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_ESTRICTED

_;ATIONAL ADVIS0.qY C0'_HITTEZ YOl_ AE._0_AUTICS

TECHI_ICAL I_0T_ _T0. 935

_? -DY..AI_ICS CY THE If?LET SYSTEV 0F A FOUK-STROEE EITGI!TZ

By _. H. Boden and Harry Schector

SUMH._Y

9

Tests were run an a single-cyllnder and a mu!ticyl-

inder four-stroke engine in order to determine tho effect

of the dynamics of the inlet system upon indicated mean

effective pressure.

Tests on the single-cylinder engine were made at

various speeds, inlet valve timin_s, and inlet pipe

io_:gths. Those tests indicated that the i:_dicated ucan

-&ffcctlve pressure could be r.-.iscd consi<orably at any

o:'.o speed by the use of a suitably io_:g inlet pipe.

Testa at other speeds with this length of piloo sho'.:cd

higher indicated mean effective pressure than ,.,_th a very

short pipe, although not so high as could bc obtained

_,ith the pipe length adjusted for each speed, A joneralrelation was discovoroi bot_leen optimum tiuc of Inlet

valve closing and pipe length; namely, that !ongor pipes

require later inlet valve closinC in order to bo full?

=ffcctive.

Tests wcr_ also made on three c'jlinder_ connccte_ to

o single pipe. With this arrangement, increased volunct-

rlc efficiency at low spc@d. _;as obtainable by using a

long pipe, but only with a sacrifice of volumetric effi-

ciency, at high speed. Volumetric efficiency at high

speed was progressively ]o,_:or as _hc pipe length was in-

crcaso_.

-% _ ,_%T_.;TaO= UCTi ON

It has long boon recognized that the volumetric ef-

Zicloncy and hence the mean offectlve pressure of an in-

ternal combustion engine is affected by the dynamics of

KESTKIOTED

@

I,:ACA T'T _,_o. 935

the inlet system. (See references I to 19.) The pre_ent

wor_< wan undertaken in order to make a more _ystematic

and extensive investigation than had been ma_e before,

and to attempt to devise a mathematical analysis which

would lead to a better understanding of the observe_ phe-

no_enao

.. "",_i .....1The work was done under the auspices of t_e .._._ .....

Advisory Committee for Aeronautics. The experimental

work was done in the Sloan Automotive Laboratory at the

_:assachusetts Institute of Technology unler the supe1"vi-

siou of rrofessor E. S, Taylor. A theoretical analysi_ of

th!_ problem may be found in Item 4 of the _!bllography,

A_PARATUS A_D METHOD

Single-Cylinder Engine

The first part of the experimental work wa_ done on

an _ACA universal test engine (bore 5 in., stroke ? in.)

equipped for variable valve timing (reference 20). Var-iables _ere controlled as follows:

_uel .......... 8V-octane aviation gasoline

Compression ratio (all runsi ............ 5.0

Cooling water temperature (all runsi . . , 175 ° to 185 ° 2"

Fuel-air ratio (each run) ...... set for best power

Spark timing (each speed) ....... set for best power

Speed ............ variable, 1000 to 1600 rpm

Inlet pipe length, variable, by increments 9 to 247 inche_

Inlet pipe diameter ......... _.56 or 1°88 inches

Inlet valve lift ............. O.31S inch

Exhaust valve lift ..... 0._76 inch

. ha stopening i i i . . . • . • °Exh'_ust closing (25 ° A.T.C.) ........... 25°

(Crank angles are measured from top deaC center un-

less other_ise indicated.)

In order that thc air flow should not be disturbc_

by the restriction of a carburetor venturi, fuel _¢as in-

jected directly into the cylinder for all te_ts.

Photographs of the general setup are given in figure

i, and a diagrammatic sketch in figure 2. (_urther ex-

perimental details will be found in reference 1S.)

_A_A T_ _o. 935

One series of runs was made with a constant "standard"

inlet valve timing as follows;

Inlet opening (25 ° B.T.Co) ............ 3350

Inlet closing (250 A.B.C.) ............. 2050

A further series was made at constant speed (160C

rpm) and variable inlet timing as follows:

Inlet opening (45 ° to 15 ° B.T.Co) .....

