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Feb 26, 2024

How Do Turbochargers & Superchargers Work In Piston Engines?

Many have heard the terms; today we break them down. Engines produce power by burning air and fuel. The air and fuel together are known as a mixture, sometimes called charge. In a piston engine, the

Many have heard the terms; today we break them down.

Engines produce power by burning air and fuel. The air and fuel together are known as a mixture, sometimes called charge. In a piston engine, the engine's power output depends on the quantity or weight of the charge that can be accepted by the cylinders in the engine.

The weight of the mixture, which can be sucked into the cylinders by the pistons, largely depends on its temperature and pressure. As an aircraft climbs, the reduced air density results in reduced air pressure, and ultimately less oxygen will enter the cylinders. For this reason, a normally aspirated engine loses power with increasing altitude.

To augment the power of a piston engine, it can be either turbocharged or supercharged.

A turbocharger comprises a turbine and a compressor assembly. The turbine and the compressor are fitted on the same shaft; thus, when the turbine rotates, so does the compressor.

A turbocharger's compressor is directly connected to the intake system of the cylinders, while the turbine is connected to the exhaust system. The compressor is also exposed to the air inlet of the engine.

Turbochargers can increase the engine power for takeoff and allow aircraft to climb to higher altitudes. A normally aspirated engine can only produce sea-level pressure of 29.92 inches of Mercury. As it climbs, the pressure falls off due to the reduced air density. With a turbocharger, an engine can generate a lot more power. For example, one the most powerful piston engines ever built, the Pratt & Whitney R-4360, which is supercharged, can produce a manifold pressure of 60 inches of Mercury on takeoff, just over double normal atmospheric pressure.

Whether an engine is turbocharged or not, it will still lose power as the aircraft climbs. However, with a turbocharged or a supercharged engine, the engine loses power at a much lower rate. This facilitates higher altitudes and attaining greater speeds.

When a turbocharger rotates, air flows into the compressor, increasing the air pressure. Compressors used in turbochargers are mostly centrifugal and are made of two main parts: an impeller and a diffuser. As the air hits the impeller, it gets accelerated and is thrown outwards of the impeller. From the impeller, the air, which has now gained significant kinetic energy, is passed into the diffuser. The diffuser consists of vanes that form divergent passages. When the airflow goes through these passages, the increased area decreases the velocity of the air, increasing its pressure. It's a simple conversion of kinetic energy (velocity) into potential energy (pressure).

A turbocharger's turbine assembly is what runs the compressor. For this, exhaust gases from the engine exhaust are directed at the turbine, causing it to turn. The amount of exhaust gases allowed to flow into the turbine is controlled by something called a wastegate.

This is the most essential part of a turbocharger as it controls its speed or rotation. The wastegate is essentially a valve mechanism that controls the amount of exhaust that flows into the turbine of the turbocharger. When the wastegate is fully closed, almost all the exhaust gases from the engine are directed to the turbine. This increases the rotation speed of the turbine and that of the turbocharger.

The wastegate needs to be appropriately controlled to prevent an over-boost situation. Overboosting is a condition where the engine experiences too much pressure, causing severe damage. There are two methods by which the wastegate can be controlled. One is by giving wastegate control to the pilot, and the other (which is the most preferable method) is to have an automatic mechanism to control it.

The Absolute Pressure Controller (APC) controls the wastegate automatically to prevent an over-boost. The APC contains an aneroid capsule that can sense the pressure at the compressor outlet of the turbocharger. It uses oil pressure from the engine lubrication to control the wastegate actuator, which features a spring mechanism. When too much pressure is detected at the compressor outlet, the APC drains oil from the wastegate actuator, causing the spring mechanism to open the wastegate, allowing some of the exhaust gases to escape into the atmosphere. When the engine needs power, oil is sent to the wastegate actuator when commanded by the APC, closing it and redirecting exhaust into the turbine.

For example, during takeoff, as the pilot opens full throttle, the engine runs at its maximum capacity, producing many exhaust gases, which can lead to an over-boost. The APC detects the increased pressure causing the wastegate to open. After takeoff, as the aircraft climbs, the compressor outlet pressure decreases. This reduction in pressure is again detected by the APC, which commands the wastegate to start closing to increase the rotation speed of the turbocharger.

At some point in the climb, the wastegate will be fully closed, and the turbocharger will attain its maximum speed. The altitude at which this happens is called the Critical Altitude. Above this altitude, the loss of engine power in a climb gets greater and greater, reaching similar values to that of a normally aspirated engine.

A turbocharger and a supercharger achieve the same goal in two different ways. A turbocharger is mostly a separate entity from the engine. Sometimes a turbocharger is also called an externally driven supercharger.

A supercharger is internally driven and is a part of the engine. Unlike a turbocharger, a supercharger does not have a turbine. It only has a compressor run by the engine's crankshaft. Because the engine itself rotates a supercharger, they are only found in very potent engines. I a less powerful engine is used, it may not be able to bear the load of a supercharger.

As a supercharger also increases the air pressure, like a turbocharger, speed must be controlled to prevent excessive pressure in the engine. In a supercharger system, the speed of it is governed by the engine power lever and the RPM or propeller control lever.

The power lever determines the amount of air directed into the supercharger's compressor. As the power lever is moved forward, more air is sent into the compressor, increasing the pressure of the air delivered to the engine. For this reason, the power lever or the throttle of a supercharged engine is sometimes called a boost control lever.

An RPM lever controls the propeller blade angle. Moving the lever forward makes the blades finer or flat, increasing the speed or the RPM. And moving the lever aft coarsens the blades, decreasing the RPM. This function can significantly affect a supercharger's rotational speed.

The combination of the power lever and the RPM lever determines the supercharger's performance.

The Automatic Boost Control (ABC) is a system in a supercharged engine that automatically controls the throttle valve to maintain a constant boost pressure.

High density on the ground and at low altitudes may result in the supercharger producing excessive pressure, which can damage the engine in the long run. In this case, the throttle valve cannot open fully to limit the amount of air entering the supercharger and, thus, the engine.

At high altitudes, pressure is lost due to reduced air density, detected by the ABC, which opens the throttle valve.

When the engine is idling on the ground, the throttle valve is partially opened so that there is no risk of overpressure. Remember that as the engine turns, so does the supercharger. On the ground, with higher pressures, combined with the pressure increment from the supercharger, an overpressure may develop inside the engine.

As the aircraft climbs, the throttle valve is opened more and more to compensate for pressure loss until an altitude is reached where the throttle valve is fully forward. This altitude is known as Full Throttle Altitude (FTH).

When an aircraft is allowed to climb at its Rated RPM and Rated boost, its FTH is known as its Rated Altitude, but pilots can either increase or decrease the FTH altitude.

If the intention is to increase the FTH altitude, the aircraft can be flown at Rated RPM with a Less than Rated boost. In this condition, as the RPM or the engine speed is maintained at its rated value, the boost controller slowly opens the throttle valve during the climb, increasing the FTH altitude.

If the intention is to decrease the FTH altitude, the aircraft can be flown at Rated boost with Less than Rated RPM. Since the RPM or engine speed is less than its rated value, the boost controller quickly opens the throttle valve during the climb, increasing the FTH altitude.

Journalist - An Airbus A320 pilot, Anas has over 4,000 hours of flying experience. He is excited to bring his operational and safety experience to Simple Flying as a member of the writing team. Based in The Maldives.