peejay's comment:- If you want to fight stretch, find a way to make the piston lighter, since that is what is stretching the rod.peejay wrote: ↑Mon Jul 01, 2019 8:39 pm If you want to fight stretch, find a way to make the piston lighter, since that is what is stretching the rod.
Some days I wonder what it would be like to have a carbon composite piston that had a stainless steel deck on top, maybe Inconel, to take combustion heat. Should be pretty cheap right? It would have to be a metal that had a very high heat resistance since it wouldn't transfer very much through the skirt.
When teaching an engine building class I always stress the need for a piston/pin combo that is not needlessly heavy. If you look through enough piston catalogs you can often find pistons as much as 25 grams below the norm. Add to the fact that stock pins are almost alway excessively overweight and you can see a direct route to a weight saving of 50 to 75 grams.
At present I am in the midst of testing my own composite pin. For a penalty of about 5 grams extra weight they are the same strength as a Ti pin but I can make a set of 8 for slightly less than the price of two TI pins.
Peejay - you brought up the subject of composite pistons. Back at the turn of the century I was heavily involved in the research and production of Carbon/carbon pistons ultimately destined for Cosworth.
There were a great many issues that had to be addressed. For instance the Carbon/carbon composite had a very low thermal expansion rate - far lower in fact than the block. This meant that when cold the pistons were an interference fit in the bores. Before starting the engine it had to be warmed to about 200 F before it would crank sufficiently easily.
Because the piston to cylinder wall clearance was so small dimensional stability at temperature and rpm was critical. Although having no rings and virtually zero clearance worked it was still subject to blowby. The goal here was zero blowby. I solved this problem with a ringless piston design that acted as if it had rings.
As a matter of interest the thermal characteristics were such that no metal heat shield on the crown was necessary . The material could withstand something like 3500 F.
The weight of a finished F1 piston was half of that of a conventional one. The cost was equally impressive if you like big numbers. The production of the first piston was well over $150,000. That price would have dropped significantly if the pistons had been ordered in quantity. I felt that meeting Cosworths demand we could produce these pistons for between $15,000 - $20,000 apiece. Just as an aside here we also made some valves.
I had to build a super accurate dyno to test these in a single cylinder test motor that emulated the valve train of the then current Cosworth F1 engines. I also made a cup car intake valve and spun it. What this 29 gram valve could do was amazing - but maybe that is a subject for my column that Mike will be introducing soon.
Another problem that plagued the initial designs was delamination. Fortunately part of the research team was a guy by the name of Burt Northam.
At the time Burt was a just retired NASA rocket motor scientist. When you watch a space launch using a NASA sourced/designed launch vehicle it is about 80% likely that it is one of Burt's designs getting the job done. Among many other things he pioneered super sonic combustion in rocket motors and that increased their capability by a very useful margin. So Burt applied his 180 plus IQ to the delamination problem and solved it. Now we were getting somewhere. Then the whole project came to a blindingly fast stop. The reason - and this is how I heard it - the cost was so high and the FIA got wind of what we were doing and banned it. How close that is/was to reality I cannot say for sure. I was being paid good money for my work on this so a cancelation of the project was not to my benefit here. But to be honest it was not just about the money - this was exciting stuff with an F1 type budget attached!
DV
