Utilization of Ethanol-Gasoline Blends as Motor Fuels

alcohol and gasoline as motor fuel has been. @T under serious consideration for many years,. In a number of countries, alcohol blends have been used w...
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Utilization of Ethanol-Gasoline Blends as Motor Fuels OSCAR C. BRIDGEMAN National Bureau of Standards, Washington, D. C.

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H E possibility of utilizing blends of ethyl It seems probable that most of the differences in conclualcohol and gasoline as motor fuel has been sions regarding the performance characteristics of the two under serious consideration for many years, types of fuel have arisen from the lack of a common basis In a number of countries, alcohol blends have been used with for comparison. Consideration of the problem indicates varying degrees of success, depending upon the service rethat there may be many different bases for comparison, quirements and the precautions taken to prevent the separabut the three following are the ones most commonly emtion of the alcohol from the gasoline in service. The difFiployed : cultiea which have been encountered result largely from the COMPARISON BASIS1. A given gasoline and the same gasoline fact that, for economic or other reasons, full advantage has blended with ethyl alcohol are used without appreciable detonanot been taken of the available technical information. Thus, tion in either case in the same engine under comparable condiif the need or desire for such fuels should arise in this country, tions without change in the carburetor or spark setting. This sufficient technical information is available to ensure that represents the case where both types of fuel are available and used indiscriminately, and where the carburetor and spark adblends containing ethyl alcohol could be utilized satisfactorily vance will tend to be set midway between the optimum settings $8 motor fuels, provided full advantage can be taken of the for the respective fuels. available technical information. For the most part, thereCOMPARISON BASIS2. A given gasoline and the same gasoline fore, the problem is whether or not it is economically feasible blended with ethyl alcohol are used without appreciable detonation in either case in the same engine under comparable condiin practice to utilize to the fullest extent this available techtions with optimum carburetor and spark settings for each fuel. nical information. For the most efficient utilization of alcohol This represents the case where only one type of fuel is available blends, the engines and accessory equipment should be deor where one type of fuel is used continuously. signed specifically for the purpose. Also it is necessary either COMPARISON BASIS3. A given gasoline and the same gasoline blended with ethyl alcohol are used in two different engines, each to reduce the water content of the alcohol so that the latter with optimum compression ratio (or compression pressure) for will not separate from the gasoline a t the lowest temperature its type of fuel, and with optimum carburetor and spark settings. likely to be encountered and to guard against further introThis represents the case where the engine is designed specifically duction of water. or to use blending agents so as to increase for the fuel. the water tolerance. The relation between enThe object of the present gine p o w e r , acceleration, and fuel consumption with paper is to summarize the Blends containing ethyl alcohol have technical information on the the two types of fuels will comparative properties of be c o n s i d e r e d from the no material advantage over gasoline as g a s o l i n e and of a l c o h o l standpoint of these three motor fuels, although they can be utiblends from the standpoint bases of comparison. lized satisfactorily if full advantage is The power which can be of their performance in servtaken of the available technical informadeveloped in a given engine ice. The economic phases tion. Small percentages of ethyl alcohol depends upon the mass of of this subject are outside oxygen consumed by the the scope of the paper and in the blend are more advantageous than fuel per unit time. The will not be taken into conlarge percentages from the standpoints of function of the fuel is to sideration. A comparison maximum power and acceleration for of the two types of fuels produce heat by combinaminimum fuel consumption, and of ease tion with the available oxywill be made on the basis of of engine starting and warming. The reengine power, acceleration, gen. For the same mass of oxygen consumed, the heat and fuel consumption; vaverse is true from the standpoints of vapor developed on combustion por lock; engine s t a r t i n g lock and of water tolerance. A comvaries only over very narand warming; and w a t e r promise may therefore be necessary from row limits for all combustolerance of the fuel. the technical standpoint in determining tible liquids. H o w e v e r , the composition of the blend most suits o m e liquids require less Engine Power, Acoxygen for combustion than able for any particular purpose. celeration, and Fuel do others, as the result of Consumption The alcohol used for blending should differences in chemical combe essentially anhydrous in order to preposition, so that in these Any comparison of the vent separation of the alcohol in service. cases more liquid will have performance characteristics By employing a suitable blending agent, to be burned in order to of g a s o l i n e a n d alcohol make a comparison on the blends must be predicated the water tolerance of the blend can be basis of equal mass of oxyupon the establishment of a markedly increased, although the ethyl gen c o n s u m e d . Accordbasis for comparison, since alcohol used must still be practically aningly, essentially the mme the properties of gasoline, hydrous unless very large percentages of power may be developed in and hence of the blend, blending agent are employed. a given engine when operatmay vary over wide ranges. 1102

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INDUSTRIAL AND ENGINEERING CHEMISTRY

ing on ally type of fuel, provided the quantities of fuel consumed are adjusted so as to consume the same amount of oxygen in each case. Maximum power is developed when the mixture delivered to the combustion chamber contains about 1 pound of gasoline for every 12 pounds of air on the one hand, or about 1 pound of pure ethyl alcohol for every 6 to 7 pounds of air on the other hand. Mixtures of air and fuel for maximum power with blends of alcohol and gasoline lie between the above limits. Figure 1 shows typical power-mixture ratio curves obtained with a given engine operating with a fixed partialthrottle opening, a fixed spark setting, and a fixed compression ratio, using gasoline and blends containing 10 and 20 per cent of ethyl alcohol as fuels Similar curves would be obtained with other engines, other throttle openings, spark settings, and compression ratios. The major effect would be to shift all of the curves up or down if the same changes were made for each fuel, or to shift the curves up or down relat'ive to one another if different throttle openings, spark settings, or compression ratios were used for each fuel. This figure will be used as a basis for the discussion of relative fuel consumption, power, and acceleration with gasoline and alcohol blends.

