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> Turbogenerators For Automotive Use?
Fallingwater
Posted: October 30, 2011 06:17 pm
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As we all know, current battery technology doesn't have nearly enough energy density to be used exclusively in any vehicle that isn't exclusively for city driving.

Some have solved the problem by designing or converting electric vehicles, and adding a small gas generator to greatly extend range and recharge the batteries where an outlet isn't available.

I'm wondering, though... what if that generator's engine was a small turboshaft? Turbine engines can make large amounts of power for their size, and they're notoriously only efficient at one particular speed while being enormously inefficient at all others, which makes them perfect for generator use (after suitable down-gearing).

I guess what I'm asking is: given a small diesel genset and a small turboshaft one, assuming both are of modern construction and running at the respective speed of optimal efficiency, with both generating the same power, which engine will be sucking more fuel, and by how much?


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Sch3mat1c
Posted: October 30, 2011 08:41 pm
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IIRC turboshafts actually lose a lot of power in the takeoff, either if you do a secondary turbine (which doesn't reduce the exhaust velocity much towards zero, thus limited efficiency) or if you gear the shaft directly (because the gears run so ridiculously fast that they burn a lot of power).

Tim


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johansen
Posted: October 30, 2011 08:43 pm
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I have been looking into manufacturing a 20-60Krpm radial inflow turbine and its associated alternator for some time.

three words: $$$

also, turbines aren't more efficient than IC unless they are above a few hundred KW

edit: no need for gears, run the alternator at 100Krpm. use a cylinder of C5 magnet with a stainless steel shaft running down the middle, and use a ferrite core for the alternator. Such an arrangement could run well under the first critical rpm for a small, under 10 kw alternator.
Edit2: use a small purpose built fan blade to pull a partial vacuum on the alternator to kill the wind resistance.


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tekwiz
Posted: October 30, 2011 09:28 pm
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There really isn't any such thing as a 'small' turboshaft engine. Commercial models start at ~150hp. Only turbojet engines are available under that power level, & they depend on thrust to take power from the engine & turn it into push.
I've heard of people retrofitting small Allison turboshaft engines into cars, however. These crank out ~400hp from an engine that compares in weight to a mid sized car engine. They aren't very efficient, though, because too much power is lost to the exhaust.
It takes a turboshaft with a multi stage power turbine to get any kind of decent efficiency, & these are only practical on bigger stationary engines. The smaller ones have only one or two power turbine stages.

@Tim: A lot of a power turbine's output energy comes from reducing exhaust temperature rather than just exhaust velocity. This principle allows aircraft to get very low temperature air for cabin climate control from tapping into the flow of cool bypass air in a turbojet engine. Temperature drops of more than 100F from a single turbine stage are not uncommon.


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VenomBallistics
Posted: October 31, 2011 12:21 am
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Jaguar has one
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paulmasoner
Posted: November 01, 2011 09:15 am
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my knowledge of circular things ends at an axial compressor(just the compressor).. but I have an alternate solution to what you describe. Rotary/wankel. Ya'll might know I am biased towards them lol, but it's a legitimate option afaik.

When designed for a single load/rpm point they can be extremely efficient and smog can be handled well. Ceramic seals and diesel and it will run forever, extremely power dense/lightweight.

IMO the wankel engine in it's historic setting is dead for good this time. But I see a potentially very bright future for small, possible single rotor, generators for series hybrid use
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tekwiz
Posted: November 01, 2011 10:35 pm
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QUOTE (paulmasoner @ November 01, 2011 12:15 am)
my knowledge of circular things ends at an axial compressor(just the compressor).. but I have an alternate solution to what you describe. Rotary/wankel. Ya'll might know I am biased towards them lol, but it's a legitimate option afaik.

When designed for a single load/rpm point they can be extremely efficient and smog can be handled well. Ceramic seals and diesel and it will run forever, extremely power dense/lightweight.

IMO the wankel engine in it's historic setting is dead for good this time. But I see a potentially very bright future for small, possible single rotor, generators for series hybrid use

Has anyone managed to make a diesel wankel engine? I always thought that the sealing requirements were beyond current materials technology.

My candidate for the next step in small generator technology would have to be the free piston engine combined with a linear permag alternator.


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paulmasoner
Posted: November 01, 2011 10:53 pm
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yep, they've been toyed with since the 60's. In the early days they were spark assisted, low compression for that very reason. Those falts have been/are being overcome in modern designs though.
There is a small diesel rotary(<20hp) currently produced being used for small business jet APU. http://www.der-wankelmotor.de/Motoren/UAV/Pats/pats.html
If you can get access to SAE pub 930683(a bit dated), there's a load of information on spark assisted diesel. I can provide direction to that paper and a couple others on the topic if anyone is interested, don't feel I should post it out in the open for obvious reasons

A curious side note, DARPA's VTOL/transformer/flying car thing had, at least initially, some pretty heavy contract work out for a 'full compression' diesel powerplant smile.gif
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Fallingwater
Posted: November 02, 2011 02:13 pm
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QUOTE (paulmasoner @ November 01, 2011 09:15 am)
Rotary/wankel.

I have never liked them. Too temperamental and fuel-hungry for my tastes - I've always felt about wankels as fancy two-strokes (yes, yes, I know they really aren't).

Then again, they do use them for aircraft, so I suppose they've made huge leaps in reliability from the days this was true:

user posted image

tongue.gif

I still don't know about using a small diesel wankel of the type you link to as a genset in a car. Doesn't exactly look like a product whose spares you'd find at the local engine shop - if it blows while you're on the go, you're going to have a hell of a time fixing it.
Also, I wonder what's the fuel consumption difference between that diesel wankel and a reciprocating diesel of equal power.

