THE DESIGN OF AN OIL ENGINE. 27 
gain on the present type of high-pressure engine. In Fig. 1 we have three typical 
indicator cards. Card A is a conventional Diesel card taken at full load. Cards 
B and C are ideal cards. The reason for expecting that such cards might be pro- 
duced will be found in the sample cards shown in the paper on “The Thermody- 
namics of the Marine Oil Engine,’ Transactions of 1914. These cards gave com- 
bustion lines as shown. ‘The height of these cards or the total power is estimated 
by allowing for the increase of air present over the Diesel card. It is of course 
reasonable to assume that for a given bore and stroke the amount of oil per unit 
of air can be kept constant and hence, as the air increases by weight the weight 
of fuel can be increased, and hence the power will be increased per unit of piston 
displacement. This has already been taken up. 
These three cards were developed as shown in Fig. 2, allowing a constant 
ratio of crank to connecting rod for all three cases. The pressure in the cylinder 
for every 20 degrees of crank angle was obtained for the three cases. With this 
data the three curves were drawn in as shown. It is evident to the eye that the 
average pressure for the combined compression and expansion strokes is greatest 
in the Diesel Card A. Now the mean effective pressures of these three cards have 
been figured and are 113, 128 and 140 for 4, B and C respectively. If we plot 
the average pressures obtained from Fig. 2 and the mean effective pressures on 
the respective clearances of the three cards, as has been done in Fig. 1, we see 
how the power increases as the clearance increases and how the bearing pres- 
sures will fall off. 
This shows that by a variation in the direction of an increase in clearance the 
stresses in the engine will decrease, the power will increase, and bearing troubles 
will decrease. It is very seldom that in the solution of a commercial problem we 
have three variables which are all benefited by the change of a single factor as is 
shown to be the case here. 
SPEED VARIATION. 
For the marine engine, and especially for the naval engine, this is a most im- 
portant feature. It is here that admission valve control comes in. In all present oil 
engines the speed is controlled by a variation of the fuel fed per stroke. This 
means that at reduced power the engine compresses an unnecessarily large amount 
of air per stroke. Unnecessary work means mechanical loss. For this reason it is 
proposed to reduce the amount of air compressed per stroke as we reduce the 
amount of fuel used per stroke. Under the Diesel school theories this is im- 
possible. It has, however, been proven possible to a certain degree by the experi- 
ments of the author. The result is that no unnecessary frictional loss is occasioned 
by the compression of an unnecessarily large amount of air. 
There is a theoretical gain in the low-power cycle which is independent of the 
mechanical gain. In the internal-combustion engine the pressure at the opening 
of the exhaust passages is still very high—from 20 to 30 pounds. Suppose we 
could collect this high-pressure working fluid and expand it in a secondary cylinder 