Comparison Basis 1 Consider first the case where gasoline and an alcohol blend are used in the same engine under comparable conditions without change in the carburetor or spark setting. Further, assume that the engine is operated with a fixed throttle opening. If the carburetor is set on the lean side and delivers a 14:l mixture to the engine, the power developed will be as indicated by line A , with gasoline producing the greatest power. If the carburetor is set on the rich side and delivers a 10:l mixture to the engine, the situation is exactly reversed as indicated by line B, for the 20 per cent blend will deliver the greatest power and the gasoline the least. At those mixture ratios where the power curves cross, identical amounts of power will be obtained, and over a considerable range of mixture ratio little difference in power is observable between gasoline and the 10 per cent blend or even the 20 per cent blend. Soticeable differences in power would, however, be observed when using 40 per cent alcohol blends with any carburetor setting normally employed for gasoline. The above comparison is based on the power developed with a fixed throttle opening and the same mixture ratio used with each fuel. I n practice, on the other hand, the operator of the vehicle opens or shuts the throttle so as to obtain the desired power. Thus, considering again line A (Figure l), if the throttle is wide open when using gasoline, no further opening of the throttle is possible and the situation is as outlined in the preceding paragraph. If the throttle is almost wide open with the gasoline fuel, further opening of the throttle will tend to increase the power developed by the alcohol blends. Finally, if the throttle is sufficiently closed when operating on gasoline, the effect of opening the throttle wider with the alcohol blends will be to raise the power produced by these blends to the same value as that obtained with gasoline. I n doing this, however, more fuel is consumed with the 10 per cent blend, and still more with the 20 per cent blend, than with gasoline. The situation is exactly reversed when the carburetor is adjusted to deliver a 10:l mixture as indicated by line B. I n this case the throttle opening may be decreased with the alcohol blends, thereby lowering the power to that developed with gasoline. Hence, less fuel would be consumed with the 10 per cent blend, and still less with the 20 per cent blend, than with gasoline. Over a considerable range of mixture ratio, opening or closing of the throttle to obtain the same power as with gasoline results in small increases or decreases in fuel consumption with the

8

Rich

9

10

II

1103

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M i x t u r e Ratio

I3

(Y

6

I6 Ltdn

FIGURE1. TYPICAL HORSEPOWER-MIXTURE RATIO CURVESFOR GASOLINEAND FOR 10 AND 20 PERCENT ALCOHOLBLENDS

blends, depending upon whether the carburetor is set lean or rich. I n service, carburetors are adjusted to deliver approximately a 12:l mixture of air and gasoline for maximum power and acceleration with a slight, though definite, tendency towards a slightly richer setting. With individual engines, carburetor settings are distributed almost equally on both sides of this average setting. Accordingly, some cars show more fuel consumption with a 10 per cent blend than with gasoline and some less fuel consumption, and on the average there is little difference in fuel consumption with either gasoline or a 10 per cent blend for the same power requirements. Likewise. some cars show slightly better acceleration with the 10 per cent blend and some slightly poorer acceleration. These statements are particularly applicable if the carburetor is adjusted to deliver a mixture halfway between the optimum mixtures for gasoline and for the 10 per cent blend, and the differences are only slightly magnified if both types of fuels are used indiscriminately in engines with optimum carburetor settings for either fuel. Somewhat greater differences are observed when comparing gasoline and a 20 per cent blend, with a definite tendency towards a greater average fuel consumption with the blend. This tendency towards a higher average fuel consumption is more marked with 40 per cent blend. Large differences in fuel consumption between gasoline and blends containing up to 20 per cent of alcohol when used in the same engine under comparable operating conditions are found only (a) when the carburetor is set to deliver either an abnormally rich or an abnormally lean mixture, or (b) when the vehicle is being operated in very hot weather under conditions where large amounts of vapor are being formed in the fuel feed lines. This latter case will be discussed more fully under the section on vapor lock. Numerous experimental investigations have shown the general correctness of the above statements, and only a limited number of illustrations will be given. I n May, 1933, series of road tests (2) was made by the National Bureau of Standards with fifteen different models of cars. The cars and the gasolines were selected with a view to being typical of products in use on the highway. Tests were run over a course of about 89 miles, with the usual precautions to assure accurate measurements of distance, car speed, fuel consumption, and such other quantities as were necessary to secure precise results. The cars were driven in all cases by experienced drivers who were not informed as to the fuel in use og

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a.

