K6nig and Sheppard [25] reached a ... Sheppard, C. G. W., and Ibrahim, E.-S. A. A., Instru- mentation .... several years ago, Oscar Lee measured a correlation be-.
Twenty-Fifth Symposium (International) on Combustion/The Combustion Institute, 1994/pp. 125-133
CARS TEMPERATURE M E A S U R E M E N T S A N D THE CYCLIC D I S P E R S I O N O F K N O C K IN SPARK I G N I T I O N E N G I N E S D. BRADLEY,~ G. T. KALGHATGI, C. MORLEY ANDP. SNOWDON Shell Research Limited Thornton Research Centre P.O. Box 1, Chester CH1 3SH, England AND
JINKU YEO Department of Mechanical Engineering University of Leeds Leeds, England
The use is described of the coherent anti-Stokes Raman spectroscopy (CARS) technique in a singlecylinder research engine to measure end-gas temperature in the regime that leads to the onset of knock. The investigations,under nominally the same running conditions, covered a large cyclic dispersion of knock, with knock occurring in about 80% of the cycles. In-cylinder pressures were measured with a transducer, and an ion gap signified flame arrival towards the end of the flame travel and just before autoignition. In general, the temperature increased by over 100 K, both with and without knock, as a result of exothermic preflame reactions. This measured elevation was supported by computations with a simple five-step model of autoignition, calibrated against measured autoignition times in a rapid compression machine. The temperatures measured just prior to knock increased with the severity of the knock. The observed severe cyclic dispersion of knock, under the conditions of the study, is attributed primarily to the cyclic dispersion of flame speeds. An increase in flame speed is associated with an increase in temperature and greater propensity to knock.
Introduction The characteristic noise of knock is caused by acoustic excitation of the gas in the cylinder and of the engine block. It arises from pressure pulses generated by autoignition at one or more sites in the end gas. This comprises runaway exothermic chemistry with rapid temperature increases of more than 1000 K. Not all autoignitions, however, generate pressure pulses sufficiently strong to lead to knock, and these might be regarded as benign. With alkane-containing fuels, there is a possibility that so-called cool flame chemistry can cause reaction at earlier, lower-temperature, stages of the end-gas compression, with the resulting temperature rise (and possibly the accumulation of reactive products) influencing the subsequent explosive autoignition [1]. The paper discusses the end-gas temperatures measured prior to knock. Although a rapid bum rate might appear advantageous, in that burning might be completed before an autoignition delay time has elapsed, an increase in this rate increases the tern~ Department of Mechanical Engineering, University of Leeds, Leeds, England.
perature of the end gas, which reduces the delay. Optimum balance between burn rate, autoignition chemistry, and spark timing, from cycle to cycle, is sought in engine management systems. The inevitable cyclic variations of burning velocity arising from turbulence make this difficult [2]. The study attributes the observed appreciable cyclic dispersion of knock to the cyclic dispersion of burn rate. Experiments were under nominally the same conditions of load, throttle, and equivalence ratio in a four-stroke engine, with a pancake-shaped combustion chamber and very little swirl. Pressure was measured by a pressure transducer, flame progress by an ion gap, and end-gas temperatures by coherent anti-Stokes Raman spectroscopy (CARS). The more rapid the burn rate, the greater was the tendency to engine knock. As the temperature increased due to exothermic, preknock reactions, statistically, so also did the severity of any subsequent knock. This behaviour also is mimicked in a reduced, four-species, five-reaction model of autoignition. Both model and experiments focus on the regime leading up to autoignition and any subsequent onset of knock, rather than the factors that govern the
125
INTERNAL COMBUSTION ENGINES
126
Pressure ansducer 40 20
(b) Filteredpressure,bar 1.4 0 -1.4 Spark,1.3BTDC Flamearrival (c) Ion-gapsignal -1.6V
CARS beams focus FIG. 1. Plan of measurement points.
-3.2V 42
knock intensity. The latter are more complex, with three modes for the propagation of reaction from exothermic centres: deflagration, thermal explosion, and developing detonation, dependent upon the temperature gradient around the centre [3].
