Third Generation Steam for North America
A 6000hp Triple Expansion Compound 2-10-0

This 2-10-0 design dates from the 1970s first being given a public airing at the 14th Pan American Railway Congress at Lima, Peru, in November 1978. It was drawn out to demonstrate what Porta felt was possible within the North American AAR loading gauge 'Plate C' based on then current knowledge, predictions and requirements. However it should be noted this machine is designated as Third Generation steam, development work would have been required to build it in the 1970s, as would still be the case today. Much of the technology embodied in this 2-10-0 has only been trialled, not used in regular service whilst other parts of the design break new ground - hence the need for development. Some of the technology shown in this locomotive is taken from then current coal fired power station technology and suitably adapted for locomotive use. It should be noted that, based on Porta's definition, Second Generation steam is capable of being built today with certainty in the outcome, no new development work is required. However Third Generation steam is, as the name suggests, a step chance from Second Generation, more akin to a paradigm shift than a simple uprating of the technology. It is worth repeating here Porta's definition of "steam generations":

6000hp Triple Expansion Compound 2-10-0

This locomotive has been referred to previously, notably by David Wardale in his book 'The Red Devil and Other Tales from the Age of Steam', however some of the material presented below has not been published previously. This locomotive is a development of a similar 6000hp 2-10-0, but second generation locomotive, designed by Porta during his time as the Head of Thermodynamics at the Argentine state technology institute, INTI, in 1970s.

The Loading Gauge

The above illustration shows the AAR Plate 'C' loading gauge this locomotive is designed for. To allow comparison on the right is the UIC "Berne" gauge which is (to be very imprecise) the common loading gauge used in mainland Europe. Click here or on the image above to open a larger version of the illustration. (33Kb)

Statistics

Boiler Pressure 60 bar (881psi)
Boiler Barrel Length 3000mm
Boiler Barrel Diameter 1600mm

Cylinder Details: Triple Expansion Compound
High Pressure Cylinder 477mm x 736.6mm (left outside)
Medium Pressure Cylinder 686mm x 736.6mm (middle)
Low Pressure Cylinder 1024mm x 736.6mm (right outside)
Maximum Piston Thrust 50.740 Kgf
Valvegear Southern*

Booster Details: Triple Expansion Compound, Lewty Type
High Pressure Cylinder 138mm x 250mm
Medium Pressure Cylinder 207mm x 250mm
Low Pressure Cylinder 309mm x 250mm
Maximum RPM 1500

Dimensions
Driving Wheel Diameter 1524mm (5')
Driving Wheel Axleload 26.4 tonnes (each)
Guiding Wheels Axleload 14.4 tonnes
Maximum height from the rail 4928mm (16'2")
Length over couplings 24280mm
Maximum Design Speed 90mph

Capacities
Fuel Capacity Sufficient for 5 hours at 6000dbhp output
Water Capacity (tender) Sufficient for 5 hours at 6000dbhp output
Water Capacity (engine) 18m3 (ballast weight and emergency supply)

Power
Continuous Rated Output 6000hp
Peak Output 7500hp
Maximum Tractive Effort 60000Kg (130000lb) (with booster cut in)

*Southern Valvegear is selected for Porta type compounds as it is a simple, heavyduty gear, it is easy to adapt to give solid, reliable transmission from the outside motion to the inside, it allows the cut off ratio between the HP, MP and LP cylinder to be easily set up and the link does not wear on each wheel revolution.

General Arrangement

General arrangement of the triple expansion compound for US service. Click to view a larger version (157kb, 3137 x 1000 pixels)
General arrangement of the triple expansion compound for US service. Click to view a larger version (157kb, 3137 x 1000 pixels)

A head on view of the 2-10-0 showing very clearly how the triple expansion is laid out. Also note how the booster engine is also triple expansion.
A head on view of the 2-10-0 showing very clearly how the triple expansion is laid out. Also note how the booster engine is also triple expansion.