INDUSTRIAL AND ENGINEERING CHEMISTRY

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any psrticular test, thus avoiding any possible prejudice in this regard. Since cars are operated on the road with a variety of carburetor adjustments or corresponding mixture ratios, these conditions were reproduced as follows: The fuels were classed in pairs, one of each pair being gasoline and the other the same gasoline blended with 10 per cent of absolute ethyl alcohol. Several carburetor adjustments were made on each of the cars and either one or both pairs of fuels were run in each car with each of the carburetor adjustments. Thus there was obtained a total of forty-three direct comparisons between gasoline and the corresponding alcohol-gasoline blend ; each pair of runs was at the same setting of the carburetor in a given car. I n making the tests, the cars were run as close to the legal speed limits throughout the course as traffic and traffic lights permitted. The driving time t o cover the course of 89.2 miles averaged 2 hours 50 minutes. Four of the fifteen cars showed, on the average, more than 1 per cent greater fuel consumption with the gasoline than with the alcohol blend, and six cars showed as much as 1 per cent greater fuel consumption with the alcohol blend. The average results are given in Table I. TABLE I. COMPARATIVE FUELCONSUMPTIOX WITH G.4SOLINE8 AND 10 PERCENTALCOHOLBLENDS F u e l No.

Description

No. of R u n s

Av. Consumption

GaZ./nile

1

2

3 4

Low-octane gasoline Fuel 1 10% alqohol High-octane gasoline Fuel 3 10% aioohol

+ +

30 30 13 13 Av. for gasolines Av. for 10% blends

0.0645 0.0643

0.0593a 0.0606Q 0.0629 0.0631

a T h e lower average number of gallons per mile was n o t d u e t o t h e use of higher octane gasoline b u t rather to t h e fact t h a t fuels 3 a n d 4 were r u n in different groups of cars.

I n the group of runs with fuels 1 and 2, the average values for fuel consumption are almost identical. I n the second group of runs with fuels 3 and 4 the alcohol blend shows a definite increase in the gallons per mile. Averaging all of the results shows that any diyerence in fuel consumption between the two types of fuels is very small. When the reports on the above series of tests were analyzed, it was found that on the average the drivers reported slightly poorer acceleration with fuels 2 and 4-name1yJ with the alcohol blends. This difference in the acceleration obtained with gasoline and a 10 per cent blend in the same engine with carburetor adjusted to an average setting for gasoline (and not for the mean between gasoline and the blend) would be observed by only a few drivers when accelerating on a level road, by many drivers when accelerating after stopping a t a traffic light, and by most drivers when accelerating on a steep hill in passing another car. The differences would be more frequently observed when using a 20 per cent blend and would be very marked with a 40 per cent blend. If both gasoline and a n alcohol blend were available commercially and were used indiscriminately by the public, carburetors might tend to be set, especially in fleets of trucks, to deliver a mixture intermediate between the optimum values for the two types of fuels, and most of the difference in acceleration, particularly with 10 per cent blends, would be eliminated. However, since many drivers would choose one type of fuel for continuous use, the tendency in this case would be to adjust the carburetor for that particular type of fuel, since rapid acceleration is one of the characteristics of automotive equipment most highly prized by the general public. Adjustment of the carburetor for the particular type of fuel would undoubtedly be made by the large fleet owners to whom minimum fuel

VOL. 28. NO. 9

consumption for the desired engine performance is very important. This introduces the second basis of comparison between the two types of fuels from the standpoint of power, acceleration, and fuel consumption.

Comparison Basis 2 Consider now the case where gasoline and an alcohol blend are used in the same engine under comparable operating conditions with optimum carburetor and spark settings for each type of fuel. The curves in Figure 1show that the same power may be obtained with an alcohol blend as with gasoline, for the same throttle opening, if the carburetor is adjusted t o deliver a richer mixture in the case of the blend. The extent t o which the mixture must be enriched depends upon the percentage of alcohol in the blend, a 20 per cent blend requiring a richer mixture for the same power than a 10 per cent blend. The maximum power obtainable with an alcohol blend with proper carburetor and spark adjustment may be slightly greater than with gasoline, for the same compression ratio and throttle opening, and may become increasingly greater as the percentage of alcohol in the blend is increased. This results from the fact that the latent heat of vaporization of ethyl alcohol is higher than that of gasoline and hence, because of increased cooling of the mixture in the intake manifold, there is increased volumetric efficiency when using the blend, the increase being dependent upon the percentage of alcohol in the blend. Sumerous experimental investigations have shown that, when the mixture ratio and the spark setting are adjusted to optimum values, the same acceleration may be obtained with a 10 per cent alcohol blend as with gasoline, a t the expense of about 3 t o 4 per cent increase in fuel consumption with the blend, The increase in fuel consumption would be approximately double this amount when using a 20 per cent alcohol blend. One illustration of the difference in fuel consumption for equal acceleration with gasoline and with a 10 per cent alcohol blend is given in Table 11. based on road tests made by the National Bureau of Standards. ~

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~~

T ~ B L E11. COMPARATIVE FUEL CONSUMPTION FOR E a u s ~ ACCELERITIOS WITH G SOLIN NE AND THE SAME GASOLINE BLENDED WITH 10 PER CEXTABSOLUTEETHYLALCOHOL Car N o

1 2

Gal. per Mile f o r E q u a l Acceleration 1 0 7 alcohol Gasoline blend 0 0508 0 0568

