Rationale for big end con-rod design seen here

[jellydonut]

Hey,

I've wondered about the reasoning behind, or rationale for, the angled big end attachment of the connecting rod in piston engines.

This is from a large Wärtsilä four-stroke marine diesel:
http://i.imgur.com/hsCI9.jpg

From a tiny Daihatsu marine diesel:
http://i.imgur.com/I7aec.jpg

My first thought was that this was somehow advantageous due to the reciprocating forces, but that's me with my layman eyes.. It was suggested to me that the reason was much simpler: easy access because marine engines are maintained from the side, and not by removing the sump. This made sense and the matter settled.

Then I saw this:

http://i.imgur.com/e0qhy.jpg

This is from the new Subaru boxer diesel. I suppose it could make sense that due to the engine shape they made them this way solely for maintenance access here as well, but I just want to ask anyway because this is like an itch I can't scratch.

Do they angle the attachment solely for practicality, or does it increase the lifespan of the con rods?

[brewnog]

Think about where the forces in the big end are acting just after TDC.
Also think about how you'd go about removing pistons on such an engine.

[jellydonut]

That's what I've been doing :uhh:

I mentioned both such thoughts - I'm just wondering if I am right about either, both or none. Certainly it is easier to remove the piston on a marine engine with this design, I was just wondering if this was the sole reason or just a side effect of a design more tolerant of reciprocating forces.

[brewnog]

Yeah, some of both.

[Ranger Mike]

First time i was in a marine engine facility was when i saw a piston/ rod combination being lowered into the engine case with an over head crane..huge..access is from the side
Subaru is not a V8,v6, V4 but horizontal engine design and again access to con rod bolt is difficult , hence the design...btw Subaru has all wheel drive and touts the horizontal engine layout for lower CG