Supercharger
Overview
 
A supercharger is an air compressor
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...

 used for forced induction
Forced induction
Forced induction is the process of compressing air on the intake of an internal combustion engine . A forced induction engine uses a gas compressor to increase the pressure, temperature and density of the air...

 of an internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the 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
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 to be burned and more work to be done per cycle, increasing the power output of the engine.

Power for the unit can come mechanically by a belt, gear, shaft, or chain connected to the engine's crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...

.

When power comes from an exhaust gas
Exhaust gas
Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.It often disperses...

 turbine
Turbine
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...

 a supercharger is known as a turbosupercharger – typically referred to simply as a turbocharger
Turbocharger
A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...

or just turbo.
Encyclopedia
A supercharger is an air compressor
Gas compressor
A gas compressor is a mechanical device that increases the pressure of a gas by reducing its volume.Compressors are similar to pumps: both increase the pressure on a fluid and both can transport the fluid through a pipe. As gases are compressible, the compressor also reduces the volume of a gas...

 used for forced induction
Forced induction
Forced induction is the process of compressing air on the intake of an internal combustion engine . A forced induction engine uses a gas compressor to increase the pressure, temperature and density of the air...

 of an internal combustion engine
Internal combustion engine
The internal combustion engine is an engine in which the combustion of a fuel occurs with an oxidizer in a combustion chamber. In an internal combustion engine, the expansion of the high-temperature and high -pressure gases produced by combustion apply direct force to some component of the 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
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...

 to be burned and more work to be done per cycle, increasing the power output of the engine.

Power for the unit can come mechanically by a belt, gear, shaft, or chain connected to the engine's crankshaft
Crankshaft
The crankshaft, sometimes casually abbreviated to crank, is the part of an engine which translates reciprocating linear piston motion into rotation...

.

When power comes from an exhaust gas
Exhaust gas
Exhaust gas or flue gas is emitted as a result of the combustion of fuels such as natural gas, gasoline/petrol, diesel fuel, fuel oil or coal. According to the type of engine, it is discharged into the atmosphere through an exhaust pipe, flue gas stack or propelling nozzle.It often disperses...

 turbine
Turbine
A turbine is a rotary engine that extracts energy from a fluid flow and converts it into useful work.The simplest turbines have one moving part, a rotor assembly, which is a shaft or drum with blades attached. Moving fluid acts on the blades, or the blades react to the flow, so that they move and...

 a supercharger is known as a turbosupercharger – typically referred to simply as a turbocharger
Turbocharger
A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...

or just turbo. Common usage restricts the term supercharger to mechanically driven units.

History

In 1860, brothers Philander and Francis Marion Roots of Connersville, Indiana
Connersville, Indiana
At the 2000 census, there were 15,411 people, 6,382 households and 4,135 families residing in the city. The population density was 1,894.5 per square mile . There were 6,974 housing units at an average density of 857.3 per square mile...

, patented the design for an air mover, for use in blast furnace
Blast furnace
A blast furnace is a type of metallurgical furnace used for smelting to produce industrial metals, generally iron.In a blast furnace, fuel and ore and flux are continuously supplied through the top of the furnace, while air is blown into the bottom of the chamber, so that the chemical reactions...

s and other industrial applications.

The world's first functional, actually tested engine supercharger was made by Dugald Clerk, who used it for the first two-stroke engine in 1878. Gottlieb Daimler
Gottlieb Daimler
Gottlieb Daimler was an engineer, industrial designer and industrialist born in Schorndorf , in what is now Germany. He was a pioneer of internal-combustion engines and automobile development...

 received a German patent
Patent
A patent is a form of intellectual property. It consists of a set of exclusive rights granted by a sovereign state to an inventor or their assignee for a limited period of time in exchange for the public disclosure of an invention....

 for supercharging an internal combustion engine in 1885. Louis Renault
Louis Renault (industrialist)
Louis Renault was a French industrialist, one of the founders of Renault and a pioneer of the automobile industry....

 patented a centrifugal supercharger in France in 1902. An early supercharged race car was built by Lee Chadwick
Lee Chadwick
Lee Chadwick is an is an English mixed martial artist. He trains with HAMMA Mixed Martial Arts Academy based in St. Helens, England.He formally trained at Team Kaobon...

 of Pottstown, Pennsylvania in 1908, which, it was reported, reached a speed of 100 mph (160.9 km/h).

