The First Airplane Diesel Engine: Packard Model DR-980 of 1928 Part 2

Summing up, the differences between the Diesel and gasoline engines start with the fact that the gasoline engine requires a complicated electrical ignition system in order to fire the combustible mixture, whereas the Diesel engine generates its own heat to start combustion by means of highly compressed air. This brings about the necessity for injecting the fuel in a well-atomized condition at the time that combustion is desired and the quant.i.ties of fuel injected at this time control the amount of heat generated; that is, an infinitesimally small quant.i.ty of fuel will be burned just as efficiently in the Diesel engine as a full charge of fuel, whereas in the gasoline engine the mixture ratio must be kept reasonably constant and, if the supply of fuel is to be cut down for throttling purposes, the supply of air must be correspondingly reduced. It is this requirement in a gasoline engine that necessitates an accurate and sensitive fuel-and-air metering device known as the carburetor.

The fact that the air supply of a Diesel engine is compressed and its temperature raised to such a high degree permits the use of liquid fuels with a high ignition temperature. These fuels correspond more nearly to the crude petroleum oil as it issues from the wells and this fact accounts for the much lower cost of Diesel fuel as compared to the highly refined gasoline needed for aircraft engines.

Weight-Saving Features

In order to be successful in aviation use, the modern lightweight diesel of the time had to have its weight reduced from 25 lb/hp to 2.5 lb/hp.

This required unusual design and construction methods, as follows:

Crankcase: It weighed only 34 lb because of three factors: Magnesium alloy was used extensively in its construction, thus saving weight as compared with aluminum alloy, which was the conventional material at this time. It was a single casting. This saved weight because heavy f.l.a.n.g.es, nuts, and bolts were dispensed with. The cylinders, instead of being bolted to the crankcase, as was normal practice, were held in position by two circular hoops of alloy steel pa.s.sing over the cylinder f.l.a.n.g.es. They were tightened to such an extent that at no time did the cylinders transfer any tension loads to the crankcase. This type of fastening actually strengthened the crankcase in contrast to the usual method. For this reason it could be built lighter. The hoops did not always function well. "The first job I ever did on the Towle was to patch the holes in the top and bottom of the hull when a cylinder blew off during run-up and nearly beheaded the pilot."[13]

[Ill.u.s.tration: Figure 22.--Rear view of engine with rear crankcase cover removed, showing valve and injector rocker levers and injector control ring mounted on crankcase diaphram. U.S. Navy test, 1931. (Smithsonian photo A48323D.)]

[Ill.u.s.tration: Figure 23.--Main crankcase. U.S. Navy test, 1931.

(Smithsonian photo A48325B.)]

[Ill.u.s.tration: Figure 24.--Rear crankcase cover and gear train: crankshaft gear drives B, which drives oil pump at F. A, integral with B, drives internal cam gear. B also drives C on fuel-circulating pump.

D, driven by crankshaft gear, drives E on generator shaft. U.S. Navy test, 1931. (Smithsonian photo A48325C.)]

[Ill.u.s.tration: Figure 25.--Master and link connecting rods. U.S. Navy test, 1931. (Smithsonian photo A48323A.)]

[Ill.u.s.tration: Figure 26.--Crankshaft with automatic-timing r.e.t.a.r.ding device on rear end of pivoted- and spring-mounted counterweights. U.S.

Navy test, 1931. (Smithsonian photo A48323B.)]

[Ill.u.s.tration: Figure 27.--Propeller hub and vibration damper. U.S. Navy test, 1931. (Smithsonian photo A48325A.)]

Crankshaft: Since this engine developed the high maximum cylinder pressure of 1500 psi, it was necessary to protect the crankshaft from the resulting heavy stresses. Without such protection the crankshaft would be too large and heavy for practical aeronautical applications.

Although the maximum cylinder pressures were 10 times as great as the average ones, they were of short duration. The method of protecting the crankshaft took full advantage of this fact. It consisted of having the counterweights flexibly mounted instead of being rigidly bolted, as was common practice. The counterweights were pivoted on the crank cheeks.

Powerful compression springs absorbed the maximum impulses by permitting the counterweights to lag slightly, yet forced them to travel precisely with the crank cheeks at all other times.

Propeller Hub: The propeller is, of course, subject to the same stresses as the crankshaft. Instead of being rigidly bolted to the shaft as was common practice, it was further protected from excessive acceleration forces by being mounted in a rubber-cus.h.i.+oned hub. This permitted the use of a lighter propeller and hub.