0 0524 0 0592

%

Increase

3 2 4 2

I n these road tests two different makes of cars were employed, and the carburetor of each was carefully adjusted so as t o give the maximum acceleration with gasoline. After the fuel consumption runs with the gasoline were completed, the carburetors were adjusted to give the same (and maximum) acceleration with a blend of the same gasoline containing 10 per cent of absolute ethyl alcohol, and fuel consumption runs were made with the blend. The difference in fuel consumption for the same acceleration was found to average nearly 4 per cent. The comparison thus far between the two types of fuels has been based on operation in an engine in which neither fuel detonates. The case may arise in which the gasoline will cause detonation, whereas detonation will not occur with the alcohol blend because of its higher octane number. I n this event none of the conclusions already reached will be changed if the operator does not object t o engine knock, since the power will not begin to decrease until detonation becomes very heavy. On the other hand, if the operator of the vehicle objects to

SEPTEMBER, 1936

INDUSTRIAL ,4KD ENGINEERING CHEMISTRY

engine knock, a wider throttle opening may be employed with the alcohol blend before detonation occurs, resulting in better acceleration and somewhat higher power with the blend than with the gasoline, still a t t!!e expense of greater fuel consumption with the blend. The octane number of pure ethyl alcohol is about 90, while that of gasoline varies from about 74 t o 80 for the first grade, about 65 to 70 for the second grade, and usually below 65 for the third grade. Accordingly, blending ethyl alcohol with gasoline will improve the octane number by amounts which cannot be stated quantitatively but which depend upon the percentage of alcohol used and upon the octane number of the gasoline. I n general, addition of 10 per cent of ethyl alcohol to a second or regular grade of gasoline will increase the octane number about 3 to 5 units, while the increase will be less when blending with the first or premium grade, and may be as high as 10 octane units when blending with a low, third-grade gasoline. As the result of this increase in octane number, it is possible to use for blending, a gasoline of lower octane number than would be used unblended.

Comparison Basis 3 S o material gain in engine performance is obtained by the increase in octane number on blending unless the compression ratio of the engine is sufficiently high to take advantage of the increased octane number. This brings us to the third basis of comparison between the two types of fuels. Consider a gasoline with an octane number of 65 and assume that addition of 10 per cent of ethyl alcohol to this gasoline raises the octane number by several unit.. Consider further that the engine used with the gasoline has the optimum compression ratio for a 65 octane gasoline and that the engine used with the 10 per cent blend has the optimum compression ratio for the blend. The effect of increasing the compression ratio is largely to shift the curve upwards (Figure 1). If the compression ratio is left unchanged with the gasoline but is increased for the blend, then for the same mixture ratio and throttle opening, a t any usual carburetor setting leaner than that for maximum power with the blend, the power obtained with the blend may equal or even surpass that obtained with the gasoline, depending upon the permissible increase in compression ratio. Thus for the same fuel consumption using engines designed specifically for each type of fuel and taking full advantage of the compression ratio in each, it may be possible to obtain the same power and acceleration with each, or even to show slightly better performance with the blend. Under these same conditions, if the optimum carburetor and spark settings are used for each fuel, somewhat better performance could be obtained with the blend but a t the expense of increased fuel consumption. This comparison is largely academic a t the moment since it implies a limit on the octane number obtainable with gasoline, which so far has not been reached. Thus, no matter what gain in performance can be obtained by raising the compression ratio with alcohol blends, a still greater gain in engine performance per gallon of fuel consumed will be obtained by using a gasoline of suitable octane number in the same equipment. In fact, hydrocarbon fuels have been made, but have not yet been used as motor fuels in automotive equipment, which have higher octane numbers than ethyl alcohol; in such cases the blends would have lower octane numbers than the unblended fuel. On the other hand, the comparison loses much of its significance from a practical standpoint if the case arose where only a n alcohol blend were available commercially, for then individual drivers would compare the performance in their own cars with that in some other car, and there would be no basis for them to compare with the performance obtainable with gasoline.

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Summary of Three Comparison Bases In summarizing the comparison between gasoline and an alcohol blend, the following conclusions can be stated : 1. When used in the same engine, particularly with the carburetor and spark set for the mean of the optimum values for the two types of fuels, little difference in power, acceleration, or fuel consumption is observed with a 10 per cent blend or even under many operating conditions with a 20 per cent blend. Satisfactory engine performance could not be obtained with a 40 per cent blend and gasoline in the same engine with any fixed carburetor setting. 2. When used in the same engine, TFith carburetor and spark accurately adjusted to the optimum settings for each type of fuel, the same power and acceleration may be obtained at the expense of about 3 to 4 per cent greater fuel consuniption with a 10 per cent blend, about double this difference with a 20 per cent blend, and still more with a 40 per cent blend. 3. When used in the same engine, a gasoline of lower octane number may be employed for blending with ethyl alcohol than is necessary unblended. 4. If suitably designed engines are used for the two types of fuels, equal or better power and acceleration per gallon of fuel consumed may be obtained from the blend when comparing a gasoline and the same gasoline blended with ethyl alcohol, within the range of present-day commercial gasolines. 5. Assuming that it is possible to produce and utilize gasoline having any desired octane number, the engine performance per gallon of fuel consumed will always be poorer with a blend of ethyl alcohol, when both fuels are used in the same engine.

