Tech Focus: Superchargers. All you ever wanted to know about superchargers but were afraid to ask. One of the best ways to extract more power from a normally aspirated BMW is to fit a supercharger but do you know the different types there are and how they work?
It’s now impossible to walk into a BMW dealer and buy a car with a naturally aspirated engine – the need for lower emissions has seen the inexorable rise of the turbocharger in the new models – but you don’t have to dip too far back into the past to find a whole host of BMWs that are suitable for a supercharger upgrade. And who wouldn’t like some more power? But what are the different types of supercharger available and how do they work?
Why centrifugal superchargers?
The Centrifugal Supercharger is the most compact system, probably the easiest to install and one of the most efficient of the compressor systems, meaning they do not heat the compressed air as much as other designs as long as they are correctly chosen. They do however suffer from the same disadvantage as the Turbocharger in that they do not produce boost at low engine speeds, and if they are geared, or sized to do so, will quickly overspeed or overboost at higher engine speeds. When selecting a suitable supercharger for an application, it is important to study the compressor airflow and efficiency “maps” that come from the supercharger manufacturer.
The characteristics of the centrifugal supercharger make them a good choice for an installation on a street car because with little or no boost at low engine rpm, there are no traction issues, the engine and transmission are not loaded heavily at low speed, front wheel drive cars do not suffer from torque steer, and being engine driven, the engines are far more responsive than in a comparable turbocharger installation. Another significant advantage of the design over the turbocharger and the positive displacement supercharger is drive efficiency.
Another advantage of the Centrifugal Supercharger is its self-contained lubrication system. This saves all the plumbing required taking an oil supply from the engines lubrication system and installing a drain back to the engine. Some units use sealed bearings, some have an internal splash or low pressure pumped lubrication, and the Rotrex units use a special traction fluid that acts like an oil for lubrication normally, but when compressed on the compressor driveshaft, acts like a much thicker drive fluid increasing “traction” between the shafts.
With the compressor effectively being run from the crankshaft, albeit through a belt and transmission system, even at low rpm when boost is not being produced, the compressor wheel is still spinning, reducing any resistance to airflow from the induction system into the engine, so with no boost being produced, the belt load is slight, therefore not robbing the engine of any significant power to drive it (parasitic load), which also helps with fuel economy. Lower drivebelt power transmission also has the benefit of decreasing lateral.
Centrifugal superchargers: the basics
In many ways, the centrifugal supercharger design has both advantages and disadvantages of the traditional Positive Displacement Supercharger (blower) and the more familiar Turbo-Supercharger (Turbo).
As the Centrifugal Supercharger is essentially a hybrid of the other two types, one could argue that your large capacity Supercharged V8 is in fact a Hybrid, and must therefore be essentially good for the environment… good luck with that argument!
With advances in materials technology over the past few years, it has been possible to build a belt driven Turbocharger, (Centrifugal Supercharger) with either gears, internal belt drive, or a sun and planet arrangement fluid drive, capable of driving a small compressor rotor up to around 200,000 rpm. The drive for the transmission is taken from the engine, usually via a belt driven by the crankshaft pulley, and then an internal transmission arrangement, either gear or fluid drive, increases the speed of the compressor to it’s effective speed range.
The centrifugal supercharger gets its name from its principle of operation. A spinning conical shaped, radial vaned wheel (the compressor) pulls air into the centre and whilst spinning, throws the air out under centrifugal force between vanes formed in the wheel, into a snail shell shaped chamber.
In this chamber, as the diameter increases, the airflow slows and increases pressure. This is exactly the same principle, in reverse, of a carburettor venturi where the decrease in area, increases the speed, decreasing the pressure of the air passing through it. As the area of the scroll increases, the pressure continually increases and the scroll design gives the internal compression ratio of the unit.
As the engine speeds increase, the compressor enters its “efficiency envelope” and it starts to make boost. This boost then starts to load the drivebelt, and hence the crank, although still producing far more power than it is taking to drive it.
The belt drive system is a very important consideration in the design of the installation. The ideal drive system will have a supercharger running in its maximum efficiency envelope derived from the compressor map, approaching maximum design speed, at the engine redline. If you overdrive the supercharger to increase boost at lower rpm, by increasing the size of the crank drive pulley, or reducing the diameter of the driven (supercharger) pulley, then you will either overspeed the supercharger, or the compressor will spin out of its efficiency envelope and either significantly heat the air charge, or start to stall. If you overdrive the supercharger, beyond its maximum design speed, then it’s lifespan will be significantly reduced or it can suffer a catastrophic failure.
