Post turbine exhaust gas temperature on 1.6 TD?

[larry104]

I've read that post-turbine exhaust gas temperatures are consistently about 300 deg F cooler than pre-tubine.  If this is so, why not install the thermocouple post turbine and subtract 300 degrees from max temp -- say 800 F instead of 1,100 F? That way, should the thermocouple tip crack off, it won't destroy the turbine and would instead blow harmlessly out the exhaust pipe.

[Chestrockwell]

Thermocouple will only break off if it is hit by a flying valve or compressor wheel.... then you have a problem.

[larry104]

What about vibrational fatigue? The harmonic frequency of the TC is probably much higher than mechanical engine vibes, though the engine vibe amplitude is pretty high.  And the TC  diameter is pretty small.  Wondering if the turbo could set up an acoustic vibration that would tweak the TC harmonic frequency? All that being said,  the chances of a TC breaking are probably pretty slim,  though if you can locate it downstream and eliminate the risk, why not?

[jtanguay]

I can't see how the thermocouple tip could break off during normal operation,  especially with 1100F temps.  The only time the tip is glowing, is at around 1500F and in which the thermocouple is at its weakest point.
Even at 1400F the tip is not glowing, and should be a safe temp provided its not run at for an extended period.

Pre-turbine is much more accurate.  You can see spikes from turbo buildup (you won't be able to see the temp that the turbo sees pre-turbine.) and that is essential.  Once the turbo is fully spooled, then yes subtract 300F from the temp and voila, but not before.

My opinion, get the best TC you can buy, and that should be insurance enough

[larry104]

I can't see how the thermocouple tip could break off during normal operation,  especially with 1100F temps.  The only time the tip is glowing, is at around 1500F and in which the thermocouple is at its weakest point.
Even at 1400F the tip is not glowing, and should be a safe temp provided its not run at for an extended period.

Pre-turbine is much more accurate.  You can see spikes from turbo buildup (you won't be able to see the temp that the turbo sees pre-turbine.) and that is essential.  Once the turbo is fully spooled, then yes subtract 300F from the temp and voila, but not before.

My opinion, get the best TC you can buy, and that should be insurance enough

I'd argue that it would be difficult to watch the gauge that closely and react to temperature spikes and drive all at the same time. :shock: Besides, the Inconel sheath itself has thermal mass and will integrate out short duration spikes.  I'm no expert, but I would think steady-state conditions under load, say on a hill, are when high EGTs would be a problem, if they are going to be.  Fwiw,  metal raised to 1,400 F gives off a bright orange glow. At 1,600 F it's bright yellow. I think Inconel 718 has about half the strength at 1,600 F that it does at room temperature, which is still quite high. Again, it's probably a rare occurence that a tip would break, but why risk it if it's unnecessary.

[Mark(The Miser)UK]

Will a hot tip snap due to vibration or will it be more malleable and only break off when rigid?

 Are vibrations that bad at highish rpm anyway? My 'Q' always seems smoother the faster I go :shock:

Isn't the purpose of a thermocouple purely to check combustion isn't melting pistons; so ideally have 4 one for each manifold branch