"Fat Boy" 12/118 thrust engine

[enginewhisperer]

it's always nice having plenty of data to go through later with a suitable amount of head scratching

[racket]

Hi Andrew

Yeh , can't have too much data .............the funny part about watching the vids is theres a lot of different things going on at any one time , so I need to concentrate on different aspects with each viewing .

The comp efficiency numbers are looking good as well as the differences between the two interstage thermocouples that are indicating the combustion is "even" between sides , the fuel pressure drop across the injectors appears to indicate a fuel burn of ~2,500 ml/m at 2.5 Bar P2.

Not a lot of differences between gasifier "numbers" when the wastegate was open or closed so maybe the turb wheel exducer is running choked .

I've started disassembly and have the freepower assembly on the bench , the water injection ring still looks OK , so will delve deeper tomorrow , I think I found the "vibration" , the two screws holding the alloy pinion side housing to the mount on the test stand were "loose" , thankfully the self locking nuts prevented anything falling off , it looks like I'd just about nipped them up but was waiting until all the other bolts had been inserted into place befor final tightening , but didn't get back to them .

Cheers
John

[racket]

Hi Guys

Did some calibrating of my P4t gauge today , its a good quality VDO manual 30 psi gauge which I've always thought should be accurate , but I used my new "Mil Spec" 2.5 Bar gauge (NOS Boost gauge off a Navy vessel) which I use for testing my fuel injection flow rates at various pressure drops to recalibrate and found the VDO gauge has been underreading , at 4 psi VDO the other gauge read 0.4 Bar-5.88 psi , 6 psi was 0.51Bar-7.5 psi , 8 psi was 0.7 Bar -10.3 psi and at the max 9 psi I registered the other day the better gauge read 0.75 Bar - 11psi , a full 2 psi extra and almost right on the theoretical 11.3 psi I calculated when I first started building the engine 4 years ago , a nice cold winters morning and she'll be spot on :-)

I had both gauges hooked up to a Tee piece with similar lengths of tubing to each gauge and fed both from the workshop air compressor , slowly bleeding air to the tee piece to bring them up slowly , repeating the exercise a couple of times just to be certain , with the VDO gauge registering 0 psi whilst the good gauge still had some pressure on it the air comp line was disconnected with a rush of air from it to provide the definitive evidence

Another good result as I wasn't happy only seeing 9 psit on the gauge considering the comp efficiency up near 80% , something always felt "wrong" .

Cheers
John

[enginewhisperer]

That's definitely good to know!

I guess these issues could show up even more with electrical sensors.

[racket]

Hi Andrew

Yeh , unless we use very reputable bits and pieces our "results" are questionable , the VDO gauge should have been more accurate, but I guess its only a 50mm automotive gauge whereas my "calibration" Budenberg gauge is 100mm and probably cost 4 times as much when first purchased , its a nice bit of gear and was perfect for the fuel flow testing at fractions of a Bar pressures .

I'd really like to use properly calibrated instruments but my budget doesn't run to them , so only expect plus or minus 5% results :-(

Cheers
John

[turboron]

John, another great example of the subconscious striving to find answers to questions. Did you wake up in the middle of night with an ah ha moment?

Thanks, Ron

[racket]

Hi Ron

LOL.......so you've been there too :-)

Watching boring TV often has the same outcome, a quick leap from the lounge chair and off to the shed .

I must go through the data of my other engine builds that also used that P4t gauge to see what they registered , I might get a welcome surprise .

Cheers
John

[enginewhisperer]

haha, all the work getting the last 5% out of things, maybe you got 10%!

[racket]

Yep , at least 10% probably a tad more

[turboron]

John, now that you have studied the performance data what level of compressor and turbine component efficiencies have you achieved? What percentage improvement due you give to the addition of the compressor diffuser vanes and the turbine inlet guide vanes over the use of the turbocharger scrolls?