The world's first series-produced cars with superchargers were Mercedes
Mercedes (car)
Mercedes was a brand of the Daimler-Motoren-Gesellschaft . DMG which began to develop in 1900, after the death of its co-founder, Gottlieb Daimler...

 6/25/40 hp and Mercedes 10/40/65 hp. Both models were introduced in 1921 and had Roots superchargers.

Types of supercharger

There are two main types of superchargers defined according to the method of compression: positive displacement
Positive displacement
Positive displacement may refer to:* Positive displacement pump* Positive displacement meter...

 and dynamic compressors. The former deliver a fairly constant level of pressure increase at all engine speeds (RPM), whereas the latter deliver increasing pressure 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 almost constant at all speeds for a given pressure, thus 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
    Roots type supercharger
    The Roots type supercharger or Roots blower is a positive displacement lobe pump which operates by pumping fluids with a pair of meshing lobes not unlike a set of stretched gears. Fluid is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust...

  • Lysholm screw
  • Sliding vane
    Powerplus supercharger
    The Powerplus was a design of supercharger used to boost the performance of car engines in the 1930s. It is a mechanically-driven positive displacement pump, operating on the sliding-vane principle.- MG cars :...

  • Scroll-type supercharger
    Scroll-type supercharger
    The scroll-type supercharger is a positive displacement orbiting-spiral supercharger. It is a compromise between the more rugged rotating lobe, and the more efficient sliding vane type superchargers, and is considered to offer the highest potential in regard to efficiency, noise and pressure...

    , also known as the G-Lader

Compression type

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 to 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 back flow. This is more effective than back flow 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 back flow is zero. If the boost pressure exceeds that compression pressure, back flow 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.

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 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 16–71 blowers used in different motor sports. 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.

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
    Centrifugal type supercharger
    The centrifugal-type supercharger is an engine-driven compressor used to increase the power output of an internal-combustion engine by increasing the amount of available oxygen by compressing air that is entering the engine...

  • Multi-stage axial-flow
  • Pressure wave supercharger
    Pressure wave supercharger
    A pressure wave supercharger is a type of supercharger technology that harnesses the pressure waves produced by an internal combustion engine exhaust gas pulses to compress the intake air. Its automotive use is not widespread; the most widely used example is the Comprex, developed by Brown Boveri...


Supercharger drive types

Superchargers are further defined according to their method of drive (mechanical—or turbine).

Mechanical

  • Belt (V-belt, Synchronous belt, Flat belt)
  • Direct drive
  • Gear drive
  • Chain drive

Other

  • Electric motor
    Electric supercharger
    An electric supercharger is a specific type of supercharger that uses an electrically powered forced-air system that contains an electric motor to drive an axial flow fan to pressurize the intake air while it rids the inlet and filter box of most restriction...

  • Auxiliary Power Unit
    Auxiliary power unit
    An auxiliary power unit is a device on a vehicle that provides energy for functions other than propulsion. They are commonly found on large aircraft, as well as some large land vehicles.-Function:...

     in some large industrial applications.

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. In principle, a positive displacement engine could be used in place of an exhaust turbine to improve low speed performance. Electric superchargers are all essentially fans (axial pumps). A form of regenerative braking has been tried where the car is slowed by compressing air for future acceleration.

Temperature effects and intercoolers

One downside of supercharging is that compressing the air increases its temperature. When a supercharger is used on an internal combustion engine, the temperature of the fuel/air charge becomes a major limiting factor in engine performance. Extreme temperatures will cause detonation
Engine knocking
Knocking in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.The...

 of the fuel-air mixture (spark ignition engines) and damage to the engine. In cars, this can cause a problem when it is a hot day outside, or when large amounts of boost are being pushed.