Combustion System

The grate is of the V-anti-clinker, grinding type. Connected to the motion of the loco the grate moves very slightly at all times the loco is underway, proportional to the cut off. The consequence of this action is to ensure any clinkers forming, however unlikely this may be given the use of the Gas Producer Combustion System (especially the cyclonic variant), are broken up but as importantly the fire is kept ash free.

This head-on view takes a section through the rear of the firebox. It clearly shows the V-clinkering grate proposed (as trialled at Rio Turbio in the 1960s) and the extensive use of combustion air preheaters. Also note the ash screws designed to remove ash from the ashpan keeping them clear and thus ensuring optimum performance of the Cyclonic Gas Producer furnace.
This head-on view takes a section through the rear of the firebox. It clearly shows the V-clinkering grate proposed (as trialled at Rio Turbio in the 1960s) and the extensive use of combustion air preheaters. Also note the ash screws designed to remove ash from the ashpan keeping them clear and thus ensuring optimum performance of the Cyclonic Gas Producer furnace.

To ensure the best combustion but also to aid the design in general the ashpan under the grate is very small. In fact ash screws are employed to remove ash to the 'real' ashpan situated between the bogies of the tender. In essence the ash screws are just like mechanical stoker screws operating in the reverse direction.

Fuel would be delivered to the grate mechanically with the final distribution likely to be a version of the Elvin or Patadon type rather than the traditional table plate and steam jet system. The reason for this being that these types of stoker head will give the best coal distribution to the grate, far better than more traditional systems using steam jets, whilst also assisting in the creation of the correct 'wash bowl' shaped fire.

This view of half of the grate area is looking down on it from above the cyclone 'igloo'. The cyclone throat is the opening at the top of the 'igloo' through which the combustion gases exit into the top of the firebox and thus the boiler tubes.
This view of half of the grate area is looking down on it from above the cyclone 'igloo'. The cyclone throat is the opening at the top of the 'igloo' through which the combustion gases exit into the top of the firebox and thus the boiler tubes.
This view of half of the grate area is looking down on it from above the cyclone 'igloo'. The cyclone throat is the opening at the top of the 'igloo' through which the combustion gases exit into the top of the firebox and thus the boiler tubes.

Click here for a view of the cyclonic gas producer furnace arrangement:

Click here for a view of the cyclonic gas producer furnace arrangement

Included, as is normal for Porta designs, is full combustion air preheating. This pre-heating uses exhaust steam to heat all air used for combustion to high temperatures before it is used as either primary or secondary air. When considering why this is an important improvement to overall efficiency it is worth remembering that air is approximately 79% nitrogen. As nitrogen plays no part in the combustion process any heat applied to it by combustion is wasted. Reducing the amount of heating to take place can significantly improve efficiency. As can be seen above four air preheaters are specified which work in series. The first receives exhaust steam from the low pressure cylinder whilst the others use exhaust steam from the high pressure cylinder.

To assist with overall efficiency and to make a useful contribution to the reduction of standby losses the ashpan dampers would be linked to the throttle, opening automatically only when required. Porta felt it was possible to reduce fuel consumption when on standby to just 10kg per hour - in traditional terms these would be just a couple of shovels of coal.

The Boiler

The boiler pressure is at 60atm (882psi). The boiler design is firetube but the firebox is based on the Winterthur type dating from 1926. The stay design proposed copied the Winterthur design but with updates in line with the latest Tross-Henschel recommendations.

The type of firebox proposed for this locomotive is based very much on the Winterthur type of 1926.
The type of firebox proposed for this locomotive is based very much on the Winterthur type of 1926.

The boiler barrel would be substantially equipped with flue tubes to accommodate the required number of superheater and re-superheater elements. However other than being designed for 60atm pressure the barrel, all welded of course, would be much like a traditional boiler. As can be seen from the general arrangement diagram the front section of the firebox crown is raised above the rest of the crown. This was first implemented by Porta in 1949. It allows an extra set of boiler tubes to be incorporated into the barrel without greatly reducing the steam space volume above the crown, a fact little appreciated or understood to this day but essential to both second and third generation steam locomotive boilers.