Experimental
Details
The Engine and Signal Processing: A single-cylinder Ricardo E6 engine was modified by inserting a plate with windows between the head and the block to allow optical access to the combustion chamber. The cylinder swept volume was 507 cm 3, with a compression ratio of 10.18 in all but the CARS calibration experiments, when it was 10.47. A Kistler Type 6001 transducer was wall mounted and water cooled. Ion-gap electrodes were mounted in the head close to the end gas, and measurements of the time interval between the spark and ion-gap currents gave the flame arrival time. The ion current pulses were characteristic of those associated with a propagating flame [4], and generally, the flame reached the probe before any knocking occurred. That flame arrival times were indicative of flame propagation was confirmed by the excellent correlation of such time with the pressure. Figure 1 shows the positions of the spark and ion gap, the pressure transducer, and the CARS measuring point. A crankangle (CA) selector system generated timing pulses, as required. Data were recorded by a high-speed acquisition system (Biodata Microlink 4000) at a sampling rate of 500 kHz, and analysed with FAMOS software. The fuel was 90% iso-octane-10% heptane, equivalence ratio 1.13, and the engine ran at full throttle, at 1200 rpm. The spark passed at 13 crank-
I
46
I
50 time,ms
I
54
I
58
FIG. 2. Measured traces during a single cycle. angle degrees, 1.8 ms before TDC (top dead centre). CARS lasers were triggered from a fixed crank-angle signal from each cycle. Figure 2 presents data acquired from a single engine cycle. Shown in Fig. 2(b) is the pressure signal digitally filtered between 5 and 25 kHz using FAMOS. Here knock is arbitrarily defined by a filtered pressure signal that exceeds 0.1 bar. Knock intensity (KI) is the difference between the maximum and minimum of this filtered signal. The conditions studied were for strong knock, with knock in about 80% of the cycles.
The CARS System: CARS thermometry [5-7] now is established in engine research [1,8,9]. In the present work, a modeless laser of improved accuracy was developed [10]. Two laser beams were focused in the measurement volume, of about 1 mm 3, where excitations of nitrogen produced a signal beam. One of the incident beams was broadband over a range of wavelengths, and this led to the creation of a broadband signal beam. This was spatially filtered and dispersed by a spectrometer onto a photodiode array detector to generate the CARS spectrum. As the temperature increases, the higher rotational levels of vibrational bands become more populated, to broaden the CARS spectrum. Temperatures were inferred by matching measured nitrogen spectra to those predicted by the Epsilon CARP PC code. Prior to this, CARS spectra were referenced using a nonresonant CARS spectrum from argon to account for
MEASUREMENTS AND CYCLIC DISPERSION OF KNOCK IN SPARKIGNITION ENGINES the spectral profile of the broadband laser beam. Instrument function parameters were derived to account for the contribution of the detection system to the spectral shape. This includes such effects as artificial broadening due to the finite width of the entrance slit to the spectrometer. Comparisons of an averaged, room-temperature, nitrogen spectrum with its computer code fit allowed the instrument function to be found. The pump beam was formed by frequency doubling the output from a Nd: YAG laser, while the probe beam was the output from a broadband modeless dye laser. The Nd:YAG laser system produced two frequency-doubled outputs: one pmnped the modeless dye laser system (80 mJ, 532 nm), and the other provided the CARS pump beam (90 mJ, 532 nm). The output from the modeless laser system (2 mJ, 1.0-nm bandwidth, centred on 607.5 nm) was combined with the green pump beam and transmitted via prisms to the engine test cell.