It is possible to estimate the temperature rise across a supercharger by modeling it as an isentropic process
Isentropic process
In thermodynamics, an isentropic process or isoentropic process is one in which for purposes of engineering analysis and calculation, one may assume that the process takes place from initiation to completion without an increase or decrease in the entropy of the system, i.e., the entropy of the...

.

Where:
= ambient air temperature
= temperature after the compressor
= ambient atmospheric pressure (absolute)
= pressure after the compressor (absolute)
= Ratio of specific heats for air =
= Specific heat at constant pressure
= Specific heat at constant volume


For example, if a supercharged engine is pushing 10 psi (0.6894757293 bar) of boost at sea level (ambient pressure of 14.7 psi (1 bar), ambient temperature of 75 °F), the temperature of the air after the supercharger will be 160.5 °F (71.4 °C). This temperature is known as the compressor discharge temperature (CDT) and highlights why a method for cooling the air after the compressor is so important.

In addition to causing possible detonation and damage, hot intake decreases power in at least one way. At a given pressure, the hotter the air the less dense it is, so the mass of intake is decreased, or for the same mass it takes more power to drive the compressor.

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. Thus a two stroke is unable to run without some form of supercharging to perform the scavenging
Scavenging (automotive)
In automotive usage, scavenging is the process of pushing exhausted gas-charge out of the cylinder and drawing in a fresh draught of air ready for the next 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 a 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.

Automobiles

In 1900, Gottlieb Daimler
Gottlieb Daimler
Gottlieb Daimler was an engineer, industrial designer and industrialist born in Schorndorf , in what is now Germany. He was a pioneer of internal-combustion engines and automobile development...

, of Daimler-Benz
Daimler-Benz
Daimler-Benz AG was a German manufacturer of automobiles, motor vehicles, and internal combustion engines; founded in 1926. An Agreement of Mutual Interest - which was valid until year 2000 - was signed on 1 May 1924 between Karl Benz's Benz & Cie., and Daimler Motoren Gesellschaft, which had...

 (Daimler AG), was the first to patent a forced-induction system for internal combustion engines, superchargers based on the twin-rotor air-pump design, first patented by the American Francis Roots in 1860, the basic design for the modern Roots type supercharger
Roots type supercharger
The Roots type supercharger or Roots blower is a positive displacement lobe pump which operates by pumping fluids with a pair of meshing lobes not unlike a set of stretched gears. Fluid is trapped in pockets surrounding the lobes and carried from the intake side to the exhaust...

.

The first supercharged cars were introduced at the 1921 Berlin Motor Show: the 6/20 hp and 10/35 hp Mercedes
Mercedes (car)
Mercedes was a brand of the Daimler-Motoren-Gesellschaft . DMG which began to develop in 1900, after the death of its co-founder, Gottlieb Daimler...

. 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
Fiat
FIAT, an acronym for Fabbrica Italiana Automobili Torino , is an Italian automobile manufacturer, engine manufacturer, financial, and industrial group based in Turin in the Italian region of Piedmont. Fiat was founded in 1899 by a group of investors including Giovanni Agnelli...

 805-405, 1923 Miller 122 1924 Alfa Romeo P2
Alfa Romeo P2
The Alfa Romeo P2 won the inaugural Automobile World Championship in 1925, taking victory in two of the four championship rounds when Antonio Ascari drove it in the European Grand Prix at Spa and Gastone Brilli-Peri won the Italian Grand Prix at Monza after Ascari died while leading the intervening...

, 1924 Sunbeam, 1925 Delage
Delage
Delage was a French luxury automobile and racecar company founded in 1905 by Louis Delage in Levallois-Perret near Paris; it was acquired by Delahaye in 1935 and ceased operation in 1953.-History:...

