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The Poultney Locomotive: A New Type of Modern Design

Increased Power with a Minimum Coal and Water Consumption


The Poultney Locomotive

The Poultney Locomotive

THE modern methods of operating railways, coupled with increased working costs and altered labour conditions, have naturally created a demand for more powerful locomotives and greater economy in working. In an endeavour to meet these requirements a new type of locomotive has been designed by Mr. E. C. Poultney, OBE, associated with Mr. A. H. Calling and Mr. H. A. Akroyd of the Yorkshire Engine Co. Ltd, Sheffield.

The new type, which incorporates the articulated principle already familiar to readers, is specially designed for moderate speed, heavy freight service. The basis of the new design is the increased power for a given steam production, obtained by using the limited cut-off principle. It is claimed that with the adoption of the Poultney type of loco great economies are effected.

The two chief objects attained by the new type are increased power in existing locomotives, with the same coal and water consumption, and locomotives of greatly increased power over ordinary types with reduced coal and water consumption.

The underlying principle of locomotive design is that connecting weight and power. Most locomotives consist of an engine and tender and of the combined weight of these, only part of that of the engine portion is available for the purpose of traction, the amount varying according to the wheel arrangement. For engines having the 4-6-0, 2-6-2, 2-8-2 or 2-10-0 wheel arrangement, the amount (in terms of per cent, of the engine weight) maybe as high as 96% or as low as, say, 70%. When the total locomotive weight is considered, however, these ratios are much less, and become roughly its follows:- For the 2-6-0, 50%; 4-6-0, 40%; 2-8-2, 45%; 2-10-0, 55%. These figures though only approximate, are sufficient to indicate that generally the total weight necessary for a given tractive effort is quite large.

In ordinary types of locomotives any increase in power is usually only obtained by augmented adhesive weight, but this is not always permissible owing to restricted axle loads. There is no reason why the weight of the tender, or part of it, should not be used, however, for the tender offers a possible means of converting its dead weight into useful effort. In the Poultney system full advantage is taken of this opportunity by applying driving gear to the underframe of the tender. This results in two special advantages, which are that for a given axle loading on the engine the tractive effort of the locomotive can be increased; and that for a given tractive effort, axle loads, and hence bearing pressures, may be decreased.

Using part, or all, of the tender weight for driving wheel weight means that, of the total adhesive weight 60 to 70% will be on the engine portion, and 40 to 30% on the tender. A general study of many locomotive examples has brought out the fact that 70% of the total power can be developed by the engine and 30% by the tender, and of the entire locomotive weight, that available for traction may run about 70% of the total, instead of not more than from 55% to 60% for locomotives of normal construction. The advantage gained by utilising the tender weight in this manner is obviously very important.

Having thus briefly outlined the problems involved from the weight utilization standpoint, it is desirable to consider boiler capacity and engine characteristics. But first of all let us deal with the adhesive factor - that is the value of the product of tractive effort divided by the adhesive weight.

The Poultney LocomotiveOn the engine portion of the locomotive, the cylinder volume will either be 25% larger, or the steam pressure may be increased by a like amount. Alternatively, these two characteristics may be modified to produce an equal effect.

Owing to the locomotive being arranged on the limited cut-off principle, it has starting ports so arranged as to give momentarily an actual cut-off of 80%, the relative tractive efforts at the cylinders being 9.47 and 11.11 or, in other words, 17% greater. If, therefore, the ordinary locomotive has an adhesive factor of 4.5 then the Poultney Locomotive will have, on the engine portion, a factor of 4.5 x 0.83 = 3.74.

On the tender portion, the cylinders used are of a suitable size to make use of the steam saved by the curtailed cut-off. It is a remarkable fact that the increase in power by this method, which is gained without any extra consumption of steam, is from 20 to 30%, and, as is obvious, no alteration to the boiler is required.

There is no difficulty in obtaining a fairly high adhesive factor when calculating on a half-loaded condition. The adhesive factors on the engine in examples thus far worked out run between 3.6 and 3.8, and there is no reason why such should not be sufficient, as numerous examples of locomotives having adhesive factors of equal value are running successfully. Further it must be remembered that the 50% cut-off locomotive has a turning torque very similar to the three-cylinder engine with equal piston strokes, and connecting-rod lengths, and tins allows a lower adhesive factor to be used.

We must now say something about this 50% cut-off, one of the main features of the Poultney Locomotive.

