Hi Paul,

Here I go with that thinking stuff again.:-)) If you were doing a turbo
compound system on your RX8, would you also change the rear axle ratio to
something taller? On a car the higher gear ratio would put more load on the
engine to maintain cruise speed, resulting in the throttle being more open.
That reduces pumping losses and more hot air to apply force on the turbine.
That should then add slightly more advantage from the turbo compound system.
With that in mind, what gear would you chose instead of the 4.44:1 they have?

Dale Davies

Lets assume the car has a frontal area of 20 square feet.
The rule of thumb is 80% of the height times the width estimates
the frontal area. Lets say 6 feet wide and 4 feet high.
Or 5.5 feet wide and 4.4 feet high.

Lets also assume the C sub d of .3! Typical for a Prius. Drag would then be
20 x .3 x 75 x 75 x .0026 or 88 pounds at 75 MPH. 75 MPH is 110 FPS.
So required HP is then drag x FPS or 8250 pound foot per second or
15 HP. Double that for tire rolling resistance and you get 30 HP needed
to maintain 75 MPH on a level road at sea level.

Automotive turbo compound is a real challenge. 90% of the time a car
cruises on the freeway or autobahn at 30 HP while the engine may be rated at 400 HP. In order to make that highly efficient you need a small engine running at about 75% of it's max power at best torque RPM. This is the rational for shutting of some of the cylinders in a piston engine.

The rear axle ratio could be the same as the current RX8 at around 3.5:1 for acceleration and hill climbing reasons.

What would be great is a 2.5 rotor Wankel engine. In other words 2 rotors for
acceleration and hill climbing and a smaller one rotor for cruise.
The unique configuration of the e-shaft makes that possible.

http://www.rotaryeng.net/RX8-TC-eng3.html

The small one rotor engine would have a tiny turbine to get the
BSFC below .36! Diesel range or below.  The fuel burn would then be about 10.8 pounds per hour or about 1.7 gallons an hour. At 75 MPH the MPG would then be
about 44 MPG.

The advantage over a hybrid is less extra weight and cost for electric motor
and battery. Not to mention the limited availability of lithium.
The advantage over the diesel is better acceleration and less
cost and weight. No carcinogenic black smoke problems either.

Paul Lamar