This book excerpt discusses the design and function of the Packard-Diesel engine as compared to earlier gasoline aircraft engine models, including increased fuel economy and reliability.
Fig. 65. Packard Diesel Aircraft Engine Elevation Partly in Section. Courtesy of Society of Automotive Engineers and Packard Motor Car Co.
Walter Taubeneck of Marysville, Washington thought our readers would be interested in this section of the book, Diesel and Other Internal Combustion Engines by Howard E. Degler, published by the American Technical Society in 1937.
The Packard-Diesel aircraft engine is of the radial air-cooled type having nine cylinders with a bore of 4-13/16 inches and a stroke of 6 inches. It gives a displacement of approximately 980 cubic inches per minute. The engine is rated at 225 horsepower at 1950 revolutions per minute and weighs 510 pounds.
An inspection of Figs. 65 and 66 (see the Image Gallery) reveals a radical departure from the usual engine in that each cylinder is provided with only one valve, which serves for both inlet and exhaust. The single rocker-arm box, which is slanted in the direction of the spiral of slipstream, contributes considerably to the clean external appearance of the Packard- Diesel engine (Fig. 65), and what is more important, to its low parasitic drag.
In the Packard engine, an advance that has been effected over previous Diesel practice consists in the ability to extend the range of engine speeds possible with the Disel cycle. Heretofore Diesel engines in stationary and marine service have been of the low-speed type, 100 to 500 rpm. Even so-called high-speed Diesels of modern type have been limited to a maximum speed of about 1500 rpm. With this engine the speed has been increased to more than 2000 rpm., which has been attained by an engine design that produces a turbulence never before approached in this type of engine. The engine design and the highly efficient and quick-acting fuel pumps that were developed to go with it are the means that produce the accelerated co-mingling of the fuel and air which brought about this greatly increased engine speed. The fuel pumps give a positive and metered supply of fuel.
The most interesting aspect of the design is undoubtedly a consideration of the features that reduced the weight of the engine to practically the same level as that of gasoline engines of equivalent power. Heretofore (1935), even Diesel engines of the so-called light-weight modern type weighed about 20 pounds per horsepower, whereas this engine weighs but one-tenth as much, i.e., 2.3 pounds per horsepower.
A minimum of weight is essential for any successful aircraft engine; new methods of construction were employed in the Packard-Diesel to reach this desired objective. Important weight economies were secured; first, by the elimination of carburetors and magnetos; and second, by an intensive simplification of design, as shown in Figs. 65 and 66. Evidence of the latter are found in the one-piece crank case construction of extremely light weight and single-valve arrangement which automatically halves the number of parts required for valve operations as used on conventional gasoline engines.
The cooling problem of a Diesel cylinder is considerably simpler than that of a corresponding gasoline-engine cylinder, since the increased thermal efficiency of the Diesel is reflected in lower heat losses to the cylinder walls than is the case with the gasoline-engine cylinder. This fact justifies the simple form of closed-end cylinder design in which the cylinder head is formed integral with the cylinder barrel, as in Fig. 65, a construction which is not considered satisfactory for gasoline engines because of the inferior cooling attained with this type of cylinder head as contrasted with the more bulky screwed-on aluminum type of head commonly used with gasoline engines.
The fact that only air is drawn into the cylinder on the intake stroke of the Diesel engine permits the novel use of a single valve for both inlet and exhaust purposes in the Packard aircraft engine. The use of one valve per cylinder contributes in a marked degree to the lightness of the cylinder itself, since the cylinder head is weakened by only one port instead of two, as in the conventional engine. The single-valve arrangement was favored not only on account of the general gain in simplicity and saving weight, but also in the interests of reliability, as this valve operated at a much lower temperature than the conventional exhaust valve due to the cooling effect of the incoming air passing over the same valve. The available time for opening and closing the valve can also be utilized, thus saving wear and tear on the whole mechanism. Bolted to the top of each cylinder is a light aluminum cylinder head carrying cooling fins and supporting the valve-operating mechanism as well as forming the combined inlet and exhaust port.
One of the distinctions between a Diesel engine and a gasoline engine is the manner of assuring a homogeneous mixture of fuel and air. It is obvious that in either case such a mixture must be created if perfect combustion is to be secured in the extremely short time available, about 0.004 second at an engine speed of 1800 rpm. In a gasoline engine this mixing of fuel and air is accomplished in the carburetor, intake manifold, rotary, distributor (if used), and finally the cylinder itself during the intake and compression stroke.
In a Diesel engine no fuel is admitted into the cylinder until practically the instant that combustion is desired; so it is obvious that, in a high-speed Diesel, special means must be provided to assure complete and efficient combustion. This is accomplished in the Packard Diesel by giving the incoming air a rapid whirling motion, so that at high speeds the circulation of the air in the cylinder is at a rate which permits of one revolution of the mass of air around the circumference of the cylinder bore in the time available for combustion. This high-velocity spiral motion is brought about by shaping the inlet port as venturi (see Fig. 65) arranged tangentially to the cylinder bore. The large diameter of a single valve assures a full volume of air to the cylinder and also offers little resistance to the speeded-up air flow.
