26 THE DESIGN OF AN OIL ENGINE. 
In order to be a commercial success th oil engine must not only have the efficiency 
but also the power. This point is a strong one in favor of the low-pressure type. 
The full overload condition of the engine means that the maximum amount of fuel 
must be used. The maximum amount of fuel depends upon the amount of air 
which can be compressed per stroke. Here we have about 7 per cent more air in 
the cylinder of the low-pressure type than in the high-pressure type of the same 
bore and stroke. It is obvious that the amount of fuel which we can use will be 7 
per cent greater in the low-pressure than in the high-pressure type. Against this 
argument it can be said that in order to produce the same ignition temperature in 
the low-pressure type as is produced in the high-pressure type, the air in the low- 
pressure type must be hotter than is the case with the high-pressure type and hence 
the weight of air will thus be reduced. This argument is a good one and must be 
considered. The answer is that in the high-pressure type the air is cooler than in 
the low-pressure type only in the beginning of the operation of the engine. It has 
been shown how, as the high-pressure type is warmed up, the air at the beginning of 
compression is much above the normal room or atmospheric temperature and the 
final temperature at the end of compression is much above that needed for ignition 
of the fuel. For this reason the comparison by weight of the air compressed in the 
two types of engines will not vary appreciably from the comparison by volume. 
This being so, the amount of fuel used per stroke in the low-pressure type will be 
about 7 per cent more than is possible in the high compared on a basis of piston dis- 
placement. 
The mechanical efficiency of the engine is the ratio of the brake horse-power 
over the indicated horse-power. The brake horse-power is the indicated horse- 
power minus the frictional loss. The frictional loss is a function of the pressure on 
the bearings and also certain losses in the auxiliaries. As far as the bearing losses 
are concerned or the main-engine friction, this is largely a function of the 
sum of the average pressure for the compression as well as for the expansion 
strokes. Without going into a detailed calculation it is evident that the sum of these 
two average pressures will be less in the low-pressure type than in the high. 
For this reason it would appear that the low-pressure type has a greater power 
for the same piston displacement with an equal thermal efficiency and a greater 
mechanical efficiency. This seems to be the ideal condition sought for in the oil en- 
gine. It means a more powerful engine and at the same time reduced bearing pres- 
sures and stresses which will be reason for expecting a longer life and reduced up- 
keep. 
Here is the proper place to take up the matter of the injection air. In the high- 
pressure type of engine about 10 per cent of the net brake horse-power is consumed 
in the air compressor. Since the pressures are lower in the low-pressure type, the 
air for injection purposes can be reduced in pressure, which will reduce the loss 
in the air compressor, which in turn will give an additional gain in the efficiency 
of this type of engine. ; 
Plate 20 shows graphically the manner in which the low-pressure engine will 
