Question 18 UFIV01 - Chief Engineer - UFIV

The Correct Answer is C ### Explanation of the Correct Answer (C) The turbo soak-back pump (or turbo auxiliary pump) is designed to ensure the turbocharger's bearings are properly lubricated and cooled during critical phases of engine operation, specifically during start-up and shutdown, when the main engine-driven lube oil pump may not be providing adequate pressure or flow, or when the turbocharger is still extremely hot after the engine has stopped. 1. **Before Engine Start-up (Pre-lubrication):** Running the soak-back pump briefly before the engine starts ensures that the turbocharger bearings are pre-lubricated. This is crucial because the turbocharger spins at extremely high speeds immediately upon engine ignition, and waiting for the main lube oil pump to build sufficient pressure could cause damage due to dry running. 2. **After Engine Shutdown (Soak-back/Post-cooling):** When the engine stops, the main oil pump stops immediately, but the turbocharger rotor continues to spin (coast down) due to inertia, and the turbocharger housing remains extremely hot (heat soak). The soak-back pump runs for a predetermined period (e.g., 5 to 30 minutes) after shutdown to continuously circulate cooler oil through the bearings. This removes residual heat (preventing carbonization of oil inside the bearings) and provides lubrication until the rotor stops spinning completely, thus protecting the high-speed bearings from heat damage. Therefore, the pump is required to run both before start-up and after shutdown. ### Explanation of Incorrect Options **A) It will run for a period of time after engine shutdown only.** This is partially correct (the post-shutdown operation is essential for cooling), but it ignores the equally important pre-lubrication function required before engine start-up to prevent dry-running wear. **B) It will operate at all times while the engine is running.** While the turbocharger needs lubrication constantly, this specific auxiliary/soak-back pump typically does not run while the engine is running. When the engine is operating, the **main engine-driven lube oil pump** provides sufficient flow and pressure to lubricate the turbocharger (often 4 to 6 bar), making the auxiliary soak-back pump redundant during normal operation. **D) It will run for a period of time prior to engine start-up only.** This is partially correct (the pre-lubrication function is necessary), but it fails to account for the most common and crucial function: the post-shutdown cooling/soak-back period required to prevent heat damage to the turbocharger bearings.