In a four-stroke/cycle medium-speed diesel engine, fuel injection commences at 10 crankshaft degrees before top dead center. After fuel injection commences, approximately how many degrees will the camshaft rotate before the exhaust valve begins to open?
• Relationship between crankshaft and camshaft speed in a four-stroke diesel engine • How many crankshaft degrees occur in one complete four-stroke cycle and where TDC (top dead center) and exhaust valve opening typically occur • Converting crankshaft degrees to camshaft degrees based on their speed ratio
• First, recall how many crankshaft degrees make up one full four-stroke cycle and where TDC sits in that cycle. • Think about typical valve timing: after fuel injection starts before TDC on the compression stroke, roughly when in crankshaft degrees does the exhaust valve begin to open near the end of the power stroke? • Once you estimate the crankshaft degrees between injection start and exhaust valve opening, convert that to camshaft degrees using their speed relationship.
• Be clear on the 2:1 speed ratio between crankshaft and camshaft in a four-stroke engine. • Estimate a reasonable range of crankshaft degrees from fuel injection start (just before TDC on compression) to exhaust valve opening (near bottom of power stroke). • After finding crankshaft degrees, divide appropriately to get camshaft degrees before comparing to the answer choices.
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