JetManJoe Turbo Jet GoKart

[andym]

Dec 28, 2023 11:46:01 GMT -5 richardm said:

Just curious. How much gain do you get from the afterburner in terms of % thrust ?

40% may be..... some claim 50%..... still its fun 

[richardm]

Not bad. They claim about 50% on fighter jets, some up to 70 %. But did you know that Concord's Olympus afterburners only increased thrust for 17%..

[slittlewing]

I don’t think I personally have ever had more than about 25% increase in thrust so far but it is indeed fun!
Military jet engines operate at way higher pressures than ours so can tolerate much higher back pressure in their AB with increased gas speeds and thrust (the multiple Mach diamonds say it all). combusting at less than 1 bar (in our engines) makes for very bad efficiency but still sounds great haha.

[jetmanjoe]

Once I build it I will let you know   that be the reason I put a load cell on the test stand with digital display on the panel.
But who doesn't want more 'controlled' flames and noise

[racket]

Hi Joe

Thrust increases as the square root of absolute temp change of our exhaust gases , so ~40% is all we can get , those higher increases are from burning bypass air which starts at a lower temperature .

We might have absolute gas temps of ~1000 K from a dry nozzle , with the A/B going the temp will go to 2000K , twice the temp , ......sq root 2 = ~1.4 , so 40 % increase, gas pressure in the jetpipe or /A/B should be producing the same backpressure on the engine otherwise the engines temperatures, rpm, pressures will change .

A fighter jet with mixed hot gases and cold bypass air might only have say 700K going into the A/B , 2000/700 = 2.85 , sq root 2.85 = 1.69 , so potentially ~70% increase.

If we measure the thrust of our sorted engine running a dry nozzle , then when replaced with a suitable A/B with its larger nozzle, you should produce a 40% increase , if you don't then you're either got an A/B combustion issue or you need to look at your gas producer parameters for changes .

Its absolutely necessary to measure thrust both dry and wet , then check all of the engine parameters to make sure the engine isn't being adversely affected , theres no point fitting an A/B unless its producing the thrust :-)

Cheers
John

[jetmanjoe]

Love your work John, another post of yours I will save to my files.

And yes I would do the dry run and get the jet humming nicely with lots of data from the test stand. Then consult with you on the math for the dimensions.

Much appreciated everyone.

Joe

[jetmanjoe]

Found an interesting diagram. Using trim POTs to set lower and upper limts for a hall sensor to operate my PWM contolled fuel pump.

I have a foot pedal hall sensor for my jet Gokart project and this would allow me to set idle speed with my foot off the pedal and also adjust the max speed at full pedal throttle.

[richardm]

The problem Im having is to get the motor started, Once running I can back the pot down and run the pump at quite a lower rpm. I ve found that all my PWM circuits have a "soft start" function that causes this.
If I connect everything first and crank up the pot I'll have the problem just described: crank it up, the power to the motor increase to a point much above its running power and it finally stars and watts decrease.But if I just crack the pot a bit and then connect the motor it starts right away at low speed, and with a normal wattage.
Ive tried adjusting lower and upper limits and frequency. No joy. So it looks like the soft start is the culprit.

I received and tried step up voltage converter yesterday. Now I have the opposite problem. My pump starts ok at the set base voltage but will get to spin too fast at the upper end of the pot...
I think I ll revert to Andy's advice and" KISS " by simply limiting the pot travel mechanically. Or maybe run the pump at a steady speed and vary the pressure regulator by some means. Im thinking an RC servo.This could open the door to more possibilities.

I know there are a lot of other ways, many of em involving programming. Not my cup of tea. Electro mechanic and some analog electronic is what I understand

[jetmanjoe]

Hi Richard,

What PWM controller are you using?

I know with the PWM Controller I bought they have the option to run a hall type sensor or POT and if the setting isn't set correctly it can cause some problems with the "usable range" of the POT.

I would turn soft start off permanently. Mine has this function too.

[richardm]

I have 3 different models On has adjustable lower and upper speed limits and adjustable frequency. Tweaking any of those did not solve the starting problem.None of em has the hall sensor option.
My simplest fix could be to mechanically lock the potentiometer so that it does not go any lower than the min speed and Insures a positive start at the lowest possible speed. Dont need the pot to travel all those 270 degrees of travel.I would have the PWM turned on first and a relay would connect it to the pump after a few seconds to " bypass" the soft start function they all have but on with I have no control.
You see the motor I have is from a reverse omosis pump Large torque at low rpm" I tested the pump and my nozzle on diesel fuel and get around 50 psi at only about 200 rpm. Im getting close to 300 psi at about 1000rpm.
So my rpm range is quite narrow.

Im also looking at the possibility of running the pump at a constant max speed and controlling the flow trough a variable pressure regulator. I need a regulator anyway since I had to disable the pump integrated one for it to supply fuel at various pressures. It coud be built with a lever of some kind( instead of an adjusting screw) and controlled by an RC servo.The servo in turn would be moved via a servo tester, no radio. This could be further developed to incorporate some compressor discharge pressure sensor and maybe temperature sensor that would act on the servo. I t could be possible to use those sensors to "trick" the servo tester. That could be useful to prevent overtemp and to get a smooth acceleration as the fuel schedule could follow the compressor discharge pressure variation.

Maybe I just trying to get to far or just dreaming...

I own a Garret Airesearch GTC 43/45 Built in 1952 I have the manuals.. No electronics Bleed air controls about every thing from the start sequence to the fuel schedules. Just springs, bellows and a few relays...

Too bad those turbo are running too fast for a practical mechanical power take off so that we could use some to run the pumps and govern speed etc ..

[racket]

Hi Richard

It is possible to make a reasonable manual throttle when running the fuel pump at constant speed jetandturbineowners.proboards.com/thread/1366/tv84-turbine-bike?page=2 , I eventually ended up with a civilised throttle on the bike

Cheers
John

[richardm]

Thanks Ill look at it!

[jetmanjoe]

Just got back from holidays and this was waiting in the mail for me. Really happy with the way the coupling turned out. Dealing with the company from China on Alibaba was great and pretty well priced.

[racket]

That should do the job nicely :-)

[jetmanjoe]

Started making the fuel tank for my test bench. Should hold 20lt usable fuel, total capacity 28lt.

Good local metal fabrication shop in town let me use their guillotine and brake press to cut and fold my tank for a carton 😏

I used 3mm Alloy sheet, gotta get some weld on threaded bosses.