Effects of Halocarbons on Engine Performance FRED G. ROUNDS AND JOHN D. CAPLAN Fuels and Lubricants Department, Research Laboratories Division, General Motors Corp., Detroit, Mich.
‘T
I I E vcell-established fact that engine knock is closely related to combustion time has led t o experimentation in many different areas. For example, attempts have recently been made to control the occurrence of knock in spark-ignition engines by shortening the length of flame travel by combustion chamber design ( I $ ) , employing multiple spark plugs ( 7 ) , and utilizing fuel injection t o ieduce the fuel residence time (1). The purpose of all these mechanical alterations has been to shorten the time that the last pait of the fuel-air charge to burn is exposed to the influence of the temperatures and pressures existing in the cylinder of the engine. In general, then, the mechanical approach to the problem has been based on the belief that, d the charge can be burned rapidly enough the preflanw reactions leading to knock will not have sufficient tune to reach a critical rate before the normal flame front completely traverses the combustion chamber and, consequently, knock will not occur. Because these mechanical changes appear t o be effective in reducing the knocking tendency of an engine, similai iesults might be obtained if some chemical additive could be found that would reduce the over-all combustion time by increasing the reaction rate of the main combustion reactions n ithout simultaneously increasing the rate of the reactions leading t o knock. It has been qhown in burner and tube experiments that the addition of various diluents to a combustible-air mixture reduces the range of air-fuel ratios which will support combustion (W,3, 6, 10). Bunsen burner evperiments with ethylene-air mixtures have indicated that the flame speed can be lowered by the use of either nitrogeii or carbon dioxide as diluents (12). Carbon dioxide.,the more effective of thp two diluents in rclducing the flame speed, has also been shown t o be the more effective in reducing the range of flammable air-fuel mixtures. The effects of these diluents upon the flame speeds in Bunsen burners are consistent with hitherto unpublished effects of diluents upon rombustion time (duration of the inflammation process) in engines. I t has been shown in these laboratories that the addition of various diluents to the intake air of an engine can increase the combustion time in the engine. Table I, for example, presents the variations in combustion time in a General XIotors laboratory single-cylinder engine when various diluents
TABLE I. EFFECT OF DILUEXTS UPON ENGINE COMBLSTION TIME AND FLAME PROPAGATION LIMITS Single-Cylinder Engine, 1000 R,P.M., 7.4 Vpl. % ’ of Diluent Addition Apparent Max. power oomb. time, output, Diliieut % of base % of base 100 100 Base fuel 102 113 Helium 102 115 Nitrogen 115 101 Engine exhaust gas 121 100 Water vapor 93 132 Carbon dioxide Carbon tetrachloride Dichloromonofluoromethane Diahlorodifluoromethane Trichloromonofluoromethane
Max. Allowable Concn. for Flame Propagation (6)
7 Vol. % methaneair mixture
2.5 Vol. %‘ gasolineair mixture
.. 37 34
41
29 24
29
..
The combustion chamber or cylinder of the single-cylinder engine used in this investigation (Figure 1) was designed to simulate a cylinder of a recent model General Motors passenger-car engine. The use of a closed air-metering system employing critical-flow nozzles permitted accurate control of the temperature, pressure, and mass rate of flow of the inlet air (6). An automotive-type carburetor, modified by elimination of the idle systrm and the addition of a needle valve upstream from the main jets, permitted accurate control of the fuel flow rate. With this combination of air and fuel controls, the air-fuel ratio could be maintained constant while the concentration of the diluent was varied. Both liquid and gaseous diluents were ‘investigated. The gaseous diluents were metered from high-pressure cylinders into the intake-air manifold upstream from a surge tank. The quantity of diluent in the air stream was determined by measuring the resulting pressure increase in the surge tank. The liquid additives were mixed with the fuel in the proper concentration to give the desired equivalent volume concentration in the vapor phase. All the liquid additives were completely miscible with the liquid fuels in the concentrations investigated. The engine operating conditions are shown in Table 11. The knocking tendency of the engine and the antiknock quality of the fuels were determined by a modified borderline-knock procedure ~~
TABLE 11. ENGINE OPERATINQCONDITIONS (Prior to addition of diluents)
..
.. 15
..
TEST EQUIPMENT AND EXPERIMENTAL PROCEDURE
36
*.
..
were added t o the intake mixture. Also shown in Table I are the maximum allowable concentrations of various diluents which will permit flame propagation in methane-air and gasoline-air mixtures (6). I n this table the concentration8 of the diluents in the fuel-air mixtures are expressed as volume per cent of the total charge. Examination of the data in Table I indicates that the lower the maximum allowable concentration of the diluent for flame propagation, the longer the apparent combustion time in the engine. If the halocarbons are as effective in the engine as in the flame propagation experiments, a compound such as dichlorodifluoromethane (Freon-12) might cause a much greater increase in engine combustion time than any of the diluents previously tested. Thus the addition of halocarbons to the inducted airfuel mixture appears to be an attractive method for altering engine combustion time as well as for checking validity of the theory behind the mechanical approach t o knock control. The effects of the various halocarbons upon engine combustion, knocking tendency, and power output were therefore investigated.
..