Maximum steam temperature (to the high pressure cylinder) would be 550°C. To aid the production of the highest degree of superheat at all times a superheater booster would be fitted. This device, as used on SAR No. 3450, the Red Devil, is a damper fitted ahead of the front tubeplate. It opens to allow gases to flow through the lower portion of tubes only under the effect of a high smokebox vacuum, in other words when the locomotive is working hard. When working lightly the damper would be closed ensuring the maximum amount of combustion gas would flow over the superheater elements.

The stay design for the 60atm boiler. Porta description of this drawing was: The shaft 1 takes the shape of a tube at 2 31/38mm welded according to the latest TROSS-HENSCHEL system.

The stay design for the 60atm boiler. Porta description of this drawing was:

"The shaft 1 takes the shape of a tube at 2 31/38mm welded according to the latest TROSS-HENSCHEL system."

Naturally for any advanced design feedwater heating is included as standard. The aim would be to raise the feedwater to the highest feasible temperature before injection into the boiler. To this end 4 closed type feedwater heaters are provided as the first stage of this pre-heating. These feedwater heaters are connected in series with the first using exhaust steam from the low pressure cylinder whilst the remaining feedwater heaters use a fraction of the exhaust steam from the high pressure cylinder. From the feedwater heaters the water would be directed to an economiser positioned in the smokebox ahead of the front tubeplate. This economiser, fitted with something like the Serve type of ribbed tube, would provide the final stage of water heating. By positioning the economiser ahead of the front tubeplate heat that would otherwise be wasted at the chimney is put to good use.

As can be guessed 'perfect' water treatment, allowing 6-12 months between washing out and internal inspection, would be a must. Despite the high steam temperatures and pressures Porta was certain he knew how to adapt his internal water treatment regime, Porta Treatment, to suit.

The entire boiler, firebox and related auxiliaries would be very heavily insulated to reduce standby losses to as close to zero as possible. Good enough would not be good enough, only the very best would do.

The Engines

The locomotive is a triple expansion compound of the Porta type. That is superheated steam, at 550°C, is expanded in the high pressure cylinder (477mm x 736.6mm) situated on the outside left of the loco. This cylinder operates as a back pressure engine. The exhaust steam is routed via a re-superheater aimed at heating the steam to above the theoretical optimum prior to being expanded in the medium pressure cylinder (686mm x 736.6mm) situated between the frames of the loco. The medium pressure cylinder, also operating as a back pressure engine, exhausts again into a re-superheater which is also designed to superheat the steam above the theoretical optimum. As can be imagined with a bank of live steam superheaters and two banks of re-superheaters the bulk of the boiler barrel would be taken up with flue tubes. The re-superheated steam from the medium pressure cylinder is now fed to the final cylinder, this being the low pressure cylinder (1024mm x 736.6mm) situated on the outside right. This cylinder is equipped with twin piston valves to ensure smooth and steady flow coefficients, low pressure drop and back pressure, all as good as practically possible.

The design ensures equality of work done by each cylinder. In other words the maximum piston thrust of 50.740 Kgf is possible from each cylinder. When operating at the maximum continuous rated power each cylinder is contributing 2000hp to the overall figure.

Clearance volumes would be kept as low as possible. Porta quoted the target figure is in the range of 9-10%.

Naturally being a three cylinder locomotive a crank axle is used. In this case it is the third driven axle. Porta had designed a crank axle more than capable of absorbing the powers produced by the centre cylinder which was fully in line with AAR recommendations. As he was keen to point out the internal combustion engine features a considerably more complex crankshaft arrangements, which are infinitely more reliable than the cranks of first generation steam locos, so why should it not be possible to replicate the performance of internal combustion cranks on modern steam locomotives?

