Let's pick this up right where the oil does its work inside the engine — Page 45, Lesson 58

Let's pick this up right where the oil does its work inside the engine. We've already seen how the lubricant reduces friction and wear, but I want you to understand the full job it's doing, because it's far more than just a slippery film. First, that oil film between the crankshaft and its bearings does something very specific: it absorbs shock loads. When the combustion pressure slams down on the piston, that force travels down the connecting rod and hits the crankshaft. Without that oil cushion, the metal would hammer against metal, and that's what causes vibration and premature wear. So the oil acts as a hydraulic cushion, damping those impacts and reducing vibration. Now, here's a clever secondary use: that same oil can be the power source for operating a hydraulic variable pitch propeller. The propeller blades change their angle using oil pressure, and the engine's own oil system provides that pressure. So the oil isn't just lubricating—it's doing mechanical work. And there's an even more important role for you as a pilot. The oil system can be used as an indicating medium—a way to tell you what's happening inside the engine. It gives you an indication of the power being developed and the condition of the engine. This is of great importance because it can give you an early warning of mechanical failure or loss of power. If something's starting to wear out, the oil pressure or temperature will change before the engine actually fails. That early warning is your chance to react. Let me make one relationship crystal clear, because it's fundamental: an increase in friction causes an increase in Friction Horsepower, and that means a reduction in the Brake Horse Power developed by the engine. Think of it this way—the engine produces power, but some of that power is consumed just overcoming its own internal friction. That consumed portion is Friction Horsepower. What's left over, the useful power you actually get at the propeller, is Brake Horse Power. So if friction goes up, more power gets eaten internally, and less is left for you. That's why lubrication matters so much. Now, the reduction of friction and wear is of prime importance, but I don't want you to ignore the secondary functions. There are five of them: Cooling, Cleaning, Protection, Hydraulic, and Indicating Mediums. The oil cools the engine by carrying heat away, it cleans by carrying contaminants to the filter, it protects against corrosion, it provides that hydraulic power we mentioned, and it indicates engine condition. All of these matter. Now let's move to the two systems that deliver this oil: the Wet Sump and Dry Sump systems. These are the two in common use. The choice of which one is used normally depends on two things: the power output of the engine and the role of the aircraft. But here's the key point—the principle of lubrication is the same in both systems. The only real difference is the method used to store the supply of oil. That's it. Everything else about how the oil circulates and lubricates is identical. Let's start with the Wet Sump system. Most light, non-aerobatic aircraft engines use this. In this system, the oil is stored in the bottom, or sump, of the engine—right inside the engine casing. This simplifies construction, which is why it's popular on light aircraft. But it has four disadvantages, and I want you to know each one. First, lubrication difficulties arise during manoeuvres. Here's what happens: the oil enters the crankcase, gets flung around by the revolving shafts, and you can get over-oiling of the engine. Inverted flight is particularly hazardous, because the oil pickup can't reach the oil, or the oil floods areas it shouldn't. So for aerobatics, this is a problem. Second, the temperature of the oil is more difficult to control, because the oil is stored within the hot engine casing. The engine is hot, so the oil stays hot, and you have less ability to cool it. Third, the oil becomes contaminated and oxidizes more easily, because of the continual contact of the oil with the hot engine. Heat accelerates oxidation, and contamination builds up faster. Fourth, the oil supply is limited by the sump capacity. You can only carry as much oil as the sump holds, and that's a fixed, limited amount. Now, the Dry Sump system overcomes all four of these problems by storing the oil in a remotely mounted tank. The tank is separate from the engine, so the oil isn't sitting in the hot casing, it's easier to cool, and you can carry more of it. As I said, the principle of oil supply is the same for both systems. A Pressure Pump circulates the oil through the engine and lubricates the moving parts. But in a dry sump system, there's an addition: Scavenge Pumps return the oil to the tank. Their job is to prevent the engine sumps from flooding. In a wet sump, the oil just drains back to the sump by gravity. In a dry sump, you need those scavenge pumps to actively pull the oil out and send it back to the remote tank. Now, the arrangement of oil systems varies widely between different aircraft engines, but the functions are all the same. So if you study one system thoroughly, you'll understand the general operation and maintenance requirements of all the others. Let me walk you through the principal units in a typical reciprocating engine oil system. We have an Oil Tank—that's for the dry sump system—Oil Filters, a Pressure Pump and a Scavenge Pump, an Oil Cooler, which is also called a radiator, an Oil Pressure Gauge and an Oil Temperature Gauge, plus the necessary interconnecting oil lines. For a wet sump system, you simply don't use the oil tank, and you have a single pump—just the pressure pump, no scavenge pump, because there's no remote tank to return oil to. That figure shows you the dry sump system laid out. I want you to trace the flow in your mind: the pressure pump pushes oil from the tank through the filters, into the engine to lubricate, then the scavenge pumps pull it back out, through the oil cooler to shed heat, and back to the tank. The gauges tell you what's happening at key points. So the big picture is this: the oil does five jobs—lubricate, cool, clean, protect, and indicate—and the only real difference between wet and dry sump is where the oil is stored. Wet sump stores it in the engine, simpler but with those four drawbacks. Dry sump stores it in a remote tank, more complex but solves all four problems.