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
flanges, 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 passing over the cylinder
flanges. 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]
[Illustration: 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.)]
[Illustration: Figure 23.--Main crankcase. U.S. Navy test, 1931.
(Smithsonian photo A48325B.)]
[Illustration: 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.)]
[Illustration: Figure 25.--Master and link connecting rods. U.S. Navy
test, 1931. (Smithsonian photo A48323A.)]
[Illustration: Figure 26.--Crankshaft with automatic-timing retarding
device on rear end of pivoted- and spring-mounted counterweights. U.S.
Navy test, 1931. (Smithsonian photo A48323B.)]
[Illustration: 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-cushioned 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.
[Illustration: Figure 28.--Cylinder disassembly, showing valve and fuel
injector. U.S. Navy test, 1931. (Smithsonian photo A48324D.)]