Tommy Cookers wrote:downsizing doesn't need higher TE to work
so sucessful (ie higher overall efficiency) downsizing doesn't prove that higher TE has been reached
I'm not sure that we are talking about the same thing here. Downsizing according to my understanding typically involves turbo charging and a reduction of the cylinder number and displacement volume. Lets assume for the sake of simplicity that the power output, the mass flow and the radiated heat would be the same for both engines. In reality the mass flow of the turbo engine is slightly smaller due to a lower fuel consumption. If we think along that line a number of efficiency mechanisms can be identified. The cylinder reduction will give you higher mechanical efficiency by reducing friction. Let us assume again for simplicity that the engine will run on the same rpm level although it typically does not. Then we can exclude further friction efficiency gains from that. How does adding an exhaust turbine and a turbo compressor affect various energy balances we can do? That is the best way to investigate the efficiency gains the downsized engine offers.
Lets look at the balance of the turbo charger first. The compressor needs energy to compress the charge air. It gets the energy from the exhaust turbine. Actually the turbine takes out more energy from the exhaust than the compressor adds to the intake air because you have losses at the waste gate and for both turbo machines. The power for running the complete turbocharger has to come from somewhere. It comes out of the energy differential (enthalpy) in the mass flow.
The mass flow of the turbocharged engine has to leave the engine at a much lower energy level than it does in the comparable NA engine that hasn't the turbine extraction or the whole contraption would not work.
At this point we can go back to the total energy balance of the engine. The energy intake with the mass flow and the radiated heat is roughly the same. So we can be certain that the turbo engine must have a higher thermal efficiency because it releases substantially less waste in form of kinetic and thermal energy at the tail pipe.
The example of the simplified downsized engine shows how the fuel saving is mainly driven by increased thermal and mechanical efficiency. It would be much clearer if we were looking at the actual figures. I would suggest to look at Ringo's thermodynamic computations of the 2014 F1 compressor and the turbine to get a feel for this.
Tommy Cookers wrote:boosting demands lower CR and gives lower expansion in cylinder, so puts more 'waste energy' into the exhaust anyway
If you look at the true exhaust at the tail pipe it doesn't. I have covered the issue above.
Tommy Cookers wrote:so it's not unreasonable to have a turbine there
there's a lot of tradeoffs
I also think it is an advantage to use turbochargers. The trade off I see mainly in complexity issues. But in terms of efficiency a turbo engine beats the NA engine.
Tommy Cookers wrote:does your turbo expert opine regarding eg whether boost should be eg 0.1 bar or 4 bar ?
nesting a piston engine inside a turbine has no clear end point (and no clear start point ?)
(when NA engines have so reduced their throttling etc losses eg Fiat)
(controlled PRT action in NA engines is one route to this)
I must admit that I do not understand your train of thought here. I opened the thread to introduce the marketing thoughts of the Garrett expert into the discussion of the turbo engines two years ago. At that time many people had difficulties with the idea of using turbo charging, particularly US people. There is nothing in the original document to answer your question, but if you put it in a different way perhaps I may be able to understand what you are driving at and continue to discuss it.