Supercharging and Turbocharging

A piston engine's power depends on the mass of air it can take into its cylinders on each stroke. As an aeroplane climbs, the air thins and a naturally aspirated engine loses power steadily. Supercharging compresses the induction air before it reaches the cylinders, restoring the density that altitude takes away and, on some engines, adding more. The compressor is either geared to the engine, a supercharger, or driven by the exhaust, a turbocharger.

For the pilot, a boosted engine brings new limits and new handling: manifold pressure has an upper limit as well as a lower one, the throttle must be moved with care, and power is held only up to a certain altitude. The basic engine is described in piston engine principles.

Naturally aspirated engines and altitude

A naturally aspirated engine, also called a normally aspirated engine, draws in air at the pressure of the atmosphere around it. At full throttle its manifold pressure is slightly below atmospheric, because of losses in the intake, and it falls as the atmospheric pressure falls with height. The mass of each charge falls with it, and so does the power. A common rule of thumb is a loss of roughly 3 per cent of sea-level power for every 1,000 ft of density altitude, leaving about 75 per cent at 8,000 ft.

Manifold pressure (MAP) is the absolute pressure in the induction manifold, downstream of the throttle. With the engine stopped, the gauge reads the ambient pressure; at idle, with the throttle nearly closed, it reads well below it. The throttle of a naturally aspirated engine can only reduce manifold pressure below ambient. A supercharged engine can raise it above.

Internally driven superchargers

A supercharger in the strict sense is a compressor, usually a centrifugal impeller, driven by the engine through gears from the crankshaft. Because it is geared to the engine, it responds instantly to the throttle, but it absorbs part of the engine's power to drive it.

This kind of supercharger has no wastegate; the throttle limits the boost. At sea level the compressor could deliver far more pressure than the cylinders can stand, so the throttle is kept partly closed, or choked, to hold manifold pressure within limits. As the aeroplane climbs and the ambient pressure falls, the throttle is opened progressively to keep the same boost. The height at which it reaches fully open is the full throttle height (FTH). Above it the supercharger can no longer compensate, and power falls with further climb. A typical internally driven supercharger can hold sea-level manifold pressure at rated power up to about 5,000 to 10,000 ft.

Up to the full throttle height, with boost held constant, power actually rises slightly with height: the colder air is denser for a given manifold pressure, and the lower exhaust back-pressure improves scavenging.

The full throttle height depends on what is being asked of the engine:

• Climbing at rated rpm with less than rated boost raises the FTH, because less boost is needed and the throttle reaches fully open higher up.
• Climbing at rated boost with less than rated rpm lowers the FTH, because the slower impeller delivers less, and the throttle must be opened further, sooner.

For the same reason, reducing rpm at a given throttle setting reduces the boost, and the throttle must be opened to restore it.

Exhaust-driven turbochargers

A turbocharger uses the energy in the exhaust gas, which a naturally aspirated engine throws away. The exhaust drives a turbine, and the turbine drives a compressor on the same shaft, which compresses the induction air. The turbocharger costs the engine almost no shaft power, but it has two drawbacks: it responds more slowly, since the turbine must spool up before boost rises, and it raises the back-pressure in the exhaust. Its shaft bearing is lubricated by engine oil.

View full schematic

Wastegate control

The wastegate is a valve in the exhaust that decides how much gas passes through the turbine and how much bypasses it straight to atmosphere. Closing the wastegate forces more exhaust through the turbine, spins the compressor faster and raises boost; opening it reduces boost.

In the usual light-aircraft installation, engine oil pressure closes the wastegate against a spring, under the control of an automatic controller. That gives a characteristic sequence:

• Before start, there is no oil pressure, so the spring holds the wastegate fully open.
• At full throttle at sea level, the wastegate is almost fully open: the engine has plenty of exhaust energy and the turbine needs only part of it. The remaining opening is held in reserve for days warmer than standard.
• In the climb, as ambient pressure falls, the controller closes the wastegate progressively, forcing more exhaust through the turbine to hold the selected manifold pressure.
• At the critical altitude, the wastegate is fully closed.

The design fails safe. If the oil supply to the actuator is lost, the spring drives the wastegate fully open, all the exhaust bypasses the turbine, and the engine behaves as a naturally aspirated one; failing closed would risk an uncontrolled overboost. If the wastegate seizes part-open, it cannot close further in the climb, and manifold pressure falls with height as on a naturally aspirated engine. Some simpler installations have a manually controlled wastegate instead of an automatic one, which demands more care with the throttle.

