supercharger

supercharger

[soo-per-chahr-jer]

Air compressor or blower used in piston-type internal-combustion engines to increase the amount of air drawn into the cylinders by the movement of the pistons during each intake stroke. With the additional air, more fuel is burned, and the engine's power is increased. In aircraft engines, supercharging compensates for the reduced atmospheric pressure at high altitudes. Development of the gas turbine, which requires a constant flow of air and fuel, led to the turbosupercharger, a centrifugal blower driven by a small gas turbine powered by the exhaust gases from the engine cylinders.

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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 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.

History

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 and reportedly reached a speed of .

Types of supercharger

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.

Positive displacement

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:

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 which 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 back flow from the engine into the supercharger until the two reach equilibrium. It is the back flow which 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 air being compressed within the supercharger itself and this compressed air, 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 and the blower takes this same designation. However because 6-71 is actually the engines 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 you 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 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. 15 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 moves 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 after cooler system and goes a long way to negating the inefficiency of the roots design in that application.

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:

Supercharger drive types

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

Mechanical:

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

Exhaust gas turbines:

  • Axial turbine
  • Radial turbine

Other:

  • 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, while positive displacement pumps usually use one of the mechanical drives. However, all of the possible combinations have been tried with various levels of success.

Automobiles

In cars, this device is used to increase the "effective displacement" and volumetric efficiency of an engine; it is a blower that pushes the fuel air into the cylinders, as if the engine had larger valves and cylinders, resulting in a "larger" engine that weighs less.

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 car was the 1921 Mercedes 6/25/40 PS (road car). The next supercharged cars were almost all racing cars, including the 1923 Fiat 805-405, 1924 Alfa Romeo P2, 1924 Sunbeam, 1925 Miller, and the Delage, 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.

Boosting (attaching a supercharger) to a stock production naturally-aspirated engine, has returned as a practice, because of the increased quality of the alloys and the precision of the machining of modern engines. In the past, boosting greatly shortened engine life, because of the extremely high temperatures and pressures created by the supercharger, however, modern engines, made of modern materials, are over-designed to be stronger than required, thus, boosting is not a serious reliability concern. Commonly, boosting is done with small cars, the added supercharger's weight is less than the weight of a larger, greater-power engine. This decreases the fuel/mileage ratio, because mileage is a function of the car's total weight, most of which is the engine. Nevertheless, adding a supercharger often voids the car's drive-train warranty. Moreover, an improperly installed supercharger, or one with excessive boost, will greatly reduce the life of engine, the differential, and the transmission, because they were not originally designed and made to operate at the additional, greater rates of speed, time, and torque.

Supercharging and turbocharging

The term supercharging technically 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.

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 where engine response and power is more important than any other consideration, such as top-fuel dragsters and vehicles used in tractor pulling competitions, positive displacement superchargers are extremely common. Superchargers are generally the reason why tuned engines have a distinct high-pitched whine upon acceleration.

There are three main styles 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 Turbo Charging technology, throttle response on turbocharged cars is nearly as good as with mechanical 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.

Picking any method of compression that cannot support the mass of airflow needed for the engine creates excessive heat in the air/fuel charge temperatures. This is true with all forms of supercharging. It is critical to not under-size the component.

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.

Turbo-spool is often confused with the term turbo-lag. Turbo-lag refers to how long it takes to spool the turbo up when there is sufficient engine speed to create boost. This is greatly affected by the specifications of the turbocharger. If the turbocharger is too large for the power-band that is desired, needless time will be wasted trying to spool-up the turbocharger.

By correctly choosing a turbocharger, for its use, response time can be improved to the point of being nearly instant. Many well-matched turbochargers can provide boost at cruising speeds. Modern practice is to use two small turbos rather than one larger one, see Sequential, Twin and Compound turbochargers below.

Centrifugal superchargers suffer from a form of turbo spool. Due to the fact that the impeller speed is directly proportional to the engine RPM, the pressure and flow output at low RPM is limited, thus it is possible for the demand to outweigh the supply and a vacuum is created until the impeller reaches its compression threshold. This is not a great problem for aero-engines that almost always operate in the top half of their power output, but it is not much help in a car.

Sequential, Twin, two-stage turbocharging and Compound turbochargers

Many efforts have been made to mitigate the effects of turbo-lag in exhaust-driven turbochargers.

Sequential Turbo Charging was used on the Toyota Supra. The MkIV Toyota Supra uses two equally sized turbos. At low RPMs the exhaust gas is flowed through solely the first turbo. Once the boost pressure reaches a pre-set level, the exhaust gas flow is directed through both turbos equally. These two small turbos are then operating in parallel.

An alternative arrangement utilizes two turbochargers of the same size, known as a "twin-turbo". Twin turbocharging can make more power than a single turbo of the same output for two reasons. One is the lower rotating mass of two smaller turbos versus one large turbo, which allows the compressor to spin up to speed much more quickly. The second is the increased exhaust outlet area available for the exhaust gas to flow out of the twin turbo exhaust manifold. Increased exhaust flow will increase power in most situations.

