Ceramic coated pistons

[mic*]

Wondering if many have done this and with what results as i have heard it mentioned.

I would not have thought it to be a good idea as it would force more heat to be dissipated through the upper cylinder and the head, resulting in a lower threshold for knock = less timing or more octane?

Comments

[oldcorollas]

depends if you ceramic coat the combustion chambers, valves(edit) and ports too eh?

more heat in combustion chamber = less compression/timing needed, but maybe same power? (since heat in gas = power).. as for octane... that depends on heat generated/present during the compression stroke (unless you have hotspots)
in theory

[myne]

For an engine to be most efficient, the heat would ideally NOT reach the pistons or head.
Ceramic coating insulates the pistons from the combustion temperatures, which keeps them cooler, which in turn keeps the mixture cooler, preventing detonation.

[mic*]

Well if you coated the chambers and ports too, id excpect higher translation of thermal energy into kinetic as well as greater heat dissipation through the exhaust on that stroke and lower cylinder temps on compression. All good things.

If the piston was just coated, the head and upper cylinder will cop a lot more heat providing much more ideal conditions for end-gases to autoignite on compression stroke (knock / detonation / pinging). That is what i would think.

Is it possible to coat the upper cylinder? I have no practical experience with ceramic coating.

[Celica RA45]

i get all my pistons coated but i only get the skirts done to stop wear as most of the skirts is where most of the friction is caused from bottom of oil ring down to bottom of shirt
you could always do the dome ,but most probaly more suited to turbo pistons

[gianttomato]

Glenn, that's molybdenum disulfide coating on the skirts (usually - defers to Stu!).

[mic*]

Again, without the dome done, the normal surface area for heat dissipation is left untouched. And my thoughts i have expressed on why i dont think it would be a good idea - for turbo or not - to coat the piston top.

Has anyone done the whole piston and has can show that they didnt need to pull their timing back?

And stu, (assuming piston only done) causing hotspots by heating the head beyond its normal operating temps is the main reason i suspected coating the piston would cause knock if timing is not retarded.

[Celica RA45]

yeah sorry dave i jumped the gun only saw coatings and missed the CERAMIC BIT

[roadsailing]

think of this way, the piston can only get rid of heat either through the piston ringsor via oil cooling from oil squiters. Much easier to keep the combustion chambers cool.

but ceramic coating the combustion chambers as well would probably be a good idea, as long as your exhaust valves can take it.

[mic*]

Umm not meaning to be blunt but where does the oil squirt the pistons??? And how does the heat get to there in the first place if the pistons are not soaking massive amounts of it out of the combustion chamber?

Stopping this heatsoak through the piston without giving the same treatment to everything else withtin the combustion chamber is gonna equal trouble. Can anyone give me a supported reason why they think or know this is not the case.

[roadsailing]

oil squirters squirt oil onto the undersides of the pistons, you can find them on smallport 4AGEs as well as other types, and lots of other engines.

the heat gets "into" the pistons as they are "sucking up heat" from the combustion process.

should have added, the pistons will also lose heat to the air drawn into the engine.

i already stated my opinion on why it won't be bad, you prove me wrong

4AGZEs have ceramic coated pistons, and AFAIK the combustion chambers are not coated, nor is any other production engine i know of.

[oldcorollas]

well this is always an interesting subject.. i don't have any specific hard evidence that it works or is better, but technically it has merit.. maybe race teams do it, bu they don't always do things for the right reasons (the other guy is doing it, so we better do it too or we'll be behind.. we don't have time to research it )

as SBS said, heat should mainly go thru surface of piston and then thru the rings to the bore. thats not a very large area, and comparing the rate of heat transfer from a large aluminium surface thru a small steel/cast iron surface... poor rings

insulating the top of the piston with a TBC will reduce heat entering the metal, because the ceramic reduces the FLUX of heat thru it to the metal surface. the metal can still conduct heat thru it to the rings.
the surface temperature of the ceramic may be roughly the same since it is in contact with the gases, and heat is still drawn away from it by the metal. ceramic also has a lower heat capacity than the metal, so there is a complex balance of heat flux occurring.
also complicated by the different strokes of the engine.
power stroke hottest
exhaust stroke, a bit less hot,
intake stroke, heat from piston/chamber surface enters incoming mixture, heating mixture and cooling surfaces
compression stroke, gas heats up and metal may heat or cool, depending on the relative temps...

so there is not much room for surface cooling in there. it may be that the heat transfer thru the rings, and also thru the head to coolant, is enough to keeep metal cool enough. this will in turn form a thin "quench"layer of gas which is cooler than the rest (since gas has even slower heat transfer than ceramic.

