Turbocharger Jet Engine Starting Problem

[turbo5cyl]

Hello all, I am new to the forum and looking for some help. I recently finished my homemade jet engine and need some help trying to start it up.

Here is a rough description of it. Its based off an Audi K03 S4 turbo. Its quite tiny but its what I had to work with. It has an compressor inducer of 33.6mm and turbine exducer of 38mm. I built my flame tube and combustion chamber as close as I could to the Jetspec recommendations. I have a 2.5" diameter and 7.5" long flame tube and an equally long combustion chamber with a diameter of 3.5". The flame tube has 24 primary holes, 5 secondary holes and 5 dilution holes of varying sizes to add up to the required area for each part. My fuel rail is a piece of pipe in the middle of the FT just passed the primary holes with 6 radial holes drilled into it. The compressor connects to my combustion chamber right where the primary holes are as well. I also have a spark plug mounted at the end of the combustion chamber right before the turbine housing for the initial ignition.

I am using propane as my fuel and i have an air compressor regulator as a propane regulator. I am not looking for high performance or thrust or anything, as long as it self sustains, I would be happy....for now.

I have tried starting it up with my shop vac (doubles as a leaf blower) and have not had much luck. This is my procedure: turn oil pump on, turn ignition on, start spooling with shop vac and finally adding propane (slowly opening valve more and more). I don't know how much propane I should be feeding in there but I have opened the valve quite a lot and have nothing happening. I dont want to go too crazy until I asked because I dont want an explosion or anything like that....Is there something I am doing wrong or should try differently? I appreciate any help

[racket]

Hi

Throw the air compressor regulator away and simply use the propane cylinder valve , our engines use a lot of fuel and most regulators simply don't have large enough flow areas .

Generally its best to ignite the propane prior to applying the starter/blower, ..........so oil pump on , ignition on, crack open the propane cylinder valve and you should hear a "whoop" as the propane ignites , apply the blower whilst continuing to open the cylinder valve , keep the blower applied until you hear the engine accelerating strongly to idle , have someone stand at a distance from the engines exhaust to keep an eye on the turbine wheel to make sure it doesn't start to glow .

How fast does your blower spin the rotor with oil pressure applied ??

Cheers
John

[turbo5cyl]

I have read before on here that these things are pigs with fuel but I figured mine wouldn't be as bad since it was quite small. But I will ditch the regulator and try using just the cylinder valve as the regulator. If they need this much fuel, I may have another issue too. The propane hose fitting that threads into the propane cylinder has an incredibly tiny hole before it leads into the rubber tubing. I feel like that is a huge restriction and also not providing enough fuel. I drilled this hole out a little bit to 1/8". Should I go to an even larger hole to allow for more flow?

The other day I had tried that (propane and ignition but no blower) and I got the whole thing lit on fire for a second before I applied the blower since propane was coming out the compressor and turbine I suppose. I wasn't a huge fan of starting it this way so I didn't continue. Most videos I've seen, propane flow is just increased while the blower is on until there is a "pop". Is that not safer to start it that way since there is no external fire?

I have my oil set to about 35psi and with the blower it gets it spinning pretty quick. Not fast enough that I can hear the spool sound but it looks like it goes quite fast (don't have a tach). Is it possible that I am feeding too much or too little air for ignition?

Thanks for the help!

[madpatty]

Hi

I wanted to share my experience.

When i built my first can type combustor engine, i also used to follow the same sequence of startup.

But after so many failed startup attempts i eventually found that the ignitable mixture was just getting blown out of the flametube and burning at the turbine exit.(yes exit not inlet)

There was a peculiar way how it burnt....it used to burn forming a vortex at the turbine exit.
I used to keep on feeding air and fuel and yet turbine never spooled up.

There is characteristic rumbling sound when flame burns properly inside the combustor.

So i changed the startup sequence.

Fuel in , combustion, slowly starting air supply so as flame doesnt goes off (rumbling sound), then crack open fuel and air both.


Cheers,
Patty

[turbo5cyl]

Hi

Ok thank you for sharing. I had a feeling that the mixture was getting blown out too because I'm feeding quite a lot of air with the blower. I will try the other way this weekend then, first getting it lit inside the chamber and then adding some air flow to the compressor. I did not know there is a rumbling sound either so I will listen for that too! Should I be able to see any flame coming out the turbine or is the combustion not happening quick enough when there is a flame at the exit?

[madpatty]

Hi

In my case the flame used to come out in both cases but when the whole mixture was burning outside, there used to be a sort of bluish flame in the shape of vortex at the turbine exit and turbine didn use to spool up no matter how much fuel or air you provide.

In case of the proper combustion i wasnt able to see any vortex at the exit (still there used to be small flame) and turbine used to spool up instantly.

Cheers.

[turbo5cyl]

Ok I see. How much do you need to spool the turbo using the blower on start up? Does it really have to spool it up to a high speed or is the blower more to feed air to the fuel/combustion in order for the combustion to start spooling the turbine on its own?

[racket]

Hi

You'll be needing an idle rpm of >70,000 , so you'll need your starter strong enough to spin the rotor to >20,000 rpm even without fuel being added , once fuel is burning in the combustor the starter/blower still needs to help accelerate the rotor to idle , the starter must not be disconnected until your compressor is producing at least a couple of psi of pressure .