10
a
Nominal cornmesaion ratio
9
7
1677
Engine Speed, R P M 1000 2000 7.5:1 7.5:l 13:l 100 131 177 27
INDUSTRIAL AND ENGINEERING CHEMISTRY
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Vol. 46, No. 8
(This is equivalent to 1.8 volume % of the total charge.) Thc, fuel flow rate was also increased t o maintain a constant ail-fuel ratio. 3. Iso-octane under the same operating conditions as in ( I ) , except that the engine was supercharged with Freon-12 from the standard manifold pressure of 27 inchea of mercury to 27.5 inches (This is equivalent t o 1.8 volume % of the total charge.) T h r air-iso-octane ratio remained essentially constant.
Figure 1.
Single-Cylinder Test Engine
(4). Borderline knock was detected with an oscilloscope by observing “double-differentiated” cylinder pressure-time traces from a condenser-type pressure indicator. Quantitative pressure-time records for combustion a n a l p i s Twre obtained using a balanced-diaphragm pressure indicator and associated electronic equipment (see Figure 2 ) . As this is a point-by-point technique, the pressure-time records xere average records for a large number of engine cycles. The diluents and fuels for this investigation came from the following sources and were generally of commercial grade. Freon-11, 12, 13. 113, 114, 115 Methylene bromide Carbon tetrachloride Chloroform and benzene Methane n-Heptane, iso-octane, and cyolopentane Diisobutylene
Kinetics Chemical Division, E. I. du Pont de Nemours 8: Co., Inc. Eastman Organic Chemicals Baker and Adamson lierck & Co., Inc. Matheson Chemical Co. Phillips Petroleum Co. Enjay Co.
RESULTS AND DISCUSSION
EFFECT O F FREOK-12 O S ESGISEPOWER. I n order t o evaluate properly the observed effects of the halocarbon additions, it was necessary t o investigate their effects upon engine power. Because the ignition timing required for maximum power is an indirect measure of combustion time in an engine, it is possible t o obtain information about the effects of a given variable on both engine power and combustion time from power-ignition timing curves. I n Figure 3 are presented power-ignition timing curves for three different cases. 1. Iso-octane (2,2,4-trimethylpentane) under the operating conditions listed in Table I1 for 1000 r.p.m. 2. Iso-octane under the same operating conditions as in ( l ) , except t h a t the engine was supercharged with air from the standard manifold pressure of 27 inches of mercury t o 27.5 inches.
It can be observed on Figure 3 that supercharging the engine with additional air and fuel gave an increase of about 2% in pon-er ovei the entire range of ignition settings. However, this supercharging did not alter the ignition setting for maximum power or, in effect, the combustion time. I n contrast, supercharging the engine with Freon-12 resulted in a large increase in p o m r (13%) a t t h e ignition timing for maximum power, but a t a fixed ignition setting corresponding to the manufacturer’s distributor setting for this engine a large decrease (11%) in power occurred. Furthermore, the addition of the Freon-12 greatly increased the ignition timing required for the development of maximum engine i This increase indicates a lengthening -p o ~er. of the combustion time. The data in Figure 3 raise a v e r y i m p o r t a n t question, “Khy did the large increase in maximum engine poLver result from the addition of the Freon12?” I n ansn-ering this question it is necessary to rationalize two observations: (1) The large increase in the timing required for maximum power indicates an increase in apparent combustion time. ( 2 ) Extending combustion time in the engine tends to lower engine efficiency. cordingly, a decrease in maximum poxer m-ould be expected from the effects of the added Freon-12 upon engine combustion time, Such a decrease in engine power did occur with carbon dioxide addition-the diluent in Table I causing the greatest increase in conibuqtion time, Consequently, the unexpected performance of the Freon-12 in the engine must be attributed to factors other than its effect upon the over-all rate of engine combustion. EFFECTOF ENGISESPEED FREON-12 COKCESTRAI n Figure 4 are shown the influence of both Freon-12 concentration and engine speed . upon the effectiveness of Freon-12 in increasing the maximum power of the engine. The corresponding ignition timing required t o obtain maximum engine power is also illustrated. From this figure several important observations can be made. Figure 2. Balanced Diaphragm Pressure Indicator Equipment
ASD
TION.
Engine speed myas very important in determining the effectiveness of the Freon-12 in increasing maximum engine pon-er. The Freon-12 p-as approximately twice as effective a t 1000 r.p.m. as a t 2000 r.p.m. a t all the concentrations investigated. This suggests that the power increase is due t o some chemical reaction involving the Freon-12 which proceeds a t a slower rate than the main combustion reactions. Thus, when the available reaction time Fas halved by doubling the engine speed, approximately twice as much Freon-12 was required t o obtain a given increase in power. An increase in the concentration of Freon-12 increased the maxi-
INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1954
1
I
,
IM
1000, RPM
I
70
a
o
1
I
I
I
l
16 24 32 40 IGNITION TIMING- DEGREE BTC
4.9
Figure 3. Comparison of Effects of Freon-12 Addition and Supercharging on Engine Power mum engine power. However, the effectiveness of additional quantities of Freon-12 decreased with increasing concentration. To obtain the increases in maximum power illustrated in Figure 4, it was necessary t o advance the ignition timing as the concentration of Freon-12 was increased. A comparison of the curves showing the maximum power increase and ignition timing required for maximum power indicates a marked similarity which suggests a correlation between the power increase and the advance in ignition timing required for maximum power.
