Maximum combustion chamber pressure and connector rod length

AI Thread Summary
The discussion focuses on calculating the change in maximum combustion pressure (Pmax) in a diesel generator's combustion chamber after reducing the connecting rod height by 0.5 mm due to pitting. Key data shared includes a stroke of 310 mm, a bore of 210 mm, and an initial Pmax of 185 bar at full load, with a compression ratio of 16.5:1. The calculations indicate that the new Pmax would be approximately 180.56 bar after the adjustment, necessitating a 2% increase in fuel volume to compensate for the cooler air's delayed ignition. Participants also discuss the importance of understanding the combustion chamber's configuration and the potential use of a metal shim to offset material removal. The conversation emphasizes the need for precise calculations and adjustments in engine parameters during maintenance.
Daveee91
Messages
8
Reaction score
2
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?
 
Engineering news on Phys.org
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.
 
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
 
  • #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.
 
  • #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?
 
  • #13
5 bars is nothing in this kind of engines. Anyway thank you very much!
 

Hot Threads

Recent Insights

Back
Top