SUPERCHARGING AND TURBOCHARGING HIGH PERFORMANCE AIRCRAFT ENGINES POWER

SUPERCHARGING AND TURBOCHARGING HIGH PERFORMANCE AIRCRAFT ENGINES

POWER MANAGEMENT • The power created by a reciprocating engine is a product of MAP (manifold absolute pressure) and rpm. • If rpm remains constant and MP is increased, power output will be increased. • If MP remains constant and rpm is increased, power output will be increased.

FACTORS AFFECTING POWER • Humidity- water vapor in the air takes the place of oxygen molecules. The molecular weight of water vapor is less than oxygen; as a result moist air is less dense than dry air. Less dense air means decreased performance. • Temperature- temperature affects air density which affects performance: T↑, d↓, P↓ • Mixture- fuel: air ratio • Ambient Pressure- the pressure altitude at the aerodrome affects air density and performance: p↓, d↓, P↓

FACTORS AFFECTING POWER • Friction loss- as the air flows through the intake system it loses pressure due to skin friction and rounding corners. This factor is based on intake system design. • Altitude- as an aircraft climbs the air becomes less dense and power decreases as a result. As we climb the throttle must be opened further to maintain climb power. • Critical altitude- the altitude where throttle is fully open in order to achieve the desired power setting. (normally aspirated engine)

POWER SETTINGS • The power setting used for cruise flight is a trade-off between fuel economy, engine longevity, and speed. • 100% power is only used for takeoff and initial climb. 100% power will only be available under certain altitude and temperature conditions. • Normal cruise power setting is usually 65%.

RULES TO PROLONG ENGINE LIFE • Always observe manufacturer operating limitations. • Make throttle, propeller rpm, and mixture changes slowly and smoothly. Abrupt changes put large stresses on engine components. • Keep rpm high during power changes to avoid high cylinder pressures and stresses. • Power increase: MPT • Power decrease: TPM

RULES TO PROLONG ENGINE LIFE • Be aware of thermal shock. Avoid large power reductions. Reduce power in increments. Manage cowl flaps correctly. • Periodically warm the engine in prolonged power off descents. • Use full rich mixture when operating at full or near full power.

RULES TO PROLONG ENGINE LIFE • Be aware of conditions conducive to the formation of carb ice. • Preheat a cold engine before start. Idle at low rpm until the oil pressure and temp. is within operating parameters. • Allow turbocharged engines to idle for a few minutes before shutdown to allow components to cool.

GIVE ME MORE AIR • Normally aspirated engine power is limited by the ambient air density. • By utilizing supercharger or turbocharger systems we can increase engine power output through larger range of atmospheric conditions. • This is done by supplying the engine with higher manifold pressures than normal aspiration.

WHY FLY HIGH? • Get above weather, icing, turbulence, and terrain. • Utilize stronger winds aloft. • Lower traffic density. • Ensure radar coverage for ATC assistance. • Go faster. Aircraft fly faster at altitude given the same power setting. The thinner air produces less parasite drag. • Conserve fuel. Aircraft engines require less fuel at altitude due to the less dense air.

SUPERCHARGERS • Usually compress the fuel/air mixture after it leaves the carburetor. • A supercharger is driven directly from the engine. • Some of the power created is offset by the power required to drive the supercharger. • The amount of supercharging done is limited by the temperatures produced to avoid detonation problems.

SUPERCHARGERS • Each increase in air/fuel mixture pressure is called a stage. • Single-stage, two-stage, multi-stage. • Superchargers may also be geared to operate at variable speeds. • Single-speed, two-speed, variable-speed. • EX: single-stage, two-speed supercharger. • Multi-speed superchargers are used to control supercharger output at different altitudes. (higher output for higher altitudes)

SUPERCHARGERS • Superchargers are usually built as an integral part of the engine. • There most common aviation application is on high powered radial engines. • The air entering the induction system is controlled by the throttle valve. • The fuel is mixed with air in the carburetor. • The fuel/air mixture enters the supercharger, where an impeller (centrifugal compressor) compresses the mixture. • This compressed mixture is fed to the cylinders via the intake manifold.

SUPERCHARGERS EXHAUS T GASES FUEL/AIR MIXTURE

ADVANTAGES/DISADVANTAGES • Advantages: • Improved performance at altitude. • More power for take-off. • Disdvantages: • Power gain is offset by power used by engine to drive supercharger. • Increased temperature of fuel/air mixture increases risk of detonation.

TURBOCHARGERS • Turbochargers deliver compressed air to the inlet side of the carburetor or fuel control unit. • Unlike a supercharger, they are driven by the exhaust gases produced by the combustion process. • In this way turbochargers harness some of the unused energy contained in the hot exhaust gases. • A ground boosted turbocharged engine will produce MP on the ground higher than ambient pressure in order to achieve its rated power. • A turbo-normalized engine will maintain sea level performance to higher altitudes.