A combination fuel pump and nozzle unit is used for injecting finely atomized fuel into the cylinder at the proper time and in accurately controlled quantities. Many solid-fuel injection engines of the so-called high speed type (maximum revolutions about 1500 rpm) are characterized by a multiple-pump unit mounted somewhere on the engine, remote from the cylinder heads in which the nozzles are located and connected to them by comparatively long capillary tubing. With such a system, satisfactory high-speed operation is difficult to obtain, for several reasons, the principal one being that the enormous hydraulic pressures necessary for high speeds cause serious surges of pressure waves in the tubing which interfere with the correct timing of the fuel injection. This also tends to make the engine run unevenly, since it is difficult to arrange the tubing to the various cylinders so that all of the tubes are of the same length. Another difficulty arises from the trapping of air in the capillary tubing, which air is difficult to expel and causes the engine to misfire. These troubles have been overcome with the fuel-injection systems of the Packard-Diesel, since the pump and nozzle are practically one unit with very short connecting passage between them, as shown at the upper right side of the cylinder in Fig. 66.
The valve and fuel-pump push rods, of which there are nine each, all radically arranged at the rear of the engine, are in turn operated by two cams which are formed integrally and each of which is provided with four lobes. These cams are driven at one-eighth engine speed in the direction opposite to the crankshaft rotation, a large integral gear being formed integrally with the cam and crankshaft gears, respectively, as shown in Fig. 65. Both the single valve and fuel pump of each cylinder are operated through the medium or rocker arms, which contact with the respective cams and are supported on a common shaft anchored in the diaphragm and which also obtains a steady bearing in suitable bosses formed in the cover casting.
The Packard-Diesel aircraft engine will operate with gasoline as a fuel but such practice is not recommended because of the fire hazard and also because gasoline has no lubricating qualities, whereas fuel oil has a considerable degree of 'oiliness' and hence the fuel-pump plungers function satisfactorily with fuel oil but not very well with gasoline unless some lubricating oil is added to the gasoline. Less power would be secured with gasoline than with fuel oil because the fuel pumps are proportioned to handle fuel oil, the heat value of which is about 23 percent more than that of gasoline on a volumetric basis.
Diesel aircraft engines are able to operate in an airplane at great altitudes without using a preheater or supercharger. Airplanes so equipped have climbed to well over 18,000 feet with the engine functioning in a normal manner without special equipment of any kind. When a plane is climbing with a gasoline engine, engine revolutions will drop somewhat as higher altitudes are reached and, although the carburetors can be 'leaned out' by means of the altitude adjustment to bring the revolutions up slightly, nevertheless, for any fixed position of the throttle, the engine speed will decrease with altitude. With the Diesel engine, however, assuming that soon after the take-off the throttle is closed somewhat to cruising position, it has been found that the engine speed increases as the plane climbs, instead of decreasing as in the case of the gasoline engine.
This interesting phenomenon is responsible for better fuel economy under flying conditions, since the engine automatically adjusts itself to burn efficiently whatever fuel is injected. The increase in speed with altitude under set throttle condition is due to the fact that a predetermined amount of fuel is injected into each cylinder and this fuel needs for its complete combustion a certain weight of oxygen. The density of air decreases with altitude so that a given weight of oxygen is contained in a larger volume of air at higher altitudes. From this it follows that the position of the throttle which would cause the fuel to combine with only half the available oxygen near sea level would give the equivalent of full-throttle operation at 18,000 feet. From the pilot's standpoint this is a very desirable characteristic, since the engine tends always to give the most economical result, and he is not required to make continual adjustments in the fuel supply for varying altitudes, as is necessary to get the most efficient result from a gasoline engine.
The late Captain L. M. Woolson (designer) summarized the advantages of the Packard-Diesel aircraft engine under the following headings.
The engine is inherently far more reliable than the gasoline engine because (a) the electrical ignition system is eliminated, and (b) a separate fuel-injection means is applied to each cylinder, in contrast to the use of a single carburetor feeding several cylinders in a gasoline engine. This means that in the Diesel engine each cylinder operates independently of the others and the failure of any one fuel pump would not affect the other cylinders.
Considerable ground and flight testing have proved that it is impossible to accidentally ignite the fuel oil used for this engine. It is virtually essential to atomize its fuel into a very fine spray before it can be ignited and at no time was it possible to start a fire under any conditions simulating the result of an airplane crash or accidental breakage of the fuel line in flight.
A conservative estimate of the fuel-consumption reduction by weight as compared with that of a gasoline engine represents a saving of 20 percent and, due to the higher specific gravity of the fuel used, a saving of over 30 percent by volume is effected.
Because of the much lower grade of fuel used and the higher efficiency of the Diesel principle, the specific fuel cost is considerably reduced.
The Diesel engine is not affected by either abnormally high or excessively low air-temperatures, as is the case with the gasoline engine. On the one hand, there is no tendency for the Diesel engine to detonate or pre-ignite as in the case of the gasoline engine, nor on the other hand has it any characteristics comparable with the icing-up of the gasoline engine carburetor in cold weather and failure of the engine to accelerate properly after a long glide.
The fact that no electrical ignition equipment is used in the operation of the engine removes one of the most troublesome obstacles to extensive radio use in airplanes. With the gasoline engine it is necessary to adopt shielding means for all magnetos, spark plugs, high and low-tension wiring, etc. This shielding reduces the efficiency of the ignition system and renders it far more liable to failure.