Cylinder Offset From Crank 1

[thundair]

Is there a diminishing return on offsetting the bore over the crank?
I have moved it 1 and 2mm toward rotation and did not get the results I expected, although it is easier then stock to push the piston down by hand . This is a parallel twin but I guess applies to all in line engines.

Is there any studies on moving the cylinder to have the highest force on the piston at the largest moment on the crank

Thanks

I don't know anything but the people that do.

 

[SMOKEY44211]

There is something to be gained by offsetting the wrist pin in the direction of rotation. On a 350 cu" size V8 there is a 5-8 hp gain by installing the pistons "backwards" manufactured with .030 offset. My observations suggest that the gain is due to decreased cylinder wall side loading as there is a corresponding decrease in fuel consumption. Be advised that an engine with this configuration is very noisy but durability does not seem to be compromised. Also note that TDC is when center of wrist pin, crank pin and main journal form a straight line.-------Phil

 

[andyv8]

The Honda 2.2L Diesel is another engine with an offset crank to reduce noise and friction.
One mathematical model by some chaps at Hyundai showed a reduction in friction at low engine speeds, but increased friction at higher engine speeds (cross-over point depends on offset). There have been some SAE papers published on the topic.

 

[SomptingGuy]

... the Volkswagen VR engines have a 12.5mm offset. That was what I meant by "big".

 

[jomor]

Our linkages book back in school had a whole chapter on this with software similation in the back.

It is packed away from my last move but I'm sure you can search 4- bar linkages and come up with something.
Maybe check martindales reference desk.

Sorry I don't have a better answer. HTH

 

[BrianPetersen]

The interesting thing about the Volkswagen VR.. engines is that the three front cylinders are offset one way, and the three rear cylinders are offset the other way.

Offsetting the piston pin away from the centerline of the piston but keeping the crank centerline on the centerline of the cylinder, has a different effect from having the piston pin centered on the piston but the crankshaft shifted off the centerline of the cylinder. The former approach requires oddball pistons. The latter approach requires oddball crankcase machining. I suspect the V8's are using a millimeter or two of the former approach. The above-referenced Toyota Aygo (and Echo and Prius, and Honda diesel) use the latter approach.

I suspect that aside from piston-slap control, any overall effect on the engine's power and efficiency will be very small.

 

[FoMoCoMoFo]

Interesting indeed...

 

[MikeHalloran]

I'm pretty sure the VW was made that way for packaging, not efficiency, reasons.



Mike Halloran
Pembroke Pines, FL, USA

 

[BrianPetersen]

Yep, the layout of the VR6 cylinders doesn't allow the bottoms of the cylinders to approach closely enough to be pointed straight at the centerline of the crankshaft, there is physically not enough space. The offset is purely to make the layout work. The consequence is that the three front cylinders are offset one way and the three rear cylinders are offset the other way. It doesn't seem to hurt (or help) the engine in any way. The VR6 has certain other inherent disadvantages, but it's not because of this.

 

[GregLocock]

F1 engines are the same. The offset is to reduce the cg height, not for any arcane power benefits

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[SomptingGuy]

Kinematically an offset crank is the same as an offset pin in terms of primary piston motion. My guess is that piston secondary dynamics (e.g. piston slap) would be different though.

 

[malbeare]

If you want to have a play have a look at

http://www.wfu.edu/~rollins/piston/offset/offset.xls

A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare
www.sixstroke.com

 

[thundair]

I know I am not very good at explaining 3 dimensional because I spent a while trying to explain it to a mechanic but with a small crankshaft assy and a few sketches he got it...
I simplified my statement by showing 15* ATDC = .085 down the hole with a 3.5 stroke and a 6" rod. That being arguably the highest pressure. Then using that .085 down the hole at 25* should give you a larger moment arm and more torque.


I don't know anything but the people that do.

 

[Lou Scannon]

I getting a sense from some of the posts that there is expectation of being able to increase power for a given displacement by virtue of a kinematic change itself. That is a fallacy. If there is any power to be gained via a kinematic change, it will be due to more favorable phasing of volume change with combustion and the resulting net effect on heat release, and/or due to reduced net friction resulting from modified thrust angle phasing through the cycle.