[THEORY] - INDUCTION, EXHAUST, FUEL SYSTEMS

The throttle bar and wastegate control the turbocharger. The wastegate is a butterfly bar, and for simple control, it's linked to the throttle. When starting, the wastegate is typically fully open, meaning the compressor isn't initially running. As the throttle is advanced, especially for takeoff at low density altitude (low altitude, high air density), the wastegate may be nearly fully open. As altitude increases, power decreases, requiring the pilot to close the wastegate to maintain power, eventually leading to a fully closed wastegate at critical altitude.

Density altitude refers to altitude, not density directly. Lower altitude means lower density altitude, which translates to higher air density. As you climb, air density decreases. During engine start, some manufacturers keep the turbocharger off. However, some systems, from sea level, immediately use turbocharging. The wastegate controls turbine speed; closing it directs more exhaust to the turbine, speeding it up. This, in turn, allows more air into the cylinders as the throttle opens.

There are different types of manual wastegate control. One allows the pilot to set the wastegate position manually from the cockpit. A simpler type uses an adjustable restrictor, which is set by a mechanic on the ground and cannot be changed by the pilot during flight. This fixed setting determines how much exhaust bypasses the turbine, making it less optimal than dynamic control but simpler. An optimally operating wastegate is fully closed to maximize efficiency.

Automatic control systems for turbochargers are now common. The wastegate actuator is often normally open and connected to a piston and spring mechanism. Oil pressure builds up to push the piston, closing the wastegate. This increases exhaust flow to the turbine, speeding up the compressor and increasing upper deck pressure. The Absolute Pressure Controller (APC), a sensor measuring upper deck pressure, regulates this automatically.

Manifold pressure is measured just outside the intake port of the cylinder. Over-boosting can cause induction piping to burst, so manufacturers define limits for maximum pressure. A manifold pressure gauge indicates this. Unlike naturally aspirated engines where manifold pressure is always lower than ambient, turbochargers can create pressures higher than ambient.

The relief valve primarily releases excessive upper deck pressure and does not directly control the wastegate. The APC and ratio controller, however, directly influence the wastegate by regulating oil flow to its actuator. Blocking oil flow builds pressure, closing the wastegate, while allowing it to flow opens the wastegate. The ratio controller measures the pressure difference between the upper deck and ambient air, crucial for maintaining cabin pressurization at higher altitudes.

More complex systems, especially for sea-level boosted turbochargers, incorporate a density controller and a differential pressure controller. The density controller compares upper deck air density with a stored nitrogen pressure. If upper deck pressure is too low, it closes the wastegate to increase boost. However, its response can be delayed, potentially leading to over-boosting. The differential pressure controller provides a quicker response by measuring the difference between upper deck and manifold pressure, preventing over-pressurization by rapidly opening the wastegate.

Beyond driving the compressor, exhaust energy can be recovered using a power recovery turbine or turbo compound system. This turbine, driven by exhaust gases, converts heat energy into mechanical energy. This mechanical energy is then fed back to the engine's gearbox and crankshaft, providing additional power to the propeller, a more efficient use of otherwise wasted exhaust energy.

Reciprocating exhaust systems come in two main types: short stack and collector. Exhaust components like mufflers and tailpipes are crucial. Inspections are essential to check for cracks, which could lead to carbon monoxide leaks. Exhaust systems also play a role in noise reduction and engine cooling.

An augmented tube exhaust system, where the exhaust pipe is not directly welded to the tube, improves engine cooling. The high-velocity exhaust gases exiting the tube create a low-pressure area (Bernoulli's principle). This low pressure sucks surrounding air from the engine compartment, increasing the flow of cooling air over the cylinders. This enhances the engine's cooling efficiency by drawing more external air through the cowling.