crank position 1

[dexion7]

why is it that in internal combustion engines the crank is always positioned directly below the pistons? if it was offset would that not give designers latitude to address issues such as detonation / piston speed / rod angle etc?

 

[chicopee]

Economy of design perhaps. With more offset, more clearance from the engine block would be needed on the offset side for the links.

 

[dvd]

Offset crank would also increase skirt friction due to higher side forces.

 

[MikeHalloran]

The current VW VR6 has the crank offset from the cylinder axes by 12.5mm or so, for packaging reasons. So at some level of detail, the crank angle/piston position transfer function is different for the two banks.
That hasn't stopped hot rodders from getting high specific power.

http://en.wikipedia.org/wiki/VR6_engine

Mike Halloran
Pembroke Pines, FL, USA

 

[SomptingGuy]

Offset crank is the same as pin offset in terms of the mechanicals (and the maths). Small-ish offsets are used to mitigate the thrust/anti-thrust forces in some engines (piston secondary motion and/or scuffing).

VW's massive crank offset is mainly imposed by packaging - the principle of a single head for a Vee engine gives the engineers some interesting self-imposed problems to solve.

Then there's engine balancing.

- Steve

 

[GregLocock]

F1 engines usually have the crank offset upwards into the V, so they are running negative offset on one bank and positive on the other. This isn't for any fancy pants reason, just to get the CG lower.

Incidentally the technical term is desaxe, and it has been used on thousands of engine designs, including steam engines in the 1800s.

In theory it can be used for performance reasons, in practice it, or its close cousin gudgeon pin offset, is now used to reduce piston knock when cold.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[ivymike]

I knew piston pin offset would give the desired result w/r/t piston slap. I didn't realize you can accomplish that with crank offset. I can see how the effects might be similar in some ways, but it would seem that the piston pin offset would be the better lever to pull.

 

[GregLocock]

In practice desaxe is designed in at the whim of the original designer, the development engineer is the one out in the frosty car park fine tuning the solution with a bag full of pistons. Incidentally there are legions of racing engine builders who will claim that turning the pistons round on various engines increases the power.





Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[ivymike]

you can find a dozen-or-so of them right here on this forum.