Ind. Eng. Chem. Prod. Res. Dev. 1982,21, 305-309
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Prevention of Carburetor I c e in Aircraft Engines by the Addition of Ethylene Glycol Monomethyl Ether to Aviation Gasoline Richard L. Newman Crew Systems Consultants, Yellow Springs, Ohlo 45387
The effectiveness of 0.15 vol % ethylene glycol monomethyl ether (EGME) as an anti-carburetor-icing fuel addiiie was investigated inflight using an airplane equipped with two venturi carbureted engines. One engine used fuel containing the addiie; the other operated on stock aviation gasoline. The results show that EGME is quite effective in preventing carburetor ice during cruising flight. Both the maximum severity of icing and the range 01 environmental conditions conducive to its formation were reduced. During reduced power, the overall rate of ice formation was reduced by the additive. There are certain environmental conditions, however, where the EGME may enhance slightly the rate of ice formation. Based on the overall results, it seems that EGME c a n be quite useful in preventing the majority of carburetor ice accidents in general aviation.
Introduction Carburetor ice remains one of the more common causes of general aviation engine stoppages. (General aviation means any non-airline civil aircraft operation. The types of aircraft discussed in this paper are, for the most part small (less that 12500 lb) piston powered airplanes.) Caused by the cooling of moist air within the carburetor, it shuts off the flow of air and fuel to the engine. while pilots are taught to detect and prevent its formation, the accident record shows that this alone is not the cure. Newman (1977)reviewed the National Transportation Safety Board files covering the years 1969 to 1975 and found a list of 468 accidents where the Safety Board found a cause of “Engine Failure-carburetor/induction system icing.” This total includes 421 accidents to single-engine airplanes, 31 to multi-engine airplanes, and 16 to rotarywing aircraft. There were 44 fatalities and 202 serious injuries represented. Seventy-five aircraft were destroyed. In his review, Newman could find no significant pattern to suggest that one aircraft design was better or worse than another. While several airplane models did have statistically higher accident rates than might be expected, the type of operation (such as flight instruction, aerial application, personal travel, etc.) appeared to be more significant. Flight instruction and aerial application (crop dusting) operations appear to be especially prone to carburetor icing. In addition to these accidents with official reasons for the engine stoppage given by the Safety Board, Newman also reviewed those accidents where the Board was unable to determine the cause of the stoppage. He concluded from examining these records that approximately one-half of all carburetor ice accidents are missed by the accident investigators. Newman concluded that the real number of carburetor ice accidents is about 140 per year, costing an estimated two million dollars. Background
Before discussing carburetor ice protection, it is necessary to review briefly the physics of ita formation and those design and operational factors that influence its formation. This review will only cover the main issues. The interested reader should consult Coles (1949)or Diblin (1971)for more detailed discussions. There are three types of induction system icing: impact icing, throttle icing, and fuel vaporization icing. Impact 0196-432118211221-0305$01.25/0
icing is formed by moisture laden air striking elements of the intake passage at temperatures slightly below freezing. It is very similar to airframe icing and forms in much the same conditions. It is most common at 20-32 OF when flying in clouds or other visible moisture. Common to all airbreathing engines, impact icing can be prevented by proper intake design, the use of sheltered air sources, or by structural heating. This last method is typical design practice in turbine engines. Throttle icing forms when moisture in the air condenses and freezes because of cooling when the air passes through restrictions, such as inlet guide vanes, throttle butterflies, or carburetor venturis. The acceleration of the air produces a static pressure drop which in turn causes a temperature drop which can approach 30 OF. Like impact icing, throttle icing is common to all airbreathing engines. It is unlikely to be significant at high power settings in reciprocating engines. Throttle icing can best be prevented by air preheat, although localized structural heating can be effective in those installations where the air can regain its temperature downstream. In float (venturi) carburetors, throttle icing acts in concert with fuel vaporization icing. Fuel vaporization icing is caused by cooling resulting from the vaporization of the fuel. As the fuel evaporates, its heat of vaporization will cool the surroundings in the absence of a heat source. In a gasoline powered engine, the average cooling of a stoichiometric mixture will be approximately 37 OF. Total cooling (vaporization plus throttling) of 70 OF has been reported (Coles et al., 1949). Vaporization icing is unique to carbureted engines since other air-breathingengines inject the fuel in much warmer regions of the engine where the cooling effect cannot reduce the temperature to the freezing point. Because of the additive effects of vaporization and throttling through the venturi, vaporization icing is especially serious in venturi carburetors. According to Coles (1949),the critical factors for ice formation are carburetor inlet temperature, humidity, and the throttle angle. Fuel temperature and the amount of excess free water were not found to be significant. The fuel/air ratio (mixture) was reported not to be significant, although Diblin (1971)disagrees. Figure 1 shows the regions of serious ice formation for a venturi carburetor at several power settings (i.e., several throttle angles) from Coles’ report. The Canadian Ministry of Transport (1976) has published a similar chart for pilot awareness. This 0 1982 Amerlcan Chemical Society
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1
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Figure 1. Conditions favorable for carburetor ice formation (adapted from Coles, 1949).
