Question 26 OSE01 - Chief Engineer - OSV

The Correct Answer is A ### 2. Why Option A is Correct **A) Airside aluminum fins on after coolers are excessively restricted.** The aftercooler (or intercooler) is located immediately downstream of the turbocharger compressor and upstream of the air box (scavenging air manifold). Its function is to cool the highly compressed air before it enters the cylinders. If the airside fins become restricted (fouled with oil, dirt, or debris), the air traveling from the compressor outlet to the air box meets severe resistance. This restriction causes a significant **pressure drop** across the aftercooler. Since the air box pressure is measured *after* this restriction, the reading will be abnormally low, despite the turbocharger potentially running at the correct speed. Low air box pressure means less air is delivered to the cylinders, leading to an overly rich mixture and the observed excessive gray/black smoke. ### 3. Why Other Options Are Incorrect **B) Scavenging air intake ports are excessively restricted with carbon deposits.** Restricting the scavenging ports reduces the amount of air that can enter the cylinder. While this leads to incomplete combustion (smoke), the restriction is located *downstream* of where the air box pressure is measured. If the air cannot easily escape the air box into the cylinder, the air box pressure measurement tends to remain high or even slightly increase, rather than dropping significantly. **C) Turbocharger exhaust turbine inlet screen is excessively restricted.** A restricted turbine inlet prevents the exhaust gases from efficiently driving the turbine wheel. This causes the turbocharger to spin slower, which reduces the output pressure of the compressor side. While this *does* lead to low air delivery and smoke, restricting the aftercooler (A) is a more common and direct cause of a low pressure *reading* specifically due to excessive flow resistance within the delivery manifold itself. Furthermore, Aftercooler fouling is typically the most frequently cited issue for excessive pressure drop in the charge air path. **D) Exhaust silencer/muffler is excessively restricted with carbon deposits.** Restricting the exhaust silencer creates high exhaust back pressure. This hinders the engine's ability to scavenge (clear spent gases) and reduces the effective pressure differential across the turbocharger turbine, causing it to slow down. Similar to option C, this indirectly reduces air pressure output. However, a physical restriction in the charge air flow path (like the aftercooler, Option A) is the more immediate and direct cause of a low pressure reading in the air box manifold.