SUPERCHARGERS AND TURBOCHARGERS

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SUPERCHARGERS AND TURBOCHARGERS
What is a super charger?
A supercharger is an air compressor used for forced induction in internal combustion engines.
Need for supercharger .
Heat loss: to the environment and in the form of flue gasses .
Increased Work O/P: to increase the work o/p per cycle by increasing the total mass flow rate.
Altitude effect: with the rise in altitude , power o/p of the IC engines fell considerably due to lesser availability of oxygen.
History .
By the late 1800s, German engineer called Krigar invented an air pump that utilized twin rotating shafts that compressed air.
In 1860 patented the design for an air mover was patented for use in blast furnaces
The combination of the pair of inventions resulted in a third, with the first functional supercharger attributed to German engineer Gottlieb Daimler supercharged an internal combustion engine in 1885.
And first supercharged race car was built by Lee Chadwick of Pottstown, Pennsylvania in 1908, which, it was reported, reached a speed of 100 mph (160 km/h).
Types Of supercharger
There are two main types of superchargers defined according to the method of compression:
Positive displacement .
Dynamic compressors.
Types of supercharger ..
Positive Displacement
Provides a constant compression with Increasing RPM
Roots Type.
Lysholm compressor.
Reciprocating Compressor
Dynamic Displacement
Rate of compression increases with increase in RPM
Centrifugal Compressor
Vane blower
Reciprocating Compressor
primitive type of compressors.
The size of the compressor is decided by the volume of the air supplied .
compressor very bulky and heavy.
isentropic efficiency 75-80%.
Speed is the limiting factor.
very rarely used these days.
Lysholm Compressor
positive displacement rotary supercharger.
The air is admitted at one end and trapped b/w the helical rotors and the casing.
The screw action of the rotors displaces the air axially.
The compression is produced internally .
Disadvantage:
Disadvantage of this compressor model is its mechanical complexity.
Centrifugal Compressor
The air enters the hub of the impeller and is turned 90 degrees by the radial vanes.
A high velocity is imparted to the air due to centrifugal action.
This high velocity air from the tip of the radial vanes is passed to a diffuser where the pressure is further raised and the supplied to the engine.
Efficiency Aspects
isentropic efficiency 70-80% .
Very high pressure can be obtained at pressure ratio 2:1.
Mechanical efficiency of about 90-95% .
Operated at speeds of 15000- 30000rpm.
Drawback:
Occurrence of surge if the compressor is working near surge limit any reduction in flow will reduce compressor efficiency.
This in turn reduces pressure ratio ,and hence further reduces air flow to abnormal limits.
Roots type Supercharger
It is named for the brothers Philander and Francis Marion Roots who first patented the basic design in 1860 as an air pump for use in blast furnaces and other industrial applications.
In 1900, Gottlieb Daimler included a Roots-style supercharger in a patented engine design, making the Roots-type supercharger the oldest of the various designs now available.
Working
Roots blower is a positive displacement lobe pump .
operates by pumping fluids with a pair of meshing lobes.
Fluid is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust.
It is frequently used as supercharger in engines, where it is driven directly from the engine's crankshaft via a belt or by spur gears.
Effects of supercharging
Power out put
Increases as work O/P Per cycle is increased.
Better and complete comb of fuel due to better scavenging.
Mechanical Efficiency:
Due to increased pressure large bearings and heavier
components are needed.
Increase in frictional forces.
Fuel Efficiency:
Otto engines use rich mixtures whereas diesel engines use lean mix.
Otto engines have higher specific fuel consumption in super charging.
Turbocharging
When the driving power for the compressor comes from the exhaust gas turbine it is called a turbosupercharger or simply turbo charger.
Types Of Turbochargers
Constant Pressure Turbocharger.
Pulse Turbocharging.
Pulse Converter Turbocharging.
Working ..
Constant Pressure
Exhaust of all the cylinders is
discharged into a common
manifold.
Pressure is higher then the atmospheric pressure.
gasses expand to a constant pressure condition and pass to the turbine
Pulse Turbocharging
The blow-down energy is converted into pulse as soon as exhaust valve open.
pulses led to the turbine through the shortest & narrowest path .
expansion done wholly at the turbine.
Drawbacks ..
Constant Pressure
Engine dimensions are increased.
Slow acceleration.
Reduced part load efficiency.
Scavenging is difficult due to constant pressure condition.
Pulse turbocharging
Energy recovery is poor at high pressure.
exhaust arrangement is complicated.
Poor efficiency at high charging
Scavenging can be disturbed if pulse takes long to commute.
Pulse converter
Different branches of the exhaust manifold are connected together in a venturi junction called the pulse converter.
Pulse system provides the max utilization of the effluent energy.
Turbine runs at full admission
Comparative Study
Supercharger
superchargers may absorb as much as a third of the total crankshaft power of the engine
thermal efficiency & power O/P of supercharger is lower.
mechanically-driven supercharger has better throttle response
Turbocharger
Turbocharger as driven by exhaust is indep. of the engine.
Higher then supercharger.
Thermal spool is the main drawback.