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Journal of Research of the National Bureau of Standards Vol. 57, No. 6, December 1956 Research Paper 2721 Scavenging Characteristics of a Two--Cycle Engine as Determined by Skip-Cycle Operation

P. M. Ku and T. F. Trimble

A method for determining the mass fraction of fresh charge in the of the two- stroke-cycle engine, from measurements of engine under normal operation and with engine fired only once in several cycles of operation is described. The results obtained from a crankcase-scavenged engine are presented.

1. Introduction 2. Nomenclature The two-stroke cycle engine, in order to develop For analytical purposes, it is convenient to reduce one power cycle for every revolution of the crank­ the two mass rates mentioned above to nondimen- shaft, must rely on the action of an auxiliary device, sional quantities. In this connection, several dif­ known as the scavenging pump, to expel the combus­ ferent systems of nondimensionalization [1,3] have tion products from the cylinder and to fill the cyl­ been proposed. Each proposed system has certain inder with fresh charge. The scavenging pump may attractive features, thus the particular choice is, at be and frequently is physically separate from the least to some extent, a matter of personal preference. engine cylinder proper. In the crankcase-scavenged A system rather similar to that employed by engine, the cylinder-crankcase- combination Taylor and Rogowski [1] is used. To nondimen- serves as the scavenging pump. sionalize the mass of fresh charge supplied by the The ideal objective of the scavenging process is, of scavenging pump per unit time, the term "scaveng­ course, to replace completely the combustion prod­ ing ratio", Rs, as defined below, is introduced, ucts in the engine cylinder with fresh charge delivered through the inlet ports, without meantime losing any s (1) of the fresh charge through the exhaust ports. In PsVdN this manner, maximum power is derived from the cylinder with the least expenditure of scavenging- To nondimensionalize the mass of fresh charge re­ pump power. In practice, such an objective is never tained in the engine cylinder per unit time, the term attainable, hence the excellence of the scavenging "scavenging efficiency", es, is introduced, process must be judged by the mass of fresh charge Mr retained in the engine cylinder per unit time, in e =- (2) relation to the mass of fresh charge supplied per unit s nVdN time by the scavenging pump. It follows from the above that two mass rates per where unit time, viz., that supplied by the scavenging pump Mg=mass of fresh charge supplied by the and that retained in the engine cylinder, are required scavenging pump per unit time, to define the scavenging characteristics of an engine. Mr=mass of fresh charge retained in the engine Of these two quantities, the mass supplied by the cylinder after port closure, per unit of scavenging pump per unit time can be directly time, measured by the use of a flowmeter placed in the ps = density of fresh charge computed on the inlet system. The mass retained in the engine cyl­ basis of inlet temperature and exhaust inder per unit time is not, however, subject to a , direct measurement. For its determination, a num­ Va= displacement volume, ber of indirect methods have been devised and N= engine speed, revolutions per unit time. 1 studied tl.,2]. In general, such methods are either In the above expressions, displacement volume extremely laborious, or liable to considerable errors. Vd is used basically as a matter of convenience, for This paper describes a rather simple method for the this is the volume used in defining the mean effective determination of an important related quantity, the pressure of the engine. The introduction of exhaust mass fraction of fresh charge in the engine cylinder pressure into density term ps is especially convenient after port closure. On the basis of tests performed for engines with late exhaust closing, in which the on a crankcase-scavenged engine, this method ap­ exhaust pressure rather than the inlet pressure peared to be quite practical and reliable. exercises controlling influence on the density of the 1 Figures in brackets indicate the literature references at the end of this paper. cylinder contents at the time of port closure.