, 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) have been 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

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
Power (physics)
In physics, power is the rate at which energy is transferred, used, or transformed. For example, the rate at which a light bulb transforms electrical energy into heat and light is measured in watts—the more wattage, the more power, or equivalently the more electrical energy is used per unit...

 are more important than any other consideration, such as top-fuel dragsters
Top Fuel
Top Fuel racing is a class of drag racing in which the cars are run on a mix of approximately 90% nitromethane and 10% methanol rather than gasoline or simply methanol. The cars are purpose-built for drag racing, with an exaggerated layout that in some ways resembles open-wheel circuit racing...

 and vehicles used in tractor pulling
Tractor pulling
Truck and Tractor pulling, also known as power pulling, is a motorsport competition, popular in America, Europe , Australia and Brazil, which requires modified tractors to pull a heavy sledge along a 35ft. wide and length of 100 metre or 300ft+ track, with the winner being the tractor that pulls...

 competitions, positive-displacement superchargers are very common.

There are three main categories of superchargers 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, Twin Screw (Lysholm), 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
Throttle
A throttle is the mechanism by which the flow of a fluid is managed by constriction or obstruction. An engine's power can be increased or decreased by the restriction of inlet gases , but usually decreased. The term throttle has come to refer, informally and incorrectly, to any mechanism by which...

 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.

Turbochargers suffer (to a greater or lesser extent) from so-called turbo-spool (turbo lag; more correctly, boost lag), in which initial acceleration from low RPM is limited by the lack of sufficient exhaust gas mass flow
Mass flow
Mass flow, also known as mass transfer and bulk flow, is the movement of material matter. In physics, mass flow occurs in open systems and is often measured as occurring when moving across a certain boundary characterized by its cross-sectional area and a flow rate. In engineering and biology it...

 (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 engine-driven superchargers, which apply boost in direct proportion to the engine RPM.

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
Intercooler
An intercooler , or charge air cooler, is an air-to-air or air-to-liquid heat exchange device used on turbocharged and supercharged internal combustion engines to improve their volumetric efficiency by increasing intake air charge density through nearly isobaric cooling, which removes...

 between the pump and the engine to reduce the temperature of the air.

In the 1985 and 1986 World Rally Championships, Lancia ran the Delta S4
Lancia Delta S4
The Lancia Delta S4 is a Group B rally car that competed in the World Rally Championship in 1985 and 1986, until Group B cars were banned from competition by the FIA. The car replaced and was an evolution of the Lancia 037 Monte Carlo. The S4 took full advantage of the Group B regulations, and...

, 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, boost was derived from the supercharger. In the middle of the rev range, boost was derived from both systems, while at the highest revs the system disconnected 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 impacted on the cars reliability in WRC events, as well as increasing the weight of engine ancillaries in the finished design.

The Volkswagen TSI engine (or Twincharger
Twincharger
Twincharger refers to a compound forced induction system used on some piston-type internal combustion engines. It is a combination of an exhaust-driven turbocharger and an engine-driven supercharger, each mitigating the weaknesses of the other...

) is a 1.4-litre direct-injection motor that also uses both a supercharger and turbocharger.

Altitude effects

Superchargers are a natural addition to aircraft piston engines
Aircraft engine
An aircraft engine is the component of the propulsion system for an aircraft that generates mechanical power. Aircraft engines are almost always either lightweight piston engines or gas turbines...

 that are intended for operation at high altitudes. As an aircraft climbs to higher altitude, air pressure and air density decreases. The output of a piston engine drops because of the reduction in the mass of air that can be drawn into the engine. For example, the air density at 30000 ft (9,144 m) is of that at sea level
Sea level
Mean sea level is a measure of the average height of the ocean's surface ; used as a standard in reckoning land elevation...

, thus only of the amount of air can be drawn into the cylinder, with enough oxygen to provide efficient combustion for only a third as much fuel. So, at 30000 ft (9,144 m), only of the fuel burnt at sea level can be burnt. (An advantage of the decreased air density is that the airframe experiences only about 1/3 of the aerodynamic drag
Drag equation
In fluid dynamics, the drag equation is a practical formula used to calculate the force of drag experienced by an object due to movement through a fully enclosing fluid....

