Here the curves for the SW900. This was somewhat more powerful than the SW8, and I think that it also had higher capacity motors and larger traction motor blowers. As may be seen, the SW900 was viewed as being self-protecting, so that MCS and CTE numbers were not quoted.

EMD SW900 Curves.gif

Very roughly, the 57 000 lbf point on the SW900 tractive effort curve corresponds to a speed of 3 mile/h. Reducing the power by 11% (corresponding to the difference between the SW900 and the SW8) would nominally result in an 11% speed reduction for the same tractive effort, say from 3 to 2.7 mile/h. On that basis, 57 000 lbf at 2.5 mile/h looks to be not unreasonable for the DH2, at least within whatever precision one might reasonably assign to these very rough back-of-the-envelope calculations imply.

If we assume that the DH2 transmission was specially designed for it (albeit using established Allison precepts), then the designers were free to size the torque converter and its cooling system to meet the mission requirements. On the question of MCS and CTE, perhaps it was thought that there was no real advantage in having a CTE larger than that which typical adhesion conditions allowed, and so they settled upon the “25%” number as being a reasonable target. And on the curve, that corresponded to 2.5 mile/h, quite reasonable for the MCS.

Returning to the SW900, 57 000 lbf at 3 mile/h corresponds to 456 hp, well down on the 900 hp available at the main generator input. Whilst transmission efficiency is lower at low speeds where traction motor currents are higher, and so consequently are ohmic losses, in part , perhaps in major part, that difference is attributable to the fact that at the bottom of the speed range, the transmission cannot fully load the engine, so the utilization factor is low. Looked at another way, at zero speed with full tractive effort the power is zero. So the power curve ramps up from zero to maximum in the lower speed range, usually with a turnover point below about 10 mile/h, and probably at the low end of the range for switching locomotives.

In the DH2 case, 57 000 lbf at 2.5 mile/h corresponds to 380 hp, again well down on the 800 hp available. Here a reasonable proportion of that difference would be attributable to power lost to heating the torque converter fluid. Unavoidably, at very low speeds, hydraulic transmissions must operate on the low efficiency part of the torque converter (or 1st torque converter as appropriate) curve. Here are a couple of examples that illustrate that point.

Voith L36r Curves.gif

SCG Torque Converter Transmission Curves.gif

The SCG transmission comprised a single-stage, two-phase torque converter-coupling and a four-speed Wilson planetary gearbox. Note that torque converter efficiency in 1st gear had dropped to 50% at around 2 mile/h. And the tractive effort curve leans to the left of the parabola below 5 mile/h.

The Voith L36r transmission was of the classic Voith triple converter form. 1st converter efficiency has dropped to 50% at around 10% of maximum speed. By the way, Voith is being generous in allowing a 100% utilization factor down to zero speed for the electric transmission comparison case. Rather it would decline somewhat. I’d say that the utilization factors in this case are “gross”, in that they reflect the total engine loading, including power lost to heat, as well a useful power.

Cheers,