Valves: A further weight saving resulted from the use of a single valve for each cylinder instead of two as in the case of conventional gasoline aircraft engines. (A diesel engine designed in this manner loses less efficiency than a gasoline one because only air is drawn in during the intake stroke.) In addition to the weight saving brought about by having fewer parts in the valve mechanism, there was an additional advantage since the cylinder heads could be made considerably lighter.

[Ill.u.s.tration: Figure 28.--Cylinder disa.s.sembly, showing valve and fuel injector. U.S. Navy test, 1931. (Smithsonian photo A48324D.)]

Diesel Cycle Features

Although Woolson designed the ingenious weight-saving features, Dorner was responsible for the engine's diesel cycle which employed the "solid"

type of fuel injection. In order to understand Dorner's contribution, a brief description of the type of diesel injection pioneered by Dr.

Rudolf Diesel is necessary. His system injected the fuel into the cylinder head with a blast of air supplied by a special air reservoir at a pressure of 1000 psi or more. Known as the "air blast" type of injection it produced good turbulence, with the fuel and air thoroughly mixed before being ignited. Such mixing increases engine efficiency, but it involves the provision of bulky and costly air-compressing apparatus which can absorb more than 5 percent of the engine's power. Naturally the compressor also adds considerably to the engine's weight.

In contrast to this, a "solid" type of fuel injection may be employed to eliminate the complications of the "air blast" system. It consists of injecting only fuel at a pressure of 1000 psi or more. Air is admitted by intake stroke, as with a gasoline engine. Turbulence is induced by designing the combustion chamber and piston so as to give a whirling motion to the air during the intake stroke. The following quotation from Dorner now becomes readily understandable. "Since 1922 my invention consisted in eliminating the highly complicated compressor and in injecting directly such a highly diffused fuel spray so that a quick first ignition could be depended upon. By means of rotating the air column around the cylinder axis, fresh air was constantly led along the fuel spray to achieve completely sootless burning-up.... In 1930 I sold my U.S.A. patents to Packard."[14]

Valve Ports: The inlet port (which was also the exhaust port) was arranged tangentially to the cylinder. This design imparted a very rapid whirling motion to the incoming air, thereby aiding the combustion process. Engine efficiency and rpm were both increased.

Fuel Injector Pumps: A combination fuel pump and nozzle was provided for each cylinder in contrast to the usual system of having a multiple pump unit remotely placed with regard to the nozzles. The former system was adopted after frequent fuel-line failures were experienced due to the engine's vibration. Woolson stated that his system prevented pressure waves, which interfered with the correct timing of the fuel injection, from forming in the tubing. Leigh M. Griffith, vice president of Emsco Aero, writing in the September 1930, _S.A.E. Journal_ stated: "Regarding the superiority claim for the simple combination of fuel pump and injection valve into one unit, without connecting piping, the author entirely overlooks the fact that the elasticity of a pipe and its contained fuel can be important aids in securing that extremely abrupt beginning and ending of injection which is so desirable."

[Ill.u.s.tration: Figure 29.--Fuel-injector disa.s.sembly. U.S. Navy test, 1931. (Smithsonian photo A48323C.)]

A major advantage obtained from combining the fuel pump and injection valve is the ability of an engine so equipped to burn a wide variety of fuels. The elimination of the above-mentioned type of high-pressure tubing reduces the possibility of a vapor lock occurring, thereby permitting more volatile fuels to be burned. This increases the range of hydrocarbon fuels the engine can utilize. It could run on any type of hydrocarbon from gasoline to melted b.u.t.ter.[15]

Another reason for combining the fuel pump and injection valve is given by P. E. Biggar in _Diesel Engines_ (published in 1936 by the Macmillan Company of Canada Ltd., Toronto): "In the Dorner pump, for example, the stroke of the plunger is changed by using a lever-type lifter and moving the push-rod along the lever to vary its movement. Unfortunately, in all arrangements of this sort, the plunger comes to a reluctant and weary stop, as the roller of the lifter rounds the nose of the cam. When the movement does finally end, the injection does not necessarily stop, as the compressed fuel in the injection pipe is still left to dribble miserably into the combustion chamber. To minimize this defect, the designer has placed the pump and injector together in a single unit."

