26-04-2014, 02:32 PM
Supercharger
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A supercharger is an air compressor used to increase the pressure, temperature, and density of air supplied to an internal combustion engine. The compressed air that a supercharger provides to an engine supplies a greater mass of oxygen per cycle of the engine to support combustion than available to a naturally aspirated engine, which makes it possible for more fuel to be burned and more work to be done per cycle, which increases the power the engine produces.
Power for the supercharger can be provided mechanically by a belt, gear, shaft, or chain connected to the engine's crankshaft. When power is provided by a turbine powered by exhaust gas, a supercharger is known as a turbosupercharger[1] – typically referred to simply as a turbochargeror just turbo. Common usage restricts the term supercharger to mechanically driven units.
History
In 1860, brothers Philander and Francis Marion Roots, founders of Roots Blower Company of Connersville, Indiana, patented the design for an air mover, for use in blast furnaces and other industrial applications.
The world's first functional, actually tested[2] engine supercharger was made by Dugald Clerk, who used it for the first[3] two-stroke engine in 1878. Gottlieb Daimler 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 reportedly reached a speed of 100 mph (160 km/h).
The world's first series-produced cars[4] with superchargers were Mercedes 6/25/40 hp and Mercedes 10/40/65 hp. Both models were introduced in 1921 and had Roots superchargers. They were distinguished as "Kompressor" models, the origin of the Mercedes-Benz badging which continues today.
On March 24, 1878 Heinrich Krigar of Germany obtained patent #4121, patenting the first ever screw-type compressor. Later that same year on August 16 he obtained patent #7116 after modifying and improving his original designs. His designs show a two-lobe rotor assembly with each rotor having the same shape as the other. Although the design resembled the roots style compressor, the "screws" were clearly shown with 180 degrees of twist along their length. Unfortunately, the technology of the time was not sufficient to produce such a unit, and Heinrich made no further progress with the screw compressor. Nearly half a century later, in 1935, Alf Lysholm, who was working for Ljungstroms Angturbin AB (later known as Svenska Rotor Maskiner AB or SRM in 1951), patented a design with five female and four male rotors. He also patented the method for machining the compressor rotors.
Capacity rating
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 (1,163 cc) each and would be used on a two-stroke diesel of 426 cubic inches (6,981 cc), 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 (5,555 cc) per revolution.
Two-stroke engines
A two-stroke engine does not have an induction stroke where low pressure can draw in air. In addition a supply of air at higher than ambient pressure is needed to blow out the burnt gases from the previous combustion cycle.
In small trunk engines this is commonly achieved by using the crankcase as a supercharger. As the piston descends during the power stroke the underside of the pistons compresses the air in the crankcase. As it nears the bottom of its stroke a valve or port will open and allow the compressed air charge to escape into the cylinder.
In larger engines other forms of supercharging are needed. These engines are likely to be using crossheads and so have limited under-piston volume. They are also likely to have a crankcase shared by several cylinders. In these cases other means of supercharging are necessary and most, if not all, of the methods listed above have been employed.
Some engines, such as large marine diesels, will use a combination of superchargers. These will use turbocharging, for its efficiency gains, at medium and high speeds. For starting and running at low speeds, when the turbocharger may be unable to supply adequate air, an electrically driven blower will be used. On these engines mechanically driven superchargers are unlikely to be employed due to fuel efficiency being a major design criterion of this engine type.