Inlet closing (15 ° to 450 A.B.C.) .....

3150 to 345 0

195 ° to 225 °

Measurements of torque were made for each run. Fric-

tion was estimated by the usual method, "motoring" at

several speeds. From the torque resulting from the addi-

tion of the measured torque to the motoring torque forthe same speed, the indicated mean effective pressure

(imep) was computed an@ plotted in figures 3 to 5.

In addition to the foregoing measurements, records

were taken of the pressure at the valve end of the inlet

pipe for all tests. In later tests where the valve tizlng

was varied, records also were taken of pressure in the

cylinder, These measurements were made by means of the

I_.I.T. indicator (reference 21)o Sample diagrams are re-

produced in figure 6.

l_igure 7 shows a comparison of t_¢o curves of indlc_ed

mean effective pressure against pipe length, from two setsof data taken at an interval of about five months. Dif-

ferent cylinder heads of the same design were used. Thedata for the curve marked "Head l" was taken in one con-

tinuous run; while each observation for the curve marked

"Head 2" was taken on a different day,

R_SUZ_S

Single-Cylinder Engine

Volumetric efficiency.- Volumetric efficiency may bedefined by the expression

W : f 7 d #o m (l)

where

._,.-_C.-_ ,.-.- "7o, 9354

mass of air taken in per unit time

f number of suction strokes per unit time

V d piston displacement of one cylinder

Pc average inlet air density

E volumetric efficiency

Volumetric efficiency could be computed from a meas-

urement of air quantity in addition to the ordinary meas-

urements of speed, inlet pressure, and temperature; butin this work it was desirable to avoid the measurement of

air, because the presence of meters or orifices would dis-

turb the dynamics of the inlet system. On account of this

difficulty, and since only relative values were required,

it was decided to estimate the quantity of air from the

_ower measurement. The indicated horsepower = 778 (W) (H) (_)" 550 x 6C x.60

where W is the mass of air consumed per hour, _ is the

heating value of unit mass of air, and _ is the indicated

thermal efficiency based on air; H and _ are substan-

tially constant since spark and fuel rate were adjusted for

best power. Under these conditions M is proportional to

indicated horsepower. From this relation it follows that

volumetric efficiency is proportional to indicated mean

pressure.

Effect of pipe length.- Varying the length of the in-

let pipe at constant speed results in curves of indicated

mean effective pressure against pipe length, as shown in

ficures 3, 4, and 5. Essential characteristics of these

curves are:

1. A gradually rising mean effective pressure as pipe

length is increased from as short as possible

to a length which gives a frequency ratio* of

slightly more than 5

"The "frequency ratio" here introduced is defined asthe ratio of inlet pipe frequency to valve frequency (in-

let pipe frequency being the fundamental frequency of the

air column in the inlet pipe when the valve is closed and

valve frequency being half the engine crankshaft frequen-

cy). Thus this frequency ratio is inversely proportional

to the product of intake pipe length and engine speed.

\

_;ACA TIT !_o. 935

2. At least three distinct peaks, at frequency ratios

of sli_Iv_.._ _ more than 5, 4, S

3. A decrease in zean effective pressure for lengths

longer than that giving a frequency ratio of G

Figure 8 is a set of records of pressv.re in the pipe

(near the inlet valve) and in the cylinder, at 1600 r!_m.

Pipe length is the single independent variable.

Essential characteristics of the pressure in the

pipe are:

i. Gradually increasin_ amplitude of the pressure

waves as pipe length is increased

2. Occurrence of a pressure peak near the time of

inlet closing, arriving later as the length

of pipe is increased

The cylinder pressure records of figure 8 show an

intcrestlng pressure minimum during the suction stroke,

which moves to the right (i.e,, comes later in the stroke)

as pipe length is increased, until the number cf pressure

waves per engine cycle changes. _Q_en this occurs, the

point of minimum pressure returns to approximately its

original crank anglo and again starts zevlng to the right

as the pipe length is increased.

