Short Piston Skirt Design

[strokersix]

Over the last several decades piston design has evolved to shorter skirts. I would like to know the limit on how short piston skirts can be and what trade-offs are involved. Tribology, stability, piston material strength requirement, cylinder wall loading, other?

Thank you.

 

[Hasbro]

Years ago I remember while researching this question, I ran into information from WW2 aircraft issues. During the time of piston engines and dog fights, I understand the allied forces lost an enormous amount of our fighter pilots from pison sezure in the act of w/o throttle dives. High altitude to low altitude temperature and oxygen quality/quantity, nitous and water injection. The metalurgests went to work and had to find a metal that would not change dimensions enough to thereby cause engine failure. They succeded with what I believe is MS75 & RR58 used by TRW and Cosworth even today. (Protected, by the way). It was really an indepth, quickly accomplished study for the war. Additionaly, It was determined that there is absolutly a ratio of piston length to pin placement. These boy's went through many skirt profiles and cams 24 hrs. a day to come up with reliable engines. Oils played a part but it was the piston that turned around the war.. We were able to build more power.
The piston skirt is a spring and it has to be designed as such. I have asked about this formula when talking tech at the vendors, but they don't convince me it's a moot point. Someday I will run into this work when I do my reviews in my library. But, these guy's are suppose to know their work more than we.

 

[strokersix]

Hasbro,

Will you please elaborate on "The piston skirt is a spring and it has to be designed as such."

Thanks.

 

[evelrod]

The consensus long years agos was the skirt stabalized the piston in the manner described, spring effect, even to the point of adding an additional ring to the bottom of the skirt. It would appear by looking at the set of JE pistons I installed a couple weeks ago that that theory is of no, or at least limited value in todays pistons. They had almost no skirt below the pin boss. In test run ups of the engine (dyno is not scheduled yet) I have significantly less blowby and leak down than with the previous Hepolite/Mahle design long skirt pistons. JE uses a 'hypereutectic' (sp) alloy that is installed with 0.0015" clearance at the skirt. Kinda tight for me but, it's working. I personally have seen a rather radical change in the pistons I have used in the last 20 years (Venolia, JE, AE, Omega, Hepolite, Mahle, TRW, Cosworth$, Jahns, etc., both forged and various cast methods) in high performance engines. I like what is happening as they seem to be a much better product as far as I am concerned.

I had thought that the original Packard study of piston problems were brought on by the use of the "war power" detent on the throttle quadrant. When used from high altitude to low and from lower boost to higher boost levels caused significant localized overheat and thus engine failure. I may not be remembering this article correctly as I believe I read it back in 1955 or there-a-bout.

It may be that Cosworth uses some sort of exotic alloy, they sure COST a lot more than any other!!! LOL

Just some more rambling thoughts this morning, sorry.

Rod

 

[Hasbro]

Stroker, One thing about my own thinking. I can know a lot about one thing, but not a lot about everything. That gets us in a lot of trouble in our evaluating, etc.
What I have noted in limited involvement concerning the piston acting as a spring, was notable in thin wall engines. In that case, the skirt tails had again, a determined point of legnth for especially cold start. OEM style. This goes away back. As a piston transfers from cold to operating values, a piston transfers heat into mass, then out to min. mass, dissapating. The metal bridge limits that growth, the slots halt the expansion,around the pin bore on into the band. If it failed to accomplish this, sezure would occurr. @, a spring in tension. ---- On into the racing venue, early attempts in providing flexable forgings to satisfy those requirments were failiars. The cloverleaf did not fill the need, as it dissapated more heat into the band delivering diameter in the process. Relieving those high mass corners created additional issues in the piston, being forced into 'beyond tensile values' and they fractured. @ spring. It's an evolving process. In this day with thick wall blocks, artifically or meatalurgically, it has allowed the piston to evolve as we see it. Metals are probbably where they are because of the cold war being over. Yet, I still see with the pistons getting increasingly ridged, that spring effect dissappearing, all caused by metalurgy. Stregnth vs. stregnth. It must fit the application. Thin,thick,flexable,ridged. My oppinion. John Haskell
Aire Research Engr.