Due to the lower loads on the drivebelt of a Centrifugal Supercharger, the belt width and speed can be reduced. This is achieved by increasing the pulley diameters by the same ratio which allows greater belt contact and wrap angle with the pulley and therefore traction between the belt and pulleys, whilst also reducing belt speeds.
You will find that as the rpm increases, the boost rises at an ever-increasing rate giving the feeling of endless surge experienced with the centrifugal supercharger. The faster you go, the harder it accelerates, and this is a very intoxicating sensation.
To increase the lower rpm boost from the centrifugal supercharger, you can either overspeed it, which is not recommended, or use a higher flow compressor. This introduces the problem of excessive boost at higher engine speeds. You would then need to have a system in place to “dump” all the excess boost at a preset maximum limit. The system will need a method of dumping excessive pressure anyway to prevent compressor surge.
Compressor surge occurs when the supercharger is producing boost and you then close the throttle. Apart from the pressure on the throttle butterfly, the pressure builds up in front of the compressor outlet with nowhere to go. The pressure then reacts back on the compressor. Then, when the throttle is reopened, the compressor has to restart pushing the stalled column of compressed air back into the intake manifold. The cure for this on a centrifugal compressor system is a simple “blow-off” valve utilising the vacuum caused in the inlet manifold during throttle plate closure, to lift the valve and vent all the pressure between the supercharger and the throttle plate.
A further consideration for discharging this pressure is the stress on the thin throttle butterfly and spindle. Consider those two tiny screws that hold it on the shaft of a 75mm throttle valve. Consider slamming the throttle shut on your engine making a reasonable 15psi boost which would equate to 106lbs of force pressing on that spindle! This level of force on a spindle not designed for it can bend the spindle or wedge the throttle plate shut, neither of which will help your power delivery, so it is important to relieve this pressure.
Choosing your compressor
When selecting your compressor size, you need to carefully examine the compressor maps. These are graphs produced by the manufacturer of Airflow from the compressor plotted against the pressure ratio produces at a series of compressor rpms. These graphs show the compressor efficiency plotted together with the compressor rpm.
So what performance could one expect from a properly installed centrifugal supercharger? Well, the torque increase is substantial, with an increase in the region of 40 percent on a low boost of just 0.4 Bar (6psi), or to put it another way, the car will accelerate as hard in 4th gear as it did unsupercharged in 3rd, and on 0.7 Bar (10.5psi) it will carry out the 0-100kph sprint 33 percent quicker, so from 8 secs to under 6 if you can get the traction. There are a lot of other things to consider to do this, such as correct fuelling, ignition and intercooling, and these will be dealt with at a later stage, but for now, do you start reading your compressor maps, or wait to see what is achievable from a positive displacement supercharger?
Positive displacement superchargers:
Having had a look at centrifugal superchargers we can now move on to the traditional supercharger, the Positive Displacement Supercharger. There are substantial differences between the Positive Displacement and the Centrifugal Supercharger, including the way in which the air is delivered to the engine, the drive system and the power delivery, as well as the effects of the above on the response from the engine. Different supercharger types can have an internal compression ratio, and be called compressors, delivering an air charge at a higher than atmospheric pressure, or may be described as ‘blowers’, where the discharged air is still at atmospheric air pressure, just more of it.
Imagine your 2000cc engine is fitted with a Centrifugal Supercharger running at 15 psi boost. Every induction stroke contains air delivered to the engine at double atmospheric pressure, so your engine will ingest the equivalent of approximately 4000cc of atmospheric pressure air for combustion. The positive displacement supercharger moves a fixed amount of air through it per rotation, and are therefore sized in cubic capacity. If we fit a 2000cc supercharger to our 2000cc engine and run it at engine speed (a 1 to 1 drive ratio), every engine rotation will cause the supercharger to force 2000cc of air into the inlet manifold. This will be air at atmospheric pressure, until it is forced into the intake, at which point the pressure will then increase due to the squeezing effect of forcing that amount of air into a smaller volume. As the engine only inhales half its cylinders per revolution, as in all 4 stroke engines, then 1000cc of induction inhales 2000cc of air, giving it the same 15 psi as in the centrifugal example above.
