[serialk11r]

Avoiding the vehicle side discussion and going to the drivetrain for a moment, after they increase the number of gears on every car to 7 minimum and the downsizing trend goes on for a little longer, I don't think there's that much more fuel economy to be gained.

Engines are often run at low efficiency but downsizing improves average efficiency and better gearing improves efficiency. A limit for the efficiency of the actual gas cycle with late intake valve closure is something like 60% (the ideal Otto cycle, you can go higher with a higher expansion ratio than compression ratio but in real life your heat ratio is not going to be good enough and effective compression ratio is limited). Pretty much no cars have the right compression ratio to hit that though and fast combustion causes very high friction, so let's just cut it to 55%. Right now engines are peaking at 37% or so, which is pretty darn good considering parasitic losses and friction eat up 20% of the indicated power. The 2ZR-FXE would probably be like 39% efficient on ethanol or race fuel and a 16:1 compression ratio (which in its operating range is like ~13:1 on most engines).

I think the multi-point laser ignition that Mazda is developing combined with a state-of-the-art (today) direct + port injection system is probably as good as it'll get.

I would think that there's perhaps only 10% more FE to gain from combustion cycle improvements left max (less for engines that are already direct injected and all) and if parasitic loads can be powered entirely by waste heat then we might be able to halve mechanical friction/load losses, for a grand total of 20%. There's not really a good way to keep the piston skirts in hydrodynamic lubrication, and the effective mu at journal bearings is really low already. My little 2150lb MR2 might barely hit 42mpg average if it had auto start-stop + such an engine with the same displacement and gearing.