Rationale for big end con-rod design seen here

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In summary: CG argument has something to do with it too?The angle of the attachment is for practicality and lifespan of the con rods.
  • #1
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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  • #2
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.
 
  • #3
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.
 
  • #4
Yeah, some of both.
 
  • #5
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
 

1. What is the rationale behind using a big end con-rod design?

The big end con-rod design is used in engines to increase the strength and durability of the engine. It allows for a larger connecting rod bearing, which can handle higher loads and pressures, resulting in a more powerful and efficient engine.

2. How does a big end con-rod design improve engine performance?

A big end con-rod design allows for a longer and wider connecting rod, which increases the leverage and reduces the stress on the piston. This results in a smoother and more efficient movement of the piston, leading to improved engine performance.

3. What factors are considered when designing a big end con-rod?

When designing a big end con-rod, factors such as engine size, power output, and intended use of the engine are taken into consideration. The material of the con-rod, its length and width, and the shape of the connecting rod bearing are also important factors in the design process.

4. Are there any drawbacks to using a big end con-rod design?

One potential drawback of using a big end con-rod design is the added weight and size of the con-rod, which can result in increased friction and resistance, leading to a slight decrease in engine efficiency. However, this is often outweighed by the benefits of improved strength and performance.

5. How has the design of big end con-rods evolved over time?

In the past, big end con-rods were typically made of cast iron or steel, but advancements in materials and manufacturing techniques have led to the use of lighter and stronger materials such as aluminum and titanium. Additionally, computer-aided design and simulation have allowed for more precise and efficient designs, leading to further improvements in engine performance.

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