A turbocharged, two-stroke cycle main propulsion diesel engine on your OSV is emitting gray to black smoke excessively from the stack. Upon comparing the measured air box pressure against a reference engine which is producing a clear stack, the measured air box pressure is determined to be too high. Which of the following conditions would most likely be the cause for the relatively high air box pressure?
• Relationship between turbocharger air flow, air box (scavenge receiver) pressure, and engine breathing on a two-stroke diesel • What happens to air box pressure when air flow into the scavenge space is restricted versus when it is free-flowing • Which components listed affect air before the air box, within the air box, or after the air box
• If the air box pressure is higher than normal while the engine is smoking heavily, does that suggest the air is being blocked before it can enter the cylinders, or that the turbocharger is not supplying enough air? • Look at each choice and ask: does this fault tend to make the turbocharger deliver less air (lower scavenge pressure) or cause air to back up in the scavenge space (higher scavenge pressure)? • On a two-stroke, where are the scavenging ports located, and how would a restriction there change the pressure in the scavenge receiver compared to a restriction at the turbocharger inlet filter or aftercooler fins?
• Identify which component in each option is located upstream of the air box and which is located at/after the air box • For each option, predict whether the resulting air box pressure change would be higher or lower and whether smoke would likely increase or decrease • Verify which fault would cause the turbocharger to work against a blocked outlet, raising air box pressure but reducing effective scavenging of the cylinders
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