Supercharger
In an internal combustion engine, a supercharger is a device which compresses the intake gas, forcing more air into the engine in order to produce more power for a given displacement. It is a form of forced induction that is mechanically powered, as opposed to a turbocharger, which is powered by the kinetic energy of the exhaust gases. However, up until the mid-20th century, a turbocharger was called a "turbosupercharger" and was considered a type of supercharger.
The first supercharged engine was built in 1878, with usage in aircraft engines beginning in the 1910s and usage in car engines beginning in the 1920s. In piston engines used by aircraft, supercharging was often used to compensate for the lower air density at high altitudes. Mechanical supercharging is less commonly used in the 21st century, as manufacturers have shifted to turbochargers to reduce fuel consumption and increase power outputs, especially with reduced engine displacements. Almost all diesels are today turbocharged.
A variant of the supercharger is the electric supercharger or e-supercharger, which uses an electric motor as its power source instead of a belt drive.
Design
Types
There are two main families of superchargers defined according to the method of gas transfer: positive displacement and dynamic superchargers. Positive displacement superchargers deliver an almost constant level of boost pressure increase at all engine speeds, while dynamic superchargers cause the boost pressure to rise exponentially with engine speed. Another family of supercharger, albeit rarely used, is the pressure wave supercharger.Roots blowers tend to be only 40–50% efficient at high boost levels, compared with 70–85% for dynamic superchargers. Lysholm-style blowers can be nearly as efficient as dynamic superchargers over a narrow range of load/speed/boost, for which the system must be specifically designed.
Positive displacement
s deliver a nearly fixed volume of air per revolution of the compressor. The most common type of positive-displacement superchargers is the Roots-type supercharger which is a blower, not a compressor because the volume of fluid transferred does not decrease during each stroke of operation. Other types include the rotary-screw, sliding-vane and scroll-type superchargers.The rating system for positive-displacement superchargers is usually based on their capacity per revolution. In the case of the Roots blower, the GMC rating pattern is typical. The GMC rating is based on how many two-stroke cylinders – and the size of those cylinders – that it is designed to scavenge, with GMC's model range including 2–71, 3–71, 4–71 and 6–71 blowers. The 6–71 blower, for example, is designed to scavenge six cylinders of each, resulting in an engine with a total displacement of ). However, because 6–71 is the engine's designation rather than that of the blower, the actual displacement of the blower is less; for example, a 6–71 blower pumps per revolution. Other supercharger manufacturers have produced blowers rated up to 16–71.
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 a dynamic compressor are:
- Centrifugal
- Multi-stage axial-flow
Drive system
- Belt
- Direct drive
- Gear drive
- Chain drive
- Variable speed ratio, variable ratio centrifugal
- Electric superchargers use electric motors rather than mechanical power sources.
Electric superchargers
Effects of fuel octane rating
Fuels with a higher octane rating are better able to resist autoignition and detonation. As a result, the amount of boost supplied by the superchargers could be increased, resulting in an increase in engine output. The development of 100-octane aviation fuel, pioneered in the USA in the 1930s, enabled the use of higher boost pressures to be used on high-performance aviation engines and was used to vastly increase the power output for several speed record airplanes.Military use of high-octane fuels began in early 1940 when 100-octane fuel was delivered to the British Royal Air Force fighting in World War II. The German Luftwaffe also had supplies of a similar fuel. Increasing the octane rating became a major focus of aero engine development for the remainder of the war, with later fuels having up to a nominal 150-octane rating. Using such fuels, aero engines like the Rolls-Royce Merlin 66 and Daimler-Benz DB 605 DC produced power outputs of up to.
