Supercharging
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You see two Ferrari F1 sports cars from the 312 series from 1970 - 74. The more modern of the two is the one on top. But how can you tell and what are the two
doing in the opening of a chapter on turbocharging?
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The lower one, of course, nothing at all, but the upper one shows the introduction of a new component in Formula 1, called the 'airbox'. Because do you think that with
such aerodynamically sophisticated vehicles, you can afford to simply install such a large amount of air resistance above the driver, if it doesn't have some other
huge effect?
And it does, otherwise it would have disappeared long ago and we should consider it a spectacular example of 'dynamic charging', here through the 'RAM-Air' effect.
No, you won't find a compressor or turbocharger in these vehicles, but rest assured, the faster the car goes, the more air is pressed into its cylinders, considerably
more than it can ever suck in itself.
Similar designs have also found their way into fast motorcycles in series production, but today you can see an opening in the radiator grille through which the air filter
is filled in almost every new combustion engine car. The advantages of internal engine aerodynamics were recognized early on. You can find out more about the very
important dynamic charging in the chapter 'Air'.
We now move on to the first candidate for a type of air pump for the combustion engine, namely the compressor. In principle, it is connected to the engine
mechanically, but can now also be driven electrically. A type of direct drive through the exhaust gases is the pressure wave or Comprex charger. You simply must not
forget it, although it only had a relatively short appearance in the automotive sector.
The central element of this system is a kind of drum with relatively small spaces running from front to back, into which fresh air flows alternately from the bottom right.
After a quarter turn clockwise, these are then pressurized with exhaust gases directly from the engine from the top left and pressed into the engine after another
quarter turn to the bottom left.
According to the manufacturers at the time, old and fresh gases do not mix, although there is nothing separating them. The fact that this does not happen is due to the
dimensions of the revolving spaces. The rotation takes place at a higher speed from the crankshaft, whereby hardly any power is transferred, and in some operating
ranges some is even returned.
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This is very different with common compressors and is also their biggest disadvantage. Here again, almost all types of air pumps are possible. We will pick the Roots
blower, consisting of two interlocking shafts with two or three blades each. The blades used to run straight over the shaft, but now wind around it, as a kind of further
development of the screw compressor.
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As with a gear pump, the element to be pumped is guided around the outside of the wall and the return path is blocked by interlocking. The two shafts come very
close to each other and the wall, but do not touch. This is achieved by a positive power transmission on the outside via a gear drive.
| 1 | Air filter |
| 2 | Roots blower (compressor) |
| 3 | Possible bypass |
| 4 | Intercooler |
| 5 | Throttle valve |
| 6 | Collector pipe |
| 7 | Mechanical drive |
While the position of such pumps seems to be quite established, the so-called G or spiral charger has already disappeared again after a short and intense presence
at VW. Here, two discs, each with a spiral-shaped passage running from the inside to the outside, mesh with their open sides.
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While one disc is stationary or integrated into the housing, the other, performs the other, driven by the combustion engine, not rotating movements, but slightly
leading away from their center. Thanks to additional sealing, channels are created that are gradually narrowed by this circular movement from the outside to the
inside, thus driving the air drawn in from the outside to the center and exit.
It was precisely the seals that sealed the unfortunate fate of the G-charger because they were passed through very thin metal layers that proved to be insufficiently
stable in the hands of customers. Let's move on to the turbo and its biggest disadvantage, the slow response ('turbo lag'), which plays no role in any of the systems
discussed so far.
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The reason lies in its greatest advantage, namely being powered by the exhaust gases of the combustion engine. With larger engines, there were no problems
because they only change their load slowly. But racing complained about this very early on because you had to know the track very well and accelerate before the
bend so that you could benefit from it in the bend. Woe betide anyone who misjudged this.
| Choice of material |
| Turbine housing | Silicon alloy, cast materials, small surface area, possibly water cooling |
| Turbine wheel | Cast iron (spheroidal graphite), cast steel, nickel, titanium alloy, ceramic |
| Compressor housing | Aluminum alloy |
| Compressor wheel | Titanium/aluminum alloy |
In contrast, heat now seems to play a less important role. The control of the boost pressure has also changed. In the past, this was always a so-called 'wastegate', i.e.
a flap to bypass the turbine wheel and thus connect directly to the exhaust tract, but now there are fine, adjustable blades with fixed pivot points. These can allow the
pressure from the outlet to have different effects on the turbine blades.
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This is then the charger with Variable Turbine Geometry, first used in diesel engines, but now also in petrol engines. The turbocharger
began its triumphal march actually in diesel engines, where it was able to provide higher torque with fuel-efficient, low-speed driving by regulating the boost pressure
very early on. In cars with petrol engines, rather the compressor began its career.
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Even the motor speed range has become larger compared to the beginning, depending on whether you want to be sporty or economical. Of course, a naturally
aspirated engine is still able to rev much higher and delight its owner. However, early gear shifting with a turbocharger is more fuel-efficient and better compatible with
today's traffic.
The problem of turbo lag remains. The VTG charger helps a little here. The best-known solution is probably register charge. In principle, this combines charge with
less delay, i.e. a compressor or a small turbocharger, with one with more delay. The exhaust gas flow is then mainly directed through one of the two, depending on
the speed.
That will of course be a lot of components, especially if you divide it into three fields, not to mention the costs. Another, probably much more complex method seems
to be to add electric drives. No, not to start the engine, but directly on the shaft between the turbine and compressor wheel. Fed from at least 48V, it could ensure that
the boost pressure increases again quite quickly.
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