QUOTE
My candidate for the next step in small generator technology would have to be the free piston engine combined with a linear permag alternator.

Hmm. This looks pretty interesting indeed. Wonder why it hasn't made it to widespread production yet...


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Sch3mat1c
Posted: November 02, 2011 04:17 pm
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Laugh, most permanent magnets die around 150C. How hot do you think that floating piston is going to get with fire on one (or both) sides? They could get away with SmCo magnets (300C I think?), but these are slightly less powerful, less common and probably more expensive than NdFeB.

Tim


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paulmasoner
Posted: November 02, 2011 04:51 pm
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FW, lol. the periphary port motors are NOTORIOUS for that as the boost gets cranked up haha

Side ports and modern/precise($) fuel and spark control though have all but eliminated those issues, fuel control for a generator on the other hand can be done very accurately while still cheap. The temperment and fuel hog aspects, both very valid historically, I dont see as issues though.. They can be made reliable enough that they are a popular choice to power small aircraft. It just never happened that way in the 'fasterer, most furiousest' scene where everyone was shooting for the biggest number possible without breaking their parents wallets or knowing what they were doing lol. I dont know off hand what fuel consumption numbers could be on diesel, but on petrol for any given load/rpm it can be done better than reciprocating engines. This was one of the factors that helped the LeMans 4 rotor to it's famous win, it made less pits for fuel. When you only care about characteristics between 7-10K rpm and WOT, or whatever your goal is, intake plenum design nets large gains over the same approach on piston motors. It's just a horrid engine for any kind of wide powerband in regards to efficiency.

Additionally on the reliability, endurance race motors are torn down and rebuilt after every race, and they need to be as they are generally trashed from a peak performance point of view. When they opened up the LeMans 4 rotor in the inspection hold post race, there was virtually no discernable wear(ceramic seals for the win!). Subsequent dyno testing showed the curves had changed so little it was within dyno error range. This kind of result has been reproduced over and over through the years, it's just not well known as the guys who didnt try to push 500+hp through a 1.3L motor don't get talked about a lot. Parts would be short as there is currently a tiny market for the motors. But they are about the simplest thing to rebuild I've ever worked on. There's really nothing at all to them, You can rebuild one with basic hand tools and a quality micrometer set. There just arent many folks who've ever had any experience with one, ie Mazda(US) dealers were/are prohibited from opening a RENESIS engine up for rebuilt or inspection, they get sent to Japan or more recently one of their 2 or 3 rebuild 'shops' they eventually setup.
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Jimthecopierwrench
Posted: November 03, 2011 02:02 am
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QUOTE
I guess what I'm asking is: given a small diesel genset and a small turboshaft one, assuming both are of modern construction and running at the respective speed of optimal efficiency, with both generating the same power, which engine will be sucking more fuel, and by how much?


The small diesels I see being built for auxilliary power are far from a fresh sheet of paper. Granted the applications (sailing vessels, class 8 and 9 tractors, stationary, reefer) aren't calling for lightweight installations and so no one seems to need to put a new sheet on the board. Last 'little' 3 cylinder one I installed, although wonderfully compact, was 600+ pounds with an iron block and head.

Fair enough that it's presumably still running somewhere in Greece, and will likely continue doing so after I expire.

But the robust design was probably 'stolen' from another application - very likely a utility tractor or small loader.

A small reciprocating diesel designed specifically to operate continuously at a given RPM and load could in theory shed a lot of weight and be quite efficient without being any less reliable.

Hell, even some of the current German oil burner offerings seem to idle on the fumes of a fat man eating a corn dog.

The '"fact" seems IMO still to be that the electric only cars for the most part are mandated constructs to appease the green wing, with the minimum amount of engineering effort and money put into programs until somebody else sorts it all out, at the same time hoping that the fad will pass the way of the 80's robot servant and they can just get back to sipper diesels and 3 quart gassers that can be sold without having to tack the price of outsourced battery technology onto the sticker. And indeed it does seem to be a passing fad, all the Eureka at what's coming press leaks nonwithstanding.

EV's came on strong here at the start - and dissapeared even quicker. The government rebate incentives didn't outweigh the fact that EV's don't work in Canadian winter (50K GTA average) commutes without extension cords laugh.gif

Course this was your original point. Sorry, I don't have the math to back it up but If I were a betting man I'd be putting my nickels on diesel over turboshaft.

I do recall a blurb about the army looking for a lightweight internal combustion wearable power pack to operate the modern infantryman and his gadgets. Dunno what ever came of that.


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tekwiz
Posted: November 03, 2011 10:10 pm
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I dunno about infantry power packs, but I do know that there are commercially available engine powered backpack welders. IIRC, under 50lbs & 150A @24VDC.


QUOTE
Laugh, most permanent magnets die around 150C. How hot do you think that floating piston is going to get with fire on one (or both) sides? They could get away with SmCo magnets (300C I think?), but these are slightly less powerful, less common and probably more expensive than NdFeB.


Not that big a big problem. 4 stroke engines derive much of their internal cooling from lubricating oil flow & splash.
This doesn't mean that 2 strokers all experience melted pistons. They don't even get hot enough to damage the tiny amounts of lubricating oil they get.
It's all in the design.


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Trouble rather the tiger in his lair, than the sage among his books.
For to you, kings & armies are things mighty & enduring.
To him, mere toys of the moment, to be overturned at the flick of a finger.

Fortuna favet fortibus.
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