1679
dition to iso-octane were investigated. The per cent increase in maximum engine power obtained with the addition of 1.8 volume % of Freon-12 to the intake charge was essentially independent of fuel type when each fuel was run a t its maximum power air-fuel ratio. On the other hand, the increase in ignition timing required for maximum power development with the four hydrocarbons when Freon-12 vias added showed considerable variation. This is important, as differences in ignition timing for maximum power suggest that the rates of reactions involving the Freon-12 and fuel vary with the molecular structure of the fuel. Since engine operation using various fuel types indicated that fuel type had little effect upon the power increase, the remainder of the tests were generally run with iso-octane as a fuel. The effect of air-fuel ratio on the maximum power increase obtained by addition of Freon-12 to the intake charge was investigated using iso-octane a t air-fuel ratios of 10.5, 13.0, and 17.5. I n all three cases, 1.8 volume % of Freon-12 was added to the intake charge. .4s might be expected, as the mixture was made leaner, the effectiveness of Freon-12 in increasing the maximum engine power was greater. This effect can probably be attributed to the increase in the ratio of Freon-12 to fuel a t the leaner mixtures.
The addition of Freon-12 did not increase the engine power a t all ignition timings. The foregoing comparisons were made a t the ignition timing for maximum engine power; however, similar comparisons a t a fixed ignition timing such as the manufacturer's distributor setting (see Figure 3), indicate that the engine power progressively decreased with an increase in the Freon-12 concentration.
-lOL
'
I
0
05
I
IO
I
15
I
20
I
25
I
30
VOLUME PERCENT D I L U E N T IN INTAKE CHARGE
Figure 5.
I
w
I
I
I
I
INCREASE IN MAXIMUM POWER-
3 $
16
w
u
8 0
1
2
3
4
5
VOLUME PERCENT FREON-I2 IN INTAKE CHARGE
Figure 4. Effects of Freon-12 Concentration on Maximum Engine Power and Ignition Timing for Maximum Power at Two Engine Speeds
EFFECT OF FUEL TYPEAND AIR-FUELRATIO. The power increases due to the addition of the Freon-12 may be associated with reactions of the Freon and fuel. Consequently, the effects of fuel type and air-fuel ratio on the power increase obtained with the addition of Freon-12 were investigated. To determine the effect of fuel type, an olefin (diisobutylene), a naphthene (cgclopentane), and an aromatic (benzene) in ad-
Effect of Various Halocarbons on Maximum Engine Power
EFFECTOF OTHER HALOCARBOW o s ENGINEPOWER.-4 number of other halocarbons and some other miscellaneous diluents were also investigated for their effect upon maximum engine power. However, because of the more limited availability of some of these diluents, their effects upon engine combustion were not so extensively investigated as were those of Freon-12. I n Figure 5 are shown the changes in maximum engine power a t 1000 r.p.m. obtained upon adding these diluents over a range of concentrations. The range illustrated for each diluent does not necessarily represent the maximum range of operation permitted. The ignition timing was adjusted to the optimum value for maximum power development with each diluent a t each concentration. HALORIETHBNES. The effect of adding more fuel to the fuelair mixture is shown by the curve for methane in Figure 5 . As the air-fuel ratio was initially a t the maximum power value, the addition of methane enriched the mixture, with an attendant loss of power. If chlorine atoms were substituted for the hydrogen atoms of methane to give either chloroform or carbon tetrachloride, the addition of these compounds to the engine gave a slight increase in power. Similarly, if bromine atoms were substituted for the hydrogen in methane to give dibromomethane, the addition of this compound also produced an increase in engine
11.;
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D U S T R I A L A N D E N G I N E E R I N G C H E M IST R Y
power. However, the increase in power was less than that obtained with chloroform. T h e increase in power observed with the addition of the fluorochloromethanes is of particular interest. A comparison among these compounds a t a concentration of 1 volume % of the charge indicates that the increases in engine poxyer were 3.5, 9, and 4.5% for CC13F, CC12F2, and CClF,, respectively. These data do not correlate directly with the chemical stability of the compounds, in that the compound of intermediate stability was the most effective in increasing maximum engine power. A similar situation is found with the tetraallrylleads, in that the compound of intermediate stability, tetraethyllead, has greater antiknock effectiveness than either the more stable tetramethyllead or the less stable tetrapropyllead. I
J
- 4
I
,
HALOETHIUES. As in the case of the fluorochloromethanes, the fluorochloroethane of intermediate stability was the most effective in increasing engine power. Thus, a t a concentration of 1 volume % of the charge, the increases in power obtained with C&13Fa, C&lzF,, and C&lFB were 5, 10.5, and 7%, respectively. However, difluorodichloromethane was more effective in increasing engine power than was trifluorotrichloroethane, in spite of the greater molar concentrations of fluorine and chlorine present in the engine with the latter compound. These limited experimental results indicate that the presence of either chlorine or fluorine atoms increased the maximum engine power. However, the effectiveness of each of these halogen atoms was not constant, but was greatly dependent upon the stability of the molecule in which i t was present. I n both the substituted methanes and ethanes, the greatest power increases were obtained with the halocarbons containing two fluorine atoms per carbon atom. EVALUATION OF CHANGES IN POWER DUE TO ADDITION OF HALOCARBONS
Two effects have been postulated as responsible for the power changes observed with the addition of halocarbons. 1. A change in the total energV -. liberated bv the fuel-air charge duringcombustion. 2. IModulation of the combustion process, thereby changing tlhe ternDora1 energv release pattern. Because fluorine has a strong affinity for xydrogen and one of the current theories of flame propagation postulates propagation by a hydrogen atom diffusion process ( l 7 ) , the loss of hydrogen atoms t o the fluorine might be expected t o alter the propagation rate. I n addition, the observed shifts in the ignition setting for maximurn power, which occurred when the halocarbons were added, are evidence that the combustion time has bem changed.
