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Post by racket on May 4, 2022 17:12:57 GMT -5
Hi Chris
Yep , my comp doesn't have a lot of backsweep compared to some and might account for why the full blown surge came in rather suddenly rather than being preceeded by "rough air" .
LOL......maybe the poor old comp was doing its best and finally said , ENOUGH :-)
It'll be interesting to see what difference the extra exducer flow area makes as it'll be the only change made this time , it was unfortunate that I had to make two changes together with the last test , but it allowed me to test a "tight" system , so at least I know where that lies now .
Cheers John
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Post by racket on May 2, 2022 0:04:43 GMT -5
Hi Guys
Turb wheel has been back in the "grinder" to have its exducer clipped further , now back to where it was with the last wheel , the exducer throat is now 37% bigger than the original which had a comp flowing 3 lbs/sec , so hopefully big enough for 4 lbs/sec , .............I just need to find what needs to be done to get it work , the G Trim Garrett wheel used in the 12/118 engine had an exducer throat area of ~5080 sq mms and flowed ~3.3 lbs/sec , with 6590 sq mms , some ~30% larger , 4lbs/sec should be easy.
Currently my poor comp efficiency from running on the surge line is requiring more pressure drop through the turb stage than optimum , producing lower gas pressures/densities that require more flow area ,........I'm "on the wrong side of the curve" :-(
With flow a bit further away from surge I hope to pickup an extra 10% comp effic improvement which will mean less pressure drop required thru the turb stage and higher pressures and densities adding to the extra mass flow..........maybe :-)
If I'm still able to run relatively cool temps I might consider a tad more off the exducer .
Rotor off to the balancer tomorrow morning , she's boxed up and ready to go .
The engine looks much "happier" inside , less heat stress :-)
Cheers John
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Post by racket on Apr 30, 2022 18:33:22 GMT -5
Hi Chris
Thank you for your thoughts , much appreciated :-)
I'm currently experiencing low comp efficiency , the T2 minus T1 is indicating effic. in the mid to high 60% , whereas earlier in the year it was ~10% greater.
Fuel pressures are much lower now than in the previous test , some of the decrease is from lower combustion temps but some from lower mass flows .
I did a set of calcs for the 4:1 PR numbers where I had 0.57 Bar in the jetpipe pitot at an "average" 660 deg C and 145 lbs of thrust , the jet nozzle is 118 mm dia , very oversized , the resulting velocity was probably ~1540 ft/sec , which would work out at a mass flow of only ~3 lbs/sec , rather than the ~4 lbs expected .
The comp temp rise was ~215 C degrees which would require ~180 C deg drop through the turb stage requiring a ~2.45 PR , our 4:1 PR becomes say 3.8 going into the turb stage , 3.8/2.45 = 1.53 coming out , my measured jetpipe total was ~1.57 , pretty close.
Unfortunately I have no idea what the comps heritage is , it was just an aftermarket wheel , but looking at it, it appears to be a "reasonable" normal comp wheel capable of 4:1 PR on a big diesel engine .
Its a 53 Trim wheel with inducer:exducer flow area ratio of ~1.7 : 1 , inducer throat area of 6390 sq mms , diffuser throats ~40% of inducer throats .
The previous NGV had total throat area of 6430 sq mms , somewhat "oversized" , new NGV ~5060 sq mms ,
The new turb exducer was clipped on a 7 degree slope which removed ~4 mm axially at the tip and changed tip angle from a tight 25 degrees to 30 degrees with throat increased from ~4800 sq mms to 5700 sq mms , the previous wheel was clipped back further to produce a 40 degree tip and a throat area of ~6590 sq mms.
Interestingly , my TV84 turb wheel was clipped to ~47 degrees ( 43 deg from axial) yet performed OK with a choked turb scroll at 4:1 PR , it was relying more on impact energy than reaction energy , I'm hoping that I might be able to replicate that scenario with even better results due to the higher comp efficiencies I was achieving earlier in the year .
My turbine wheel is "undersized" for the comp I'm using , a "marriage of convenience" that needs to be massaged gently to get it to work :-)
Thank you again for your thoughts
Cheers John
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RPM
Apr 29, 2022 16:22:29 GMT -5
misterg likes this
Post by racket on Apr 29, 2022 16:22:29 GMT -5
Hi
Some of the early experiments were conducted with a magnetised nut , not sure how they magnetised them but might be worth a try
Cheers John
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Post by racket on Apr 28, 2022 5:21:34 GMT -5
Hi Mark
Yep , its provided me with another "boundary" , I now know the "surge line" of the comp , so as long as I can move the flow a bit more to the right side , things should be OK .
That surge scared the crap outa me as I thought it was the engine getting into difficulties, something failing , at least I know my surge slot is working , but theres a limit to how far I can abuse it .
Hopefully the comp efficiency will improve with a tad more flow , which should help temps even more , lots to consider :-)
Cheers John
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Post by racket on Apr 28, 2022 4:36:57 GMT -5
Hi Guys,
She went into severe surge at ~5minute mark in the video ,it was rather loud , so I removed the sealing band I had around the surge slot exhaust , this allowed me to get up to 3.5 Bar before surge started again at ~7min 10 sec in the video .
