|
|
Post by starterjetkart on Aug 12, 2021 17:54:44 GMT -5
I'm hoping to get some insight from the pros here.
For years I've been reading people say Jetspecs is a "crude program, it's outdated, the measurements are very forgiving and overall leaves alot of room for error". That being said with my current (and first) engine I followed jetspecs to the decimal point. Smallish 58mm turbo, the combustor and flametube dimensions weren't massive, I was able to find a nice way to mount it on my Kart and not be a huge monstrosity. I'm in the beginning stages of designing and building a much larger engine based on a 98mm inducer. The flame tube measurements per jetspecs are massive. Combustor shell will obviously follow suit. I've been studying all these bigs build for years and correct me if I'm wrong, these combustors look not much larger (definitely not longer) than what i've built for my 58mm. I specifically remember one team of guys that claimed to "specialize" in building smaller, more efficient combustors. I'm looking for insight. Am I off base? Were these combustors in these big engines in fact very large and the internet subtracts 12"? Can it be done? What would the guidelines be in using a much smaller flametube than what jetspecs spits out? I know the reason for the length is for pressure and temperature drop along the path of a long flame tube. What's the catch, what are the tricks? Ideally I would use my combustor shell, swap in a new flametube with correct hole areas for the larger turbo taking care to work the right magic to not overheat the new turbine. The new engine is based on a Garrett T18A98 - 98mm inducer.
Thanks guys, Your knowledge and experience are everything to me.
|
|
|
|
Post by racket on Aug 12, 2021 20:46:44 GMT -5
Hi Stephan
LOL.......I wouldn't say "a lot of room for error" , more likely a lot of room for "variables" that invariably creap in with a lot of crude builds , it tries to cover all eventualities so that guys end up with a working engine, a large generously proportioned flametube will be more efficient than an undersized one
Jetspecs is "crude" for very good reasons , when Jesse and I worked on its construction it became obvious that there was no simple solutions to the huge variations of builds, but mainly because of the large number of engines with long skinny flametubes that simply wouldn't work.
A 98 mm inducered engine can use a flametube with a ID of 175 mm - 6.8 inch ( 8.5 inch ID outer can) and a length at that 6.8" diameter of ~10 inches before the tapered section to the turb scroll inlet , if using a single fuel injection point .
Flametubes can be made shorter if there are multiple injection points , effectively splitting up the cross section into many "small flametubes" , but we need a lot more sophistication with everything , my current engine will have a flametube only ~4 inches long despite a 128 mm comp inducer , but there are hundreds of holes in the flametube and 9 injection points , earlier engines had 18 injection points , but the annular flametube has an annulus width of only ~60 mm , but an overal flametube cross section of 3 inducer areas .
If you use less than 3 inducer areas for your flametube cross section then theres a good chance of having problems, flametubes can be oversized but not undersized .
Cheers
John
|
|
|
|
Post by starterjetkart on Aug 12, 2021 21:05:41 GMT -5
As always, amazing info. Thank you John.. Just what I needed.
Let me start by saying I definitely don't mean to offend by poking at jetspecs. I was unaware you personally had your hand in its creation. I actually believe I've seen you write yourself it can be a crude tool. Way back when I first drew up plans for an engine I had to hand calculate everything and when jetspecs came out is was a godsend. It confirmed my math was close and also allowed me to quickly toss numbers at the program to compare design changes and turbo options. In an attempt to "fool the science" to allow myself a smaller combustor I guess I did poke at an invaluable tool you've provided all of us all these years.
At this point I definitely would like to say with a single point injection. Great thoughts for future designs. I definitely understand the fact a larger combustor is acceptable but not a smaller one. I had overbuilt all my components on this engine hoping to have room to grow with a larger turbo later, but I didn't anticipate finding such a large one that likely won't work with my size combustor. All the more reason to start fresh on the new build and keep this engine alive.
As always, thank you for crunching some numbers or me.
|
|
|
|
Post by racket on Aug 12, 2021 22:10:44 GMT -5
Hi Stephen
LOL, no offence taken :-) it is a crude device but invaluable over the years as its saved me a lot of typing .
The "problem" with single point injection is it takes time/distance for the fuel spread and for the air jets to enter the flametube , the injection holes need to be large enough to get sufficient depth of penetration but that limits their number whilst maintaining their percentage of hole area , but fewer injection holes means lots of area where theres no air to mix with the fuel spray , hence the need for more length , a length/diameter ratio of ~1.5:1 is minimal , but with our annular flametubes our "L" can be rather short because our "D" is also small , but theres more "segments" in an annular FT .
