Journa l of Research of the Nationa l Burea u of Standards Vol. 57, No.6, December 1956 Research Paper 2721

Scavenging Characteristics of a Two-Stroke-Cycle Engine as Determined by Skip-Cycle Operation

P. M. Ku and T. F. Trimble

A method for determin ing t he mass fraction of fresh charge in t he cylinder of t he two­sLrok e-cycle engine, from m easuremen ts of engine power under normal operation and wi th engine fired only once in several cycles of operation is described. T he r esults obtain ed from a crankcase-scav enged en gine are presented .

1. Introduction

The Lwo-stroke cycle engine, in order to develop one power cycle for every r evolution of th e crank­shaft , mu sL rely on the acLion of an auxiliary device, known as the scavenging pump, to expel th e combus­t ion products from th e cylinder and to fill the cyl­inder wi th fresh charge. Th e scavenging pump may be and frequenLly is physically separate from the engin e cylinder proper. In th e crankcase-scavenged engine, the cylinder-crankcase-piston combination serves as the scavenging pump .

The ideal objective of the scavenging proccss is, of course, to replace compleLel.v th e combustion prod­ucts in Lh e engine cylinder wiLh fresh cbarge delivered throu gh Lhe inleL porLs, wiLhou t mean t ime losing any of the fresh charge through the exhaust ports. In this manner , maximum power is derived from th e cylinder wi th th e leasL expenditure of scavenging­pump power . I n practice, such an ob.i ective is never attainable, h ence the cxcellence of the scave nging process must be judged by t he mass of fr esh charge r etained in th e engine cylinder per uni t time, in rela tion to the mass of fresh charge supplied per uni t t ime by th e scavenging pump .

It follows from the above tha t two mass rates per uni t time, viz ., th at supplied by th e scavenging pump and th at retained in the engine cylinder , are required to defin e the scavenging characteristics of an engine. Of these two quantities, th e mass supplied b~r the scavenging pump per unit t ime can be directly m easured by the use of a flowmeter placed in th e inlet system. The mass retained in the engine c~-l­incler per un it time is not, however, subj ect to a direct measuremen t. Fo r its determination, a num­ber of indirect me thods have bee n devi sed and studied [1,2).1 In general, such met.h ods are ei ther extremely laboriolls, or li abl e to co nsiderable errors. This paper describes a rather simple method for the determination of an importa nt rela led qu antit.\" th e mass fraction of fresh ch arge in the engin e cylin der after port closure. On the basis of tes ts perfonnecl on a crankcase-scavenged engine, this method ap­peared to be quite practi cal and reliable.

1 Figures in brackets indicate the literature references at the end of th is paper.

2. Nomenclature

For analy t ical purposes, it is convenient to redu ce th e two mass rates men tioned above to nondimen­sional quantities. In this connection, several cEf­feren t systems of nondimensionalization [1 ,3] have been proposed. E ach proposed sys tem has cer tain a t tractive features, t hus the par ticular choice is, at least to some extent, a matter of personal preference.

A s~Ts tem ra ther similar to tha t employed by Taylor and Rogowski [1] is used . To nondimen­sionalize the mass of fresh charge supplied by the scavenging pump per unit time, th e term "scaveng­ing ratio" , Rs, as defined belOW, is introduced,

R _ ]o.£g • s-PsVdN

(1)

T o nonclimensionali ze the mass of fresh ch arge re­tain ed in th e engine cylinder per uni t t ime, th e t erm "scavenging e ffic ienc~-", es, is in trodu ced,

where

Af T es= V 7o. T

Ps ai ' (2)

Mg=mass of fresh ch arge supplier] b~' Lhe scavenging pump pel' uni t time,

Mr=mass of fresh charge retain ed in th e engine cylinder after por t closure, per uni t of t ime,

ps= density of fresh charge compu ted on Lb e basis of inlet tpmpcrature and exhaust pressure,

Va= displacemenl volume, N = engin e speed , revolut ions p CI' uni t time.

I n the above expressions, displacement volume Ild is used b asically as a ma LLer of conve nience, for this is the volume used in definin g the mean crfect ive pressm e of the engine. The introdu ction of exh aust pressure into density term Ps is especiall convenien t for engines with la Le exhaust closing, in which the exhaust pressure rather than the inlet pressure exercises controlling influence on th e density of the cylinder contents at the time of port closure.

