Volkswagen's VR6 and W-engines



    24-valve VR6

    When the whole world is fascinating with 4-valve engines, Volkswagen’s VR engine (both VR6 and V5) still relies on sohc 2-valve head until the arrival of the second generation VR6 in July 1999. You may wonder why it takes 8 years to bring the VR6 a 4-valve head. In fact, there was a very big technical difficulty behind the development.

    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. 
     

     
    However, sohc 4-valve is not a perfect design. Firstly, it concentrates 3 or 4 elegant, narrow cams to every cylinder, thus relatively complex. Secondly, the most ideal position of a rocker arm / cam set is exactly vertical above the valve it controls. Otherwise the movement may generate a lateral moment which waste power, introduce friction and eventually brings down the rev. For sohc 4-valve, because the ideal position of the rocker arms for intake and exhaust are exactly the same, a small distance shift is introduced to one of them or both of them, thus result in the aforementioned drawback. In fact, all the high performance Honda (from Civic SiR to Type R) employed dohc instead of the sohc of the standard car.

    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.


    W12 engine

    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.


    W16 engine

    Similar to W12, W16 is made by mating two VR8s together, although at the moment Volkswagen group has not shown any VR8. The VR8 consists of 2 banks of 4-cylinder, mated at 15° just like VR6. The two VR8s then join together at 72°. In other words, W16 is just a W12 with one more cylinder added to each bank.

    W8 engine

    The W8 engine was first introduced in Volkswagen Passat W8. As it is produced in the same production line of other modular family members, the basic architecture is the same as W12 and W16. In other words, it is a W12 with 1 cylinder deleted from each bank, or simply half of a W16. W8 consists of a pair of 15° VR4 engines joint to a common crankshaft at 72°.


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