by a third differential gearset to drive the main output shaft. Each differential input shaft was provided with a mechanism to prevent backwards rotation when the couplings were drained. Since this could cause the drive mechanism to lock up if the locomotive were to be pushed backward a vacuum operated clutch was included in the geartrain. The effect of this arrangement was that the gear ratio between an engine and the output shaft depended on how many engines were driving the transmission. Gear ratio selection was accomplished not by "changing gear" in the conventional sense, but by filling or draining the hydraulic couplings to connect or disconnect the engines from the transmission. With only one hydraulic coupling filled with oil and the other three engines disconnected and their respective input shafts to the transmission locked by the one-way clutches, the single engine drove the output shaft through an effective gear ratio of 4:1. With two engines driving, the effective gear ratio was 2:1; with three engines, 1.33:1; and with all four engines, unity. In other words, the effective gear ratio of the transmission was the inverse of the number of engines driving it. Unlike the transmission of a car, there was no overall torque-multiplication effect from selecting a lower gear. The 4:1 mechanical advantage afforded to the single engine driving in first gear was cancelled out by the fact that there was only one engine operating, so the maximum output torque from the transmission was the same as was available in top gear with all four engines operating. The same argument applies to second and third gears. The transmission of this locomotive, therefore, unlike almost all other locomotive transmissions, did not provide any means of matching the torque characteristics of the engine(s) to the requirements of the locomotive; it did not provide for an increased torque output at low speeds for starting and hill climbing. It served only to match the output speed of the engine(s) to the requirements of the locomotive. The requirement for high starting torque was met in the Fell not by the transmission characteristics but by altering the torque characteristics of the engines themselves. Normally a diesel engine aspires charge at a mass flow rate proportional to its rotational speed; the faster it rotates, the more charge it can aspire, and this leads to a power output curve which rises more or less linearly with rotational