVENGE RATIO
Fig. 3.13 LogJl-SE)-SR plot for the modified Yamaha cylinder No. 6.
NAME UNIFLOW SCRE YAM1 YAM6 CD QUBCR
TYPE
PORTED UNIFL0V 3 PORT LOOP 5 PORT LOOP 5 PORT LOOP CLASSIC CROSS QUB CROSS
COMMENTS
VERY 0 0 0 D SCAVENGING GOOD SCAVENGING QUITE GOOD SCAVENGING BAD LOOP SCAVENGING NORMAL CROSS TYPE QUB RADIAL PORTS
SLOPE,M - 1 . 7 8 2 7 - 1 . 6 7 0 9 - 1 . 6 9 9 3 - 1 . 3 5 1 6 - 1 . 0 1 0 4 - 1 . 6 3 2 5
INTERCEPT, C 0 . 2 0 9 4 0 . 1 8 9 9 0 . 3 0 5 3 0 . 1 4 3 5 - 0 . 1 1 7 0 . 1 3 9 7
Fig. 3.14 Experimental values of slope and intercept from log (l-SE)-SR plots.
Manipulating this equation shows:
SE=l-e
lM*
SR+C) (3.3.4)Therefore, the modeller can take the appropriate M and C values for the type of cylinder which is being simulated from Fig. 3.14 and derive realistic values of scavenging efficiency, SE, and trapping efficiency, TE, at any scavenge ratio level, SR. What this analysis thus far has failed to do is to provide the modeller with information regarding the influence of "perfect displacement" scavenging, or
"perfect mixing" scavenging, or "short circuiting" on the experimental or theoreti-
cal scavenging behavior of any of the cylinders tested.
The Basic Design of Two-Stroke Engines
3.3.2 A simple theoretical scavenging model which correlates with experi- ments
The problem with all of the simple theoretical models presented in previous sections of this chapter, is that the theoretician involved was under some pressure to produce a single mathematical expression or a series of such expressions. Much of this work took place in the pre-computer age, and readers who emanate from those slide rule days will appreciate that pressure. Consequently, even though Benson or Hopkinson knew perfectly well that there could never be an abrupt transition from perfect displacement scavenging to perfect mixing scavenging, this type of theoretical "fudge" was essential if Eqs. 3.1.8 to 3.1.24 were to ever be realized and be "readily soluble," arithmetically speaking, on a slide rule. Today's computer and calculator oriented engineering students will fail to understand the sarcasm inherent in the phrase, "readily soluble." It should also be said that there was no experimental evidence against which to judge the validity of the early theoretical models, for the experimental evidence contained in paper (3.20), and here as Figs.
3.10-14, has only been available since 1985.
Consequently, once the need was established, the production of a simple theoretical model of scavenging which could be solved on a microcomputer and did not contain a step change in flow behavior, was a relatively straightforward process.
This is given as Prog.3.2 called BLAIR SCAVENGING MODEL and in the Computer Program Appendix as ProgList.3.2. This model of scavenging does not contain a step-function behavior but allows the scavenging process to proceed in a continuous fashion from start to finish, albeit containing the necessary elements of perfect displacement scavenging, mixing scavenging and short-circuiting. Such a theoretical postulation would be considered logical from the more advanced CFD calculations carried out by Sweeney(3.23) and discussed in Sect. 3.4.
The proposed model of scavenging flow is illustrated in Fig. 3.16, with reference to the geometry shown in Fig. 3.15. The variations of perfect scavenging and short- circuiting are functions of scavenge ratio. At any given level of scavenge ratio, SR, a proportion of the entering increment of scavenge air, dVas, is going to be short- circuited with value S, while the remainder of the entering air goes into the perfect scavenge volume, PSV. Of the air which enters the perfect scavenge volume, a proportion of value Y stays, and the remainder, proportion 1-Y, strips off the
"surface" of the perfect scavenge volume and enters the mixing volume. The calculation method is very simple, as an examination of the computer program written in BASIC in ProgList.3.2 will reveal. The calculation proceeds to enter air increments of dVas equal to 1, up to any desired maximum value, with the cylinder volume set at V=1000. Hence the incremental change of scavenge ratio, dSR, is dVas/V, and the new scavenge ratio, SR, is increased by dSR. The entering increment of airflow, dVas, is distributed in various directions:
(a) the amount to be short-circuited, dSC, where dSC=S*dVas, (b) the remainder to be distributed, dVrem, where dVrem=dVas-dSC, (c) the amount perfectly scavenged, dVd, where dVd=Y*dVrem,