As most of our readers know, the amount of steam supplied to a cylinder during each stroke of the piston varies according to the extent of the movement of the cylinder valves. This movement is controlled by a shifting link embodied in the valve gear, and can be regulated by moving a lever conveniently situated in the engine cab. “Cut-off” is a term applied to the act of shutting off the steam supply to the cylinders, and the instant of its operation is determined by different conditions of valve-setting.

The statement that an engine works at “50% cut-off” means that steam is allowed to pass from the boiler into the cylinder whilst the piston covers exactly half the distance between one end of the cylinder and the other. It is then shut off, but the steam imprisoned in the cylinder continues to expand and drives the piston forward to the end of its stroke. If the driver places his lever to the 25% cut-off position, steam is then admitted to the cylinder through one quarter only of the piston stroke, and so on.

In the Poultney Locomotive the maximum cut-off is fixed at about 50% of the stroke. By thus restricting the maximum cut-off, the driver is prevented from working the engine at uneconomical coal and water rates. A saving in steam is therefore effected over that required by an ordinary type of locomotive, and if this can be utilised there is obviously a potential gain in power.

In the matter of boiler power it is evident that the actual power obtainable from any boiler depends on the evaporation and on the use made of the steam generated. The evaporation per square foot of heating surface may be large, but the economic performance of the engines may be such that the power developed may be relatively low.

The idea underlying the 50% cut-off is the economic utilisation of the steam produced. The cylinders being larger - or, alternatively, the pressure higher - a given power can be produced at a relatively earlier cut-off. Thus by using the steam expansively, a less quantity is used per unit of power developed, and, therefore, a boiler of a given evaporative capacity can actually furnish more power at the engines. The accuracy of this statement has been well demonstrated in actual practice by the Pennsylvania 50% cut-off engines.

The Poultney Locomotive is thus based on principles already proved, because it is a 50% cut-off locomotive, the feature being that the steam economised in the cylinders of the engine portion is used in further cylinders on the tender. Any objections to the use of more than two cylinders have lost their force at the present time, as three and four-cylinder engines are becoming increasingly essential for the requirements of modern traffic conditions - indeed, in many instances, six cylinders have proved to be necessary.

The Poultney LocomotiveThe use on the engine portion of cylinders of such size that, combined with the available steam pressure, they can furnish sufficient power to absorb a given percentage of the adhesive weight and at the same time be operated at a maximum of 50% cut-off, effects a saving in steam due to the short cut-off, less the added volume due to the increased cylinder capacity, of 30%. The steam so economised is used in the further cylinders on the tender, the volume of which up to cut-off (50%) is equal to the volume of the steam saved by the. engine cylinders. In this manner a boiler of given capacity can furnish the additional power developed by the tender. The engine portion produces closely 70% of the locomotive power, and the tender the remaining 30%.

The combined heating surface of the boiler on the Poultney Locomotive is 3,718 sq ft, and as the grate is 65 sq ft, the ratio is about 57 to 1. Actually, the boiler factor (i.e., 59372 ÷ 3718) is 15.9, but when compared with a 90 per cent, cut-off locomotive using the same volume of steam per hour, this factor is 15.9 x 0.70 = 11.1 or, in other words, 30 per cent less. The figure 11.1 coincides with average practice in the design of high-powered freight locomotives, and thus, any question as to the adequacy of the steam supply for engines built on this system is refuted.

The general arrangement of this particular design is well shown, and it is certainly a very practical idea to place the heater equipment on the tender, as generally the heaters are fixed on the top of the tank so that the condensate may pass to the tank through a suitable filter. This is one of several minor but none-the-less important features that in the aggregate add very greatly to the efficiency of the loco.

Further to improve the performance of the Poultney Locomotive, the exhaust from the tender engines is employed to heat the feed-water, and the main engine exhaust produces the required draught. This is an advantage because the hot feed lowers the demand on the heating surfaces - that is to say it reduces the heat transfer necessary and consequently lowers the fuel rate and improves the boiler efficiency.

Assuming an ordinary locomotive to cut-off at 90% in full gear (which is the longest practical admission rate, and hence that at which the greatest tractive effort will be produced) and that the value 100 represents the cylinder capacity, then the following comparative conditions will be obtained when such an engine be compared with one built on the Poultney system. For the ordinary engine the cylinders are assumed to work at a maximum cut-off of 90%. Then if the piston area be designated by 100, the steam volume is 100 x 90 = 9,000. For 50% maximum cut-off of equal power the area is 125, or 25% more, the steam volume 125 x 50 = 6,250, and the steam saving 9,000 - 6,250 = 2,750. The figure 2,750 represents the steam volume - that is, the volume up to cut-oil for the tender cylinders, which are therefore 2,750/50 = 55 area.

