Science/Engineering/Technical forums

The nice thing about ceramics in the hot section, besides temperatures, is the reduced inertia which helps with turbo lag or spool up time on gas turbine powered cars.

As far as coupling to an IC engine, the exhaust pressures are relatively low - it's all about mass flow and less about pressure. So, I think it would be hard to recover any significant amount of power. You'd probably be better off to power a generator.

Hi, Joe or anyone? Do they make generators that will spool up to 60k RPM? Besides the issue of flying apart or seizing bearings, will the eddy currents etc. render it useless anyway? I have thought of hanging a generator off the main shaft of the turbocharger/turbine engine for some time now . Maybe it could be magnetically or fluid coupled. Thoughts? -bill

The ones that I worked with were geared down from the power turbine to run at 3600 rpm (60 Hz) or 50Hz if that's you're thing. Rolls-Royce has a generator system that runs the generator (syncronously) directly off the low pressure spool. It's based on the Trent engine used on Boeing 777's, but it puts out over 60MW of power. Aircraft APU systems commonly run at 3ph/400 Hz, but I'm not sure what the gen's spin at, 24,000rpm???

A couple of options might to run it through a turboprop reduction box. That would get you closer to normal generator speeds. Or if you're not looking for a lot of power, driving a dynamotor might be an option since they're built to run at higher speeds and frequencies, but not 60krpm. I think either way it would still have to be driven by a power turbine instead of directly by the compressor/gasifier spool.

The fastest motors I know are the Dyson vacuum cleaner motors that run at 104,000 rpm. I do not think they would last very long if they were run constantly as generators.

'Turbo-compounding' is the term applied when mechanical geared shaft is taken directly off a turbocharger (rather than a turboprop, which is designed to drive a prop and is usually axial flow type rather than centrifugal, for efficiency). Some trucks even have [had] turbo compounding that feed directly to the transmission, but this tends to work only on long straight flat roads at constant speeds.

This idea gets close to where I think the internal combustion engine will eventually end up - I think the future of the ICE is as a gas-supply to drive a turbine when it is in a low-efficiency regime.

So the way this would evolve is; STEP 1: turbos in cars get bigger to the point that they are used to drive electric generator sets, rather than directly connected to an intake compressor. (Intake compressors would be electric and be driven independently of, but electrically from the turbine-generator).

STEP 2: The turbo-generators then evolve in size (and become axial-flow) to the point where fuel can be injected directly into the turbo under certain load regimes for extra, and efficient, power - hence we end up with a 'turbine' engine bolted on to the back of an ICE.

STEP 3: Then the ICE is disconnected from direct transmission to the wheels, and instead those are driven electrically from the turbine. The piston engine remains, as a gas-feeder to keep the turbine running, and to fire it up, when it is not in an efficient regime of operation.

All elements of this have, actually, already been done. There is nothing actually radical about this, apart perhaps from the integration and overall 'technology roadmap'. During the war, there was a British tank that ran off two Daimler engines that did not directly drive a transmission but whose exhaust powered two turbines that were mated to a transmission.

Joe, Hi. That kinda puts it out of the scope of being simple enough for me to pull off, but probably not for you ,my machinist friend, or some of the rest of the guys on the forum. There is a machinist that lives near me that has built three R/C miniature turbines so far. I think he has about $500 in the castings and then machines the rest himself. He says he uses ceramic bearings made in Germany. #618, and gets them from Boca Bearings, for the 54 mm turbine. The last one he built put out 17 lbs. at 160K. He uses 5% DTL lite oil in Jet-A. I believe he gets about 20 runs before he has to replace the bearing. I tried attaching a video of the flight he sent me, but PHP doesn't like the extension. I wanted him to put it on youtube to save some space, but he says he was waiting for a better recording. I will send anyone his e-mail address, or the video, if they are interested.
54MM? ISO 63 turbo pump? Could you use the small Pfieiffer turbo for a compressor for a miniature turbine? No, I think I am answering my own question. The blades are probably the wrong angle, clearance too close, and aren't made to run in "air". Besides; sucking down a "gnat" would probably destroy the thing. -bill

Chris, Hi. Thanks for the info. It is all rather interesting to me. I know you can buy electric "superchargers" now. Probably not very efficient as compared to belt driven ones though. Sir James Dyson Huh? Maybe my next vac should be a Dyson. He really seems to think out his product. So for extra power you would dump in more fuel; kind of like "kicking in the afterburner", so to speak. What about a rotating honeycomb ceramic re-generator? Would that still have any promise. I recall they tried that on some of the turbine cars. OK, I'll shut up now. -bill

Hi Bill, I first wanted to build a small R/C one when I saw one as a kid back in the late '80's. The bearings have always been a problem, but the ceramics last way better than the old slip rings used too. You're probably right about the turbo pumps, plus they don't really have any shape to the blades. The RR/Allison 250 turboshaft engine only has a 4-5" inlet. It has maybe 7 stages and then a radial flow compressor as the last one and it puts out about 700 Shaft Hp. So, even just having a few stages maybe 2 or 3 on a radial flow compressor (they have a higher compression ratio per stage) would net a lot of extra power. Yeah, the regens were a hot item on the turbine powered cars to improve efficiency and are probably still worthwhile if you're not concerned about weight. I think the cars had a relatively low compression ratio, so the regens were a way to get a little more performance. I don't know if they use them on the smaller generator units, but the big installations sometimes use a waste heat recovery system downstream. I think because of volume/surface area scaling they already have more heat than they know what to do with inside the engine.

I'd be interested in seeing the video if you want to send it to me.

Hey guys,

Sorry for my late input into the particular topic, but I have some first hand experience so I'll try to lend what I can.

In middle school I began constructing and testing several pulse jets and ram jets, I found them absolutely mystifying so I decided to build a turbojet. I ended up buying a small turbo from a mitsubishi spider (for the sake off cost) and it has troubled me ever since. I would suggest checking the nominal operation speeds for the turbine before buying a turbo charger. For instance, the turbo I have is rated for pretty high speeds, which gives me a lot of head room if I'm looking to run at high levels, but is a major pain in the butt during start up. It takes three air compressors in order to spin the blades fast enough to get the thing started. I guess common sense would now dictate the acquisition of a larger turbine over a smaller one. The largest problem that I have had is the flame tube, and the only practical solution I have found is to computer model the tube for you construct it. You can either do this through a CAD software (such as Solidworks) with testing protocol for things such as air flow, or a pre-made program specifically designed for our purpose. A great software I have found for this purpose was actually located in an instructable (http://www.instructables.com/id/How-to-build-your-own-Jet-Engine/step8/Making-the-flame-tube/)

Any way you slice it though, diy jet engines are great fun.
-Chad Ramey

Heh
Heh

young son turned up with this in the weekend

input diameter 2.7 inches
and another on the way
both going units --they get replaced at 200,000kms --go figure
no end float and turns very nicely