Polysphere

Polysphere (pronounced pol-ee-sfeer)

(1) In mathematics, a product of spheres.

(2) In mechanical engineering, a design of combustion chamber formed by the two shallow concave domes under the intake and exhaust valve seats.

1955: A compound word, the construct being poly + sphere.  Poly is from the Ancient Greek πολύς (polús or polys) (many, much), from the primitive Indo-European polhiús (much, many) from the root pele (to fill), akin to the Old English fela (many).  Sphere is from the Middle English spere, from the Old French spere, from the Late Latin sphēra, from the Classical Latin sphaera (ball, globe, celestial sphere), from the Ancient Greek σφαῖρα (sphaîra) (ball, globe), of unknown origin.  Despite spread of the myth by some medieval writes, sphere is not related to superficially similar Persian سپهر‎ (sepehr) (sky).  Poly, in modern English (especially in industrial and scientific application) use became a word-forming element meaning "many, much, multi-, one or more" with derivatives referring to multitudinousness or abundance.  It was equivalent to the Latin multi- and should properly be used in compounds only with words of Greek origin but this, etymologically slutty English ignores.  Polysphere is a noun and polyspheric is an adjective; the noun plural is polyspheres.

Chrysler, the poly, the hemi and the hemi which is really a poly

Chrysler didn’t invent hemispherical combustion chambers but they certainly made a cult of them.  In internal combustion engines of the mid-late twentieth century, the hemispherical combustion chamber was one of the best designs with with to provide an efficient burn-space while minimizing thermal loss and permitting the use of large diameter canted-valves to optimize intake and exhaust flow.  The early Chrysler Hemi V8s (1951-1958) were the most powerful of their generation but there were drawbacks.  To take advantage of the large valves at diverging angles, the valve train assembly was both bulky and heavy, needing two rocker shafts rather than the single units used with in-line arrangements.  Adding to the cost and complication were the inherently more expensive casting and machining processes required to produce the hemispherical shape of the combustion chambers in the cylinder heads.  To enable the mass-production of a less expensive V8 to use in their lower-priced lines, Chrysler created new cylinder heads with polyspheric (two shallow concave domes under the valves and named the “Poly”) combustion chambers and a less elaborate system of valve activation which needed only a single rocker shaft.  Although less powerful than the Hemis, the Polys were cheaper and lighter although it wouldn’t be until the 1960s that Chrysler standardized engines across their divisions; an early adoption of such economies of scale might have saved the corporation more money than retaining an exclusively Hemi-headed line would have cost.

The Hemi, 1951-1958 & 1964-1971 (left), the polyspheric, 1955-1967 (centre) and the new "Hemi" which is really a swirl Chamber, 2003- (right).

However, the Poly proved a cul-de-sac.  In an era of cheap petrol, larger capacity engines proved a more attractive route to horsepower than sophisticated combustion chamber design and the Hemis were retired in 1958, replaced by larger engines with wedge-shaped chambers, used by other manufacturers and much more suited to mass-production.  Consigned to the grave with the Hemis were almost all the Polys, only the 318 V8 (5.2 litre) retained as a rare oddity until 1967.  The Hemi would return, available between 1964-1971 as a 426 cubic inch (7.0 litre) race engine (there were also some reduced displacement versions to satisfy local rules) which, for homologation purposes would in 1966 be released in slightly detuned form detuned for street use.  The name however held such an allure that it was revived in 2003 for Chrysler's new (and perhaps final) generation of V8s although in the narrow technical sense, Hemi is now more a marketing than an engineering term because the twenty-first century combustion chambers are something of a hybrid of hemispheric and polyspheric, the general term describing them for the last fifty-odd years being swirl chambers, a design which makes possible a high out-output of power, low emissions and an economy in operation which would have been thought impossible to achieve as recently as the 1980s.

Lindsay Lohan with polyspheric hair.  Polyspheric hair styles are possible, the classic example of which is the symmetrical “twin dome” look which is difficult exactly to achieve and harder still to maintain for more than a brief time.  They’re thus seen usually only at photo-shoots or for one-off events but the design element is popular with asymmetric styles.

When Chrysler in 1964 introduced the 273 cubic inch (4.5 litre) V8 as the first of its LA-Series (that would begat the later 318, 340 & 360 (the V10 Magnum used in the Dodge Viper is also as descendent)), the most obvious visual difference from the earlier A-Series V8s was the noticeably smaller cylinder heads.  The A engines used as skew-type valve arrangement in which the exhaust valve was parallel to the bore with the intake valve tipped toward the intake manifold (the classic polyspherical chamber).  For the LA, Chrysler rendered all the valves tipped to the intake manifold and in-line (as viewed from the front), the industry’s standard approach to a wedge combustion chamber.  The reason for the change was that the decision had been taken to offer the compact Valiant with a V8 but it was a car which had been designed to accommodate only a straight-six and the wide-shouldered polyspheric head A-Series V8s simply wouldn’t fit.  So, essentially, wedge-heads were bolted atop the old A-Series block but the “L” in LA stood for light and the engineers wanted something genuinely lighter for the compact (in contemporary US terms) Valiant.  Accordingly, in addition to the reduced size of the heads and intake manifold, a new casting process was developed for the block (the biggest, heaviest part of an engine) which made possible thinner walls.  With the exception of the Hemis, the new big-block engines used wedge-heads and the small block polyspheres (the A-Series) were replaced by the LA except for an export version of the 313 (5.1 litre) which in small numbers was manufactured until 1965 and the 318, the last of which was fitted in 1967.  Confusingly, the replacement LA engine was also a 318, a product of carrying over certain components, both the 318-A & 318-LA sharing the same bore & stroke.  In an example of production-line rationalization, when Chrysler Australia bored out their 245 cubic inch (4.0 litre) Hemi-6 to create the 265 (4.3), the bore chosen was the same as the 318s so pistons could have been shared with the V8 although for technical reasons this wasn't actually done.  The Australian "Hemi" straight sixes used another variation of the combustion chamber in that chambers sat in upper third of the globe, hence the "low hemispherical" slang which wasn't wholly accurate but Ford's Boss 429 V8 had already been dubbed the "semi-hemi" and linguistic novelty was becoming hard to find.