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| Jet engine Ignition system Build with LT375 / TL494 / ZVS driver Lockup |
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| TurboTom:
Unfortunately, I can't go too deep into detail of my latest ignition exciter design since it was contract work and hence I'm bound by an NDA. What I can tell is that the whole electronics of that 1J+ exciter weighs just round about 160g and the size equals approx. a cigarette box that's 50% thicker than standard. It's completely solid-state. If you ever disassembled an aircraft turbine engine ignition exciter, you would be surprised how small actually the series ignition coil is. Basically, it's a ferrite rod of a few centimeters length and maybe 10mm diameter with a single layer of 0.3mm enamelled copper wire (well isolated) as the secondary and on top a few turns of copper ribbon, wired as an auto-transformer. For the details, please have a look at the schematic of a replacement I made maybe 15 years ago to revive an ignition exciter of a GTP30-67 turbine engine where the step-up transformer secondary isolation broke down. I ended up with almost all of the original high voltage stuff and made a flyback converter to charge the storage cap. The HV rectifier diode (D5) is rather critical and the mentioned one wouldn't work reliably. Problem is the considerable peak and also high average current that causes it to heat up. After that, when the spark cap fires, the induced plasma across the spark plug is quite conductive so there will be an LC oscillator formed of the storage cap, the ionized spark gap (can be considered as a conductor), the secondary of the series ignition coil and finally the ionized spark plug. The problem that results is that during the negative half-wave of the oscillation, the (already hot) rectifier diode starts to conduct and, depending on the characteristics of the output coil of the flyback transformer, will carry a substantial current. A diode that performs quite well is the Sanken UX-FOB that's normally used in microwave oven with an inverter PSU. These diodes don't sell cheap but they will permanently withstand the abuse... Anyway, that's basically an insight how ignition exciters had been built in the 1950s to maybe 1980s, of course with varying methods of charging the energy storage cap (electromagnetic trembler, single-transistor blocking oscillator, voltage limited flyback converters of discrete or integrated designs). In early units the high voltage rectifier was just a cold cathode gas-filled diode tube, later silicon rectifiers were used. All these exciters contained a specifically made switching spark gap that was designed to break down at approx. 2kV. The ugly thing about these is that they were usually pressurized with tritium gas to aid ionization (they always contain a "radioactivity" warning sign, yet it isn't possible to measure any radiation externally. That's a good indicator for tritium with its relatively low energy beta radiation that's completely shielded by the glass envelope. These switching spark gaps are more or less unobtainium, and as I mentioned before, conventional surge arresters won't last too long. I once found a (Chinese) company who had these special switching spark gaps in ther portfolio but I quickly decided against using them after I heard the price...IIRC must have been some 500EUR a piece, and the minimum order quantity was 50 since they made them specifically to the required break-down voltage. Once in a while you can find suitable spark tubes on the bay, though... FYI I attached the schematic of the original TS-21 ignition exciter (two such sections in one box, no series ignition coil in this one but a sense transformer instead). BTW, I'm quite impressed by your work, I once intended converting a TS-21 to turboprop as well but finally decided against it due to the low efficiency of that engine. It's funny I've also got a Turnigy "Aquastar" brushless motor sitting in a box and waiting to become a starter generator on a KHD/BMW/RR T312 engine. Yet, if this motor will be permanently capable of the 50,500 rpm of the TS-21 gas generator is (in my opinion) at least questionable. Cheers, Thomas |
| Amper:
I know how the old ones operate but it is difficult to get info on anything not vacuum based. I havent seen many cut open exciters on the internet so my experience is limited there. My assumption allways was that the output transformer is more of a security feature not to have charged capacitors on any output ports and also limiting the current by inductance. I just wanted to throw in a ferrite rod later and hope it will flatten the pulse enough. About the discharge tubes i have to agree, i have previously worked with the smaller 8x8mm ones and had some failing so i switched to the large 25kA rated ones. Obviously they are not designed for a long life. How long do you expect them to last? Since im not going to use it in an aircraft in the near future i would be fine changing them from time to time which is actually the reason i mounted it on screw terminals in an accessible spot. regarding full solid state: Wouldnt it be possible to do away with all HV stuff and just feed the transformer 24V from a very low esr cap? Discharge a stack of ceramic capacitors using some of the modern crazily powerful mosfets might do but even driving it like a normal transformer with a lot of power should be possible at few hundred kHz. Yes, the TS is not the most efficient engine but its actually better than expected. I got mine 3 years ago and a friend of mine got the second one. Since at the time we were both in the akaflieg stuttgart we