24 THE DESIGN OF AN OIL ENGINE. 
We know that a small four-cycle oil engine, when warm, will fire on a compression 
of 125 pounds. It is certain that under the same conditions—namely, after the en- 
gine has been warmed up by operation—the larger sizes will be able to run on pres- 
sures as low as 150 to 175 pounds. 
This latitude may be objected to as being altogether too great to be tolerated in 
a design which is to be built. Just as the main bearings have to be scraped to fit 
the shaft, so it is possible to fit the cylinders with a proper clearance. This is not 
a very difficult matter; that is, in case of an error in estimate, no parts need be 
scrapped. There are several ways in which considerable adjustment can be made 
in the cylinder clearances. In the original engines put out by the American and 
British Manufacturing Company, the shaft was raised or lowered by means of 
wedges upon which the bearings rested. This seems to offer a little too much 
chance for meddling on the part of irresponsible parties, and it is suggested that 
the connecting rods may be lengthened or shortened to make the needed variation. 
In addition to this method, it is possible to get the same result by taking a chip from 
either the head or the cylinder, or a slightly thicker gasket might be used in place of 
the original gasket. Even this change in the gasket might be objected to, while 
absolutely no objection can be found to taking a chip from the head or the cylinder 
in case the engine is found to have too great a clearance and in case the clearance 
be not large enough to taking a chip from the connecting rod. In fact, with a ma- 
rine type connecting rod, all the variation which is necessary can be made by 
either lengthening or shortening the rod. This matter of adjustment will not be 
a serious one, and when the designer understands that his estimated clearance can 
easily be altered, in case of need, without the waste of any machined parts, greater 
confidence should exist in his mind in regard to the final product. Having shown 
how the clearance can easily be altered to suit the needs of the engine, the neces- 
sity of wide experience is done away with. 
The clearance will vary for the different types to be built. For instance, a 
small high-speed engine will run on a lower compression since the length of time 
is a minimum during which the compressed air at its maximum temperature is 
exposed to radiation losses under the worst condition. On the other hand, in a 
large and slow-speed engine, while the air at the end of the stroke has a longer time 
in which to lose sufficient heat to prevent ignition, the space in which the air is 
compressed is much greater and hence the facilities for heat losses are less. It is 
very probable that there will not be a very great difference in the clearance which 
can be used. The time element in the high-speed engine is to a very great degree 
counteracted by the increased clearance of the larger engine. 
INITIAL TEMPERATURE OF THE CYLINDER. 
The data presented in “The Thermodynamics of the Marine Oil Engine” prove 
conclusively that the full pressure of compression is not needed after the engine 
has been run long enough to get warmed up. Since the engine will warm up in a 