Vol. 46,No. 8
CHANGES I N ENERGY LIBERATION.I n order to evaluate thc possibility that the presence of the halocarbons changed the total energy liberated during combustion, a thermochemical analysis was made of t,he over-all combustion reactions M-ith and without Freon-12. Using the heats of formation given in Sational Bureau of Standards tables ( 1 4 ) and an equilibrium constant of 4.1 for the water gas reaction, t,he heat of reaction for t,he combustion of iso-octane a t an air-fuel ratio of 13 to 1 was calculated a t 77" F. to be -980 kcal. pel. gram-mole. I n order to calculate the heat of reaction with t h e Freon-12 present, i t was assumed t,hat the water gas equilibrium constant was unchanged and that the halogens combined preferentially wit,h hydrogen. As the heat of formation of Freon-12 is not stated in the 'cables, it was est'imated by the Etructural group contribut,ion method of Boyer, Anderson, and Watson (9) to be -105 Itcal. per gram-mole. K h e n this value was used for t h e heat of formation of the Freon-1.2 and heats of format,ion for Lhe other reactants and products were taken from the tables, thc. heat of reaction for the combustion of iso-octane with 1.8 volume 70 Freon-12 added to the intake charge was calculated at 77" 17. to be -1024 lrcal. per gram-mole. LTpon comparing the heats of react,ion for the combustion of iso-octane n-ith and without Freon-12, -980 and -1024 kcal., respectively, it is seen that' the postulation of a change in encrgy liberation may be well founded. The thermochemical analyses wcre made a t 77' F. , however, and t,hus t,he difference in heat's of' reaction may riot even qualitatively reflect Trhat may occur at engine temperatures (200' to 5000" F.). In order t'o determine if the calculated increase in encrgy libcration \vas even qualitatively valid under engine conditions, presssure-time records obtained with a balanced-tliphragm indicator were analyzed by a method developed by Rassweiler and \\.ithrn'Lr (16). It is possible to calculate t,he incrementab pressure development due to the combuetion of the fuel-air charge and eliminate that pressure development due to piston motion during each increment, and t o adjust the incremental valves of t'he pressure development due to combustion to equivalent pressure developments based on a constant cylinder volume. Tn the calculation for this investigation increments of two crank angle degrees were used and the clearance volume oi the engine was used as the basis for the adjustments to const,ant volume. By summing up these increments of constant volume pressure development, the total pressure development due to combustion of the fuel-air charge is obtained. By employing the foregoing method of analysis, the following pressure developments and corresponding engine power developments tvere obbined for t'he iso-octane and for the iso-octane with 1.8 volume % addition of Freon-12 t,o the intake charge a t 1000 r.p.m. and maximum power ignition t,imings. Pressure Lb./sq. inch %
_ _ _ I
Iso-octane Iso-octane
+ 1.8 vol. Yo Freon-12
515 584
100 113
Power,
%
100 113
A comparison of these two values indicates that the presence of the Freon-12 resulted in approximately a 13% increase in the pressure development due t o combustion. 9 s these pressure development values are based upon incremental constant volume combustion and as the heat liberated a t constant volume for a perfect gas is equal to S W C , ( V / B ) dP, nhere W is weight, C is the heat capacity a t constant volume, V is the volume, 12 is the universal gas constant, and P is the pressure, then if constant mass and heat capacity are assumed, the heat liberated is directly proportional t o the constant volume pressure development. Consequently, within the limits of these assumptions, the per cent increase in pressure development reflects the per cent increase in energy liberated. The foregoing analysis of' enrrgy release was made with the
August 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
ignition timing set for maximum power development with and without Freon-12, A similar type of analysis can be made for the identical engine operating conditions and Freon-12 concentration but with the spark fixed a t the manufacturer’s distributor ignition timing of 6.5’ before top center. This analysis gives the following results: Pressure,
Power,
100 116
100 89
%
Iso-octane Iso-octane
+ 1.8 vol.