[Ill.u.s.tration: Figure 30.--Mechanism for r.e.t.a.r.ding valve and fuel-injection timing during starting (see also fig. 26). U.S. Navy test, 1931. (Smithsonian photo A48324E.)]

[Ill.u.s.tration: Figure 31.--Upper--valve and fuel injector cam; lower--fuel-injector cam used for starting. U.S. Navy test, 1931.

(Smithsonian photo A48325.)]

Starting System: On November 1, 1961, C. H. Wiegman, vice president of engineering of the Lycoming Division of Avco Corporation wrote to the Museum in part as follows:

Early in the development it became quite evident that cold starting was a problem. This was finally worked out by Packard through the use of glow plugs and speeding up the injectors during the cranking period. It had been felt that during the slow cranking process we were not vaporizing the fuel through the nozzles and that if we could speed up the injection pumps during this period of cranking a better vaporization could be obtained. Our tests showed that we were right, and that the engine could be started quite easily at minus 10 F through the use of glow plugs. The method used for speeding up the injection pumps was accomplished by utilizing a crankshaft cam during the cranking period. The starter would s.h.i.+ft the running cam out of position allowing the crankshaft cam to take over. After the engine fired, the starter was disengaged and the running injector pump cam would a.s.sume its original position. The starting cam would be run at engine speed during cranking, and the running cam at 1/8 reverse engine speed during engine operation.

The s.h.i.+fting was accomplished by a pin-in-slot and spring arrangement to change the indexing of the cams to starting position and return.

An Eclipse electric starter with an oversized flywheel was used....

This was powered by a double-sized battery.

Development

Air Shutters: The first engines had no provision for throttling the intake air. This allowed the engine to run on its own lubricating oil when the throttle was in idle position. As a result the engine idled too fast, thereby causing either excessive taxiing speeds or rapid brake wear. This inability to idle slowly also caused high landing speeds since the propeller did not turn slowly enough to act as an airbrake.

Figure 1 shows the first model. Note that the tubular air intakes on top of the cylinders have no valves. Figure 32 shows a later model. Note the b.u.t.terfly valves in the U-shaped air intakes. Here they are shown fully opened. When the throttle was placed in idle position these valves automatically closed and prevented air from flowing past them. Air could then only enter from the back of the intakes. Since less air could flow into the cylinders, the force of their explosions was reduced, which, in turn, lowered the idling revolutions per minute. Figure 28 shows a cylinder from a more advanced model. Note the circular opening between the air intake and the intake/exhaust housing. A barrel type of valve fitted into this opening. One of these valves can be seen just below and to the left of the cylinder. When the throttle was placed in idle position this valve rotated to a position which cut off almost all of the airflow into its cylinder. This increased the vacuum formed toward the end of the intake stroke, thereby causing more resistance, which reduced the idling rpm to that of a gasoline engine.[16]

[Ill.u.s.tration: Figure 32.--Front left view of engine from Packard Motor Car Co. 50-hour test, 1930, showing b.u.t.terfly valve type venturi throttles. (Smithsonian photo A48325E.)]

[Ill.u.s.tration: Figure 33.--Front left view of engine from U.S. Navy test, 1931, showing spiral oil cooler. (Smithsonian photo A48324A.)]

Crankcase: It was strengthened by having external ribs added. Note the contrast between the first engine, figure 2, and a later model, figure 32.

Oil Cooler: The drum-shaped honeycombed cooler was replaced by a spiral pipe type located between the engine cowl and the crankcase. Figure 3 shows an example of the former type of cooler located at the top of the engine between two of the cylinders. Figure 33 ill.u.s.trates the latter type located between the cowling and the crankcase.

Cylinder Fastening: Early models had their cylinders strapped and bolted to the crankcase. Later ones had them only strapped. Figure 2 shows a bolt-fastened clamp between two of the cylinders on the first engine.

Figure 19 shows a later model without any bolts holding down the cylinders.

Pistons: The pistons used in the 1929 engine had one compression ring and one oil sc.r.a.per ring above the piston pin, and one oil sc.r.a.per ring below it. There were three grooves, two above the piston pin, and one below it.[17] Pistons used in 1930 had two compression rings, one oil sc.r.a.per ring above the piston pin, and one oil sc.r.a.per ring below it.

There were four grooves, three above the piston pin, and one below it.[18] The 1931 pistons had one compression ring above the piston pin, and one compression ring and four oil sc.r.a.per rings below it. There were four grooves, one above the piston pin, and three below it.[19]