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Link to original content: https://web.archive.org/web/20100126054913/http://www.autozine.org/technical_school/engine/tech_engine_packaging.htm
Undoubtedly, engines for small cars have to be mounted transversely, unless it is BMW 3-series Compact which has a long long bonnet (hence poor poor space efficiency). But even mounting transversely can’t guarantee the installation of a V6. The width of V6 (excluding accessory) is at least doubled from inline-six, depends on the incline angle (usually 60° or 90°), so it engages a lot of length of the engine compartment. Moreover, the hot exhaust pipes in either side of the Vee also prevent any other components from placing too near, thus need more clearance. Therefore most small cars cannot accommodate V6.
In
1991, a breakthrough was achieved by Volkswagen. It developed a
narrow-angle
(15°) V6 displacing as much as 2.8 litres and installed it to the
generation
3 Golf. As everybody knows, this is the so-called "VR6". As seen in the
picture, the VR6 is really very compact, nearly as narrow as any inline
engine and not much longer than a straight-4. It could be fitted to
many
small cars, including Polo (which didn’t because of price reason). It
is
also supplied to Mercedes-Benz V-class, whose short front end cannot
fit
Mercedes’ own V6.
VR6 could be as
narrow as
15° without cylinders overlapping is because adjacent cylinders are
widely spaced from each other, as seen in the following pictures. This
inevitably increase its length but the result is still just equals to 4
and a half cylinder, versus a V6’s 3 and a half. For most small cars
this
is short enough.
 |
Comparision between a straight-4, V6 and a VR6's cylinder block, viewing from above. The V6 has the length equals to 3 and a half cylinder in-line. The VR6 approaches 4 and a half, however, it is a lot narrower. |
Another feature of VR6 is very important for our further study of 24-valve VR6 and W-engines. It is: the VR6 is asymmetric. For conventional V6, one bank of cylinders mirrors another bank, that means, air intake from the center of the V and exhaust pumps out from outside of the V. (Not vice versa, because the inside of V cannot accommodate the very hot exhaust pipes.) Now please see the illustration in below ....
Left : Symmetric design of V6
Right : Asymmetric design of VR6
... the VR6 has the air
intake from one side and exhaust from another side for ALL cylinders,
no
matter in which bank, so it is not a symmetric design. Normally,
induction
manifolds take place at the top of the engine thus waste no space, it
is
the hot exhaust pipes that engage a lot of space (or length) of the
car,
especially is a certain clearance should be provided to avoid
overheating
to surrounding components. Now VR6 concentrate all the exhaust pipes to
one side of the engine, thus save space.
The same cannot be implemented to conventional V6s because their adjacent cylinders are packed so close to each other thus provide no space for induction / exhaust pipes running to the same side.
Valve Gear
The first generation VR6 has 2 valves per cylinder, single overhead camshaft (sohc) serving each bank just like any conventional 2-valve V6s, although the 2 camshafts are so close that they look as if a twin-cam design.
Cylinder Head / Block
However, in many ways the VR6 is constructed like an in-line engine. Thanks to the narrow angle, the two banks are merged into a single cylinder block. Also, a single cylinder head houses the valve gears for all 6 cylinders. In contrast, a conventional V6 consists of 2 blocks and 2 heads. As a result, VR6 is not only smaller, but also lighter. It would have been cheaper as well if not employ 7-bearing crankshaft.
Technical Difficulties
When I heard the
rumour about
the 24v VR6 about 2 years ago, the first question arouse in my mind
was:
how to fit 4 camshafts into the small piece of cylinder head ? It is
virtually
impossible, especially is that some space has to be preserved for
replacing
spark plugs. If not having 4 camshafts, then it must be an sohc design
serving 4 valves per cylinder, just like many Japanese cars, say, Honda
and Mitsubishi.
 |
Honda’s SOHC 4-valve engine.
Lelf : Each camshaft has 4 closely packed cams for each cylinder. The cams activate valves via rocker arms. Right: the complex rocker arms. |
 |
But the most important reason that the sohc 4-valve not desirable is that it doesn’t allow the adoption of cam-phasing variable valve timing. Shift the camshaft 20° in advance leads to the intake valves open and close earlier, but so do the exhaust valves. Therefore there is no gain in performance.
Using cam-changing VVT like VTEC or MIVEC may introduce real performance gain, but as already discussed in the Variable Valve Timing section, it doesn’t improve drivability at low speed thus European car makers are not very interested in.