k!C
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CO
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Figure 2. Conditions known favorable for carburetor icing (adapted from chart, courtesy of Aviation Safety Bureau, Canadian Ministry of Transport, 1976).
chart, reproduced in Figure 2, incorporates some accident data to supplement ita theoretical basis. It shows a somewhat greater extent than does Figure 1. The National Advisory Committee for Aeronautics (NACA), during the 1930s and 1940s, did a considerable amount of work investigating both airframe and engine icing. The engine icing studies culminated in the previously cited reports by Coles and his co-workers. The final report concluded that air preheat (from 34 to 42 Btu/lb of air, depending on the power) was the preferred method of ice prevention. Other anti-icinglde-icingtechniques were also discussed in these NACA reports. The heating of the induction system structure is mentioned as a possibility. While the inlet duct itself might be difficult to heat, oil-heated throttles were tested by NACA and these kept the throttle butterflies, but not the barrels, free from ice.
Alcohol de-icing was not favored because of ineffectiveness and possible fire hazards. Alcohol fuel additives were mentioned as a British technique, but no effectiveness data were reported. In the late 1960s, the National Research Council of Canada (NRC) studied the effect of fuel additives and non-adhering carburetor surfaces (Gardner and Moon; 1970). Their results showed that several combinations of anti-icing additives were extremely effective in preventing carburetor ice. The NRC tests were all conducted on a dynamometer with a 283-in.3 automotive engine and a typical updraft aviation carburetor. Several alcohols were tried with limited success. Two glycols were found to be effective. Hexylene glycol was the most effective, but it was quite sensitive to the concentration in the fuel. Ethylene glycol monomethyl ether (EGME) was almost as effective and was insensitive to reduced concentrations. In either case, the optimum concentration was 0.15% by volume. In addition to the glycols and alcohols, several proprietary carburetor detergents were found to assist in preventing the ice from adhering. Teflon spray was also tried to prevent ice adhesion. Two separate mechanisms seem to be present. The glycols (and alcohols) serve to depress the freezing point of the water condensing in the carburetor, thus forming (at worst) a spongy, soft ice. The detergents and the Teflon spray, on the other hand, were reported to form a hard, dry ice which adhered poorly to the carburetor structure. Gardner and Moon (1970) reported a synergistic effect and stated that the combination of EGME and either detergent or Teflon spray “eliminated all ice deposition.’! The recommendation of Gardner and Moon was to incorporate 0.10 to 0.15 vol % of EGME into aviation gasoline with or without the detergent or a Teflon-coated throttle. The Canadian specification for aviation gasoline was modified as a result of these tests to allow the addition of EGME on an optional basis. At present, it is not believed to be available in any aviation gasoline sold in Canada (Saunders, 1977). Flight Testing Two objectives were identified for a set of carburetor ice flight tests. The first objective was to validate the range of environmental conditions conducive to the formation of carburetor ice. These conditions had previously been published by NACA (Figure 1)and by the Canadian MOT (Figure 2). An initial objective to validate these figures in flight was set for the program. The second objective was to confirm or reject the effectiveness of EGME as an anti-carburetor-icingadditive. This fuel additive had been well tested in ground tests at relatively severe icing conditions, but had not been tested in flight. Further, i t was hoped that additional insight could be gathered concerning the effect of throttle position, mixture control setting, and other pilot techniques on the formation and detection of carburetor ice. Aircraft Description. The airplane used in these tests was a Piper PA-23-150 twin-engine, low-wing airplane equipped with two AVCO-Lycoming 0-320-A air-cooled engines. These engines are representative of the small single-engine fleet. The test airplane was equipped with two newly remanufactured engines and the engine instruments were calibrated. In addition to the normal engine instruments, ALCOR exhaust gas temperature analyzers and Richter carburetor air temperature probes were installed and calibrated.
