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Valve question

Started by Trevor Williams, August 26, 2008, 08:14:09

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Derek Coghill

I've had no valve problems so far; haven't had the heads off since the 24hr but can do so and have a look easily enough. Modified standard valves in mine as far as I remember.

Where are the little nicks from, Trev? The edge of the valve, the sealing face or elsewhere?

Paul Robertson

Quote from: mark turner on August 27, 2008, 11:42:06
I had a conversation with another club member last weekend who is running stainless steel valves and has
done so for several seasons and claims to have had no problems since fitting them!Could this be the answer
or do we investigate further?
Bully, i have sent the file to my engineering works, just waiting for price and time scale.
I would think this would be the best idea.

Trevor Williams

Derek
Edge of the valve around the circumference. One valve has minute cracks starting to move away from the nicks
Some days, it's REALLY difficult being me!

Marc Fenner

Im talking to a guy at the moment who has titanium valves made for his 2cv projects. Trying to find out the price of  them as a set for me and also as a bulk buy. They may be able to do stainless ones too.

Derek Coghill

Trev, next question; has it been touching the piston?

Trevor Williams

Derek
Absolutely not, only damage is from the bits of valve that went round the combustion chamber
Some days, it's REALLY difficult being me!

Francis Rottenburg

Any correlation to the weather conditions?  Most of the recent 24 hours have been either wet or bloody monsoon so engines would have been running very cool.  I understand you had a good deal of dry weather in which case temperatures might have been similar to 2004.  What fans were people running on the failed engines?  Seems odd that the problem disappeared and suddenly came back again.

Marc Fenner

The other thing is fuel at the circuit ???

Andrew Bull

We have been fortunate enough to have a valve that has finally revealed its failure on the bench rather than in the engine.  Sammie rebuilt Mark Fenners engine and found one broke in her hand as she was grinding it in.  Therefore we can actually look at something that doesnt resemble Kryton's chewing gum.

The failure faces clearly show striations (clamshell lines) propogating from Two points approximatley 180 degrees apart, on the outside edge. This is the classic sign of Fatigue Failure.   Then in the centre there is a clear brittle fracture, obviously at the point when it could not cope.  This valve must have been on the point of separation, with just enough left to hold it together when it was removed from the engine.

Fatigue failure occurs with initial crack initaition.  The origin of the crack can be any microscopic surface flaw, mark, scratch, defect or stress raiser.  In our case i beleive the surface finish on the 'Machined' wasted stem valve is the major contributor as it is turned, thus creating hundreds of annular rings around the stem.  Perfect stress raisers, when combined with flexural (Bending) loading on the stem, and thus giving an indication as to why we have two origins.

Then propogation, where the crack has repeated breaks during each loading cycle.  This creates the tell tale striations formed from this failure mode.

Then Brittle fracture which essentailly is the final 'give up' when there is no longer enough material left to be strong enough to withstand the normal axial loading through the centre axis of the stem.

In a nutshell, the valves are machined to create wasted stem, This leaves marks which are stress raisers.  the valve has pull forces and bending forces acting upon it.  As it bends time and again it creates a crack where a machining mark is (or scratch / mark). This crack grows with each bending cycle, and then it fails completely. 

I hope this may provide some more direction. 

A couple of ways to reduce potential crack initiation would be to shot peen the machined surfaces.  This reduces stress raisers and work hardens the surface where cracks occur.  Or, if the wasted area was ground this would create a much smoother surface again reducing stress raisers on the stem.


Any comments? 

Andrew Bull
Tête Rouge Racing with Habitabull & Propolis
Rookie Champion       2022
team champions         1998 2000 2008 2009 2013
24hr winners               2005 2006 2009 2010 
championship winner 2006 2007 2008 2009 2010 2011 2013

Martin Harrold

I can confirm the concept of what Andrew is talking about. A distant relative who was a top boffin at Farnborough led the team that fished all the failed Comets out of the Irish Sea and elsewhere many, many, years ago. He built a pressure tank and eventually worked out that the reason for failure was cracks caused by flexing of the structure which emanated from the corner point of the then square corners of the window apertures. That's why all planes now have rounded window apertures.
  Smooth metal surfaces and openings are much less prone to stress and fatigue failure.
  He wrote the official history of Farnborough - and if anyone is interested I will be happy to lend them a copy.
2CV Team LION
2CVParts.com Champions 2014, 2015, 2018, 2019, 2021 & 2022
CITROEN 2CV 24 Hour race winners, 2014, 2015, 2016, 2017, 2020, 2021 & 2022
m: 07973 303982

Steve Cowell

as Pete said,we in car 29 had 1 valve head drop off, and 2 others nick the edges  ???
The older I get,the faster I was (not)

Derek Coghill

<Any comments?>

Wouldn't it be a bit difficult to grind the waisted-stem valves? How about sanding/polishing once they've been made? As regards shot-peening hardening the surface, if it's already prone to cracking won't that make it more so?

No comments, just more questions!

Andrew Bull

Work Hardening the surface would help to reduce the chance of Crack initiation.  As for grinding the area there should be no problems as there would not be any undercuts so a shaped wheel could be quite adequate.  Obviously i'm not talking about doing the grinding ourselves, but it could be an alternative manufacturing method. 
Andrew Bull
Tête Rouge Racing with Habitabull & Propolis
Rookie Champion       2022
team champions         1998 2000 2008 2009 2013
24hr winners               2005 2006 2009 2010 
championship winner 2006 2007 2008 2009 2010 2011 2013

Derek Coghill

What about valves that people already have? The obvious thing would be not to use them, but then they'd have to buy (or make) more.

Paul Robertson

Following discussions with the committee we have asked mark to organise enough stainless valves for 100 engines.This has reduced the cost per valve considerably and they should be no more than £15 per valve.
As for delivery time we don't know at the moment,we may all have to play a bit of russian roulette at mallory.
Paul