Paul,
 I wanted to bring up this thought I had some time back about a possibly
simpler way to turbo compounding. This wouldn't be as good as your use of
the traction drive to improve the drive ratio to optimum, but it could
work. We all agree that some kind of exhaust energy recovery is a great
idea. My thought was to use just the turbine from a regular turbocharger
and couple it directly to the crankshaft of the aircraft using a belt,
(v or poly-v) to allow some give but still recovering considerable
energy in the correct RPM range. The system might require a small
gearbox as well but I think you could improve the BSFC in cruise quite a
bit.

The reason I thought about this is that many of the guys were
having such trouble with mufflers that they were thinking of using a
turbo without the compressor even hooked up as just a muffler. OK that
would be a rather expensive and heavy way to provide muffling alone ,
but what if you could do a cheap test version of the turbo compound? Use
a single reduction ratio with a belt final drive and no compressor
section. This is using a standard radial inlet turbocharger turbine
section. At low RPMs the turbine wouldn't do any large amount of work,
but let it be wasted it is just doing a muffling job down there. As the
RPM increased to cruise or takeoff RPM the turbine would be in it's
"proper" range and start to drive the crank providing actual work.

For starting you could de-tension the belt using a movable idler if that was
even needed. With a direct connection the turbine would be prevented
from overspeeding, the problem in some of the stock turbo supercharging
systems. This system would suck on a car, but as Paul has often said,
"We fly within a narrow rev range." I am attaching a crude sketch of
what I'm talking about.
Bill Jepson

Bill, can you render that Solid Works "crude sketch" :) file and send us the render?

We almost have this idea on the web site but it includes the compressor.
http://www.rotaryeng.net/18-complete-reduction--engine-3D.jpg
The hard and expensive part is building the gear box as the turbine RPM is in
the 100,000 RPM category. Even 50,000 RPM is a significant problem.

Less of a problem with our new beryllium aluminum alloy rotors with the
110 pound engine (less turbine)  turning at 23,000 RPM. The rotors shown are
solid  aluminum alloy rotors with out internal oil cooling or weight savings.
The turbo charged engine is only 855 HP at 11,000 RPM without the turbo
compound. I don't think the FIA would like it running at 23,000 RPM :) Come to
think of it Ferrari, Mercedes, BMW, Toyota and Honda would not like it either.

Paul Lamar

Bill,

I think I understand where you are going with this. With a direct belt
drive the engine will limit the turbo charger rpms to lets say  6K rpm.
It may even bog the engine down.  At those speeds the little turbine
will produce no significant power.    Running it at 80,000 rpm however
may put out several foot lbs of torque.

A traction drive is the only reduction method we have found to date that
can handle those rpms. Even with the larger 7:1 reduction box from
Rotrex's larger units, the unhindered output will be at
11,000 rpms or so.  From there to the shaft speed the required reduction
ratio is still a question for me, and why I wanted to know the specs on
the Curtiss Wright R3350 TC cyclone engine.

Doug in Japan

R3350 turbine speed is 16,000 RPM at cruise and 19,000 RPM at takeoff power.
Gear ratio is 6.52:1. Turbines are spin tested to 27,500. Diameter is
11.45" with the length of the buckets being 1.375" The temperature of
the hub is kept to 700 F. The turbines are generating over 200 HP each
at takeoff power on early engines. Later engines the turbines made 300 HP each.

From a Curtiss Wright booklet "Facts about the Wright Turbo Compound".
At the 200 HP level the engine power was increased 21.4%.

With this size turbine a current Formula 1 engine would need no gear reduction
between the engine and the turbine. The Formula 1 engine would go from about
800 HP to about 970 HP. 1950's tech.

I am currently working on an TC article for Race Tech mag if they want to
publish it.

Paul Lamar

Paul,

With all due respect please make another forum to discuss race car
applications, or title the post as such.
If I may be so bold to go back into the air for a moment then according
to the illustration you posted once, Fig. 13 Turbo Compound engine heat
balance, at cruise power 1680 Hp, and .056 F/A ratio, the engine is at
2400 rpms.  If the turbines are spinning at 16,000 rpm cruise, then at a
6.52 step down ratio, the output would be 2454 rpms.  This is so close
to 2400 I am assuming the cruise prop shaft rpm is the same.

Ditto the same result calculating take-off power of 2900 rpms as stated
in  Fig. 12.

This then would be the calculated step down ratio including the fluid
coupling wich would be slipping somewhat.  It is not an 100 % efficient
connection.  I understand that the fluid coupling is absolutely
necessary when gears are used as part of the step down system. As an
aside the planetary traction wheel drive system is a fluid coupling as
such and it also slips a bit. I loses 2% of its efficiency. I suspect
the older fluid coupling would be more in the range of 90% efficient. To
allow for this 10% slippage and perhaps other anomalies of the engine
rpms stabilizing I suspect the actual step down ratios of the gear set
would be higher.

Am I making mountain out of an molehill?  Is it as simple as saying the
gear down ratio is what it is, based on experiments to determine the
best balance between the exhaust turbines and prop shaft rpms and the
available torque from the exhaust turbines factoring in slippage?
Obviously this system balance power curve is not a straight line and in
airplanes we know the best step down ratio was biased toward the cruise
rpms where the engine was going to spend most of its time.  In other
words just off idle the system was not so efficient. At full power it
was perhaps a little off of peak efficiency.  We also can assume the
16,000 exhaust turbine rpm is the design rpm at which the most work can
be extracted.  We know this in part as axial turbine operate best in a
narrow rpm range.

Since the turbo-compound system matches prop rpms  work is measured in
increased torque from the exhaust turbines. This  allows the prop shaft
to carry more load or coarsen the pitch, otherwise it may overspeed.
Critical factors limiting the operation are turbine temps,  engine back
pressure, deck pressure and ????

Doug in Japan.

First the prop shaft is geared on the R3350. Some engines used .4375 and some
used .355.

I am not sure what you are driving at Doug. The fixed nozzle turbine is
not all that narrow a device. Here is a relevant chart.
The DD15 uses a fixed nozzle and it derives a TC benefit from 1000 RPM to 2000 RPM.

Some of these messages are sent to the Formula 1 engineering list
so you may see some comments from time to time relevant to Formula 1.

Paul Lamar

Paul

My point was is the 6.52:1 ratio a gear set ratio or the ratio from the
turbine through the fluid coupling?  Said another way is the ratio
actual or calculated?

Doug in Japan.

I think it is the actual ratio of the gears used since it is
to a couple of decimal places.

Paul Lamar