had hopes to get our engines flying some day so he decided to build a turbojet from it for the usual glider applications. I meanwhile allways liked the idea of turboprops and the TS construction is ideal in many ways. The output pretty much exactly matches a rotax 912 which opens up a lot of accessories. At some pint i noticed that the propeller bearing of a thielert 1.7 aviation diesel EXACTLY fit the output flange of the TS2 which made it easier. Sadly at that time due to other more important projects i didnt have the time to continue and my friend decided that he wanted a turboprop as well, so after i obtained two sets of prop bearings he put his engine together and did first runs with propeller in the late summer of 2017 iirc. Since last summer he is now doing his masters thesis on characterizing the engine. He did full instrumentation on it, i did the electronics and if everything goes well he should be done soonish. Problems he found was especially high oil consumption but that could be for other reasons and it can be solved. Most interesting so far is that its extremely sensitive to changes in pressure of the supplied air. It was run in a laboratory with an eddicurrent brake and the gasses were supplied by ducting for flow and exhaust measurement. Even just a very small pressure drop on the inlet of a few dozed mBar reduced the shaft power by half(!). This is due to the fact that the pressure drop over the second turbine is very very small and is directly influenced this way. This also means though that adding a booster stage will increase efficiency dramatically! Regarding the starter generator, my friend used the motor of an align T-Rex 700 though its an outrunner and it will rupture the bell at the speed necessary if not using it only as a starter motor. I choose the aquastar because it was the cheapest alternative too the lehner motor i actually want to use. Since im still not sure about the exact configuration i decided that it would be a good idea to first get a cheap motor for the beginning and not ruin one that costs half the price of the engine itself... The rotor will not survive the 50.5k in stock configuration, the kevlar is really crappy and dray. I rewound it using aviation carbon roving and it should have a burst speed of over 100k now. I tested it to 60k and it was perfectly fine doing that and didnt even require rebalancing. In terms of bearings i have a little experience with home made rc jet engines and repairing turbo molecular pumps. Using partially home made bearings with permanent lubrication one of my pumps is at almost 100h of operation at 75k min⁻¹ and while the forces involved are much smaller its very possible to use such bearings on the aquastar and get decent lifetime. A T312 would be a dream to have but i dont have to tell you that they are very very rare compared to the TS and im not yet willing to sell both kidneys for one ; ) EDIT: Can you give me any numbers on pulse duration and power or is it nda relevant as well? EDIT2: Habe jetzt erst gemerkt, wer du bist, meinen kumpel kenst du wohl schon, ihr habt öfter mal Telefoniert :D |
| Amper:
Ane heeere we goooooo! https://youtu.be/j4iKh5aaWNQ I had to wire a DIP 494 to the pads because i couldnt wait to get the right ones in the mail and to my absolute shock this thing worked on the first try. Amazing. Now i have current regulation and voltage limit so i cant overcharge the cap if the sparkgap should decide to die. Pulse frequency can be much higher then in the video but for now im sticking to only 20kHz to drive the small transformer because its still only fr4 instead of aluminium pcb. Once i have the right parts ill put everything together and hope for the best. |
| Amper:
And another Update: TurboTom was perfectly right, the GDT caused another set of unexpected problems. It turned out that it works very nicely with a certain spark plug impedance but if it changes due too soot or moisture or some voltage levels dont hit the sweet spot it can start to go into simmer mode not letting the charge circuit do its thing. For this reason i decided to go full solid state as he recommended. The scheme used now is a boost converter using my original tl494 board just reduced to one 600V IGBT driving the most basic booster. The output charges roughly 15-20uF of film caps which are then switched into a toroidal inductor / transformer with a single winding and a tap going to ground via a capacitor. I got the idea from this great pdf: http://www.gasturbineworld.co.uk/Small%20Gas%20Turbines%203%20starting.pdf The result of the bulky test setup is amazing performance and a more inductive pulse that will not eat the spark plug as quickly as the pure capacitor version.I took a video at 50Hz trigger frequency but up to 100Hz was possible and only limited by the speed of the relay used to trigger the thyratron. Thesound you can hear in the videos btw is not at all representative. This thing is seriously loud and can not be operated in the lab without hearing protection. Its comparable to a spark gap teslacoil. I will post updates when i continue as there is still some issues to solve. https://youtu.be/ypEAAUoQ-iY |
| spec:
Hi Amper Just read through your thread: jet engines, Royer oscillators, ignition systems :-+ I see that you have been having a bit of trouble with the circuit of your OP. With that circuit, there are issues at the macro and at the micro level. One of the micro level issues is the gate drive arrangements, so I have quickly knocked out a revised gate drive circuit, as shown on the attached schematic, for you to try out, if you wish: |
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