Forced Induction

Ram Air Intake

Ram air intake is perhaps the simplest kind of forced induction. When the car is travelling at speed, air will be forced into the intake manifold through the ram air inlet which usually locates at the top of bonnet. This generates a slightly higher pressure than normal aspiration. The higher intake pressure not only accelerates engine breathing but also leads to a slightly denser air filling the combustion chambers, very much like a light supercharging. Of course, the latter helps generating more power.

You can clearly see ram air inlet in the bonnet of Ferrari 550 Maranello. Don't confuse it with inlet for intercooler, this car is not turbocharged !

Ram air intakes can be easily seen in motor racing. The air box on every Formula race car and the roof-mounted air intakes on many GT or endurance race cars are ram air devices. A Formula One engineer said a typical air box can contribute to an additional 20 horsepower at 200 km/h (124 mph). It's not a big gain, but it's free, with almost no drawbacks. At low speed, however, the effect of ram air is negligible.

Advantage Power gain at high speed
Disadvantage Negligible gain at low speed
Who use it ? Many American muscle cars in the 1960s-70s; Ferrari 550/575M, McLaren F1, Lamborghini Diablo SV/GT, Bugatti Veyron, Lotus Exige, Porsche 911 GT3 RS etc.


Before turbocharging arrived in the 1960s, supercharging had been dominating the forced induction world. Supercharging, also called mechanical charging, appeared in the early 1920s in Grand Prix cars as a means to boost power without enlarging engines. Since the compressor is driven directly by the crankshaft, it has the advantage of instant response compare with turbocharging. On the downside, the charger itself is rather heavy and energy inefficient, thus it cannot produce as much power as turbocharger. Especially at high rev, it generates a lot of friction hence energy loss and prevent the engine from revving higher.

A typical supercharger transforms the output characteristic very much - it flattens the torque curve and shifts its peak towards the lower end of spectrum. Consequently, power becomes much easier to access. On the flip side, the engine becomes less enthusiastic for rev. A lower redline and declining high-rev output means the supercharged engine does not encourage the driver to work harder at throttle or gearchange. For these reasons, supercharging is quite well suited to nowadays heavy sedans, espeically those mated with automatic transmission. On the other hand, sports cars find little merits to use it.

The noise, friction and vibration generated by mechanical supercharger are the main reasons preventing it from being used in luxury cars. Although Mercedes-Benz did introduce a couple of supercharged fours (dubbed "Kompressor") into its C-class, it had never tried the same idea on larger models. In 2010, the Kompressor engines on C-class were replaced with turbocharged ones, ending the company's mass use of superchargers.

The advancement of other technologies, such as variable valve timing, direct-injection, light-pressure turbos and advanced turbo diesel, also threatened the very existence of supercharging. Volkswagen dropped its G-Ladder supercharger in the mid-1990s in favour of light-pressure turbo. General Motors used to produce the most supercharging engines thanks to its supercharged 3800 V6. Following the retirement of that engine in the late 2000s, its usage of supercharging is limited to the small batches of high-performance Cadillac CTS-V and Corvette ZR1.

Roots type supercharger

Roots type supercharger is named after its inventor, the Roots brothers. It was invented as an industrial air pump well before Daimler and Benz invented motor cars. Surprisingly, the ancient design is still being used in most numbers today.

The Roots supercharger consists of two rotors, usually with 3 lobes each. They rotate in counter direction to pump the air from inlet to outlet (see the first picture). This motion does not compress the air inside the supercharger. However, as the supercharger supplies air faster than what the engine can consume, high pressure is built up in the intake manifold.

The first picture shows the classic Roots type supercharger has its inlet and outlet located above and below the main body respectively. While it is simple for understanding, it is not necessarily an effective design. Why? because when air enters the supercharger, it actually hits against the rotor lobes, which are running at direction opposite to the air flow. Therefore this supercharger construction is far from efficient.