All the best with the build :-)
Cheers
John
|
|
|
|
Post by starterjetkart on Aug 12, 2021 22:14:36 GMT -5
Hi Stephan LOL.......I wouldn't say "a lot of room for error" , more likely a lot of room for "variables" that invariably creap in with a lot of crude builds , it tries to cover all eventualities so that guys end up with a working engine, a large generously proportioned flametube will be more efficient than an undersized one Jetspecs is "crude" for very good reasons , when Jesse and I worked on its construction it became obvious that there was no simple solutions to the huge variations of builds, but mainly because of the large number of engines with long skinny flametubes that simply wouldn't work. A 98 mm inducered engine can use a flametube with a ID of 175 mm - 6.8 inch ( 8.5 inch ID outer can) and a length at that 6.8" diameter of ~10 inches before the tapered section to the turb scroll inlet , if using a single fuel injection point . Flametubes can be made shorter if there are multiple injection points , effectively splitting up the cross section into many "small flametubes" , but we need a lot more sophistication with everything , my current engine will have a flametube only ~4 inches long despite a 128 mm comp inducer , but there are hundreds of holes in the flametube and 9 injection points , earlier engines had 18 injection points , but the annular flametube has an annulus width of only ~60 mm , but an overal flametube cross section of 3 inducer areas . If you use less than 3 inducer areas for your flametube cross section then theres a good chance of having problems, flametubes can be oversized but not undersized . Cheers John Let me ask you a hypothetical. I'll switch a gears a bit here. I currently have a 5" flametube, 14" long before the convergence into the turbine housing. What would you recommend as the largest inducered engine to mate with it? Perhaps I should add a stepping stone between where I am and where I want to be with the 98mm engine. We spoke previously about my current engine. I was unaware of the limitations in efficiency of what I was working with. I'd like to swap in a turbo with my current setup and let it live as is and start over with the next big engine. Thanks so much. |
|
|
|
Post by racket on Aug 13, 2021 2:32:49 GMT -5
Hi Stephen
5inch dia = 19.63 sq ins , divide by 3 = 6.54 sq ins for the inducer = 2.88" 73 mm dia ...................nah , not much bigger than your current engine , go with the 98mm job , its 80% bigger flow than a 73mm .
If you want to power a kart and have any sort of performance from just thrust power, then 98mm is ballpark.
Cheers
John
|
|
|
|
Post by starterjetkart on Aug 13, 2021 6:55:35 GMT -5
Hi Stephen 5inch dia = 19.63 sq ins , divide by 3 = 6.54 sq ins for the inducer = 2.88" 73 mm dia ...................nah , not much bigger than your current engine , go with the 98mm job , its 80% bigger flow than a 73mm . If you want to power a kart and have any sort of performance from just thrust power, then 98mm is ballpark. Cheers John Awesome. That's what I needed to hear. Thanks again so much. Back to the drawing board! |
|
|
|
Post by racket on Aug 13, 2021 17:00:52 GMT -5
Hi Stephan
Or you could put a freepower wheel downstream of your current engine and go for shaft horsepower rather than thrust ;-)
Cheers
John
|
|
|
|
Post by starterjetkart on Aug 13, 2021 22:17:14 GMT -5
Hi Stephan Or you could put a freepower wheel downstream of your current engine and go for shaft horsepower rather than thrust ;-) Cheers John I would love to get into freepower. I've done some digging but I can't find anything substantial on the subject. Seems there are many guys doing it but limited info on how they got there. Are they building these things from scratch? Is there a donor unit that can be borrowed from another application? I'm in the dark on Freeport. I get the concept. I understand what's happening. No idea how to get there. 🤪 |
|
|
|
Post by racket on Aug 14, 2021 1:42:31 GMT -5
|
|
|
|
Post by starterjetkart on Aug 14, 2021 10:53:32 GMT -5
Ok, you've got me digging deep now. As if my head weren't already spinning before! I'm conjuring ideas rapidly. Couple questions: For 1, if I were to use the exhaust scroll and matching turbine wheel from a donor turbo I would cannibalize, fabricate a ball bearing support as you did in the "simple" kart build, what are the guidelines for size in searching for the donor the turbo? I read further down you recommend twice the size of the gas producer to resist surging. Is there any concrete guideline here? Math to get an exact number and aim for that with what I can find? For 2, and I feel this may piggyback Q1, I see you have a bypass allowing much of the gas produced to exit around the freepower scroll. My hunch is the power turbine was too close in size of the gas procedure and therefore "half" of the gas produced is allowed to flow outside so as to resist the surging. Stop me here if I'm off. If I'm correct: I assume sizing the free power turbine correctly would allow a fully closed circuit between the 2 in a perfect world. Final question (for now!), what is the approximate size of inducer (airflow) of the gas producer on that "simple" kart? I'm just curious if my current 58mm based gas producer is worth playing with or should I bump up to a ~73mm unit from here that will play nice with what I've already built. What can I expect performance wise with what I have vs a ~73mm based gas producer? I want to do it all! But I'd love to make the most of what I have prior to getting heavily entrenched in the 98mm thrust only engine build. Thanks John! |
|
|
|
Post by racket on Aug 14, 2021 17:55:54 GMT -5
Hi Stephen
Actual sizing of the freepower will depend on both the mass flow as well as the "energy" leaving the gas producer , the "energy" consists of both the temperature and total pressure and will determine velocities through the wheel and scroll.
