Starting out with a VZ21 and looking for a little guidence

[tomahawktom]

Hi y'all, I just recently graduated and I'm looking to making a turbocharger jet engine as a personal project.

In doing my research about making the combustion chamber I keep seeing the 30%-20%-50% "rule" as it were for the flame tube, and I was wondering if someone could shed some light on why that rule of thumb exists. I understand that the primary and secondary holes are for the fuel and air mixing, and the tertiary for cooling the gases, but where did those percentages come from? Are they just what's been found to yield the most complete burn while also keeping the turbine intact? I'm planning on writing a report for this project so if anyone could also give a good source I would very much appreciate it!

Secondly I'm looking for some safety guidelines to follow. I'd prefer not to melt my turbine, and least of all frag my entire engine, so any advice here is more than welcome, especially in the realm of proper fuel delivery.

Thanks!

-Thomas

[wannabebuilderuk]

I'm sure john will be along soon to give you a proper explanation but the 30/20/50% is basically the total area of the holes which adds up to the same total area of the compressor inducer.

You want as much of the combustion completed as possible in the combustion chamber for maximum energy converted into heating the surrounding air whilst also keeping the flame away from the turbine wheel. Having 30% in the primary allows for a good mixing of air/fuel which gives a better combustion before using the flame to heat up the extra air going through the tertiary holes which is what drives the turbine.

[racket]

Hi Thomas

Thanks Ben :-)

Yep , I can take the blame for the 30/20/50% "rule ??"...........the 30 part is a bit higher percentage than what you'll find most places , it came about because of builders using lotsa small Primary holes which didn't flow as well as the larger Secondary and Tertiary ones , also we tend to run higher turb inlet temps than the guys building RC model aircraft turbine engines so we need more air for the extra fuel .

The 20 Secondary air is provided to finish off any combustion whilst not chilling the combustion products too much and causing other issues .

The 50 is to cool the combustion products to a level that the turb survives at, they are all rough guidelines , combustion issues could require some minor changes , but generally they'll work OK

Checkout jetandturbineowners.proboards.com/thread/680/diy-turbines click on the icons for the info .

If you intend using the VZ21 turbo , you'll probably have no end of problems, its an extremely small turbo with limited component efficiencies making it difficult even to self sustain , a bigger turbo should be better if the turbine exducer is 20% greater in area than the comp inducer area , hole out 20% bigger than hole in .

Cheers
John

[tomahawktom]

Hi Ben and John, thanks for the reply!

I suppose coming from an engineering education background I'm expecting everything to have hard numbers and reasoning, but I do understand that rules and conventions are just as valid when they've been shown time and again to work well. I appreciate the insight. I've actually already checked out that link and will definitely be referring to it for my project!

I actually was thinking a smaller turbine would be easier to deal with, mainly in the realm of manufacturing the combustion section. I'm still up for trying to get the VZ21 to work (since it is already in transit haha), but if I do choose to use a better suited turbine I'll be sure to look for one with a 20% larger exducer.

Another question I've run into after calculating some reasonable safety margins, should I be designing around a target pressure ratio? I don't have a target torque or thrust value but looking on a spec sheet for the VZ21 it gives an airflow range at a pi_c = 2.0, so I figured that would be a good value to shoot for, assuming I am thinking along the correct line?

-Thomas

[racket]

Hi Thomas

The comp map maxes out at 2:1 PR , this is very low , and its efficiency is rather poor at that point , I'd be looking at trying for ~1.75 PR where the effic is highest , this will provide your best chance of producing an engine that works .

Unfortunately pressure ratio alone doesn't provide safety as rpm can vary enormously depending on where on the map the flow is , but considering the turb stage it comes with I don't think you need worry too much as you'll be closer to surge than choke .

Because of your low pressure ratio ( 11 psi P2 ) capability , you'll be idling at virtually full power , do not try idling too low , a turbo based engine with "brass" bearings with more friction than a ball bearing turbo , generally needs at least 5 psi P2 at idle even for larger turbos , your small one will be even more particular , a good strong starting system required to get those rpm up to >100,000 for self sustain , idle could be 150K

Cheers
John

[tomahawktom]

Hey John,

Which compressor map are you looking at? I just found one for the RHB31 specifically since my original source said they were similar and now I'm seeing where you got that 2.0 PR max and 1.75 efficiency island.
(https://geometroforum.com/sprint-turbocharger-compressor-map-t6457.html)

My first mistake was probably using this one document that claims it is close to the RHB31, then says RHB3, and shows compressor maps and spec sheets for a range of turbos, including the RHB3. Thats where I was getting a max PR of 2.7 and a design PR of 2.0. (https://www.ecotrons.com/files/ECOTRONS%20Turbo%20VZ21%20technical%20specl.pdf)

I'll have to look for an appropriate leaf blower or equivalent for getting to those RPMs. Looks like from my first Comp map Ill need 6 lbs/min of air, which is around 75 cubic feet/min, so I should get something with at least that output? I imagine Ill likely want something more than that since Ill have to overcome the inertia of the turbo machinery.

