Piston Engines - Performance and Power Augmentation — Page 143, Lesson 180

Let’s pick this up right where the turbocharger story gets interesting. We’ve already seen how the wastegate works — when it’s fully open, nearly all the exhaust gases pass straight to atmosphere, bypassing the turbine. As the wastegate closes, those gases get directed onto the turbine wheel, which spins the impeller and compresses the intake air. The maximum rotor speed happens when the wastegate is fully closed, and that occurs at what we call the critical altitude for that engine and that turbocharger combination. Critical altitude is defined as the height above which maximum boost, or manifold pressure, can no longer be maintained. So below that altitude, the wastegate can keep closing to hold your boost; above it, the wastegate is already shut and you simply can’t get any more manifold pressure. Now, who controls the wastegate? It can be controlled manually by the pilot, but in most turbocharger systems we fit automatic controls to prevent overboosting the engine — that is, pushing manifold pressure beyond the engine’s safe limit. In an automatic control system, the wastegate is mechanically connected to a single acting actuator. The position of that actuator depends on the opposing forces of a spring and engine oil pressure. Here’s the key relationship: spring force tends to open the wastegate, and oil pressure tends to close it. So the oil pressure inside the actuator regulates the wastegate position according to what the engine needs at that moment. To vary that actuator oil pressure, we use different types of controllers. There are three you need to know: the Absolute Pressure Controller, or APC; the Density Controller, or DC; and the Differential Pressure Controller, or DPC. We’ll concentrate on the Absolute Pressure Controller first, then come back to the others. So, the APC. Some simple turbocharger systems use just this single controller, and its job is to prevent compressor outlet pressure from exceeding a specified maximum. That’s the pressure of the air coming out of the compressor, after it’s been compressed. The APC is illustrated in Figure 11.4. Here’s how it works. The APC uses an aneroid capsule — that’s a sealed, flexible bellows — which is sensitive to compressor outlet pressure. That capsule controls the oil bleed from the wastegate actuator. By bleeding oil away from the actuator, it changes the oil pressure acting on the actuator, and that in turn controls the wastegate position. The whole loop is designed to maintain the required compressor outlet pressure. Once the compressor outlet pressure is held steady, the throttle then controls manifold pressure downstream. So the APC is holding the compressor outlet pressure constant, and the pilot’s throttle sets the manifold pressure. Now, one important operating point: at low power settings, full oil pressure is applied to the wastegate actuator, which closes the wastegate. Think about that — at low power, you want the wastegate shut so that all the exhaust goes through the turbine, keeping the turbo spinning and giving you boost even when the throttle is barely open. That’s the APC doing its job at the low end of the power range. So to tie it together: the wastegate diverts exhaust to the turbine, the actuator positions the wastegate based on spring versus oil pressure, and the APC senses compressor outlet pressure with its aneroid capsule to bleed oil and hold that pressure at its maximum. That’s the absolute pressure controller in a nutshell.