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Types of supercharger
There are two main types of supercharger defined according to the method of compression: positive displacement and dynamic compressors. The former deliver a fairly constant level of boost regardless of engine speed (RPM), whereas the latter deliver increasing boost with increasing engine speed.
Positive displacement
An Eaton MP62 Roots-type supercharger is visible at the front of this Ecotec LSJ engine in a 2006 Saturn Ion Red Line.
Lysholm screw rotors. Note the complex shape of each rotor which must run at high speed and with close tolerances. This makes this type of supercharger quite expensive. (This unit has been blued to show close contact areas.)Positive displacement pumps deliver a nearly fixed volume of air per revolution at all speeds (minus leakage which is nearly constant at all speeds for a given pressure and so its importance decreases at higher speeds). The device divides the air mechanically into parcels for delivery to the engine, mechanically moving the air into the engine bit by bit.
Major types of positive displacement pumps include:
Roots
Lysholm screw
Sliding vane
Scroll-type supercharger, also known as the G-lader
Piston as in Bourke engine
Wankel engine
Positive displacement pumps are further divided into internal compression and external compression types.
Roots superchargers are typically external compression only (although high helix roots blowers attempt to emulate the internal compression of the Lysholm screw).
External compression refers to pumps which transfer air at ambient pressure into the engine. If the engine is running under boost conditions, the pressure in the intake manifold is higher than that coming from the supercharger. That causes a back flow from the engine into the supercharger until the two reach equilibrium. It is the back flow which actually compresses the incoming gas. This is a highly inefficient process and the main factor in the lack of efficiency of roots superchargers when used at high boost levels. The lower the boost level the smaller is this loss and roots blowers are very efficient at moving air at low pressure differentials, which is what they were first invented for (hence the original term "blower").
All the other types have some degree of internal compression.
Internal compression refers to the air being compressed within the supercharger itself and this compressed air, already at or close to boost level, can be delivered smoothly to the engine with little or no backflow. This is more efficient than backflow compression and allows higher efficiency to be achieved. Internal compression devices usually use a fixed internal compression ratio. When the boost pressure is equal to the compression pressure of the supercharger, the backflow is zero. If the boost pressure exceeds that compression pressure, backflow can still occur as in a roots blower. Internal compression blowers must be matched to the expected boost pressure in order to achieve the higher efficiency they are capable of, otherwise they will suffer the same problems and low efficiency of the roots blowers.
Positive displacement superchargers are usually rated by their capacity per revolution. In the case of the roots blower, the GMC rating pattern is typical. The GMC types are rated according to how many two stroke cylinders, and the size of those cylinders, it is designed to scavenge. GMC has made 2-71, 3-71, 4-71, and the famed 6-71 blowers. For example a 6-71 blower is designed to scavenge six cylinders of 71 cubic inches each and would be used on a two-stroke diesel of 426 cubic inches which is designated a 6-71 and the blower takes this same designation. However because 6-71 is actually the engines designation, the actual displacement is less than the simple multiplication would suggest. A 6-71 actually pumps 339 cubic inches per revolution.
Aftermarket derivatives continue the trend with 8-71 to current 14-71 blowers. From this you can see that a 6-71 is roughly twice the size of a 3-71. GMC also made -53 cubic inch series in 2, 3, 4, 6 and 8-53 sizes as well as a “V71” series for use on engines using a V configuration.
Roots Supercharger Efficiency Map. This generalized roots blower efficiency map shows how a roots blower's efficiency varies with speed and boost.Roots Efficiency map
For any given roots blower running under given conditions, a single point will fall on the map. This point will rise with increasing boost and will move to the right with increasing blower speed. It can be seen that at moderate speed and low boost the efficiency can be over 90%. This is the area in which roots blowers were originally intended to operate and they are very good at it.
