[3]

A. C. Alkidas Engine Research Department,

General Motors Research Laboratories, Warren, Mich. 48090

Heat Transfer Characteristics of a Spark-Ignition Engine Transient heat flux measurements were obtained at four positions on the cylinder head of a four-stroke single-cylinder spark-ignition engine. Tests were performed for both fired and motored operation of the engine. The primary engine operational variable was engine speed. The results showed that the heat flux varies considerably with position of measure-ment. At fired conditions, the initial high rate of increase of heat flux at each position of measurement correlated with the calculated time of arrival of the flame at that position. Finally, as expected, the peak heat flux was found to increase with increased engine speed.

A | v ,

I n t r o d u c t i o n In internal combustion engines, accurate heat transfer information

is becoming increasingly important with the great emphasis on engine efficiency and because of the demonstrated strong influence of heat transfer on exhaust emissions. In addition, heat transfer is important in calculating heat release rates and flame propagation from pres-sure-time data and in other engine simulation studies.

Experimental heat transfer studies in diesel engines have been performed by a large number of investigators, such as Eichelberg [1], Sitkei [2], Annand [3], Woschni [4], LeFeuvre, et al. [5], Whitehouse [6], Flynn, et al. [7], and Dent and Suliaman [8], to name a few.

In contrast, investigations of the heat transfer in spark-ignition engines have been surprisingly few. Overbye, et al. [9], measured the heat flux at several positions on the cylinder head of a CFR engine. Tests were performed at near stoichiometric air-fuel ratio and an engine speed of 830 r/min. The effects of intake manifold pressure, turbulence and wall deposits on surface heat flux were investigated. Overbye presented an empirical heat transfer correlation for the motored operation of the engine. However, this correlation failed when applied to fired conditions. Oguri [10] measured the instantaneous heat flux at one position on the cylinder head of a spark-ignition en-gine. Comparison of his measurements with calculated results ob-tained using Eichelberg's correlation [1] showed that this correlation was good in the expansion stroke but failed in the compression stroke. He then proposed an empirical correlation similar to that of Elser's [11], which showed marginal agreement with his experimental re-sults.

Presently, because of the limited number of heat transfer investi-gations in spark-ignition engines, most of the analytical modeling studies of these engines utilize empirical heat transfer correlations obtained in diesel engines. However, because of the radically different combustion characteristics in these two types of internal combustion engines, one should not necessarily expect the heat transfer correla-tions obtained in diesel engines to be applicable to spark-ignition engines.

The objective of this investigation was to study experimentally the unsteady heat flux characteristics of a single-cylinder spark-ignition engine. The transient heat flux at four positions on the cylinder head and the transient cylinder gas pressure were measured coincidentally. The primary engine operational variable was engine speed. All tests were performed at constant air-fuel ratio and volumetric efficiency. With the exception of a set of special tests, spark timing was kept at Minimum advance for Best Torque (MBT). In order to determine the influence of combustion on the heat transfer characteristics of the engine, comparative heat flux measurements were made at both motored and fired conditions.

T h e o r y of H e a t F l u x M e a s u r e m e n t s The surface heat flux in the chamber of a reciprocating internal

Contributed by the Heat Transfer Division for publication in The JOURNAL OP HEAT TRANSFER. Manuscript received by The Heat Transfer Division August 25, 1979.

combustion engine consists of two components: the steady-state component, which can be calculated from time-averaged temperature measurements at two known positions within the metal wall (see, e.g., [12]), and the unsteady component, which can be calculated from the cyclic surface temperature variation.

The principal assumption used to calculate the heat flux is that the heat flow through the walls of the combustion chamber is one-di-mensional. The method of calculating the unsteady heat flux from transient surface temperature measurements has been well docu-mented in several studies [9, 13]. Briefly, the procedure is as fol-lows:

1 The experimental surface temperature variation during the engine cycle is represented by a Fourier series of the form

Tw(t) = Tw(0) + N Y. Wn cos nut + Bn sin not) (1)

where Tw(0) is the time-averaged surface temperature, and the coefficients of the Fourier series An and Bn for rc = 1 to A/ are evalu-ated from the experimental surface temperature-time data. The re-maining symbols used in equation (1), along with all the other symbols in this paper, are defined in the Nomenclature.