That is why the modern Roots superchargers you can find on production cars have a rather different construction, as shown in the second picture. The inlet locates at the front (rather than top) of the supercharger body, while the outlet locates at the bottom surface, but close to the back of the body (i.e. opposite to the inlet). Moreover, the rotor lobes are made twisted (see the third picture for a better view). Relocating the inlet has a couple of advantages. Firstly, it is easier to package. Especially when the supercharger sits on top of the engine, a front-facing inlet saves space under the bonnet, whereas a top-facing inlet may need a power dome to accommodate the intake piping. Secondly, the front-facing inlet avoids the aforementioned drawback of the classic design. Air enters the rotor chambers in axial direction, so it does not go against the rotor lobes. To promote the axial flowing of air, the outlet is opened at the opposite end and twisted rotors are used. As the twisted rotors rotate, they pushes the air from the inlet side towards to outlet side. Consequently, a smooth flowing is obtained and efficiency is enhanced.

Roots type supercharger consumes a lot of power at high speed. Therefore, when boost of power is not required, for example, when the car is cruising on highway, the supercharger is better to be disengaged from the engine. The third picture shows the Kompressor on Mercedes four-cylinder engine has an electromagnetic clutch to actuate the engagement / disengagement.

Another solution adopted by some superchargers is the use of by-pass valve to establish a link between the inlet and outlet. This largely reduces pumping loss, but the moving mechanicals still consumes power due to friction.

Advantage Cheap to build
Disadvantage Low efficiency, low boost, consumes lots of power at high rev, ugly noise.
Who use it ? Mercedes M111 and M271 4-cylinder, GM 3800 V6 and most affordable supercharged engines.

Eaton TVS supercharger

Eaton TVS (Twin-Vortices Series) is a Roots type supercharger, but it has a number of significant improvements over previous Roots superchargers. Therefore we are going to spend an extra section to discuss about it.

Eaton has been the largest producer of superchargers for many years. The aforementioned Roots type superchargers on Mercedes four-cylinder engine and GM 3800 V6 were all its products. However, what really catch our attention - and would probably reverse the declining trend of superchargers - is its new TVS supercharger. It was first introduced to Chevrolet Corvette ZR1 and Cadillac CTS-V in 2008. The same year saw Audi selected it to power its new 3.0 TFSI V6. The latter is a testament of its much improved performance and refinement.

Compare with the previous generation (Gen 5) Eaton superchargers, the TVS has many advantages. First of all, it can provide 20 percent higher air flow, hence higher boost and power; Secondly, it consumes less power at high rev. For example, the one on Corvette ZR1 consumes 75 horsepower at peak power, versus 115 hp on the Gen 5. This translates to higher efficiency, of course. Thirdly, it generates less heat thus depends less on intercooling. Lastly but not least, it largely eliminates the ugly noise traditionally associated with Roots-type superchargers.

From mechanical point of view, the TVS has two significant changes from conventional Roots type superchargers: 1) Its rotors have 4 lobes instead of 3 lobes; 2) The twist angle of its rotors is increased from 60 degrees (on Gen 5) to 160 degrees.

Let's see how these changes affect its performance.

The TVS produces higher air flow because the 4-lobe rotors enable larger total volume than the old 3-lobe rotors. It's only simple geometry.

Regarding accoustic quality, by doubling the tooth of drive gears, mechanical noise is lifted to higher frequency range which is less annoying to human ears. Besides, better internal air flow management reduces air noise.

Regarding higher efficiency, it is contributed by two factors. The first is the larger air inlet on TVS. The illustrations below show a comparison between conventional Roots supercharger and TVS on how they go through one cycle of operation. The cycle consists of 3 phases - Expansion, Seal and Discharge. It is the Seal phase that dictates the size of air inlet. The illustration shows that the TVS can use considerably larger air inlet because its 4-lobe design reduces the span of Seal phase to 90 degrees (vs 120 degrees on the 3-lobe design). This allows the inlet to open wider. If the conventional supercharger used the same large inlet as TVS, the Seal would never happen, and the chamber would bridge between the inlet and outlet thus release all the high pressure at the outlet side.

Conventional Roots Supercharger                            
TVS Supercharger                                   

So what is the benefit of a larger inlet? The most obvious is less restriction on the rate of air flow. But we are now talking about efficiency rather than the amount of air flow, so there must be another reason... actually a very complicated reason. It takes some more explaination...