Small turbos are very difficult to turn into successful gas turbines as the bearing drag is disproportionately high compared to a larger turbo , this bearing drag makes the turbine wheels job of accelerating the rotor during spoolup very difficult, unlike the RC gas turbines with their ball bearings and virtually no rotor drag .

How big is your propane cylinder?

Ideally we need to use a 20 lb - 9 kg BBQ cylinder , smaller camping cylinders have small valve passageways.

Can I suggest you apply a little bit of blower air during ignition attempts , hold the blower away from the comp entry but close enough to force a little bit of air through the engine , have a feel at the turbine exit to judge the required holdoff distance before feeding in the propane , you only need to just crack open the cylinder valve to get a combustable mix at the sparkplug , once you hear ignition , add more fuel and apply full blower and keep the blower in place until the engine reaches idle which could be 10 seconds or more depending on how quickly it accelerates .

But make sure your turbine wheel doesn't start to glow, if you see flames coming out of the turbine , reduce the propane a tad until the flames retreat inside.

Cheers
John

[turbo5cyl]

Hello,

Let me start by saying I managed to get it started and self sustaining! Thank you for all the help! I added a nozzle to my shop vac to increase the air speed into the turbo and help it spool up more which made a big difference and I also got rid of the compressor regulator I was using before. I tried what you guys suggested which was first getting ignition without the blower and that did not quite work. I would have ignition but only for a moment since the all the air in the chamber would be burned up and would not let the propane say lit. I tried adding a little bit of air and that seemed to be better and eventually I ended up using all the blower fully right from the start and then adding propane. That ignited and sustained and after a few seconds I was able to remove the blower as well!

I can see how a small turbo would be harder to spool up since there is a small compressor but I managed to spool it up enough I am using a 20lb BBQ cylinder to provide the propane by the way.

Only one issue I have. On startup there was blue flames coming out the turbine so I reduced the propane a tad and they retreated inside but I still had the turbine wheel glowing a bit. There was a red glowing ring on the turbine wheel so only the tips of the turbine were slightly glowing. Is that still too much heat or is it only too much when the whole wheel is glowing? How is it that I can reduce the turbine temp if it is too hot? I do not have a jet pipe installed on it either.

[racket]

Hi

Depending on what P2 you are idling at the comp and turb efficiencies will vary and that has a big bearing on the temperature required to keep things spinning around .

What P2 did you produce at idle ??

A P2 of ~5 psi will generally produce the lowest temperatures , below 5psi the temps rise and above 5psi they'll generally start to rise as well .

It sounds like you've experienced the usual sort of problems we all have with getting a new engine started .

As long as the turbine glow is only a dull red and you don't try pushing the rpm and P2 ( boost), you'll be OK .

Congrats on getting her going , a few more spoolups to get the feel of things and you'll be right :-)

Cheers
John

[turbo5cyl]

Oh interesting, I did not know you an actually drop the turbine temperature by increasing the rpm and P2 but I suppose that makes sense if I think about it for a minute. To be honest, I do not have a gauge hooked up for the P2 but I will get one installed tonight and let you know what P2 I have and I will try and aim for 5 psi. I am happy I was able to get it running at least! I do not have too much knowledge about these jet engines but I still wanted to try and build one What is it that happens if the turbine temp. gets too high? Will the turbine itself let go and fly apart or is there something else that happens?

[racket]

Hi

If temps get too high the turbine wheel inducer tips tend to fly off , the hub stays intact as its temperature takes longer to rise .

Tips flying off isn't such a big deal as the scroll will/should contain them , but "bits" will fly out the exhaust , .......... LOL, isn't that right Patty ;-).

Yep , get a "low " pressure gauge hooked up to the combustor outer wall , a 30psi gauge is all you need , don't go for a 100 psi one as they aren't always accurate at the low pressures you're wanting to measure accurately .

Cheers
John

[madpatty]

Hi

If temps get too high the turbine wheel inducer tips tend to fly off, the hub stays intact as its temperature takes longer to rise.

Tips flying off isn't such a big deal as the scroll will/should contain them, but "bits" will fly out the exhaust, .......... LOL isn't that right Patty ;-).

Yep, get a "low " pressure gauge hooked up to the combustor outer wall, a 30psi gauge is all you need, don't go for a 100 psi one as they aren't always accurate at the low pressures you're wanting to measure accurately.

Cheers
John

Exactly Racket.

Tips flying off the outlet don't look nice though but once you are into it everything seems fine after some time.

Reminds me of the numerous failed attempts.

Cheers,
Patty

[turbo5cyl]

At least having only the tips fly off is better than the whole turbine wheel flying apart!

I mounted a gauge on the compressor housing last night and did another test. It's a 100psi gauge since I did not have any other around the house but I ordered a 30psi gauge too. At idle, it was slightly lower than 5psi, maybe like 3psi. This time, the turbine did not get red, did not even start to glow. I spooled it up a little bit as well, up to about 10psi and even at this point, my turbine wheel was JUST starting to get a slight glow (I could barely see it and it was dark out). I had to go back to my compressor regulator too since I have no control with just the tank valve but it actually works really well now. In the beginning when it wouldn't start, I was actually feeding too much propane and not enough air since when it was at 10psi last night the valve was barely open.

I tested the 100psi gauge against another gauge I had that is lower pressure and it seems to be quite accurate at low pressure too

Now that I have it running, I think I may build the jet pipe.

Thanks for all the help!

[racket]

Hi

Thats great news , the worst is behind you , those first few starts are the ones that give us all the most worry .

Cheers
John