V8 engines
May I say all V8s are 90° ? really, I haven’t heard any non-90° V8. 90° is the only configuration achieving good balance for V8. However, there are two types of crankshaft arrangements, which delivers very different characters. They are cross-plane crankshaft and flat-plane crankshaft. Most of the world’s V8s are cross-plane V8, including all American V8s and all sedans’ V8s. However, the most exotic European sports cars, including Ferrari, Lotus and TVRs, employ flat-plane V8s.
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No matter which kind of V8s, they have 5 main bearings. A cylinder in bank A shares the same crank pin with the corresponding cylinder in bank B, therefore the crankshaft of V8 is actually simpler than V6. Both V8s generate no vibration in vertical, transverse directions or between bank and bank.
Cross-plane V8
However,
for cross-plane V8s, there is vibration from end to end of the engine,
this is because the first piston of bank A is not in the same position
as the last piston of bank A (the same goes for bank B), unlike an inline-4
engine. No problem, the 90° V8 solves this problem by introducing an
extra-heavy counter weight to every cylinder. The counter weight is heavy
enough to balance the weight of crank throw, con-rod and piston of that
cylinder, thus resulting in lack of vibration.
Now you must be wondering
why such counter weight is not used in other kinds of engines. It is because
this counter weight must be used in 90° V-type engines which have shared
crank pins. It our previous study, you’ll find only V8 fulfills all these
requirements. Why are there such requirements? good question. As you know,
all engines have counter weights just enough to balance the weight of crank
throws and part of the connecting rods, leaving the remaining weight of
connecting rods and the whole, all-important pistons unbalanced. This is
because the rotating counter weight can only balance rotating mass. Unluckily,
the whole piston moves vertically rather than rotates about the crankshaft,
while the CG of con-rod is somewhat rotating but also somewhat going up
and down. If we insist to use heavy counter weight, it will cause side
shake.
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Considering the illustration.
Assume the counter weight in vertical position is heavy enough to balance
the crank throw, con-rod and pistons. When the crankshaft rotate 90°,
the counter weight is repositioned to the right, but the piston doesn’t
go to the left, and the con-rod just partially moves to the left. Only
the crank pin moves completely to the left. Now you can see the system
is not balanced. The counter weight will generate a net force towards the
right.
However, for 90° V8, when such a heavy counter weight moves to the right, the piston from another bank will cancel it completely, because their movement are exactly opposed at this moment. (see illustrations below) The same result can be found for the counter weight moving to the left. Therefore 90° cross-plane V8 employs full-weight counter weights can achieve near perfect smoothness. |
Flat-plane
V8 is named according to the shape of the crankshaft, which is in a flat
plane. It is very much like two inline-4 engines mated together. In particular,
it achieves end-to-end balance because the first piston and last piston
of a bank is exactly in the same position, so are the center two pistons.
This is just the same as straight-four engines, therefore the sound of
flat-plane V8 is usually somewhat like a pair of four-pot engines screaming
simultaneously, unlike the rumble-bumble of cross-plane V8s.
