THE DESIGN OF AN OIL ENGINE. 25 
very short time under the fuel cycle, obviously it is important to decide how to 
bring the engine to a condition of design under which it will not only start but, 
after starting, will be in the most ideal condition for operation under the fuel cycle. 
The engine built in large units must be jacketed when running on the fuel cycle. 
Vessels of any considerable displacement must have steam aboard for auxiliary 
purposes. Under these circumstances the most practical means for meeting this 
condition is to provide the water jackets with steam pipes, so that steam can be 
admitted to the cylinder jackets in order to warm the cylinders preparatory to the 
first explosion. The term “explosion” is used here with a purpose. Combustion 
and explosion differ chiefly in the time element. The explosion we usually asso- 
ciate with a rising pressure, and it is this rising pressure which is needed in the oil 
engine. By using steam to heat the cylinders it is possible to produce a card such 
as is shown on Fig. 1, Plate 16, marked “Ideal Card of the Low-Pressure Type.” It 
is only after the engine cylinders have been warmed in some manner that such a 
card is possible at the initial stroke. Since only a small amount of steam will be 
needed to warm the cylinders, it seems highly desirable to install a steam plant for 
this purpose, even if there be no other use for steam on the vessel. 
EFFECT ON THE EFFICIENCY OF THE ENGINE. 
The ultimate efficiency of the engine is a combination of the thermal and me- 
chanical efficiencies. The thermal efficiency depends upon the number of expan- 
sions. The mechanical efficiency depends upon the ratio of the mean effective pres- 
sure to the average pressure during the compression and expansion strokes. 
Tt was explained in the Transactions for 1914 how the thermal efficiency was 
not reduced by reducing the pressure of compression and increasing the clear- 
ance while advancing the fuel injection. Fig. 1, Plate 17, shows two cylinders of 
the same size, but one is of the low-pressure type and the other of the high-pres- 
sure type. Besides the expansions there is the matter of radiation. The greater 
the clearance the less chance for radiation loss. Fig. 2, Plate 16, contains some 
comparisons to bring this out. Here we see that while the volume of the clearance 
of the Diesel or high-pressure type is but .30 of the same volume for the low-pres- 
sure type, the surface of the Diesel clearance is about .78 of the surface of the low- 
pressure type. This shows how the opportunity for radiation losses is much greater 
in the high-pressure type. 
Before considering the mechanical efficiency it is quite important to under- 
stand the effect produced upon the power by the reduced compression. Referring 
again to Fig. 1, Plate 17, we find the relative volumes of air compressed per stroke 
in the two types of engines. For convenience the two engines were taken as each 
being 12 inches bore and 12 inches stroke. It is suggested as desirable to place a 
blower on the intake pipe in order to improve the scavenging. Assuming perfect 
scavenging, the volume in the low-pressure type at the end of the suction stroke 
will be .g1 cubic foot against a corresponding volume of .85 cubic foot for the high- 
pressure type owing to the fact that the low-pressure type has a greater clearance. 
