Supercharger

For other meanings, see supercharger (disambiguation)

A supercharger (also known as a blower) is an air compressor used to force more air (and hence more oxygen) into the combustion chamber(s) of an internal combustion engine than can be achieved at ambient atmospheric pressure.

The additional mass of oxygen-containing air that is forced into the engine improves on its volumetric efficiency which allows it to burn more fuel in a given cycle - which in turn makes it produce more power. A supercharger can be powered mechanically by belt, gear, or chain-drive from the engine's crankshaft. It can also be driven by a gas turbine powered by the exhaust gases from the engine. Turbine-driven superchargers are correctly referred to as turbo-superchargers - or more commonly as turbochargers.

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 later 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:

Roots
Lysholm screw
Sliding Vane
Scroll-type supercharger, also known as the G-lader
Piston
Wankel

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 6 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.

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
Multi stage axial flow

-Comprex superchargers do not fit neatly into either dynamic or positive displacement categories. The Comprex design uses the exhaust gas to directly compress the incoming charge.

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

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, the device is used to increase the "effective displacement" and volumetric efficiency of an engine, and is often referred to as a blower. By pushing the air into the cylinders, it is 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-Chrysler fame) became the first person to patent a forced-induction system for internal combustion engines. His first superchargers were based on a twin-rotor air-pump design first patented by American Francis Roots in 1860. This design is the basis for the modern Roots type supercharger.

It wasn't long before the supercharger was applied to custom racing cars, with the first supercharged production vehicles being built by Mercedes and Bentley in the 1920's. Since then superchargers (as well as turbochargers) have been widely applied to both racing and production cars, although their complexity and cost have largely relegated the supercharger to pricey performance cars.

Boosting, or adding a supercharger to a stock naturally-aspirated engine, has made a comeback in recent years due largely to the increased quality of the alloys and machining used in modern engines. In the past, boosting would dramatically shorten engine life due to the extreme temperature and pressure created by the supercharger, but modern engines produced with modern materials provide considerable overdesign; thus, boosting is no longer a serious reliability concern. For this reason boosting is commonly used in smaller cars, where the added weight of the supercharger is less than the weight of a larger engine delivering the same amount of power. This also results in better gas mileage, as mileage is often a function of the overall weight of the car, a sizeable percentage of which is weight of the engine. Nevertheless, adding boost to a car will often void the drivetrain warranty. Also, improperly installed or excessive boost will greatly reduce the life expectancy of both the engine as well as the transmission (which may not have been designed to cope with additional torque).

Supercharging and Turbocharging

A 2004 MINI Cooper S, with intercooled 1.6 l Eaton Roots-type supercharger. Note the hood scoop which supplies cool air to the intercooler.

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 gasses.

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. Cars that whine in this way include the Ford Mustang Cobra, Mercedes SLR and the MINI Cooper S.

There are three main styles of supercharger for automotive use:

Centrifugal turbochargers - driven from exhaust gasses.
Centrifugal superchargers - driven directly by the engine via a belt-drive.
Positive displacement pumps (such as the Roots and the Lysholm (Whipple) 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. With the latest Turbo Charging technology, throttle response on turbocharged cars is nearly as good as with mechanical powered superchargers. 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. The Lysholm style blowers are nearly as efficient as their Centrifugal counterparts.

Keeping the air that enters the engine cool is an important part of the design of both superchargers and turbochargers. Compressing air makes it hotter - 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 undersize the component.

Turbochargers also suffer (to a greater or lesser extent) from so-called turbo-lag in which initial acceleration from low RPM's is limited by the lack of sufficient exhaust gas pressure. Once engine RPM is sufficient to start the turbo 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-lag is often confused with the term Turbo-spool. Turbo Lag refers to how long it takes to spool the turbo 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 powerband that is desired, needless time will be wasted trying to spool 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.

Centrifugal turbochargers suffer from a form of turbo spool. Due to the fact that the turbine speed is directly proportional to the RPM, 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 turbine reaches its compression threshold.

Sequential and Twin 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 Turbo Charging 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 "Compound Turbo charging". This is gaining popularity for diesel engines. Tractor engines which compete in tractor pulling use compound turbo charging in some classes. Compound Turbo Charging can create boost levels above 200psig. Compound turbochargers are set up in various fashions. The most popular set up is to use one smaller and one larger turbo. The larger turbo compressor blows into the smaller turbo compressor. The exhaust is set up to first enter the turbine of the smaller turbo, and then into the turbine of the larger turbo. Compound Turbo Charging has little "turbo lag" and can create high power levels.

There are also acts of combining both turbocharging, and a positive displacement supercharger. By compressing air first in the turbocharger, and feeding it to the supercharger. By running more compression in the turbocharger, efficiency is improved as superchargers are less efficient.

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

Aircraft

A more natural use of the supercharger is with aircraft engines. As an aircraft climbs to higher altitudes the pressure of the surrounding air quickly falls off—at 6000 m (18,000 ft) the air is at half the pressure of sea level. Since the charge in the cylinders is being pushed in by this air pressure it means that the engine will normally produce half-power at full throttle at this altitude.

Altitude effects

A supercharger remedies this problem by compressing the air back to sea-level pressures, or even much higher. This 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) lost, the engine is delivering 1000 hp (750 kW) when it would otherwise deliver 750 hp (560 kW), a net improvement of 250 hp. And while the supercharged engine delivers as much or more thrust as it did at sea level, the airframe only experiences half the aerodynamic drag due to the low atmospheric pressure at high altitude. For this reason supercharged planes are able to fly much faster at higher altitudes.

