Maybe this is my lack of English only, but I would have thought turbo charger and supercharger serve the same purpose, and it is just the driving force that's different?

Please, do enlighten me!

Yes, and there have been production examples in light-duty vehicles. The 1.4 R4 TSI "Twincharger" engine used such an arrangement to reduce low RPM turbo lag (i.e., provide boost quicker following low-rpm tip-in). The downside is increased cost and complexity and some increased parasitic losses from the mechanical supercharger drive. Improvements in turbochargers (ball-bearings, variable geometry) and improvements in engine control (e.g. optimization of variable valve timing) can also reduce lag and improve low rpm torque. In the end, VW didn't need the added complexity and eventually superseded the configuration with one using a variable-geometry turbocharger. In some high-speed diesel truck applications, series turbocharging is sometimes used to similar effect.

Some high-speed, uniflow scavenged 2-stroke diesel engines also use a combination of a mechanical positive-displacement supercharger and a turbocharger but I believe in those cases, it is primarily done to improve low-rpm scavenging rather than just to increase available intake boost pressure across the engine speed range or during low-speed transients.

Dear Joseph, I think yours is a very comprehensive and clear answer, and I see my understanding wasn't all that wrong. Avoiding tubo lag is about the only explanation I could have thought of for such a configuration.

Apparently you think of two chargers. In that case, indeed, a series of two smaller turbo chargers appear to be the most viable concept. If you think of a single, combined-concept charger, I recall the work on the Comprex charger; capable of combining mechanical (or: electrical) and pressure wave charging. The idea is to reclaim energy from exhaust gasses, while using a mechanical (or: electrical) drive. However, when mining energy from exhaust gasses, I think even with a mechanically driven charger, some lagging might result without additional control. As far as I know the Comprex idea is here for decades already.

See for instance: http://www.kfz-tech.de/

To my believe, the comprex concept is open to further evolution. The drive part could be innovated with either (variable-transmission) mechanical or electronically-controlled electrical drive. The materials could be innovated using ceramics or (shape-changing?) smart materials. The pressure wave-based loading principle could be innovated using acoustic or electrostatic loading, or other means. Of course that also consumes some energy but a vehicle still produces a lot of energy waste that could be used.

I don't know why this has not (?) been done yet. Maybe a conventional turbo-charger is just a fairly good compromise between performance, simplicity and construction cost. A comprex is a more complex part to construct and robustness is a major design factor.

I am somewhat familiar with the Comprex exhaust pressure supercharging although I haven't reviewed any research in that area in decades and I wasn't aware of it's application as a compound drive. I recall that there were some issues with hot exhaust dilution of the intake air with some of these systems.

Daimler/Detroit Diesel Corporation have used turbo-compound engine configurations (using the exhaust turbine to extract additional energy via coupling with the crankshaft) to achieve very impressive thermal efficiency with large, Class 8 long-haul trucks. The drive system is probably too expensive to be applicable to passenger cars but the high initial cost of Class 8 tractors coupled with the potential fuel savings in such an application due to the very long trip distances hauling heavy loads makes this approach attractive for some applications.

I observed that my above-given answer contains a remote URL not directly going to the Comprex pages I referred to. Here's a better link:

http://www.kfz-tech.de/Engl/ComprLader.htm

Thanks for the link. From an application engineering standpoint, I would think heating the intake charge and loss of some control of exhaust gas recirculation would pose emissions challenges for application of a comprex system to SI or diesel mobile applications, even when taking into consideration the more modern controls now available that are mentioned in the linked discussion, such as ETC in SI applications.

Both can be employed together but it will increase complexity and cost, even though efficiency also will be improved.

Exhaust mixing into the intake isn't necessarily a bad thing as long as it can be controlled and cooled. Cooled EGR has been beneficial at reducing NOx emissions from lean combustion, such as diesel, and can reduce reliance on spark-retard and fuel enrichment during knock-limited operation with spark-ignition vehicles. The keys, however, are control and cooling. That would be fairly difficult to implement with Comprex and there are simpler boosting methods that can be applied to range-extender engines. Lotus Engineering has acheived impressive efficiency even without boosting with their Fagor line of two and three-cylinder range-extender engines and the engines clearly have EU6/US-Tier3/LEV-III capability in addition to having low CO2 emissions. Range extending applications allow tailoring the engine for peak efficiency over a very narrow band of engine speed/load operation - something we can't really do over the broader range of operation necessary for conventional IC powertrains or even HEVs. Such engines are also simple, light-weight and relatively inexpensive - important considerations for EV range extending applications.

Yes.You can Use both.The concept of supercharger and Turbocharger are slightly different.Infact when you go for turbo charging it involves the concept of suercharging indirectly but not the other way round.

Send the charge at high pressure in to the cylinder using a supercharger since the exhaust will be at high pressure you can operate a turbine using these high pressure exhaust gases which will develop work not only sufficient to run the supercharger but also develops some excess power.

If you use only supercharging(without turbo charger) it consumes roughly 3-5% of power develloped by the engine however sp power devloped by the engine and other performance factors like volumetric efficiency etcc increases.The exhaust is releases at high pressure without utiliing its energy.