4080G3- 56--2 325

For the purpose of the present paper, it is helpful to note that the quantity Mr is related to the cylinder volume at the instant of port closure , as follows:

(3)

where e= mass fraction of fresh charge in the engine

cylinder after port closure, px= average density of the cylinder contents at

the instant of port closure, Vx= cylinder volume at the instant of port

closure. Combining eq (2) and (3), the scavenging efficiency may then be written

(4)

This paper deals with a method for the determina­tion of the quantity e, and eq (4) shows the relation between this quantity and scavenging efficiency. The ratio Vx/Vd is obviously determined by port timing. No attempt has been made to determine the ratio Px/ Ps in the present investigation. How­ever, some qualitative remarks may be in order here. The ratio Px/ Ps is, in general, influenced by heat transfer between the fresh charge and the engine parts, flow resistance at the inlet and exhaust ports, and the dynamics of flow in the scavenging system. For engines with late exhaust closing, if the heat transfer, flow resistance, and flow dynamics effects were negligible, then the ratio Px/ Ps obviously would become unity. Thus, Px/Ps provides a measure of the combined effect of heat transfer, flow resistance and flow dynamics in this instance.

For engines with late inlet closing, the relation may best be examined by writing

where Pi= density of fresh charge at inlet temperature

and inlet pressure, pt= inlet pressure, absolute, p .=exhaust pressure, absolute.

Applying this expression to eq (4),

Here the ratio p dp. accounts for the static effect of inlet pressure, as against the exhaust pressure used in the definition of scavenging efficiency. For engines with late inlet closing, the ratio Px/ Pi provides a measure of the combined effect of heat transfer, flow resistance, and flow dynamics . In the ideal case where these effects are negligible, the ratio Px/Pi reduces to unity.

3. Principle of Skip-Cycle Method

In the normal operation of a two-stroke cycle engine, when the engine fires every cycle, the cylinder is full of combustion products at the begin-

ning of the scavenging process. At the end of the scavenging process, only a fraction of the combustion products remains in the cylinder. Now, consider the case when the engine is allowed to fire for one cycle and then skip, or run without firing, for several cycles. Obviously, any combustion products that remain in the cylinder must come from the firing cycle. Accordingly, the progressive scavenging ac­tion of the skipped cycles results in progressively less combustion products, or progressively more fresh charge, in the cylinder after each skipped cycle.

The above condition can be rather satisfactorily represented by a simple mathematical relationship . Let j denote the mass fraction of residual gas re­tained in the cylinder during normal operation. The mass fraction of fresh charge during normal operation is then

e= l - .f. (6)

Let it be assumed that the conditions of heat trans­fer, flow resistance and flow dynamics are not signifi­cantly influenced by skip cycling- an assumption which appears to be justified by the experimental results reported herein. In that case, the total mass of retained cylinder contents would remain substantially constant, and likewise the mass fraction of cylinder contents to be passed from one cycle to the next. Consequently, if the engine is allowed to fire only once in n cycles of operation, then for each of the firing cycles, the mass fraction of combustion products remaining in the cylinder would be jn, and the mass fraction of fresh charge in the cylinder after port closure would be

(7)

Figure 1 gives a plot of (l - jn) j ( l - j) versus f, for several values of n. The curve for n= ex:>

represents the limiting case where the cylinder con­tains nothing but fresh charge.

0 . I

I ,.J~ "

1 1lL I "

I 1/1/ 1/// .I

60

0 I

/; V/v .0

Jt~ 1/ v / /

0

~ t=:== L----------I--- ' ~

~ " ~ r:::::--- I ,-...--. 10

ID

FIGURE 1. Plot of (1 -fn) / (1 - f) versus f.

326

The skip-cycle method for the determination of E is based on the premise that at constant fuel-air ratio and bes t-power ignition timing, the indicated thermal effi ciency of a given internal combustion engine is very nearly constan t over a wide range of operating conditions. The indicated mean effective pressure of a two-stroke cycle engin e supplied with a premi;{ed fuel-air charge, a nd operating normally, can be exp rC'ssed, in terms of scavenging effic i e n c~T defin ed previou sl~-, as follows,

(8)

where

imep = illdicated mean effective pressure (nor­mal operation),

F = fuel-air ratio Ec= heating valu~ of the fuel ,

1) ;=indicated thermal effi ciency based on fu el quantity retained in t he cylinder,

J = m ec hanical equivalent of heat.

Appl~'ing eq (4) to this expression ,

(9)

Under skip-c.\Tcle operation, eq (9) can be modified to read

(10)

where imepn is t he imep of tJIC firing cycle wiLh firing occurring once every n cycles of operation. The quantities Px, F, and 1) i a re constant, under the as­sumptions made previously.

