To all DIYGTers,

Thanks for the new membership to DIY gas turbines.   I have followed the garage GT movement for quite some time now.   Of late I have come to this group because I am currently exploring the possibility of prototyping an innovative gas turbine design in this similar fashion -- that is employing automotive turbo charger tech.   This makes sense to myself and team mate because peak thermal efficiency is not terribly important at this stage of development.  Computer simulations have given us TE numbers and will further help us to extrapolate TE from this first prototype. 

The prototype will be primarily expected to address operational characteristics/dynamics and efficiency changes through a typical (0 to 110) power band.  A GT built from turbo charger(s) should suffice in getting us to those answers. 

In our case when I reference GT, I literally mean gas turbine and not a jet engine.   I would prefer to tap net power rotationally.  In a lab test bed, a jet engine has no effective way to accurately set load, and then chart power output not to mention creating other problems.  

This true GT can be by either a single or dual shaft design.  A key characteristic of the turbo products to be employed is that the shaft must be changed out or possibly extended.  Essentially I would like to mount the equivalent of two turbo chargers on one shaft.  If the same shaft can also accommodate a generator then a single shaft design would be adequate for test purposes.  Indeed that direct load control of the turbo (by gen.) will have its obvious control and starting advantages thus out weighing the advantages of two shaft (free turbine) for the purposes of our current tests. 

I have seen several of the group working on free turbine designs; I am surprised that I have not seen any attempts on single shaft turbo generators (built from auto turbo chargers).  Is it because shaft change out/extension is so difficult or Is it because of shaft speed?  

In our case, as mentioned, we are less concerned about peak TE right now in prototype so we can keep rpms low to match generator specs rather than target optimum area of compressor and turbine maps.  For present priorities we are also not so concerned about lost enthalpy at exhaust. 

If dynamics and control are proven in first prototype then a next generation prototype would target the expected overall efficiencies.   That GT proto would have heat recovery as well as actual GT components.  The cost of that prototype would be magnitudes more than this first round but well worth proceeding to if results from first round are positive. 

So, as I seriously consider the garage approach to our first proto, my first round of questions I am tossing into this forum is about shaft change out/extension for off the shelf turbo chargers.  It looks, from here, like this shaft change out will be difficult (not to mention will void the warranty).   We will need to take apart the CHRA (core section) of two turbo chargers.  Will bearings be tricky?   We will be replacing two short shafts of each with a single long shaft.  This will run through the two CHRA units connecting the two disc sets in series. We will actually be throwing out one of the discs in the first set.   I won't say which one but you can probably guess when I tell you that there will be no concern for the shaft being subject to the hot gas stream. 
The two CHRAs (in series) must be mounted together as one integral unit.  Should this be accomplished through housing?  We may be modifying housing for other reasons.  I expect balancing will be a pain.   And especially if we include that generator on this same shaft.  Has anyone done anything even close to this?  Sure would like to hear from you.

Any advice, any of you may have will be of great help.  I am on the learning curve here when it comes to turbo chargers and what you can do with them.  Feel free to ask followup questions keeping in mind that my answers to design details will be limited at this stage without appropriate NDAs.  Hope I inspire an interesting thread.

One final note:  If we go down this DIY path to a first prototype we may decide to farm out some work - assuming the budget allows.  For an example: some one who has built successful single can combustors may be of interest to us, if they are also interested in such a contract.   But that is down the road....

Thanks from new member,
Bruce