Piston Engines - Performance and Power Augmentation — Page 147, Lesson 186

Let’s pick up the turbocharger story right where the wastegate leaves off. I want to walk you through the full operating sequence of a turbocharged piston engine, from the moment before start, all the way up to the climb, because this is the heart of how power augmentation actually works in practice. First, the key components you need to keep straight: the wastegate, the Wastegate Actuator, the Aneroid Capsule, the Bleed Valve, the Absolute Pressure Controller (APC), and the compressor. The wastegate is a valve in the exhaust path that diverts exhaust gas either through the turbine or around it, past it, to atmosphere. The Wastegate Actuator is the device that moves the wastegate, using oil pressure and an internal spring. The Aneroid Capsule is a pressure-sensing element, and the Bleed Valve is a valve in the Absolute Pressure Controller that controls oil flow. The APC is the brain that senses compressor outlet pressure. Now, the sequence. Before start, the wastegate must be fully open. Why? Because if it’s closed, the exhaust gases can’t flow freely to atmosphere, and the engine would be very difficult, if not impossible, to start. That opening is achieved by the spring inside the Wastegate Actuator, which forces the wastegate fully open. So at rest, the spring holds it open. Immediately after start, there’s a problem: there probably isn’t enough exhaust gas yet to spin the turbine fast enough to create the required pressure at the compressor outlet. So what happens? The Aneroid Capsule is expanded, and that closes the Bleed Valve in the Absolute Pressure Controller. Closing the bleed valve traps oil inside the wastegate actuator, and that trapped oil pushes the actuator’s piston, which closes the wastegate fully. So right after start, the wastegate slams shut to force all exhaust through the turbine, building up speed. When you open the throttle, now there’s enough exhaust gas to turn the turbine at a speed that lets the compressor achieve more than the required pressure at its outlet. That increased pressure is sensed at the Absolute Pressure Controller. The APC responds by releasing oil through the bleed valve from inside the wastegate actuator. With the oil released, the internal spring starts opening the wastegate again. The wastegate continues to open as you open the throttle further, until at full throttle at Sea Level ISA pressure, it’s almost fully open. Now here’s a subtle but critical point: that extra wastegate opening is there to cater for days when the ambient pressure is greater than ISA. If the wastegate couldn’t open further, there would be no way to reduce turbine speed to keep the compressor outlet pressure within limits. So the wastegate isn’t just a simple on-off valve; it’s a modulating control that dumps excess exhaust to keep pressure in check. From the moment of take-off, and throughout the climb, the pressure at the compressor inlet falls, and that causes the compressor outlet pressure to fall too. This drop in outlet pressure is signalled to the APC, which closes the bleed valve, trapping oil in the wastegate actuator, and that progressively closes the wastegate. So as you climb, the wastegate gradually shuts to force more exhaust through the turbine, maintaining compressor outlet pressure. Eventually, the wastegate will be fully shut, and no more increase in turbine speed is possible. That point is called the Critical Altitude. Above that, the compressor outlet pressure will fall, and the inlet manifold pressure and engine power output will fall in sympathy. Now, from the moment of take-off, engine power decreases with every foot of climb — that’s typical of a turbocharged engine. But after Critical Altitude, the decrease gets greater, approximating that of a normally aspirated engine. So the turbocharger gives you sea-level power up to Critical Altitude, then you degrade like a normally aspirated engine. Let me show you the relationship visually. This figure shows how wastegate position, engine power, manifold pressure, and turbocharger rpm all relate to each other through this sequence. And compares the power curves of a normally aspirated engine versus a turbocharged engine, so you can see the flat power output up to Critical Altitude, then the fall-off. Now, the excerpt also mentions Alternative Turbocharger Control — other types of turbocharger control are described, and there’s a Figure 11.7 for that. But the text doesn’t give the details of those alternatives here, so we’ll leave that for when we get to it. Let me make sure you’ve got the sequence locked in: before start, wastegate open, spring holds it. After start, not enough exhaust, so aneroid expands, bleed valve closes, oil traps, wastegate closes fully. Open throttle, pressure builds, APC senses it, bleed valve opens, oil releases, spring opens wastegate progressively. At full throttle sea level ISA, almost fully open. Climb, inlet pressure falls, outlet falls, APC closes bleed valve, oil traps, wastegate progressively closes. Eventually fully shut — that’s Critical Altitude. Above that, power falls like a normally aspirated engine. That’s the complete operating cycle