now, as for the head getting hotter if the piston is coated. this assumes that there is a delicate balance between the piston temp, the gas temp and the head temp. i would suggest that the balance is not so delicate as to tip the head to melting if you don't lose heat to the piston.
remember that ALL the heat absorbed by the piston and head MUST be removed via the radiator/oil cooler.
the gas may be a bit hotter, but i dont think enough to compromise the head. you may need a slightly cooler plug tho.

now if you coat everything (as in the pics below), you will keep more heat in the gas. how much? i don't know, but more importantly, you lose less heat to the cooling systems.

because of the higher gas heat retention, perhaps you may need to lower compression ratio slightly (sicne the dynamic compression will be a little higher with more heat retained), but then again....

if during the intake and compression strokes, the piston surface is hotter than the gas, you lose heat to the gas. HOWEVER, if you have ceramic coated surfaces, with lower heat transfer and lower heat capacity, you could lose LESS heat to the incoming gas, AND still have lower surface temperatures as a result which means you are LESS likely to have pre-ignition...... hmmmmmm

anyway, what we can do is to calculate the temps involved, the heat flux that occurs at given temps etc.. even just basic modelling would be ok, but you would need to know the gas temps at each stage during each stroke for it to be even vaguely accurate.

anyways, i have plans to do the whole works for the next engine.. total cost for everything (including extractors) was around $500 for a 4cyl (iirc)... not too bad

HPC..
http://www.hpcoatings.com.au/pages/piston.htm


as you'll notice, the only UNCOATED part of the piston is around the top rings = area fro heat loss FROM piston to the engines cooling system
edit: and as for piston coating... one main benefit is to reduce the heat that MUST be transferrred thru the rings... i can't think of any part that is in more danger from failing than the rings and ring lands from heat. they are bloody hot already and under high stress, and although combustion gas pressure is what makes them seal, losing tension from the metal material itself can't be a good thing..


edit2: i'm not sure how the ceramic coating would go if you are using an o-ring between gasket and head.. but if o-ring is between block and gasket, maybe it should be ok...

[mic*]

Sorry Shiny but i dont think im on your wavelength. I know that they squirt the bottom, thats my point, the heat is dissipated through them and with a coating i imagine it wouldnt for the most part.

Im not getting why you say it wont increase incipient knock with all other parameters unchanged.

And i guess the point of the thread is that i think there is many potential gains in using these coatings well... not that i have explored yet, but i am not going to do a bottom end re-build only to have to pull it apart again coz i need to retard my timing something shocking to stop the detonation and RSI.

Can the head & cylinders be coated effectively???

[oldcorollas]

Sorry Shiny but i dont think im on your wavelength. I know that they squirt the bottom, thats my point, the heat is dissipated through them and with a coating i imagine it wouldnt for the most part.
Im not getting why you say it wont increase incipient knock with all other parameters unchanged.
And i guess the point of the thread is that i think there is many potential gains in using these coatings well... not that i have explored yet, but i am not going to do a bottom end re-build only to have to pull it apart again coz i need to retard my timing something shocking to stop the detonation and RSI.

Can the head & cylinders be coated effectively???

only the TOP and SIDES of the pistons are coated.. the underside is still bare and would allow heat to be removed effectively still.

it may increase timing based knock, but probably not pre-ignition.. you need to seperate those two..

if the gas is hotter, it burns faster, meaning you need less ignition timing for the same power... which is a good thing!! you only need more timing because the burn rate is slow, and because a certain angle ATDC gives best power when peak pressures arrive at that time.. so the timing is varied to allow peak pressure to occur at the right time...

i would be very happy to find i needed less timing when using coatings means there is a faster burn rate = more energy being kept in the gas
perhaps higher octane fuel is required (or lower compression) due to higher gas temps, but alternatively, you need less compression and timing to get gas to same temp/pressure so you get same power anyways...

edit: yes chambers and valves can be coated. cylinder walls are not coated as they must be bare to seal effectively to the rings....

then again, some alloy block cylinders have SIN? AlN? coatings.. which effectively meanms a ceramic surface, and they still seal... i think it was a porsche engine i'm thinking of...

[mic*]

^^^^^^ Answered by stu. Thanx

Can clearly see the sound logic behind doing what you suggest - coating the works. Still would feel very skeptical bout coating just pistons, but i think you answered the point of contention to some degree in that the cooling system in the head has a fair capacity to absorb the additional heat that the piston would force through it.

Stu, why would you go along the track of lowering the compression in a coated engine rather than retarding the timing back to the new ideal point? I thought compression efficiency does not plateu until about 17:1 ?

Coz of intention to boost?