_ ords taken from the_igure 9 is a set of pressure ..c

inlet pipe at 1220 rpm.

Effect of speed.- Figures i0 and ii show the pressure

in the pipe for various engine speeds. Similarity bet_een

these records and those of figures 8 and 9 is evident.

When speci and pipe length are varied together so as to

give a constant frequency ratio, the result is a set of

very similar curves _ith amplitude increasing appzoxizately

in proportion to thc speed. Figure 12 is such a set of

curves. This suggests a plot of indicated mean effective

_rossure against frequency ratio for various values of

speed as shown in fi_ure 13. The first peak of indicated

mean effective pressure, which occurs at a frequency ratio

of approximately S, decreases notably as speed increases;

while the peak at 5 shows a much smaller variation, not

consistently in either direction.

Effect of valve timing.- The effect of the time of

N_.C/ T;" :7o. 9S.5

inlet-closing on indicated mean effective pressure is il-

lustrated in figures 4 and 5 which show indicated mean ef-

fective pressure against pipe length for various valve

timings. It will be noted that late inlet-closing im-

proves the performance with long pipe lengths; while

early closing is better for shorter pipe lengths. This

is the result of later arrival of the pressure wave after

bottom center which was noted in figure 8. Figure 14 is

a series of records taken with inlet closing for best

power in each case. Note that the closing of the inlet

valve is slightly ahead of the pressure peak in all cases

except with the shortest three pipes where the pressurepeak is very close to bottom center. This is due to the

effect of piston motion. If a series of compression

curves is plotted, starting with various inlet pressuresat bottom center and these curves are superlmpozed upon

the pressure wave in the pipe, a point of tangency is ob-tained between one of the comprosslon curves and the pres-

sure in the inlet pipe. Provided the inertia of the air

in the valve is negligible, this will be the point atwhich the valve must be closed in order to entrap a maxi-

mum quantity of air. Such compression curves have boondrawn in figure 14. The actual valve closing is somewhat

later than the points of tangency because the valve re-

quires a finite time to close and is effectively closedsomewhat earlier. The difference between actual closing

and effective closing appears to be about l0 ° at 1600 rpm.

This quantity would be expected to increase with speed.

A direct comparison of the pressures in the cylinder and

the pipe for different valve timings appears in figure 15.

The curves marked "A" (fig. 15) indicate that a difference

In timing may alter considerably the pressure in the pipe.

In this case reverse flow induced by the piston motion

apparently reinforces the pressure waves.

Effect of inlet pipe diameter.- Pigure 16 is a plot

of indicated mean effective pressure against inlet pipe

length for a pipe approximately half the area of the pipeused in the other tests. The suppression of the _eaks ofthe curve is notable in the case of the small diameter

pipe.

_igure 17 shows the pressure waves in the small di-

ameter pipe, for various lengths. On comparing thesewith figure 8, it is noted that the amplitude is greater

in the case of the small pipe and that the preszure wave

arrives later after bottom center, which indicates thata later timing might be expected to give higher indicated

NACATN No. 935

mean effective pressure with the small pipe. Figure 18 isa direct Comparison of the pressure in the pipe and thecylinder for a small and a large pipe of the same length,

The points noted here are evident in this figure.

APPARATUS AND IIETHOD

Multlcylinder Engine

i•i?¸

Additional experimental work was carried out to de-termine the effect of having several cylinders connected

to the same inlet pipe. The engine used for these tests

was a slx-cyllnder Chrysler automobile engine. Engine

data follows:

GI21_Z2Serial _o, .................. 3_e inchesBore . .................Stroke ............. 4314 inches

o, _o=Isplaoement ......... 2 0 cubi Valve timing: 40 o B,B,C.

Exhaust opens ............... 4 o A.T.C.Exhaust closes ............. 70 B.T.C.

Inlet opens .............. 55 ° A.B.C,Inlet closes ..............

The cylinders of this engine were manifolded together in

two groups of three within the cylinder-block casting as

shown in figure 19, One group of three cylinders was

separated from the other and was provided with separateinlet and exhaust connections. This grou_ was used merely

as an air pump, the remaining group of three cylindersbeing used to supply power to operate the air pump (fig.