The Cummins wheel I used rotates in the right direction for chain drive as its a "reverse"rotation , also its inducer diameter is fairly large so the tip speed is higher for the limiting rpm of the karting chain of ~20,000 rpm.
Your 58mm engine will be flowing a bit under 1 lb/sec which would be OK for the Cummins turbo , ST/VT50 turbo , the shafting is fairly thick on it as well which is a good thing .
You need the biggest gearing ratio possible , so at least the 8:1 I used .
Yep , my gas producer's flow was too big for the freepower , hence the "dump" jet nozzle , the TV84 had a 88mm -3.5" inducer, it was the same turbo used in my bike which required the Allison heli turbine wheel for the freepower .
A 73 mm inducered gas producer will be flowing too much for most turbo turbines used as a freepower , also the power output would seriously stress the karting chain , it was on its limits with my setup , your 58 mm unit would be a better match for the chain .
My thoughts would be to use your existing gas producer with a ST/VT50 turbo freepower , then at a latter date upgrade to a 98 mm gas producer and a "heavy duty" freepower and gearbox setup for some real performance :-)
Cheers
John
|
|
|
|
Post by starterjetkart on Aug 23, 2021 20:01:21 GMT -5
Hi Stephen Actual sizing of the freepower will depend on both the mass flow as well as the "energy" leaving the gas producer , the "energy" consists of both the temperature and total pressure and will determine velocities through the wheel and scroll. The Cummins wheel I used rotates in the right direction for chain drive as its a "reverse"rotation , also its inducer diameter is fairly large so the tip speed is higher for the limiting rpm of the karting chain of ~20,000 rpm. Your 58mm engine will be flowing a bit under 1 lb/sec which would be OK for the Cummins turbo , ST/VT50 turbo , the shafting is fairly thick on it as well which is a good thing . You need the biggest gearing ratio possible , so at least the 8:1 I used . Yep , my gas producer's flow was too big for the freepower , hence the "dump" jet nozzle , the TV84 had a 88mm -3.5" inducer, it was the same turbo used in my bike which required the Allison heli turbine wheel for the freepower . A 73 mm inducered gas producer will be flowing too much for most turbo turbines used as a freepower , also the power output would seriously stress the karting chain , it was on its limits with my setup , your 58 mm unit would be a better match for the chain . My thoughts would be to use your existing gas producer with a ST/VT50 turbo freepower , then at a latter date upgrade to a 98 mm gas producer and a "heavy duty" freepower and gearbox setup for some real performance :-) Cheers John Hi again John. I purchased a good core VT50, it should arrive mid week. Couple quick questions I had for you. 1) do you have a rough shaft HP estimate I can expect from this setup? I'm looking into components for the freepower to drive and I want to be sure I'm not over or under sizing. I'd rather stay light of course so as to not waste any more HP than I need. I'm considering a jack shaft setup to obtain that 8:1 ratio and also possibly a cvt/comet clutch setup if I can get my ratios correct before and after it. 2) Do you see any issue with a 180 degree turn out of my 58mm and into the freepower? Of course the shortest and most direct path is ideal. But if I can use heavy enough gauge tubing and insulate it do you think there would be a major loss? I ask because my entire kart is setup for a rear facing exhaust outlet for thrust and afterburning. It wouldn't be the end of the world to reverse the entire engine and its systems, but it would be alot of starting over and time...which I never have much of. As always, Thanks so much, Stephan |
|
|
|
Post by racket on Aug 23, 2021 20:25:10 GMT -5
Hi Stephan If you go with the standard kart sprockets as I did with my kart jetandturbineowners.proboards.com/thread/40/2-shaft-turbine-kart-build then thats sufficient gearing . My mass flow rate was roughly 1.1 lbs/sec , you'll have ~80% of that , I calculated ~30 HP for mine , so 25 HP isn't out of the question . 180 degree is OK , just keep the diameters large enough, the VT50 turb inlet will dictate size . Cheers John |
|
dieselguy86
Veteran Member
Joined: September 2014
Posts: 186 
|
Post by dieselguy86 on Aug 24, 2021 15:09:00 GMT -5
Hi John,
Here's a random thought. In a case like this where someone is trying to use what they have. Could a larger than ideal compressor be used on the gas producer, and use a wastegate to bypass the extra that the turb can't process, and basically feed some higher density air to the power turbine? Wondering if it would help make up for the mass flow deficit.
Our would it be a wash since the turbine will be sucking more energy than normal to power the oversized comp?
-Joe
|
|
