November, 1925
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1115
Motor Design and Fuel Economy’ By C. F. Kettering GENERAL MOTORREWARCH CORP.. DETROIT, MIcn.
gain for increasing utility rather than obtaining more miles development in transportation ever witnessed in the per gallon. While the above may seem so obvious as to require no world’s history. This development is the almost universal use of the automobile by the American public. mention, failure to recognize such points has misled many At present enormous quantities of motor fuel are being inventors to propose schemes which, although engineeringly consumed by these automobiles. Authorities are in agree sound, have resulted only in waste effort. Increased miles ment that to supply these quantities of motor fuel for all per gallon cannot be obtained a t the expense of utility, except time in the future constitutes an outstanding problem for through extreme price change of motor fuel. This may be represented by the following equation: serious consideration. There appears to be an abundance of fuel distributed (f)Miles per gallon = ( f )Fuel price; with e5ciency constant (f)Utility throughout the earth’s crust, but only a small proportion of this meets the exacting requirements of the automobile From this it follows that increased efficiency can only result engine. When one considers that motors are in use which in both increased miles per gallon and increased utility since require that forty separate fires be built within each cylinder to expect any other result violates the equation. per second of operation, one readily perceives that the flints This formula is intended only as of very broad and general and dry moss of our forefathers are inadequate substitutes for application, since utility is composed of many and various the high-tension spark and gasoline vapor of the present day. items and these items are extremely variable under different The one essential property of a satisfactory motor fuel is to conditions. More durable finishes and ease of washing are meet this need in volatility-all other requirements can be items of utility which obviously have no direct relationship obtained by the addition of modifiers. to miles per gallon or efficiency. (That an indirect relationThough there appears to be no immediate danger of our ship does exist will be apparent if one considers that many subterranean fuel tanks of petroleum running dry, it is evi- people might purchase a car of lower performance and condent that every gallon of petroleum taken out of the ground sequently higher economy to obtain advantages of finish or leaves one gallon less to be taken out in the future, and some ease of washing.) In hilly sections of the country power is of day-no one knows just when-our petroleum reserves will higher utility value than in level sections. Rough roads have diminished to the point where we can no longer supply make a virtue of heavy, durable construction, which becomes our motorists with a sufficient quantity of cheap, volat’ile, only a burden on smooth roads. The formula will, however, motor fuel from this source. withstand the test of practical application and serve as a No economically satisfactory substitute is immediately useful guide in appraising various proposals €or increasing the available. Many years of research may be necessary before economy of automotive transportation. the actual development of such a substitute can start. More Hypothetical Motor Car Design for Economy years of development may be necessary before practical results of a quantitative nature are obtained. If a petroleum The factors affecting miles per gallon are so diverse and so shortage should occur during these years, the enormous interrelated that in order to obtain a comprehensive picture utility of automotive transportation would be seriously of the whole it will be well mentally to construct an automohampered. Such a catastrophe can be largely avoided, or bile to yield a maximum mileage. at least greatly modified, if motor car fuel economy can be This hypothetical car will have the following charactermaterially increased. istics: It is the purpose of this paper to present some of the recogIt will be very small, with narrow tread and short wheel base. nizable improvements in motor and car design which can be It will be very light, made of the best materials for strength effected and to consider some of the simple underlying prin- obtainable. ciples involved. It will look rather odd, being stream-lined throughout to reduce windage, and i t will have no top, windshield, or mudWhat Is Fuel Economy?
HE past two decades have seen the most remarkable
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Before proceeding with the more technical points involved it will be well to digress and briefly determine just what it is that we are trying to accomplish. Does increasing fuel economy mean simply the obtaining of more miles per gallon? I n the narrow sense it does, but broadly considered it implies increasing efficiency and increasing utility. For example, a man could trade his $5000 limousine, giving 8 miles per gallon, for a light, cheap runabout delivering 24 miles per gallon, and put $4000 in the bank. Obviously this transaction is highly economical, but it is seldom executed since the reduced utility of the cheaper car more than offsets the gains in economy. Likewise, increasing efficiency, in the strict engineering sense, does not necessarily imply increased miles per gallon, since the motorist may elect to utilize such a Resented as a part of the Symposium on Motor Fuel and Oil Conservation a t the Intersectional Meeting of the American Chemical Society, New York City, September 29 t o October 2, 1925. 1
guards. It will have a conventional but small radiator. It will have a small high-compression motor, adapted to run on gasoline treated with an antiknock material, The carburetor will be quite complex, practically no one but the designer will be able to adjust it. The ignition system will be complicated to such an extent that the spark advance will always be correct for each condition of speed and load. It will have no fan or electrical equipment. The transmission will have four speeds forward and one reverse, so designed that when in high gear all other gears will be disconnected. The rear axle will be so geared that only on the level can the motor pull the car with the high gear. The brakes will be carefully made and adjusted so that when not in use they are entirely free of any contact with the wheels.
Changes Necessary to Produce Salable Car This is not to be considered as the care of the future. I n looks, weight, size, etc., it is the care of the long since past.
