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From engine to wheels


To supercharge an engine is not always necessary to use a turbocharger and these Audi and Kawasaki prototypes are the clear demonstration of this idea


Nicodemo Angì

We read in the Encyclopedia of Science and Technology under the heading compressor: "machine for the compression of materials, mainly used for gasses and soil compression."

The compressor we are interested in is obviously the one that compresses gas, an action that "forces" the same amount of gas molecules into a smaller volume.

But why is there a need of compressing air into an engine?

The reason is simple: if the air present in the intake manifolds has a pressure higher than the atmospheric pressure, combustion will develop a greater thrust on the piston (there is more oxygen available to the fuel), thus increasing the power and torque delivered by the engine.

Obviously, since nothing is free, compression requires mechanical action that has to be supplied by an external device: every compressor does indeed need an engine to set it in motion.


Do we need a turbine? Not really…..!                        

In turbochargers, so prevalent in the automotive industry, this engine is a turbine driven by exhaust gases and that is why this unit is called turbocharger.

Compressors can be of different types, and those commonly used in vehicles are centrifugal.

The name comes from the fact that the air to be compressed reaches the center of the compressor’s impeller: once there, the blades drag the air towards its edge thanks to the centrifugal force. This movement "accumulates" the air outwardly (in a structure with a characteristic spiral shape) increasing the pressure; a duct then leads the compressed air to the intake manifolds.

Turbochargers have different advantages, for example they are able to recover much of the energy produced by exhaust gasses when these leave the combustion chamber, which would otherwise be lost. The unit is quite functional, since the overall efficiency can exceed 70%: 70% of the energy present in the exhaust gas is used to  compress the air present in the intake manifold.

One of the greatest flaws of turbochargers, on the other hand, is that their flow rate increases as the turbine’s rotation increases: if the maximum flow rate is reached (let’s say) at 120,000 rev / min., at 60,000 rpm it will only be a quarter or so. This is why the so-called turbo-lag takes place, which is a noticeable delay between the request and the actual delivery of power, which can also be rather abrupt.

To solve the problem of the so-called turbo-lag, many solutions have been devised: for example reducing the diameter of the compressors so that less exhaust gas is needed to spin it, putting more compressors in succession or parallel, and adopting a variable geometry of the valves responsible for conveying the exhaust gasses in the turbine. Furthermore, in recent months, a number of alternative approaches have emerged, such as Audi’s Tdi-e solution.

In some prototypes, equipped with single or double turbochargers, a compressor driven by an electric motor has been further added.

The legacy of Formula 1 Power Units and their MGU-H, an electric motor / generator connected to the shaft of the turbocharger, is easily recognizable. It absorbs power from the turbine shaft when it functions as a generator, and it is used to control the speed of the turbocharger to match the air requirement of the engine, accelerating it to compensate the turbo lag.

It is noteworthy how in Audi prototypes electric compressors virtually have no impact in the energy balance of the car because when it is not used to compress air it acts as a generator that charges a battery pack.


Electric or gear driven? Your choice!                                        

Another interesting point is that the system is powered by a 48 V system, designed to decrease the intensity of the power flowing through the cables, thus reducing losses.

The transition to 48 V must be viewed for future applications, because in the presence of high absorbing systems - such as brake-by-wire or electric heating - the power drawn is so high that it will waste a lot of energy from the power cables.

The electric compressor starts spinning already when the engine is idling, providing an immediate engine response at any speed.


Alternative supercharging, so to speak, could also be extended to motorcycles.

General interest in this development is definitely lower than it is for the automotive sector, however, it is worth talking about it because it is very interesting.

Supercharging in motorcycle engines has always been very difficult to apply due to the reduced space available and the nature of the engines, which greatly depend on great air intake, where the turbine of a compressor could represent an obstacle.

The solution developed by Kawasaki is a centrifugal compressor driven not by a turbine but  directly by the engine via a mechanical transmission.

The engine in question was seen as a prototype in the recent Tokyo salon, and although Kawasaki did not provide a lot of information about it, a patient research and reverse engineering has allowed us to understand many things, including the presence of a two speed gearbox that varies the transmission ratio between the engine shaft and the impeller of the compressor.

The strategy devised by the Japanese manufacturer has the purpose to keep the compressor always around its ideal operating range, in order to increase performance and minimize delays in response to acceleration.

It seems reasonable therefore, also in view of the Formula 1 experience, to expect great innovations in the field of a supercharging that goes in the direction of riding pleasure coupled with great performance.

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