% Freon-I2
%
From these two analyses it is evident that part of the increase in power observed a t the maximum power ignition timing might be attributed to the increased energy liberated with the Freon-12 present. However, the large decrease in power observed with Freon-12 addition a t the manufacturer’s ignition timing occurred in spite of the increased energy liberated. I n order to account tor this latter effect, it is necessary to evaluate the postulation that the addition of the Freon-12 alters the temporal energy release pattern. CHANGESI N TEMPORAL ENERGYRELEASEPATTERN. In Figure 6 are shown curves comparing on a constant volume basis the relative rates of pressure development due to combustion for iso-octane and iso-octane with 1.8volume % addition of Freon-12 to the intake charge. In each case the fuel-air charge was ignited a t the ignition timing for maximum power. These two rurves show that the major difference lies in what may be called the “ignition lag period.” For example, with Freon-12 added, more than 24 crank angle degrees were required for the combustion process to reach a measurable rate of pressure development, whereas, without the Freon-12, a measurable rate of pressure development occurred after about 4 crank angle degrees. Thus, the presence of the Freon-12 caused a large increase in the ignition lag but did not appear to alter the curve grossly for the remainder of the combustion period. In contrast to the rate curves shown in Figure 6, those in Figure 7 for the identical cases, but with a fixed ignition timing equal to the manufacturer’s distributor setting, show a considerable displacement of the two curves throughout the entire combustion period. I n this cose the ignition lag is not so pronounced for the rate curve representing the Freon-12 addition. This suggests that because ignition occurred a t higher pressures and temperatures, the Freon-12 had started to decompose earlier in the engine cycle and thus was not so effective in retarding the initial stagee of combustion. From the curves shown in Figures 6 and 7 it is evident that considerable differences in the temporal energy release pattern are caused by the Freon-12 addition a t the two types of ignition timing. I n order to evaluate these differences quantitatively from the standpoint of their effect upon engine power, it is necessary to consider how the time in the engine cycle a t which energy release occurs affects the power output. The efficiency with which an engine converts the heat energy released by combustion of the fuel-air charge into work depends upon the expansion ratio a t which this energy is liberated. Thus, the energy liberated must be modified by a factor indicative of the efficiency with which this energy conversion process occurs if the work output of the engine is to be determined. To make this modification in this investigation, the efficiency of the conversion process for each incremental quantity of heat energy released was evaluated using the theoretical relationship between thermal efficiency and expansion ratio for the air standard Otto cycle. However, a measured value of the polytropic exponent of expansion was used in place of the theoretical isentropic exponent. Efficiency = 1 - l/(R)” where R is the expansion ratio and n is the measured polytropic exponent of expansion. I n the foregoing equation the value of R for inrrpmental energy release for each 2 crank angle degrees was
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calculated from the cylinder volume corresponding to the average crank angle a t which the energy was liberated. Thus an efficiency of conversion of heat into mechanical work was obtained corresponding to each incremental energy release. When the value of the incremental energy release was multiplied by the calculated efficiency of conversion, the product obtained represented the portion of the energy released converted into work. Upon summing up these products for the entire combustion cycle, a value was obtained that represented the total heat energy converted into work during the cycle.
I
I
$ I
a
.z
!
I
WITH L R E O N - I Z
I 0 VOL % FREON-12. 1000 RPM
I
l
l
1
CRANKANGLE - DEGREES
Figure 7. Effect of Freon-12 on Rate of Pressure Development Due to Combustion Manufacturer’s ignition timing
The analysis of the pressure time records for iso-octane and iso-octane with 1.8 volume % addition of Freon-12 to the intake charge for the two types of ignition timing is summarized in Table 111. The calculated values of energy converted into work are in qualitative agreement with the observed engine power developments for both types of ignition timing. Furthermore, comparisons of the calculated values of energy converted into work with the summation of the energy release permit an estimation of the effect of the Freon-I2 on the conversion efficiency. For instance, a t the maximum power ignition setting, the presence of the Freon12 did not appreciably change the efficiency of the conversion of heat into work and the increase in engine power was due for the most part to the increase in total energy liberation caused by the Freon-12 addition. However, a t the fixed ignition timing corresponding to the manufacturer’s distributor setting, the presence of the Freon-12 lowered the conversion efficiency so greatly that a decrease in engine power occurred even with the large increase in total energy available as a result of the presence of Freon-12. From these comparisons it is evident that proper analysiq of the pressure-time records makes it possible to attribute the ob-
TABLE 111. COMPARISON OF PERCEST POWER OUTPLT,ENERGY RELEASE,AND ENERGY CONVERTED INTO WORK FOR ISO-OCT4NE .4ND ISO-OCTANE WITH
1.8% B Y VOLUME CHARGE
OF
FREON-12
IN INT4KE
(1000 r.p.m., iso-octane = 100%) Iso-octane Ignition timing for maximum power Power output (observed) Energy release Energy converted into work (calcd.) Ignition timing a t manufacturer’s distributor setting Power output (observed) Energy reiease Energy converted into work (calcd.)
+
Iso-octane
Freon-12
100 100
100
113 113 114
100 100 100
89 116 85
Vol. 46, No. 8
INDUSTRIAL AND ENGINEERING CHEMISTRY
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TABLE IT’.
QU31IfARY O F
(1000 r.p.m.)