Heating of intake air
One disadvantage of forced induction is that compressing the intake air increases its temperature. For an internal combustion engine, the temperature of the intake air becomes a limiting factor in engine performance. Extreme temperatures can cause pre-ignition or knocking, which reduces performance and can cause engine damage. The risk of pre-ignition/knocking increases with higher ambient air temperatures and higher boost levels.Comparison with turbocharging
engines use energy from the exhaust gas that would normally be wasted, compared with a supercharger which mechanically draws power from the engine. Therefore turbocharged engines usually produce more power and better fuel economy than supercharged engines. However, turbochargers can suffer from turbo lag, where the exhaust gas flow is initially insufficient to spin the turbocharger and achieve the desired boost level, thus leading to a delay in the throttle response. This is often a result of a turbo charger which is too large for the engine displacement. For this reason, supercharged engines are common in applications where throttle response is a key concern, such as drag racing and tractor pulling competitions.In design, superchargers have relatively simple piping from the air intake, through the supercharger and into the engine. Turbochargers have more complicated piping where the intake operates as with the supercharger but the exhaust must also pass through the turbocharger, requiring both intake and exhaust piping to come close together in an often crowded engine bay. As well, turbocharged engines are more prone to heat soak of the intake air, as extremely hot exhaust and turbo components are directly alongside the intake air system, although this can be solved through the use of an intercooler. Turbocharged engines utilize intercoolers more often than supercharged engines as a result.
Comparison for aircraft engines
The majority of aircraft engines used during World War II used mechanically driven superchargers because they had some significant manufacturing advantages over turbochargers. However, the benefit to the operational range was given a much higher priority to American aircraft because of a less predictable requirement on the operational range and having to travel far from their home bases. Consequently, turbochargers were mainly employed in American aircraft engines such as the Allison V-1710 and the Pratt & Whitney R-2800, which were comparably heavier when turbocharged, and required additional ducting of expensive high-temperature metal alloys in the gas turbine and a pre-turbine section of the exhaust system. The size of the ducting alone was a serious design consideration. For example, both the F4U Corsair and the P-47 Thunderbolt used the same radial engine, but the large barrel-shaped fuselage of the turbocharged P-47 was needed because of the amount of ducting to and from the turbocharger in the rear of the aircraft. The F4U used a two-stage inter-cooled supercharger with a more compact layout. Nonetheless, turbochargers were useful in high-altitude bombers and some fighter aircraft due to the increased high-altitude performance and range.Turbocharged piston engines are also subject to many of the same operating restrictions as those of gas turbine engines. Turbocharged engines also require frequent inspections of their turbochargers and exhaust systems to search for possible damage caused by the extreme heat and pressure of the turbochargers. Such damage was a prominent problem in the early models of the American Boeing B-29 Superfortress high-altitude bombers used in the Pacific Theater of Operations during 1944–45.
Turbocharged piston engines continued to be used in a large number of postwar airplanes, such as the B-50 Superfortress, the KC-97 Stratofreighter, the Boeing 377 Stratocruiser, the Lockheed Constellation, and the C-124 Globemaster II.
Twincharging
In the 1985 and 1986 World Rally Championships, Lancia ran the Delta S4, which incorporated both a belt-driven supercharger and exhaust-driven turbocharger. The design used a complex series of bypass valves in the induction and exhaust systems as well as an electromagnetic clutch so that, at low engine speeds, a boost was derived from the supercharger. In the middle of the rev range, a boost was derived from both systems, while at the highest revs the system disconnected the drive from the supercharger and isolated the associated ducting. This was done in an attempt to exploit the advantages of each of the charging systems while removing the disadvantages. In turn, this approach brought greater complexity and affected the car's reliability in WRC events, as well as increasing the weight of engine ancillaries in the finished design.Twincharged engines have occasionally been used in production cars, such as the 2005–2013 Volkswagen 1.4 litre and the 2017–2018 Volvo B4204T43/B4204T48 2.0 litre four-cylinder engines.
History
In 1849, G. Jones of Birmingham, England began manufacturing a lobe pump compressor to provide ventilation for coal mines. In 1860, the Roots Blower Company in the United States patented the design for an air mover for use in blast furnaces and other industrial applications. This air mover and Birmingham's ventilation compressor both used designs similar to that of the later Roots-type superchargers.In March of 1878, German engineer Heinrich Krigar obtained the first patent for a screw-type compressor. The design was a two-lobe rotor assembly with identically-shaped rotors, however the design did not reach production.
Also in 1878, Scottish engineer Dugald Clerk designed the first supercharger which was used with an engine. This supercharger was used with a two-stroke gas engine. Gottlieb Daimler received a German patent for supercharging an internal combustion engine in 1885. Louis Renault patented a centrifugal supercharger in France in 1902.