P2 Locomotive Company and related matters

Fascinating that you are persevering with the Lentz OC valve gear and the Franklin Patents derivative of the same.
The fact that the construction of this locomotive can continue for some time before a decision has to be taken with respect to the valve gear type to be used is very much a bonus.

Lentz gear never delivered long term satisfaction here in the UK. The high leakage experienced was one of the main reasons for the abandonment of the gear by Chapelon. This was before the invention of elastic valve seats. So at least these allow you to cross one major hurdle.

It proved very difficult to achieve the clearance volumes that the LNER required. Large clearance volumes are not so much of a problem if you are designing a compound. So will you be able to significantly improve on the original, as modified, figures?

The problem faced in achieving a finely adjustable cut-off is another matter.

The LNE did some very good work on BC gear. So if you have to switch to this it would not be a cause for undue concern.

It will be very interesting indeed to follow the thinking and the decisions taken on the valve gear. The rest of the locomotive could be seen as the "easy bit".

 

As you say several of the problems found at the time can be overcome with modern materials and design methods. We believe the problems with the original infinitely adjustable cut-off are solvable and the issue of clearance volumes has been looked at and our engineer is happy we can reduce them to an acceptable amount for the new locomotive.

Lots of work has gone into looking at other locos away from the P2 both in the UK and abroad to help in the design and development to get us to a finished product.

At present the work we are doing allows us to defer a decision on the valve gear until next year, however we are slowly getting there with the Lentz and my personal opinion is we will solve it and fit it.

Andy

 

It wouldnt surprise me if - once a rolling chassis is in place - attention is switched to the boiler. so theres plenty of time to come to a decision on, and then develop,the design of the cylinders and associated motion

 

242A1, I have only just noticed after re-reading this that you mentioned the Lentz OC gear, we are looking at the RC gear as fitted to the original locos. Franklin also made both OC and RC versions.

 

The principal claimed virtue of British Caprotti is that you can have the admission cut-off as short as you like without restricting the exhaust. Does the same apply to Lenz, oscillating or rotary?

On the subject of infinite variability versus stepped: although ideally you have the regulator wide open and adjust the cut-off to get the amount of power you want, how much does it really matter if you have to choose a step that's a bit longer cut-off than you want and compensate by slightly closing the regulator?

And one more question! If British Caprotti can achieve infinite variability without point contact and consequent high wear, why is it difficult to do the same with Lenz?

 

Your last question is exactly what we are looking at. It is possible with materials technology improvements the point contact may not be a problem with lesser wear envisaged.

According to contemporary details I have, with the stepped cams the drivers found that in position x the engines would want to be let in/out just a fraction more to get the best performance, however by adjusting to position w or y the engines would romp away rather than the fine control the driver wanted. Trying to drive on the regulator was not ideal as the engine was not designed like this and it was often again not possible top get the fine control. Likewise the engines steamed best with the regulator in one position.

I know from my time on the mainline that drivers on Tornado (and other locos) often adjust the reverser by very small amounts when they know the road whilst leaving the regulator well alone and this I am certain is how the P2's should have been.

 

Sure, but my point was that (if I understand correctly) Caprotti achieves infinite variability without needing to restrict contact to a small area, so the materials aren't so critical. But I acknowledge that, as usual in engineering, there will be pros and cons to every option, including in this case the non-trivial aspect of fitting it all into the available space.

 

The space is one of the issues in this arrangement. I confess to not being an absolute expert on all of this. I'm a researcher that is learning it all as I go. As I understand it (although I could be wrong here) the BC system is a little more fragile than we would like, correctly working and developed Lentz would be the better option for want we aim to achieve.

Andy

 

My typo, I do apologise. I did intend to go for RC. I understand what your concerns are re the BC gear. In truth I am with LDP on this matter - poppet valve gears are not robust enough for railway service and offer no advantages over well designed piston valves.

So, good luck on this one. You might need it.

 

Caprotti gear uses two cams per inlet valve, each with a constant profile giving lift over maybe 40% of a complete revolution. There's a rocker lever between the two of them, so the valve is only lifted if both the cams are lifting. By changing the phase angle of the cams relative to each other, you can get anything from 0% to 80% admission for a given power stroke. Numbers illustrative only. Anyway, because the cam profile is constant, it's nice and easy to keep contact stresses manageable. The complexity is in the cam phasing gear, although I suppose the rocker could be a weak point too.