408063—56 2 325 For the purpose of the present paper, it is helpful ning of the scavenging process. At the end of the to note that the quantity MT is related to the cylinder scavenging process, only a fraction of the combustion volume at the instant of port closure, as follows: products remains in the cylinder. Now, consider the case when the engine is allowed to fire for one Mr=ePxVzN, (3) cycle and then skip, or run without firing, for several cycles. Obviously, any combustion products that where remain in the cylinder must come from the firing e=mass fraction of fresh charge in the engine cycle. Accordingly, the progressive scavenging ac­ cylinder after port closure, tion of the skipped cycles results in progressively less Px= average density of the cylinder contents at combustion products, or progressively more fresh the instant of port closure, charge, in the cylinder after each skipped cycle. Vx=cylinder volume at the instant of port The above condition can be rather satisfactorily closure. represented by a simple mathematical relationship. Combining eq (2) and (3), the scavenging efficiency Let / denote the mass fraction of residual gas re­ may then be written tained in the cylinder during normal operation. The mass fraction of fresh charge during normal operation is then e.=e(Px/Pa)(Vx/Vd). (4) This paper deals with a method for the determina­ e=W. (6) tion of the quantity e, and eq (4) shows the relation between this quantity and scavenging efficiency. Let it be assumed that the conditions of heat trans­ The ratio Vx/Vd is obviously determined by port fer, flow resistance and flow dynamics are not signifi­ timing. No attempt has been made to determine cantly influenced by skip cycling—an assumption the ratio px/ps in the present investigation. How­ which appears to be justified by the experimental ever, some qualitative remarks may be in order here. results reported herein. In that case, the total The ratio px/ps is, in general, influenced by heat mass of retained cylinder contents would remain transfer between the fresh charge and the engine substantially constant, and likewise the mass parts, flow resistance at the inlet and exhaust ports, fraction of cylinder contents to be passed from one and the dynamics of flow in the scavenging system. cycle to the next. Consequently, if the engine is For engines with late exhaust closing, if the heat allowed to fire only once in n cycles of operation, transfer, flow resistance, and flow dynamics effects then for each of the firing cycles, the mass fraction were negligible, then the ratio px/ps obviously would of combustion products remaining in the cylinder w become unity. Thus, px/ps provides a measure of would be/ , and the mass fraction of fresh charge the combined effect of heat transfer, flow resistance in the cylinder after port closure would be and flow dynamics in this instance. n For engines with late inlet closing, the relation en=l-f . (7) may best be examined by writing Figure 1 gives a plot of (1—jn)l(l—/) versus /, Px/ps= (Px/pi) (pi/ps) = (px/pi) (Pi/Pe), for several values of n. The curve for n= <» represents the limiting case where the cylinder con­ where tains nothing but fresh charge. Pt=density of fresh charge at inlet temperature and inlet pressure, ^f=inlet pressure, absolute, 2>e=exhaust pressure, absolute. Applying this expression to eq (4),

es=e(px/pi) (pjpe) (Vx/Vd). (5)

Here the ratio pt/pe 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/pt 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 Pxlpt 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- I FIGURE 1. Plot of (1 —fn)/(l — /) versus /. 326 The skip-cycle method for the determination of The engine employs conventional loop scavenging. c is based on the premise that at constant fuel-air The total transfer port area at the crankcase is ratio and best-power , the indicated 1.27 in.2, the total inlet port area is 1.41 in.2, and thermal efficiency of a given internal combustion the total exhaust port area is 0.89 in.2 The is very nearly constant over a wide range of valve has a maximum opening of 1.32 in.2 The operating conditions. The indicated mean effective port timing of the engine is shown in figure 2. pressure of a two-stroke cycle engine supplied with The engine was connected to a small electric dy­ a premixed fuel-air charge, and operating normally, namometer. The engine speed was measured by an can be expressed, in terms of scavenging efficiency electric , accurate speed control being ac­ defined previously, as follows, complished with the aid of a stroboscope directed upon a striped flywheel. An MIT hydraulic scale imep^JesPsFEcTji, (8) [4] was used for dynamometer measurement. Figure 3 is a schematic diagram of the experimental where set-up. As indicated, air was supplied by a centrif­ imep=indicated mean effective pressure (nor­ ugal blower, through appropriate controlling and mal operation), measuring devices, to the engine rotary valve inlet F= fuel-air ratio, (herein defined as the engine inlet). Fuel (a 20:1 gasoline-oil mixture) was supplied under pressure, Eccheating value of the fuel, 77 i=indicated thermal efficiency based on through appropriate controlling and measuring de­ fuel quantity retained in the cylinder, vices, to the inlet surge tank, where it was mixed J= mechanical equivalent of heat. Applying eq (4) to this expression,

imep=JePx(Vx/Vd)FEc71i. (9) Under skip-cycle operation, eq (9) can be modified to read

imepn=Jenpx(Vx/Vd)FEerii, (10) where imepn is the imep of the firing cycle with firing occurring once every n cycles of operation. The quantities px, F, and rjf are constant, under the as­ sumptions made previously. Combining eq (9) and (10), and introducing the values for e and en as given by eq (6) and (7), n imepn=== e=l—f i _ imep €„ 1—/

