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.
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 100 miles per hour (160 km/h).
Types of supercharger
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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.
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:
* 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 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 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. 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 aftercooler system and goes a long way to negating the inefficiency of the Roots design in that application.
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:
* Multi-stage axial-flow
* Pressure wave supercharger
Supercharger drive types
Superchargers are further defined according to their method of drive (mechanical—or turbine).
* Belt (V-belt, Synchronous belt, Flat belt)
* Direct drive
* Gear drive
* Chain drive
Exhaust gas turbines:
* Axial turbine
* Radial turbine
* Electric motor
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.
1929 "Blower" Bentley from the Ralph Lauren collection. The large "blower" (supercharger) is located at the front, in front of the radiator, and gave the car its name.
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 4½ 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.
Supercharging versus turbocharging
Supercharged Mercury Capri
Positive-displacement superchargers may absorb as much as a third of the total crankshaft power of the engine, and, in many applications, are less efficient than turbochargers. In applications for which engine response and power are more important than any other consideration, such as top-fuel dragsters and vehicles used in tractor pulling competitions, positive-displacement superchargers are very common.
There are three main categories of supercharger for automotive use:
* Centrifugal turbochargers — driven from exhaust gases.
* Centrifugal superchargers — driven directly by the engine via a belt-drive.
* Positive displacement pumps — such as the Roots, Lysholm (Whipple), and TVS (Eaton) blowers.
The thermal efficiency, or fraction of the fuel/air energy that is converted to output power, is less with a mechanically-driven supercharger than with a turbocharger, because turbochargers are using energy from the exhaust gases that would normally be wasted. For this reason, both the economy and the power of a turbocharged engine are usually better than with superchargers. The main advantage of an engine with a mechanically-driven supercharger is better throttle response, as well as the ability to reach full-boost pressure instantaneously. With the latest turbocharging technology, throttle response on turbocharged cars is nearly as good as with mechanically-powered superchargers, but the existing lag time is still considered a major drawback, especially considering that the vast majority of mechanically-driven superchargers are now driven off clutched pulleys, much like an air compressor.
Roots blowers tend to be 40–50% efficient at high boost levels. Centrifugal Superchargers are 70–85% efficient. Lysholm-style blowers can be nearly as efficient as their centrifugal counterparts over a narrow range of load/speed/boost, for which the system must be specifically designed.
Keeping the air that enters the engine cool is an important part of the design of both superchargers and turbochargers. Compressing air increases its temperature, so it is common to use a small radiator called an intercooler between the pump and the engine to reduce the temperature of the air. Turbochargers also suffer (to a greater or lesser extent) from so-called turbo-spool, in which initial acceleration from low RPMs is limited by the lack of sufficient exhaust gas mass flow (pressure). Once engine RPM is sufficient to start the turbine spinning, there is a rapid increase in power, as higher turbo boost causes more exhaust gas production, which spins the turbo yet faster, leading to a belated "surge" of acceleration. This makes the maintenance of smoothly-increasing RPM far harder with turbochargers than with belt-driven superchargers, which apply boost in direct proportion to the engine RPM.
The Rolls Royce Merlin, a supercharged aircraft engine from World War II.
Superchargers are a natural addition to aircraft piston engines which are intended for operation at high altitudes. As an aircraft climbs to higher altitude, air pressure and air density decreases. The efficiency of a piston engine drops because of the reduction in the weight of air that can be drawn into the engine; for example the air density at 30,000 feet (9,100 m) is 1/3rd of that at sea level, thus only 1/3rd of the amount of air can be drawn into the cylinder and not enough oxygen can be delivered to the engine cylinders to provide efficient combustion for the fuel/air mixture; so ,at 30,000 feet (9,100 m), only 1/3rd of the fuel can be burnt. One advantage of the decreased air density is the airframe only experiences about 1/3rd of the aerodynamic drag, plus there is decreased back pressure on the exhaust gases.
A supercharger can be thought of either as artificially increasing the density of the air by compressing it - or as forcing more air than normal into the cylinder every time the piston moves down.
A supercharger compresses the air back to sea-level equivalent pressures, or even much higher, in order to make the engine produce just as much power at cruise altitude as it does at sea level. With the reduced aerodynamic drag at high altitude and the engine still producing rated power, a supercharged airplane can fly much faster at altitude than a naturally-aspirated one. The pilot controls the output of the supercharger with the throttle and indirectly via the propeller governor control. Since the size of the supercharger is chosen to produce a given amount of pressure at high altitude, the supercharger is over-sized for low altitude. The pilot must be careful with the throttle and watch the manifold pressure gauge to avoid overboosting at low altitude. As the aircraft climbs and the air density drops, the pilot must continually open the throttle in small increments to maintain full power. The altitude at which the throttle reaches full open and the engine is still producing full rated power is known as the critical altitude. Above the critical altitude, engine power output will start to drop as the aircraft continues to climb.
Effects of temperature
The downside of supercharging is that compressing the air increases its temperature. When a supercharger is used on an aircraft, the temperature of the fuel/air charge becomes a major limiting factor in engine performance. Extreme temperatures will cause detonation of the fuel-air mixture and damage to the engine. This caused a problem at low altitudes, where the air is both denser and warmer than at high altitudes. With high ambient air temperatures, detonation could start to occur with the manifold pressure gauge reading far below redline.