These conclusions are based on the use of anhydrous ethyl alcohol for blending with gasoline under conditions where no vaporization occurs in the fuel feed lines. The case where vaporization does occur will be considered in the section on vapor lock. If the ethyl alcohol used for blending contains water, then for equal engine performance with gasoline the fuel consumption with the blend will be still further increased by an amount proportional to the percentage of water in the blend. On the other hand, if blending agents are used to increase the water tolerance of the ethyl alcohol-gasoline blend, the amount and nature of the blending agents may have a marked effect. Thus some blending agents, such as propyl alcohol, butyl alcohol, or benzene, produce more energy per gallon than ethyl alcohol, and the differences between gasoline and the blended fuel may be diminished. Other blending agents have marked knocking tendencies and their use might. greatly reduce the gain in octane number resulting from the addition of ethyl alcohol to the gasoline.

Vapor Lock with Alcohol-Gasoline Blends Vapor lock is defined as the interference with fuel flow in the fuel feed system as the result of formation of vapor or gas and is manifested by loss in engine power, by poorer acceleration, and in extreme cases by engine stoppage. I t is caused by the use of a fuel which is so volatile that boiling occurs in the fuel feed lines and by the fact that the capacity of the fuel lines for handling vapor is limited. For the same fuel used in a number of different engines operated under comparable conditions a t the same atmospheric temperature, vapor lock may or may not occur in any particular engine, depending upon the temperatures reached in the fuel feed lines and upon the capacity of the fuel system for handling vapor. Extensive investigations (3) conducted a t the Kational Bureau of Standards over a period of several years showed that marked differences exist in the temperatures and the vapor-handling capacities of the fuel lines in different models. Accordingly, in order to secure anything like universal freedom from vapor lock, it is necessary to give most consideration to those makes and models of cars in which the fuel line temperatures are high and the vapor-handling capacities of the fuel systems are lorn. Under these conditions the Reid vapor pressure is the most useful criterion for vapor lock

INDUSTRIAL AND ENGINEERING CHEMISTRY

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with gasoline, and control of the Reid vapor pressure is a satisfactory means of assuring freedom from vapor lock. If vapor-locking tendency could be considered independently of other properties of the gasoline, use of a product of low vapor pressure would entirely eliminate the vapor lock problem. Unfortunately, this cannot be done, particularly in the spring and fall months when both warm and cold days are likely to be encountered and therefore a compromise must be reached between freedom from vapor lock and ease of engine starting. Thus a decrease in vapor pressure t o avoid vapor lock also decreases the ease of engine starting; vice versa, an increase in volatility and hence vapor pressure to improve ease of engine starting increases the likelihood of vapor lock difficulties. In practice, therefore, the vapor pressure of gasoline is maintained a t as high a value as possible compatible with reasonable freedom from vapor lock. This practice, furthermore, is advantageous from the standpoint of efficient utilization of available petroleum resources in that i t makes feasible the utilization of as large a percentage as possible of the volatile, high-octane fractions of gasoline. For the same reasons, it is necessary t o maintain the vapor pressure values of alcohol blends as high as possible. In order to obtain comparative data on the vapor-locking tendencies of gasoline and of alcohol blends, a series of road tests were conducted by the National Bureau of Standards in which each car was vapor-locked with both types of fuel. Eleven representative makes of cars were employed and were operated with fuels (of both types) of successively increasing vapor pressure until vapor lock just occurred a t a constant speed of 40 miles per hour. The gasolines used ac av

ID

e

e

points in the fuel feed system to obtain the vapor-locking temperature. A detailed description of the procedure used in all of the vapor lock road tests by the National Bureau of Standards has already been published (3) and need not be repeated here. After vapor lock mas obtained with the gasoline, the tests were repeated in the same car under the same operating conditions using 10 per cent alcohol blends of increasing vapor pressure until vapor lock occurred. A comparison of the results obtained with the two types of fuels in the eleven cars is made in Figure 2. Each point represents a different car and indicates the Reid vapor pressure of the gasoline and of the 10 per cent alcohol blend which caused vapor lock under the same operating conditions at the same atmospheric temperature with that car. While the operating conditions were identical with each car, the atmospheric temperature necessarily differed with the tests on the various cars since the investigation extended over severaI weeks. On account of the low atmospheric temperature prevailing during the runs with a number of the cars, it was necessary to use fuels with higher vapor pressures than those of commercial gasolines. However, sufficient basic information has been obtained on the vapor lock problem to make it certain that similar differences between the gasolines and the alcohol blends would have been found if the same cars had been operated a t a higher atmospheric temperature on fuels of lower average vapor pressure. Figure 2 shows that the difference between the vapor pressures of the gasoline and the alcohol blend when vapor lock occurred differs with the various cars. This is t o be expected since cars vary considerably in the capacity of their fuel systems for handling vapor formed in the fuel lines, and a car with a low vapor-handling capacity is less affected by differences in the distillation characteristics of the fuel, as differentiated from vapor pressure differences, than is one with a high vapor-handling capacity. Taking into consideration these differences between cars, the results obtained indicate that, for the same freedom from vapor lock, the Reid vapor pressure of a 10 per cent alcohol blend should be about I pound per square inch lower than that considered permissible for gasoline. Another method of comparing the two types of fuels, from the standpoint of vapor lock, is to consider the fuel line temperatures required to produce vapor lock, usually called the ‘vapor-locking temperatures.” Such a comparison is made in Figure 3 where experimental vapor-locking temperatures for three different gasolines and 10 per cent alcohol blends with these gasolines are plotted against the vapor-handling capacity ( V / L ) of the fuel system. The vapor-handling capacity is defined as the ma.ximum volume of vapor per unit volume of liquid flowing through the fuel lines which can be handled by the fuel system without occurrence of vapor lock, Gasolines A, E, and C have Reid vapor pressures of 5.2, 8.7, and 11.0 pounds per square inch, respectively, while the 10 per cent alcohol blends with these gasolines have vapor pressures of 6.2, 9.4, and 11.9 pounds per square inch. The addition of 10 per cent of alcohol to gasoline considerably lowers the temperatures which can be permitted before vapor lock occurs. Further, the lowering in vapor-locking temperature depends upon the characteristics of the gasoline with which the alcohol is blended, and, in general, the lowering increases with increase in vapor-handling capacity of the fuel feed system. These effects of fuel characteristics and vapor-handling capacities are further illustrated in Figure 4 where the comparison is on the basis of vapor pressure. Figure 4A shows comparative information obtained on gasolines characterized by steep distillation curves. The curves are for vapor-handling capacities of 10, 30, and 50, and represent lines of equal vapor-locking tendency for gasolines with steep distillation i