Ind. Eng. Chem. Prod. Res. Dev., Vol. 21, No. 2, 1982 307
Table I. Instrumentation Summary data description
instrument
units
pressure altitude ambient temperature wet bulb temperature manifold pressure (each engine)
aircraft altimeter ft Fisher Scientific psychrometer “F
engine speed (each engine)
engine gauges
throttle position (each engine)
scale on throttles
engine gauges
in. Hg
carburetor temperature (each engine) Richter Aero Equip. indicator inlet temperature (each engine) exhaust gas temperature
aircraft gauges ALCOR, Inc. Analyzer
The same fuel was used in each engine, except that after the initial testing (to confirm that each engine was equally susceptible to carburetor icing), 0.15% by volume EGME was added to the left engine’s fuel. The EGME was added using Prist aerosol spray cans. Periodically during the testing, the fuel was analyzed for EGME concentration. On several occasions, both engines were run on EGME-treated fuel to ascertain the de-icing capability of the additive. On other occasions, untreated fuel was fed to both engines to verify that each was still equally susceptible to carburetor ice. Instrumentation. All data were hand-recorded. The flight parameters of interest are shown in Table I. Aside from the free air temperature and the wet-bulb thermometer readings, all test data were recorded from the aircraft instruments. The temperature/dew point data were obtained from a psychrometer held out of the pilot% clear view window. Modifications to the dew point calibration chart were made to account for the increased air flow over the wet bulb. Test data were recorded at 1Bmin intervals except when the icing rate made shorter intervals necessary. Test Method. Once the altitude and geographical location for the test had been decided upon, the airplane was flown to the particular area where the desired environmental conditions were likely to be established. Upon arrival, the desired flight conditions were established (throttle, propeller, and mixture settings). The airplane was flown in these conditions until a power loss rate could be measured or the absence of ice confirmed before terminating the test. All flight procedures were “normal” PA-23 operating procedures (Piper Aircraft Handbook, no date; AVCOLycoming, 1973) except as noted. The carburetor heat function was tested during the engine run-up prior to takeoff to ensure that heat was available. Following this check, carburetor heat was not used except as a check for ice formation (or as a last resort for safety of flight). The accumulation of carburetor ice was monitored by recording the manifold pressure drop and variations in the mixture (from exhaust gas temperature variations) as functions of time. Once a power loss had developed to a significant degree, carburetor ice was verified by applying full heat for a sufficient time to remove the ice and noting the increase in manifold pressure following removal of the heat. The amount of power loss (measured by the change in manifold pressure from before to after the application of heat) was a measure of the amount of carburetor ice.
calibration FAA certified repair station calibration ASTM certified thermometers
calibrated against FAA calibrated altimeter using standard atmosphere table rPm calibrated using reed-type vibration indicator arbitrary none; equal power for same throttle position verified using engine instruments “C checked at room temperature with ASTM thermometer “F not calibrated “F (change) calibrated using ALCOR procedures; referenced to 65% power
Table 11. Carburetor Icing Classifications carburetor icing severity none trace light moderate heavy severe
rate of power loss none detected less than 0.5 in./h 0.5 to 5 in./h 5 to 10 in./h 10 to 20 in./h more than 20 in./h
estimated time to engine failure
__
more than 20 h 2 to 20 h 1to2h 30 min to 1h less than 30 min
On occasion, the engines did not show any measurable loss in power but did exhibit roughness or variations in mixture. If the addition of heat corrected this problem, then the carburetor ice was recorded as “trace.” In addition, if the rate of accumulation was less than 0.5 in. of manifold pressure drop per hour, or if the total loss of power was 0.1 in. or less, then “trace” was also reported. At various intervals throughout the test program, samples of EGME treated fuel were withdrawn from the fuel tank and analyzed for EGME concentration using a freezing point depression method (Houston Chemical). Except for one analysis, the concentrations determined by this analysis agreed with the predicted values using the aerosol spray can. On one occasion, the concentration was much higher than predicted. One gallon of fuel was drained, and then a second 500-mL sample was obtained. This sample analyzed as predicted. Results The power loss rate was found by dividing the amount of power regained (following application of heat) by the exposure time. This results in a rate of ice formation in terms of inches of manifold pressure lost per hour. These rates have been grouped into arbitrary classifications of icing severity. We can also estimate the time to engine failure from these rates. Early in the testing, we found that a manifold pressure of about 15 in. of mercury was necessary in order to have enough heat available to clear the ice from the carburetor. Based on a low altitude encounter, a loss of approximately 10 in. will reduce the power to this level. Table I1 lists the severity classifications in terms of manifold pressure loss and the approximate times to engine failure. The effect of altitude, throttle position, and power developed are all interrelated in a normally aspirated engine, such as the test engine. For this reason, it is difficult to separate the effects of variations in all of these parameters. The variable that appears to be the most significant is the throttle position. For this reason, we have grouped the
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heavy ir.--
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t o obtain l i g h t i c i n g envelope
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0
deg F
Figure 3. Conditions found favorable for carburetor ice formation during high power operations (flight test results).
