1934 Mercedes-Benz W25 supercharged Straight-8 at Goodwood Festival of Speed 2009
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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 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.
When power comes from an exhaust gas turbine a supercharger is known as a turbosupercharger – typically referred to simply as a turbocharger or just turbo. Common usage restricts the term supercharger to mechanically driven units.
In 1860, brothers Philander and Francis Marion Roots 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 engine supercharger was made by Dugald Clerk, who used it for the first 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, it was reported, reached a speed of 100 mph (160 km/h).
The world's first series-produced cars with superchargers were Mercedes 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 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 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:
- Lysholm screw
- Sliding vane
- Scroll-type supercharger, also known as the G-Lader
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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.
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.
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. 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 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.
- = 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 (0.69 bar) of boost at sea level (ambient pressure of 14.7 psi (1.01 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.
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.
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.
In 1900, Gottlieb Daimler, of Daimler-Benz (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.
The first supercharged cars were introduced at 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) 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 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 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 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 (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 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, 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) is a 1.4-litre direct-injection motor that also uses both a supercharger and turbocharger.
Superchargers are a natural addition to aircraft piston engines 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 30,000 ft (9,100 m) is 1⁄3 of that at sea level, thus only 1⁄3 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 30,000 ft (9,100 m), only 1⁄3 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. 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 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 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 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 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 (3,700 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 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.
- Main article: Turbocharger
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 (110 kW). Without a supercharger, the engine would produce 750 hp (560 kW); with a supercharger, it produces 1,000 hp (750 kW), a total increase of 400 hp (750 hp — 150 + 400), or a net gain of 250 hp (190 kW). 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 to a supercharger driven by the engine itself, a turbocharger 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, 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 and Republic P-47 Thunderbolt 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 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 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 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 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 from refineries in America and the East Indies. The German Luftwaffe 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 66 or the Daimler-Benz DB 605DC developed as much as 2,000 hp (1,500 kW).
- Boost gauge
- Jet engine
- Ram-air intake
- History of the internal combustion engine
- Glossary Index
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-  D&W Performance
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- Northrop 1955, p.111
- Payton-Smith 1971, pp. 259–260.
- Mankau and Petrick 2001, pp. 24–29.
- Griehl 1999, p. 8.
- Price, 1982. p. 170.
- Berger & Street, 1994. p. 199.
- Mermet 1999, pp. 14–17.
- Mermet 1999, p. 48.
- White, Graham. Allied Aircraft Piston Engines of World War II: History and Development of Frontline Aircraft Piston Engines Produced by Great Britain and the United States during World War II. Warrendale, Penn: Society of Automotive Engineers, Inc.; Shrewsbury, England: Airlife Publishing Ltd.; 1995. ISBN 1560916559, ISBN 1853107344.
- How Superchargers Work, by Bill Harris. HowStuffWorks.com.
- Vintage Superchargers. vintagesupercharger.com.
- HE HARNESSED A TORNADO – and developed a modern airplane supercharger large article Popular Science June 1941
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