Thanks, Ron

[racket]

Hi Ron

The "measured" compressor efficiency seems to be a bit too high , at 0:13 on the vid www.youtube.com/watch?v=cammpUfAYio at a PR of 1.7 on a 22 deg C - 295K morning with lotsa humidity there should be a temp increase of 48.3 C degrees at 100% , I had a T2 minus T1 of 57.2 , so 48.3/57.2 = 84.5% with TOTs of 465 and 501 deg C .

At 1:03 on the vid at a 3:1 PR with TOTs of 611 and 631 the T2 minus T1 was 133 C degrees , at 100% comp effic we would have a rise of 108.9 C deg , 108.9/133 = 82% effic .

At 1:22 on vid with wastegate closed the TOTs increased to 673 and 677 deg C ( nice and close) , and T2 minus T1 dropped back a tad to 129 C degrees , unfortunately I'm not sure how the P2 was affected , but from what I can see on the vid at 1:16 it didn't change much , so maybe the exducer was choking at this point and "isolating" the gasifier .

At 1:30 the TOTs had stabilised a bit at 720 and 693C with T2 minus T1 of 124.4 C degs , so even if the PR had dropped back to 2.9:1 the comp would have been at 84% .

At 2:21 with ~3.5:1 PR and TOTs of 716 and 735 deg C the T2 minus T1 was 148.9 , so ~85% effic .................the comp thermocouple is shielded from flametube radient heat but is a bare wire thermo so quick to respond whereas the TOT thermos are sheathed and need a few more seconds to heat up .

In the past I've had troubles getting a reliable T2 , a possibility is static electricity from fast moving air , I don't really know.

As for turbine efficiency , because the relationship between NGV throat area and exducer throat area has been changed when I changed the turb wheel from the severely clipped one to a standard wheel, thanks Monty, even though it has a larger exducer dia the efficiency has probably been compromised , ......... I normally calculate with an 80% turb effic and a 5% flametube drop in pressure, theres going to be an "uneven" distribution of pressure drop in my turb stage as the NGV is overly large , whilst the tighter exducer will be running choked ...........I really need a larger turbine wheel to maximise the potential of the comp wheel

My original calcs were done with 78% comp and 80% turb with a 900C TIT , assuming I am only having a max temp rise of ~150 C in the comp , the turb stage drop will only be ~130 C , so currently I'm running at less than design temps .

The one parameter I really like is the P4t , it gives the energy for making power/thrust , so at ~11 psit and an average TOT of ~725 C I'm more than satisfied as it'd be possible to "tighten" things up a tad to increase temps and P4t ,......... we all need a pitot tube in the gasifier's exhaust .

During a previous test with the 107mm jet nozzle and no freepower bits , at a 3.5 PR TOTs were 743 and 696 C with a T2 minus T1 of 152 C degs with comp effic of 80% and "revised" P4t of 10 psit , when cranked up to 3.8 PR the test stand started to "lift" , so I'd imagine P4t probably went closer to 11- 12 psit

Cheers
John

[turboron]

John, based on your results would it be safe to assume the compressor diffuser vanes raise its efficiency from 75% to 80%? Would the data say that the turbine IGVs raise the turbine efficiency from 70% to 80%? I have seen textbook statements that the diffuser vanes will raise the peak efficiency while reducing the range of the compressor. I have see previous on-line comments that turbine IGVs are worth 10% on efficiency.

Thanks, Ron

[racket]

Hi Ron

Yep , they're reasonable assumptions

Cheers
John

[racket]

Hi Guys

Thought I'd better fit a diffusing cone in the exhaust, mounted with a few straightening vanes, to protect the rear of the wheel from the heat of the exhaust , I'll try and feed some bleedair into the cone and blow it on the central securing bolt area where it can then find its way out through the ~2.5 mm gap between wheel and cone .

Its been a fiddly job trying to hammer the cone out of 1.2mm stainless , but its a reasonable shape , the mounting vanes are 1.5mm stainless and they'll have pop rivets holding them to the exhaust pipe .





Cheers
John

[Johansson]

Really nice!