Piston Engines - Lubrication — Page 45, Lesson 60

Let's talk about the oil tank in a piston engine lubrication system. This is a really important component, and I want you to understand exactly what it does and why it's built the way it is. First, the construction. Oil tanks are made of sheet metal. They're suitably baffled and strengthened internally. The baffles are internal walls that stop the oil from surging around violently during manoeuvres, which could damage the tank. So the strengthening and baffling are there to prevent damage from oil surging. Now, where is it placed? The tank is placed wherever possible at a higher level than the engine. Why? To give a gravity feed to the pressure pump. So the oil flows down to the pump by gravity, which helps the pump do its job. The tank forms a reservoir of oil large enough for the engine's requirements, plus an air space. That air space is critical, and it allows for four specific things. Let me go through them. First, the increased oil return when starting the engine. When the engine is stopped after a previous run, the walls of the crankcase are saturated with oil. That oil drains into the sump and stays there until the engine is started again. When you start it, the scavenge pump returns that oil to the tank. So the tank needs extra space to accept that sudden return of oil. Second, the expansion of the oil. As the oil absorbs heat from the bearings, it expands and takes up more volume. The air space accommodates that greater volume. Third, 'frothing' due to aeration of the oil. Air gets mixed into the oil, creating foam, and that froth takes up more space. Fourth, the displacement of oil from the variable pitch propeller and other automatic controlling devices. When those devices operate, they push oil back into the tank, so the air space absorbs that displaced volume. Now, inside the tank there's a very clever component called the hot pot, or hot well. It forms a separate compartment within the tank. Its purpose is to reduce the time taken to raise the temperature of the oil when starting the engine from cold. It does this by restricting the quantity of oil in circulation when the oil is cold and viscous, meaning thick and slow-flowing. Let me explain how it works. The hot pot is a cylinder of metal fitted above the oil outlet to the engine. So the oil must be inside the hot pot to be able to reach the pressure pump. When you start the engine, the level of oil in the hot pot drops, uncovering a ring of small diameter ports. These ports offer great resistance to the flow of cold, thick oil, so very little passes to the inside of the hot pot. Meanwhile, the oil returned from the engine goes to the inside of the hot pot and is recirculated. Here's the clever part. As the hot oil is returned to the tank, some of its heat raises the temperature of the walls of the hot pot. The oil in the immediate vicinity is heated and thins out, so the ports offer less resistance to the flow of the thinner oil. Progressively, more and more oil is brought into circulation. So the hot pot is a self-regulating system that gradually warms the oil and brings it into full circulation. The oil is filtered by the suction filter before passing to the pressure pump. Now, there's a special provision for feathering propellers. When feathering propellers are fitted, the lower ring of feed ports to the hot pot are placed above the bottom of the tank. This provides a feathering reserve of oil even if the main tank has been emptied through the normal outlet. That would occur if the main feed pipeline developed a leak or completely failed. So you always have a reserve of oil available for feathering the propeller, which is a critical safety feature. Let me also cover how the scavenge oil returns to the tank. It's passed by an internal pipeline over a de-aerator plate to the inside of the hot pot. The plate separates the air from the oil to reduce frothing. And the tank is vented through the crankcase breather to prevent oil losses during excessive frothing conditions. So the de-aerator plate and the vent work together to manage the air in the oil. Finally, let's look at the suction filter. A coarse wire mesh filter is fitted between the tank and the pressure pump. It's designed to remove large solid particles from the oil before it enters the pressure pump, and so prevent damage. So it's a first line of defence, catching the big debris before the oil reaches the sensitive pump. Let me bring it all together. The oil tank is a reservoir with a gravity feed to the pressure pump, an air space for expansion and return surges, a hot pot to manage cold-start oil circulation, a de-aerator plate to reduce frothing, a vent to prevent losses, and a suction filter to protect the pump. Each part has a specific job, and they all work together to keep the engine properly lubricated.