232 HEAVY-OIL ENGINES FOR MARINE PROPULSION. 
3. At about the beginning of the return or expansion stroke the liquid 
fuel is sprayed through a specially designed valve, into the hot compressed 
air. Due both to the high pressure and the high temperature, the small 
particles of fuel are burned almost instantly, thereby increasing the tem- 
perature and maintaining the pressure about constant during the first part 
of the stroke. Fuel is sprayed in until enough has been supplied to enter 
into combustion with the oxygen of the air. When this point is reached, 
usually at about one-tenth the stroke, the fuel valve is closed and during the 
remainder of the stroke the hot products of combustion expand, doing work. 
4. On the next stroke the products of combustion are expelled. 
Typical indicator cards of both the Otto and the Diesel cycle are shown 
in Figs. 1 and 2, Plate 118. 
The cycle above described is the usual four-stroke cycle, and is the one 
on which the engine was first developed, and hundreds of thousands of horse- 
power are in use to-day. It has certain advantages as well as disadvantages. 
The Diesel cycle is also used on the two-stroke principle as is the case 
with engines of the Otto cycle, but here again the Diesel cycle has a great 
advantage. During the process of scavenging at the end of the expansion 
stroke the Otto cycle, employing a mixture of air and fuel, loses a part of 
the unconsumed fuel through the exhaust ports. The Diesel type of engine, 
on the other hand, is scavenged by a charge of pure air and, therefore, no 
fuel is wasted in this way. The fuel does not enter the cylinder until after 
the end of the compression stroke, and every particle of fuel injected is 
burned in the cylinder. 
From the foregoing it will be seen that the main points of difference 
between the Otto and Diesel cycle may be summarized as follows: 
Fuel.—The Diesel engine may employ practically any form of com- 
bustible liquid, while the Otto cycle must use a gas or vapor. 
Compression.—Theoretically, the higher the compression, the greater 
the efficiency. There are certain practical limits to this, and the - Diesel 
engine is enabled to attain this limit since only pure air is compressed. In 
the Otto cycle the compression is limited by the danger of premature explo- 
sion due to raising the temperature of the mixture by compression. — 
Explosion.—Since the fuel is sprayed into the Diesel engine at a rate 
just sufficient to maintain a constant pressure, there is no explosion and, 
consequently, no sudden shocks, nor instantaneous changes of temperature. 
This is conducive to smooth running and long life of working parts. The 
Otto cycle, on the other hand, is based on an explosive action. 
From the foregoing it is seen why the Diesel type of engine has shown 
itself so well adapted to the use of heavy oils. 
