Piston and valve rings.

I read somewhere that such rings lasted 20 to 30 times longer in IC engines than those in steam powered ones. On the face of it, it would suggest that the lubrication oil as delivered to steam loco cylinders was woefully short in quantity that would be required. Granted that if much larger quantities were supplied some drastic re-thinking of the oil separators in the steam line to exhaust steam injectors would be required.

However, when one reads of the likes of 9Fs and 'Britannia's' needing replacement rings after only 5000 miles, yet lorry-sized Diesel engines being capable of running for 500,000 miles due to improved lubricants, it does give food for thought.

Discuss.

 

Funnily enough I’ve been discussing the subject of cylinder lubrication today but not in this context. Our discussion was centred on carbon build up on valves, pistons and passageways. I was told by a technical chap at Morris lubricants that heritage railways generally over lubricate the cylinders and this is the main cause of carbon build up. If this is true, and there appears to be a good argument along these lines, under lubrication shouldn’t be the reason for ring wear.
I don’t have an answer to your question but IC piston design is quite different to steam loco piston design and IC engines are single acting, which may well be significant.

Edit. More thought on the subject; IC engines are cooled to a fairly low temperature so, although the combustion gases are relatively high the metalwork and oil will be relatively cool. With a superheated steam loco this is far from the case and the temperatures involved are not good for oil. The last thing you want to be doing is cooling the cylinders on a steam engine; in fact the opposite would be beneficial in some respects.

 

As 50,000 miles were achieved with locomotives which had a driving wheel diameter of 2' 10" something is very wrong. If the oil was fed through atomisers then it was degraded before it reached the point of application.

Piston head design and piston ring design are factors, so is the quality of the cylinder liner material. The material chosen for steam locomotive cylinders ought to match the quality used in diesel engine liners - if you use an inferior material you cannot expect similar results. You have other factors which have a negative impact on the service life of the components in question, priming being one of them. There is quite a bit written about this subject.

 

Not quantity but quality of lubrication. A single acting IC engine has lubrication oil delivered directly to the cylinder wall, even the piston wall, on one side of the rings, and a miscible fuel with lubricant properties delivered to the other side. It's little wonder that ring life is orders of magnitude better.

Oil consumption and cylinder/ring life were a major preoccupation in the steam age, together with secondary maintenance issues like carbon deposits, but the fundamentals of a double acting cylinder and imiscible lubricants are wrong from the POV of lubrication. Inevitable but wrong.

My ignorance on matters of tribology is almost complete, but it seems to me that if there was a cheap water soluble lubricant that didn't degrade at superheated steam temperatures, and it could be contrived to be fed through the piston to the cylinder walls between the rings, well then there might be some massive improvements.

 

That would be great. Just four problems to solve: cheap, water soluble, OK at superheated steam temperatures, and system for feeding it through the piston. Has our resident DPhil chemist any bright ideas?

 

I suggest that there are quite a few differences to take into account but firstly with GW piston valve locos we expect the rings to last 20,000 to 25,000 miles.

Differences from the IC situation:
1. Tolerances 1.- IC engines are mass produced to much closer tolerances than are possible with steam locomotives as built. Much more development work goes into the materials used.
2. Tolerances 2. - steam locos are frequently re ringed with worn bores. There are limits to the allowable wear but it is still considerable. Reboring each time would be wasteful and expensive.
2. Water - there is the inevitable presence of water condensed from steam in the large cold mass of steam loco cylinders at the start of a new shift. Priming is an enemy of lubrication washing away the oil film.
3, Cylinder cock use - every time the cylinder cocks are used with significant steam on the cylinder and valve chest walls are steam cleaned and some coating of oil lost. That cannot be entirely avoided but crews which leave the cocks open EVERY time they start clearly don't understand what they are doing.
4. Operator - steam locos rely on operator diligence particularly where sight feed lubrication is used, though this system offers advantages over the mechanical alternative.
5. Closed system - steam loco cylinders are open to exhaust gases & small detritus when coasting. IC cylinders are closed.

 

Just on that aspect: why do steam loco cylinders but not IC cylinders wear and need reboring and eventually new liners?

 

IC cylinders & rings do wear & the cylinders are rebored and fitted with oversize pistons and rings where needed.

 

ICE cylinders do, or did, need rebores and, occasionally, new liners although there are variables. 'Wet' liners are generally used with engines in HGVs but are now increasingly used in smaller petrol engines and are easily replaced. 'Dry' liners require the bore to be opened out to allow them to be forced in. But these days, bores do last a lot longer and, when they have worn to maximum size or ovality, are considered scrap, with oversize pistons unavailable. But we're talking 21st Century ICE, not 19th or 20th Century steam technology.

 

Writing as someone who has been watching and travelling on steam hauled trains for nearly 70 years, it seems to me that the use of cylinder cocks has greatly increased in the recent steam era both on the mainline and heritage lines. Do others also think this?

 

Opening cylinder cocks of engines that have been standing for a period is vital for ejecting any condensate, perhaps some drivers are over cautious which is understandable as they are not doing it day in day out. On footplate trips I’ve had in recent times I’ve also noticed a change In the way firemen work. As an example, on a recent trip between Woking and Basingstoke the fireman was using the injector intermittently to maintain boiler water level. It was a long time ago but on a similar footplate trip in 67 I seem to remember the fireman setting the injector feed to match the steam consumption in the boiler and just leaving it. Experience probably told him how far to cut back the feed, a skill that is now lost or am I wrong?

