I. B&D Cycles and Events Lecture

I. B&D Cycles and Events Lecture - combustion P...

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Unformatted text preview: combustion P expansion H exhaust [valve intake 099” vatve : . open " 7- Air—\Intake I ‘ __ ' valve exhaust ' closed valve ct d Use TDC BDC V piston (2) Figure 9.3 Four-stroke IC engine: (1) intake stroke; (2) compression stroke; (3) power stroke; and (4) exhaust stroke. “:2; combuaiion gxpanQon v” exhaust pt)“. I (099" intake I ,5 r port I : ’ , comp-reason Open = exhaust , _ . ll pan J a I Lmtake ' closed I 30” Closed TDC BDC V intake all exhaust port parts port open closed open (1} {2} Figure 9.2 Two-stroke 1C angina: (1} intake—compression stroke; (2) power—exhaust stroke; Cooling air —~—r Scavenge port "W Deflector -I- Figure 1-1 ‘\ u - Cross scavenge b 7 Loop scavenge e. 777 Exhaust porL Crankcase intake port c — Loop scavenge (Schnuflc type) Uniflow scavenge d — Exhaust valve c - Opposed piston Figure 2-3 A crankcase—scavenged two—stroke cycle engine. Basic types of scavenging arrangements. Table 2-1 Classification of different scavenging methods and their applications Method Cross Loop, MAN—type Loop, Schniirle—type Unifiow, exhaust valve Uniflow, opposed piston Advantages Good scavenging at par— tial throttling and low speeds Low engine volume for multicylinder arrange— ments Low manufacturing cost Good scavenging at WOT Low surface-to-volume ratio combustion chamber Low manufacturing cost Good scavenging at WOT and medium engine speed Fair scavenging at part throttle and other than medium engine speeds Low manufacturing cost Very good scavenging at WOT for high stroke—to— bore ratio Excellent bsfc Very good scavenging at WOT for high stroke~tod bore ratio Drawbacks High bsfc at high throt- tle opening and high speeds High tendency to knock limits compression ratio Poor scavenging at part- throttle operation High hst’c at part throttle operation Need for exhaust valves; thus more complex and "higher manufacturing cost Need for mechanical coupling between two crankshafts Applications Small outboard engines, and some other spe— cific applications (see Table 1—4) Large—bore marine CI en- gines SI engines for a large vari— ety of applications (see Tables 1—2 and 1—4} Large—bore low—speed CI marine and stationary engines (Table 1-4) Sometimes used in large— bore low—speed CI ma— rine engines Combustion 390 2000 51a p Exhaust kPa 200 Compression Expansion 1000 100 U TC BC TC BC TC 1.0 0.5 I Unburned 0 TC BC TC BC TC —360° -180° 0° 180D 360° Crank position and angle FIGURE 1-8 Sequence of events in four-stroke spark-ignition engine operating cycle. Cylinder pressure p (solid line, firing cycle; dashed line, motored cycle), cylinder volume WV," are plotted against crank angle. ax’ SO] EOI mfi i BC TC BC 160 120 80 (atm ) 40 SOI __l _|_ | | 1 BC TC BC —180° 790° 0° 90° 180° Crank angle FIGURE 1-15 Sequence of events during compression, combustion, and expansion processes of a naturally aspirated compression-ignition engine operating cycle. Cylinder volume/clearance volume V/K, rate of fuel injection m”, cylinder pressure 19 (solid line, firing cycle; dashed line, motored cycle), and rate of fuel burning (or fuel chemical energy release rate) r31” are plotted against crank angle. and mass fraction burned xb Pressure rc 90° 270“ p I": e 1» $ TC 2'70" TC Crank angle FIGURE 1-16 Sequence of events during expansion, gas exchange, and compression processes in a loop-scavenged two-stroke cycle compression-ignition engine. Cylinder volume/clearance volume V/K , cylinder pres- sure p, exhaust purl open area A”, and intake port open area Aiare plotted against crank angle. Piston Scavenge p011 Exhaust P011 Figure 1-3 Cylinder-pressure versus cylinder, volume trace of a two-stroke cycle engine. ...
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