Maximum combustion chamber pressure and connector rod length

Click For Summary
SUMMARY

The discussion focuses on calculating the change in maximum combustion chamber pressure (Pmax) when modifying the connecting rod length in a MAN L21/31 diesel engine. The original Pmax is 185 bar at full load, with a compression ratio of 16.5:1. After reducing the connecting rod height by 0.5 mm, the new Pmax is calculated to be 180.56 bar, necessitating a 2% increase in fuel volume to compensate for the cooler air's ignition delay. Key parameters include a stroke of 310 mm and a piston area of 346 cm².

PREREQUISITES
  • Understanding of diesel engine mechanics and operation
  • Familiarity with combustion chamber dynamics
  • Knowledge of compression ratio calculations
  • Experience with fuel injection adjustments
NEXT STEPS
  • Research the impact of compression ratio on diesel engine performance
  • Learn about fuel injection volume adjustments for modified engines
  • Study the effects of combustion chamber volume changes on engine efficiency
  • Explore methods for calculating Pmax in diesel engines
USEFUL FOR

Marine engineers, diesel engine technicians, and anyone involved in the maintenance and optimization of diesel generator performance on ships.

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?
 
Reply
  • Like
Likes   Reactions: DeBangis21
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.
 
Reply
  • Like
Likes   Reactions: DeBangis21
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
 
Reply
  • Like
Likes   Reactions: Lnewqban
  • #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.
 
Reply
  • Like
Likes   Reactions: Daveee91
  • #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?
 
Reply
  • Like
Likes   Reactions: Tom.G
  • #13
5 bars is nothing in this kind of engines. Anyway thank you very much!