PRINCIPLES OF NAVAL ENGINEERING 



Otto cycle is practically at constant-volume 

 throughout the phase, combustion in the modified 

 diesel cycle takes place with volume practically 

 constant for a short time, during which period 

 there is a sharp increase in pressure, until the 

 piston reaches a point slightly past TDC. Then, 

 combustion continues at a relatively constant 

 pressure, dropping slightly as combustion ends 

 at d. For these reasons, the combustion cycle in 

 modern diesel engines is sometimes referredto 

 as the constant-volume constant-pressure cycle. 



Pressure-volume diagrams for gasoline and 

 diesel engines operating on the 2-stroke cycle 

 would be similar to those just discussed, except 

 that separate exhaust and intake curves would 

 not exist. They do not exist because intake and 

 exhaust occur during a relatively short interval 

 of time near BDC and do not involve full strokes 

 of the piston as in the case of the 4-stroke cycle. 

 Thus, a pressure-volume diagram for a 2-stroke 

 modified diesel cycle would be similar to a dia- 

 gram formed by f-b-c-d-e-f of figure 22-7. The 

 exhaust and intake phases would take place be- 

 tween e and b with some overlap of the events. 

 (See fig. 22-2.) 



The preceding discussion has pointed out 

 some of the main differences between engines 

 which operate on the Otto cycle and those which 

 operate on the modified diesel cycle. In brief, 

 these differences involve (1) the mixing of fuel 

 and air, (2) compression ratio, (3) ignition, and 

 (4) the combustion process. 



Action of Combustion Gases on Pistons 



Engines are classifiedinmany ways. Mention 

 has already been made of some classifications 

 such as those based on (1) the fuels used (diesel 

 fuel and gasoline), (2) the ignition methods (spark 

 and compression), (3) the combustion cycles 

 (Otto and diesel), and (4) the mechanical cycles 

 (2-stroke and 4-stroke). Engines may also be 

 classified on the basis of cylinder arrangements 

 (V, in-line, opposed, etc.), the cooling media 

 (liquid and air), and the valve arrangements 

 (L-head, valve-in head, etc.). The manner in 

 which the pressure of combustion gases acts 

 upon the piston to move it in the cylinder of 

 an engine is also used as a method of classi- 

 fying engines. 



The classification of engines according to 

 combustion-gas action is based upon a consider- 

 ation of whether the pressure created by the 

 combustion gases acts upon one or two surfaces 

 of a single piston or against single surfaces of 



two separate and opposed pistons. The two 

 types of engines under this classification are 

 commonly referred to as single-acting and 

 opposed-piston engines. 



SINGLE-ACTING ENGINES. -Engines of this 

 type are those whichhave one piston per cylinder 

 and in which the pressure of combustion gases 

 acts only on one surface of the piston. This is a 

 feature of design rather than principle, for the 

 basic principles of operation apply whether an 

 engine is single acting, opposed piston, or 

 double acting. 



The pistons in most single-acting engines 

 are of the trunk type (length greater than 

 diameter). The barrel or wall of a piston of 

 this type has one end closed (crown) and one 

 end open (skirt end). Only the crown of a trunk 

 piston serves as part of the combustion space 

 surface. Therefore, the pressure of combustion 

 can act only against the crown; thus, with respect 

 to the surfaces of a piston, pressure is single 

 acting . Most modern gasoline engines as well as 

 many of the diesel engines used by the Navy are 

 single acting. 



OPPOSED-PISTON ENGINES.-With respect 

 to combust ion- gas action, the term opposed 

 piston is used to identify those engines which have 

 two pistons and one combustion space in each cyl- 

 inder. The pistons are arranged in "opposed" 

 positions— that is, crown to crown, withthecom- 

 bustion space in between. (See fig. 22-8.) When 

 combustion takes place, the gases act against the 

 crowns of both pistons, driving them in opposite 

 directions. Thus, the term "opposed" not only 

 signifies that, with respect to pressure and piston 

 surfaces, the gases act in "opposite" direction, 

 but also classifies piston arrangement within the 

 cylinder. 



In modern engines which have the opposed- 

 piston arrangement, two crankshaft (upper and 

 lower) are required for transmission of power. 

 Both shafts contribute to the power output of the 

 engine. They may be connected in one of two 

 ways; chains as well as gears have been used 

 for the connection between shafts. However, 

 in most opposed-piston engines common to Navy 

 service, the crankshafts are connected by a 

 vertical gear drive. (See fig. 22-8.) 



The cylinders of opposed-piston engines have 

 scavenging air ports located near the top. These 

 ports are opened and closed by the upper piston. 

 Exhaust ports located near the bottom of the 

 cylinder are closed and opened by the lower 

 piston. 



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