Given the steam temperatures aimed at very advanced tribology (the science and technology of friction, lubrication and wear) is necessary. This could, for example, include the use of synthetic lubricants. Porta type, multi-ring, lightweight and articulated piston valves would be used. These valves are proven to give very low steam leakage even when approaching replacement. Further improvements were proposed.

It should also be stated that the entire steam circuit, including the cylinders, would be very heavily insulated - exaggerated insulation - so as to both minimise heat loss and reduce condensation within the system. Additionally pre-heating of the cylinders via steam jacketing is used in a similar way to Chapelon's 160A1 but in a fabricated rather than cast manner. These, combined with the overall cylinder design, would reduce wall effects to a negligible amount.

Insulation internal to the cylinders would also be used to lessen the effect of heat loss through conduction - for example across a piston head, that is from the live steam to exhaust side.

It is not explicitly stated in any of the available literature but it is assumed the design would also include counter pressure braking.

For more information in general on the Porta type of compounding and much of the thinking behind it readers are advised to consult this book.

The Lewty Booster Engine

As for the main set of engines on the locomotive the booster is a triple expansion type. It is unclear from the available information if there was resuperheating between the stages of expansion but is it suspected there was not. As can be seen from the drawings the high pressure cylinder (138mm x 250mm) and the medium pressure cylinder (207mm x 250mm) are both fed steam from single piston valves whilst the low pressure cylinder (309mm x 250mm) is fed via a pair of piston valves.

Maximum drawbar tractive effort, with the booster cut in, was quoted as 60000kg (130000lb). Quite a figure. However it is worth remembering that horsepower, not tractive effort, pulls trains.

Exhaust Ejector

As drawn the exhaust system is labeled at 'Double Kilpor'. It is believed 'Kilpor' is a bit of a 'Portaism'. Instead of using the correct spelling, Kylpor, the misspelling was done on purpose so as to underline how to correctly pronounce Kylpor.

It is perhaps interesting that given the date of the design that the Kylpor rather than the Lempor was specified. When initially drawn out a Lempor desinged from scratch, based on the 1969-74 development work on FCGB No.1802, had yet to be completed. So the Kylpor was specified. Shortly afterwards such a Lempor design was worked out for SAR 19D No.2644 so it can be assumed that if this locomoive had been built in the late 1970s/early 1980s the exhaust ejector would have been a Lempor. Today it is likely the locomotive would be built with a Lemprex exhaust ejector, the development of which started in the late 20th century but has yet to be completed. Either way Porta is on record as stating that 'even better' ejector performance would be necessary, in other words nothing is the end point of development.

It should be obvious, but it is none the less worth stating, that this machine is designed to exhaust to atmosphere. It was no condensing machine. Porta is on record as thinking that whilst very effective condensing at below atmospheric pressure was feasible for railway locomotives he was unconvinced by the economics of it. Would the complexity of condensing coupled to cheap fuel (biomass) make the extra 4-5% overall locomotive efficiency be economically justifiable?

The Chassis

This 2-10-0 design has a maximum axleload of 26.4 tonnes. The driven wheel diameter of 1524mm (5') gives a design speed of 90mph based on the AAR 504rpm standard.

Maximum use of all adhesions aids is made. So in addition to the high adhesion tyre profile the loco would have rail cleaning jets using live steam to clean the rails in addition to sanding gear alongside other devices designed to assist adhesion.

To assist with guidance of the locomotive the outer end driven axles are linked to the next axle in by Beuginot levers. These act on the axleboxes of these axles steering them into corners.

Other features of the chassis would be fairly standard, for a Porta design - rugged bar frames, very substantially mounted horns, the use of roller bearings at every point, full mechanical lubrication meaning there would be zero lubrication to be attended to by the loco crew, all reciprocating parts would be lightweight, axlebox wedges would be self adjusting etc etc.