2 The one-dimensional unsteady heat conduction equation

ot a ox2

is solved subject to the following boundary conditions: T(0,t) = Tw(t) T(8, t) = T(S)

(2)

(3)

where T(b) is the steady-state temperature of the solid at a distance 5 from the surface (x = 0).

From equation (2) with conditions (3) a steady periodic solution was obtained for T{x, t) and, from it, the heat flux at the surface, x = 0, was evaluated as follows:

qw(t) dT -K(0,t)

dx + K

K [T(0) - T(8)\

[An (cos nut sin nut) 2a

+ Bn (sin nut + cos nut)] (4)

The first term in the above expression of the surface heat flux is in-dependent of time. It represents the steady-state component of heat flux. The second term is time dependent and represents the unsteady component of heat flux.

Equation (4) is used to calculate the surface heat flux from the harmonic synthesis of the experimental surface temperature fluctu-ation data. As an example, Fig. 1 shows a typical experimental tem-perature-crankangle curve and Fig. 2 shows the corresponding cal-culated variation of heat flux through the engine cycle.

Journal of Heat Transfer MAY 1980, VOL. 102 / 189 Copyright 1980 by ASME

Downloaded From: http://heattransfer.asmedigitalcollection.asme.org/ on 03/19/2015 Terms of Use: http://asme.org/terms

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iW

388

386

384

38?

1 1

~ INTAKE

J

1

COMPRESSION

IGNITION -.

1

EXPANSION

TDC 1 1 1

1

EXHAUST

~~

-

-

1

0 720 180 360 540 CRANKANGIE, DEGREES

Fig. 1 Measured surface temperature variation with crankangle

E S 5 X

Typical variations of heat flux with crank angle at the four positions of measurement on the cylinder head are shown in Figs. 5 and 6 for engine speeds of 1000 and 2000 r/min, respectively. Comparison of these results at each engine speed condition tested shows that the magnitude of the peak heat flux varies with position of measurement. Differences in this magnitude were as high as 1.3 X 106 W/m2, the highest peak heat flux being 75 percent greater than the lowest. In all cases the peak heat flux was highest at position HT2, which is the position second closest to the centrally located spark plug.

The observed spatial variations of the peak heat flux may be at-tributed primarily to spatial variations of the temperature and ve-locity fields in the combustion chamber. Spatial variations of the heat flux on the cylinder head have also been observed by Overbye, et al. [9] in a spark-ignition engine and by Annand and Ma [14], LeFeuvre, et al. [5] and Whitehouse [6] in diesel engines.

TIP :OF PROBE

0.025 mm

R I B B O N E L E M E N T S

HEAT FLUX PROBE

Fig. 4 Design of heat flux probes

Table 2 Test parameters Air-Fuel Ratio Volumetric Efficiency* Speed Intake Air Temperature Coolant Temperature Oil Temperature

18 40 percent 500-2500 r/min 306 K 358 K 364 K

* Defined as the ratio of the actual mass of air supplied to the cylinder per cycle to the theoretical mass of air necessary to fill the displacement volume at 288 K and 101 kPa.

3.0

-2.0 -

rf 1-0 ~

0,0

1 1 r - i I I I I

~ 8 FLAME ARRIVAL A HTl - HT2 o HT3

^ HT4

a \ H-1000 r/min / \ V.E. - 40 %

J _ V A / F = 18 r.-A- 4 A \ >O M B T

I J J l M 1 1

P V * w > . * -MAX *si.B

1 1 1 1 1

350 360 370 380 390 400 410 420 CRANKANGLE,DEGREES

The most significant characteristic of the above results, however, is that the initial high rate of increase in heat flux at each position occurs in the same sequence as the distance of the heat flux probes (measuring position) from the spark plug. Thus, the heat flux rises significantly first at position HTl , followed in sequence by positions HT2, HT3 and finally HT4.

