Piston Engines - Lubrication — Page 52, Lesson 66

I want to walk you through the lubrication system of a piston engine, and we're starting with how we measure and understand the oil itself. This is a critical chapter because, as we'll see, oil temperature and pressure are your best windows into the health of the engine. Let's begin with the basics of how we monitor the oil. The temperature of the oil is measured at the inlet to the engine pressure pump. That's the pump that pushes oil through the engine, and we're reading the temperature right where it enters. Most aircraft use an electrical sensor for this, which sends the temperature to a gauge on the flight deck. A normal operating temperature is in the region of 85°C. Now, oil pressure is a different measurement, taken at the outlet side of that same engine-driven pressure pump. The pressure depends on the size and loading of the engine, with 50 to 100 psi being a typical value. The sensor for pressure can be electrical, or it can be a direct-reading mechanical system. Both the temperature and pressure sensing systems are covered in detail in Engine Instruments, Book 5, so we won't go deep into the sensors here. Here's the key regulatory point: it is mandatory that oil temperature and pressure are indicated on the flight deck. Oil quantity, on the other hand, may be displayed. If it's not displayed, there must be a facility for checking the quantity before flight, either with a dip stick or a sight glass. Why is all this so important? Correct oil temperature and pressure during engine operation are perhaps the most important indicators the pilot has of engine condition. If you see indications outside the operating limits, that could be a sign of impending engine failure. So these gauges aren't just nice to have—they're your early warning system. Now let's get into the heart of this excerpt: viscosity. Aircraft engines operate under varying loads, power settings, and outside air temperatures, so they need oils with differing properties. The thickness of the oil is a very important factor, and it's known as the oil's viscosity, or its grade. Here's the precise definition: viscosity is the measure of a fluid's internal friction, or its resistance to flow. A liquid that flows freely has a low viscosity—that's thin oil. One that is sluggish has a high viscosity—thick oil. The critical relationship here is that viscosity changes with temperature. An increase in temperature will reduce viscosity, and vice versa—a decrease in temperature increases viscosity. Think about what happens when you start an engine from cold: the oil is thick. As the engine runs and heats up, the oil thins out. Now, the engine's operating temperature varies considerably, from the moment it's started cold to running at high power for long periods. The oil's viscosity must stay within required limits to do its job across that whole range. This range of temperature over which the oil maintains its required viscosity is termed its viscosity index. Let me explain how we grade these oils. There are various standards used to determine viscosity, and they all provide a datum—a reference point—by which different oils can be compared. The method is consistent: they measure the time taken for a fixed quantity of oil at a given temperature to flow through an orifice, or jet, of a given size. The faster it flows, the thinner the oil. In aviation, there are two standards generally employed. The first is the Society of Automotive Engineers, or SAE. The second is the Saybolt Universal system. Both systems use numbers to indicate viscosity, and here's the rule: the lower the viscosity number, the thinner the oil. There's a neat relationship between the two systems. Looking at the commercial SAE numbers and the Saybolt Universal numbers side by side: SAE 30 corresponds to Saybolt 60, SAE 40 to Saybolt 80, SAE 50 to Saybolt 100, and SAE 60 to Saybolt 120. You can see the SAE number is exactly half that of the Saybolt Universal system. Finally, there's a practical application to all this. Lighter loaded engines use a low viscosity, or thin oil. Higher powered engines with higher loading use a thicker oil. That's the takeaway: match the oil grade to the engine's demands. So to tie it together: you monitor oil temperature at the pump inlet and pressure at the pump outlet, both mandatory on the flight deck. Viscosity is the oil's resistance to flow, it changes with temperature, and the viscosity index tells you the range over which the oil stays within limits. The SAE and Saybolt systems give you numbers to compare oils, with SAE being half the Saybolt value. And the engine's loading determines whether you need a thin or thick oil.