To understand how it improved efficiency, perhaps we should study how a conventional Roots supercharger waste energy first. As I have mentioned, part of the energy is wasted through mechanical friction. (TVS reduces that loss by using tighter machining rotors and friction-reducing coating.) However, an even higher percentage of energy loss is due to the internal flow of the supercharger. Let's look back to the illustration above. At the start of expansion phase, you can see a new space is created between the rotors (the blue area). This space enlarges quickly (the second picture) and sucks air from the inlet to fill that space. In fact, the rotor is spinning so fast (up to 20,000 rpm) that this space enlarges at enormous speed. As a result, the part vacuum created is also enormous and the air rushes into this space at very high velocity. You can imagine what happens when this high-speed air flow hits the end of the space - it finds no way to go, then compresses, bounces back, hits the incoming air following it and causes great turbulences. Such rapid change of velocity and pressure transforms the kinetic energy to heat. In fact, Roots-type superchargers waste a lot of energy in the form of heat. It takes charge cooler and intercooler to lower the temperature, but then wasted is wasted.

Now, the TVS has a considerably larger inlet. This mean the air flow goes through the inlet will be slower. Slower air flow means less kinetic energy, hence less energy is transformed to heat.

Another factor contributing to its improved efficiency is the larger twist angle of rotors. With 160º twist angle, in Expansion phase the rotor takes 160
º of rotation to realize full volume of chamber (blue area in picture). This mean the rate of expansion is much slower than the case of 60º twist angle, where full volume is realized with just 60º of rotation. Slower expansion reduces the part vacuum thus lowers further the velocity of intake air flow. Consequently, the aforementioned waste of kinetic energy is also reduced.

Advantage Pretty high efficiency, acceptable noise.
Disadvantage -
Who use it ? Chevy Corvette ZR1, Cadillac CTS-V, Audi S4, Jaguar XFR, Lotus Evora S.

Lysholm (screw type) supercharger

A typical Lysholm supercharger has a 3-lobe male screw and a 5-lobe female screw, though other combinations are possible.

Lysholm is also called screw type supercharger. Although its theory was invented as early as 1878, it was not until 1930s that Alf Lysholm improved and realized the idea. The supercharger consists of 2 screws, one with male threads and another with female threads. They closely mesh together. When they rotate, air is captured between the screws and the housing while being pushed from the inlet towards the outlet. Moreover, the space gets smaller and smaller as it moves forward, so Lysholm performs internal compression and enables higher boost pressure than Roots type supercharger.

Apart from high boost pressure, Lysholm supercharger has advantages of high efficiency, wide rpm range and compact size, so it is the first choice for performance cars. On the downside, it is very expensive to build, because the tightly meshed nature of the screws means they need very high precision machining.

The high costs hampers its popularity. Few production cars are known to have it equipped, e.g. Mazda Millenium Miller Cycle, Ford GT, Mercedes-AMG's 3.2 V6 (pictured) and 5.5 V8. It is more popular in aftermarket. Many car tuners prefer to use it to boost performance.

Advantage High efficiency, high boost pressure, wide rpm range.
Disadvantage Expensive to build
Who use it ? Mercedes "55" AMG series (including SLR), C32 AMG, Ford GT.

Centrifugal supercharger

Centrifugal supercharger is very similar to turbocharger, except that it is driven by the crankshaft instead of exhaust gas. This mean it has only one turbine. Like turbochargers, its turbine needs to spin at very high speed (up to 60,000 rpm) to produce maximum boost pressure. To make this possible, it incorporates step-up gears to multiply rev from crankshaft.

The boost pressure grows exponentially with rev. As a result, centrifugal supercharger produces little boost at low to medium rev. It works best at high rpm. Unlike a turbocharger which has waste gate to keep boost pressure constant at high rev, centrigual supercharger does not have such device. Therefore its output characteristic shall be designed to produce maximum boost at the engine's peak output. This also limits the boost it can provide at lower rpm.

Like turbo, centrifugal supercharger uses impeller to spin the air outward by centrifugal force. Diffuser vanes guide the air flow to the outlet. The simple construction leads to advantages of lightweight and compact size. Therefore it is easy to be fitted to existing production cars as aftermarket device. Moreover, because the boost level at low rpm is negligible, the engine does not need a lot of modifications (not even reduce compression ratio) to avoid knocking.

Because of its low inertia and friction, it has the highest efficiency among all superchargers. Moreover, it does not hamper the revvability of engine. On the downside, its power delivery is extremely peaky. Such characteristics can be seen on Koenigsegg supercar.

Advantage Highest efficiency among mechanical superchargers, high boost pressure, small and light.
Disadvantage Weak boost at low to medium rev
Who use it ? Koenigsegg, Farbio.

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