Piston Engines - Lubrication — Page 52, Lesson 64

Let's pick up with the scavenge pump, because that's the piece that makes a dry sump system actually work. The scavenge pump's job is to return the oil by pumping it from the sump back to the tank. Now, think about what happens when the engine is stopped: all the oil in the crankcase drains down into the sump. When you start the engine again, there's a big slug of oil sitting there. If the scavenge pump were the same size as the pressure pump, it wouldn't be able to remove that oil fast enough. So, to maintain a dry sump, the scavenge pump has to be bigger than the pressure pump. In practice, the scavenge pump capacity is 25% to 50% larger than that of the pressure pump. That's the key number to remember. Now, the oil cooler. We've already stressed that oil cools the internal components of the engine. But if the oil itself gets too hot, it could fail as a lubricant. So, to stop its temperature rising too high, we introduce a cooler into the system. The oil cooler consists of a matrix, or tube block, which spreads the oil in a thin film and subjects it to cooling air. The matrix is built up of round tubes. The ends of these tubes are expanded and shaped to form hexagons, which gives a flat surface for soldering the tubes together. The whole matrix is bonded into the oil cooler jacket by soldering those flats of the tubes to the inner shell of the cooler jacket. Here's the problem with that design: when you start the engine from cold, the cooler matrix is full of cold, thick oil. Forcing that thick oil through the small oilways of the cooler would require a very high pressure, and that could damage the cooler. So, to prevent that damage, an anti-surge valve is fitted to by-pass the matrix when the oil is cold. That valve lets the cold oil skip the cooler until it warms up. Now, the temperature of the oil is affected by three factors. First, the amount of heat generated in the engine, which is the power. Second, the temperature of the cooling air. And third, the rate at which air flows through the cooler. In some light aircraft, the flow of air through the cooler is simply dependent on the forward speed of the aircraft in flight, and the airflow from the propeller while the aircraft is on the ground. That creates a problem in certain flight conditions, like a climb, where you're using high power but low forward speed. In that case, you have to be careful to prevent overheating the oil. The flight manual will recommend climb speeds that should ensure adequate cooling. Higher powered aircraft are fitted with shutters behind the cooler to control the flow of air through it. These shutters are closed at start-up to let the engine oil temperature rise quickly, because cold oil increases internal friction. Then they're opened to maintain the temperature. In flight, as the air temperature reduces at altitude, the shutters will close off again. Control of the shutters can be manual or automatic. One more point on coolers: diesel engine lubrication systems are typically wet-sump, and they would definitely include an oil cooler, because of the need to dissipate the additional heat generated by the diesel engine. Finally, the lubrication monitoring instruments. We've explained the importance of maintaining the correct oil temperature. The other two parameters of the oil system that are monitored are pressure and quantity. So, temperature, pressure, and quantity — those are the three things you're watching on your instruments.