citação:Originalmente colocada por J.R.
Mas actualmente são todas.... quase de certeza.
Dá-se o nome de "pent-roof" porque não são completamente hemisféricas (redondinhas
).
Chamber Design
One of the characteristic chambers that people are familiar with is the Chrysler Hemi.
The engine had a chamber that was like a half of a baseball. Hemispherical in nature and in nomenclature, too. The two valves were on either side of the chamber with the spark plug at the very top. The charge burned downward across the chamber. That approach worked fairly well in passenger car engines but racing versions of the Hemi had problems.
Because the chamber was so big and the bores were so large, the chamber volume also was large;
it was difficult to get the compression ratio high. Racers put a dome on the piston to increase the compression ratio. If you were to take that solution to the extreme and had a 13:1 or 14:1 compression ratio in the engine pistons had a very tall dome. The piston dome almost mimicked the shape of the head's combustion chamber with the piston at top dead center.
One could call the remaining volume "the skin of the orange." When ignited the charge burned very slowly, like the ripples in a pond,, covering the distance to the block cylinder wall.
Thus, those engines, as a result of the chamber design, required a tremendous amount of spark advance, about 40-45 degrees. With that much spark advance detonation was a serious possibility if not fed high octane fuel.
Hemis tended to be very sensitive to tuning. As often happened,
one would keep advancing the spark, get more power and all of a sudden the engine would detonate. Because they were high output engines, turning at high RPM, things would happen suddenly.
Hemi racing engines would typically knock the ring land off, get blow by, torch the piston and fall apart. No one then understood why. We now know that the Hemi design is at the worst end of the spectrum for a combustion chamber.
A nice compact chamber is best; that's why the four valve pent roof style chambers are so popular. The flatter the chamber, the smaller the closed volume of the chamber, the less dome you need in the piston.
We can get inherently high compression ratios with a flat top piston with a very nice bum pattern right in the combustion chamber,
with very short distances, with very good mixture motion - a very efficient chamber.
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