 

Also, are you comparing like with like? A short stop in the middle of a 60-mile run where the engine has been fully warmed and worked is going to be very different to a 25mph pootle with long gaps

 

With a GWR loco the cylinder cocks are not generally needed for the rest of the day once the cylinder block is hot and the regulator does not blow through. There is therefore very little if any accumulation of water in the cylinders. Yes the cocks might be opened whilst stationary but are not needed with steam applied and as mentioned earlier steam cleaning of the bores is bad practice. Well attuned driver observation of the chimney exhaust appearance & sound is though essential.

 

The subject of tribology and the steam locomotive is one is one of those areas which needed further development.

Most of us are aware of the problems brought about by the use of broad rings on piston valves. These present a heavy wall loading on the cylinder which absorbs a significant amount of power to overcome it and also presents a load to the valve gear which needs consideration. Achieving a satisfactory lubrication film while using these rings was essentially impossible and worse they did not offer a solution to the leakage issue.

Narrow rings presented a solution which introduced less wall loading, and hence wear providing your lubrication system was adequate and functioning and also much reduced leakage, which was good for efficiency and improved power output.

The introduction of long travel valve gear would also prove beneficial but it also brought about concerns concerning increased wear. Eventually matters were resolved well enough and this is the situation that we are dealing with when it comes to our preserved locomotives. Resolved well enough but getting on for one hundred years ago.

If we look at piston valve heads they have four or six rings and work in cast iron valve liners. Where and how does your lubrication enter the system? Those who have carried out work in this area maintain that the oil should be delivered directly and injected between the valve rings and it should never be mixed with steam.

Looking at the valve head again, how is it supported? It will in all probability be heavier than it needs to be and is it made from the best modern material or the same old same old because that is the way it has always been done?

A revised valve head design offering some weight saving would be beneficial and you could always progress further by introducing more sealing elements, say aim for ten or twelve rings per head. The valve liners will need attention since they ought to be produced from a better material, a pearlitic chrome iron is recommended.

One of the reasons for delivering the oil between the rings is that the pressure is lower in that area since the delivery of the vital lubricant is not so opposed by steam pressure.

The selection of the rings is also important and some recommend that some of the rings are made of bronze.

The elimination of abrasive materials is also important so make sure of your water treatment (antifoam) and avoid materials entering via the blast pipe.

Improving the lubrication and wear situation would involve some changes but few if any would be visible. That being said there are other issues and others have already touched on these. The steam locomotive suffers from condensation and in dealing with this condensation we have the cylinder drain cock issue. Drain cocks when operated should not contaminate the railhead but this is not a matter under current discussion. We have all seen the damage brought about by failing to clear the condensate. You can drain it off but this process can make the lubrication problem worse. If the cylinders could be kept warm enough to avoid the problem it would be beneficial to say the least but the fitting of enhanced insulation is not without its problems. The loading gauge raises its ugly head once more. Some have used steam jacketing but again you need the space. On a new design you can use smaller cylinders and a higher boiler pressure to compensate but this is not an option available. In theory pressure release valves ought to work, they are fitted but they don’t appear to solve the vanishing cylinder head and bent motion problem. Bearing in mind the fact that you do not want your precious lubricant film degraded or even removed perhaps this whole area deserves more investigation.

There are books and papers which are helpful. The Advanced Steam Traction Trust is one source, Porta has written on the subject, so has David Wardale and there are others. Given that this work dates from many years ago why are we discussing this now?

 

Yes I do .All the wonderfull photo's taken in steam days of trains leaving stations ,particularly terminal stations, where the loco' would have been on the train for 20 mins or so,would'nt have been possible if the present day usage of cyliner cocks had been the norm'then.Loco' s would have been in a cloud of steam for some way after starting.In any case at a lot of stations, there were signs up saying .. Keep engines quiet ..That included cyinder cocks as well as saftey valves.

 

Presumably because further research might still be needed and/or lessons that were learnt are not being applied.

 

Because someone was curious and asked the question which is kinda the sort of thing forum discussions are for?

 

A lot of drivers are drain cock happy without good reason. Once a loco is warmed up there is no need to use them in the normal course of operation. I certainly don’t. There’s even less need with slide valves where the only benefit in using them is to stop water being ejected out of the chimney. Indeed, many small industrial locos don’t have cylinder drain cocks, only valve chest ones. The reason slide valve locos don’t need them is because the valve is pushed off the valve face if there is water present in the cylinder.

 

I watched the first departure from Bridgnorth on Sunday morning and wondered if the driver was ever going to shut the drains. But, the understandable caution may be stem from standing instructions issued by the management. Damage due to water entrapment is not easily fixed and potentially disastrous. We are dealing with museum pieces and I wouldn't criticise a driver for this.

 

All in all, it’s no wonder steam traction was superseded by electric and diesel traction!

As has been observed, ad infinitum; there are wider differences in skill of operators, than in steam locomotives.