THE DESIGN OF AN OIL ENGINE. 35 
The air compressor is not included, nor the piping. The weight has been fig- 
ured out to show that the engine comes somewhere near to the limiting figure. On 
the other hand, the brake horse-power was set for 450. Allowing go for the correct 
effective pressure on the pistons to give the brake horse-power, the calculations for 
brake horse-power will be as follows :— 
132.7 X 90 X 900 X 6 
at 2 — — 406 B. H. P. 
33,000 X 4 4 
132.7 is piston area in square inches. 
90. is the M. E. P. factor. 
goo is the piston speed. 
6 is the number of pistons. 
33,000 is the foot-pounds per horse-power per minute. 
4 is the divisor to account for the four-cycle type. 
It is expected that this design will be severely criticized. If there were nothing 
about this design to criticize the writer would not be inclined to put in the time nec- 
essary to prepare this paper. It is, however, felt that this design can be defended 
upon all lines of attack. The first serious criticism will perhaps be on this factor of 
go in the power calculation. The normal practice is a mean effective pressure of 
about 100 and a mechanical efficiency of 75. This will give a factor of 75. This 
factor of go is used because the mean effective pressure will be increased on account 
of a lowered pressure of compression, and the mechanical efficiency will be raised 
for the same reason. It is felt that a mean effective pressure of over 120 and a 
mechanical efficiency of over 80 would be perfectly reasonable. This increase in mean 
effective pressure and in the mechanical efficiency will be explained somewhat by 
Plates 16 and 17, which were prepared after the paper on “The Thermodynamics 
of the Marine Oil Engine” had been submitted, in an attempt to meet criticism on 
that paper. These were not presented in argument on that paper because the 
criticism was not forthcoming. 
The first question to be settled in the design of an oil engine should be what 
pressure of compression shall be selected for the full-compression pressure. The 
writer is intolerant of restraint. The oil engines in use today have jacketed heads 
and use a pressure of compression of 500 pounds. This is used because Dr. Diesel 
used it. Dr. Diesel took the suggestion of du Rochas and made it practical by fix- 
ing the pressure of compression. How he arrived at this is unknown, Too great 
credit cannot be given to Dr. Diesel for his progressive spirit. Too great condemna- 
tion cannot be given to his imitators who, with additional data on hand, still copy 
the cycle which Diesel hit upon as a result of his groping around in the dark. In 
this submarine engine the compression at full load was fixed at 200 pounds. The 
clearance was found to be 15.6 per cent and the height of the clearance to be 24 
inches. This design was worked out, not alone to illustrate to the Navy Department 
certain ideas in oil-engine construction, but also along lines upon which the author 