. Plus, there is decreased back pressure on the exhaust gases. On the other hand, more energy is consumed holding an airplane up with less air in which to generate lift.)
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 continuously 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

As discussed above, supercharging can cause a spike in temperature, and extreme temperatures will cause detonation
Engine knocking
Knocking in spark-ignition internal combustion engines occurs when combustion of the air/fuel mixture in the cylinder starts off correctly in response to ignition by the spark plug, but one or more pockets of air/fuel mixture explode outside the envelope of the normal combustion front.The...

 of the fuel-air mixture and damage to the engine. In the case of aircraft, this causes 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 the red line.

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
Carburetor
A carburetor , carburettor, or carburetter is a device that blends air and fuel for an internal combustion engine. It is sometimes shortened to carb in North America and the United Kingdom....

 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: intercooler
Intercooler
An intercooler , or charge air cooler, is an air-to-air or air-to-liquid heat exchange device used on turbocharged and supercharged internal combustion engines to improve their volumetric efficiency by increasing intake air charge density through nearly isobaric cooling, which removes...

s and aftercoolers, anti-detonant injection
Water injection (engines)
In internal combustion engines, water injection, also known as anti-detonant injection, is spraying water into the cylinder or incoming fuel-air mixture to cool the combustion chambers of the engine, allowing for greater compression ratios and largely eliminating the problem of engine knocking...

, 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 12000 feet (3,657.6 m), 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
Intercooler
An intercooler , or charge air cooler, is an air-to-air or air-to-liquid heat exchange device used on turbocharged and supercharged internal combustion engines to improve their volumetric efficiency by increasing intake air charge density through nearly isobaric cooling, which removes...

 radiator where it was cooled before being compressed again by the high-pressure stage and then aftercooled in another heat exchanger
Heat exchanger
A heat exchanger is a piece of equipment built for efficient heat transfer from one medium to another. The media may be separated by a solid wall, so that they never mix, or they may be in direct contact...

. 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.

Turbocharging

A mechanically driven 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 hp. Without a supercharger, the engine would produce 750 hp (560 kW); with a supercharger, it produces 1000 hp, a total increase of 400 hp (750 hp — 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
Specific power
In physics and engineering, surface power density or sometimes simply specific power is power per unit area.-Applications:* The intensity of electromagnetic radiation can be expressed in W/m2...

 and power-to-weight ratio
Power-to-weight ratio
Power-to-weight ratio is a calculation commonly applied to engines and mobile power sources to enable the comparison of one unit or design to another. Power-to-weight ratio is a measurement of actual performance of any engine or power sources...

, but also increases the engine's specific fuel consumption
Specific fuel consumption
Thrust specific fuel consumption or sometimes simply specific fuel consumption, SFC, is an engineering term that is used to describe the fuel efficiency of an engine design with respect to thrust output...

. This increases the cost of running the aircraft and reduces its overall range.

As opposed to a supercharger driven by the engine itself, a turbocharger
Turbocharger
A turbocharger, or turbo , from the Greek "τύρβη" is a centrifugal compressor powered by a turbine that is driven by an engine's exhaust gases. Its benefit lies with the compressor increasing the mass of air entering the engine , thereby resulting in greater performance...

 is 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, helping a turbocharged engine to compensate for changing altitude.

The 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 and the Pratt & Whitney R-2800
Pratt & Whitney R-2800
The Pratt & Whitney R-2800 Double Wasp is a two-row, 18-cylinder, air-cooled radial aircraft engine with a displacement of 2,804 in³ , and is part of the long-lived Wasp family....

, which 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 Corsair
F4U Corsair
The Vought F4U Corsair was a carrier-capable fighter aircraft that saw service primarily in World War II and the Korean War. Demand for the aircraft soon overwhelmed Vought's manufacturing capability, resulting in production by Goodyear and Brewster: Goodyear-built Corsairs were designated FG and...

 and Republic P-47 Thunderbolt
P-47 Thunderbolt
Republic Aviation's P-47 Thunderbolt, also known as the "Jug", was the largest, heaviest, and most expensive fighter aircraft in history to be powered by a single reciprocating engine. It was heavily armed with eight .50-caliber machine guns, four per wing. When fully loaded, the P-47 weighed up to...

 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 as a result of damage due to the increased heat, thereby increasing maintenance costs.