Ground-boosted and turbo-normalised engines

Turbocharged engines fall into two groups:

• A turbo-normalised engine, a practice called turbonormalizing in American usage, uses the turbocharger only to restore what the climb takes away. It holds roughly sea-level manifold pressure, and so roughly sea-level power, up to its critical altitude.
• A ground-boosted engine is certified to run at manifold pressures above the sea-level value, so it gives more than its unboosted power even on the ground.

In both, the wastegate closes progressively during the climb, and once it is fully closed the critical altitude has been reached.

Intercooling the charge air

Compressing air heats it. Hot charge air is less dense, which costs part of the gain the compressor was meant to deliver, and it brings the engine closer to detonation. An intercooler, a heat exchanger between the compressor and the induction manifold, removes much of that heat. It restores charge density, and so power, and widens the margin from detonation. The mechanism of detonation is explained in mixture control and abnormal combustion.

Boost pressure and rated power

Boost pressure is the pressure the compressor delivers to the induction manifold. Most light aircraft show it directly as manifold pressure in inches of mercury. Some engines, older ones in particular, have a boost gauge reading in pounds per square inch above or below standard sea-level pressure instead, so zero boost corresponds to about 29.92 inHg, and each pound of boost is roughly 2 inHg of manifold pressure.

Static boost is the reading of the boost or manifold pressure gauge with the engine stopped, which is simply the ambient pressure. It is used in a ground power check: with the throttle set so that the manifold pressure equals static boost, the engine should give its reference rpm within a small tolerance, about 50 rpm. A result outside the tolerance points to a cylinder down on power, an ignition or carburation fault, or a wrongly set propeller fine-pitch stop.

An engine's rated power is the power it is certified to deliver at its rated manifold pressure, or boost, and rated rpm. In the FAA's definitions, rated maximum continuous power is approved for unrestricted periods of use, while rated takeoff power is limited to periods of not more than 5 minutes for take-off. Exceeding either the manifold pressure or the rpm limit is not allowed.

Critical altitude and full throttle height

The critical altitude of a turbocharged engine is the highest altitude at which the turbocharger can still hold the rated manifold pressure. The FAA definition is framed more generally, as the maximum altitude at which, in the standard atmosphere, a specified power or manifold pressure can be maintained at a specified rpm. Below it, a turbocharged aeroplane's climb performance stays much closer to its sea-level values than that of a naturally aspirated one; above it, manifold pressure and power fall with further climb, and performance declines as it would on a naturally aspirated engine. See climb performance.

Full throttle height is the equivalent term for an internally driven supercharger, where the throttle rather than a wastegate is the regulating device.

Near the critical altitude, with the wastegate fully closed and no regulating reserve left, manifold pressure may drift up and down without any control being moved. This bootstrapping is a self-sustaining oscillation: a small change in exhaust flow changes turbine speed, which changes boost, which changes the exhaust flow again.

Overboost and turbocharger care

Overboost is a manifold pressure above the engine's limit. It is most likely on take-off, and especially on a cold morning: the dense air lets the compressor build pressure quickly, while the automatic controller, with cold, viscous oil, is slow to open the wastegate. A rapid throttle movement, or a manual wastegate, makes it more likely. Overboost stresses the cylinders and can cause detonation. Some engines have an overboost relief valve, but the pilot still advances the throttle smoothly and slowly, watches the manifold pressure and respects the red line.

Turbochargers also need care after the flight. The turbine runs extremely hot, and stopping the engine immediately after high-power operation also stops the oil flowing through its bearing. Residual heat can then bake the stagnant oil into carbon, turbocharger coking, which progressively destroys the bearing. The flight manual therefore calls for a period at idle before shutdown.

A turbocharger failure in flight is a significant loss of power at altitude. With the compressor seized, the engine can at best give naturally aspirated power for that height, and the seized turbine can obstruct the exhaust and cost more. The pilot descends and plans for a precautionary landing. Because a boosted engine runs a hotter charge at higher pressure, the mixture, cowl flaps and cylinder head temperature need closer attention than on a naturally aspirated engine throughout the flight.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.