Reid V.F! o f 10% Alcohol 8fends FIGURE 2. COMPARISON OF REIDVAPOR PRESSURES OF GASOLINES AND 10 PER CENTALCOHOL BLENDSWHICH OF CARS CAUSEDVAPORLOCKIN ELEVENMAKES

were prepared by blending butane with a refinery gasoline of 5.6 pounds per square inch vapor pressure in order to obtain the desired vapor pressure characteristics for vapor lock. The alcohol blends were prepared by blending 10 per cent of absolute ethyl alcohol with these gasolines of various vapor pressures. The test procedure involved making successive runs in the car selected using gasolines of increasing vapor pressure until definite evidence of vapor lock was obtained when operating a t a constant speed of 40 m. p. h. The car was then stopped and a sample of the gasoline removed from the tank for determination of its Reid vapor pressure. During the runs, temperature measurements were made a t various

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INDUSTRIAL AND ENGINEERING CHEMISTRY

curves and for 10 per cent alcohol blends with gasolines of this type. Figure 4B represents similar information on gasolines with flat distillation curves and 10 per cent alcohol blends with gasolines of this same type. Since the two types of gasolines chosen represent extremes, they cover the complete range of present-day commercial gasolines. Figure 4 shows that the lowering in permissible vapor pressure for 10 per cent alcohol blends depends upon the characteristics of the gasoline used. Thus the differences are very large when the gasoline has a steep distillation curve and may become comparatively small when a gasoline with a flat distillation curve is used for blending. On the average, with the gasolines marketed a t the present time, the difference in permissible vapor pressure for the same freedom from vapor lock amounts to about 1 pound per square inch. In order to obtain this equality of vapor-locking tendency, a gasoline of lower vapor pressure would have to be used for blending than would normally be employed without blending. The extent to which the vapor pressure of the gasoline used for blending would have to be lowered is greater than 1 pound per square inch since the vapor pressure of the blend is higher than the vapor pressure of the blending gasoline. Representative data on the increase in vapor pressure on blending are shown in Table 111. The average increase is 0.8 pound per square inch.

TABLE111. REID VAPOR PRESS~RES OF GASOLINES AND OF 10 PERCENTALCOHOLBLENDS WITH THESE GASOLINES -Reid Fuel No.

Gasoline -Pounds

415

10.3 12.4

423

Vapor Pressure at 100' F.-

107 alcohol

glend per square inch-

Increase

0.9 0.7 Av. increase 0 . 8

11.2

13.1

Accordingly if i t is desired that a 10 per cent alcohol blend shall have no greater tendency to cause vapor lock than a gasoline fuel when operated under similar conditions in the same engine, i t is necessary to use for blending purposes gasoline with a vapor pressure about 1.8 pounds per square inch lower than would be used without the addition of alcohol. No definite information has been obtained on the vapor-locking tendencies of blends containing 20 or 40 per cent of ethyl alcohol, although there is reason to believe that a gasoline of higher vapor pressure could be used for blending than is permissible with 10 per cent blends. Thus the increase in vapor pressure on blending 20 per cent of alcohol with gasoline is approximately 0.5 as compared with 0.8 pound per square inch for a 10 per cent blend; in the case of a 40 per cent blend, there is a decrease in vapor pressure of about 0.5 pound per square inch on blending. Further, the temperature a t which a 20 per cent blend commences to boil in the fuel lines is higher than that for gasoline and is still higher in the case of a 40 per cent blend. Accordingly, it might beanticipated that there would be little difference on the average between the vapor-locking characteristics of gasoline and the same gasoline blended with 20 per cent of ethyl alcohol. With a 40 per cent blend, the vapor-locking tendency of the blend should be considerably less than that of the gasoline from which the blend was prepared.

