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Thread: Vanos 101

  1. #1
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    Vanos 101

    The VANOS system used on our S62 motors is really quite remarkable and I thought some of you might enjoy exploring its inner workings.

    VANOS stands for VAriable NOckenwellen Steuerung (variable cam control) and is BMW's nomenclature for a system to adjust valve timing to optimize an engine's performance under varying conditions.

    There is a VANOS system to control intake valve timing and another VANOS system to control exhaust valve timing. When both are used on an engine, the system is called DOUBLE VANOS. The Z8's engine incorporates DOUBLE VANOS.

    The following explanation is over-simplified but hopefully adequate for discussion purposes. The purpose of the intake system on an engine is to combine air with fuel and deliver the mixture to the cylinders where it can be compressed, ignited by a spark to create power, then sent to the exhaust system for elimination. In order for the air/fuel mixture to enter and leave the cylinders, it must pass through openings in the cylinder heads. These openings have valves in them to control whether they are open or closed. Whether the valves are open or closed is determined by the camshafts which have lobes on them to force the valves into the open position, while springs force the valves to close.

    The important thing to understand is that on most engines (without VANOS) the rotation of the camshafts, thus the timing of the openings and closings, is fixed in relationship to the rotation of the crankshaft and therefore the positions of the pistons in the cylinders. The trouble with a fixed relationship is that as the engine runs at different speeds, the optimum timing of the delivery of the air/fuel mixture and its subsequent release into the exhaust system varies. This means that a fixed valve timing system can only deliver optimum conditions for combustion at one specific rpm; every other rpm will be a compromise.

    Enter DOUBLE VANOS! By developing a way to vary the relationship of the camshafts' rotation to the crankshaft's rotation, BMW can optimize the timing of the delivery of the fuel/air mixture and its release into the exhaust system. This is accomplished by a moveable camshaft drive gear with a helical timing gear. As the camshaft drive gear moves in or out, the helical cut of the timing gear forces the camshaft gear to rotate forward or backward slightly. This changes the timing of the camshafts' rotation relative to the fixed rotation of the crankshaft, thus allowing the valve timing to be advanced or retarded. The onboard engine computer is put in charge of deciding what that timing should be to optimise engine performance and it uses hydraulics to move the shaft in or out. The Z8's DOUBLE VANOS system can adjust the intake valve timing by 54 degrees while the exhaust timing can be adjusted by 39 degrees. The adjustments take less than 250 milliseconds.

    The benefits of having DOUBLE VANOS on the Z8 include high torque at low and medium speeds, reduced emissions, better fuel consumption and smoother idling. When combined with the Z8 engine's individually controlled throttle butterflys, engine performance can be controlled to an unprecedented degree, resulting in what many consider to be the finest production engine BMW has ever built.

    Grease Monkey

  2. #2
    Great write-up, thanks! I never knew exactly how the Vanos does what it does, but I've not driven another n/a motor that has such an even spread of torque feeding into power so smoothly and purposefully. The GT3 and the M6 both are much more rev-happy powerband motors, and my SL has enough torque to potter and cruise, but it isn't endowed with the power to make it brisk in a sporting manner.
    Andrew Macpherson

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  3. #3
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    GM, I'm intrigued by the description above. However, in high performance applications (which the Z8 is not), it seems to me that the degree of freedom of the DV is fairly limited, e.g., the valve timing of the 4 left bank (or right) cylinders can only change together. Is this a limitation, in particular, when considering the trend towards individually controlled throttle butterflys? Also, the added weight of the system and reduced engine reliability (or added complexity) is a point against such artifacts. I rather shift more often!
    62050 - Z8

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    Good questions. As far as I know, no one has figured out a way to control the intake and exhaust valve timings on an individual cylinder basis in a V8 engine. You talk about complexity, such a set-up would have to resemble the inner workings of a swiss watch to handle the chores. Since I don't believe the benefits would justify the costs involved, I don't think you will ever see such a development on production engines. The reason is related to those individually controlled throttle butterflys. Since the onboard computer can precisely control the mixture delivered to each cylinder, on an individual basis, there really is no need to vary the valve timing from one cylinder to the next. The optimal valve timing (for a given camshaft profile) is a fixed relationship directly related to engine speed and it is the same for every cylinder. If the air/fuel mixture can be precisely adjusted to provide the most efficient combustion process in each individual cylinder, there would be no benefit to individually variable valve timing. There are other approaches used in modern engines but they are less sophisticated than Double Vanos. For example, in some Honda engines, when the RPMs exceed a predetermined level, control of valve timing is transferred to a second camshaft with a higher performance profile. This results in more efficient breathing at higher RPMs but the transition is quite abrupt and no where near as refined (nor as effective) as BMW's approach. On the other hand, it is certainly less complicated and perhaps more durable. Interestingly enough, many race engines (including Dinan's S62) eschew variable valve timing for the very reasons you cited, i.e., added weight, parasitic power loss, and concerns about reliability under race conditions. However, it must be pointed out that a race engine runs at or near maximum RPM most of the time so the major benefit of variable valve timing, i.e., better low RPM torque in a high revving engine, is of little to no value. For street purposes, variable valve timing offers real benefits, and high performance auto manufacturers like Porsche, Ferrari, and Lamborghini all incorporate it in their engines. While the Double Vanos system can be removed from your Z8's engine, I think you would miss it, no matter how often you are willing to shift. Consider the consequences: if you optimize the fixed camshaft profile (timing, lift, duration, and overlap) to provide higher performance at high RPMs, your engine's combustion process will only be optimal at a single RPM level, it will make considerably less power in the range where most engines run on the street, acceleration will be uneven (sometimes referred to as cammy), idle will be rough, and emissions will be measurably higher. Plus, keeping your Z8's engine at "full boil", i.e., up on the cam most of the time, might get a little old. When I was growing up, I used to fantasize about inventing a "rubber camshaft" that would change its shape in response to RPMs, so I think DOUBLE VANOS is a dream come true!