autoigmition in engines. The regime of cool flames and negative temperature coefficients is entered towards the end of the compression stroke. There have been several other simplified descriptions of the chemistry [14-17]. The current, particularly simple, five-reaction scheme models the regime of negative temperature coefficient, which obtains towards the end of the compression stroke. It gives a good representation of this regime and of the autoignition delays in rapid compression machines [18]. The present rate data, in the form of Arrhenius "A" values and activation temperatures Ta, given in Table 2, were derived from experimental data from the rapid compression machine at Lille [19] for the same fuel, and kindly supplied by Professor L. R. Sochet and Dr. R. Minetti. The five reactions of the scheme are as follows: F + 12.3502
Validation of CARS Temperature Measurements: The measurement technique was validated in three ways. In a high-temperature/high-pressure cell containing nitrogen, the measured (steady) temperatures exhibited a normal distribution, and as can be seen in Table l(a), there is good agreement between the mean values of these and those measured by a thermocouple. The 95% confidence interval also is given and is tolerably small. Nonsteady calibrations were made at the centre of the engine combustion chamber at different crank angles, in both a motored engine and in one running on propane-air, but not knocking. About 80-85% of the spectra gave a sufficiently good spectral match. Measured mean temperatures and the 95% confidence intervals are given in Tables l(b) and (c), which compare mean values with those calculated from the ideal gas law using the mean value of the measured pressure, known volume, and molar mass from the measured air consmnption rate. It was assumed that no leakage of the charge occurred. There is, again, very good agreement between these temperatures and mean CARS temperatures.
Reduced Reaction Scheme for Autoignition The delay time for autoignition must be greater than the time of arrival of the flame in the end gas to avoid autoignition. Because of the complexities of complete autoignition reaction schemes, there is a long tradition of reduced schemes, although more complete reaction schemes are now being treated [11]. Gray and Yang [12] employed a reduced scheme for the analysis of cool flame oscillations, whilst the seminal work of Halstead et al. [13] used a generic formulation to simplify the prediction of
127
i
F + 02 C
b
C
I
direct reaction
chain initiation
chain branching
~P
,P
,P
,C
,2C q
C + C
d
quadratic termination linear termination.
(1) (2) (3) (4) (5)
Here F represents the fuel, C chain earners, Oz oxygen, and P the products of combustion. The 90% iso-octane-10% heptane fuel stoichiometry is taken to be C7.9H17.8 + 12.3502 ~ 7.9CO2 + 8.9H20.
(6)
The empirical form of the direct oxidation expression at high temperature follows that recommended by Westbrook and Dryer [20]:
die] ~- -
k~[e]~[Od b
(7)
where [ ] indicates mole density. The constants a and b are assigned values of 0.25 and 1.5 and d [02] - _ 12.35kd [F]0.25[O211.5. dt
(8)
The total scheme comprises
d[F] dt
-
kd[F]~
1"5 - ki[F][O2]
(9)
INTERNAL COMBUSTION ENGINES
128
TABLE 1 Calibration of CARS temperature measurements (a) Using high-temperature/high-pressure cell
Pressure (bar)
Thermocouple temperature (K)
CARS temperature (K) mean
1.0 6.4 12.5 21.6 1.0 5.53 11.07 15.15
296 296 296 296 1205 693 790 596
296 298 297 297 1209 694 807 594
CARS temperature (K) 95% confidence interval • • • • • • • •
15 8 10 14 11 17
(b) Using motored engine Motored Temperature (K) Crank angle (deg)
Calculated
CARS mean
CARS 95% confidence interval
663 683 693 703
405 497 549 593
412 504 542 609
• 1O • 12 • • 17
(c) Using engine fired with propane/air Firing Temperature (K) Crank angle (deg)
Calculated
CARS mean
CARS 95% confidence interval
663 683 713
451 569 706
458 574 719
• 12 • •
arl_~c, = k,[e][Q] dt
(lo) + (k b - kl)[C ] - 2kq[C] 2
d[02] _
dt
12.35ka[F]0"25102]l'5
- k~[F][Od
and the energy equation, for a volumetric internal energy, E dE d--7 = kahd [F]~
(11)
+ (kbh b + k q h q [ C ]
+ kihi [F][O2] + klhl)[C] - Q.
(12)
Here h is the heat of reaction and Q is the volumetric
MEASUREMENTS AND CYCLIC DISPERSION OF KNOCK IN SPARK IGNITION ENGINES
129
TABLE 2 Constants for the five (d-l) reaction autoignition nmdel (Arrhenius "A" constant in mole m s units; activation temperature, T,, in K; heat of reaction, h, kJ/mole)
A Ta h
d
i
b
q
1
7.0 10~ 15,000 5,200
2.0 10v 20,000 0
5.2 101~ 11,500 100
2.5 102 0 0
1.0 1014 17,500 50
35 30253
2015:~. 1 0 -
505
.... , .... 600 650 700 i
. . . .