EXHAUS T INTAKE AIR CARBURETOR

TURBOCHARGERS • The turbocharger consists of a compressor assembly, exhaust gas turbine assembly, and a pump and bearing casing. • The compressor assembly is made up of a housing which directs air flow and a compressor wheel (impeller). • The exhaust gas turbine assembly is made up of a housing which directs exhaust gas flow and a turbine wheel. • The center casing contains a housing which directs cooling oil around the shaft linking the turbine and compressor. The shaft is suspended by bearings which reduce the heat created by

TURBOCHARGERS • The impeller/compressor, turbine wheel, and connecting shaft together are called the rotor. • At no time in the process do the exhaust gases come into contact with the compressed air. • Turbocharger output is controlled by the wastegate.

WASTEGATE • The wastegate controls the amount of exhaust gases directed to the turbine wheel of the turbocharger. • If the wastegate is open all the exhaust gas is vented overboard through the exhaust system bypassing the turbocharger. (zero boost) • As the wastegate is closed, more and more exhaust gas is directed to the turbocharger until the wastegate is fully closed. (max. boost) • There are manual and automatic wastegates.

FIXED WASTEGATE • Without a wastegate, the amount of boost that a turbocharger creates varies with the pressure of the engine's exhaust. This happens because exhaust pressure varies with relation to the engine's speed (measured in RPM's). This implies that as an engine reaches higher RPM's, increasing amounts of boost will be created by the turbocharger. The problem with this is that an engine can only accommodate a given amount of boost. • With this type of installment the pilot is responsible for maintaining MP within limits through throttle setting.

MANUAL WASTGATE • A manual wastegate is controlled by the pilot through linkage and a flight deck control. • On some installations it is possible to overboost the engine during takeoff if the wastegate is not managed properly. • As the aircraft climbs to its normally aspirated critical altitude, the pilot begins to close the wastegate through the climb. • The altitude at which the wastegate is fully closed and MP pressure can no longer be maintained is the critical altitude. • During descent the pilot must open the wastegate in increments to ensure engine operating

AUTOMATIC WASTEGATE • Automatically adjusts wastegate position based on throttle position. • This is achieved through a density controller and a differential pressure controller. • Engine oil pressure is used to control a wastegate actuator. • Oil pressure moves the wastegate to the closed position, while a spring moves it to the open position. The density controller and differential pressure controller adjust the amount of oil which is bled away form the actuator to control wastegate position.

AUTOMATIC WASTEGATE • Deck pressure- pressure between compressor discharge and the throttle valve. • Manifold pressure- pressure in the intake manifold downstream from the throttle valve. • Critical altitude- The altitude at which the wastegate is completely closed, and manifold pressure will start to drop if the climb is continued. From this point it acts as a normally aspirated engine.

DENSITY CONTROLLER • The density controllers job is to limit maximum MP preventing overboost. • It limits deck pressure while the aircraft is below the turbochargers critical altitude. • If deck pressure becomes too high, the density controller bleeds more oil from the wastegate actuator causing the wastegate to open.

DIFFERENTIAL PRESSURE CONTROLLER • The differential pressure controller senses the air pressures on either side of the throttle plate (deck pressure/manifold pressure) and acts to maintain an optimum balance between a low turbocharger workload and a quick spool-up time. • It controls oil bled from the wastegate actuator to maintain a pre-set pressure differential of approx. 2”-3”. • This ensures the system will respond quickly and smoothly to throttle changes. Helps control bootstrapping.

TURBOCHARGED ENGINE TRAITS • Bootstrapping- any change in engine rpm or temperature will change the amount of exhaust gas flowing to the turbine. This will cause an increase or decrease in boost. The resultant fluctuation of MP is called bootstrapping. It is most pronounced when the wastegate is fully closed. • Overboost- manifold pressure exceeds the limits of the engine. • Overshoot- a turbocharged engine is more sensitive to throttle changes than a normally aspirated engine. Smooth throttle control is needed to avoid MP drift. • Cool down- the rotor of a turbocharger is subject to intense temperatures due to the high rpm. A cool down period of idle operation before shutdown is necessary with most installations.

INTERCOOLER • The compression of air by the turbocharger creates a temperature rise in the induction air which could result in detonation. • An intercooler cools the induction air after compression. • The intercooler acts as a air to air heat exchanger.

TURBO/SUPERCHARGED

PRESSURIZATION • The compressed air form a turbocharger is commonly used to provide a source of pressurized cabin air.