A V configuration is probably the most compact, for one. I think one
important reason the V type is most common is because it allows for a good
intake manifold design. With an inline four cylinder, if you've only got one
air intake/carburetor, it's going to be closer to the middle two cylinders than
the outside two. Therefore, the intake runners will be longer for two of the
cylinders than the other two. For optimum cylinder charging however, you want
to tune the intake runner lengths (the pipes that go from the carburetor to the
cylinder head intake port) to a specific length for maximum cylinder charging
at a certain rpm, and you want them to all be as close to the same length as
possible. If the lengths are different, you end up with two pairs of cylinders
that make peak torque at two different speeds. That might make a flatter
torque curve, but in high rpm racing it's generally more desirable to have one
really high torque peak, as long as the rules allow enough transmission gears.
(Some engines offset this somewhat by using wierd camshafts that have different
valve timing for different cylinders.)
Of course, a single carburetor V-8 is little more than two inline four
cylinder engines joined together, so I don't know exactly what point I was just
trying to make there :-) Anyway, horizontally opposed engines require more
than one carburetor/intake unless it doesn't need to spin very fast.
From a book I'm looking at now:
"In multi-cylinder engines the cylinders and their disposition are arranged to
eliminate as many of the primary and secondary forces and moments as possible.
Complete elimination is possible for: in-line 6 cylinder or 8 cylinder engines,
horizontally opposed 8-cylinder or 12-cylinder engines, and 12-cylinder or
16-cylinder V engines."
Those engines will be the smoothest and can probably be run to the highest
rpm before something breaks. Of course, it doesn't say that a V-8 is one of
the good ones, so..... ?
A couple years back I wrote some engine simulation stuff, but didn't get into
multi-cylinder models for a bunch of reasons I won't bore you with :-) I do
recall realizing that the cylinder bank angle would have an impact on a couple
of things. The cylinder firing angles was one. Since you'd want cylinders to
fire at the same angle relative to each other, one firing at 0 degrees, the
next firing at 90 degrees (compared to the first one), the third at 180, etc.,
all the way around, you probably will get a crazy wobble with a bank angle of
10 degrees instead of perfectly flat, 60, or 90 degrees. This book talks
briefly about a twin cylinder horizontally opposed engine that fires at 180
degrees, 540, 180, 540, etc., rather than 180,360,180,360, (or is that
180-0-180-0 ?) which would cancel out the forces and run nice and smooth, as a
bad example. It's sort of like having the pedals on a bicycle at 90 degrees to
each other instead of 180, it makes things a bit wobbly and the engine won't
spin very fast without breaking something. However, if it's cheap to build it
this way, it fits where you want it, and it doesn't need to spin at 100,000
rpm, it might be just fine.
One other thing that bank angle and V/inline/etc. arrangment may have an
impact on is the timing of the opening/closing of the valves and the pressure
pulses that move through the intake and exhaust runners. It's desirable to
have them in tune (by selecting appropriate lengths), and I think changing the
bank angle can effect both this and balance at the same time. Anyhow, these
all interelate along with how the throws on the crankshaft itself are arranged.
That doesn't really answer your question, but it's the best I can do right
now!
>What are the advantages with a pushrod? I understand they have a bit of
Pushrod engines are cheaper to build, that's probably the only benefit. The
pushrods are relatively flexible compared to the camshaft or tappet, so the
valve lift doesn't follow the cam profile as well as an overhead system might.
There's also more mass to move around, so you need stiffer springs, which wears
out the camshaft faster and also causes power loss from the added friction.
Perhaps NASCAR and the others might have mandated them to keep costs for the
teams down. Otherwise, they all would spend millions more designing four/five
valve overhead cam systems, only to get restricted somehow to keep the speeds
down. Just my opinion though, I don't really know for sure.
Todd Wasson
---
Performance Simulations
Drag Racing and Top Speed Prediction
Software
http://PerformanceSimulations.Com
