Piston Engines - Lubrication — Page 52, Lesson 69

Right, let's pick this up. We're now looking at the viscosity of the oil and how it's graded, and then the different types of oil you'll actually find in a piston engine. So, we've established that viscosity is the oil's resistance to flow. Now, the key point here is that the climate the engine operates in has a direct influence on what viscosity you need. Think of it this way: a light aircraft operating in the UK during winter might use an 80 grade oil, and in summer it would use a 100 grade. The choice is dependent on the average ambient temperature. So the higher the number, the higher the viscosity, the thicker the oil. A high viscosity, or thick, oil is what you'd want for certain conditions, but the ambient temperature is the deciding factor. Now, here's a potential problem. If you use too high a viscosity oil at too low a temperature, you can cause problems during starting. The oil is just too thick to flow properly when it's cold. To solve this, we have oils that have two viscosity values. You'll see them marked like SAE 15W/50. These are called Multi-grade Oils. They give you the characteristics of low viscosity at low temperatures—so the engine starts easily—and high viscosity at higher temperatures—so the engine stays protected when it's hot. The "W" stands for winter, by the way. Now, let's talk about the types of oil themselves. The type of oil used in aircraft piston engines is normally mineral based. If that oil contains no additives at all, it's called a Straight oil. But to meet certain requirements of engine operation, additives can be added. These take the form of anti-oxidants, detergents, and oiliness agents. When you add those, the oil is called a Compound oil. So how do you tell them apart? The two oils are identified by the viscosity numbering system, and if it's a compound oil, by the addition of letters or lettering. For example, a bottle or can containing a straight oil with a viscosity of 80 would have only the number 80 marked on it. But a compound oil of the same viscosity might be marked AD 80 or W 80. The actual lettering varies with the manufacturer. The letters AD stand for Ashless Dispersant, and it's an oil with specific qualities for cleaning. So that's your clue—if you see letters, it's a compound oil. Now, when would you use each? Generally, straight oil is only used when running in new engines, or for specific engine installations. As I said, piston engines normally use mineral based oils. However, some engine manufacturers have trialed and approved the use of Semi-synthetic Oils. That's a newer development, and it's something you'll see in Figure 3.3, which shows the types of oil compound, multigrade, and straight oil side by side. So to tie it all together: you've got straight oils with no additives, compound oils with additives like the ashless dispersant, and then the multi-grade oils that handle both cold and hot conditions. The grade number tells you the viscosity, and the letters tell you it's a compound.