Principles of naval engineering - Historic Naval Ships Association

PRINCIPLES OF NAVAL ENGINEERINGOtto cycle is practically at constant-volumethroughout the phase, combustion in the modifieddiesel cycle takes place with volume practicallyconstant for a short time, during which periodthere is a sharp increase in pressure, until thepiston reaches a point slightly past TDC. Then,combustion continues at a relatively constantpressure, dropping slightly as combustion endsat d. For these reasons, the combustion cycle inmodern diesel engines is sometimes referredtoas the constant-volume constant-pressure cycle.Pressure-volume diagrams for gasoline anddiesel engines operating on the 2-stroke cyclewould be similar to those just discussed, exceptthat separate exhaust and intake curves wouldnot exist. They do not exist because intake andexhaust occur during a relatively short intervalof time near BDC and do not involve full strokesof the piston as in the case of the 4-stroke cycle.Thus, a pressure-volume diagram for a 2-strokemodified diesel cycle would be similar to a diagramformed by f-b-c-d-e-f of figure 22-7. Theexhaust and intake phases would take place betweene and b with some overlap of the events.(See fig. 22-2.)The preceding discussion has pointed outsome of the main differences between engineswhich operate on the Otto cycle and those whichoperate on the modified diesel cycle. In brief,these differences involve (1) the mixing of fueland air, (2) compression ratio, (3) ignition, and(4) the combustion process.Action of Combustion Gases on PistonsEngines are classifiedinmany ways. Mentionhas already been made of some classificationssuch as those based on (1) the fuels used (dieselfuel and gasoline), (2) the ignition methods (sparkand compression), (3) the combustion cycles(Otto and diesel), and (4) the mechanical cycles(2-stroke and 4-stroke). Engines may also beclassified 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 inwhich the pressure of combustion gases actsupon the piston to move it in the cylinder ofan engine is also used as a method of classifyingengines.The classification of engines according tocombustion-gas action is based upon a considerationof whether the pressure created by thecombustion gases acts upon one or two surfacesof a single piston or against single surfaces oftwo separate and opposed pistons. The twotypes of engines under this classification arecommonly referred to as single-acting andopposed-piston engines.SINGLE-ACTING ENGINES. -Engines of thistype are those whichhave one piston per cylinderand in which the pressure of combustion gasesacts only on one surface of the piston. This is afeature of design rather than principle, for thebasic principles of operation apply whether anengine is single acting, opposed piston, ordouble acting.The pistons in most single-acting enginesare of the trunk type (length greater thandiameter). The barrel or wall of a piston ofthis type has one end closed (crown) and oneend open (skirt end). Only the crown of a trunkpiston serves as part of the combustion spacesurface. Therefore, the pressure of combustioncan act only against the crown; thus, with respectto the surfaces of a piston, pressure is singleacting . Most modern gasoline engines as well asmany of the diesel engines used by the Navy aresingle acting.OPPOSED-PISTON ENGINES.-With respectto combust ion- gas action, the term opposedpiston is used to identify those engines which havetwo pistons and one combustion space in each cylinder.The pistons are arranged in "opposed"positions—that is, crown to crown, withthecombustionspace in between. (See fig. 22-8.) Whencombustion takes place, the gases act against thecrowns of both pistons, driving them in oppositedirections. Thus, the term "opposed" not onlysignifies that, with respect to pressure and pistonsurfaces, the gases act in "opposite" direction,but also classifies piston arrangement within thecylinder.In modern engines which have the opposedpistonarrangement, two crankshaft (upper andlower) are required for transmission of power.Both shafts contribute to the power output of theengine. They may be connected in one of twoways; chains as well as gears have been usedfor the connection between shafts. However,in most opposed-piston engines common to Navyservice, the crankshafts are connected by avertical gear drive. (See fig. 22-8.)The cylinders of opposed-piston engines havescavenging air ports located near the top. Theseports are opened and closed by the upper piston.Exhaust ports located near the bottom of thecylinder are closed and opened by the lowerpiston.554