,
. . . .
i
750 800 Temperature, K
. . . .
i
850
. . . .
i
900
. . . .
i
950
FIG. 3. Variation of computed autoignition delay time after instantaneous compression of stoichiometric mixture from 1 bar and 350 K.
sumption that the composition and temperature in the end gas were spatially uniform. Up to the point of sparking, the computed chemical heat release was very small, and the polytropic index for the compression was found from the known pressure-volume relationship. Thereafter, the end gas was compressed by both the piston motion and the flame propagation, and as long as the heat release was negligible, the polytropic index was assumed to have the same value of 1.3. This enabled the end-gas temperature to be calculated from the measured pressure. However, in the later stages, the computations usually revealed some heat release and end-gas temperatures in excess of the polytropic values. This also was observed e~cperimentally.
Experimental Results and Discussion heat loss rate, determined from the polytropic index of compression in the absence of reaction, and with CARS temperatures were measured at three variable ratio of specific heats. Optimisation of the crank-angle timings, A, B, and C, as shown in Fig. various rate parameters was guided by Morley's cor- 2(a). A range of knocking behaviour was observed, relation of the research octane number of alkanes with extensive cyclic variations. For each timing, 250 with the rate of the branching reaction [21]. This spectra were collected. For the last set, C, at 28.8 CA correlation has been extended to include the termi- degrees after TDC, a significant proportion of the nation rate [22], and the ratio of the rate constants, measurements was after the onset of knock. However, only measurements before the onset were conkb/kt, was chosen on this basis. A simplified, but informative, application of the sidered. Table 3 summarises the mean temperature scheme is the computation of the variation of antoig- and pressure at each crank angle, along with the renition delay time with compression ratio. This is spective 95% confidence intervals. Results are shown in Fig. 3 plotted against compression temper- grouped according to whether there was knock durature. This assumes instantaneous polytropic com- ing the cycle. The knocking cycles tended to be hotpression of a stoichiometric mixture of the fuel, with ter in the later stages of compression than the nonindices, n, of 1.30 and 1.33, from a pressure of 1 bar knocking cycles, as shown by set C. This also is and 350 K to the given temperature. The delay time demonstrated in the CARS measurements of Nakada is the total time to the onset of explosive autoignition. et al. [9]. A cool flame may occur during this period. After the The cyclic dispersion of the knock most probably negative temperature coefficient regime, the delay arises from the stochastic nature of the turbulence, time decreases substantially as a consequence of the which influences the flame speed [2]. The temperamodelled "high-temperature" direct reaction. It also ture of residual gas from the previous cycle and its is apparent that an increase in heat loss to the walls, mixing also are important in governing the precomassociated with a decrease in the polytropic index, pression, and hence end-gas, temperature [9]. Figure can increase the delay time. 4 shows how a shorter flame arrival time, the time When applied to the engine conditions, solutions for the flame to reach the ion gap after sparking, in to Eqs. (9) through (12) were obtained for the mea- t u r n correlates with a higher pressure. The mean sured pressure-volume-time relationship, on the as- temperature at the start of compression was esti-
130
INTERNAL COMBUSTION ENGINES TABLE 3 Summary of CARS temperatures [mean temperatures (K) and pressures (bar); 95% confidence intervals in parentheses] Time after TDC
Set
Nonknocking cycles
CA (deg)
(ms)
Temperature (K)
Pressure (bar)
Temperature (K)
Pressure (bar)
10.7
1.48 3.15 4.0
827 (30) 822 (57) 890 (43)
22.5 (09 23.4 (0.22) 22.6 (0.3)
844 (20) 804 (33) 951 (41)
24.3 (0.21) 26.4 (0.27) 269 (09
22.8
28.8
60] ~200]
36 9
I0 < 0.2 bet,
+
KI
