The purpose of this paper is to describe, from a purely nontechnical standpoint, the evolution of one of the latest American aviation engines, with a view to pointing out the principles utilized in a new and rational design. In the aircraft engine today, we find the application of scientific engineering to the highest degree. Thermodynamics, machine design, and the science of materials are here welded into a noteworthy product. The background for all this, however, is nothing more mysterious than common sense, and this fact is well illustrated in the new Packard engines which powered the two PN-9’s and the PB-1, the Navy’s twin-engined seaplanes designed for the San Francisco- Hawaiian flight.
Two of these engines with reduction gears installed in PN-9 No. 1 established a world’s seaplane endurance record of twenty- eight hours and thirty-five minutes at the Naval Aircraft Factory, Philadelphia. The same two engines, in Commander Rodgers’ plane, flew nonstop for approximately twenty-five hours and twenty-three minutes and were reported to have functioned perfectly as long as the gasoline held out. Two similar engines in PN-p No. 2, Lieutenant Snoddy’s plane, functioned perfectly, this plane being forced down by broken oil gauge lines not a part of the engine but a part of the installation system. Two of the 800 h.p. Packard engines were installed in the PB-i and gave very excellent performance. The difficulties encountered were entirely due to the installation and not in any way to the engines. By their performance to date, the engines have conclusively proved that they are highly dependable even when run at greater than their designed power.
Aircraft engine development is measured largely in pounds per horsepower. When we realize that this development is grouped into the last twenty years, and that the modern aircraft power plant weighs three pounds per horsepower, ready to run, against thirty-seven pounds per horsepower for a 25,000-horsepower destroyer and 160 pounds per horsepower for a turbo-electric battleship, we begin to appreciate the extent of aircraft engine progress. Much of this progress has been due to the use of special materials. Some of it has been due to higher thermal efficiencies, but in the case of the new Packard engine, which is to be described here, a great deal more is the result of calm study and rational arrangement.
The highest powered, and at the same time the lightest, engine in pounds per horsepower in service in the United States today is the Packard 1A-2500 engine, designed and built by the Packard Motor Car Company, Detroit, Michigan. As the name 1A-2500 indicates, this engine is of the first series of aircraft engines and has a piston displacement of 2,500 cubic inches. It is rated at 800 horsepower at 2,000 revolutions per minute on a dry weight of 1,113 pounds. It therefore weighs 1.39 pounds per horsepower dry. A better comparison of engines is had when they are compared on the basis of the total power plant weight. On this basis, the Packard engine weighs 2.35 pounds per horsepower and the Liberty engine 3.3 pounds per horsepower installed in an airplane ready for flight. As has been stated, this saving in weight is materially due to features of design, and it is proposed to point these out in a very general way at this time.
It will be remembered that the Packard Motor Car Company was intimately associated with the Liberty engine development. On the basis of its experience with the Liberty engine, it has evolved two new engines—the 1 A-1500, a 400-500 h.p. engine, and the 1A-2500, a 600-800 h.p. engine. (Plates I, II, and III.) In designing these engines, the company has availed itself of its Liberty engine experience but has started out with a clean sheet with the purpose of building new engines to succeed the postwar type and to be the last word in light weight construction, using available materials and adhering to sound principles of design.
Experience indicated that the 12-cylinder V type watercooled engine offered the best field for this development work, since such an engine has important advantages in smooth running, small frontal area, high speed capabilities, and compact design. The first step in a problem of this kind was the analysis of the fundamental principles controlling the design of the lightest possible engine. The immediate conclusion was that the engine must have the greatest possible piston displacement for the smallest possible exterior dimensions. This, then, is the fundamental requirement and the starting point of the design.
This meant that the cylinder bores must be as close together as possible, that the cylinder height must be as restricted as possible, and the crank case dimensions be limited to the minimum. If the cylinder bores were to be kept close together, it naturally followed that the various types of cylinder construction should be analyzed. If the cylinder height was to be kept low, it was necessary to consider the features of length of stroke, length of piston, height of the valve gear, and length of the connecting rod.
The first step, then, involved establishing the desired ratio of bore to stroke. This was selected at a value of slightly over one, since experience has shown that a “square” engine, one having the bore and stroke approximately equal, gives better high speed results than one like the Liberty, for instance (5"x?"), in which the stroke exceeded the bore. This improvement is due partly to the tact that the square engine permits larger valves, and partly to the fact that in this engine the piston friction is less than that with the longer stroke engine.
