richardm
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Joined: June 2022
Posts: 261
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Post by richardm on Apr 13, 2024 9:20:43 GMT -5
Those are impressive numbers for a PR! I was referring to what I know about centrifugal gas turbines and jet engine I ve read the the RR NENE reached a max of 4:1 PR But that raise another question in my mind: Those high PR turbos are feeding a piston engine wich ,I guess , implies a quite different mass flow and pattern. I mean a jet engine is basically unrestricted flow trough compared to a piston engine Could this explain the higher PR ? Higher pressure at a lower flow?
I have limited knowledge about shockwave. My understanding is that the shockwave represent the ultimate pressure the medium can reach in given conditions. And it is followed by an area of low pressure ..
Just look at those pictures of aircraft doing supersonic fly by .. Looks like a "solid cone" ( the shock wave) followed by some condensation ( the low pressure area) Now one could picture a compressor blade instead of the plane. The blade pushing a wall of air followed by an area of low pressure ...
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richardm
Senior Member
Joined: June 2022
Posts: 261
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Post by richardm on Apr 13, 2024 11:15:17 GMT -5
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Post by racket on Apr 13, 2024 16:56:14 GMT -5
Hi Richard
The Nene did a pretty good job considering its rather crude compressor by todays standards .
High compression isn't really needed for a pure jet producing thrust , we end up with too much wasted energy going out the nozzle , but better if theres extra turb wheels to extract power to drive a fan or produce SHP and reduce that exhaust speed and temp .
A 4:1 PR is about the upper limit for a pure thrust engine , we only want enough jetpipe energy to choke the jetnozzle
Cheers John
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Post by racket on Apr 13, 2024 18:00:24 GMT -5
Hi Richard
I should add a warning to Members about trying to run high pressure ratios using turbo parts.
Unless the heritage of the turbo is known , we should keep to more sensible RPM/ tip speeds/PRs ............ the DIY Turbine Community LIMITS were 1450 ft/sec tip speed and 1450 deg F going INTO the turb stage, take a few hundred degs off for turb outlet temps .
With small turbos designed for wide maps and limited PRs , 20 psi P2 would be a reasonable limit , for larger truck sized turbos ~30 psi .
If the turbo is a high quality new unit then follow whats on the comp map and any warning from the manufacturer .
Generally speaking , theres not much point pushing the RPM/tip speed/PR limits as the potential thrust gains aren't there for most turbos We need to be careful if the turb stage is "oversized" as its possible to go into the choke region where overspeed at modest PRs is possible
We don't want exploding turbos :-(
Cheers John
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richardm
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Joined: June 2022
Posts: 261
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Post by richardm on Apr 13, 2024 18:36:00 GMT -5
Agree 1450 ft sec is a speeding bullet. Ive seen busted APU turbines .. surrounding full of bullet holes despite a turbo housing is much thicker and should contain debris but then they ll be shouting out the back end.
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Post by racket on Apr 13, 2024 18:57:04 GMT -5
Hi Richard
Yes , most reputable turbos should contain the shrapnel , but that hole at the back can still allow stuff out , the competition turbos have "cross bars" in the scroll exhaust to prevent a detached turb wheel from exiting and spinning off into the crowd .
Cheers John
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richardm
Senior Member
Joined: June 2022
Posts: 261
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Post by richardm on Apr 14, 2024 4:20:41 GMT -5
That I didn't know. Can't Imagine the destruction. We tend to underestimate the energy involved in what we see as just a small chunk of metal.
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richardm
Senior Member
Joined: June 2022
Posts: 261
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Post by richardm on Apr 14, 2024 4:32:07 GMT -5
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Post by racket on Apr 18, 2024 0:07:51 GMT -5
Hi Guys
The sun was shining and with a forecast of thunderstorms this afternoon , a test run was planned after charging batteries and heating the oil tank .
Drove out to my test area , got everything setup.
Checked fuel to oil pressure interlock was working , set the prestart fuel pressure , cameras all turned on .
Hit the starter toggle switch ...........nothin :-(
Bummer , it was working OK at the last test .............then I noticed the oil pump wasn't spinning very fast .............OK , oil pressure interlocked with fuel pump which is interlocked with the starter .