It is seen that as n approaches infinity, en approaches unity for any value off less than unity, in which case e=imep /imep m as a limit. This relationship, though simple, is unfortunately difficult to apply in practical determination of e, because it is difficult to FIGURE 2. Port timing of the engine used in the experiment. extrapolate with any reasonable accuracy for the value of imepm from readings taken with the neces­ sarily limited values of n in any practical experiment. Experience indicates that it is far more satisfactory to solve for the value of / from the ratio of imepn/ imep for finite values of n, and then obtain e=l— /. Figure 1 can conveniently be used for this operation. 4. Experimental Equipment A small single-cylinder, air-cooled, crankcase- scavenged engine was used for the experimental investigation. The principal dimensions of this engine are as follows: 2. 500 in. Stroke 2. 000 in. 3 Exhaust Displacement volume 9. 818 in. Blower Cylinder 7. 05 Crankcase compression ratio 1. 44 FIGURE 3. Schematic diagram of the engine setup. 327 with the inlet air. The air rate was measured by where calibrated sharp-edged orifices of the ASME flange £7= a constant, taps design [5]. Two orifices were used to cover the Pb=dynamometer reading taken with engine range of the experiment. Calibrated float-type flow­ firing, meters were used for fuel rate measurement. Again, Pf= dynamometer reading taken With engine two instruments were used to cover the entire ex­ motoring. perimental range. The engine was enclosed in a duct, and was cooled Under skip-cycle operation, the indicated mean effec­ by air supplied by a blower. The stations for the tive pressure of the firing cycle, imepn, was computed measurement of and temperatures are indi­ from the dynamometer readings as follows cated in the figure. All temperatures were measured by means of thermocouples. The pressures were imepn=n C(Pb+Pf), measured by mercury and water manometers. Figure 4 shows the ignition circuit used in the ex­ where n, as defined earlier, is the number of cycles periment. The circuit was designed basically to of operation per firing cycle. operate as a conventional automotive ignition sys­ tem. There were four sets of breakers: the main 5. Results and Discussion breaker, a, operating at speed, and the auxiliary breakers, b, c, and d, operating at reduced Three series of runs were made, with the engine speeds. The reduced speeds were obtained by mount­ speed, inlet pressure and exhaust pressure varied in­ ing the , b, c, and d, in a gearbox giving dependently. With the exception of the variable speeds in the progression of 1, K, and %. The whole whose effect was being measured, all operating vari­ assembly was then driven through chain and sproc­ ables were maintained constant at values shown kets by the crankshaft. The sprockets could be below: changed to furnish a speed ratio of either 2 or 3 between the crankshaft and shaft b. In this fashion, N, engine speed rpm__ 1,800 the auxiliary breakers could operate at either }{, }{, Pi, inlet pressure (inches of mercury, absolute) "Hga__ 29.92 ±0.01 and y$ of crankshaft speed, or %, %, and %2 of crank­ pe, exhaust pressure "Hga__ 29.92 ±0.03 shaft speed. Ti, inlet temperature °F_. 100 ±1 The main breaker was used to control the spark Ta, cooling air outlet temperature advance at all times. The spark advance was read °F__ 130 ±3 by an automotive timing light. To obtain normal F, fuel-air ratio 0.080 ±0.001 operation (n=l), switch A should be closed, in which case a spark would be produced every time the main The highest engine speed used in this experiment breaker was opened. For skip-cycle operation, the was 3,000 rpm. Limitation of the dynamometer, as switch giving the desired value of n should be closed well as vibrations of the installation, did not permit and all other switches opened. In that case, the operation at higher speeds. primary circuit would be completed only for the Figure 5 shows the variation of imep, ihp, and air­ cycle for which ignition was desired. For that cycle, flow rate, Mg, of the engine, under best-power spark- a spark would be produced at the instant the main advance conditions, over a fairly wide range of en­ breaker was opened. gine speed, inlet pressure, and exhaust pressure. Figure 6 shows the variation of imepn versus spark The indicated mean effective pressure, imep, was advance under skip-cycle conditions, at 1,800 rpm. computed from dynamometer readings taken with Note the progressive decrease in best-power spark engine firing and motoring, by the familiar relation advance, and the progressive increase in imepn, as the value of n was increased.2 An indication of the imep = C(Pb+Pf), degree of reproducibility of the experiment can be had by noting that results of three sets of runs, made on three different occasions, have been included in Reduction Normal Gear Ratio Position this figure. 1 close From data presented in the form of figure 6, the open 2x open values of best-power spark advance and imepn under 4x open best-power spark-advance conditions, could be de­ rived for each set of operating conditions. In this Spark manner, figures 7 and 8 were constructed. Note Plug

70 -—8

50 5 60

50

40 4 40

30

30 3 20

10

20 2 0

10 I

0 0 _l I L J I I I I L_ 500 1000 1500 2000 2500 3000 27 28 29 29.92 29.92 N, rpm Pj , Hgo Pe . Hgo FIGURE 5. Performance characteristics of the engine. , Operating conditions (unless otherwise specified): 1,800 rpm,fpt=pe=29.92"_Hga, j7\=100° F, fuel-air ratio=0.080. best-power spark advance.