Combining eq (9) and (J 0), and introducing the values for e and en as given b~- eq (6) and (7),

imepn e 1- f " imep = en= 1- f' (11)

It is seen that as n approach es infinity, en approaches unity for any value of f less than unity, in which case e= imep!imep m as a limi t. This r elationship, though simple, is unfortunately difficult to apply in practical determination of E, because it is difficult to extrapolate with any reasonable accuracy for the value of imep m from r eadings taken with the neces­sarily limited values of n in any practical experiment. Experience indicates that it is far more sa tisfactory to solve for t he value of f from the ratio of imepn! imep for filliLe values of n, and then obtain e= I - f. Figure 1 can convenien tly be used for this operation .

4. Experimental Equipment

A small single-cylinder, air-cooled, crall kcase­scavenged engine was used for the experimental investigation. The principal dimensions of t his engine are as follows:

Bore __________________________________ 2. 500 in . Stroke __ ____________________________ 2.000 in. Displacement volum e __ _______________ 9.818 in .3

Cy linder compression ratio ______________ 7. 05 Crankcase compression raLio _____________ l. 44

The engine employs conventional loop scavenging. The total transfer port area at t he erank:ca e is 1.27 in .2, the total inlet port arca is ].41 in.2, and the total exhaust port area is 0.89 in .2 Th e rotary valve has a maximum opening of 1.32 in.2 The port timing of the engin e is shown in figure 2.

The engine was connected to a small electric dy­.namometcr. The enginc speed was measured by an electric tachometer, accurate speed con trol being ac­complished with the aid of a stroboscope directed upon a striped flywheel. An MIT hydnwlic scale [4] was used for dynamom eter torque measurement.

Figure 3 is a schematic diagram of the experimental set-up . As indi cated , air was supplied by a centrif­ugal blower, through appropriate controll ing and measuring devices, to the engine rotar~T valve inlet (herein defmed as the engine inlet ). Fucl (a 20:1 gasoline-oil mixture) was supplied under pressure, through appropriate controlling and measuring de­v ices, to the inlet surge tank, where it was mixed

T DC

BDC

Fw l ' RE 2. Port timing oj the engine used 'in the experiment.

Air

Throttle

ASME Orif ices

Cooli ng Air

Flo_ meters

l Elhausl Blo we,

FIGU R E 3. Schematic diagram of the eng'ine setup.

327

with the inlet air. The air rate was measured by calibrated sharp-edged orifices of the ASME flange taps design [5] . Two orifices were used to cover the range of the experiment. Calibrated float-type flow­meters were used for fuel rate m easurement. Again, two instruments were used to cover the entire ex­perimental range.

The engine was enclosed in a duct, and was cooled by air supplied by a blower. T he stations for the measurement of pressures and t emperatures are indi­cated in the figure. All temperatures were m easured by means of thermocouples. The pressures were measured by mercury and water manometers.

Figure 4 shows the ignition circuit used in the ex­periment. The circuit was designed basically to operate as a conventional automotive ignition sys­tem . There were four sets of breakers: the main breaker, a, operating at crankshaft speed, and the auxiliary breakers, b , c, and el, operating at reduced speeds. The reduced speeds were obtaired by mount­ing the camshafts , b , c, and d , in a gearbox giving speeds in th e progression of 1, K and X. The whole assembly was then driven through chain and sproc­kets by the crankshaft. The sprockets could b e changed to furnish a speed ratio of either 2 or 3 between the crankshaft and shaft b . In this fashion , the auxiliary breakers could operate at either ~, X, and Ys of crankshaft speed, or ?~ , ?t, and 7\2 of crank­shaft speed.

The main breaker was used to control the spark advance at all times . The spark advance was r ead by an automotive timing light. To obtain normal operation (n = l ), switch A should be closed, in which case a spark would b e produced every time the main breaker was opened. For skip-cycle operation, the switch giving the desired value of n should b e closed and all other switches opened. In tha t case, th e prim ary circuit would be completed only for the cycle for which ignition was d esired. For th at cycle, a spark would be produced at the instant the main breaker was opened.

The indicated mean effective pressure, imep, was computed from dynamometer readings taken with engine firin g and motoring, by the familiar relation

Timing Light

Spark

"'""

Breaker Reducl ior, Gear Ralio

I , 2, 4,

FIGURE 4. Circuit diagram of the ignition system.

Normol Position

close open open open

where 0 = a constant,

P b= dynamometer reading taken with engine firing,

P r= dy n amometer reading taken with engine motoring.