Diluent Iso-octane CCla Iso-octane CCbF? Iso-octane
++ Iso-octane + CClF3 Iso-octane + CzClzF? 180-octane + CzC12Fa Iso-octane + C?ClFs Iso-octane + CBrSHs
Concn. of Diluent, Vol. % of Intake Charge f,’8 1.8 1.8 1 8 1.1 1.1
1.1 1.8
tained a t the manufacturer’s distributor ignition setting without Freon-12. It is criEnergy Time for Time for Converted 5% of 95% of dent that the presence of the into Work, Charge t o Charge t o Freon-12 in the intake charge % of Burn, Burn, Iso-octane ”c of 70 of materially reduced the octane (Calcd.) Iso-octane Iso-octane number of the primary refer100 100 100 104 133 119 ence blend required to produce 114 200 150 the equivalent levels of knock85 152 118 150 108 185 free poTver obtainable without 105 110 107 108 140 117 the addition of Freon-12. 111 124 134 This is a rather surprising 94 250 188 observation in light of tlir effect of Freon-12 on comhubtion time. Two additional comparisons of octane number iequiremerit can be made: the requirement of the engine TTith and nithout Freon-12 a t the manufacturer’s ignition timing and the requircment a t the ignition timings for maximum poner development with and rvithout Freon-12. These data are also summarized in Table T’. From these two comparisons it is observed that the presence of the Freon-12 decreased the octane number requiiement a t the manufacturer’s ignition timing, but increased it a t the respective ignition timings for maximum power. Tbeze latter two comparisons mere not made a t the same level of powei output and thus reflect the combined effects of Freon-12 on engine power and engine knocking tendency. EFFECT OF F ~ ~ n s - 1 OY2 FUELOCTANE R ~ T I U G SAs . the octane rating of a fuel in comparison with the octane number requirement of the engine determines whether the engine is going to knock on that fuel, the effect of adding Freon-12 to the engine on the octane ratings of fuels was also investigated. The uwal method of determining the octane rating of a fuel is to compare the fuel with a series of primary reference blends (mixtures of “180-octane” and n-heptane) under the same engine operating conditions until the unknown fuel has been bracketed in terins of knocking tendency by two of the primary reference blends. Utilizing this method, the octane ratings of two commercial gasolines and two special reference fuels were determined with and without Freon-12 added to the engine (Table VI). T h r special reference blends are nonleaded mixtures of “iso-octane,” diisobutylene, and n-heptane and correspond in BSTlf Research and Motor octane number ratings t o the two commercial gasolines.
PRESSURE-TIME RECORD ANALYSES
Ignition Power Energy Timing, Output Release, Degrees 5% of 1s;% of before Top octane Iso-octane Dead Center (Measured) (Calcd.) Max. power, 22 100 100 Alas. power, 30 101 103 I f a x . power, 44 113 113 N f r . set, 6 1 / 2 89 116 1Iax. power, 40 108 110 Max. power, 25 105 105 Max. power, 25 111 111 Max. power, 27 113 109 Max. power, 52 101 95
served changes in pon-er to changes in the total eneigy liberated and to changes in the temporal energy release pattern, APPLICATION OF PRESSURE-TIRIE ANALYSIS TO RESGLTS WITH OTHER HALOCARBONS
The pressure-time records obtained Tyith some of the other halocarbons investigated have been analyzed (Table IV). I n addition, an indication of their effects upon combustion time is given by the data for the time required to burn 5 and 95% of the fuel-air charge. The 5% mass burned time is indicative of the “ignition lag period,” vihereas the 9570 mass burned time is indicative of the total combustion time. For the most part, the data in Table IV indicate that the observed changes in power resulting from the addition of the halocarbons were due almost entirely to changes in the energy liberated. Most of the analyses in Table IV are for cases where the ignition timing was adjusted for maximum power development. However, when the ignition timing 7%as adjusted to the manufacturer’s distributor setting during the addition of halocarbons, a power decrease was observed. These decreases in po\Ter were due to a decrease in the efficiency of converting the released energy into ivork. An example of this is given by the data for Freon-12 in Table IT’. Table IV indicates that all the halocarbons increased the time required to burn 95% of the charge. The bromine-containing compound was especially effective in this respect. From the tabulation of the time required for 5% of the fuel-air charge to burn, it is also evident that most of the obeerved increase in combustion time occurred during the “ignition lag period.” EFFECT O F FREON-12 OV KNOCKIVG TENDEVCY OF EhGINE AWD ANTIKNOCK QUALITY OF FUELS
Wechanical methods of altering the combustion time in an engine have been shoan to be effective in changing the knocking tendency of the engine and the use of chemical additives to alter the combustion time might be expected to have similar effects. Changes in engine operation xhich alter the poryer output, such as changing the ignition timing, supercharging, and changing the throttle opening, all change the knoching tendency of the engine. As the presence of Freon-12 increases the combustion time and increases the engine power, both of T< hich tend to increase engine knocking tendency, the addition of Freon-12 would be expected to increase the engine knocking tendency. EFFECTO F FREON-12 O X ENGIKEK N n C K I S G TESDEYCY. Because of the attendant effect of Freon-12 upon engine power, isolation of its effects upon engine knocking tendency and antiknock quality of fuels is difficult. However, as the primary function of an engine is t o produce power, comparison of the knocking tendencies of a n engine a t a given p o w r output level is of particular interest. I n Table V are summarized the constant power octane number requirements of the engine used in t h k investigation a t t a o different power levels: (1) the maximum power obtainahle without Freon-12 and (2) the p o i ~ e rlevel ob-
TABLE 1’. EFFECTO F FREOS-12 O N ENGINEKNOCKISG TESDEXCY (1000 r.p.m., 1.8 vol. 7c rreon-12 added to intake charge) Octane S o . Requirement, Primary Reference Blends Without Witli Freon-12 Freon-12
Power comparisons Maximum power ivithout Freon addition ( l l l l b . / s q . inch, indicated mean effectivepressure) Power a t manufacturer’s ignition timing without Freon addition (100lb./sq. inch, indicated mean effective pressure) Ignition timing comparisons RIanufacturer’s ignition timing ( 6 . 5 O BTC) Respective maximum power ignition timings (24’ B T C vvjthout Freon) (44’ BTC with Freon)
93
92.5
77
Less than 60
77
Leas t h a n 60 97
93
~
~
~~~
From the data in Table VI. it is spen that the addition of thcx Freon-12 to the engine decreased the octane ratings of all of the fuels ewept blend 7-9. However, the decreaies in ratings fol the two commercial gasolines wcrevery much greater than for blend 5-8. The large difference in the effects of the Fieon-12 on the c0mInc.rcia1 gasolines and the S-blends are of interest in that the major difference between the two types of fuels is that the commririnl gasolines contain tetraethyllead, Thus the data in Tnhlc 1-1
INDUSTRIAL AND ENGINEERING CHEMISTRY
August 1954
TABLEVI.
