07-02-2013, 10:05 AM
HIGH PERFORMANCE AIRCRAFT ENGINES
HIGH PERFORMANCE .ppt (Size: 1.21 MB / Downloads: 264)
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
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
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.
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.
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.
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.