Something is restricting flow , most likely the turb exducer as I only clipped back the exducer by a limited amount , changing the original exducer tip angle from a "tight" 25 degrees to a more normal 30 degrees , the old turb wheel was clipped back to achieve a 40 degree angle with extra throat area .
The old setup was at one extreme , the new setup is at the other , I need something in the middle .
Temperatures today were very low , down in the 400 deg C range at idle , indicating lotsa gas deflection , too much actually , I'd prefer temps ~500C at idle .
Otherwise , the engine worked OK , rather nice to get her up to a 4.5 :1 Pressure Ratio , thats the highest I've ever taken one of my engines :-)
The T2 minus T1 temps were also indicating poor comp efficiency which can happen if flowing too close to the surge line , another 10% flow might fix things .
Lotsa "numbers" to digest and calcs to make before I pull her apart to make any mods , the easiest modification will be a bit more ground off the exducer , maybe another 5 degree angle change .
Cheers John
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Post by racket on Apr 27, 2022 22:34:01 GMT -5
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RPM
Apr 27, 2022 17:40:54 GMT -5
Post by racket on Apr 27, 2022 17:40:54 GMT -5
I use a laser tach off ebay , like this one www.ebay.com.au/itm/384120560884?hash=item596f5e9cf4:g:2BUAAOSwFEFgiRob , comp snout/nut painted half gloss white , half matt black , with a crescent shaped piece of the reflective tape that comes with the tach fitted to the radial face of the comp outboard of the nut on the half white segment, this way you can get a good reflection if the tach is in an axial position to the shaft . But if you need more than 99,999 RPM then you'll need to look at more complex/expensive units like the ones Joe mentioned Cheers John
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Post by racket on Apr 26, 2022 18:55:18 GMT -5
Try using 1.2 mm , its easier , my sheetmetal guy recommended it to me , they hate using anything thinner .
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Post by racket on Apr 22, 2022 22:59:27 GMT -5
Hi Guys
Test stand back on the trailer , so time to check everything is working .
Oilpump OK , fuel pump interlock OK , fuel pump "screaming" OK , ignitor working OK ..............starter motor solenoid OK , starter motor nope , just some "magic smoke" :-(
Pulled the starter apart and found two brushes jammed , the couple servicing the field coils .
Unjammed them and gave the armature commutator a skim , she works fine now , spooling the rotor to 10,000 rpm under 70 psi of cold oil pressure , about 0.1 Bar of P2 produced by the comp, gave it two spoolups just to be sure , the starter gets a bit hot after two tests , so plenty of power going through her .
I'll make up a fuel brew and wait for the weather to improve , can't wait to see what the mods produce :-)
Cheers John
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Post by racket on Apr 22, 2022 22:48:06 GMT -5
Some serious braking power now :-)
Nice bits
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Post by racket on Apr 22, 2022 0:41:28 GMT -5
Hi Guys
Engine back on the test stand .
Had an alignment problem with the ignitor when I tried to fit it to the engine , don't know why as it fitted OK when I tried it after modifying the NGV , maybe a compounding up of a few variables .
Anyway , outer can off and the ignitor sealing arrangement ground off the flametube , outer can replaced and position of ignitor marked on flametube , can removed and hole modified and sealing arrangement tack welded back on , outer can back on to check , and ignitor slipped into place , so can off , fully welded arrangement and can replaced permanently :-)
Weather isn't looking good for a testrun until next week , so plenty of time to test everything is working properly before loading onto the trailer ..
Cheers John
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Post by racket on Apr 19, 2022 0:24:36 GMT -5
Hi Guys
Rotor is back from being balanced :-)
Also picked up some new silicone O'rings for sealing the NGV/shaft tunnel joint , reassembly begins in the morning.
Cheers John
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Post by racket on Apr 18, 2022 23:51:38 GMT -5
Hi Joe
I think I'd be putting the holes at maybe 0.75" distance to prevent that high pressure spray from contacting the walls
Cheers John
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Post by racket on Apr 11, 2022 17:59:15 GMT -5
Hi Scott
Yep , lotsa different methods used by the manufacturers , there are considerations of lube pressure that come into their designs , because the "bush" rotates at around half shaft speed the lube holes in the bush act as centrifugal pumps preventing oil getting to the shaft unless the pressure is high enough to overcome the pumping force .
The crescent groove in the shaft tunnel over a segment of the bush OD creates a "wedge" that increases the lube pressure force attempting to get oil through the hole/s which then allows a lower oil supply pressure to be used.
The designs with non rotating "floating" bushes can get away with lowish pressures as well .
If an engine is designed with only a modest oil pressure when its up to temperature , that pressure might not be high enough for the turbos requirement if its pumping high boost levels/rpm , hence the need to use "wedgeing" or very thin sections in the holes to minimise pumping pressures .
The main thing is to get plenty of oil to all surfaces to keep the shaft's dynamics under control as well as its temperature
Ideally a cast iron housing for the bushes is whats required for long life , but thankfully our relatively short duty cycles let us get away with alloy .
Looking forward to video of the A/B on full song pushing the kart along at a high rate of knots, she's a beast :-)
Cheers John
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