From the above, the piston area of the Poultney engine is 125 + 55 = 180, and the percentage of work done on the tender is 30. The total volume to cut-off being the same, thus 100 x 90 = 9,000 and 180 x 50 = 9,000.

As the engine portion represents the same power as when working with the smaller cylinders at the long cut-off, the power developed by the tender is the theoretical increase in power obtainable with the Poultney system. Alternatively, instead of making the engine cylinders larger, an equal result may, of course, be obtained by increasing the boiler steam pressure by 25%.

The arrangement of the Poultney Locomotive is similar in all essentials to an ordinary engine and tender, and the arrange-ment is as follows:- The two, or more, cylinders on the engine have a maximum cut-off of 50% as have also those on the tender.

The exhaust from the engine portion cylinders is utilised for the blast, and that from the tender cylinders is employed for heating the feed-water, which is supplied to the boiler by a pump operate by superheated steam, when the locomotive is running.

Due to the above arrangement, the engines on the boiler portion are always proportioned so that they develop not less than 67 to 70% of the total locomotive power. When the steam pressure is increased, then of the total piston area designated by the value 164, 70% should be oh the boiler portion leaving 49.2 on the tender. The cylinder volumes must therefore be 1:2.31, the larger value being of course that representing the cylinders on the engine.

From the foregoing it will be seen that the following cylinder ratios obtain. Increasing the engine portion cylinders 25% means a cylinder ratio of 125:55 or 2.27:1. Increasing the boiler working pressure 25% means a cylinder ratio of 114.8:49.2 or 2.33:1.

We come now to the manner in which the Poultney loco combines the good points of both compound and simple types.

It will be evident from what has been said of the 50% cut-off locomotive that in its characteristic features it is similar to a compound haying 1:2 cylinder ratios. It also resembles the compound in that it should, when compared with an ordinary long cut-off engine, preferably carry a higher steam pressure. The reason for this is that at all times it uses the steam expansively - in fact, it cannot be operated at extravagant steam rates. In full gear the ratios of expansions are in the proportion of 2 and 1.11 and 1.25 for 50% cut-off, and 90% and 80% cut-off engines respectively.

In order to effect easy starting at all crank angles, a special piston valve has been designed. The valve is of the narrow ring type having two heads in which, on the steam side, are cut starting ports communicating with the steam ports. The relation that these ports bear to the main ports is such that actually steam is not cut-off before 80% of the piston stroke.

The starting ports in the valve are out of balance - that is to say, there is a greater port area through the valve at the front end in each direction of working. Thus when the locomotive is moving with the engine part leading, more supplementary steam is admitted at the front ports of the engine cylinders than at the back ports. Likewise, when the tender is leading, the front ports of the tender cylinders receive more starting steam. In this way quick and adequate starting is assured.

The starting ports are arranged to move over specially-formed bars in the annular ports, so that although the area of the former is considerable, their effectiveness rapidly diminishes as the valve moves to the point of cut-off. Therefore, an undue amount of steam is not admitted, and, as speed increases, the wire drawing action set up quickly discounts the auxiliary steam admitted, and the indicator cards show only a 50% cut-off in full gear operation. In practice the 80% cut-off effect is neutralised as soon as the engine speed has increased beyond about 14 revs, per minute.

IThe Poultney Locomotiven its characteristics the Poultney Locomotive is equal to a locomotive of the compound type but has the same freedom in running as the simple. This is made possible by the fact that the excessive back pressure present in the high-pressure cylinder of a compound is absent in the Poultney engine. More power is provided at all rates of admission than in the case of an ordinary locomotive and, by reason of having a power-driven tender, it runs equally well in either direction. The fact that the power is divided means lower axle loading for a given tractive effort, and this in turn means fewer coupled axles.

Compared with the usual forms of articulated locomotives, the pipe connections required for locomotives built on the Poultney system are very simple. There is required only one flexible joint subject to high pressure steam and none for exhaust steam.

The tender feed-piping is similar to that generally employed, with the difference that the feed from the heater installation is under pressure. Obviously there need be no difficulty in this, as the same arrangement as that used for the high pressure steam supply to the tender unit may be used.