It can be seen, as is so typical of Porta machines, that the boiler looks small in comparison to historic locos. The small size and thus weight dictates the use of a chassis mounted water tank as ballast but also as an emergency supply of water. The high pitched boiler would make access to the inside motion much easier than on historic machines. Overall the design gives much better accessibility for maintenance than the best of previous US steam coupled, of course, to much greater individual component and general reliability. This reliability would ensure maintenance costs would be just a fraction of the figure for a diesel of equivalent output.

The Tender

Fuel, whatever that may be but today it can be assumed to be biomass, held in the tender would be sufficient for 5 hours operation at the maximum rated output. Water capacity would be the same. The tender would also be set up with a considerable area set aside as a 'hotwell' receiving exhaust steam condensate from auxiliaries. The effect being a further preheating effect on feedwater. Additional water, 18m3, would be available in emergencies from a tank mounted alongside the boiler.

Situated between the bogies, drawn as three axle bogies, is the true ashpan. This ashpan, fitted with a simple dump mechanism, is fed with ash from under the grate by the ash conveyer screws.

The overall guiding design principle behind the tender is to minimise the weight leaving the maximum energy available for hauling revenue earning traffic.

Auxiliaries

All auxiliaries would, just like the locomotive, be designed or modified to give the highest efficiency levels possible. This would include providing superheated steam where appropriate. It is very likely two manifolds would be provided - a saturated steam manifold for items such as whistles and a superheated steam manifold feeding items such as air brake compressors. With the throttle valve being situated above the steam chest of the high pressure cylinder, as per marine practice, arranging a supply of superheated auxiliary steam would be fairly straightforward. For safety reasons the manifolds would not be located within the cab as has been traditional. As few controls as possible would actually have valves etc in the cab. However the cab is likely to be equipped with a considerable number of gauges and other performance monitoring equipment.

The Cab

Crew conditions on this locomotive would be in-line with the very best of diesel or electric locomotives. The very heavy insulated boiler would mean heat radiation into the cab would be virtually eliminated. Cab air conditioning and heating would be provided as would other facilities for the crew such as a stove and a toilet. The actual jobs of firing and driving would be accomplished from a sitting position, all controls being within easy reach. Foot pedals for sanding etc would also be made use of. A full ergonomic assessment of the control positions would be undertaken as part of the detail design process.

Performance

Performance curves for the proposed 2-10-0.
Performance curves for the proposed 2-10-0.

Thermal efficiency would be 21% when fully warmed up. The rated (continuous) drawbar horsepower for the design is 6000hp which would allow it to replace in service a diesel locomotive with an engine rating of 9000hp. Peak output from the 2-10-0 is calculated to be at least 7500dbhp.

When writing about this design in 1987 Porta commented: "As is seen, the designer has been able to keep the design within a traditional configuration whose success supported the test of time. Why should it be changed?"

At the same time Porta commented that the design would now require further development work to meet ever tightening emission regulations despite it being specified to have a Cyclonic Gas Producer Combustion System. Thus it can be inferred Porta was referring to exhaust scrubbing systems such as particulate capture in addition to the application of ALL applicable technology to improve combustion efficiency. It should be remembered it is always better to deal with a problem at source than try to clean up the results of the problem elsewhere.

References:

Porta, L.D.: Steam Locomotive Development in Argentina - Its Contribution to the Future of Railway Technology in the Under-Developed Countries. 1969
Porta, L.D.: The Contribution of a New Steam Motive Power to an Oilless World. 1987
Porta, L.D.: Steam Locomotive Power: Advances made During the last Thirty Years. The Future. 1990
Porta, L.D.: Fundamentals of the Porta Compound System for Steam Locomotives. 2000 (Also see here.)
Porta, L.D.: Steam Locomotive Power for the XXIst Century. 2001.

Wardale, D.: The Red Devil and Other Tales from the Age of Steam. 1998
Various e-mails & meetings between the webmaster and McMahon, S.T. 2006
-2007

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