TURBO HP/ not engine HP

There's No Such Thing As A Free Lunch:
The horsepower generated by the turbine section of a turbo is non-trivial: O(20-50 hp). However, 100% of it is going back into compressing intake air. They spin up until the power consumed by compressing intake air exactly balances the power generated by the exhaust gasses flowing thru them. If you tried to extract that HP, your intake airflow would go away & so would the turbine's output.

BTW, turbos spin a lot faster than 10,000 RPM. Some of the smaller ones spool up closer to 50,000 RPM or more at max. boost!

 

It has been done in piston aviation motors but the turbine wheel has to be very large in diameter and room for it may not be available in a passenger car.

It is not a new idea, it is well known and has been around at least since the 40's.

In aviation applications it was connected directly to the crankshaft.

IIRC it was used on some variants of the 3350. And it seems to me that it's real benefit would be in constant speed/ constant power situations like aviation applications.

 

They are almost all spinning over 100K these days...at least all the ones I looked at putting on my car, some over 200!

I think you could net about 50-100 hp depending on engine size at full throttle at the engines hp peak. But cruising the engine is only putting out about 15 hp, so there may be 2 hp available in the exhaust, if the turbine were size to collect it. Then you would need several turbines of different sizes and some kind of a flow controler to direct the exhaust to the proper one/combination of them at the given exhaust flow rate. Many peole have tried this over the years...none have really been practial.

The practical solution is a turbocharger. It lets you down size the engien so it gets good fuel milage and still delivers the required hp.

 

"Turbo compounding" is the term.

Pic here: http://www.pilotfriend.com/aircraft%20performance/aero_engines/images/17hist.jpg

 

because of the , relatively , small size of turbine wheel used in automotive
turbos , power is more usefully produced by having the small and light
turbine wheel spin at incredible rpms (eg , 100K rpms and above).

this in turn , allows the compressor wheel to also rotate at a velocity
enough to generate the necessary +ve pressure for turbo-charging.

to hook up that same small turbine wheel to drive a crankshaft would be
such a brake on the wheel to totally impede the compressor's ability to
produce the necessary rpms to create pressure.

Variable-vaning is only meant to deal with the inherent problem with
all turbo applications - ie , lag.

a more practical way around lag is actually a reliable sequential setup.

turbo people have been messing around with variable-vaning for a long
time but its complexity and demanding operating environment (to put it
mildly) , makes it unreliable.

gimmick.

 

"Search" really is your friend!..... A few reasons for the revival:

1. Ferrari (Carlo Chiti, et al, 1961) "initiated the development of an impulse turbine for his 120 deg V6 F1 engine" - The exhaust turbine would be geared to the engine's output shaft.

2. From the same article: "F1 rules framers of the future" are (in summary) seriously looking at it.

3. The most successful aviation engine of this design, the "Wright turbo compound" was 18 cylinder radial, and they built 12,000 of 'em between 1950-58.

[From Forza mag, Feb '09 btw - Ferrari tried some weird stuff over the years! - Two strokes, "shared" combustion chambers, "W" format - "Been there, done that".....]

4. I figure a big reason this "exhaust compounding" worked well in aviation was, as RD noted above, their "constant load/speed" nature. However, *today*, with electronics controlling the whole shebang, I reckon it was worth reviving this thread

Cheers,
Ian

 

Copied from everything2:

"The turbo compound was developed in the 1940's as a way to increase the power output of large piston engines.

The only production engine to eventually use turbo compounding was the Wright R-3350-32W. These engines were installed on Lockheed L-1049 Super Constellations and Douglas DC-7C's. They were finicky and hard to maintain properly but when well maintained they were good engines.

The "turbo" part of turbo compounding works just like a turbocharger in that there is a turbine in the flow of the engine's exhaust, Wright called these "PRT's" or "Power Recovery Turbines." The pressure from the expansion of the exhaust gas turned the turbine which was connected by a torque converter and transmission back to the engine crankshaft. Each R-3350 had three PRT's. R-3350 turbo compound engines were able to convert 20 to 30 percent of otherwise wasted heat energy into horsepower. Where a normal turbosupercharged R-3350 could produce 2,800 horsepower, the R-3350-32W turbo compound engine produced 3,500 horsepower.

Today some European truck manufacturers are claiming this as a revolutionary advance... but like most piston engine technologies it's been around the block a few times."


My dad used to work on those monsters, he's forgot more about them than I'll ever know. But these were long before the 1950's. Without the Wright R-3350, the B-29 would never have had engines. In fact the B-29 was the first military contract that called for 3350's.

Back in the turbo F1 era, I remember reading an article in R&T discussing this subject. I dont recall the author, but it was suggested that there was a great deal more HP available at the tubine shaft than was being developed at the crankshaft, all due to pumping losses within the engine. The problem was how to extract it and use it. It was discussed using the engine simply as an energy source to drive the turbine, and why it wouldnt work well in a car.

As noted above, airplanes and ships can make good use of turbines because they operate at more or less constant speed and dont require instant power changes. Cars cant use that kind of power when they are rapidly accelerating and decelerating. Cars need a gearbox with changeable ratios, and a power source with a wide rpm power band. Which is why tubine powered cars have not yet materialised, turbines are to slow to spool up and too slow to spool down.

VanDoorne of the Netherlands, inventor of the snowmobile transmission and the CVT, has been working on the idea of a turbine powered car for decades, and has had Fiat and Ford, among others help fund research to that end. But nothing has yet really materialised beyond a few prototypes.

 

The idea of turbo-compound aircraft engines was carried to its logical (or illogical, depending on point of view) in the Napier Nomad engine. This used a flat 12, sleeve valve diesel as a gas generator for what was in effect a power turbine section of a turbo prop engine. I think that well over 50% of the power was generated by the turbine rather than the piston engine. In any event, it is the most fuel efficient "piston" aircraft engine ever built.

There is an example in the Science Museum, London. A good description is in L.J.K. Setrights' "The Power To Fly".

BTW, the Science Museum has a stunning exhibit of aircraft engines, I've spent days in there.