K) 20 30 40 50

10 20 30 40 50 0 10 20 30 40 50 10 II 12 SPARK ADVANCE, DEGREES

FIGURE 6. Imepn versus spark advance, for varying degrees of FIGURE 7. Best-power spark advance and imepn at best-power skip cycling. o spark advance as affected by skip cycling, at several engine Operating conditions: 1,800 rpm, pt=pe=29.92" Hga, T»=100 F, fuel-air ratio= speeds. 0.080. The three different symbols denote results of three runs under identical Operating conditions: p»=;p«=29.92" Hga, T = 100° F, fuel-air ratio=0.080. conditions, made on three different occasions. t 329 P i/Pe Pj/Pe .0.92 ^-__----~-%-0.96 £-1.00 -.^0.975 ^0.92 5

o. 60

Symbol P,, "Hga Pe,"Hga o 29.92 29.92 o— Varied' 29.92 -o 29.92 Varied

1 0.975 W $ 0.96

FIGURE 8. Best-power spark advance and imepn at best-power spark advance as affected by skip cycling, at several inlet and • -O 0.92 exhaust pressures. I 2 3 4 5 6 7 8 9 10 II 12 Operating conditions: 1,800 rpm, 7\=100° F, fuel-air ratio=0.080. n FIGURE 10. Mass fraction of fresh charge in the cylinder, at several values of Pi/pe. Operating conditions: see figure 8.

0.7 1 1—- I 1 1 1—1 T"~ T 1 "1 I 1 1 r— \ 0.6 ^ X -o \ [ / \ W 0.3 r

Symbol rpm Pj,"Hga Pe,"Hga ' 1 1 1 T ™|i i i 1 1 0.2 r O 1800 29.92 29.92 3000 O 1800 Varied 29.92 1 A J _ +, 2400 -O 1800 29.92 Varied \ -i—1~*— i 1800 X X ? 0.1 r X Varied 29.92 29.92 1 * X 1200 - 1 - 1 i i i 1 1 1 L_ 1 _J 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

FIGURE 11. Mass fraction of fresh charge in the cylinder as FIGURE 9. Mass fraction of fresh charge in the cylinder, at a function of scavenging ratio, for range of engine speed, several engine speeds. inlet pressure and exhaust pressure investigated. Operating conditions: see figure 7. Operating conditions: 7\=100° F, fuel-air ratio=0.080. 330 7 and 8. Note that because the variable px was The assistance provided by Edward Bryant in eliminated in this method of presentation, the imepj setting up the experimental equipment and in oper­ imep curves in figure 10 appeared in a systematic ating the engine is gratefully acknowledged. order with respect to the ratio pi\pe. Figures 9 and 10 also present the values of e, as 6. References derived from the imepjimep data given in the same figures, by the method outlined in section 3. Note [1] C. F. Taylor and A. R. Rogowski, Scavenging the two- that the value of e for each case remained substan­ stroke engine, SAE Trans. 62, 486 (1954). [2] P. H. Schweitzer, Scavenging of two-stroke cycle diesel tially constant over the range of n investigated. engines, ch. 16 (The Macmillan Co., New York, N. Y., This constancy of the value of e is gratifying, in view 1949). of the complexity of the scavenging process and the [3] P. H. Schweitzer, Scavenging of two-stroke cycle diesel simplifying assumptions made. By averaging the engines, ch. 3 (The Macmillan Co., New York, N. Y., 1949). results obtained at several values of n, a rather [4] Hydraulic scale for measurement of engine dynamometer reliable value for e may thus be derived. forces, Laboratory Equipment Bulletin No. 6, Diesel The results presented in figures 9 and 10 are sum­ Engine Manufacturers Association (January 3, 1949). marized in figure 11. It is interesting to note that [5] Fluid meters—their theory and application, Report of ASME Special Research Committee on Fluid Meters, the mass fraction of fresh charge in the cylinder was, pt I (1937). within experimental accuracy, a function of scaveng­ ing ratio only, regardless of the individual values of engine speed, inlet pressure, or exhaust pressure, within the range of the variables investigated. WASHINGTON, April 5, 1956.

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