Ceramic Engine Parts - Does this look gimmicky?

TugboatEng · May 11, 2022

https://www.3printr.com/first-entirely-ceramic-piston-for-chaos-ultracar-5555832/

I don't have much knowledge of ceramics. This article popped up on my news feed, I can understand complex computer modelling can lead to unexpected optimisations of shape. The part that gets me is the wild beam geometry yet my Caterpillar diesel engines have more interesting optimizations of the big end of the rod.

Also, any guesses on the ceramic? I didn't think they had very good tensile strength. I see 69ksi listed for alumina. Maybe that's equal to what is normally used in connecting rods?

btrueblood · May 12, 2022

Ceramics can have incredibly high tensile strength...just typically no ductility. And in as-fired form, the tensile strength becomes more of a probability distribution...

3DDave · May 12, 2022

Pistons are often aluminum, so not sure that the strength is problematic. Aluminum also has a pretty good rate of thermal expansion and low elasticity relative to most engineering metals and, of course, limited heat resistance. As long as no one smacks it with something sharp I expect that manufacturing cost and production of relatively thin features without distortion are the biggest obstacles.

That con rod is something though. I have seen where different optimization strategies yield remarkably similar results, so I accept that such forms are not aberrations. Typical manufacturing methods drive shapes - con rods could be hollow, but those are tougher to make. With direct deposit additive manufacturing the shapes are less constrained.

I wonder if that thick strand is the oil line to lube the wrist pin.

TugboatEng · May 12, 2022

I don't know about wrist pin lube. Most automotive engines so not feed pressurized oil to the wrist pin. Lube is pumped from the cylinder wall to the wrist pin by the scraper ring in some designs.

3DDave · May 12, 2022

Sure - but with this they could. Apparently con rod channels/galleries are on some diesels with gasoline engines using the spray and pray method.

enginesrus · May 12, 2022

Ceramics are used on some metal cutting tools and can hold up real well.

gruntguru · May 13, 2022

I'm calling BS. I see lots of slender columns seeing large compressive forces.

je suis charlie

EdStainless · May 13, 2022

If these are fired to full density, and they are a toughened grade of alumina then this should work.
The joint doesn't need lube, it just needs to stay below some critical temp.
In fact if they made the pin from a different ceramic it might work fine dry.
The compression strength of these materials is fantastic.
Fracture toughness is about zero though, it does get better at high temp.
One issue with these 'geometry optimized' designs is how much redundancy is built in.
Ones that I have seen work used 4x or 5x load path designs.
Less than that and minor build flaws can doom you.
But my beef with AM is still there.
They can't tell you before they make the parts what the microstructure or properties will be at any specific location in any specific direction.
That isn't a process for making parts with low redundancy.
It works in many aerospace parts because they are replacing a heavy part that is 50x overbuilt with AM that is only 25x overbuilt.
Or an assembly of 32 parts with a single AM one in order to get special geometries and load isn't a design criteria (fuel nozzles).

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P.E. Metallurgy, consulting work welcomed

gruntguru · May 13, 2022

Look at the piston skirts and visualise the loading they will see. No chance.

je suis charlie

EdStainless · May 13, 2022

You do realize how absurdly low the strength of Al pistons is, don't you?
Most Al alloys start with RT yield strength of 25-40ksi, and at 500F they are <1/4 of that.
Fairly low bar to clear.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

TugboatEng · May 14, 2022

This brings up an interesting thought on the evolution of engines. Early engines were cross-head designed and had full bearings to support the side loading generated by the rod. Newer engines are trunk piston type and use the piston skirt. The newest engines are nearly shirtless. I guess the development of high silicon alloys for pistons has reduced the running clearance which reduces the amount of work the skirt needs to do. Ceramic must take that s step further.

enginesrus · May 14, 2022

Only engines I know that use almost no skirt are racing types. But then they usually only need to last the race.

inline6 · May 14, 2022

looks like same nonsense as here

https://tiremeetsroad.com/2019/11/2...ing-rods-for-evos-can-handle-3000-horsepower/

gruntguru · May 15, 2022

EdStainless said:
You do realize how absurdly low the strength of Al pistons is, don't you?
Most Al alloys start with RT yield strength of 25-40ksi, and at 500F they are <1/4 of that.

Methinks I could crush the skirt on that ceramic piston between thumb and forefinger . . . and then there is impact toughness. Imagine gently tapping the skirt of an aluminium piston with a small hammer - then repeat the exercise with the ceramic one shown.

je suis charlie

EdStainless · May 16, 2022

At 500-60F I could fold the skirt of an Al piston over with a very small hammer.
At that temp a ceramic skirt will withstand some serious loads and impact.
I am not suggesting that this particular design is good, just that the concept of ceramic engine parts is very valid and actually fairly well understood. Remember the Army Adiabatic Deisel program? There is decades of data from that and other work.
The strength and toughness of ceramics when hot is truly impressive.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed

gruntguru · May 16, 2022

Then we are arguing about two different things.

je suis charlie

TugboatEng · May 16, 2022

There aren't gremlins with tiny hammers in most engines. Assuming the piston survives transportation and install, the conditions within the cylinder are quite protected.

gruntguru · May 16, 2022

My point really, is that the skirt design looks a long way from one that was FEA optimised.

je suis charlie

romychaj · Jul 18, 2022

type of ceramic: probably some doped zirconium oxide. flexural 950 mpa, tensile 250 mpa, k1c 10 mpa(..), modulus like steel.

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