Having decided on this “square” cylinder for the above reasons, it now became desirable to see how closely the cylinder centers could be grouped together. Naturally, the closer the cylinder centers the shorter the engine could be made. Plate IV shows there are three types of cylinder construction from which to choose: Fig. I, the individual steel cylinders with sheet steel water jackets, as in the Liberty engine; Fig. 2, the aluminum block construction in which a steel sleeve, not in contact with the water, is screwed into an aluminum jacket, and Fig. 3, the aluminum block construction in which a steel sleeve making contact with the circulating water is screwed into the aluminum jacket. These three types are illustrated diagrammatically by the chart, Plate IV, and a glance immediately shows that the steel cylinder-sheet steel jacket construction lends itself best to the closest possible cylinder centers. Fig. 4 is the type ultimately employed.
Since the heart of the engine is its cylinder, it may be well to pause at this time and consider some of the factors that enter into cylinder construction. In Fig. 1, the Liberty construction, it will be noted that the whole cylinder head is of steel. This makes a rather difficult forging and one subject to such distortion in operation as to cause leaky water jackets and cracking of the head. It has been necessary to reinforce Liberty cylinder heads. In Fig. 2, the typical Hispano-Suiza construction, it is necessary for the heat to flow from the steel sleeve across a junction resistance to the aluminum jacket, and this is a path of relatively high resistance. The aluminum jacket necessarily has thick walls. Fig. 3 is the Curtiss construction in which the circulating water is in direct contact with the sleeve and one of the objections to the Hispano-Suiza construction is removed. It will be noted that in the Curtiss construction, there are two cam shafts for the four valves. Fig. 4 is the construction which has been adopted for the Packard engine, in which everything below the valve seats is of steel. The complicated portions above the seats are of cast aluminum. But one cam shaft is required for the four valves and the jacket space is very small. This latter feature reduces not only the weight of the cylinder itself but of the water which it contains, and since this extra water serves no useful purpose, this is a direct saving of considerable importance. Fig. 4, then, combines to a great degree all the advantages of Figs. 1, 2, and 3, and eliminates their disadvantages.
The new cylinder construction permits such close cylinder centers that to utilize this advantage, radical departures had to be made from standard bearing design, since shortening the crankshaft reduces the length of the main and connecting rod bearings. The company, therefore, undertook an extensive series of bearing tests at high speeds and high loads and determined an important fact: If the bearings are made stiff and rigid enough, and if generous lubrication is supplied, it is possible to run bearings successfully under conditions some three times as severe as has normally been attempted. The copious oil supply obviously acts as a cooling medium for the bearings. The rigid backing is required for the babbitt bearing metal to keep the load uniformly distributed and add stiffness to the whole structure.
Having arrived at the most suitable cylinder design to permit the bores being brought together as closely as possible, and having proven by means of bearing tests that higher loads were reasonable, the next step was to study minutely each phase of the design problem with a view to disposing of the material to best advantage, it being borne in mind that the object was to produce the lightest possible engine for the particular displacement chosen. The first problem was to design a cylinder head and valve gear construction. The cylinders adopted did not differ materially from the Liberty engine insofar as the barrel proper was concerned, but the head was made flat to take care of the valve gear desired. The head is provided with four integral valve ports—two inlet and two exhaust—and these occupy most of the space in it. The jacket is wrapped around the cylinder and has one vertical seam. From this, it is apparent that all of the high pressure parts of the cylinder are made of steel permitting the lightest possible construction.
Having developed the cylinder construction, it next became necessary to devise a valve housing. Since this was not a highly stressed part, an aluminum block casting interchangeable between the right and left banks was adopted. This valve housing performs the following functions:
- Forms the intake passages connecting the twelve inlet ports of each bank of cylinders to the carburetors.
- Connects the four exhaust ports of two adjacent cylinders to a single exhaust opening. (Siamese ports.)
- Forms a support for the cam shaft bearings.
- Carries the valve guides supporting the valves.
- Collects circulating water for the individual cylinders.
- Stiffens up the entire cylinder assembly and thus has all the advantages, in this respect, of the block construction.