moderate i c i n g l o w power
moderate i c i n g medium power
40 Temperature
60
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100
deg F
Figure 5. conditions found favorable for carburetor ice formation during idle power operations (flight test results).
-l i g h t i c i n g -low and medium Dower
--
um i
1 4 0
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Figure 4. Conditions found favorable for carburetor ice formation during high power operations with EGME added to fuel (flight test results).
data by throttle position, rather than by altitude or power developed. The division between “high power” and “medium power” is a throttle setting of 2 units. This corresponds to a developed power of 73% at 3000 ft altitude. The dividing line between “low power” and ”medium power” is a throttle position of 1.5 units, corresponding to the engine developing about 60% power at 3000 f t altitude. The results of the carburetor icing tests (at cruise power) are summarized in Figures 3 and 4. Figure 3 shows the carburetor icing severities for an engine operated on stock aviation gasoline and should be interpreted as in in-flight verification of the NASA and Canadian data in Figures 1 and 2. Figure 4 shows the severity data for an engine operated on gasoline containing 0.15% volume percent EGME. As can be seen from a comparison of Figures 3 and 4,EGME is quite effective in reducing both the se-
0
20
40 Temperature
-
60 deg F
80
100
Figure 6. Conditions found favorable for carburetor ice formation during idle power operations with EGME added to fuel (flight test results).
verity and the range of conditions of carburetor ice. The icing data obtained during idle power descents is not as clearcut. Much less data were taken since this was not a major point of emphasis. These data were taken during landing approaches-thus no steady-state conditions could be maintained. The test for carburetor ice was made following landing and the taxi to parking. The exposure averaged about 5-10 min and the power was not controlled during this time. Recognizing these limitations and the sparse data, Figures 5 and 6 show the icing envelopes for the descent power case. In general, the descent data show a benefit of EGME, but not as great as for the cruise power case. Of interest are two regions where the presence of EGME appears to increase the rate of icing slightly-at T = 60 OF/D.P. = 60 O F and at T = 30 OF/D.P. = 15 OF.