FREOK-12 O N FUELOCTANE NUMBER RATINGS (1000 r.p.m., 1.8 vol. Yo Freon-12 added t o intake charge) EFFECT O F
Octane Rating without Freon-12
Fuel Premium grade commercial gasoline (90 5 R, 82 0 M) Regular grade commercial gasoline (84 5 R 77 5 M) Blend 7-S’(90 0 R 80 5 h‘l) Blend 5-9 (85.0 R,’ 78.5 bf)
Octane Rating with Freon-12
92
83
85
77 5
88.5
90 0 82 5
84 5
suggest that the Freon-12 may be canceling the antiknock effect of the tetraethyllead. The ASTM Research rating of the premium gasoline was 83 octane number before the tetraethyllead was added, which is in agreement with the rating obtained with the Freon-12 present. That the various halogens (fluorine and chlorine) are antagonists for tetraethyllead has been reported by Livingston (1%). The implication that Freon-12 is antagonistic toward tetraethyllead is also of interest from three different standpoints in respect to the mechanism of the antiknock action of tetraethyllead. 1. The Freon-12 or its products may cause the decomposition of the tetraethyllead too early in the engine cycle, thereby decreasing its effectiveness. 2. The Freon-12 or its products may combine with the active uroducts of tetraethvllead (lead or lead oxide). Dreventinr- the hormal course of reactions. 3. The Freon-12 or its products may combine pieferentially with hydrogen or its reaction products. Inasmuch as tetraethyllead is effective as an antiknock when hydrogen is used as a fuel ( 8 ) ,the removal of the hydrogen or its reaction products may lower the tetraethyllead effectiveness. ~
From this discussion it is evident that the addition of Freon-12 to the intake charge of an engine may serve as a valuable tool for further investigations of the mechanism of the antiknock action of tetraethyllead. EFFECTO F FREOS-12 ON KNOCK-LIhfITED ENGINEPOWER. Since it has been shown that the addition of Freon-12 changes the engine octane number requirement, fuel octane number ratings, and engine power output, some criterion of comparison is required which includes all three of these effects in order to evaluate tke “net” effect of Freon-12 on engine performance. Such a criterion is knock-limited power output on various fuels. In Table VI1 are shown for 10 different fuels the per cent changes in knock-limited power obtained when Freon-12 is added to the engine. An examination of the data indicates that with all of the fuels the addition of the Freon-12 permitted an increase in knock-limited power and that the magnitude of the increase in power is dependent upon both the fuel and the presence or absence of tetraethyllead. MISCELLANEOUS EFFECTS O F HALOCARBONS ON ENGINE OPERATION
I n addition to their effects upon engine power, combustion, octane requirements, and octane ratings of fuels, the halocarbons also influenced some other aspects of engine operation. The most interesting of these effects was the large increase in carbon deposition which occurred when the halocarbons were used. This effect mas first detected when it was observed that the engine would stop running after a few minutes of operation with the halocarbons. Examination of the spark plug indicated that it had been “fouled” by a light fluffy deposit which appeared to be mainly carbon. Figure 8 compares a new spark plug with one that had failed after running with Freon-12. Subsequent dismantling of the engine indicated that the heavy carbon deposition was not limited to the spark plug but was prevalent throughout the combustion chamber. Unlike normal combus-
1683
tion chamber deposits, these deposits could be removed by running the engine without any halocarbons added for about 5 minutes. This heavy carbon laydown observed when employing the halocarbons as diluents is of particular interest in view of the report that the products of pyrolysis of halogen compounds promote smoke formation by removal of hydrogen from the fuel molecules, thereby promoting polymerization reactions (16). In addition to increased carbon formation, when the halocarbons were used the cooled exhaust gases from the engine were very corrosive. I n fact, holes were corroded through a section of standard gage 2-inch water pipe in 4 hours of operation. Analyses of the exhaust gases indicated the presence of halogen acids. In addition, inspection tests of the solventextracted mid-continent base lubricating oil employed during the tests indicated more rapid than normal sludging of the oil and also a marked increase in the neutralization number. However, inspection of the combustion chamber surfaces and valves after over 30 hours of operation with the halocarbons disclosed no abnormalities. The partial persistence of the observed changes in engine power after the halocarbon Figure 8. “Fouling” of additions were discontinued Spark Plug Due to Addiis also of interest. Thirty tion of Freon-12 minutes after the addition of Freon-12 was discontinued, the maximum power of the engine was still 3% greater than the normal poiver. However, after some additional time, depending upon the original concentration of Freon-12 employed, the power returned to the normal level. This “hangover” effect appears to be similar to the persistence of the antiknock action of tetraethyllead for a number of engine cycles after changeover from a leaded to a nonleaded fuel. That this partial persistence was due to some halocarbon remaining in the intake system is considered extremely doubtful, as tests n-ith several inert gases gave no similar effects. Some of the halocarbons or their decomposition products may have been absorbed on the combustion chamber deposits and subsequently released. SUMMARY
Some of the effects of employing halocarbons as diluents in the intake charge of a spark-ignition engine on engine performance
TABLE VII.
