Question about nozzles

[willisdaye]

Is there a rule of thumb or calculation to determine the nozzle output diameter for maximum thrust? Also, are there any dangers in having the output too small, such as overheating or over pressurizing? I'm using a Garrett t04e. Sorry for the noob question.

[racket]

Hi

Its extremely important that the nozzle is not too small , you can easily wreck an engine from overtemp or surge .

Nozzle diameter needs to be sized so as to produce the desired jetpipe temperature (~600 deg C ) at full power output .

Generally advisable to fire up the engine the first time without a jet nozzle , then if the TOTs are at coolish temps of <500 deg C , a jet nozzle can be installed , start with one around the same diameter as the comp wheel inducer dia ( 1.8" yes/no ??) , this will increase temperatures and depending on how high those temps are the nozzle can be reduced/increased a tad at a time until the desired temp in the jetpipe is achieved.

Cheers
John

[willisdaye]

So I shouldnt go over like 650*C? Also, I have one of those ranged infrared thermometers, will that work, and what part of the engine should I be measuring the temperature of?

[racket]

Hi

Your temperature drop across the turbine wheel varies between idle and full power , so if our limiting turbine inlet temperature ( T I T ) is 1450 F - 788C , at idle you might only have a 40 deg C drop across the turbine so a 748 C turb outlet temp ( TOT) is acceptable , but at full power you might need that 188 C drop in which case a 600C temp in the jetpipe is the max .

If your turbo is new and modern , and a reputable brand , not a copy , and not a diesel turbo that can have a lower temp turb material , then temps can be increased , but 650C in the jetpipe would be a max if unsure of the turbos heritage.

I don't like infrared thermos as you'll be reading material that has cooling air on one side and will most likely be reading a temperature below the hot gas on the other side , but if you must use it then try different parts and use the highest temp reading .

If you stand well back from the engine and keep temperatures below a point where you observe the turbine wheel starting to turn a dull red you should be OK

Cheers
John

[willisdaye]

Thanks. How will changing the nozzle diameter affect the engine's thrust output?

[racket]

Generally , the smaller the nozzle the greater the thrust , but only up to a point where you reach maximum safe temperatures , past this point thrust will keep increasing with temperatures until you eventually start to shed turbine wheel blade tips.

If the nozzle is too small the engine can go into surge and wreck itself , the nozzle has to be just the right size if the engine is to survive whilst making maximum thrust .

[willisdaye]

That makes sense. I think I'll start with a larger nozzle and make it smaller if it's still cool enough.

[racket]

Yep , thats the safest way , something the same diameter as the comp wheel inducer :-)

[willisdaye]

Thanks. One last thing, how long should the nozzle be? will changing the length of it affect anything?

[racket]

Hi

You probably only need a total length of jetpipe and nozzle of ~3 inches , long pipes/nozzles only create losses from skin friction .

Keep the angle of the nozzle fairly shallow , like this jetandturbineowners.proboards.com/attachment/download/221 and you won't go far wrong :-)

Cheers
John

[willisdaye]

Thanks. Is that threaded hole for an afterburner?

[racket]

The threaded hole is for one of the two thermocouples that measure the TOT, theres Afterburner stuff latter in the build thread jetandturbineowners.proboards.com/thread/78/garrett-gt6041-powered-kart

[willisdaye]

Thanks for the help!

[racket]

Thats what we're here for , all the best

Cheers
John