Another style of turbo charging is called "two-stage", or "compound", turbocharging. This is gaining popularity for diesel engines. Tractor engines which compete in tractor pulling use two-stage and even three-stage turbocharging in some classes. Multiple stage turbocharging can create boost levels above 200 psi.

Two-stage turbocharging is set up in various fashions. The most popular set up is to use one smaller and one larger turbo. The larger turbo's compressor stage blows into the smaller one's. The exhaust is set up the other way round: it first enters the turbine of the smaller turbo, and then the turbine of the larger turbo. Two-stage turbocharging has little "turbo lag" and can create high power levels.

There are also layouts that combine a turbocharger and a positive displacement supercharger. Compressing the air first in the turbocharger, then feeding it to the supercharger improves efficiency in these designs as superchargers on their own are less efficient.

There is also another type of compound system called turbocompound. In these systems, a turbine section like that of a turbocharger is not used to power a compressor stage, but simply converts the energy from the exhaust into kinetic energy that is then used to add power to the crank shaft.

Still other combinations are possible - for example, there are after-market kits for several supercharged cars to add a turbocharger before, after or in parallel with the supercharger. In this manner, the supercharger operates alone at lower RPMs and the turbo either takes over from or adds to the supercharger once there is sufficient exhaust gas pressure available.

Aircraft

Altitude effects

Superchargers are a natural addition to aircraft engines for operation at high altitude. As an aircraft climbs to higher altitude, the pressure of the surrounding air quickly falls off. For example, at 5,486 m (18,000 ft) the air is at half the pressure of sea level. As a result, the engine produces half as much power, but the airframe only experiences half the aerodynamic drag.

A supercharger compresses the air back to sea-level 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.

Effects of temperature

The downside of supercharging is that compressing the air increases its temperature. When a supercharger is used on an aircraft, manifold air temperature becomes a major limiting factor in engine performance. Extreme temperatures will cause pre-ignition and/or 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. Pilots were taught to watch their manifold pressure gauge and not push it past redline, yet the manifold pressure gauge ignores the effect of temperature on engine performance and life. Several solutions to this problem 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 to bypass one stage as needed. Some systems had a cockpit control to open or close 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 prohibitively costly and complicated. Ultimately it was found that for most engines a single-stage two-speed setup was most suitable.

Comparison to turbocharging

It is interesting to compare all of this complexity to the same system implemented with a turbocharger. A supercharger inevitably requires some energy to be bled from the engine to drive the supercharger. On the single-stage single-speed supercharged Rolls Royce Merlin engine for instance, the supercharger uses up about 150 horsepower (110 kW). Yet the benefits outweigh the costs, for that 150 hp (110 kW), the engine is delivering 1,000 hp (750 kW) when it would otherwise deliver 750 hp (560 kW), a net gain of 250 hp. The overall additional 250 hp required 150 hp to make, with the accompanying increase in fuel burn. This is where the principle 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.

On the other hand, a turbocharger is driven using the exhaust gases. 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.

Yet the vast majority of WWII engines used superchargers, because they maintained three significant manufacturing advantages over turbochargers, which were larger, involved extra piping, and required exotic high-temperature materials in the turbine and pre-turbine section of the exhaust system. The size of the piping alone is a serious issue; consider that the Vought F4U and Republic P-47 used the same engine but the huge barrel-like fuselage of the latter was, in part, needed to hold the 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 WWII, all automobile and aviation fuel was generally rated at 87 octane. This was 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.

Research into "octane boosting" via additives was an ongoing line of research at the time. Using these techniques, less valuable crude could still supply large amounts of useful gasoline, which made it a valuable economic process. But the additives did not have to simply make poor quality oil into 87 octane gasoline; the same additives could also be used to boost the resulting gasoline to much higher octane ratings.

Higher octane fuel burns slower at the same temperature than low octane fuel, reducing the risk of detonation. As a result, the amount of boost supplied by the superchargers could be increased. In 1940 a batch of 100 octane fuel was delivered from the USA to the RAF. This allowed the boost on Merlin engines to be increased to 48 inHg (160 kPa) and the power to rise by more than 10% (from 1030 to 1160 hp, or 770 to 870 kW). By mid-1940 another increased boost yielded 1310 hp (980 kW). Supercharging by itself could not have achieved these improvements; however, when married with fuel improvements, the engine could respond to both. By the end of the war fuel was being delivered at a nominal 150 octane rating, on which the Merlin could reach about 1,700 hp and, with additional water injection, as high as 2000 hp.

In comparison the German oil industry had ready access to light crude from Romania and other European sources, and spent very little effort on octane boosting techniques. As a result their engines were all rated to use "B4" fuel at 87 octane, or the slightly higher 96 octane "C3". This limited the amount of boost they could use with their supercharger, which initially were of a higher level of development than their English counterparts. By 1941 the altitude advantage they had at the beginning of the war was erased, and as the war progressed their engines fell further and further behind. Their only solution was to build much larger engines, thereby constantly disrupting their assembly lines in order to introduce new models, leading to a chronic shortage of engines throughout the war.

The result was that late in WWII, British aircraft engines generally had higher critical altitudes than their German counterparts, which meant that British airplanes were generally able to outperform German ones in most situations.

See also

References

  • 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.

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