Piston Engines - Performance and Power Augmentation — Page 147, Lesson 189

Let's talk about how we control the turbocharger on a piston engine, because that's what this section is really about. We've got three different control arrangements here, and I want you to see how each one handles the job of managing manifold pressure. Start with the simplest one. We have a fixed orifice exhaust bypass. Picture this: the exhaust gases flow out of the engine, and some of them are always going to be diverted to drive the turbocharger turbine. There's a fixed opening, a fixed orifice, so a proportion of the exhaust always goes to the turbo. Now, with this setup, the manifold pressure is controlled strictly by the throttle valve. You open the throttle to get more manifold absolute pressure, or MAP, which we also call boost. As you open that throttle, the turbine speed increases, and the throttle input pressure and the MAP respond to that chain reaction. Here's the catch: because the bypass is fixed, rapid movement of the throttle will probably cause overboosting with this type of system. You slam the throttle open, and the turbo spools up faster than the system can compensate, and you get too much boost. That's the limitation of the simplest design. Now, let's move to the more sophisticated systems. There are two further controllers that you may encounter as a pair, a dual setup. We have a Density Controller and a Differential Pressure Controller. These are only fitted to a more sophisticated system. Let's look at the Density Controller first. The density controller will limit the maximum MAP, or boost, below the critical altitude when the throttle is opened fully. So think about critical altitude as the altitude where the turbocharger can no longer maintain sea-level manifold pressure. Below that altitude, this controller steps in to cap the boost. The density controller is fitted with two bellows that sense compressor outlet pressure and temperature. These bellows are filled with dry nitrogen. Now, here's the clever part: the nitrogen allows the pressure to increase as the temperature increases. Remember, as the wastegate closes, the turbo runs faster, and compressor rpm can be up to 110,000 revolutions per minute. That's fast. So the bellows are sensing both pressure and temperature at the compressor outlet. The effect of having a density controller is that the maximum available pressure will increase up to critical altitude, and in doing so, it will reduce the normal loss associated with the increased charge temperature at a constant pressure. In plain terms, as the air gets compressed, it heats up, and hot air is less dense, so you lose power. The density controller compensates for that temperature effect so you get more consistent power as you climb. Now, the second controller, the Differential Pressure Controller. This one operates at all positions of the throttle other than the fully open position. Its job is to reduce the compressor outlet pressure if a lower manifold pressure is required. So when you're not at full throttle, and you want less boost, this controller manages the wastegate to bring the pressure down. Here's the key point you must remember: only one of these two controls will be in use and controlling the wastegate position at any moment in time. They work as a pair, but they never fight each other. Either the density controller is in charge, or the differential pressure controller is in charge, depending on the throttle position and the flight condition. So to sum up the whole picture: we have three ways to control the turbocharger. The fixed orifice bypass, which is simple but prone to overboosting on rapid throttle movement. Then the sophisticated pair — the density controller, which limits boost below critical altitude at full throttle by sensing pressure and temperature with nitrogen-filled bellows, and the differential pressure controller, which manages boost at all other throttle positions. And at any given moment, only one of those two is actually controlling the wastegate.