A supercharger is only able to supply a limited amount of pressure because the compression increases the air temperature, and the engine is limited in maximum charge-air temperature before engine knock occurs. Intercoolers and aftercoolers are often used to get around this problem. The boost is typically measured as the altitude at which the supercharger can still supply sea level pressure (101 kPa or 1013 mbar) and is referred to as the critical altitude.

Altitude efficiency

Below the critical altitude the supercharger is capable of delivering too much boost and must therefore be restricted lest the engine be damaged. Unless other measures are taken, this means that at least some of the power driving the supercharger is wasted. Also, due to the denser air at lower altitudes, the supercharger is not operating at its best efficiency, and this can cause an additional load on the engine.

For the early years of the war this was simply how it was, and this led to the seemingly odd fact that many early-war engines actually delivered less power at lower altitudes. This was because the supercharger was still using up power to compress air that was not delivering any power back. As the war progressed two-speed superchargers were introduced using better controllers and, notably, hydraulic clutches, that allowed the boost to be managed over a wide range of altitudes by operating at low rpm down low and at high rpm at higher altitudes. This generally "flattened out" the power below the critical altitude.

Throughout WWII British superchargers generally had higher critical altitudes than their German counterparts and, when combined with the higher octane fuels that the Americans supplied, this allowed far higher boost levels, which meant that British airplane engines were generally able to outperform German ones in most situations.

Improving 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.

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 "B2" 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 progessed 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 thoughout the war.

Multiple stages

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. Ultimately it was found that for most engines (excepting those in high-performance fighters) a single-stage two-speed setup was most suitable.

A final improvement was the use of two compressors in series, which were introduced to solve combustion problems. Compressing a gas always causes its temperature to rise, and highly compressed fuel-air mixtures may prematurely ignite or may detonate or both. In order to avoid combustion problems the "two stage" design was used. After being compressed "half-way" in the low pressure stage the air flowed through an intercooler radiator where it was partially cooled down before being compressed the rest of the way in the high pressure stage and then aftercooled in another air/air or coolant/air radiator (heat exchanger). At low altitudes one stage could be turned off completely. The two-stage Merlin was losing 400 hp (300 kW) to turn the supercharger but developing between 1500 and 1700 hp (1125 to 1275 kW) at the propeller shaft, depending on model.

It is interesting to compare all of this complexity to the same system implemented with a turbocharger. Since the turbo is driven off the exhaust gases, simply bypassing some of the exhaust pressure is sufficient to drive the compressor at almost any desired speed. In addition the power in the exhaust would otherwise be wasted (except to the extent that the exhaust itself provided thrust) whereas in the supercharger that power is being taken directly from the engine. Thus at low altitudes the turbo robs nothing and, as the altitude increases, it can use just as much power as it needs and no more. Better yet the amount of power in the gas is the difference between the exhaust pressure and air pressure, which increases with altitude, so turbochargers generally have much better altitude performance.

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. 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.

Supercharging in jet engines

Supercharging is not confined to internal combustion engines — jet engines rely on supercharging as one of the main methods to improve thrust and fuel efficiency.

For example, adding an additional (i.e. zero) stage to a compressor will not only increase the overall pressure ratio of the cycle, but induce more airflow into the unit, by supercharging the entry plane of the original compressor. Stage 1 entry conditions can be used to compute the supercharged core flow:

$w_{new}=w_{old}*{\frac {P_{stage1new}}{P_{stage1old}}}*{\sqrt {\frac {T_{stage1old}}{T_{stage1new}}}}$

where:

$w\,$ ~ core mass flow

$P_{stage1}\,$ ~ stage 1 entry total pressure

$T_{stage1}\,$ ~ stage 1 entry total temperature

Alternatively, the compressor delivery conditions can be used to calculate the new core flow:

$w_{new}=w_{old}*{\frac {P_{delnew}}{P_{delold}}}*{\sqrt {\frac {T_{delold}}{T_{delnew}}}}$

Ideally, the corrected (i.e. non-dimensional) speed of the original compressor should be maintained, by raising the mechanical shaft speed by a factor ${\sqrt {\frac {T_{stage1new}}{T_{stage1old}}}}$ .

However, if stress considerations prevent any shaft speed increase, the corrected speed of the original compressor will decrease, resulting in only a relatively modest increase in airflow.

Converting a turbojet into a turbofan, by adding a fan spool, also supercharges the compression system, thereby raising core flow.

Many of the large turbofan engine series (e.g. Pratt and Whitney PW4000) have gained core flow by adding one or more stages to the front of the gas generator, usually in the LP (or IP) compressor. If the fan flow is not increased, the bypass ratio will decrease.

Supercharging can also be achieved by improving the aerodynamics of the existing blading. Core flow will increase if the original compressor outlet (corrected) flow size is maintained.

Either way, raising core flow increases core power and, thereby, the net thrust or shaftpower of the engine. Raising overall pressure ratio tends to improve specific fuel consumption (i.e. fuel efficiency).

References

Allied Aircraft Piston Engines of World War II, Graham White 1995: Airlife Publishing Ltd, England and Society of Automotive Engineers, Inc., in the USA. ISBN 1-85310-734-4
http://auto.howstuffworks.com/supercharger.htm

External links