From what I can gather, Lenz gear and its offspring (like Franklin gear) have a single cam per valve, with the profile varying along the length. If you want to change the cutoff, you move the cam along its axis and bring a different profile under the cam follower. If you want lots of steps, either each step is going to be very narrow (meaning high contact stresses), or the cam is going to be very long. For an infinitely variable cam, when the valve is closing the profile of the cam will be sloped both in the direction of rotation and along the length of the cam, thus putting a lateral load on the follower.

High hardness/low friction surface treatments and exotic high strength steels are both areas that have made considerable progress since 2001 was outshopped, so perhaps the Lenz gear can be persuaded to behave properly this time round.

 

That's the gist of what I wanted to write but couldn't put into words. Well done that man.

 

Thank you 8126. You have most kindly supplied enough information with respect to the two main players in this valve gear saga to give forum members an insight into the problems to be faced. The point contact on the variable profile cam of the Lentz gear was the insurmountable problem faced by the LNE.

The BC gear suffers from "quart in a pint pot" syndrome in that it is difficult to make the gear sufficiently robust within the confines of the space available. And for the Lentz gear to have manageable stress and wear you end up with a rather long cam. The OC form of the gear opens the valve swiftly and fully and then maintains the opening for the period required. And you drive it with conventional valve gear. The cam does not have a variable profile.

My biggest concern is having a large enough steam chest volume. Leakage has been dealt with, clearance volumes can be addressed, there should be no concern about the basic valve setting. Getting a form of gear to function reliably offering the equivalent port area opening of a modern pv fitted design and maintaining inlet steam pressure throughout the admission period, well I just hope that this can be achieved. Could turn out to be quite a financial millstone though.

 

It may be worth adding that I understand that BC gear, as per DoG, doesn't suit the P2 due to differences in the disposition of the inside cylinder.

 

I have never seen details of how Lenz valve gear works, so the descriptions above were very interesting. If I have understood it correctly the problem with a continuously variable implementation is the need for the cam follower to move in two axes over the cam, therefore requiring a very small contact point?

I wonder if the modern materials being considered would include ceramics? I seem to remember that the Wankel engine concept only became viable when ceramic rotor tips were developed for it.

 

PS I meant to add that the images of 2007, for example on the articulated trailer, are fantastic, so my admiration goes out to Andy for his work.

 

The Wankel is a clever and innovative design which exists in a world where the critical mass is with reciprocating engines, so will never be a mainstream option. For me the problem is that it has to operate at quite high revs to get the best out of it, although being very well balanced that should not really be a problem.

I have no idea of the reliability of the Mazda rotaries vs conventional ic engines, or how many failures are due to the rotor tips. I was just pointing out that the use of ceramics in this component made the design possible due to the hardness of the material.

 

One would like to know what the key features of the Reidinger infinitely variable Lenz gear (RC?) fitted to the Crabs in 1953 are... anybody ?
Only missed out on the standards by virtue of the fact that the BC Gear had already been tested as road worthy on a couple of Black 5's .
Given the somewhat bumpy environment on board a steam loco and hard usually equating to Brittle the material challenge is considerable, there are still people in Japan who can make steel things with extremely hard surfaces at one with a softer core... (Samurai swords) but i suspect these are a bit pricey...

 

Quite so Mr Class8Mikado, a Katana would also be a wee bit too sharp for this application. However, Nitriding is a process that has been around for many years that creates a case hardened surface on components such as crankshafts, cams, followers and gears, there are probably quite a few such pieces in your car. There are other hardening processes that also may be applicable.

It seems that one of the main stumbling blocks with the fitment of Poppet Valves to a locomotive in the UK could be lack of space and the design compromises that this would create. The Pennsylvania Railroad T1 class were fitted with Franklin Valve Gear for poppet valve operation, but even these locos, with potentially plenty of room for adequate construction, were not immune to valve gear failures; this was probably due to the excessive speeds that the locos were sometimes driven, but no loco can put up with being thrashed, no matter how well designed and built. The picture attached is a of T1 with the cylinders exposed for testing.

It is interesting to note that there is a scheme in the US to build a replica of a T1, perhaps some communication with our friends over the water may provide some answers to the valve operation question for 2007.

Cheers

Alan