Oil Engines. 



439 



and this compresses to a pressure of about 40 lb. per square 

 inch the whole air and vapour contents of the cylinder and 

 vapouriser into the vapouriser and the small passage connecting 

 it and the valve chamber to the cylinder. This compression of 

 the charge raises its temperature, and the vapouriser being very 

 hot, the charge ignites and burns explosively. That is to say, 

 the vapour burns at great velocity, using the oxygen of the air 

 and causing the instantaneous expansion of the cylinder 

 contents. As, however, the outward movement of the piston is 

 arrested by its connection with the engine crank, which it can 

 only move slowly in comparison with the rate at which the air 

 would expand if free to do so, the air rises in pressure to, say, 



Fig. 1. — Diagram of Hornsby-Ackroyd Engine. 



200 lb. per square inch, more or less, according to the load on 

 the engine and the action of the governor. This pressure acting 

 on the piston forces it outward again, giving motion to the 

 crank and fly-wheel, which stores up the work done on this 

 stroke. From this short description of the operations it will be 

 seen that only one out of four strokes of the piston is a working 

 stroke, so that each working stroke has to give two revolutions 

 to the fly-wheel, which must, therefore, be heavy in order that it 

 may store sufficient energy to keep the engine running at its 

 proper speed until the working stroke recurs. 



It will, of course, be understood that the vapouriser must be 

 heated by extraneous means to enable the engine to be started. 

 After it has once been started the heat due to compression of 

 the charge and its combustion keeps the vapouriser at the 