To relate the observed heat flux characteristics to the events oc-curring in the combustion chamber, flame propagation computations were performed using the heat release model of Krieger and Borman [15] modified by Lancaster [16] to calculate the position of the flame (spherical geometry) at each crank angle. In Figs. 5 and 6, the calcu-lated crank angle for the arrival of the flame at each position is de-noted by the full circle on the respective curves, and the crankangle of peak cylinder gas pressure is denoted by PMAX- It is apparent that the calculated arrival of the flame at each position approximately coincides with the beginning of the initial high rate of increase of heat flux at each position. In fact, because the high rate of increase of heat flux at each position must be caused by the arrival of the flame at that position, Figs. 5 and 6 actually suggest that the flame propagates somewhat faster than the computations indicate. Furthermore, the flame apparently does not retain a spherical geometry, but moves faster towards position HT2, as indicated by the measured higher heat flux at the computed time the flame reaches this position in com-parison to the other positions.

The location of the flame at a given crankangle computed by the heat release model is influenced by the heat transfer rates calculated from the particular empirical heat transfer correlation used. Low heat transfer rates result in slow burning rates. In general, in accordance with the above observations, the heat transfer rates obtained using Woschni's correlation are lower than the corresponding measure-ments. This is especially true during the initial stages of combustion as shown in Fig. 7, where the heat flux measured at position H T l is compared with the "area-averaged" heat flux calculated using Woschni's correlation. The vertical dotted line in Fig. 7 shows the calculated arrival of the flame at position HTl . Before the flame reaches position H T l the probe is in contact with unburned gas at relatively low temperatures; whereas, after the flame reaches the lo-cation of measurement, the probe is in contact with combustion gases at near adiabatic flame temperatures.

The initial high rate of increase of the heat flux and the magnitude of the peak heat flux are strongly influenced by the gas pressure and local burned gas temperature. This is demonstrated in Fig. 8, which shows the variations of surface heat flux at position H T l with crank angle for three spark timing settings: MBT, 10 deg advanced and 10 deg retarded. As shown in Fig. 8, spark timing strongly influences the rise of the heat flux as well as the magnitude of the peak heat flux. On the other hand, during the last portion of the expansion stroke and

3.0 -

-2.0

2: 1.0

0.0

_ , , ! ! i i i r * * FLAME ARRIVAL

A HTl / i i , -/ /

4 A.

\ HT2 H HT3

\ v HT 4 j A" . A ^ ^

N = 2000 r / m j n

/ / / i\-~(J

:l i 1 / /' /

' Us | p " ^ T D C .JU 1 1 1

/ A \ \ V.E. =40%

vMo. MBT

vS\ % \

X N x PMAX

I i 1 1

Fig. 5 Variations of surface heat flux with crankangle at the four positions of measurement (1000 r/min, 24 deg BTDC)

340 350 360 370 380 390 400 410 CRANKANGLE, DEGREES

Fig. 6 Variations of surface heat flux with crankangle at the four positions of measurement (2000 r /min, 29 deg BTDC)

Journal of Heat Transfer MAY 1980, VOL. 102 / 191


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WOSCHNI CORRELATION

370 390 410 430

CRANKANGLE, DEGREES

470

3.0

2.0 -

1 1 T 1

-

-

A *f*\ if -\ u /H

it O f / jl t w *

A S f ^ r ^ ! 0 i

r - i i i

A - MBT

o 10 A D V -

D _ . _ 10 RET

POSITION: HT1 "

A / F - 18

N = 1500 r/min

V.E. =40% -

V. V ^ % g

i > i i

Fig. 7 Comparison of transient heat flux measurements at position HT1 with values calculated using Woschni's correlation