As far as boiler feed piping is concerned, it is of a simpler character than that required for other well-known articulated designs, and freedom from unnecessary complications in this respect is of course very desirable.

As we mentioned at the beginning of this article, the principal object of the Poultney system is an increase of up to 30% in the power of an existing locomotive without increasing either the axle load or the size of the boiler. This object is achieved by the use of additional cylinders on the tender and taking advantage of the weight of the tender for adhesion. The steam demand is no more than that of an ordinary locomotive of 30% less power and therefore no larger boiler is required. To obtain this increase in power, however, the combined areas or volumes of the cylinders in the Poultney system are about 50% larger than those of an ordinary locomotive.

Although at first sight it might be supposed that the power gained by the increase in cylinder capacity might balance the steam saved by the shortened cut-off, this is not the case. An examination of the principle of the tractive effort formula based on mean effective pressure in the cylinders proves this. Cylinder clearance, pressure drop and back pressure will be ignored for simplicity, as they do not substantially affect the comparison. The constant C for the. M.E.P. varies as 2/E + 1 where E is the expansion ratio.

Working these values out for different ratios of cut-off we get:-

Cut-off 90% 85% 80% 75% 70% 60% 50%

M.E.P. .947 .919 .888 .858 .8235 .75.6666

To obtain a co-efficient for the tractive effort available at the rail or drawbar, some allowance for machinery friction must be made. This obviously varies with different types of engines, but the assumption of an average of 10% loss is customary. The above values then become:-

Cut-off 90% 85% 80% 75%

Co-efficient .852 .827 .799 .772

Cut-off 70% 60% 50%

Co-efficient .741 .675 .5999

It will be seen that the values for 90% and 50% are confirmed by American and Continental useage, where co-efficients of .85 and .6 are adopted, whilst those for 70 and 75% agree with the common British practice of using .75.

These conditions prevail at about 70 revolutions per minute, see accompanying diagrams which shows that an enormous

increase of tractive effort is claimed at starting. This presents something of a problem to the uninitiated, as if the cut-off is restricted to 50% the engine would not be able to start. The difficulty has been ingeniously overcome, however, as already described.

It is interesting to find that the restriction of the maximum cut-off to 50% does not cause uneven turning moment as has been suggested by critics. On the contrary, it gives more even turning, as an investigation of the tangential turning effort exerted by a 90% and 50% cut-off locomotive will show. In fact, the latter is practically equal to that of a three cylinder locomotive.

The blast from the engine cylinders is quite strong enough to make steam for both engine and tender, because the steam taken for the tender cylinders goes back to the boiler in the form of heat - where it raises practically an equal amount of steam - by means of the feed water heater. The exhaust from the engine cylinders alone is used for the blast and there is therefore an even heat, which is not the case with articulated engines in which the steam from all cylinders is directed up one chimney.

The objection to running ordinary locomotives tender first does not apply to the Poultney Locomotive as the tender is self-propelled and there is no tendency for it to be pushed off the road.

Incidentally it may be mentioned that the water level in the tank is very little disturbed when working on even the steepest grades.

No centre pivots are employed, the connection between the engine and tender units being of a type and design well established. In this, as in all other features of the Poultney Locomotive, experiments and fanciful ideas form no part in its constructive characteristics.

The “Poultney” locomotive is the most recent successful attempt to design a locomotive for heavy traffic purposes which, while conforming to the requirements of the British loading gauge and retaining the ability to negotiate comparatively sharp curves, possesses greater power than locomotives of the conventional type. Earlier efforts to solve this problem resulted in the production of the “Fairlie” and the “Garratt” locomotives.

The most powerful “Garratt” yet built attracted a great deal of attention when it appeared in the Railway Centenary Procession at Darlington in July 1925. This monster differs from the “Poultney” loco in being a “double-ender” in appearance and in reality, consisting virtually of two 2-8-0 locos each with cylinders 18½-in in diameter by 26-in stroke, supplied with steam generated by a boiler 7 ft in diameter. The pioneer “Garratt” was designed and built in 1909 for service on the Tasmanian Government Railways.

The original “Fairlie” articulated locomotive had as its outstanding features a central firebox with two barrels projecting longitudinally and two smoke boxes and chimneys,, while the tanks were located alongside the boiler. Later the design, was greatly modified and improved and the double-barrel boiler was abolished.

The Poultney Locomotive

The Poultney locomotive

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