Principles of naval engineering - Historic Naval Ships Association

Chapter 22. -DIESEL AND GASOLINE ENGINESduring a cycle of operation. This oversightsometimes leads to confusion when the operatingprinciples of an engine are being considered.This discussion points out the relationshipof events to strokes by covering the number ofevents occurring during a specific stroke, theduration of an event with respect to a pistonstroke, and the cases where one event overlapsanother. The relationship of events to strokescan be shown best by making use of graphic representationof the changing situation occurring ina cylinder during a cycle of operation. Figure22-1 illustrates these changes for a 4-strokecycle diesel engine.The relationship of events to strokes is morereadily understood, if the movements of a pistonand its crankshaft are considered first. In part Aof figure 22-1, the reciprocating motion andstroke of a piston are indicated and the rotarymotion of the crank during two piston strokes isshown. The positions of the piston and crank atthe start and end of a stroke are marked "top"and "bottom, " respectively. If these positionsand movements are marked on a circle (part B,fig 22-1) the piston position, when at the top ofastroke, is located at the top of a circle. Whenthe piston is at the bottom of a stroke, the pistonposition is located at the bottom center of thecircle. Note in parts A and B of figure 22-1 thatthe top center and bottom center identify pointswhere changes in direction of motion take place.In other words, when the piston is at top center,upward motion has stopped and downward motionis ready to start or, with respect to motion,the piston is "dead."The points which designate changes in directionof motion for a piston and crank arecommonly called top dead center (TDC) andbottom dead center (BDC).If the circle illustrated in B is broken atvarious points and "spread out" (part C, fig.22-1), the events of a cycle and their relationshipto the strokes and how some of the eventsof the cycle overlap can be shown. TDC andBDC should be kept in mind since they identifythe start and end of a stroke and they are thepoints from which the start and end of eventsare established.By following the strokes and events as illustrated,it can be noted that the intake event startsbefore TDC, or before the actual down stroke(intake) starts, and continues on past BDC, orbeyond the end of the stroke. The compressionevent starts when the intake event ends, but theupstroke (compression) has been in processsince BDC. The injection and ignition eventsoverlap with the latter part of the compressionevent, which ends at TDC. The burning of thefuel continues a few degrees past TDC. Thepower event or expansion of gases ends severaldegrees before the down (power) stroke endsat BDC. The exhaust event starts when the powerevent ends and continues through the completeupstroke (exhaust) and past TDC. Note the overlapof the exhaust event with the intake event ofthe next cycle. The details on why certainevents overlap and why some events are shorteror longer with respect to strokes will be coveredlater in this chapter.From the preceding discussion, it can beseen why the term "stroke" is sometimes usedto identify an event which occurs in a cycle ofoperation. However, it is best to keep in mindthat a stoke involves 180° of crankshaft rotation(or piston movement between dead centers) whilethe corresponding event may take place during agreater or lesser number of degrees of shaftrotation.The relationship of events to strokes in a2-stroke cycle diesel engine is shown in figure22-2. Comparison of figures 22-1 and 22-2 revealsa number of differences between the twotypes of mechanical or operating cycles. Thesedifferences are not too difficult to understand ifone keeps in mind that four piston strokes and720° of crankshaft rotation are involved in the4-stroke cycle while only half as many strokesand degrees are involved in a 2-stroke cycle.Reference to the cross-sectional illustrations(fig. 22-2) will aid in associating the event withthe relative position of the piston. Even thoughthe two piston strokes are frequently referredto as power and compression, they are identifiedas the "down stroke" (TDC to BDC) and "upstroke" (BDC to TDC) in this discussion in orderto avoid confusion when reference is made to anevent.Starting with the admission of air, (1) figure22-2, we find that the piston is in the lower halfof the down stroke and that the exhaust event (6)is in process. The exhaust event started (6') anumber of degrees before intake, both startingseveral degrees before the piston reached BDC.The overlap of these events is necessary inorder that the incoming air (1') can aid inclearing the cylinder of exhaust gases. Note thatthe exhaust event stops a few degrees before theintake event stops, but several degrees after theupstroke of the piston has started. (The exhaust545