1.0

0.0 320 340 360 380 400 420 440 460

CRANKANGLE, DEGREES

Fig. 8 Effects of combustion timing on surface heat flux at position HT1. (1500 r/min, MBT = 26 deg BTDC)

prior to opening of the exhaust valve the heat flux is relatively unaf-fected by the spark setting. Advancing the spark setting causes the initial rate of increase of heat flux to occur earlier in the cycle because of the earlier arrival of the flame at the position of measurement. It also augments the initial rate of increase in heat flux and the magni-tude of the peak heat flux because of the higher magnitudes of gas pressure and temperature achieved during combustion.

In contrast to fired conditions, for motored operation of the engine, the four heat flux probes do not exhibit the orderly sequence of the initial high rate of increase of heat flux that characterizes the fired conditions. Figure 9 shows the variations of heat flux with crank angle at the four positions of measurement for motored operation of the engine at a speed of 1500 r/min. The increase in heat flux during the compression stroke occurs simultaneously at all positions. In addition, despite random fluctuations in the heat flux results, it also appears that the peak heat fluxes at the four positions on the cylinder head occur at approximately the same crankangle. The irregular fluctua-tions in the calculated values of the heat flux at motored conditions are caused by the relatively low signal-to-noise ratio of the surface temperature measurement.

In general, the magnitudes of the peak heat fluxes during motored operation of the engine are an order of magnitude less than the cor-responding peak heat fluxes during fired operation of the engine.

The effect of engine speed on surface heat flux in a fired engine is shown in Fig. 10. This is a plot of the variation of surface heat flux at position HT1 with crankangle at three different speeds. The surface heat flux increases with increasing engine speed. This increase is more noticeable during the combustion period. During the post-combustion period of the expansion stroke the increase in heat flux with speed is smaller.

Engine speed affects the heat transfer to the surface of the com-bustion chamber primarily via its effect on the convective heat transfer coefficient. Increasing the engine speed increases the char-acteristic velocity of the flow [17,18], which accordingly increases the convective heat transfer coefficient and hence the heat flux.

Conclusions Based on the transient heat flux measurements presented, the

following conclusions were reached: 1 At fired conditions, the beginning of the initial high'rate of in-

crease of heat flux at each position of measurement correlates rea-sonably with the calculated time of flame arrival. In comparison, for motored operation of the engine, the increase of heat flux during the compression stroke occurs simultaneously at the four positions of measurement.

0.4

0.3

x 0.2

0.1

o.o u^ 320 340 360 380 400

CRANKANGLE,DEGREES Fig. 9 Variations of surface heat flux with crankangle at the four positions of measurement for motored operation of the engine

3.0

2.0

1.0

0.0

1000 r/mii D 1500 r/mii

340 360 380 400 420 440 460 480

CRANKANGLE, DEGREES

Fig. 10 Effect of engine speed on surface heat flux at position HT1

192 / VOL. 102, MAY 1980 Transactions of the ASME


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2 T h e p e a k hea t flux varies cons iderab ly wi th posi t ion of mea-s u r e m e n t on t h e cylinder head . T h i s spat ia l var ia t ion is cons idered to be pr incipal ly a t t r i bu t ab l e to spat ia l variat ions of the t e m p e r a t u r e a n d velocity fields in t h e combus t ion chamber .

3 Advanc ing t h e spark t iming increases t h e peak h e a t flux a n d advances t h e t ime t h a t th is occurs in the engine cycle. Dur ing the last s tage of t h e expans ion process pr ior to open ing of t h e e x h a u s t valve, t h e m a g n i t u d e of t h e h e a t flux is i n d e p e n d e n t of spa rk se t t ing .

4 T h e p e a k hea t flux a t each posi t ion of m e a s u r e m e n t increases wi th increasing engine speed.

Acknowledgments T h e a u t h o r would like especially to acknowledge t h e efforts of J .

