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


Ok. How would you propose to couple and decouple the "main" engine when
needed? The trick will be to do it seamlessly. With cylinder deactivation and
drive by wire this is fairly simple. When engaging a whole other engine would
be difficult. The rotary has low rotational braking forces. It would likely be
easier to build a 3 rotor with one rotor being 50 or 60mm wide for full time
use. The advantage would be that current machining equipment can be used. Then
the cut rotors need lube while reducing air intake to reduce pumping loses. An
R100 size rotor should be around 125HP with Renesis end housings. If narrowed
to 40 or 50mm the power would be reduced and a larger throttle
position required.

I welcome your thoughts,

Dale Davies

One way clutch. AKA  Sprag clutch.

Paul Lamar


This makes me wonder ... why hasn't Mazda already done this if it is
that obvious?????  They could make a 50 MPG Mazda 6.

SD

Me too. Back in the late 1940's, 1950's and 1960's over 9000 turbo compound aircraft engines were built. They reached a TBO of 3000 hours in airline service.

As far as  turbo compound piston engines are concerned they  are very hard on exhaust valves and much energy is therefore lost in the exhaust valves. Needless to say the rotary has no exhaust valves so in theory it would be significantly more efficient.

Paul Lamar