Hi All,
Its unclear to me if Turbo-Charging a Prius is only an exercise in power enhancment. What does Turbo-Supercharging do? - it increases the fuel-air charge density utilizing waste energy in the exhaust stream. The emisions control feed-back loop should increase the fuel to match the increase air density, just as it reduces fuel to match low air density in the mountains. This is one of the side benefits of emisions controls, altitude adaptability.
Now denser air and more fuel to match the number of oxygen molecules in that air results in more energy made per revolution, or in other words more torque. This also generates allot more heat of combustion, which in the same volume gets hotter. Being hotter, the Tin/Tout ratio might be made larger and thermal enegy conversion more efficient. It can go either way.
Turbo-chargers were originally developed to allow aircraft to be able to have lighter sleeker engines (and less aerodynamic drag). One of the two reasons (the other being laminar flow wings) the P-51 fighter plane from WWII had such great range was its sleek turbo-charged engine. There is a double wamy in this story as well. The turbo charged engine increased the aircraft ceiling and the air being thinner up higher required the plane to have to fly faster to stay in the air and this was easily possible as thinner air has less drag. Both higher speed and longer range were the desired for the long-range aircraft. The P-51 also had a prop-pitch control. The ultimate outcome of this might one day be turbo-recuporated (apply exhaust heat energy recovery to turbine engines) passenger air-liners.
If the Prius control system is smart enough to know that the engine has more torque, and then adjusts the drag from the MG1 appropriately so that it generates more electricity for MG2, the Priius should accelerate quicker.
The question of will it improve fuel economy is harder to figure out. Turbo-charged engines can be designed to have better efficiency. This is why most over-the-road trucks are turbo-diesels (besides the mountain crossing advantages). The Prius control might not close loop on engine torque however. But just have hard-coded the expected torque for a give RPM and intake air temp. If a torque loop was closed, then the control would turn the CVT ratio up and the ICE would run slower and should have less pumping losses. This might work against deriving heat energy from the exhaust however, in that the turbo might not be aerodynically efficient at the lower exhaust velocity. The true fuel economy improvement has to come from recovering waste energy in the exhaust. A special turbo might be needed that spools up to high speeds at even lower exhaust flows than a standard turbo for a 1.5 liter engine to get improved fuel economy.
If a lighter weight tubo-charged engine replaced a heavier atmospherically asperated engine, that would definately save gas. The chasis could be made lighter, and the whole vehicle would be much lighter.
The other issue is that the Atkinson compression cycle is going to be pushing against the pressure that the turbo generates. This seems to indicate the Atkinson partial power advantage would be reduced. Unless there happens to be an intaking cylinder for every cylinder pumping air out the intake during a compression cycle and a large plenum between the cylinders.
I think one way that may be more certain to get more power or economy out of the Prius from an engine modification would be a turbo-alternator. The turbo-alternator has the turbine like the turbo-charger, but rather than spooling up a supercharging air compressor, its spools up yet another Brushless Motor/generator to generate electricy from waste heat energy. With an electronic power converter, the voltage and current from the wildly varying alternator output could be matched into the battery pack. That would make the Prius a series-parallel-series (!) hybrid electric vehicle. I beleive I have heard that large diesel powered electricty generators have tubo-alterntors after the turbo-charger. These plants can get about 50 % energy conversion efficiencies.
Here is a final parting thought on this Just like the Prius has the warp-stealth mode, turbo-alternator Prius could have a warp suck mode. And actually use the turbo as an air pump (runing the generator as a motor) to exhaust cylinders to negative pressure. Which when the exhaust valve closes and the intake opens, could aid in supercharging the intake by ram-tuned induction. Puegot brothers look out!
