VR Engine displacement 3

[SomptingGuy]

Does anyone know what figure is quoted for the stroke (and therefore displacement) of a VR engine (e.g. a VW 2.8l VR6)? Is the stroke quoted as twice the crank throw or is it the measured stroke, taking into account the massive offset between the cylinder axis and crank centre-line.

- Steve

 

[Tmoose]

http://sandbox.enjoybeing.net/pics/vw/3.2-vr6-block.jpg

Looks like the cylinders are angled to point ~ at the crank centerline, so 2X offset and measured stroke would be the same

 

[SomptingGuy]

Nope, there is something like a 12.5mm offset. I was just wondering if the stroke (quoted as 90mm) takes that into account or not.

- Steve

 

[BrianGar]

Steve, the bore centreline 'V' angle is 15 degrees and they both(banks) lie equally above crank centreline(7.5 degrees off plumb). The stroke is indeed 90mm, the top of the pistons are machined at an angle(75 degrees in relation to skirt on the outer sides) so that they sit level with block face( At Tdc)
The crown moves 45mm from Tdc to Bdc.

The big pain with these is the fact that pistons need to be fitted with a special ring compressor as the rings meet the block deck at 75 degrees when inserting the piston to bore.
I made a turned tapered bush the size of bore and sliced at 75 degree's to aid fitting. Placed on deck the piston can be slid ito the tapered bush and into the cylinder without worry.

If you need further info on block structure let me know.

Brian.G

 

[ivymike]

The crown moves 45mm from Tdc to Bdc
that would be a stroke of 45mm?

So the actual stroke is 90mm... what is the radial dimension of the crank throw - the distance between the crankpin axis and the main bearing axis (was thinking about calculating it, but didn't get around to it)?

 

[BrianGar]

Sorry steve, mis type there, should read tdc to 90 degrees after tdc. What I mean is the crown moves the true amount of the crank stroke.

 

[ivymike]

...but the crank is offset upward from the intersection of the bank planes, so there should be a slight difference between the piston stroke and (2x the crank throw), regardless of the angle at the top of the piston.

 

[SomptingGuy]

(An offset gives a stroke longer than 2x the crank throw)

- Steve

 

[BrianGar]

Give me a few hours to check this out guys, I have a vr6 short block in the workshop, I strongly remember the crown moving 90mm when measured. Perhaps the crank differs due to offset. I didn't mean to mis-inform above, but Im pretty certain the 'crown stroke' is 90mm.

Brian.

 

[TrackRat]

According to VW Ag engineering, the cylinders on the 15 degree VR6 2.8-3.2L are centered on the crank. The newer 10.6 degree VR6 3.2/3.6L DI engines the cylinders are offset from the crank centerline. On the 10.6 degree engines VW quotes "effective stroke" vs. true stroke for the 15 degree VR6 engines. I have no idea how offset the 10.6 degree cylinders are so I can help on that.

 

[BrianGar]

Had a fast look there, and SomptingGuy seems to be not too far off with his 12.5mm offset, The block is bare yet but I will throw in crank and a rod/piston pair later. From a quick look with some string and a vernier the bore centre lines look to be meeting well below crank centre, Ill go out on a limb and say that they are striking tangentially a circle of an approx diameter 25mm if you can imagine the main journal that size.
Another thing about the vr6 is that the big end journals are offset by 22 degrees. Hence the exhaust note, Id know one a mile away.

 

[TDIMeister]

I believe TrackRat is correct in that the VR engines with 15° bank angle do not have cylinder offsets but the 10.6° ones in the 3.6 L, in order to clear the larger diameter pistons within the narrow bank-angle package, do. I'll look into this.

I have a spreadsheet that can calculate the true stroke resolved to each °CA; however, what needs to also be known that's not always published is the piston-pin offset if there is any in order to get a correct calculation.

 

[izzmus]

Are the piston pins also offset? Are the offset differentally?

Seems that it'd play hell with piston position bank to bank if they were not!

 

[ivymike]

uh, why wouldn't you offset them all in the same direction and by the same amount, as is normally done? (~0.5mm toward the thrust side if memory serves, but it's been a long time)

If the piston tops are asymmetric as described above, then the only real disadvantage of pin offset would be that it would necessitate different piston part numbers for the two banks (because the crown slope is thrustward on one bank and the opposite way on the other bank, but you'll offset the pins uniformly to thrust).

 

[TDIMeister]

OK, I checked out a VW SSP (Self-Study Programme) PDF that I have on the W-engine family, so this info can only apply authoritatively to that engine. However, the W engines are basically 2 15° VR engines siamesed with a 72° major bank angle, so it stands to reason that things like cylinder offset that apply to each minor bank should also apply to the VR engines on which they're based.

http://bit.ly/aFxLBL

The above link to a page on the SSP document (there is no copyright mark, I checked) shows that the minor bank does indeed have a 12.5mm cylinder offset, but what is incredible is that the offset are on opposite sides of the crankshaft centerline (!!!), with the intersection point below said crankshaft centerline. This means that the motions of the left-minor bank will be slightly different than the right-minor bank!! This also means that the articulating angle of the connecting rods are different at given crank angles and, particularly close to TDC, in fact on opposite sides of the cylinder axis.

Given a 90.3mm stroke of the first-generation 2.8 VR6, 164mm con rod length, ±12.5mm cylinder offset and assumed zero piston-pin offset, maximum true stroke is 90.585mm. On the right bank with positive cylinder offset (toward the anti-thrust side) the maximum stroke length occurs at 186° CA (referenced to TDC), and the pistons' closest approach to true TDC occurs at +3°CA (0.0016mm below true TDC). On the left bank with negative cylinder offset, the same maximum stroke length of 90.585mm occurs at 174° CA and closest approach to true TDC occurs at -3°CA (also 0.0016mm below true TDC).

On another note, a slight piston pin offset (usually 0.5 to 2 percent of piston diameter) is introduced mainly for NVH purposes in order to smooth out the transition of the piston skirt position, which wants to stick to the anti-thrust side on the upward portion of the stroke and to the thrust side on the downward part.

Interesting stuff!

 

[TDIMeister]

Sorry, wrong page to the document uploaded and linked. Here is the correct one:

http://bit.ly/are5NC

 

[SomptingGuy]

That fits with my understanding, I just didn't have any evidence. Thanks.

- Steve

 

[TDIMeister]

A quick calculation on my spreadsheet illustrates the profiles of the VR6 as described in my previous post above:

Displacement:

http://bit.ly/aDqcPv

Velocity:

http://bit.ly/bRyAmY

Acceleration:

http://bit.ly/91Ri58

The velocity and acceleration curves my be flipped from what some people are used to because I've employed the convention that a downward stroke displacement is negative.

 

[vagman2]

A Bugatti W engine:

http://files.engineering.com/getfil...7a14-4352-9f29-bbeb50a82edb&file=DSC01936.JPG

 

[TrackRat]

An engine of beauty... all 1001 HP of it! <LOL>

They hand build these at the VW engine plant in Salzgitter, Germany. It's a very interesting factory and one people can and should tour if they get a chance.