How did Volkswagen overcome these difficulties ?
Volkswagen's Solution
 |
Piech’s
ingenious engineers
solved the problems by introducing a revolutionary concept:
Twin-camshaft
per bank, one for intake, one for exhaust, but totalled also 2
camshafts.
Yes, believe your eyes. Sometimes 2 x 2 = 2.
Don't believe ? look at the photo beside. Use a single naked eye to look at the farther camshaft. You'll see the rocker arms pressing valve springs, the direction of springs project to the valves of a cylinder belonging to another bank. If you are not sure, see my illustration in below. |
Now it is clear. Camshaft A controls the intake valves of bank A as well as bank B. Similarly, camshaft B controls the exhaust valves in bank B and bank A. In other words, every cylinder is served by both camshafts, hence a twin-cam engine.
If you still
remember, a
feature of VR6 is that it is asymmetric,
this
enable the exhaust valves in both bank remains in a distance accessible
by a common camshaft. In fact, the distance is the same as in intake
valves
/ cam set. This ensure equal efficiency of intake and exhaust. Without
the narrow angle and the asymmetric configuration, the share of
camshaft
would have been impossible.
Such design allows cam-phasing variable valve timing to be installed. In the 24-valves VR6, the intake camshaft has VVT. In the future, the exhaust camshaft may also introduce VVT, just like BMW’s Double Vanos.
If it were a conventional V6, it would have needed 4 camshafts, 4 cam-phasing mechanism to implement this. Also required is 2 cylinder blocks and 2 cylinder heads. VR6 needs just half of them.
It is also interesting to see the new VR6 has the same no. of camshaft as its 2-valve predecessor. It is one of the most remarkable invention.
Having
learned the VR6, it is not difficult to understand the W12. As VW said,
the W12 engine shown in the mid-engined W12 supercar is virtually a
combination
of two VR6s. This is confirmed by its 5.6-litre displacement. It is
constructed
by mating two 15° VR6 in an inclined angle of 72°. In fact it
is
the earliest VR engine having 4-valves head, although this car was
never
put into production.
The
W configuration would have been never realised if not the invention of
VR6. Audi had been researching its own W-engines for years (even showed
in the Avus concept car, but the engine was fake) but eventually pulled
out the plug. I remember sources said it failed to solve the exhaust /
ventilation problems. It was basically formed by 3 banks of 4-cylinder
in-line. The problem was how to run the exhaust pipe for the center
bank
without overheating the surrounding and without wasting too much space.
It
seems that Volkswagen’s approach is not benefited by Audi’s experience,
because the Volkswagen unit is based on the VR6 which was under
development
well in the 80’s. Benefited by VR6’s asymmetric
design, exhaust of the left VR6 runs out from the left side, while
exhaust
of the right VR6 runs out from the right side. Therefore the exhaust
system
is just the same as any Vee engine.
The only short-coming of W-engines is that they require very thin connecting rods, as the crankshaft is much shorter than V-engines. While VR6 uses con-rods with 20mm thickness, the W-engines run with 13mm ones. This prevent it from becoming racing engines. Tight cylinder heads may also limit its breathing and ventilation.
As
18 is not dividable by 4, you know the W-18 is not derived from VR
engines.
In fact, it follows the old Audi philosophy of mating 3 banks of
6-cylinder,
running the common crankshaft. The drawback is: among the 3 banks there
are 2 large Vee angles. My estimation is 60° each, hence a total of
120°. For comparison, the W16 is just 15/2 + 72 + 15/2 = 87°,
therefore
the W-18 is a lot wider. In terms of length, the W16 has the same
length
as a VR8, that is, about the length of 5 cylinder. The W-18 is as long
as an inline-6.
As seen in the photo, the W-18 used by Bugatti EB-218 concept car is very big and complex. Two of the banks mate like a conventional 60° V6 while the remaining bank lays down to horizontal level. Complex induction manifolds and exhaust pipes run between the banks. (note that the exhaust pipes were not fitted to this prototype, otherwise it would have looked even more complex.)
Obviously, W-18 is not as clever as W-16. Although there is no problem of fitting in the jumbo Bugatti saloon, I must question its purpose. Is it more powerful than a V12 can achieve ? No. Is it smoother than the theoretically ideal V12 ? No. Is it shorter than a V12 ? No. Is it narrower than a V12 ? On the contrary. Is it cheaper to be built ? Never.
No wonder Volkswagen eventually decided to terminate the W18 project.