20). Thus the three cylinders under investigation weremotored without firing. They were arranged so that var-

ious lengths of inlet pipe could be attached to the inletopening common to the three cylinders. The air exhausted

by the three cylinders was led to a receiver, an& the flow

(volume per unit time) was measured by an NACA Roots type

supercharger. This flow, corrected for temperature, wasusod to calculate the volumetric efficiency. (Correction

for pressure was found unnecessary.) This method of test

was justified by Mueller (reference 14), who showed thatthe curve of volumetric efficiency against pipe length,

obtained by this method on a single-cylinder engine, wassimilar to the curve of indicated mean effective pressure

against pipe length when the engine was firing, The

iJ

HACA TN _o. 3S5

comparison of these curves taken from reference 14 is re-

produced in figure 21. Indicator diagrams were made of

the pressure in one of the three motoring cylinders, and

in the inlet pipe, as near as cossible to the engine.

RESULTS

i:ulticylinder Engine

Curves of volumetric efficiency against speed for

various pipe lengths, and of volumetric efficiency against

pipe length" for various speeds are reproduced in figures22 and 23. It is instructive to note that, while volumet-

ric efficiency at lo_¢ speed may be improved %y the use of

a long pipe, the improvement is at the expense of volumet-

ric efficiency at high speed. Volumetric efficiency at

high speed was not improved by additional inlet pipe

length.

i study of the pipe pressure diagrams (fig. 24) re-

veals the interesting fact that all show the same fre-

auency, the frequency of the suction strokes.

Figure 25 showing pressures in a 72-inch _ipe for

various speeds indicates phase shift as expected from

theoretical considerations. The last diagram of this fig-

ure shows the higher modes of vibration of the air column

in the pipe being excited, as would be expected.

CO_TCLUSl 07S

On the basis of the foregoing experimental investi-

gation, the following conclusions may be drawn:

i. The pressure waves in the inlet pipe are of suf-

ficient amplitude to affect the charging process (and

hence the mep) considerably.

2. For a single-cyllnder engine running at a given

speed, there are three distinct optimum lengths of inlet

pipe, with which the mean effective pressure is of the

_"Pipe length" for the mu_ticylinder engine is meas-

ured from the flange on the cylinder block.

:.;._.CA T_" i_'o. 985

order of 15 to 20 percent higher than it is when no inlet

pipe is used. These optimum pipe lengths are slightly

less than 30 c/_q where c is the velocity of sound in

feet per second, N is the engine speed in revolutions

per minute, and q is _, 4, or 5.

3. For a single-cylinder engine, a fixed pipe length

slightly less than 6 c/N m (where Hm is the highest

operating speed) gives a higher mean effective pressure

than no inlet pipe, for all operating speeds, although

not as high as can be obtained with pipe length adjusted

for each speed.

4. For a given engine speed and inlet pipe length,

full advantage of the pressure waves in the pipe can be

obtained only by suitable inlet valve timing, especially

by proper choice of the time of inlet closing; longer

pipes requiring latcr closing.

5. _Wnen three cylinders are connected to a single

pipe, the best volumetric efficiency at high speed is ob-

tained with the minimum length of pipe.

Massachusetts Institute of Technology,

Cambridge, Mass., January 1938.

:'ACA -i ::_. _'5 i0

_EFE_EECES

i. Xoester, E. _. : Luftkompressoran.

1304, p• 109•

Z.V.D.!., 3d. 48,

2. 3orth, :¢.: Untersuchungen Eber den Verbrennungsve _''_'-_.-_in der Gasmaschine. Z. _' D.!., _&. 52, 190_, p 5 °_

S. Voissel, P. : Resonanzerscheinungen in der Saugleitung

yon Compressoren und Gasmotoren. Z.V.D.I., Bd. 5_,

1912, p. 720; VDi-Forschungsheft 106.

4. C-ramberg, k. : "¢irk-angsweise und 3erechnung der ";in&-

kessel yon Xolbenpumpen. z._r°D. !. , Bd. 55, 1911,

PP. 842 and 888; VDI-Forschungsheft 129.

