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Supercharger
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A supercharger is an air compressor used for forced induction of an internal combustion engine. The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally-aspirated engine, which allows more fuel to be provided and more work to be done per cycle, increasing the power output of the engine.
A supercharger can be powered mechanically by a belt, gear, shaft, or chain connected to the engine's crankshaft. It can also be powered by an exhaust gas turbine.
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A supercharger is an air compressor used for forced induction of an internal combustion engine. The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally-aspirated engine, which allows more fuel to be provided and more work to be done per cycle, increasing the power output of the engine.
A supercharger can be powered mechanically by a belt, gear, shaft, or chain connected to the engine's crankshaft. It can also be powered by an exhaust gas turbine. A turbine-driven supercharger is known as a turbosupercharger or turbocharger. The term supercharging refers to any pump that forces air into an engine, but, in common usage, it refers to pumps that are driven directly by the engine, as opposed to turbochargers that are driven by the pressure of the exhaust gases.
History
The first functional supercharger can be attributed to German engineer Gottlieb Daimler, who received a German patent for supercharging an internal combustion engine in 1885. Louis Renault patented a centrifugal supercharger in France in 1902. An early supercharged race car was built by Lee Chadwick of Pottstown, Pennsylvania in 1908, which, it was reported, reached a speed of .
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
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:
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 that 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 backflow from the engine into the supercharger until the two reach equilibrium. It is the backflow that 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 the compression of air within the supercharger itself, which, 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; the blower takes this same designation. However, because 6-71 is actually the engine's 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, one 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 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. Fifteen 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 move 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:
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
Other:
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, whereas positive displacement pumps usually use one of the mechanical drives. However, all of the possible combinations have been tried with various levels of success.
Automobiles
In cars, this device is used to increase the "effective displacement" and volumetric efficiency of an engine; it is a blower that pushes the fuel air into the cylinders, as if the engine had larger valves and cylinders, resulting in a "larger" engine that weighs less.
In 1900, Gottlieb Daimler, of Daimler-Benz (Daimler AG), was the first to patent a forced-induction system for internal combustion engines, superchargers based the twin-rotor air-pump design, first patented by the American Francis Roots in 1860, the basic design for the modern Roots type supercharger.
The first supercharged cars were introduced in the 1921 Berlin Motor Show: the 6/20 hp and 10/35 hp Mercedes. These cars went into production in 1923 as the 6/25/40 hp (regarded as the first supercharged road car) and 10/40/65 hp. These were normal road cars as other supercharged cars at same time were almost all racing cars, including the 1923 Fiat 805-405, 1923 Miller 122, 1924 Alfa Romeo P2, 1924 Sunbeam,, 1925 Delage, and the 1926 Bugatti Type 35C. At the end of the 1920s, Bentley made a supercharged version of the Bentley 4˝ Litre road car. Since then, superchargers (and turbochargers) are widely applied to racing and production cars, although the supercharger's technological complexity and cost have largely limited it to expensive, high-performance cars.
Boosting (attaching a supercharger) to a stock production naturally-aspirated engine, has returned as a practice, because of the increased quality of the alloys and the precision of the machining of modern engines. In the past, boosting greatly shortened engine life, because of the extremely high temperatures and pressures created by the supercharger. However, modern engines, made of modern materials, are over-designed to be stronger than required, thus, boosting is not a serious reliability concern. In a common scenario, boosting is done with small cars, the added supercharger's weight is less than the weight of a larger, greater-power engine. This decreases the fuel/mileage ratio, because mileage is a function of the car's total weight, a significant part of which is the engine. Nevertheless, adding a supercharger often voids the car's drive-train warranty. Moreover, an improperly installed supercharger, or one with excessive boost, will greatly reduce the life of engine, the differential, and the transmission, because they were not originally designed and made to operate at the additional, greater rates of speed, and torque.
Supercharging and turbocharging
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