P . Myers , who in i t i a ted t h e surface t e m p e r a t u r e m e a s u r e m e n t p ro -g r a m a n d he lped in t h e design of t h e hea t flux probes , and those of R. W. Bil insky and D. T . French , who provided the technical suppor t t o t h e expe r imen ta l p rogram.

R e f e r e n c e s 1 Eichelberg, G., "Some New Investigations on Old Combustion Engine

Problems," Engineering, Vol. 148,1939, pp. 463 and 547. 2 Sitkei, G., "Beitrag zur Theorie des Warmeuberganges im Motor,"

Konstruktion, Vol. 15,1962, p. 67. 3 Annand, W. J. D., "Heat Transfer in the Cylinders of Reciprocating

Internal Combustion Engines," Proceedings of the Institution of Mechanical Engineers, Vol. 177, No. 36, 1963, p. 973.

4 Woschni, G., " A Universally Applicable Equation for the Instantaneous Heat Transfer Coefficient in the Internal Combustion Engine," SAE Trans-actions, Vol. 76, 1967, p. 3065.

5 LeFeuvre, T., Myers, P. S., and Uyehara, O. A., "Experimental Instan-taneous Heat Fluxes in a Diesel Engine and Their Correlation," SAE Paper No. 690464,1969.

6 Whitehouse, N. D., "Heat Transfer in a Quiescent Chamber Diesel Engine," Proceedings of the Institution of Mechanical Engineers, Vol. 185, 1970-1971, p. 963.

7 Fiynn, P., Mizusawa, M., Uyehara, 0 . A., and Myers, P. S., "An Exper-imental Determination of the Instantaneous Potential Radiant Heat Transfer Within an Operating Diesel Engine," SAE Paper No. 720022,1972.

8 Dent, J. C , and Suliaman, S. L., "Convective and Radiative Heat Transfer in a High Swirl Direct Injection Diesel Engine," SAE Paper No. 770407, 1977.

9 Overbye, V. D., Bennethum, J. E., Uyehara, O. A., and Myers, P. S., "Unsteady Heat Transfer in Engines," SAE Transactions, Vol. 69, 1961, p. 461.

10 Oguri, T., "On the Coefficient of Heat Transfer Between Gases and Cylinder Walls of the Spark-Ignition Engine," Bulletin of the JSME, Vol. 3, No. 11,1960, p. 363.

11 Elser, K., "Der Instationare Warmeubergang in Dieselmotoren," Mitt Inst. Thermodyn., Zurich, No. 15,1954.

12 Mattavi, J. N., "A Miniature Sensor for Measuring Heat-Transfer Rates in Engines," SAE Paper No. 741078,1974.

13 Wendland, D. W., "The Effect of Periodic Pressure and Temperature Fluctuations on Unsteady Heat Transfer in a Closed System," NASA Report CR-72323, Mar. 1968.

14 Annand, W. J. D., and Ma, T. H., "Instantaneous Heat Transfer Rates to the Cylinder Head Surface of a Small Compression-Ignition Engine," Pro-ceedings of the Institution of Mechanical Engineers, Vol 185, 1971-1972, p, 976.

15 Krieger, R. B., and Borman, G. L., "The Computation of Apparent Heat Release for Internal Combustion Engines," ASME Paper No. 66-WA/DGP-4, 1966.

16 Lancaster, D. R., Krieger, R. B., Sorenson, S. C , and Hull, W. L., "Effects of Turbulence on Spark-Ignition Engine Combustion," SAE Paper No. 760160, 1976.

17 Semenov, E. S., "Studies of Turbulent Gas Flow in Piston Engines," Combustion in Turbulent Flow, Ed. L. N. Khitrin, Translated from Russian, 1963.

18 Lancaster, D. R., "Effects of Engine Variables on Turbulence in a Spark-Ignition Engine," SAE Paper No. 760159,1976.

Journal of Heat Transfer MAY 1980, VOL. 102 / 193


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Documents

[3]