1116
INDUSTRIAL AND ENGIhTEERIAl%CHEMISTRY
This car is unsalable, so let us consider what must be done, even though economy be sacrificed to some extent in order to sell it. First, we must put on top, windshield, inud-guards, electrical system, and fan. We still obtain very good mileage but yet have a nearly unsalable product. A closed model will aid a little in salability and, surprising to say, decreases the mileage but very little despite the increased weight, since the wind resistance is less than in the open type. But it is still almost unsalable. This car does not ride well except when loaded with passengers. Analyzing this problem we find that it was necessary to have stiff springs so that when it mas loaded with people the springs would not be deflected beyond their capacity. This can be corrected, either by the metallurgists, who have been working long on the problem, or by increasing the general weight of the car so that the ratio of spring loading when empty to spring loading when full is not so great. The car now rides fairly well-longer wheel base will make it entirely satisfactory-but it has the objection of a narrow tread which does not fit the snow ruts in winter nor the mud ruts of back roads. m e , therefore, widen the tread to the conventional size. It still has remarkable miles per gallon, but d e s resistance is still too high, because it will climb no hills in high gear, it will not accelerate in high gear, and it lacks that reserve power so much desired by the motoring public. These may be corrected by putting in a larger motor and changing the gears in the rear axle so as to give the motor more leverage on the rear wheels. The miles per gallon are reduced enormously by these changes, for, whereas prior to these changes we ran along level roads with the motor pulling a t its highest torque and delivering about one horsepower-hour for one-half pound of fuel used, we now run along level roads with the motor only pulling one-third to one-fourth of its full torque under which condition it requires between 0.7 and 0.8 pound of fuel per horsegower-hour delivered. What causes this decreased efficiency? We find that the friction of the motor, which is largely the friction of the pistons and rings moving up and down in the cylinders, remains really constant, so that while it is only about a 10 per cent loss a t full load it becomes a 30 or 40 per cent loss under the conditions of normal and actual driving. I n addition to this, the actual act of throttling a motor by restricting the flow of mixture from the carburetor causes the motor to do actual work in sucking in its fuel air charge. This is called the pumping loss. It becomes quite evident that we can entirely circumvent this situation by installing two motors in the car, one the small-powered motor to run at full load all the time and another, larger motor to operate only when climbing hills or accelerating. This is not a t all practical. Methods and devices have been proposed of virtually building two motors in one by having two distinct cylinder heads, a large one for full load, and a small one for part load. These have been called constant compression motors. They have not been commercially successful to date, but may eventually be worked out. Constant compression motors, box-ever, obviate only the pumping losses, since the same pistons alld cylinders continue to dissipate as a friction loss a large percentage of the mechanical energy produced when running under light load conditions. A further development aiming in general to reduce the light load losses is the variable transmission. If no effort is imposed on the driver to change gears and no undesirable gear noises are produced, then all objections to gear changing are eliminated except the high speed of the motor. If, by
VoI. 17, No. 11
using a large enough motor it becomes unnecessary to speed it up unduly on climbing a hill, but merely to slow it down on the level so that it operates a t nearly full torque and consequent high efficiency, then a practical method of obviating the low-load losses has been accomplished. Several such devices are being diligently studied a t the present time and it is hoped that some of them may emerge from the experimental stage before petroleum becomes scarce. A compromise of the above is to be found on some cars in the shape of “over gearing.” This has been attempted a t the rear axle by having two pairs of bevel gears, of different ratio, adapted so that only one pair may be engaged a t a time or by having an extra high gear in the transmission. In either eaie a lever is provided so that the over-gearing device niay be engaged when the operator decides he has a long level stretch of road ahead. To date, these have not met with any great favor by the motoring public, largely because it has been extremely difficult for the manufacturers to produce a sufficiently good mechanical product. Recognizing that the public are not going to wait for us to solve any of the above problems before buying automobiles, we therefore must accept the inevitable and install the large motor in our hypothetical car with its attendant loss of miles per gallon. We now have a salable product. The miles per gallon obtained by this car are considerably better than the ordinary run of cars of the same class, because we have a highly efficient carburetor, a high-compression motor, a spark advance that is both automatically set to speed and load, a disengaging transmission and excellently free brakes. The question naturally arises-why, if these things are practical, are they not in common use? As a matter of fact, several cars are equipped already with carburetors of the type mentioned. Expense and difficulties of adjustments are all that is holding back their general use. High compression must wait for a nearly universal distribution of an antiknock fuel. Such distribution is being rapidly extended, although it cannot become universal or solve the high-compression problem until certain difficulties that hare been met are recognized as having been overcome. The full automatic spark advance is deeply involved in conflicting patent rights. It is of very little practical importance, however, owing to the increasing use of cylinder heads of the Ricardo type. It is also expensive and delicate of adjustment. Disengaging transmissions seem entirely feasible except for the additional expense involved in their construction. Particularly well designed brakes would probably be no better in the hands of the public than those in use. Proper adjustments of present design will yield satisfactory results. Until some practical invention of great merit makes its appearance, the above are probably the most hopeful design ppshibilitie5 for economy. Outstanding among them is the high-compression motor development, which at present seems to be well under way.
Calendar of Meetings American Institute of Chemical Engineers-Cincinnati, Ohio, December 2 to 5 , 1923; Berlin, N. H., June 21 to 23, 1926. American Chemical Society-7lst Meeting, Tulsa, Okla., April 5 to 9, 1926. American Electrochemical Society-Chicago Beach Hotel, Chicago, III., April 22 to 24, 1926. Association of Chemical Equipment Manufacturers-2nd Chemical Equipment Exposition, Cleveland, Ohio, May 10 to 15, 1926.