Today, most general aviation
General aviation
General aviation is one of the two categories of civil aviation. It refers to all flights other than military and scheduled airline and regular cargo flights, both private and commercial. General aviation flights range from gliders and powered parachutes to large, non-scheduled cargo jet flights...

 aircraft are naturally aspirated. The small number of modern aviation piston engines designed to run at high altitudes in general use a turbocharger or turbo-normalizer system rather than a supercharger driven from the crank shaft. 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. Also, depending on what monetary inflation factor one uses, fuel costs have not decreased as fast as production and maintenance costs have.

Effects of fuel octane rating

Until World War II all automobile and aviation fuel was generally rated at 87 octane
Octane rating
Octane rating or octane number is a standard measure of the anti-knock properties of a motor or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating...

 or less. This is the rating that was achieved by the simple distillation of "light crude" oil. 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, while maintaining a reasonable compression ratio.

Octane rating
Octane rating
Octane rating or octane number is a standard measure of the anti-knock properties of a motor or aviation fuel. The higher the octane number, the more compression the fuel can withstand before detonating...

 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 and detonation
Detonation
Detonation involves a supersonic exothermic front accelerating through a medium that eventually drives a shock front propagating directly in front of it. Detonations are observed in both conventional solid and liquid explosives, as well as in reactive gases...

 better than does low-octane fuel. 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 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 was delivered to the British Royal Air Force
Royal Air Force
The Royal Air Force is the aerial warfare service branch of the British Armed Forces. Formed on 1 April 1918, it is the oldest independent air force in the world...

 from refineries in America and the East Indies. The German Luftwaffe
Luftwaffe
Luftwaffe is a generic German term for an air force. It is also the official name for two of the four historic German air forces, the Wehrmacht air arm founded in 1935 and disbanded in 1946; and the current Bundeswehr air arm founded in 1956....

also had supplies of a similar fuel.

Increasing the knocking limits of existing aviation fuels became 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 Rolls-Royce Merlin
Rolls-Royce Merlin
The Rolls-Royce Merlin is a British liquid-cooled, V-12, piston aero engine, of 27-litre capacity. Rolls-Royce Limited designed and built the engine which was initially known as the PV-12: the PV-12 became known as the Merlin following the company convention of naming its piston aero engines after...

 66 or the Daimler-Benz DB 605
Daimler-Benz DB 605
|-See also:-Bibliography:* Gunston, Bill. World Encyclopedia of Aero Engines. Cambridge, England. Patrick Stephens Limited, 1989. ISBN 1-85260-163-9* Jane's Fighting Aircraft of World War II. London. Studio Editions Ltd, 1989. ISBN 0-517-67964-7...

DC developed as much as 2000 hp.

See also

  • Boost gauge
    Boost gauge
    A boost gauge is a pressure gauge that indicates manifold air pressure or turbocharger or supercharger boost pressure in an internal combustion engine...

  • Jet engine
    Jet engine
    A jet engine is a reaction engine that discharges a fast moving jet to generate thrust by jet propulsion and in accordance with Newton's laws of motion. This broad definition of jet engines includes turbojets, turbofans, rockets, ramjets, pulse jets...

  • Turbofan
    Turbofan
    The turbofan is a type of airbreathing jet engine that is widely used for aircraft propulsion. A turbofan combines two types of engines, the turbo portion which is a conventional gas turbine engine, and the fan, a propeller-like ducted fan...

  • Turbojet
  • Twincharger
    Twincharger
    Twincharger refers to a compound forced induction system used on some piston-type internal combustion engines. It is a combination of an exhaust-driven turbocharger and an engine-driven supercharger, each mitigating the weaknesses of the other...

  • Ram-air intake
    Ram-air intake
    A ram-air intake is any intake design which uses the dynamic air pressure created by vehicle motion to increase the static air pressure inside of the intake manifold on an engine, thus allowing a greater massflow through the engine and hence increasing engine power.The ram air intake works by...

  • History of the internal combustion engine
    History of the internal combustion engine
    Although various forms of internal combustion engines were developed before the 19th century, their use was hindered until the commercial drilling and production of petroleum began in the mid-1850s...


External links

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