Boost is given in terms of pressure ratio which is the ratio of absolute air pressure before the blower to the absolute air pressure after compression by the blower. If no boost is present the pressure ratio will be 1.0 (meaning 1:1) as the outlet pressure equals the inlet pressure. 15 psi boost is marked for reference (slightly above a pressure ratio of 2.0 compared to atmospheric pressure). At 15 psi boost Roots blowers hover between 50% to 58%. Replacing a smaller blower with a larger blower moves the point to the left. In most cases, as the map shows, this will moves it into higher efficiency areas on the left as the smaller blower likely will have been running fast on the right of the chart. Usually, using a larger blower and running it slower to achieve the same boost will give an increase in compressor efficiency.
The volumetric efficiency of the roots type blower is very good, usually staying above 90% at all but the lowest blower speeds. Because of this, even a blower running at low efficiency will still mechanically deliver the intended volume of air to the engine but that air will be hotter. In drag racing applications where large volumes of fuel are injected with that hot air, vaporizing the fuel absorbs the heat. This functions as a kind of liquid after cooler system and goes a long way to negating the inefficiency of the roots design in that application.
Dynamic
Dynamic compressors rely on accelerating the air to high speed and then exchanging that velocity for pressure by diffusing or slowing it down.
Major types of dynamic compressor are:
Centrifugal
Multi stage axial flow
Pressure wave supercharger
Supercharger drive types
Superchargers are further defined according to their method of drive (mechanical—or turbine).
Mechanical:
Belt (V belt, Toothed belt, Flat belt)
Direct drive
Gear drive
Chain drive
Exhaust gas turbines:
Axial turbine
Radial turbine
All types of compressor may be mated to and driven by either gas turbine or mechanical linkage. Dynamic compressors are most often matched with gas turbine drives due to their similar high-speed characteristics, while positive displacement pumps usually use one of the mechanical drives. However, all of the possible combinations have been tried with various levels of success.[attachment=1:10yxp9pa]roots charger.jpg[/attachment:10yxp9pa][attachment=0:10yxp9pa]Lysholm_screw_rotors.jpg[/attachment:10yxp9pa]
Types of supercharger
There are two main types of supercharger defined according to the method of compression: positive displacement and dynamic compressors. The former deliver a fairly constant level of boost regardless of engine speed (RPM), whereas the latter deliver increasing boost with increasing engine speed.
Positive displacement
An Eaton MP62 Roots-type supercharger is visible at the front of this Ecotec LSJ engine in a 2006 Saturn Ion Red Line.
Lysholm screw rotors. Note the complex shape of each rotor which must run at high speed and with close tolerances. This makes this type of supercharger quite expensive. (This unit has been blued to show close contact areas.)Positive displacement pumps deliver a nearly fixed volume of air per revolution at all speeds (minus leakage which is nearly constant at all speeds for a given pressure and so its importance decreases at higher speeds). The device divides the air mechanically into parcels for delivery to the engine, mechanically moving the air into the engine bit by bit.
Major types of positive displacement pumps include:
Roots
Lysholm screw
Sliding vane
Scroll-type supercharger, also known as the G-lader
Piston as in Bourke engine
Wankel engine
Positive displacement pumps are further divided into internal compression and external compression types.
Roots superchargers are typically external compression only (although high helix roots blowers attempt to emulate the internal compression of the Lysholm screw).
External compression refers to pumps which transfer air at ambient pressure into the engine. If the engine is running under boost conditions, the pressure in the intake manifold is higher than that coming from the supercharger. That causes a back flow from the engine into the supercharger until the two reach equilibrium. It is the back flow which actually compresses the incoming gas. This is a highly inefficient process and the main factor in the lack of efficiency of roots superchargers when used at high boost levels. The lower the boost level the smaller is this loss and roots blowers are very efficient at moving air at low pressure differentials, which is what they were first invented for (hence the original term "blower").
All the other types have some degree of internal compression.