Since this single aluminum casting performs all these various functions, considerable saving in weight is secured. Much more volume is available for intake and exhaust passages than would be the case if several pieces were bolted together by means of flanges, as in the Liberty.
This brings into the new construction one of the chief advantages of the aluminum-steel construction. Furthermore, both the cylinder and valve housing constructions permit of the largest possible valve being used for any given size of flat combustion chamber. By locating the two inlet and the two exhaust valves of each cylinder on a transverse axis rather than on a longitudinal axis, as has been common in the past, it is possible to open four exhaust or four inlet valves into the same passages, which are thus more generous in size than if adjacent passages were supported by a dividing wall. Furthermore, this construction makes it possible to use a single cam shaft instead of the two cam shafts frequently used with four-valve engines.
While from the standpoint of weight saving the cylinder and valve housing constructions just described are undoubtedly responsible for the major savings, there are, nevertheless, many other points which have received careful scrutiny. The engine accessories have been carefully planned to simplify the design and eliminate superfluous weight. This has been accomplished by reducing the number of accessory drives from five, ordinarily found, to two. Usually, a water-cooled aircraft engine requires drives for two magnetos, one water pump, one triple gear oil pump, and one fuel pump. In the new engines, the two magnetos have been replaced by a double magneto, and all the pumps for Water, oil, and fuel have been combined into a single unit requiring but a single drive. Furthermore, the magneto is so located that it can be driven by the cam shaft drive already provided, so that its drive is had with no added complication.
This magneto is not a single magneto, but a double one in which all the rugged parts, which never give trouble, are combined in one unit while the troublesome parts are duplicated. There is, then, no loss in reliability for a considerable saving in weight. This weight saving is not confined to the saving in the magneto alone, but includes the saving in weight and space otherwise required for driving and mounting the second magneto.
In the new engines, two separate high tension distributors are mounted in the V in such a position that the wiring to the sparkplugs is much simplified and the leads are greatly reduced in length. Furthermore, the ignition is designed so that the magneto van readily be replaced by a generator, if battery-generator ignition is desired, the same distributors being used for both systems. The water, oil, and fuel pump unit, referred to previously, also incorporates an oil strainer compartment and an adjustable oil pressure relief valve so that this unit performs many functions in a very simple and compact assembly. Needless to say, the combining of these various assemblies in a single group results in appreciable weight saving as well as greater advantages from the standpoint of maintenance and accessibility.
A feature of considerable interest is the valve system of the new engines. Instead of the usual double centrifugal valve spring construction generally adopted, a series of small piano wires are grouped concentrically around the valve stem. There are ten of these around each valve in the large engine. These springs have proven practically indestructible in contrast with the usual large diameter heat-treated spring. The improvement lies almost wholly in the fact that the small diameter springs have a natural period of vibration so high that they do not synchronize with high speed engine vibrations which cause the conventional spring to flutter and break.
Another feature in the new engines is the means adopted to cool the exhaust valves. Experience has shown that the output of an aircraft engine is limited, among other things, by the temperature of the exhaust valves. Some improvement has been effected recently through the use of special alloy steels for this high temperature work, but resort has been had here to oil cooling of the exhaust valves to accomplish the same purpose. By a simple mechanical means, lubricating oil is flushed through the exhaust valves, carrying away much of the heat and making it possible to control these temperatures.
It is, of course, undesirable, in a paper of this length, to dwell on the minor refinements in design which have resulted in such important reductions in weight and space. 1 he main lesson to be learned, however, is that considerable advance has been made not by any revolutionary changes or freak construction, but.by a painstaking study of details coupled with appreciation of the limiting factors controlling aircraft engine design in the present state of the art. The new engines have established new standards of performance on a horsepower weight basis and, at the same time, have passed tests which indicate that they are far more rugged than some of their predecessors, which weighed considerably more and delivered far less power.
It was, at one time, believed that reliability could not be had without excessive weight. This theory has long since been discarded. This new development demonstrates the unsoundness of this belief. More weight has been saved in this design by intelligent arrangement than could possibly be saved by expensive research in the field of special materials. This development is considered to be an excellent illustration of the principle that engineering requires more of design and less of invention. No matter how much the technical arts may be applied, the foundation of any engineering design is rational arrangement.