[serialk11r]

I advise you to think about the thermodynamics. It will make sense.

Even at part load, the exhaust gas is at quite high temperatures. If you had a frictionless, adiabatic Otto cycle engine with valves that had no restriction, the efficiency would be what you can find on Wikipedia for example: 1-1/CR^lambda. Computing this for typical compression ratios gives a bit over 60%. So what does this mean? It means that 30-40% of the fuel energy would be rejected to the exhaust under ideal conditions. In a real engine some of the heat is absorbed by the cooling system before this can happen, but exhaust gas temperatures under full load can be well over 700C, in fact I think catalytic converters are built to tolerate 900C for short periods of time or something. You can look up charts used for engine development that show exhaust gas temperature vs. load. Anyways about 20-30% of the fuel energy is expelled out the exhaust at temperatures over 400C even at part load.

Perhaps the best comparison would be this, an Atkinson cycle engine vs. Otto cycle engine at the same intake volume, with same compression ratio. The Atkinson cycle engine with full expansion will have zero excess pressure left. The difference in the work done by the Otto cycle and the Atkinson cycle is the work that can be extracted by adiabatic expansion of the gas to atmospheric pressure. Usually the gas leftover in the cylinder is at 3-5bar pressure when the exhaust valve opens. So how much energy can you get out of this? If you go run the calculations, you'll see that it's under 10% of total power. Yet there is about 50% of total power going out the exhaust in the form of heat energy.

A real life example: the 2ZR-FXE has lowest BSFC of about 220g/kWh. The 2GR-FSE, which has a somewhat shorter intake cam (thus higher throttling losses) has a lowest BSFC of slightly under 230g/kWh. 4% difference in efficiency. By contrast, the older, lower tech 1MZ-FE which had a similar "wasteful" intake manages 237g/kWh, still under 10% worse than the 2ZR despite having an older design with less efficient combustion. So under ideal scenarios, you can't get more than single digit improvements in efficiency just with a turbine, at full load. However regenerating the heat alone even at poor efficiencies stands to make a larger gain at part load, where there is less pressure wasted but still a lot of heat wasted.

Hope that explanation helped.