Specific Steam Consumption is defined as the steam consumed by a locomotive's cylinders per unit output of power. It is typically measured in kg/kWh or kg/KJ.
A locomotive's Specific Steam Consumption carries important implications as may be deduced from one of Porta's favourite equations:
|=||Steam Production (kg/h)
Specific Steam Consumption (kg/kWh)
Thus for any given boiler output, a locomotive's power can be increased by reducing its specific steam consumption - in particular, by increasing its cylinder efficiency and reducing steam leakage. Or as Porta put it, "the power is limited by [the amount of steam supplied by] the boiler, while the function of the cylinders is to extract the maximum work from the steam supplied".
In Section 4 of his "Compounding" paper, Porta makes the observation:
"In steam locomotives, one should note that all the losses, except for incomplete expansion, are approximately constant for a given rotational speed. Hence, the aim is to have a longer cut-off but, given that this steeply increases the incomplete expansion losses, a compromise results at 20% to 30% (15% to 20% for the author's proposals). Thus, the claims for poppet valves concerning their ability to work with very short cut-offs are illusory as they do not lead to low specific steam consumption because of these constant losses.
But there are economic reasons too. The Americans, who have the perverse habit of hooking as many cars as possible to their locomotives, force them to work at long cut-offs to get as high an α coefficient as possible so as to have a good use of the (expensive) adhesion weight. This of course leads to a high specific steam consumption, hence the need for massive evaporation, hence a massive boiler, hence idle axles to support a huge firebox, hence a gigantic tender, hence plants to supply coal en-route, hence immense coal stocks, hence diesel locomotives with a higher thermal efficiency (under test conditions) even if they cost twice as much and justify the Gulf War to supply them with oil."
In the same paper, Porta also refers to Specific Steam Consumption in relation to the TE-Speed diagrams below, which appear under the heading "Boiler Size". He introduces the diagrams as follows:
"The operating variables of any locomotive working with the throttle full open can be defined, for a fully warmed up condition, by (any) two of them. For example: tractive effort vs. speed; steam production vs. speed; cut off vs. speed, etc. In Fig. 32A, for example, the constant cut-off lines have been plotted on a TE vs. Speed diagram. There is a line corresponding to the maximum cut-off, and various lines for the various running cut-offs. As a first approximation, they are straight lines whose inclination is greater, the greater the imperfection of the internal streamlining. In ordinary locomotives in which the internal streamlining is poor, the lines have an envelope: no combination of speed and cut-offs make it possible to invade the zone M (Fig. 32B).
In Fig. 32A, the lines corresponding to constant evaporation have been drawn (lines 3) and also the lines for constant specific steam consumption (lines 5). They show a zone (hatched) in which this consumption is minimal (zone 6) and also a zone (zone 7 cross-hatched) in which it decreases (very much in the case of single expansion engines) due to the increase of the incomplete expansion losses. There is also a zone in which the various constant losses (leakage, wall effects) increase specific steam consumption - this is important in the case of shunting engines. Obviously, the aim of the designer is to provide a maximum area covered with consumptions differing as little as possible from the optimum."
Figs. 32A and 32B: Characteristic Lines
Notes on Fig 32: In Fig. 32A, Straight lines (1) are constant cut-off lines, (2) being the one corresponding to full gear. The various hyperbola-like lines (3) correspond to constant evaporation. Selecting one of them, such as (4) allows the provision of a definite boiler size. The hatched area (R) corresponding to the overload concept.
Curves (5) refer to constant specific consumption, the hatched area (6) indicating the combination of speed-tractive effort in which the consumption is at a minimum. Area (7) refers to low speed, low tractive effort characteristic of shunting services.
So far, the above refers to engines designed with good internal streamlining (a RARE case indeed!). Fig 32B is the common case in which the cut-off lines are so much inclined that they have an envelope (8): this corresponds to the American concept of "capacity power"; no combination of speed and tractive effort allows getting into the M region. Within the envelope area, the specific steam consumption is very high: this explains the huge size of American boilers and tenders.
Wardale also refers to Drawbar Specific Steam Consumption in his book, defining it (on page 273) as:
|Drawbar Specific Steam Consumption|| =
||Indicated Specific Steam Consumption
|1 -||Power to move locomotive
He goes on to point out that:
"Drawbar Specific Steam Consumption is therefore influenced by the power required to move the locomotive and as the measured values of this parameter were thought to be too high, the drawbar Specific Steam Consumption data [for the Red Devil] was distorted, especially at higher especially at higher speeds and lower steaming rates. (From this equation, it can be readily seen that the drawbar Specific Steam Consumption of a locomotive which was not capable of generating high power relative to its weight, was bound to suffer at high speed, however good the indicated Specific Steam Consumption was - e.g. Duke of Gloucester."
In the Fundamental Design Calculations for the 5AT (see FDC 1.3), Wardale gives figures of minimum indicated Specific Steam Consumption for the Duke of Gloucester as 12.2 lb/hp-h and for the SNCF 141P Class 4-cyl. compound 2-8-2 as 11.2 lb/hp-hr, as compared to 11.24 lb/hp-h (= 5.1 kg/hp-hr or 1.9 kg/MJ) for the 5AT.