-Thomas

[racket]

Hi Thomas

A leafblower will be too large and won't provide enough energy transfer as they'll be unable to work at such low flows , I'd suggest a high pressure supply from a workshop air compressor feeding a suitable nozzle that feeds the extremely high speed onto either the comp or turb wheel tangentially .

The very small RC micro turbines which are of a similar size mass flow can be started with small blowers but thats only because of their ball bearings and minimal rotational friction and relatively long spoolup time frames.

LOL...............I have one of these small turbos , I really should do a build with it so that I can speak with some authority , my smallest engine has flowed ~18 times as much as your build, so I'm only going on past experiences of others, extremely small builds like yours will present unique problems ,

Cheers
John

[drumwilldrum]

Hi Thomas and John,

I have just recently built an engine using the VZ21, more because I had that turbo spare from another project (turbo bike) than anything else.

Followed jetspecs for the combustor and using propane injection; John as you rightly note the first issue seems to be getting it spooled up to a speed where the tiny compressor can generate any real pressure.

The compressor wheel is so small it is difficult to even get an pipe through the diffuser section to strike the wheel tangentially, that aside I have only managed to spin it up to around 3psi measured at the scroll outlet - the issue seems to be the volume of air and pressure out of the starting nozzle being greater than the compressor can generate and exiting via the compressor inlet rather than flowing through the engine, so the engine gets no assistance in spooling from the turbine end. My next step is to try a smaller nozzle, then if that fails try directing the starting air at the turbine instead. After that I guess I will move to a BLDC starter.

I also have a replacement bearing housing, and a bearing sleeve for the rotor which replaces the plain bushings with ceramic bearings (designed for my turbo bike project) which I will move to if I cant get the stock turbo to work.

Good to see others are trying this.. strength in numbers. I will try to share pictures later.

Thanks
Will

[racket]

Hi Will

The turbine impingement would be the best option using a suitable nozzle brazed onto the scroll, the comp impingement has the problem of pressure losses across the flametube lowering any help from the turbine .

Cheers
John

[tomahawktom]

Hi Will,

Thanks for mentioning your issue with spooling up the turbine, I'll keep an eye on that issue. Thankfully I work at a manufacturing shop and they use a lot of pneumatic tools, so I'm hoping I have a shot at getting the compressor up to speed with compressed air.
How important is it to have the air hit the compressor tangentially? Watching another guy do a small turbo jet build it looks like he was able to spool it up no problem with an air compressor and a plastic hose placed right up to the intake. Then again, I'm fairly certain he wasn't using a VZ21, which we've noted has it's own problems.

This discussion is now also making me think I won't need the 15 PSI waste gate that came with it. But then again, better to have and not need.

Currently my next problem I'm wanting to work out is a rough estimate on how hot the propane will burn, but so far the largest unknown is what my A/F mixture will be. Is that something I can roughly estimate for the temp calculations, or will that have to come later when I choose an injector? I'm hoping to avoid arbitrarily picking an equivalence ratio like phi = 1.5 since my gut tells me the mixture will run significantly more lean.

-Thomas

[racket]

Hi Thomas

Don't worry about your mixture strength , as long as your flame tube has the correct hole sizes and positioning it automatically sorts things out , just inject enough fuel to keep the temp coming out of the turbine wheel below 1250F - 675 C.

No wastegates required , you'll only overtemp the engine if you use one .

If you have access to a large compressed air supply then it would be possible to use a hose sealed against the comp housing inlet, similar to what we do with our leafblowers on larger turbos.

Cheers
John

[tomahawktom]

Hey John,

Thanks for letting me know! If anything I'll keep any temp calculations for after the turbine works, then I'll at least be working from known values.

I was hoping not to need the wastegate. The VZ21 compressor exducer and turbine inducer are not parallel, but taking the wastegate off should allow me to rotate the assembly to make them parallel.

And here's hoping for the compressed air to work!

-Thomas

[racket]

Hi Thomas

If you can supply enough air it'll work :-)

Cheers
John

[tomahawktom]

Alright, I'm still slowly piecing everything together. This project is absolutely as complicated as you want it to be, and man I definitely am not in the shallow end haha.

Just a quick check in too, the P2 that we use for Post Compressor air pressure, would that be the equivalent of "Boost" pressure if we we're using a turbocharger for normal automobile applications? I recall the air pressure after the turbine in normal axial jet engines being P3, since station 3 is after the low and high pressure compressors, so I want to make sure I'm getting my terminology right to reduce confusion.

[racket]

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