KiYOCK-LIlfITEDENGINE POWER WITH ADDITION O F FREON-12
INCREASES I N
(1000 r.p.m., 1.8 vol. % Freon-12 added t o intake charge) % Increase in Fuel Knock-Limited Power 95 octane number primary reference blend 11.5 90 octane number primary reference blend 7.0 85 octane number primary reference blend 6.0 80 octane number primary reference blend 6.0 75 octane number primary reference blend 8.0 70 octane number primary reference blend 15.0 Commercial premium grade gasoline 0.5 Commercial regular grade gasoline 2.0 9.0 Blend 7-5 Blend 5-S 5.0
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
1684
have been determined. In general, addition of the halocarbons lengthens engine combustion time, increases the total energy liberated, and alters the temporal energy release pat’tern. As a result of these changes, the ignition timing required for maximum power development’ is increased, and the maximum power output of the engine is increased, but the power output at’ ignition timings less than that for maximum power may be increased or decreased. Addition of the halocarbons decreases the knocking tendency of the engine under conditions of constant power output or ignition t,iming, but increases the knocking tendency a t the ignition timing for maximum power. On the other hand, the halocarbons greatly lowered the octane number rating of fuels containing tetraethyllead but had little effect upon the octane rat,ing of lionleaded fuels. These observations indicate t’hat the addition of the halocarbons may be 8. useful tool for investigating the auliknock mechanism of tet’raethylleatl. The effectiveness of the various halocarbons in altering engitic performance varied markedly wit,h the type and number of halogen atoms present but showed no direct correlation with the stability of the compound. The addition of the halocarbons to the intake charge of an engine for power increases of short duration ~ o u l have d some value in racing applicat,ions if it were not)for the attendant heavy carbon formation which fouls the spark plugs and the production of corrosive exhaust products.
Vol. 46, No. 8
R. E. Schwind for his assistance ill conducting the c ~ p o i n i c n t a l n-ork, and to the members of t,he Fuels and Lubricants IIel):ti,tment of the General Motors Research Laboratories for thpir awistance in preparing thii: manuscript. LITERATURE CITED
Barber, E. RI., Reynolds, R., and Tierney, W.T., S.d.L“.Q ~ r o ~ , i Trans., 5, 25 (1951). Burgoyne, J. H., and Richardson. J. F., Fuel, 28, 150 (1949). Coleman, E. H., Ibid., 31, 445 (1952). Coordinating Research Council, Inc., “Handbook,” Sew T n r k , J. J. Little and Ives Go., 1946. Cornelius, W.,and Caplan, J. D., S.B.E. Quart. Truna., 6, (iCii, (1952).
Coward, H. F., and Jones, G. IT., C . 8.Bur. AIines, H i t l l . 53 (1953).
Diggs, D. R., SAE Trans., 61, 402 (1953). Downs, D., Walyh, A. D., and \.T’heeler, R. W.,
Tmtib.
/Zo!/.
SOC.(London), A243, 463 (1951).
Hougen, 0. 1., and Watson, IS.AI., “Chemical Proce,~sI’riii-. ciples,” Part 2, p. 758, S e w York, John Wiley & Sons. 1 ‘ M i . Jorissen, W.P., and Hermans, J. J . , Rec. t r a y . c h i m . , 52, 271 (1933).
Linnett, J. W.,and Hoare, M. F.,Trans. E‘aTaday
Soc., 4 7 , 1 iO
(1951).
Livingston, H. T C , I’m.ENG.CHEM.,43, 663 (1951). RIathews, V. P., and Turlay, J. D., S.A.E. Journal ( 7 ~ r 61, 478 (1953).
Natl. Bureau Standards, Circ. 500 (1952). Rassweiler, G. 31..and WithroiT, L. L., S.A.E. Journal (7’ 42, 185 (1938).
Schalla, R. L., and AIcDonald, G. E., IND.EXG.CHILW.,45,
ACKNOWLEDGMENT
1497 (1953).
The authors wish to express their appreciation to L. L. Wit’hrow f o i . his advice and encouragement, during the investigation, to
Tttnford, C.. and Pease, R. N., J . CAem. Phys., 15, 861 (19471. RECEIVED for reviev March 2 5 , 1954.
ACCEPTEDM a ) 10, I!):