Under skip-cycle operation, the indicated m ean effec­tive pressure of the firing cycle, imepn, was computed from the dynamometer readings as follows

imepn=n C(P b+Pf) '

where n, as defined earlier , is the number of cycles of operation per firing cycle.

5 . Results and Discussion

Three series of runs were made, with the engine speed, inlet pressure and exhaust pressure varied in­dependently. With the exception of the variable whose effect was being measured, all operating vari­ables were maintained constant at values shown below:

N, engine sp cecl ____ _____ ___ _ rplll __ 1, 800 pi, inlet prcssure (i nches of mercury,

absolute) ______ _____ __ " H ga__ 29.92 ± 0. 0l P e, exhaust pressure __ __ ____ _ " H ga_ _ 29. 92 ± o. 03 T i , inlet temperature ___ __ ___ __ °F __ 100 ± l T a, coolin g air outlet t emperature

°F __ 130 ± 3 Ji' , fuel-air ratio______ ___________ __ 0.080 ± 0. 001

The high est engine speed used in this experiment was 3,000 rpm. Limitation of the dynamometer, as well as vibrations of the installation, did no t p ermit operation at higher speeds.

Figure 5 shows the variation of imep, ihp, and air­flow rate, M g , of the engine, under best-power spark­advance conditions, over a fairly wide range of en­gine speed, inlet pressure, and exhaust pressure.

Figure 6 shows the variation of imep n versus spark: advance under skip-cycle conditions, at 1,800 rpm . Note the progressive clecrease in best-power spark advance, and th e progressive increase in imep n, as the value of n was increased.2 An indication of the degree of r eproducibili ty of th e experiment can be had by noting that results of three sets of runs, made on three different occasions, have been included in this figure.

From data presented in the form of figure 6, the values of best-power spark advance and imep n under best-power spark-advance conditions, could b e de­rived for each set of operating conditions. In this manner, figures 7 and 8 were constructed. Kote hO"lv the imepn curves ·with varied exhaust pressure cross each other, and cross the curves with varied inlet pressure (fig. 8). This b ehavior clearly shows that a variation of exhaust pressure affected not only ~, but also Px, in eq (9).

The imepn/imep curves shown in figures 9 and 10 were derived from the imepn data presented in figures

2 'I'be variation of ihe best· power spark ad vance reflects ihe elYect of , on flame speed. The variation of ;.mePn is expected on the basis of eq (11) .

328

L

E '" "- a.

.0

~ c;, a. '" ='E .c E

6 0 6 8 0

70

50 5 60

5 0

4 0 4 40

30

30 3 20

10

20 2 0

10

0 0

o 500 1000 1500 2000 2500 3000 27

N, rpm

. ..-x Ime p'-.......... ............. x --'

...----

28 29

Pj , "Hgo

29.92 29.92 31 32 33

FIGURE 5. P e1jonnance characteristics of the engine. Operating conditions (unless otherwise specified): 1,800 rpJl1 ,·p.=p. =2D.92")Tga,~T.= l OO° Y, fuel·air ratio = 0.080. best·power spark advance.

n' I

80

60~

100

80

60

.~

f 40

100 n=4

80

60

40

100

80

60

40 0 10 20 4050 01020 40 50

SPARK ADVANCE, DEGREES

FIGUR E 6. I mepn versus spark advance, for varying degrees of skip cycling.

Operating conditions: 1,800 rpm, p.=p.=2D.92" llga, Ti= lOOO F, fuel·airratio = 0.080. 'fhe three different symbols denote resnlts of three runs under identical conditions, made on three different occasions.

120

100

80

'" a.

c 60 n. <> E

40

20

0

::s::: lI'J 4 0 0:",

it'" <f)ffi 0:'"

?t°. 20 0'" Q. U , Z

f- " <f» ", 0 CD" 0

'pm

4 • 3000

~+-+---+ 2400 0

~/o "_ "- 0 ~ 1800

x 1200

~:==:-. i 60n

0- /"

V' Symbol 'pm

600 x 1200 0 180 0

+ 2 400 4 3000

1_ J rpm

2;3~ 4 J 'f~~O ~~t~~§! ~~~~ • ' _ . _ x ~.::: :~gg

.. - - 600 I ! ! ! !

3 5 6 8 9 10 II 12

n

FIGURE 7. Best-power spark advance and imepn at best-power spark advance as affected by skip cycling, at several engine speeds.

Operating conditiom: ]Ji=p.=29.D2" lIga, T i= 1000 F, fuel·air ratio = O.080.

329

L

'" a.

c a.

