Piston Engines - Performance and Power Augmentation — Page 143, Lesson 182

Let’s pick this up right where the turbocharger system leaves off. We’ve already got the turbine and the impeller on that common shaft, and we’ve got the wastegate controlling how much exhaust gas spins the turbine. Now I want to walk you through the control loop that actually keeps that compressor outlet pressure—what we call the deck pressure—at the designed value. The key component here is the APC, the Automatic Pressure Controller. Think of it as the brain of the whole system. The APC uses an aneroid capsule, which is a sealed, flexible bellows that is sensitive to compressor outlet pressure. When that pressure changes, the capsule expands or contracts, and that movement is what controls an oil bleed valve inside the APC. Now, where does the oil come from? It’s taken directly from the engine lubrication system. That same oil does double duty: it’s the hydraulic fluid that moves the wastegate actuator piston, and it also cools and lubricates the turbocharger bearings. So the oil is both the muscle and the lifeblood of the turbocharger. Here’s the sequence of events when the pilot moves the throttle. I want you to follow this loop carefully, because it’s the heart of throttle sensitivity. Step one: the pilot moves the throttle, which establishes a different pressure drop across the throttle valve, and that also varies the MAP—the manifold absolute pressure. Step two: the APC senses that change and repositions its oil bleed valve. Step three: the new bleed valve setting changes the oil flow, which establishes a new pressure on the wastegate actuator piston. That piston then moves the wastegate butterfly valve to a new position. Step four: the new wastegate position changes the amount of exhaust gas flowing to the turbine. Step five: that changes the amount of supercharging provided—in other words, it changes the deck pressure. Step six: this new deck pressure then changes the pressure drop across the throttle valve again, and the sequence returns to step two. It keeps cycling until an equilibrium is established. So the system is a closed feedback loop. The APC is constantly chasing the compressor outlet pressure, adjusting the wastegate to hold that pressure at the designed value. When the wastegate diverts all exhaust gases through the turbine, that’s the maximum boost condition. Now, some systems have additional safety features built in. There’s typically an Overboost Warning Light, and if the boost is exceeded, an Overboost Relief Valve—also called a Dump Valve—will open and relieve the deck pressure to ambient. That’s your protection against overboosting the engine. The net result of all this closed-loop control is an effect called throttle sensitivity. When you compare this to a normally aspirated engine—one without a turbocharger—the turbocharged engine’s MAP setting will require frequent resetting. That’s because the system is so responsive to throttle changes. Every time you move the throttle, the whole loop re-establishes itself, and the MAP doesn’t just settle where you put it; it needs constant trimming. So the takeaway is this: the turbocharger isn’t just a turbine and impeller bolted on. It’s a complete control system—APC, oil bleed valve, wastegate actuator, butterfly valve, and safety dump valve—all working together to hold deck pressure at the designed value, and that’s what gives you that characteristic throttle sensitivity.