Supercharging and Turbocharging Flashcards
Supercharger
A piston engine with an engine driven compressor
May be used to :
1. Boost the manifold pressure at sea level
2. Compensate for reduced density at altitude by maintaining the power available at sea level
Measuring Power Output
In an engine with a CSU the pressure of the mixture in the manifold is a measure of the power output
Power can be monitored on the RPM and manifold pressure gauge
Maintaining Power at Altitude
Pumps high pressure air into the inlet manifold using a compressor, allowing the engine to burn extra fuel and produce extra power
Allows engine to maintain power in the climb
Rated Boost
The highest manifold pressure the engine can tolerate without the danger of detonation
Ground Boosting
When manifold pressure can exceed 30 inches
Can tolerate a manifold pressure greater than normal sea level pressure without the risk of detonation and can boost the inlet manifold pressure above the normal sea level pressure
Engine develops more power at sea level and maintain better power output at altitude
MAP will progressively reduce in a climb, however it will reduce from a higher manifold pressure
Altitude Boosting
To allow the engine to develop the power required at altitude the compressor is able to overboost the engine at sea level and damage it at sea level if full throttle is used
Open the throttle partially at sea level and continue to open it as you climb
At a point in the climb, the throttle will be fully open and max MAP will still be maintained, as you climb higher the MAP will then start to reduce
Detonation Risks
As the supercharger compresses the mixture, the MAP increases and the temperature rises
If the manifold air pressure is too high (overboosted) the temperature limits may be exceeded and there is a risk of detonation
High manifold pressure and low RPM
To Avoid Detonation
Use only approved power settings
Use correct leaning procedures
The Compressor
Air enters at the centre of a rapidly rotating impeller
Inertia causes the air to be thrown outwards, accelerating in the process
It is slowed by passing through diffuser vanes and the kinetic energy is converted to pressure energy
The compressed air then enters the manifold
PLANE = BHP
P = Power L = Length of the stroke A = Area of the piston N = RPM E = Number of cylinders
Increasing the Power
Mixture
RPM
Manifold pressure
Decreasing the Power
Manifold Pressure
RPM
Mixture
Normally Aspirated Engines
Best manifold pressure = 29.92 inches of mercury (1013.25hpa) at sea level
As the aircraft climbs this will gradually decrease at approx 1hpa per 30ft
Power Augmentation Process
Pushing more air into the engine
Impellor
Works 6-12 times faster than the engine
Rated Altitude
The maximum height at which manifold pressure can maintain 30 inches
Manifold pressure increases to rated altitude as the exhaust gases exit more easily as the density of the air decreases and can be higher than 30 inches
Automatic Boost Controller
Controls the amount of air going into the cylinder
Ensures you don’t exceed 30 inches otherwise the cylinder may pop
Boost gauge = tells an increase in PSI according to 30 inches
Types of Superchargers
Geared (internally driven)
Turbochargers (driven externally by exhaust gases)
Geared Superchargers
The impeller is driven from the crankshaft via gears
A clutch arrangement enables them to operate at different speeds
Speeds of Geared Superchargers
Single-speed
Two-speed
Variable speed
Gear Ratios
Usually between 6:1 and 12:1 to give the required boost
It is important to make smooth RPM changes at avoid dramatic acceleration or deceleration of the impeller
Single-Speed Superchargers
Have a high gear ratio (high impeller RPM) which provides optimum performance at high altitude but lower performance near sea level
Two-Speed Superchargers
Have two optimum altitude ranges
Means you can change gears to gain a greater manifold pressure at higher altitude (a greater BHP for longer)
Variable Speed Superchargers
Maintain an impeller speed close to optimum throughout the altitude range of the aircraft
Are more complex in design
Turbocharger
Driven by the exhaust gases by placing a turbine in the exhaust system, which creates a better way to control the speed of the impeller
Exhaust gases drive the turbine and then exit to the exhaust pipe, the turbine controls how fast the compressor turns
Waste Gate
Varies the amount of exhaust gas that drives the turbine
Either fixed or variable
Open Waste Gate
All the exhaust gas goes overboard and the turbine ‘free-wheels’ with the engine working as if normally aspirated
Closed Waste Gate
As the gate is closed, more exhaust gas is directed over the turbine, increasing the compressor speed and boosting the manifold pressure
Advantage of Turbochargers
The compressor does nothing until the throttle butterfly is open
As the throttle is opened further, the waste gate closes and the turbine delivers power to the compressor
Fixed Waste Gate
A fixed proportion of the exhaust gas goes to the turbine all the time
Overboosting is possible at low altitude
At no stage is all the gas available to drive the turbine so the boost is limited
Variable Waste Gate
Movement is controlled by:
- A pilot operated lever
- Throttle operated
- Automatic operation
A Pilot Operated Lever Waste Gate
Allows you to adjust the wast gate manually
During the climb the pilot can gradually close the waste gate and maintain MAP
Throttle Operated Waste Gate
The first portion of throttle movement opens the throttle butterfly
Once the throttle is fully open, any further movement of the throttle closes the waste gate and powers the turbine
Automatic Operation Waste Gate
Controlled automatically to maintain MAP
Waste Gate Control
Fully closed at critical height
Spring opens the waste gate
Oil pressure closes the waste gate
At higher altitude settings, the oil pressure increases, which closes the gate, as the oil aneroid capsule increases pushing the needle down stopping oil flow and creating a build up
Density Controller
Takes into account pressure and temperature
Advantages of Increased Manifold Pressure
Can carry more weight
Require less runway
Full Throttle Height
The height at which a certain power is available with only full throttle
Full Throttle Height In a Supercharged Engine
The altitude at which the rated boost is available at a certain RPM (any lower MAP setting will also have a higher full throttle height)
Handling Techniques
Do not exceed rated boost - only use recommended MAP/RPM settings
Avoid sudden throttle movements
Avoid sudden shutdowns