Internal compression refers to the air being compressed within the supercharger itself and this compressed air, already at or close to boost level, can be delivered smoothly to the engine with little or no backflow. This is more efficient than backflow compression and allows higher efficiency to be achieved. Internal compression devices usually use a fixed internal compression ratio. When the boost pressure is equal to the compression pressure of the supercharger, the backflow is zero. If the boost pressure exceeds that compression pressure, backflow can still occur as in a roots blower. Internal compression blowers must be matched to the expected boost pressure in order to achieve the higher efficiency they are capable of, otherwise they will suffer the same problems and low efficiency of the roots blowers.
Positive displacement superchargers are usually rated by their capacity per revolution. In the case of the roots blower, the GMC rating pattern is typical. The GMC types are rated according to how many two stroke cylinders, and the size of those cylinders, it is designed to scavenge. GMC has made 2-71, 3-71, 4-71, and the famed 6-71 blowers. For example a 6-71 blower is designed to scavenge six cylinders of 71 cubic inches each and would be used on a two-stroke diesel of 426 cubic inches which is designated a 6-71 and the blower takes this same designation. However because 6-71 is actually the engines designation, the actual displacement is less than the simple multiplication would suggest. A 6-71 actually pumps 339 cubic inches per revolution.
Aftermarket derivatives continue the trend with 8-71 to current 14-71 blowers. From this you can see that a 6-71 is roughly twice the size of a 3-71. GMC also made -53 cubic inch series in 2, 3, 4, 6 and 8-53 sizes as well as a “V71” series for use on engines using a V configuration.
Roots Supercharger Efficiency Map. This generalized roots blower efficiency map shows how a roots blower's efficiency varies with speed and boost.Roots Efficiency map
For any given roots blower running under given conditions, a single point will fall on the map. This point will rise with increasing boost and will move to the right with increasing blower speed. It can be seen that at moderate speed and low boost the efficiency can be over 90%. This is the area in which roots blowers were originally intended to operate and they are very good at it.
Boost is given in terms of pressure ratio which is the ratio of absolute air pressure before the blower to the absolute air pressure after compression by the blower. If no boost is present the pressure ratio will be 1.0 (meaning 1:1) as the outlet pressure equals the inlet pressure. 15 psi boost is marked for reference (slightly above a pressure ratio of 2.0 compared to atmospheric pressure). At 15 psi boost Roots blowers hover between 50% to 58%. Replacing a smaller blower with a larger blower moves the point to the left. In most cases, as the map shows, this will moves it into higher efficiency areas on the left as the smaller blower likely will have been running fast on the right of the chart. Usually, using a larger blower and running it slower to achieve the same boost will give an increase in compressor efficiency.
The volumetric efficiency of the roots type blower is very good, usually staying above 90% at all but the lowest blower speeds. Because of this, even a blower running at low efficiency will still mechanically deliver the intended volume of air to the engine but that air will be hotter. In drag racing applications where large volumes of fuel are injected with that hot air, vaporizing the fuel absorbs the heat. This functions as a kind of liquid after cooler system and goes a long way to negating the inefficiency of the roots design in that application.
Dynamic
Dynamic compressors rely on accelerating the air to high speed and then exchanging that velocity for pressure by diffusing or slowing it down.
Major types of dynamic compressor are:
Centrifugal
Multi stage axial flow
Pressure wave supercharger
Supercharger drive types
Superchargers are further defined according to their method of drive (mechanical—or turbine).
Mechanical:
Belt (V belt, Toothed belt, Flat belt)
Direct drive
Gear drive
Chain drive
Exhaust gas turbines:
Axial turbine
Radial turbine
All types of compressor may be mated to and driven by either gas turbine or mechanical linkage. Dynamic compressors are most often matched with gas turbine drives due to their similar high-speed characteristics, while positive displacement pumps usually use one of the mechanical drives. However, all of the possible combinations have been tried with various levels of success.[attachment=1:10yxp9pa]roots charger.jpg[/attachment:10yxp9pa][attachment=0:10yxp9pa]Lysholm_screw_rotors.jpg[/attachment:10yxp9pa]