A supercharger optimized for high altitudes causes the opposite problem on the intake side of the system. With the throttle retarded to avoid overboosting, air temperature in the carburetor can drop low enough to cause ice to form at the throttle plate. In this manner enough ice could accumulate to cause engine failure, even with the engine operating at full rated power. For this reason, many supercharged aircraft featured a carburetor air temperature gauge or warning light to alert the pilot of possible icing conditions.
Several solutions to these problems were developed: intercoolers and aftercoolers, anti-detonant injection, two-speed superchargers, and two-stage superchargers.
 Two-stage and two-speed superchargers
In the 1930s, two-speed drives were developed for superchargers. These provided more flexibility for the operation of the aircraft, although they also entailed more complexity of manufacturing and maintenance. The gears connected the supercharger to the engine using a system of hydraulic clutches, which were manually engaged or disengaged by the pilot with a control in the cockpit. At low altitudes, the low-speed gear would be used in order to keep the manifold temperatures low. At around 12,000 feet, when the throttle was full forward and the manifold pressure started to drop off, the pilot would retard the throttle and switch to the higher gear, then readjust the throttle to the desired manifold pressure.
Another way to accomplish the same level of control was the use of two compressors in series. After the air was compressed in the low pressure stage, the air flowed through an intercooler radiator where it was cooled before being compressed again by the high pressure stage and then aftercooled in another heat exchanger. In these systems, damper doors could be opened or closed by the pilot in order to bypass one stage as needed. Some systems had a cockpit control for opening or closing a damper to the intercooler/aftercooler, providing another way to control temperature. The most complex systems used a two-speed, two-stage system with both an intercooler and an aftercooler, but these were found to be prohibitive in cost and complicated. In the end, it was found that, for most engines, a single-stage two-speed setup was most suitable.
Comparison to turbocharging
A supercharger has to take its drive power from the engine. Taking a single-stage single-speed supercharged engine, such as the Rolls Royce Merlin, for instance, the supercharger uses up about 150 horsepower (110 kW). Without a supercharger the engine would produce 750 hp (560 kW); with a supercharger the engine now produces 1,000 hp (750 kW), a total increase of 400 hp (750hp - 150 + 400), or a net gain of 250 hp. This is where the principal disadvantage of a supercharger becomes apparent: The engine has to burn extra fuel to provide power to turn the supercharger. The increased charge density increases the engine's specific power and power to weight ratio, but also increases the engine's specific fuel consumption. This increases the cost of running the aircraft and reduces its overall range.
As opposed superchargers driven by the engine itself, turbochargers are driven using the exhaust gases from the engines. The amount of power in the gas is proportional to the difference between the exhaust pressure and air pressure, and this difference increases with altitude, allowing a turbocharger to compensate for changing altitude without using up any extra power.
The vast majority of WWII engines used mechanically driven superchargers, because they maintained three significant manufacturing advantages over turbochargers. Turbochargers, used by American aero engines such as the Allison V-1710, the Pratt & Whitney R-2800 were larger, involved extra piping, and required rare high-temperature alloys in the turbine and pre-turbine section of the exhaust system. The size of the piping alone was a serious issue; the Vought F4U and Republic P-47 used the same engine but the huge barrel-like fuselage of the latter was, in part, a result of the necessary piping to and from the turbocharger in the rear of the plane. Turbocharged piston engines are also subject to many of the same operating restrictions as gas turbine engines. Turbocharged engines also require frequent inspections of the turbocharger and exhaust systems for damage due to the increased heat, increasing maintenance costs.
Today, most general aviation aircraft are naturally aspirated. The small number of modern aviation piston engines designed to run at high altitudes generally use a turbocharger or turbo-normalizer system rather than a supercharger. The change in thinking is largely due to economics. Aviation gasoline was once plentiful and cheap, favoring the simple but fuel-hungry supercharger. As the cost of fuel has increased, the supercharger has fallen out of favor.
Effects of fuel octane rating
Prior to the opening of World War II, all automobile and aviation fuel was generally rated at 87 octane. This is the rating that was achieved by the simple distillation of "light crude" oil, and was, therefore, the cheapest possible fuel.
Engines from around the world were designed to work with this grade of fuel, which set a limit to the amount of boosting that could be provided by the supercharger.
Octane rating boosting through additives was a line of research being explored at the time. Using these techniques, less valuable crude could still supply large amounts of useful gasoline, which made it a valuable economic process. However the additives were not limited to making poor-quality oil into 87-octane gasoline; the same additives could also be used to boost the gasoline to much higher octane ratings.
Higher-octane fuel resists auto ignition better than low-octane fuel, reducing the risk of detonation. As a result, the amount of boost supplied by the superchargers could be increased, resulting a simultanous increase in engine output. The development of 100 octane aviation fuel, pioneered in the USA before the war, enabled the use of higher boost pressures to be used on high-performance aviation engines, and was used to develop extremely high power outputs - for short periods - in several of the pre-war speed record airplanes. Operational use of the new fuel during World War II began in early 1940 when 100-octane fuel begun to be used by the British Royal Air Force and the German Luftwaffe.
Increasing the knocking limits of existing aviation fuels become a major focus of aero engine development during World War II. By the end of the war, fuel was being delivered at a nominal 150-octane rating, on which late-war aero engines like the the Rolls-Royce Merlin 66 or the Daimler-Benz DB 605DC developed as high as 2,000 hp (1,500 kW).