Piston ring fluttering

Lack of information

The basic principles of the piston ring fluttering, seems to be well known by engine builders and part manufacturers. Also symptoms of ring flutter are relatively clear and logical. Ring dynamics can be even predicted accurately by taking all the parameters in account in special simulation tools. But still there seems to be conflicted information available and lack of numerical guidelines for those who are looking confirmation for potential ring fluttering related problems or help for choosing piston rings for their specific engine setup.

 

Simplified description of compression ring fluttering

In the normal engine operation, the cylinder pressure on compression and on power cycle should keep the first compression ring steadily pressed downwards against the bottom of the piston ring groove. The pressure above the piston ring will find its way back of the piston ring, which again pushes the piston ring against the cylinder wall and securing the sealing. In standard applications, the pressure will be directed via the ring to groove clearance. In some applications this occurence may be helped by arranging more direct paths for the cylinder pressure to penetrate behind the piston ring using specific gas ports. The gas ports may be drilled vertically from the piston top or grooved laterally on the top surface of the piston ring groove.

The cylinder pressure fights against inertia of the piston ring and cylinder wall friction. Inertia of the piston ring is related to piston acceleration and to weight of the piston ring. If these opposing forces become greater the piston ring loses its contact to the bottom land of the piston ring groove, thus pressure behind the ring will escape under the ring and the force against the cylinder wall is lost. In standard application also the pressure path behind the piston ring will be closed and the pressure area above the ring is reduced making the problem even worse and last longer on the power stroke. This malfunction is called as piston ring fluttering.

 

Causes of piston ring flutter

When the piston ring fluttering happens, rings lose their contact and result will be excessive blow-by. Furthermore the oil control does not work right anymore and these malfunctions cause long list of problems, including following:

- Power losses by excessive blow-by
- Ring and groove wear or failure
- Oil in burning chamfer will provoke knocking which may cause several mechanical failures
- In endurance applications high oil consumption may lead to lubrication problems and related failures
- High blow-by may pressurise the crank case and cause oil leakages and further power losses

 

Challenges to predict the potential problems

There are a great number of different parameters that have effect on the engine speed when the ring flutter starts. The most important parameters are the acceleration of the piston, weight of the rings, and the pressure above the piston ring. To not make things too easy, also load conditions, piston ring type, friction, ring side clearance, piston to cylinder clearance, ring end caps, piston design and several other parameters are making the difference.

Having previous experiences from the ring fluttering in the same engine type than in consideration would be valuable information, but that is not always the case. Taking all the parameter in account would lead to complex piston ring dynamic simulation, but this is not realistic option for most of the engine builders either.

 

Simple method to get on the ballpark

So far the simpliest method I have found is based on early research done by J. Hepworth related to relationships between simple engine dimensions and practical operating speeds of racing engines. I found these via notes of Derek Taulbut who also collected more evidences to support the old theory.

http://www.grandprixengines.co.uk/note13.pdf

The piston ring thickness leads to typical weight, the cylinder pressures of natural aspirated race engines are relatively similar from application to other. Hepworth simplified the equation to relative value coming from acceleration and ring thickness. From some practical examples, it was found that 4060 g*mm would be around the level where the piston ring flutter starts on typical race engines. This can be taken as indicative guideline and is valid for typical cast iron rings. Steel rings may be example 10% lighter or even more.

Here is a simple calculator which needs just stroke, connection rod length and 1st compression ring thickness as input to estimate how high the engine may be revved with steady sealing.

 

 

Please note that this is not absolute limit, but after this the sealing might be more or less compromised. From my own experiences, I’ve found that at least Alfa 8V Twin Spark engine start to consume some oil at the same rpm where the calculator predicts problems to arise..

If you want to know in more details what really happens when ring start to flutter, including gas flow behaviours and ring moments, I would recommend reading following paper written by Federal Mogul and Cummins. I enjoyd it, so thanks to writers!

Understanding the fundamentals of piston ring axial motion and twist and the effect on blow-by

Kimmo @ Alfa Tune Ltd

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