132 SECTIONAL ADDRESSES 
years of persevering experiment, the first practical engine was constructed. 
It may be that a satisfactory internal combustion engine, utilising powdered 
coal as fuel, will be one of the developments of the future ; but so far oil 
has proved to be the only fuel capable of satisfactory employment in 
engines of the Diesel type. 
We may consider for a moment the conditions under which the fuel 
was to be burnt in the working cylinder, conditions which—whether the 
fuel be in colloidal or liquid form, and whether its injection be by high- 
pressure air or by mechanical means—are fundamentally the same to-day. 
The Diesel cycle relies, for the ignition of the fuel delivered to the cylinder 
during the working stroke, upon the temperature resulting from the high- 
compression pressure to which the charge of air is submitted, a pressure 
of some 500 lb. per sq. in. Ignition begins at about top dead centre, 
and continues for a definite part of the power stroke. The burning of 
the charge takes place at roughly constant pressure, the process being 
sometimes designated as the ‘ slow combustion ’ or ‘ constant pressure ’ 
cycle. In comparison with other available types of prime mover, the 
earliest practical Diesel engines were thermally very efficient, and this, 
together with the manifold advantages of oil for fuel, has led to an 
increasing exploitation of the oil engine down to the present day. 
The advantages of the Diesel engine for ship propulsion—resulting in 
a decided economy as regards fuel, space, weight—were recognised early 
in the present century. A milestone in the history of technical develop- 
ment is denoted by the launching of the Se/andia twenty-three years 
ago, a vessel 370 ft. long, fitted with a four-cycle engine. It is interest- 
ing to record that this ship is still in service and very successful results 
are still being obtained. During the last twenty years considerable 
developments in the building of marine Diesel engines have taken place. 
The supply of high-pressure air for fuel injection purposes has always 
been a difficult problem. With normal designs, a pressure of at least 
1,000 lb. per sq. in. must be available, while the compressors must keep 
in tune for long periods of service, involving a considerable maintenance 
charge for suction and delivery valves, cooling coils and compressor pistons. 
Again, several years before Diesel’s patent, Ackroyd Stuart had made use 
of mechanical injection on a low-compression engine, but, in spite of the 
various advantages of this system of injection, there was for a long time no 
serious attempt to apply it to the Diesel engine. Not only does the 
employment of mechanical injection remove the disadvantages associated 
with high-pressure air, but dispensing with the compressor drive from the 
crankshaft improves engine balance, reduces the overall length and weight 
of the power unit, and increases the overall thermal efficiency of the 
engine. A notable feature of the mechanical injection engine is the 
ability of the governor to take sole charge of the engine throughout the 
entire power and speed range, without the necessary complications by 
having the blast air supply under control when running under a varying 
speed range.’ 
More recent developments in engines operating on both the ‘ constant 
pressure ’ and ‘ mixed pressure ’ cycles have led to gradual but persistent 
reductions in the weight and size of the components, thus enabling speeds 