Turned everything off and had a think ............the ancillaries battery was being charged right up until I hooked up the trailer to the car along with the oil tank heater.
Turned on the oil pump , produced its 70 psi for several seconds then started dying away :-(
Packed up and came home .........I'll look at it all tomorrow, ......some days we shouldn't get out of bed .
The rain is bucketing down, thunder and lightning all around ........... LOL , at least the weather forecasters got it right :-)
Cheers John
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Post by enginewhisperer on Apr 18, 2024 0:28:55 GMT -5
what's the history of the battery? it might have just given up. They seem to do that sometimes. They'll work normally and then suddenly have no capacity.
It could still be a connection or cable problem though.
I had a similar thing with a 12v water pump the other day. It would run briefly at normal power and then die off. It's run from a large car battery with a solar charger. It appeared that the solar charger unit was causing issues. The battery would hold voltage but the output to the pump would drop to nearly nothing. Decided to fix it another time.
The next time I tried it it seemed to work fine!
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richardm
Senior Member
Joined: June 2022
Posts: 261
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Post by richardm on Apr 18, 2024 4:58:04 GMT -5
Just saying . Most of us do use car batteries because they readily available and cheaper But a car battery is not designed for sustained loads, deep discharge and recharge . They can supply a lot of power in a flash to start a car engine (min my place down to minus 30 c. But leave you headlight on for a few hours and that's it for you battery You may recharge it but it will never be the same, That will keep it from fully recharge and things will go down the hill from that point. A battery holding voltage doesnt mean much. It may well show a bit over 12 volts but have no Amp capacity It happens when there little lead compound left to react with acid. Its called sulfating
What we all need but dont use are deep cycle batteries.These have thicker plates better insulation and hold more chemicals. This is what must be used for solar panels windmills Youl l find those on RV boats etc. were long lasting low to high loads fast or slow charge conditions are present
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Post by racket on Apr 18, 2024 18:28:42 GMT -5
Hi Richard
A new 50Ah deep cycle battery already on order , here next week .
Combined with my 75Ah deep cycle battery that should take care of things .
The 2 X 12V car batteries will be for the starter only.
Its surprising how time flies , its only my batteries dying that provide some indication that the years are going past :-(
Cheers John
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richardm
Senior Member
Joined: June 2022
Posts: 261
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Post by richardm on Apr 19, 2024 10:12:50 GMT -5
Hi John! I bet that will fix your problem! Let us know Ideally your car batteries should be recharged after each start or at least after each test run..dont leave em sitting without a full charge Here I ve bee keeping a fully charged one for years one a small "intelligent" charger. Still top shape after all that time and a few uses. Many of us do that as a spare or as a booster for those "too cold won't star" winter nights.
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Post by racket on Apr 19, 2024 15:59:30 GMT -5
Hi Richard
Yep , always charge up after a test run .
I have an old 12V deep cycle battery from a golf cart that only died in the last year , it was originally purchased in 1996 , a good quality German made one
Look after them and they last :-)
Cheers John
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Chuks
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Joined: August 2015
Posts: 498
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Post by Chuks on Apr 26, 2024 17:01:03 GMT -5
Hi Richard Yep , shock waves are my current "unknown", just how much compression is there across a shockwave ?? I looked at some numbers for my comp today and with the high inducer tip angle , to reduce incidence to zero between blade and incoming air at the RPM I'm running , the air going in would need to be at ~900 ft/sec and the relative tip velocity is ~M-1.5 , so well into transonic conditions over a large portion of the inducer. But the throat area of the inducer can't swallow the amount of air that the inducer is biting off at the density available . So is there an increase in density across the shock that then allows the inducer to swallow what is being bitten off ................I don't know . LOL.........to answer your question about 5:1 being too great a PR for a single stage comp , nope , some of the tractor pulling guys use 100psi of boost from a single turbo , 7.7:1 PR :-) Cheers John hi John, this comment made my day "7.7:1 PR with just a single billet wheel" dont worry I wont be trying that anytime soon. Got a reply from Andy on the turbo parts pricing. new Engine build around the conner. Cheers.
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