PD-01 Jet Engine

[madpatty]

Hi Smithy,
Can you give me some more description about the thermocouples fiited on the go-kart so that i can check for their availability through the internet....and check if they are available around me..

Cheers,
Patty

[smithy1]

Hi Patty,

I'm not really sure where John got them from....but they are connected to a good quality multimeter similar to a "Fluke". John may be able to give more info.

Cheers,
Smithy.

[racket]

Hi Patty

The thermometer is a YCT YC 821 , they've been superseded by the YC824 www.fmfranklin.com.au/data/yc821.pdf , the K type thermocouples are sheathed types .

Cheers
John

[finiteparts]

Patty,

A common problem with cheaper thermocouple gauges is that they don't have a good control or measure of the cold junction temperature. Since a thermocouple generates a voltage due the presence of a thermal gradient across two dissimilar metals, not knowing what temperature the cold junction is at generates an error.

Two stroke aircraft engines are quite picky about the EGTs they run, so a good EGT set-up is key. I found out the hard way that some gauges "assume" the cold temperature to be fixed and so your gauge is only accurate at that "calibration" temperature. When your cold junction is above that calibration temperature, your gauge will read low since the temperature gradient that generates the voltage is lower...conversely, the opposite is true for the case where the reference temperature is colder than the calibration temp. Your gauge will read high since the thermal gradient (measured temp - reference temp) is larger.

So make sure that your gauge is referencing the actual ambient temperature and not a set calibration temperature.

I hope that helps.

~ Chris

[madpatty]

Hi Chris,
I think i have understood what you are trying to explain...but i have a question:-

The ambient conditions are variable And how to check that my thermocouple is referencing the ambient temperature and not a fixed temperature?

Cheers,
Patty

[finiteparts]

Patty,

It should be stated in the documentation for the gauge. If it doesn't say anything about reference compensation, you can look inside the gauge (if you want to take it apart) and see if there is anything that measures the cold junction temperature, like an RTD or thermistor. So many gauges out there are not intended to be used at the upper temperature range of the thermocouple and so this cold junction issue is not a big deal...but when you are measuring at the upper range of a type-k thermocouple, it is more important.

Even though putting in a cheap RTD to do this should be expected, I found a bunch of cheap EGT gauges don't have them.

Omega has a lot of good info on thermocouples that you can read to get up on the theory...

www.omega.com/temperature/Z/zsection.asp

Good luck!

Chris

[madpatty]

Hi Chris,
All i know about the guage is:-

It is digital industrial grade temperature controller with range of 0-1200 degrees celsius....

those controllers are used in boilers etc so i can expect them to be quite accuate....
The part which i think is faulty is the thermocouple probe...because i had been using the same temperature controller(obviously as temperature gauge) since the very beginning....

The think which is disturbing me now is when i compare the engine runs with the previous ones....the temperature and turbine blade colour is not coinciding when compared to previous runs......i know that is not a proper method of the comparison....

Yesterday only i ran the engine and at 12Psi P2 the temperature that was appearing on the Temp. Controller was only 620 degrees celsius......(that one using the same thermocouple which showed 111 degrees celsius of the boiling water temperature)......
But when i compare the colours(video shoot with the same camera) it appears near 700 degrees celsius.....

Should there be any colour of the turbine blades at around 600degrees celsius??

Cheers,
Patty

[madpatty]

Hi Racket,
I wanted to share some data that i acquired from a recent engine run.....it was using the impeller (54/83) with turbine (70/58)....

I know the compressor maps are invalid with the diffusers we use but still the results are kind of contradicting and more on the choke side rather than what we were thinking that the compressor will be near its surge line in this setup...
It will be helpful if you can interpret what is happening.

P2(Psi)  RPM

7          72900
9          81067
10        85000
11        88363
11.5     91500
12        92400
14.5     99400

cheers,
Patty

[racket]

Hi Patty

Either your tach isn't reading correctly or your P2 gauge isn't reading correctly or you're running in surge .

At 99,400 rpm you should be reading a pressure closer to 22 psi, rather than 14.5psi.

I doubt you'll be running in choke.

If you've done the piece of string test at the comp inlet to check for flow reversal to see if you are running in surge and theres no "reversal" , then its either the tach or P2 gauge thats not reading correctly .

The P2 gauge can be easily checked against another gauge of known accuracy .

The tach is less easy to check .

Possibly most likely the gauge as the pressure you should be reading is staying at ~50% greater than the measured pressure for the rpm you're reading , its "linear", whereas if the tach is consistently "over reading" then the pressure inaccuracy wouldn't be "linear". ............In reality, I haven't the faintest idea .

Cheers
John

[madpatty]

Hi Racket,
If the compressor is in surge then shouldn't the rpm be very much less than these readings and pressure considerably higher??
For example:-
If in surge then rpm should have been around 60000 at 7 psi.

Tach is OK checked when checke with against a 30000 rpm motor.

Cheers,
Patty

[madpatty]

Hi Racket,
I just rechecked the tachomoter on a motor with no load speed of 30000 rpm with a hex nut coupled to the motor shaft....
One face coloured white showed 29900 rpm...two alternate faces showed 59984 rpm and three alternate faces whitened showed rpm in vicinity of 90000....so i guess i can rely upon the rpm meter...

I will check my pressure guage and will soon post the result.

Cheers,
Patty

[madpatty]

Hi Racket,

I checked my pressure gauge at 5,10,15,20,25,30 psi against a calibrated air pressure supply and all the readings were more than exact...not even a point here or there.....

Cheers,
Patty

[racket]

Hi Patty

Gotta be surge :-(

Now surge with a swept back comp tipped wheel might not be as "dramatic" as with a purely radial comp wheel , it could be "softer" and more "internal"

If the engine is surging , your pressure reading, if taken from a large "plenum" like a combustor can, will be an "average" pressure being discharged from the compressor stage , the pressure within the comp stage itself could be fluctuating wildly to pressure well above and below the gauge reading .

Is your pressure gauge "dampened"??

Or is it undampened and the needle able to "bounce around" freely ??

P2 gauges , at least during initial development , need to be undampened so that any pressure fluctuations are easily observed so as to warn the operator of potential problems .

Cheers
John

[madpatty]

Hi Racket,
No the pressure guage is not dampened...

Cheers,
Patty

[racket]

Hi Patty

Does the needle bounce around during steady state operation ??

Is there any restriction in the connection between gauge and engine ??

Cheers
John