Why do turbocharger jets produce such little thrust?

[noshell]

Hi all
Why is this the case?
There are many miniature jets that use centrifugal compressors and produce tremendous thrust figures. Why are turbocharger based jets poor in this area?
Somebody told me once that they can produce good shaft hp, just not much thrust. I have no idea if this is true or not though. I am mainly just curious.

[racket]

Hi

Turbo based jets AREN'T poor in this area , its simply a case of mass flow and thrust per pound of flow ...............plus the RC aero engines tend to fiddle the numbers a bit ;-)

A turbo based engine can produce both good thrust and good shaft horsepower , my TV84 turbo bike jetandturbineowners.proboards.com/thread/1366/tv84-turbine-bike with a 3.5"inducer and ~1.8 lbs/sec flow produced ~110 lbs of thrust at a rate of 61 lbs of thrust per pound of mass flow , this is vastly better than the rate of the early fighter jets that used centrif compressored jet engines , when it was converted into a freepower machine the bike produced 115 RWH on the dyno after all the losses through the transmission etc etc , it was a very crude build .

Anders bike used a TV94 turbo rotor in his 306 kph bike , even though the engine was not a turbo based one , if a TV94 turbo or HX82 size turbo had been used for the gas producer it would have performed nearly as well , any shortcomings would have been from the slightly lower efficiencies of the comp/turb stages using scrolls .

Checkout the numbers here www.amtjets.com/pdf/Titan_specifications.pdf 88 lbs of thrust from 1.46 lbs./sec = 60 lbs/lb I did better 25 years ago with an old secondhand turbo ..............our engines aren't toys .

Cheers
John

[noshell]

Interesting, i'd been thinking maybe the radial turbine wheel was a hinderance.

In that case... Why are they often very underpowered? Jetspecs (which most people use thanks to you) proposes an oversized combustion chamber so i dont see this being the limitation... I dont get it

I havent had a chance to really wind up my latest engine, but my first engine basically produced no thrust. it had a measly 29mm inducer but it honestly produced fuck all, yet as a turbo it is good to flow 130hp worth of air. I realise in a jet alot of this air isnt used for combustion, but surely a good portion of that air should be useable to burn fuel and therefore make power.

[richardm]

I won't go into numbers equations etc. But about one third of the compressed air combines to fuel in burning making hot gases the rest is for cooling those gases. From all the potential energy contained in that burnt fuel ( I say potential because the combustion is not always at its best) about two thirds are converted by the turbine in rotary motion to drive the compressor.
So you're left with only one third of that energy to do useful work, either thrust or turning a power turbine

So starting from a 29 mm inducer, allowing for some design flaws, and adding inherent inefficiencies, divided by 3 leaves you with not much...Or as you say F.A.

[noshell]

So in Daniel maths
I work it out to be 130hp worth of air, two thirds of which is "wasted" One third of this burns with fuel so 43.3 left.
Then say my whack design will be 25% efficient = 10.8hp

It was honestly nowhere near that 😅

Where are my likely shortcomings? As i say i followed jet specs exactly for this. She ran on butane.

[richardm]

You're conception of 130 hp of air is misleading you It means that this turbo can supply enough boost to support a 130 h engine Its not 130 hp of air power.
The piston engine will develop 130 hp or so not the turbo.

[ripp]

for comparison, x45 turboprop with an axial turbine generates 5.5 kw = 7.5 Hp (approx. 30 mm inlet).

As a jet engine 45N

www.xicoyturbines.com/TP45.html

www.xicoyturbines.com/X45.html

[richardm]

True Axial turbines are a better choice but more expensive and not suitable in a turbo charger

[racket]

A 29mm turbo has severe limitations with component efficiencies in a turbo thats produced as cheaply as possible, there are "scale effects", wheel clearances have a huge effect, bearing drag from the "brass" is a greater proportion of losses .

Personally I wouldn't use anything less than a 50 mm inducered turbo to make a DIY engine from .

Radial turbines are better than axial ones at small sizes if we can control the clearances and use similar stators rather than a scroll with its large surface areas and losses compared to an axial NGV .

LOL.............you were successful in getting that small turbo to run , thats an achievement in itself , don't feel bad about the lack of thrust , or compare it to the HP output of the engine it came off.

When I was developing my TV84 engine and it only produced 110 lbs of thrust whilst burning huge amounts of fuel I couldn't understand why when the turbo came off a 460 HP diesel engine , but once you do the maths , things become clearer , I spent 2 years going around in circles trying to get more thrust before finally doing the maths , ...........felt a bit of an idiot afterwards for wasting so much time :-(

But the piston engine burns all the oxygen , and runs a higher compression ( expansion) ratio than our DIY engine so is able to extract more energy from those expanding gases .

