Now that Dodge has started putting Hemi
engines back in cars a lot of people are raving about them. However, a lot of
people have no clue about them. So here is an explanation on how a Hemi engine
operates.
The word Hemi is short for Hemispherical,
or round. The thing that is hemispherical or round is the top of the combustion
chamber. The combustion chamber is where the fuel enters from one valve, is
used, and leaves through another valve as exhaust. In the combustion chamber
is the piston. Most engines today use a flat combustion chamber area. So now
you know the difference and want to know why it is so great and all that. Well,
surface area has a lot to do with it. From chemistry we all know that the larger
the surface area the less energy is present. Well in Hemi engines, the amount
of surface is a lot less than in regular engines. With this less surface area,
fuel has less places to hide and can be burned more effeciently and less heat
from that fuel being burned can escape. And it is the heat from the fuel burning
that normally escapes. Not so much in a Hemi engine, that heat stays around
longer, letting the Hemi reach peak pressure much more effeciently than a typical
engine design. Another factor in the Hemi is the size of the valves. I had mentioned
earlier that it had two valves, one for fuel to enter and one for exhaust to
leave. Since the top of the combustion chamber is round, you have much more
room for valves, so the valves can be bigger. The bigger the valves, the better
the air flow, the better the air flow, the more power etc... Okay, now here
are the bad things with the Hemi, you know they have two valves per cylinder
now. The thing is, they will never be able to have four valves per cylinder
like the Cobra engines and many other high-po engines. True Hemi engines will
never have four valves per cylinder because the angles to get them to fit on
something round would be crazy, and a bitch to design and make work. So here
they come with the pent-roof "Hemi" design. It is angled with straight
angles so that four valves can fit on the combustion chamber. It is not a true
Hemi engine because it is no longer hemispherical, but rather it is angled.
Another drawback to the Hemi engine is many engineers would rather have a smaller
combustion chamber, because less heat is lost over all and the pent-roof design
is more compact so that helps out.
Hope this can explain it well. I had
to dig some stuff up on it to fill in what I did not know. So don't think ALL
of this is coming from my fingers.
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