  5. #5
    Z8 Madness
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    Very interesting. Thanks for the reply.

    I recall my S2000 having the abrupt shift of power at around 6000 RPM. To me, it felt kind of good -- similar to squeezing the N2O button. In regard to you rubber camshaft, I still fantasize about electro-magnetic valve activation (similar to an irrigation sprinkler solenoid).
    62050 - Z8

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    The above discussion has been limited to adjustable valve "timing" because of the fixed nature of the camshaft profiles involved. If we eliminate the use of camshafts altogether and pursue another way to control valve actuation and timing, one that will allow all of the variables (timing, overlap, lift, duration) to be individually controlled, things get really interesting. Rather than electro-magnetic valve activation, what you want to look at is hydraulic valve actuation. With this system, each valve is opened and closed by its own dedicated hydraulic actuator which is controlled by the engine's ECU. This means timing, overlap, lift and duration of each valve can be individually controlled and in concert with individual cylinder air/fuel mixture (butterflys or direct injection) and spark timing control, the combustion process can be fully optimized for any given RPM and/or environmental condition (cylinder temp, octane rating, air density, load, etc.). Sound too good to be true? Actually, such systems already exist, albeit in experimental form, however there are numerous obstacles to be overcome before such systems can be employed in production engines. Cost is currently the biggest problem but theoretically, such a system, if mass produced, could be made affordable. Other problems include the extraordinary amount of R&D time auto companies would have to spend trying to optimize all of the controllable variables; there would literally be millions of cam profiles to test under a multitude of varying conditions, for each and every engine/car combination. Next would come the ECU requirements demanded by such a system which would far exceed anything found in autos today. Plus, a fail safe mechanism would have to incorporated since an electrical failure or computer glitch could have potentially devastating affects on the relationship of the valves to the pistons. Then we have to consider the source of power for such a system which in this case would be a hydraulic pump driven by the crankshaft. This would rob power from the engine and if it ever failed, the consequences could again be disastrous. While an intriguing idea, given the trend towards alternative motive power, I'm not sure we will ever see hydraulic valve actuation in our cars. One other aspect of valve actuation worth mentioning is the system used in F1 engines. Often referred to as "pneumatic valve actuation", this system still uses traditional camshafts to open the valves but relies on pneumatic pressure to close them. Because of the extremely high RPMs F1 engines can turn (currently restricted to 19,000 but they are capable of even higher levels), the use of springs to close the valves has proven inadequate. Instead, small cylinders containing compressed gas are used to close the valves. Unfortunately, these valves tend to leak which requires F1 cars to carry a reservoir of compressed gas to constantly refill the cylinders and if they ever run low, the valves will hit the tops of the pistons and you get to see one of those awesome clouds of white smoke appear from the end of the car as the engine disintegrates. Necessary for F1, not very practical for the street. All things considered, I think DOUBLE VANOS is the best option available at this time.

  7. #7
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    Hydraulic valve actuation was a feature on the engine proposed by Preston Tucker in the late 1940's for use in the Tucker Torpedo - it never made it into production - one of its limitations was the speed of actuation limited the engie to very low RPM. Daimler Benz used desmodromic valave acuation in the 1955 300 SLR and W196 F1 car - this is a method of using a secondary cam follower to positively close the valves.

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    Once again, Tucker was simply ahead of his time. The current prototype hydraulic valve actuation systems are working at speeds up to 15,000 RPMs with expectations of reaching 20,000 RPMs! Desmodromic valve actuation is alive and well in the motorcycle world.

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    There seems to be another alternative: http://www.coatesengine.com

    Make sure to check out the photo gallery. http://www.coatesengine.com/photo_gallery.html
    62050 - Z8

  10. #10
    Wow, now that really is cool - thanks for sharing!
    Andrew Macpherson

    Expert Z8 Inspections, with full support for both Z8 sale and purchases.

  11. #11
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    I have driven by that plant for many summers on the way to the Jersey shore and always wondered what the plant was. Great to learn that they have been working on something exciting.

  12. #12
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    Quote Originally Posted by redz8 View Post
    There seems to be another alternative: http://www.coatesengine.com

    Make sure to check out the photo gallery. http://www.coatesengine.com/photo_gallery.html

    It appears their V-8 upper end sits atop a small block Ford...Interesting
    2000 Red over black
    heavily modified for performance. Although, not to the level of GM's car

  13. #13
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    Question for GM,

    So what was the "Vanos problem" with the early Model s62 engines (build date before 11-30-2000) and how does this tie into the carbon build up issue over time that I have heard, all Z8s will experience as the miles go higher and higher (like over 50,000 miles).??
    Best Regards,

    Jeff
    61995 Silver /// Rot - Original Owner
    Z8 Club of Germany e.V. #102

  14. #14
    Carbon Build-up issue link

    Vanos link.

    Good all purpose S62 link.
    Andrew Macpherson

    Expert Z8 Inspections, with full support for both Z8 sale and purchases.