Maximum combustion chamber pressure and connector rod length

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Discussion Overview

The discussion revolves around calculating the change in maximum combustion chamber pressure (Pmax) in a diesel engine following modifications to the connecting rod length and the associated impacts on the combustion chamber volume. Participants explore the implications of these changes on engine performance, focusing on theoretical calculations and practical considerations related to maintenance and modifications in marine diesel engines.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • One participant seeks assistance in calculating the change in Pmax when the connecting rod height is decreased by 0.5 mm, which increases the combustion chamber volume.
  • Several participants emphasize the need for the compression ratio to perform accurate calculations.
  • Questions arise regarding the specifics of the modifications, including how to grind the big end to accommodate replacement bearing shells and the implications of planing the cylinder head.
  • A participant provides detailed calculations based on the engine's specifications, estimating a new Pmax of approximately 180.56 bar after the modifications.
  • Concerns are raised about the potential need to adjust fuel injection volume and exhaust temperature balance in response to changes in compression ratio.
  • Another participant suggests using a metal shim to compensate for the material removed during grinding, indicating a practical approach to the issue.

Areas of Agreement / Disagreement

Participants express varying degrees of agreement on the need for specific data and calculations, but no consensus is reached on the best approach to compensate for the changes or the implications of the modifications on engine performance.

Contextual Notes

Participants note that the calculations depend on several assumptions, including the accuracy of the compression ratio and the specific engine model. The discussion also highlights the complexity of the relationship between combustion chamber volume and pressure, indicating that multiple factors must be considered.

Who May Find This Useful

This discussion may be useful for marine engineers, diesel engine technicians, and individuals involved in engine maintenance and modifications, particularly in understanding the implications of changes to connecting rod dimensions on combustion performance.

Daveee91
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Dear All.
Good evening

I work with diesel generatora on board of ships. Our major value that is considered after oh and during the maintenance checks is Pmax - maximum cobustion pressure in combustion chamber. It had to be equeal (~ +- 5 bar between each cylinder). I am familiar with all calculations of engine power and pressures etc but little bit rusty now (10 years ago I graduaeted)

During oh connecting rod feet and marine head had to be grinded (pitting)
Coule you please help me with calculating the change of Pmax when we f.e decrease connecting rod height for 0,5 mm. So the comustion chamber will be 0,5 mm bigger

Data:
- stroke - 310 mm
- bore - 210 mm
- Pmax - 185 bar on 100% of load

What data more do I need?
 
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Welcome, @Daveee91 !

We need to know the compression rate.
How can you shorten the connecting rod due to pitting?
 
Sorry for not being exact. Pitting was found on the contact surfaces - big end bearing housing and connecting rod feet, therefore It was grinded 0,5 mm
 
Lnewqban said:
Welcome, @Daveee91 !

We need to know the compression rate.
How can you shorten the connecting rod due to pitting?
I will recheck copression rate in documentation and come back tomorrow.
 
Daveee91 said:
What data more do I need?
After reducing the length of the connecting rod, how do you grind the big end to accept the replacement bearing shell?
What size bearing shells did you remove?
What size bearing shells will you install?

What exactly is "marine head"?
Is the combustion chamber in the top of the piston, or in the cylinder head? If the combustion chamber is in the head, then planing the head may cancel the shorter connecting rod.

What is the make and model of diesel engine?
 
Baluncore said:
After reducing the length of the connecting rod, how do you grind the big end to accept the replacement bearing shell?
What size bearing shells did you remove?
What size bearing shells will you install?

What exactly is "marine head"?
Is the combustion chamber in the top of the piston, or in the cylinder head? If the combustion chamber is in the head, then planing the head may cancel the shorter connecting rod.

What is the make and model of diesel engine?
Generally you do not change the size of bearing shells unless the crankpin journal is grinded to under size.

1719259593114.png

Bottom part is marine head (big end bearing housing) upper connecting rod. Upper part contact surface will be grinded 0,25 mm and bottom also 0,25.

Data that I have posted is from MAN L21/31.

Picture is from Yanmar engine but it is similar
 
1719260043910.png


As you can see in the picture combustion chamber is on the top of the piston closed with almost flat cylinder head which is sitting on the liner.

Guys I know it is possible to compensate and probably should be compensated!
There is a cylinder head gasket on the top of the liner on which cylinder head sits. It is 1.0 mm and can be ordered different but I would like to know how to calculate change in Pmax when I change the parameters as I wrote before.
 
Daveee91 said:
... but I would like to know how to calculate change in Pmax when I change the parameters as I wrote before.
To calculate the change in the compression ratio, you must know the compression ratio of the standard engine before modification.

For the change in compression ratio, you will need to adjust the fuel injection volume, and check that the exhaust temperature balance is within specification.
 
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Baluncore said:
To calculate the change in the compression ratio, you must know the compression ratio of the standard engine before modification.

For the change in compression ratio, you will need to adjust the fuel injection volume, and check that the exhaust temperature balance is within specification.
1719261238786.jpeg

Found some more data. Hope it will help
 
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  • #10
Stroke = 310 mm.
Piston area per cyl. = 346 cm²
Swept volume per cyl. = 10.7 litre
Compression ratio. = 16.5 : 1
Pmax - 185 bar on 100% of load
From that we get ...
Minimum volume; 10.7 litre / 16.5 = 648 cc at TDC.
Increase in cylinder volume; 346 cm² * 0.5 mm = 17.3 cc
New minimum volume; 648 cc + 17.3 cc = 665.3 cc
New maximum volume; 10700 + 17.3 = 10717.3
New compression ratio; 10717.3 / 665.3 = 16.11
Volume change ratio; 16.11 / 16.5 = 0.976
New Pmax; 185 * 0.976 = 180.56 bar.
The cooler air will take longer to ignite fuel.
Increase fuel volume by about 2% to compensate.
 
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  • #11
Baluncore said:
Stroke = 310 mm.
Piston area per cyl. = 346 cm²
Swept volume per cyl. = 10.7 litre
Compression ratio. = 16.5 : 1
Pmax - 185 bar on 100% of load
From that we get ...
Minimum volume; 10.7 litre / 16.5 = 648 cc at TDC.
Increase in cylinder volume; 346 cm² * 0.5 mm = 17.3 cc
New minimum volume; 648 cc + 17.3 cc = 665.3 cc
New maximum volume; 10700 + 17.3 = 10717.3
New compression ratio; 10717.3 / 665.3 = 16.11
Volume change ratio; 16.11 / 16.5 = 0.976
New Pmax; 185 * 0.976 = 180.56 bar.
The cooler air will take longer to ignite fuel.
Increase fuel volume by about 2% to compensate.
Thanks you so much!

Is possible to calculate this using some formula?
 
  • #12
Daveee91 said:
Is possible to calculate this using some formula?
I do not know. It is very rare to do this calculation.

Why do you not use an 0.5 mm metal shim, to make up for the material removed?
 
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  • #13
5 bars is nothing in this kind of engines. Anyway thank you very much!