LOL...............don't blame Jetspecs , its trying to cover all sizes of turbos , all levels of build sophistication or lack thereof , all levels of component efficiencies etc etc etc :-)

Cheers
John

[noshell]

That makes a bit more sense. Jetspecs is not being assigned the blame for lack of power haha
I am also suspecting that liquid fuel will produce more power than butane, off the top of my head im sure it is far more power dense.
I was hoping for 10/20hp from my latest 40mm inducer sized engine. I suppose it makes what it makes and i can always swap the turbo. Im not interested in it being fast, i just want something useable.
One thing i have noticed that is to our advantage is that turbochargers are capable of relatively high rpm compared to turbine engines i am looking at. Although i realise the size of the orafice is ultimately the limiting factor.
Do you have any material for me to aid with "the math"?

[racket]

LOL..................poor old Jetspecs , it really needs updating , but where to start , there are now many more thousands of turbo configurations than when it was made :-(

Nope there shouldn't be any difference between butane and liquid fuel as long as the temps going into the turbine are the same at the same rpm.

A 40mm turbo is still a very small turbo , the mass flow is only ~0.4 lbs/sec , but 10HP should be achievable .

Our RPM limits are basically the same as turbine engines with similar sized comps , its tip speeds that limit things generally ..............small wheels have higher rpm than larger ones .

Theres a couple of good books ............

www.ebay.com/itm/195999223574?epid=1002506501&itmmeta=01HTVMBJW4FDWAVSFN13NZ41VS&hash=item2da276cf16:g:jWsAAOSwNuxlEcCx&itmprp=enc%3AAQAJAAAA0KFuKliE2%2Fzyy3zigs3B2OdYj01PXhecJpFnufb4LrFaPHf2BkjsgfM1d0pKWzy5SlxvJdMv0gxOIctcJ8hJqjJWFmAkCpRKXH5EEbtip2VYdK%2BbxXAytSOp4AmNc7x%2B74JeAjpPsGHr1io8758Ajdmhatmq7Mp02agwu%2B%2B%2BarXCLy%2BBXbafP8tBBoSV%2BCEj1ZpkDj53350XvquPZzmmTWNJPh06kIWTmGGrZhJwOjzDa9StKFP%2F0PBjNLHfhoQy3BOvmuqhpmhklD6e5jGjyY4%3D%7Ctkp%3ABk9SR5KurvTWYw

And the best of all
www.ebay.co.uk/itm/256428361869?itmmeta=01HTVMN9EV3QG1RJKT65CZPNBX&hash=item3bb452488d:g:3sQAAOSwNRZl35GQ&itmprp=enc%3AAQAJAAAA4Jl9swfpwHuaLEIs0ewzT%2BLlIQ09ZfNP56zjq%2F%2B0tPhxmvhvEpxibhCgRx0nuVTXNROirC%2BOYWN8u5FbBugI86pfEb3kJ8ItOmsUi7ws%2FeXkQwEKjGDPyGD0OLUVqW0mjrlN8dkIaytWXDRdDmRqU6R1hkmcy%2FAX6hluIkxgdyNt4U3sa4VU9LTtR0kgSzuqJOqg8wjiwFsG9msj82gQTzFjcEXYAuLU%2BAh7xxQsab5Cqj2q6VF5a6%2BxYz84VIodwueXtyqAFW2kAi0UZezf1JjqpkLAGvIsSxxq%2BqoT7TiG%7Ctkp%3ABk9SR8CX1fTWYw

You might find Thomas Kamps book as an Ebook somewhere

Cheers
John

[enginewhisperer]

Realistically, most turbine engines have much higher mass flows than most turbochargers.

Even the RC turbojets have very large compressor wheels for the engine's overall size and weight when compared to a turbocharger.
It would be hard to get more than the engine's weight in thrust out of a turbocharger setup.

In the end you're using 1/3rd of the air to generate power, at a low pressure ratio. It's just not very efficient, so you need a big engine to make much power.


The jet nozzle will also affect thrust massively - but if your turbo is not well suited to being a jet engine you won't be able to increase exhaust velocity enough with the nozzle before you run into compressor surge or turbine inlet temperature issues.

[noshell]

This kind of thing perpetuates my original thoughts
youtu.be/6Mwkk8Yo40E?si=5J23x6f2BGoDsDuN
The guys website says it produces a measly 8hp.

I wonder if its not very optimized though.

[racket]

RC Don did some beautiful builds , but ran his engines at modest temps and pressures , hence the modest outcomes