5. Berth, :,'.: $chwingungs- und ._esonanzerscheinungen in

der P.ehrleitungen yon Kolbengebl_sen. Z.V.D.l.,

Bd. 60, 1916, pp. 565, 591, and 611.

6. Hatthe_s, Wobertson, and Oardiner, Arthur [I. : Increas-

ing the Compression Pressure in an Engine by Using

a Long Intake Pipe. I_ACA T![ ITo. 180, 1924.

7. Capetti, Antonio: Effect of Intake Pipe on the Volu-

metric Efficiency of an Internal Combustion Engine.

_ACA TH No. 501, 1929.

8. Stier, Ernst: Sp_lung und Aufladung bei gweitaktuotoren.

Z.V.D.I., 3d. 73, Heft 39, Sept. 28, 1929, pp. 1389-

18gl.

9. Klusener, Otto: Saugrohr und Liefergrad. VDI-Scnder-

heft, Dieselmaschinen, Heft 5, 1932, pp. 107-!10.

10. List, Hans: Increasing the Volumetric Efficiency of

Diesel Engines by Intake Pipes. _ACA TH l_o. ?O0,

1933.

I!. Dennison, E. S. : Inertia Supercharging of Engine

Cylinders. Trans. A,S.I:.E., vol. 55, no. 5, 1933,

p. 53.

12. Lutz, 0. : Resonanzschwlngungen in den _ohrleitungenyon Eolbenkraftmaschinen. 3oricht aus dem Labora-

torium der Verbrennungskraftmaschinen (Stuttgart),

Heft 3, 1934.

I_ACATITNo. 935l!

Ik

13. i_aier, ,r., and Lutz, 0. : Resonanzerscheinungen in

&er ._ohrleitung yon Verbrennungskraftmaschinen,

Bericht aus dem Laboratorium der Verbrennungskraft-

maschinen (Stuttgart), Heft 3, 1934.

14. _ueller, .R. K.: Inertia Supercharging: An Analysisof the i:otion in the Intake Pipe of a Four Cycle

Engine. M. S. Thesis, }i.I.T., 1934.

15. Schmi&t, T. : Schw!ngungen in Auspuffleitungen yon

Verbrennungszotoren. .Vors,_hung auf &em Gebiete des

Ingenieur_,esens, B&. 5, 1934, p. 22_; VDl-Son&er-

heft, Dieselmaschinen, Heft 6, 1936, pp. 79-89.

16. Lutz, 0. : Grun&s_tzliche Betrachtungen _Iber den

Sp_llvorgang bei Zweitaktmaschinen. Forschung aufdem Gebiete des Ingenieur_,esens, B&. 5, 1934, p.

275; VDI-Son&erheft, Dieselmaschlnen, Heft 6, 1938,

pp. 90-104.

17. Pischinger, A.: Bewegungsvorg_uge in Gass_ulen

insbeson&ere beim Auspuff- und Sp'dlvorgang yon

Zweitaktmaschinen. Forschung auf dem Gebiete &es

Ingenieurwesens, Bd. 6, Sept.-0ct., 1935, DP. 245-

°57 and l_ov.-Dec., 1935, pp. 273-260; VD!-Sonder-

heft, Dieselmaschinen, Heft 6, 19_6, pp. 104-12&.

18. Boden, R. H.: Dynamics of ti-e !n_.uction Systez of an

Internal Combustion Engine. Ph. D. Thesis, Y.I._.,

1936.

19 Shen, v. C.' The Effect of Inlet Pioe Length on ,Tol-

umetrio Efficiency in a ".[,&lticyiinder ._n__..e. • •

Thesis, I'.I.T., 1937.

20. :_'are, Harsden: Description of the IT..%,C.A, Universal

Test Engine and Some Test .Results. I[ACA .Rop. _o.

250, 1937.

21. Taylor, E. S,, and Draper, C. S. : .% iTe_z High-Speed

Engine Indicator. Y_ech. _ng,, vol, 55, no. 3,

_arch 1933, pp, 169-171.

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