Ind. Eng. Chem. Prod. Res. Dev. 1902, 21, 309-314
On several occasions, after carburetor ice was allowed to form in the carburetor of one or both engines while using stock fuel, the fuel supply was switched to a tank containing EGME-treated fuel. In every case power was immediately regained. Thus EGME-treated fuel would also serve as a de-icing agent. On five occasions, carburetor ice was encountered with both engines operating on stock fuel, but with different mixture settings. As discussed earlier, Diblin (1971) and Coles et al. (1949) do not agree of the effect of the fuel/air mixture. While the experimental samples size is small, the engine with the rich mixture lost power 39% faster (on the average) than did the engine with the lean mixture. The complete flight test report is available (Newman, 1979). Conclusions The incorporation of 0.15 vol % of EGME into aviation gasoline greatly reduces the formation of carburetor ice during cruise power flight. Both the extent of conditions favorable to ice formation and the maximum severity of ice formation are reduced. The use of EGME should prevent virtually all cruise or climb caburetor ice accidents for pilots not flying in clouds. Since over 60% of carburetor ice accidents occur during these portions of flight (Newman, 1977), this reduction would be no small benefit. There do appear to be certain environmental conditions which make the use of EGME less favorable during descents. Certain dew point/temperature combinations may produce slightly more ice with EGME added to the fuel than without. These conditions need to be examined further before drawing any conclusions regarding the effectiveness of EGME in preventing carburetor ice forma-
309
tion during descent conditions. In any event, the de-icing capability of EGME-treated fuel makes ita addition attractive, even for descent conditions. Acknowledgment This work was sponsored by the Federal Aviation Administration under Contract DOT-FA78WA-4165. Literature Cited AVCOlycoming “Operator’s Manual: AVCO-Lycomlng 0-320, 10-320. AI0320. and LIO-320 Serbs Aircraft Engines”; AVCO-Lycomlng Pubikatkn 60297-16, 2nd ed.; Dlvislon of AVCO Corporation: Willlamsport, PA, March 1973. Canadlan Mlnlstry of Transport Aviation Safety Letter 1976. Issue 6. Coles, W. D. “Investigation of Iclng Characteristics of Llght Alrplene Engine Induction Systems”; National Advlsory Commktee for Aeronautlcs Report TN-1970, Washlngton, DC, Oct 1949. Coles, W. D.; Rdlln. V. G.; Mulholland, D. R. “Iclng Protection Requirements for Reciprocating Engine Induction Systems”; National Advisory Commktee for Aeronatics Report TR-982, Washlngton. DC, June 1949. Diblin, J. Fwlng July 1971. 88(1), 82-83. Cierdner, L.; Moon,0. ”Alrcraft Carburettor Icing Studies”; Natbnal Research COUnCll of Canada Report DME-LR-536, Ottawa, Ont., July 1970. Houston Chemlcal Company, “Procedure for Determining Concentration of Prist Fuel Addithre (PFA-55MB) In Fuel by Freezing Point Method”;kuston Chemlcal Co.: Houston, ca. 1977. Newman, R. L. “Carburetor Ice: A Revlew”; Crew Systems Report TR-7719, Nov 1977; available from National Technical Information Services as Report N80-23280. Newman, R. L. “FNght Test Results of the Use of Ethylene Glycol Monomethyl Ether ( E M ) as an AntLCarbwetor-Iclng Fuel Addltlve”; Federal Avlatlon Admlnistration Report AWS-79-1, Washlngtm, DC, July 1979. Plper Aircraft Corporation “Owner’s Handbook for Operation and Maintenance of the Plper Apache Model PA-23 Airplane”; Plper Aircraft Publlcatbn: Lockhaven, PA (no date). Saunders, P., Canadlan Ministry of Transport, private communlcatlon, 1977.
Received for review December 3, 1980 Revised manuscript received October 26, 1981 Accepted February 23,1982
Process for Chemical Separation of the Three Main Components of Lignocellulosic Biomass Emmanuel 0. Kouklos’ School of Chemical Enginesrlng, Purdue University, West Lafayette, Indkna 47907
George N. Valkanas School of Chemlcai Englneerlng, Natlonal Technical Unlversity of Athens, Athens, OreeCe
The combination of acid prehydrolysis with chemical deligniflcation is examined as a potential method for the
chemical fractionation of lignocellulosic biomass. Experiments with wheat straw show that hemicellulose can be quantitatively separated by prehydrolysis; however, the structure of the residue is substantially modified. Delig nification of this residue with the conventional, alkaline methods (soda, kraft) leads to significant losses of polysaccharides and a degraded celiulose. The efficiencyof the separation can be increased by an unconventional deiignification, such as chlorination by chlorine gas. In this case, the sugar losses are minimal and further degadation of cellulose during deiignification can be avoided, while lignin is quantitatively recovered from the pulping liquors.
Introduction Biomass utilization has recently become the object of considerable investigations in the areas of chemical and
* School of Chemical Engineering, National Technical University of Athens, 42 28th Octobriou Street, Athens 147, Greece. 0196-4321/82/1221-0309$01.25/0
energy engineering. Lignocellulose conversion is of partic& interest, since the major available forms of biomass, namely @Cultural residues and Wood, me Predominantly lignocellulosic. The properties of lignocellulosic structure have been associated with a series of technical and economic problems that can only be solved by multi-stage processes, involving the separation of hemicellulose, cel0 1982 American Chemical Society
