THE DESIGN OF AN OIL ENGINE. 23 
The four-cycle is unquestionably superior to the two-cycle because it allows a 
greater variation. Ina four-cycle engine by a simple matter of admission valve 
control any desired pressure of compression can be obtained. This is possible in 
the two-cycle only by making the normal pressure the low pressure. In other words 
in the two-cycle the pressure of compression can be varied only by increasing the 
pressure of the scavenging air. Any regulation of the pressure by a reduction of 
the scavenging air below the normal would seem to be very difficult to obtain, that 
is, the normal compression should be that pressure which would result from having 
a full cylinder of air at atmospheric pressure. It is quite difficult to see how any 
pressure lower than this is practical in the two-cycle engine. A higher pressure 
than this can be obtained by increasing the pressure of the scavenging air. 
In the condition of the engine today there are many points which will remain a 
matter of opinion till settled by exhaustive tests. With a four-cycle engine these 
points in some instances can be settled by the use of such a type of valve action that 
the same engine can be run on different cycles or types of indicator cards. From 
such tests much valuable data could be obtained, and so, if for no other reason, it 
seems wise to adopt the four-cycle type. 
Even the most ardent enthusiasts for the two-cycle type do not claim it comes 
nearer than nine-tenths of the efficiency of the four-cycle type. 
Also the matter of simplicity must be considered. Is it simpler to have a 
given number of cylinders with a more elaborate system of valves in the head or 
is it better to increase the number of cylinders by adding scavenging cylinders? 
The author holds that the four-cycle type is the more simple. 
Notwithstanding this opinion, considerable attention has been given to the 
two-cycle type in order to make as many of the author’s theories applicable to the 
two-cycle as is possible. It is hard to see how the weight of the four-cycle type can 
be reduced by making a two-cycle, and it seems that the simplicity of valve gear 
is more than offset by complications in the engine as a whole, and also that a great 
amount of flexibility is sacrificed by adopting the two-cycle type. 
COMPRESSION. 
Notwithstanding Diesel’s theory, the compression pressure in the oil engine is 
primarily carried to the high point for the purpose of producing a temperature of 
ignition. The designer should consider this feature very carefully to see if a lower 
pressure cannot be used with better results. 
We know that a pressure of compression of about 500 pounds will give a tem- 
perature sufficient to ignite the fuel spray. If this pressure can be reduced, the 
result will be lower-bearing pressures and reduced strains in the crank shafts. 
Since this means a lighter and more durable engine, it becomes of vital interest to 
know just how low a pressure of compression can be used. It has been fixed that 
from 450 to 500 pounds will be an average figure to use for the conventional type 
of engine. This pressure can be very greatly reduced by warming the cylinders, by 
steam for instance, before attempting to start the engine. The estimated pressure 
of compression will not vary from the actual pressure by any considerable amount. 