You can see then, at whatever engine speed this occurs at, from idle to the redline, there is a pressurised intake manifold, and therefore boost pressure available, even at idle! This gives the PD supercharged car virtually instantaneous throttle response, unlike the higher engine rpm reliance of the CF supercharger, and the even worse Turbocharger response. This then gives the engine fitted with a PD supercharger, the characteristics of an engine, bearing more in relation to the supercharger output than the actual engine size, so our hypothetical 2000cc engine, running at 1 Bar boost will actually drive like a 4000cc engine, with corresponding torque and power.
The Roots blower
The most common type of PD supercharger is known as the Roots type supercharger and this design has certainly been around the longest. Designed around 1860 by the Rootes Brothers as a simple industrial air pump and not fitted to an internal combustion engine until 1900 by Daimler, it has been further developed recently by the Eaton Corporation to make it more user friendly on standard road vehicles, rather than all out race applications. Recent developments by Eaton have included operational noise reduction and increased thermal efficiency using improved production techniques utilising the latest computer design CAD software and computer controlled CNC manufacturing. When comparing efficiencies, last month we looked at the CF supercharger achieving efficiencies around the 78 percent mark. With the old style Roots blowers, this can be as low as 40 percent or less, and even the significant improvements made by Eaton, have only increased this to around 60 percent.
The design is effectively a pair of rotors, either 2 or 3 lobe, intermeshed and rotating within an enclosed case. The rotors almost seal between each other and the walls of the casing using very close tolerances to draw a fixed amount of air through, and positively push it out the other side. There is a small leakage of gases between the rotors and the casing so the boost is slightly lower at idle due to this “blow-by”. These rotors are driven off the engine by a toothed belt, the speed of which is controlled by the drive pulley size ratio, exactly like the Centrifugal Supercharger. One of the most significant advances is the use now of rotors twisted 60 degrees longitudinally to almost eliminate the pulsing, and associated noise of the traditional straight rotor design.
The PD supercharger became hugely popular in the USA with Hot Rodders when General Motors fitted a small Roots style blower to the small 3 cylinder diesel engines. These engines had a cylinder capacity of 3.5 litres or approximately 1163cc per cylinder. Known in the USA as 71 cubic inches, (71 cu/in = 1163cc) this became known in tuning circles as the GM 3-71 and then clearly the six-cylinder diesel supercharger became the GM 6-71. These were designed with a “redline” rpm of 5200 rpm but by the time the modifiers had redesigned their components and were using their own rotors and gears, this had risen to over 15000 rpm and an 8-71 in an attempt to get sufficient airflow from them for the large capacity race motors. Eventually, PD superchargers were purpose built for high performance installations by manufacturers such as Mooneyham who added Teflon seals and using 500mm long twisted rotors built blowers with the designation 14-71 to increase the airflow from lower rotor speeds, down from 15500 rpm to 10000 rpm for reliability. Such monster blowers running at a heady 45 psi boost and 25 percent overdrive on an 8.0 litre V8 engine are now making 7000 bhp and over 6000 ft/lbs of torque!
Positive displacement superchargers:
So, it would seem then that the PD supercharger is better in all respects to the CF supercharger at first glance, but there are some disadvantages to the design. Having boost, and the associated power and torque, from idle, seems an ideal situation, but all the time there is boost, there is a parasitic power draw on the engine to drive it, and that takes fuel to compensate, so fuel economy will suffer. Your 2000cc car could then be as fuel efficient as a 4000cc car. Not much fun if you are stuck in traffic rather than enjoying a country lane blast.
Having boost at idle and very low rpm also puts a heavy load on engine components. Not a problem when the engine was designed to accept the blower, such as a Jaguar XKR or Mercedes SLK, but retro-fitting one on your 316i and loading the engine 50 percent more at idle than one could expect from an M3 will certainly take its toll on components, especially big-ends getting thumped with double their design load. A solution to this problem can be by means of a drive clutch, similar to the electromagnetic clutch found on air conditioning compressors, or a cone type clutch as used by Mercedes in the early 1920s. The Roots type PD supercharger is also less thermally efficient that the CF superchargers and heat production is the main enemy of power production. Efficiency can be increased by injecting water or methanol into the blower which helps to seal the clearances between rotors and casing, also evaporating whilst travelling through the blower giving an intercooling effect, but this is just not practical on a road car, so keeping boost pressure low are the better option.
