Rationale for big end con-rod design seen here

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SUMMARY

The discussion centers on the design rationale for the angled big end attachment of connecting rods in piston engines, specifically in large Wärtsilä four-stroke marine diesels and Subaru boxer diesels. The primary advantage of this design is improved maintenance access, particularly in marine engines where side access is essential. While some participants speculate that this design may also enhance the lifespan of the connecting rods by better accommodating reciprocating forces, the consensus leans towards practicality as the main reason for the angled design. The discussion highlights the importance of maintenance considerations in engine design.

PREREQUISITES
  • Understanding of connecting rod mechanics in piston engines
  • Familiarity with marine diesel engine maintenance practices
  • Knowledge of Subaru boxer engine design
  • Basic principles of reciprocating forces in engine components
NEXT STEPS
  • Research Wärtsilä four-stroke marine diesel engine specifications
  • Explore maintenance techniques for Subaru boxer engines
  • Study the effects of reciprocating forces on connecting rod design
  • Investigate alternative connecting rod designs and their maintenance implications
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Engine designers, marine engineers, automotive technicians, and anyone involved in the maintenance and optimization of piston engines will benefit from this discussion.

jellydonut
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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:
hsCI9.jpg


From a tiny Daihatsu marine diesel:
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:

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?
 
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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.
 
That's what I've been doing :rolleyes:

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.
 
Yeah, some of both.
 
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
 

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