Piston vs. Caprotti Valves 6

Piston vs. Caprotti Valves – The Final Discussion? Part 6

See previous page for Wardale’s responses to John Duncan’s Comments 21 to 24.
See next page for Wardale’s responses to John Duncan’s Comments 26 to 28.

Note: The full text of this correspondence can be downloaded in PDF format under the title:
Wardale responses to Caprotti proposals 3 Sept 2009.


John Duncan’s Comments # 25 – Quoting the following passages from Porta.

Ing. L.D. Porta – Piston Valves vs. Poppet Valves

Why did the Argentine Railways choose poppet valves instead of piston valves for their standard express passenger steam locomotives in the 1950’s?

Former Central Argentine Railway Company (Now Bartolome Mitre Railway) Class PS 11, 3 cylinders, 4-6-2. So successful was this locomotive design that upon the Nationalisation of the Argentine Railways, the Authorities decided to standardize the PS 11 class for express passenger service on three railways, the Roca, the Barolome Mitre, and San Martin Railways. The Vulcan Foundry in England supplied 50 additional locomotives with the new version of British Caprotti rotary cam poppet valve gear during 1950 to 1953. Gauge 5′ 6″, driving wheels 6′ 2½” diameter, cylinders (3) 19.5″ diameter x 26″ stroke, Boiler pressure 225 psi, tractive effort at 85% boiler pressure: 38,068 lbs. The PS 11s hauled a maximum of 750 tons and 500 ton trains at an average of 67.5 mph, running at 100mph on occasions. They were free running because of the poppet valves falling of their seats when the throttle was shut allowing bypass from both ends of the cylinders.

From:- Ing. L.D. Porta’s Paper, Fundamentals of the Porta Compound System for Steam Locomotives (first circulated privately in October 2000)

1. Introduction – 4th Paragraph: “When the Author started his locomotive design exercises, they were very much based on the famous ex-FCCA (Argentina) PS 11 single expansion, three cylinder Pacifics with Caprotti poppet valves, whose performance was much superior to the older PS 8: double heading was eliminated!”

2:1 Leakage, 3rd Paragraph: “Poppet valves are heavy offenders where leakage is concerned. The various claims against this statement have never been sustained by measurement or serious reasoning. Chapelon measured heavy leaks as reported in his book, (La Locomotive a Vapeur, Bailliere et Fils, Paris 1938.) The reason is that, except in the CAPROTTI gear, the valves seat on the cylinder block, itself subjected to widely different (and varying) temperatures, and hence distortions. This aspect is so important that Stumpf developed elastic seats, and the corresponding theory for them. (STUMPF, J. Die Gleischstrom – Dampfmaschine, Dritte Auflage, Druck von R. Oldembourge, Munchen 1922).

Quotation from Ing. L.D. Porta’s paper on; “Fundamentals of the Porta Compound System for Steam locomotives“, included in “Advanced Steam locomotive Development”, one of three technical papers by Ing. L.D. Porta, published by Camden Miniature Steam Services.


This appendix has been written with the Author himself as addressee. Therefore, the intention is not to present proof for any aspect which is itself deserving of a thorough discussion elsewhere.

The controversy is between poppet valves incorporating a number of proposed improvements, and the Author’s piston valve design.

It is claimed that poppet valves can provide a fine adjustment for the valve events: this potential advantage cannot be made actual if the decision for the optimum valve events is poorly defined.

Poppet valves do leak considerably, even if in stationary tests no steam appears at the cylinder cocks. For example, DABEG valves sit against the cylinder block, but the latter is subjected to considerable distortions under the effect of superheated steam. Stumpf realised this and developed elastic seats.  CHAPELON measured considerable leakage for the poppet valves, far more than the author measured for his piston valves.

The valves are very expensive to manufacture from a solid special steel forging and cannot be repaired in the event of breakages. These come about because the contacting velocity at the moment of closure must be limited. A significant improvement is brought about by duplicating the number so as to have smaller valves (FRANKLIN).

The mechanism for valves such as CAPROTTI, FRANKLIN, COSSART, RC and others is precision made, hence expensive to manufacture, and is not immune to wear. When this occurs, the precision is lost, and so also is the initial valve setting, with an uncertain loss of performance – the Author’s piston valves, on the other hand, are guaranteed to give full power and economy from shopping to shopping.  In the case of poppet valves, expensive spare parts need to be considered, as is the case with diesel engines. All in all, poppet valves, even if improved, ARE NOT HEAVY DUTY EQUIPMENT.

Wardale’s response: Quoted passages from Porta.

  1. Why did the Argentine Railways choose poppet valves instead of piston valves for their standard express passenger steam locomotives in the 1950’s? Well, they didn’t, really, the PS11 was an already existing class that was simply duplicated. It could well have been that their Caprotti gear was indeed found better than the piston valves driven by Walschaerts gear on plain bearings, each one with an individual oil cup, that they were then using, which has no relevance to a comparison with Porta type valves as subsequently developed, driven by Walschaerts gear on roller bearings

    The standard locomotives they purchased after the war for the Belgrano Railway, for example, including passenger 4-6-2’s, all had piston valves. If quantity were the criterion, piston valves would easily be the winner. And I do question the accuracy of the clip you quote from: for example, the Roca Railway used a different type of 4-6-2, the 12(E?) Class with piston valves, by memory (Atkins writes of the PS11’s on page 89 of ‘Dropping the Fire’, “many of these evidently saw little service on those lines previously unfamiliar with poppet valve gear.”) And the possibility of trains travelling at 100 mph on Argentine track without leaving the rails is positively unbelievable, and I am more than skeptical that it ever happened. I made an extensive tour of the Argentine railways in 1973, when steam was still much in evidence, and no PS 11s were to be seen.

  2. That the Caprotti design may have overcome the leakage problem has been accepted, point (18) earlier.
  3. The quote from Porta, “Poppet vs. Piston Valves” is not an advert for the former. Rather the reverse, as you would expect!

See previous page for Wardale’s responses to John Duncan’s Comments 21 to 24.
See next page for Wardale’s responses to John Duncan’s Comments 26 to 28.