The Twin-screw supercharger
There is another type of PD supercharger design we should mention and this is the Lysholm Screw (Twin Screw) type blower. We describe it as a ‘blower’ because, as with the Roots type blower, it was originally designed as an industrial air pump around 1784 and consists of two interconnected helix shaped rotors, one with a male profile, the other a female, similar to a worm gear, meshing together and driven exactly the same as a Roots type blower, but with the air being drawn lengthways along the unit, rather than across as in the Roots. As the air travels along the unit it is compressed, therefore the unit can be considered a compressor rather than a blower.
Externally, they appear very similar to the Roots and the original units used oil inside to help seal the rotors and the case clearances but in 1934, production techniques had improved and a Swedish engineer, Alf Lysholm patented the design as a supercharger. This design would appear to be the future of supercharging with names such as Sprintex, Whipple, Opcon and the well-known IHI of Japan now manufacturing this design.
The TS supercharger is very flexible in its application because it can produce high boost levels similar to CF compressors, both in CF superchargers and turbochargers, yet still produce significant low rpm boost whereas the Roots type SC will struggle with boost pressure ratios over 15psi. As with all PD superchargers, the boost level from the TS is controlled by the drive ratio from the engine. If set for high maximum boost, then it will also produce high boost at idle. There will be a slight reduction of boost levels at very low rpm due to air bypassing the rotor and case clearances as in the Roots, but because of the ability to run tighter clearances, this still makes the TS the king of low speed boost so why do we not just run massive boost, and hence power with high speed drive ratios? Surely, too much power is still not enough? Well, there are some problems when running too much low speed boost and this gets progressively worse the fewer cylinders you have. Consider the fact that on a small four-cylinder engine, let’s say a 318i, you only have two power strokes per engine revolution. Now with a substantial amount of boost, those power strokes are significantly more violent than they were originally. Although happening around ten times per second at idle, there is a cyclic load on the crank where the crank is violently accelerated by the power stroke and then decelerated as it loses its torque to output the power.
The idle speed you see and measure is therefore the average of this speed. As each power stroke is the only time there is any torque input to the crank, and the rest of the time it is zero, this causes rough running even at boost levels of just 0.4bar boost, similar in effect to an ultra-light-flywheel. However, consider the same situation on your 760i with its 12 cylinders and you have an additional four fi ring cycles per revolution with an overlapping torque input, meaning the crank has barely started to decelerate before the next torque input from a cylinder on its power stroke.
Twin-screw supercharger benefits
As with the PD supercharger, there is a slight reduction in boost at idle due to leakage past the rotor seals but this is very quickly gained as soon as the rpms start to rise. The drive components for the TS supercharger are similar to the Roots with the only significant difference being the direction of flow and the rotor profiles. The smaller diameter rotors in the TS supercharger have the additional benefits of allowing higher operational speeds due to the reduction in centrifugal loadings, the internal compression ratio allowing higher pressure ratios with less friction, and therefore heat, all contributing to an improved volumetric and thermal efficiency and as we discussed earlier, the less heat and higher efficiencies we can get, the more power we can produce.
The twin-screw supercharger’s volumetric efficiency can be as much as 95 percent at lower boost levels while still maintaining around 80 percent at high boost levels of about 30psi which is a significant level of boost. This makes them as efficient at high boost as a centrifugal is while in its most efficient operational speed. Thermal efficiency stays consistently around 70 percent making it superior to the Roots and almost as efficient as a turbocharger. However, due to the fact that it compresses the air, even at idle, the supercharger runs warm to the touch. With the latest CAD and production facilities, these are now manufactured to run without oil to seal at less than 0.04 mm helix/helix and helix/case clearances.
This produces a supercharger that is more efficient than a CF supercharger (up from 78 percent to around 85 percent) has no air pulsing like the Roots blower, is small and light, easy to drive with less parasitic power draw than a Roots, produces boost from very low rpm like a Roots, yet continues to produce highly efficient boost at high rpm, just like a centrifugal supercharger or turbocharger.
This will explain why this supercharger system is the most popular of the aftermarket kits available where the supercharger is mounted longitudinally along the line of the inlet manifold in straight-four and straight-six engines whereas the centrifugal supercharger is the main choice for the V-engines where the Roots type blower would look awesome, but would in all cases protrude through the bonnet, which is hardly a subtle modification.
However, a pair of TS superchargers either side of a V8 or V12 would be a brilliant modification if space permitted. Such an application was on Aston Martin V8s in the 1990s which produced more power 25 years ago than a new 6.0-litre V12 Vanquish does today!