Gas Turbines - Lubrication — Page 300, Lesson 381

I want to walk you through the lubrication system of a gas turbine engine, and I want to start with a component that does double duty — the torque meter pump. In the diagram you're looking at, this pump is used to boost engine oil pressure to a much greater figure. In some turboprop engines, that figure can be as high as 600 pounds per square inch. That's a very high pressure, and it's not just for lubrication — this pressure is utilized to balance the axial thrust of the helically cut gears within the propeller reduction gear. Let me unpack that. The propeller reduction gear contains gears cut at an angle — helically cut — and those angled teeth create a force that pushes the gears along their axis of rotation. That's axial thrust. The high oil pressure from the torque meter pump is used to counteract, or balance, that thrust so the gears stay properly positioned. Now, here's the clever part. As has already been explained, measuring the torque meter oil pressure will give an accurate indication of the torque being transmitted to the propeller. The harder the engine is working to turn the propeller, the higher the oil pressure. Reference figures which take account of the ambient temperature and pressure allow the pilot to find the minimum torque pressure which the engine should be capable of producing in any set of conditions. So the pilot has a chart or reference that tells them, for the current outside temperature and pressure, what the lowest acceptable torque pressure reading should be. If the engine can't produce that minimum pressure, something is wrong. Now let's follow the oil on its journey. When the oil has completed its tasks of lubricating, cooling, cleaning and acting as a hydraulic medium, it falls into collecting trays or compartments which communicate with the scavenge pumps. So the oil does four jobs — it lubricates, it cools, it cleans, and it acts as a hydraulic medium, meaning it transmits force. After all that, it drains down into trays, and those trays connect to the scavenge pumps. The scavenge pumps are mounted in the same oil pump pack which contains the oil pressure pump. So you have one pump pack, and inside it there's the pressure pump that pushes oil into the engine, plus the scavenge pumps that collect the oil coming back. Although there is only one pressure pump, the oil pump pack may contain several scavenge pumps. This will ensure that the method of lubrication remains a dry sump system. A dry sump system means the oil isn't stored in a pool at the bottom of the engine — it's actively collected and returned to a separate tank. The scavenge pumps push the oil through an air-cooled oil cooler in this particular engine. Different engines may have different types of oil cooler fitted. Whatever the type of oil cooler, its job is to drop the temperature of the oil after its journey through the engine. The oil has absorbed heat as it passed through the hot engine, so the cooler brings that temperature back down before the oil is reused. The next stage for the oil is the de-aerator tray. Here, any air bubbles which will have been collected in the oil are allowed to escape, and the oil falls to the oil tank. In this case, the tank is contained around the engine intake. So the oil tank wraps around the air intake of the engine — that's a common arrangement on turboprops, where the intake is annular, a ring shape, and the oil tank sits around it. Any air pressure which has been built up within the engine lubrication system, through leakage from seals or from the de-aerator — and that's where the passage cuts off, but you can see the system is designed to manage that built-up air pressure as well. Let me tie the whole loop together for you. The pressure pump pushes oil into the engine under high pressure — boosted even higher by the torque meter pump for the reduction gear. The oil lubricates, cools, cleans, and acts as a hydraulic medium. It drains into collecting trays. The scavenge pumps — several of them, in the same pump pack as the single pressure pump — pull that oil out, keeping the system a dry sump. They push it through the oil cooler to drop its temperature. Then it goes to the de-aerator tray, where air bubbles escape. Finally, the oil falls into the tank around the engine intake, ready to be picked up by the pressure pump again. That's the complete circuit.