Turbo-powered alternator
 

Why not use a cutout for the turbo so you can vary the amount of exhaust gases/pressure applied to the turbo? You could set it at a point where it would spin the alt within a certain rpm range. After all, an alternator has some resistance when putting out current. This would keep rpm down some. perhaps we could open the cutout when cruising and close it when driving with variable revs/load? This would keep the turbo rpm pretty stable. Just an idea....

A wastegate does this normally. Of course you have no intake pressure to push on the wastegate actuator anymore... I imagine you rig something up though.

you don't want the turbo to go slow or your response to power goes way down, big lag. What you don't want is to get to much back pressure that the turbo is restrictive because trying to keep the exhaust housing A/R small .

What turbo manufactures have done to improve the peak efficiency around wide rpm is .
1)variable geometry exhaust housing, this not used much in gas as temps to high but idea is a variable vanes/deflectors in the housing that can keep efficiency up over engine rpm band .
2) twin scroll housing, this is were the exhaust housing has two different A/R compartments,. one for low speed response and larger for high rpm . there a flap to control when it opens (there still normal wastgate still used) .RX7 used this for many yrs on there turbo 13b engines (87-91).
3)Sequential turbos , this is were there 2 turbos, its similar to twin scroll idea. One turbo is run at low speed/boost and then second kicks in . Later 92+ RX7 went to this system and so did Toyota supra turbo's at end.

There also some work on electric assist turbos which can use the turbo shaft to power systems like alternator .

here link to article on it .

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daox, I believe you still can have it.

if you cap off the part of the turbo that usually goes to the engine intake.

if you leave it open , then ye sure, you'd lose it.

if necessary, u can leave a hole in the cap you've installed.
this would now give a measure of control on the turbo, and the hole would keep the turbo from becoming too restricted.(a valve could be used to adjust the size of the hole too)

please correct me if i've made a mistake

To use a Turbo as an alternator would be quite difficult in my opinion,
The main reasons being:
1 / That turbo's are designed to spin from 30,000 & 100,000rpm and hence balancing is critical and any mistakes in setup will result in catastrophic failure, that's why there are specialists that just do turbo's.
2 / People have the misconception that exhaust gasses are just wasted energy, in some ways yes, but they do serve a major function for the engine even after they leave the cylinders. Exhaust designers try to maximise the flow of exhaust gasses down the pipes so the momentum actually creates a suction at the engine end which helps the engine clear the exhaust and draw in fresh air. So if you remove the exhaust, put any restrictions in (turbo) or cool it down too much, the engine will run less efficiently and any energy you extract will be lost through poor engine performance.
If you can hold the heat in the exhaust, it will flow much better and the engine will breathe more freely, remember the engine is only providing power 25% of the time the other 75% it is acting like a compressor, so clearing the back pressure in the exhaust is a crucial aspect to energy efficiency.
Probably better to look at insulation on the exhaust components, esspecially in the colder climates, this may also be a significant factor when people notice their FE is down in cold weather.
Improve the exhaust flow, engine runs more efficiently & FE goes up.

on #1 may I correct something here..
when hooking up such a setup you have a very small pully on the turbo and a very large one on the alternator. so the only part of this setup thats going 100k rpm is the turbo pully. even the belt isn't speeding, so the alternator certainly isn't. also you hook it up to bypass the exhaust gasses when the turbo gets up to a too high speed...

on #2. the racing guys say superchargers waste energy from the engine , and turbos don't, that why ultimately there's more power to be had from a turbo cuz the power driving the turbo would otherwise be wasted, unlike the supercharger. I'd imagine these racing guys usually know their stuff, and if thats true then the turbo alternator idea should be quite sound.

I'm not saying the idea is not possible, but it may not be practical with current technology. Anything spinning at such high speed needs extremly fine balancing and you'll need specialised pulleys & belts to ensure minimal losses.
Turbo's do draw power from an engine, but they push more air in so more fuel can be burnt and hence provide more power. Just like putting a wind generator on the roof it provides power, but creates drag. Put a turbo in the exhaust and yes it can provide power, but forms a restriction, which means less exhaust is drawn out of the cylinders, so less air in, so less efficiency.
Why do you think "racing guys" polish the inside of their exhaust manifolds & put on extractors, insulate the manifold etc. all these are to increase exhaust gas speed and let the engine breathe more freely.
It definately can be done, but the question is will you lose more efficiency from the engine than you gain in electricity.
Even the idea of using seebeck cells to generate electricity from the exhaust has an expense, to get any effective output you need to move heat through them, the more heat you move from the exhaust, the slower it flows and same issue again.
That's just my opinion, from what I have read and experienced myself.

I'm attracted to the idea of a turbo-alternator. One could feasibly use the turbo alternator to either (a) eliminate the need for a normal alternator, or (b) use it to power an electric assist motor, without the penalty of needing lower compression pistons, resulting in less efficiency when the turbo isn't running.

I assume you believe that putting a Turbo on the exhaust is free energy, if that is the case then I'll leave you to it, I think I've said enough on that topic.

Now when you talk about an electric assist motor, that's another idea and I think that it is more viable than the Turbo idea.

"TIGERS (Turbogenerator Integrated Gas Energy Recovery System) is a water cooled switched reluctance generator coupled to an exhaust driven turbine. It is capable of operating in exhaust temperatures > 900ºC, at speeds up to 80,000rpm, delivering a shaft power of 6kW"

Controlled Power Technologies... TIGERS