Loki

iTrader: (1)

Interesting video by Engineering Explained, interesting in that he found actual numbers for conditions that deposits form and burn off.


Coles notes:

• carbon deposition is worst around 200C / 414 F
• carbon removal starts around 325C / 617 F
• piston surface is around 300C. Anything exhaust related is way above that and self-cleans. Anything intake relate is going to be cooler, right in the carbon deposition zone.
• so the theory behind getting the temperature/pressure up to self-clean carbon deposits makes sense, but some engines were found to have the opposite effect.

My thoughts on applying this to rotary:

• our rotor surface should be cooler than a piston, it's huge heatsink. This is supported by carbon crust on used rotors, which means apex seal notches are likewise cool and happy to collect deposits
• hot exhaust gas does enter the apex seal notch (in the Renny it's cleverly used to push the apex seal up), as does cold fuel-rich intake mixture, so that area goes through a hot/cold cycle with every revolution, which sounds like perfect breeding ground for deposits.
• does redline a day keep carbon away? All signs point to yes, but you need to heat soak the rotor, so just a redline isn't going to cut it. You need to drive at high load (not necessarily high rpm) for extended periods. This can be achieved just by accelerating at WOT in high gear, like when you pass someone on the highway while in 6th [ see further notes below]
• alternatively, take it really easy on the car and you might keep the whole thing below deposit-forming temperature

If anyone has data on apex seal notch temperatures and such, we might be able to dial in the recommendation even better.

Loki

iTrader: (1)

So I poked around last night. Based on Mazda's papers, the apex seal temp at steady state full load operation up to 5000rpm is between 200-250C. Right in the deposit forming zone. I can't imagine the rotor notch would be any hotter than the seal.

We need over 325 to self-clean or below 200 to not create deposits in the first place.





At partial load, the temperatures stay below deposit forming temperature (!). If the theory is right, that might explain some reports of longevity of motors that aren't wrung out regularly. Unfortunately the graph doesn't continue to 9000rpm, where a track car would spend extended amounts of time. Also oddly, it levels out between 4-5k. Does that mean the temperatures stay constant beyond that? More research needed.

Also unfortunately I can't find data for side seal temperature, which is just as likely to carbon up.

Also also it's interesting that the corners of the apex seal are significantly cooler than the center under all conditions. Those corners are quench zones, so buffered from hot gases. Potentially this is the reason apex seals wear more in the center : bowing out due to uneven heat expansion.

UnknownJinX

Is this for the Renesis specifically or for the older peripheral exhaust design? Does that make a difference?

IamFodi

iTrader: (1)

Very interesting indeed. Thanks for posting.

Any leads on papers that discuss the compositions of those deposits?

Loki

iTrader: (1)

That's a good point, I think the paper pre-dates the Renny. Which may not mean it doesn't apply to the Renny:/ I'll try to find out more.
It's a double-edged sword. The higher EGT of side port could help get into self-cleaning temperature at WOT, but by the same token push regimes that currently are below deposit-forming temperature, into deposit-forming.

Oh to have an engineering lab...

UnknownJinX

To support this point, I found something out of the Rotary Engine book written by Kenichi Yamamoto himself.


Now, of course, this is an old book(the cover still says Toyo-Kogyo, which is Mazda's very old name), but combining what people have seen on teardowns, could still very well be true. The rotor temperature is basically a very good breeding ground for carbon...

Hopefully, the magic sauce layer they have on the S2 rotor is working, LOL.