Part 2 (1/2)

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]