Suppose the diesel generator set drive engines are of the type shown in the illustration on your towing vessel. Assuming the engine is naturally aspirated, within an individual cylinder in terms of piston stroke and position, under what circumstances are the intake and exhaust valves both open? Illustration MO-0163
β’ Four-stroke diesel cycle: intake, compression, power, exhaust β’ Valve timing and valve overlap in a naturally aspirated engine β’ Piston position at TDC (top dead center) and BDC (bottom dead center) during stroke transitions
β’ Walk through each of the four strokes and picture whether the intake or exhaust valve must be open or closed to make that stroke work correctly. β’ At what point in the cycle do designers keep the exhaust valve slightly open while the intake valve is just starting to open, and why would that help cylinder scavenging? β’ For each answer choice, ask yourself: would having both valves open at that moment help or hurt compression or power production?
β’ Be clear on what happens right before and right after each top dead center (TDC) and bottom dead center (BDC) in a four-stroke cycle. β’ Verify during which stroke the fresh air is entering and during which stroke burned gases are leaving, then find where those two events slightly overlap. β’ Eliminate any option where having both valves open would destroy compression or let the fresh charge blow straight out the exhaust.
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