1107

aoo 190

4'

;180 a

170 4

E

160

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Y

g

I50

-I I

1110

4 130

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0

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10

1 I I I

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I 20

30

YO

50

60

YL FIGURE 3. COMPARISON OF ,EFFECTS OF VAPORIZATION IN RAISING THE VAPOR-LOCKING TEMPERATURES OF GASOLINES AND OF 10 PER CENTALCOHOLBLENDSWITH THESEGASOLINES

When used therefore in the same equipment under similar operating conditions, high rather than low percentages of alcohol in the blend are desirable from the standpoint of vapor lock. However, as pointed out previously, the use of high percentages of alcohol involves increased fuel consumption, and, as will be pointed out later, engine starting becomes difficult with blends containing large amounts of alcohol. One other point should be mentioned in connection with 10 per cent alcohol blends. If a gasoline of lower vapor pressure is used for blending, the lowering of 1.8 pounds per square inch in vapor pressure means that about 1.8 per cent less butane will be used in the gasoline. Since butane has a very high octane number absence of this amount of butane would appreciably lower the increase in octane number obtainable by blending with ethyl alcohol. On the other hand, with a 40 per cent blend, it would be feasible to increase the butane content of the gasoline used for blending and thereby enhance the increase in octane number on blending. In the comparisons made thus far, it has been assumed that the gasoline and the alcohol blend were used in the same engine without alteration in fuel-system design or instsllation. To a large extent the necessity for using gasolines of low vapor pressure in present-day engines and the extensive vapor lock trouble which occurs if the vapor pressure is raised slightly are caused by deficiencies in fuel-system design and installation. It is possible to design and install fuel systems which will maintain the fuel a t lower temperatures than with existing installations in many cars and thereby permit the use of fuels with higher vapor pressures. If, therefore, a comparison is made between gasoline in present-day cars and an alcohol blend in a car with an improved fuel system, it would be possible to use blends of considerably higher vapor pressure than is considered permissible today for gasoline. This, in turn, would mean improved octane rating and greater ease of engine starting. On the other hand, if gasoline without any alcohol were used in these cars with improved fuel systems, the comparison between gasoline and the alcohol blend would be as outlined previously with all vapor pressures raised to a higher level.

INDUSTRIAL AND ENGINEERIKG CHEMISTRY

1108

VOL. 28, NO. 9

so

hot weather under conditions favorable to vaporization in the fuel lines wa5 obtained during a series of road tests conducted 10 in June, 1933, by the National Bureau of Standards in coao operation with the American Automobile Association. The gasoline used had a vapor pressure of about 9 pounds per 01 I 18 square inch, and comparative runs were made 0 50 with this gasoline and with a blend of this gaso30 io 16 16 line and 10 per cent of absolute ethyl alcohol in 0 four different makes of cars operated over a 10a mile course a t an atmospheric temperature of s IY IY '0 approximately 95" F. The average fuel conia ia sumption with the gasoline was 0.065 gallon per mile; with the alcohol blend it was 0.0624, an increase of 4 per cent in fuel consumption with 10 IO the blend. Although no measurements of fuel line temperatures were made during the tests I) 8 6 9 IO fa f+ I6 18 20 6 8 10 12 1'1 16 and there was no definite proof of vaporization in the fuel feed lines, it seems probable that a t Reid \IP. OF 10% Alcohol Blends Reid V. C 8f 10%Alcohol Blends least an appreciable part of the difference in FIGURE 4. COMP.4RIsON OF PERMISSIBLE VAPOR PRESsURE.5 FOR EQUAL fuel consumption observed was due to vaporiFREEDOM FROM V.4POR LOCKWITH ( A ) GASOLINES OF STEEP DISTILLATION zation. CURVES AND 10 PERCENT ALCOHOL BLENDSWITH THESEGASOLINES AND (B) GASOLIXES OF FLATDISTILLATION CURVESAND 10 PERCENTALCOHOL Summary of Comparisons from BLENDSWITH THESEGASOLINES aa

30

yl

Y

'

The three curves represent different vapor-handling capacities of fuel systems.

One additional point, which is not generally recognized, should be stressed since it involves differences in fuel consumption resulting from differences in the extent of vaporization in the fuel feed lines. If the fuel system has a high capacity for handling vapor, considerable vaporization may occur in the fuel lines before the operator of the vehicle observes any loss in power or acceleration such as is characteristic of vapor lock. Thus, a t present, the fuel system in the average car on the road is capable of handling about 30 volumes of vapor per volume of liquid without loss in power or decrease in acceleration a t speeds up to 40 or 50 miles per hour. In some cars, the fuel system will handle fifty times as much vapor as liquid before any evidence of vapor lock occurs. Most of the vapor is vented off through the carburetor float bowl and represents fuel used without any returns in the way of horsepower developed. If two gasolines are considered, one of such characteristics that the car is on the verge of vapor lock and the other of such characteristics that it is not boiling in the feed lines, then for a n average car there will be a difference of about 12 per cent in fuel consumption; in the extreme case a difference of 20 per cent in fuel consumption may be observed. Differences such as those stated may be found between commercial gasolines, and similar differences may be observed when comparing gasoline and alcohol blends. Referring to Figure 3 and considering fuel E in comparison with a blend of this same fuel with 10 per cent of alcohol, if the temperature of the fuel in the fuel feed lines is 130" F. when operating a car with a vapor-handling capacity of 40, then about 5 volumes of vapor per volume of liquid will be formed with the gasoline and about 35 volumes of vapor per volume of liquid with the blend, in both cases without any evidence of vapor lock. This difference in vaporization is equivalent to somewhat more than 10 per cent difference in fuel consumption. It should be emphasized that this occurs only during operation in hot weather when the fuel is boiling in the feed lines. Such differences in fuel consumption between gasoline and 10 per cent alcohol blends with the same gasoline are minimized if the blend is made with a gasoline of reduced vapor pressure. On the other hand, the situation is reversed with 40 per cent alcohol blends, for there would be less fuel consumption due to vaporization with the blend than with the gasoline used in preparing the blend. One example of comparative tests on fuel consumption in

