Turbocompounding ... gear ratio

[phoenix221]

I have been doing some research into turbo compounding of a Mazda 13b rotary engine. The exhaust speed on these engines is very high, and the gases are very hot since they contain unburned fuel that continues to burn as it exits the engine. This means that there is a lot of energy that can potentially be recovered!
I was recommended the use of an Iconel blowdown turbine to be connected to the excentric shaft. Since the choice of turbine type is critical, I am wondering whether this is the best implementation? Ideally, I would like to adapt an existing turbine and not have to build it from scratch

I am also told that one of the most critical implementation issues has to do with the gear ratio selection since the turbine cannot be directly connected to the crankshaft. I would like to optimise the gear ratio for a constant rpm around 6000.

I am looking for help in choosing the correct turbine and gear ratio for the target 6000rpm operation. Any ideas or tips?

Thank you

 

[SMOKEY44211]

Pheonix221 I am working on a similar project so I'll share what I've been able to learn so far. All the books I've read on the subject recomend tip speed to be 1/2 of the working fluid velocity. You'll need to know what the anticipated exhaust mass flow at you're target rpm. You didn't say in your post if this was in a steady state cruise (rpm seems high) or WOT acceleration. I am not aware of any off the shelf units that can be easily adapted to this type of application. For my purposes I chose to go with a single large rotor fastened directly to the crank and size the exhaust conduit to achieve the desired exhaust gas velocity. If you do decide to go the geared route and end up looking at fabricating your own, you may want to consider an axial flow design with appropriate staging to make best use of the exhaust energy available.--------Phil

 

[phoenix221]

Thanks Smokey44211

>You didn't say in your post if this was in a steady state
>cruise (rpm seems high) or WOT acceleration

I would like to optimize it for a steady state cruise scenario. 6000rmp is well within the operating range of these engines. They redline above 7000rpm

>recomend tip speed to be 1/2 of the working fluid velocity

What is the reasoning behind this?

>For my purposes I chose to go with a single large rotor
>fastened directly to the crank and size the exhaust conduit
>to achieve the desired exhaust gas velocity

I would be open to any solutions, especially the ones that make the entire installation simpler. Would you be willing to elaborate on the specifics of what you are doing? You can email me directly if you wish...

Thanks

 

[SBBlue]

Just a brief note on turbocompounding. . . . .

1) Caterpillar, Volvo, and Scania all found it necessary to couple the turine to the crankshaft with both gear reduction and a hydraulic coupling.

2) For a spark ignition throttled engine at cruise an exhaust turbine will offer little to no power recovery.

 

[phoenix221]

>Just a brief note on turbocompounding. . . . .
>
>1) Caterpillar, Volvo, and Scania all found it necessary to
>couple the turine to the crankshaft with both gear reduction
>and a hydraulic coupling.

Why?

>2) For a spark ignition throttled engine at cruise an exhaust
>turbine will offer little to no power recovery.

Why not? I'm curious since this statement conflicts with everything I have read so far. Please elaborate...

Thank you

 

[Turbinator]

At lower engine RPMs and little load situations (cruise) the turbine, if connected directly to the crankshaft, will be be of no benefit and will be a drag on the engine. A turbine requires a certain amount of exhaust flow before it spools up, or in the case of turbocompounding starts making positive torque on the crankshaft.

 

[GraviMan]

"The exhaust speed on these engines is very high, and the gases are very hot since they contain unburned fuel that continues to burn as it exits the engine"

Interesting. Is that particular to this engine, or is it a common feature of rotaries? It can happen in piston engines, but only really when running rediculous BMEPs. I had wondered about rotary diesels, and was going to investigate higher compression ratios. This post has given me second thoughts!

Mart

 

[phoenix221]

>Interesting. Is that particular to this engine, or is it a
>common feature of rotaries?

I know for sure that this engine has these characteristics. Other rotaries may or may not be the same.

As I understand it, it has to do with the location of the exhaust ports, etc. There is also some unburned fuel in this mixture which continues to combust as the gases are vented out, I suppose this also has an effect as the gas continues to heat up and expand.

I have read that the exhaust gases can move supersonic!... it explains why rotaries have quite a "roar" when the mufler is broken

Coincidentally, while these are undesired features it tends to make this rotary particularly ideal for setups where recovery of the energy from the exhaust is an attractive proposition... or so I am told

 

[phoenix221]

>At lower engine RPMs and little load situations (cruise) the
>turbine, if connected directly to the crankshaft, will be be
>of no benefit and will be a drag on the engine. A turbine
>requires a certain amount of exhaust flow before it spools up,
>or in the case of turbocompounding starts making positive
>torque on the crankshaft.

SBBlue, that's good news! ... I thought you had specific knowledge about the rotary in question given the operating parameters I outlined in my original post...

You are correct, there is a break even point, but I understand that one can control that by various means, e.g. turbine design, gear ration, etc. I am willing to take a hit at lower rpms if I can boost my engine output at the target (high) rpm.

Thanks anyway...

 

[GregLocock]

I wouldn't get too excited about supersonic exhaust speeds. I seem to remember we had a problem on the little Volvo (340?) with supersonic exhaust speeds in the downpipe. You may need to research 'choking' to see why you don't want to go there. Plus it makes a farty noise, to get technical.



Cheers

Greg Locock

 

[SMOKEY44211]

I wrote a rather legnthy respnse to some of you questions (took about an hour and a half to bang it out)but when I went to post it dissappeared. The feature that shows member profiles seems to be down for the past couple of days. I would be open to exchanging info via email. My address is Smokey44211 at hotmail.------Phil

 

[SBBlue]

If you look at the thread "Boost Questions" you should find the explanation why 1) turbochargers in stoichometric (gas) engines are only really useful at full throttle.

Since throttling is used to control the engine power, at cruise conditions the power output will only be maybe 25-30% and the pressure of the exhaust gas in the cylinder when the exhaust valve opens will be near atmospheric pressure. Because of this, an exhaust turbine will not extract a significant amount of energy.

 

[phoenix221]

> If you look at the thread "Boost Questions" you should find
> the explanation

I searched for the string "Boost Question"... no matches...

 

[SBBlue]

Try it again. I just tried it and the search took me straight to the thread. It's located about 22 positions down on this present thread as we speak.

Is your goal higher efficiency or higher energy density?

 

[HomoFaber]

scania`s gear red.-hydr.coupling-flywheel has the gear ratio about 28:1