E

100

80

60

40

20

o

60

~ <> L' , 9"~ ..... _ 1.'- ... ~

Sym bol Pi, "Hgo

0 - 29 . 92 0- Vor ied '

-0 --- 29 .92

- -g---=.-o- --8:t~;~><~~--- -o I

Pf!' "Hgo

29.92

29.92 Var i ed

1·.,·, / 0.925

~~0.92 _i} - 0.975

1'::.0·96 1.00

2 4 6 7

n 9 10 II 12

FIGU RE 8. B est-power spa/'k advance and i mepn at best-power spa/'k advance as affected by skip cycling, at several inlet and exhaust pressures.

Operating conditio ns: 1,800 rpm , '1 ';= 100° F , fuel-air ratio=0.080.

1.8

a. 1. 6 OJ

E

" c: a. 1.4 Q)

E

1. 2

1.0

0 .7

\U 0 . 6

0 .5

F I GURE 9.

2 3 4 5 6

o +

7

n

rpm

600 1200 1800 2400 3000

9

lvlass fraction of fresh charge several engine speeds.

10

in

Operating conditions: ~E'e figure 7.

rpm

t 600

i 1200 0

r 1800 2400 3000

II 12

the cylindel', at

Pj / Pe

2 . 4 ______ 0 0 . 92

2 . 2 ;:-: 0 . 925 0-

2.0

1 a. -0 0.96 OJ -- -l E 1. 8

" -o?: f 0.975 c:

Cl. 0 E 1.6 0- __ 0---0 1.00

/0 0

1.4 I Symbol Pi. wHga Pe , "Hga 0

0 29.92 2 9.92 0- Varied 29.92

1. 2 -0 29.92 Var ied

1.0

0 . 7 Pi I Pe

- 0 -- 0 --0--- 0 - _-0 1. 00 0 . 6 0

--0- 0- 1 0 .975 \JJ ---0 -0 g 0.96

0 . 5 i --0- 0- f 0 . 925 ---0 -0

-0 0 .9 2 0 .4

3 6 8 9 10 II 12 n

FIG U R E 10. M"ass fraction of fresh charge in several values of pd p ,.

the cylinder, at

Operatin.g condit ions: see figu re 8.

0.7

0.6

0.5

0 .4

\JJ

0.3

Symbol rpm Pj."Hgo Pel "Hgo

0.2 0 1800 29.92 29 .92

0- 1800 Vari ed 29 .92

-0 1800 29.92 Vari ed

0.1 X Vari ed 29.92 29.92

a a 0. 1 0 .2 0 .3 0 . 4 0 .5 0.6 0 .7 0.8

Rs

F IGUR E 11. Nlass fraction of fresh charge in the cylinder as a function of scavenging ratio, for range of engine speed, inlet pressure and exhaust pressure investigated.

Operating conditions: Ti=lOOo P , fuel-air raLio=0.08~ .

330

7 and 8. Note that because the variable Px was eliminated in this method of presentation, the imepn/ imep curves in figure 10 appeared in a systematic order with respect to the ratio p dp •.

FigUl'es 9 and 10 also present the values of f , as derived from the imepn/imep data given in the same figures, by the method outlined in section 3. Note that the value of f for each case remained substan­tially constant over the range of n investigated. This constancy of the value of f is gratifying, in view of the complexity of the scavenging process and the simplifying assumptions made. By averaging the results obtained at several values of n, a rather reliable value for f may thus be derived .

The results presented in figures 9 and 10 are SUlll­

marized in figure 11. It is interesting to note that the mass fraction of fresh charge in the cylinder was, within experimental accuracy, a function of scaveng­ing ratio only, regardless of the individual values of engine speed, inlet pressure, or exhaust pressure, within the range of the variables inves tigated .

The assistance provided by Edward Bryant in setting up the experimental equipment and in oper­ating the engine is gratefully acknowledged.

6 . References

[1] C. F. Taylor and A. R . Rogowski , Scavcnging the two­stroke engine, SAE Trans. 62, 486 (1954).

[2] P. H . Schweitzer, Scavenging of t wo-stroke cycle diesel engines, ch. 16 (The Macmillan Co., New York, 1 . Y ., 1949) .

[3] P . H. Schweitzer, Scavenging of two-stroke cycle diesel engines, ch. 3 (The Macmillan Co., New York, N. Y. , 1949).

[4] H ydraulic scale for measurement of engi ne dynamometer forces, Laboratory Equipment Bulletin No.6 , Diesel Engine Manufacturers Association (January 3, 19'19).

[5] Fluid meters-their theory and application, Report of ASME Special R esearch Committee on Fluid Meters, pt I (1937) .

WASHINGTON, April 5, 1956.

331