Vapor Lock Standpoints

In s u m m a r i z i n g the comparison between gasoline and a n alcohol blend from the standpoint of vacor lock, the following conclusions can be stated: 1 . When used in the same engine, a 10 per cent alcohol blend is more likely to give trouble from vapor lock than the gasoline

wit'h which the alcohol is blended. Little difference would be expected with a 20 per cent blend, and the situation would be reversed with a 40 per cent blend, since there is greater freedom from vapor lock difficulties with the blend than with the gasoline from which the 40 per cent blend was prepared. 2. The same freedom from vapor lock may be obtained for a 10 per cent blend and gasoline if the vapor pressure of the gasoline used for blending is lower than that of the gasoline with which the blend is compared. In this case there is some loss in octane number due to use of less butane, and there is increased difficulty in engine starting. On the other hand, with a 40 peicent blend a gasoline of increased vapor pressure may be employed with enhancement of the octane number above that obtained by blending with alcohol. 3. If the fuel system is designed t o handle alcohol fuels of increased vapor pressure, the same freedom from vapor lock can he obtained in this improved equipment using a 10 per cent alcohol blend as would be obtained in present-day fuel systems with the gasoline used for blending. On the other hand, gasolines of higher vapor pressures than those permissible at present could be used in these improved fuel systems. 4. Under hot weather conditions where vaporization is taking place in the fuel feed lines without definitely causing vapor lock, a 10 per cent alcohol blend may show markedly greater fuel Consumption than the gasoline used for blending. These comparisons are based on the use of anhydrous ethyl alcohol without employing any blending agents to increase the water tolerance of the blend. If blending agents are used, there may be an effect on the vapor-locking characteristics of the blend depending upon the nature and amount of blending agent employed.

Engine Starting and Warming Reference has been made to the necessity, with present-day automotive equipment, of compromising between freedom from vapor lock on the one hand and ease of engine starting on the other. -4lthough it is possible to improve fuel systems and therefore improve the situation as far as vapor lock is concerned, no analogous improvement in fuel system design to increase ease of engine starting appears feasible a t present. Accordingly, consideration of engine starting with different fuels is almost entirely confined to consideration of the

SEPTEAIBER, 1936

INDUSTRIAL I N D EUGINEERING CHEMISTRY

differences in the volatility characteristics of the fuels a t engine rtarting temperatures. Estensil-e in\-estigations on engine darting (4) with gasoline- covering a wide range of volatility have made possible the accurate prediction of the starting characteristics of any gasoline from a knowledge of its distillation characteristics. The accuracy of this correlation between engine starting and fuel diptillation characteristics in the caFe of gasolines is dependent upon the fact that the change in vapor pressu:e with temperature is similar for all hydrocarbons and particularly for mixtures of hydrocarbons. With ethyl alcohol, the change in vapor pressure with temperature is considerably greater t,han with bhe hydrocarbons, and accordingly the correlation worked out for gasolines is not applicable to ethyl alcohol blends. Ictually, engine starting with alcohol blends is possible only a t temperatures somewhat higher than would be predicted on the basis of the relations commonly used for gasoline. M'hile some work has been done on engine starting with 10 per cent alcohol blends, insufficient work has been done on blends containing various percentages of alcohol t o make possible any general formu1at)ion of a relation between their distillation characteristics and engine starting. To compare the starting characteriqtics of gasolice and alcohol-gasoline blends, tests were made in 1933 on one engine a t various temperatures and with various amounts of choking (111a series of gasolines covering a wide range of volatility, and 011 an equal number of fuels prepared by blending 10 per cent of absolute ethyl alcohol with each of these gasolines. The temperatures a t which 10 per cent of the above gasolines and Illends were evaporated by the A. S.T. AI. distillation method are shown in Table IV. The 10 per cent point temperatures are given here since they are commonly used as criteria for engine starting with gasoline..

1109

Unfortunately, the temperature range covered iii the starting tests was not very wide, and it was nece?.sary bo extrapolate in order to obtain the low temperature values given in Table T'. This extrapolation was accomplished by drawing a line through the plotted points parallel to the line obtained by computation from the relations previously established for starting with gasolines of various distillation charact>eristics. On this account, the actual value? for starting temperatures recorded in Table V may be somewhat in error, but the relat'ive differences between starting wit)h the gasolines and the 10 per cent blends uliould be essentially correct.

TABLEV. COMP.~R.ITIVE TEMPERATURES FOR SATISF.~CTORY EXGISE%r.IRTIXG WITH G . ~ S O L I K.