Diesel Mechanical v Diesel Hydraulic
Or, if a loco has a fluid coupling in its transmission, why isn't it a diesel hydraulic?
This is an interesting question and the answer is simple but then gets "fudged"! In a fluid coupling, an impeller, driven by the engine flywheel, propels oil onto the turbine (connected to the transmission) so the force is, briefly, going through a hydraulic medium. But the physical characteristics of a fluid coupling is that it only transmits the same torque/rpm as the engine inputs. Now that is not entirely true, as there are some losses ("heat rejection"), but in essence the fluid coupling works in the same manner as a dry friction plate clutch. The fluid within the coupling is not under pressure, but woe betide the driver who leaves the loco stationary, in gear with the engine running as a temperature build up results in the fusible plug melting and throwing all the oil out.
In a torque converter, again, fluid is thrown from an impeller to a turbine, but is deflected along the way by static blades within the housing and the fluid is under pressure. The physical design of the various components and the resulting flow creates a very different outcome. Although the engine may input 'x'lb/ft torque at 'y'rpm, the output from the converter will be 'a' lb/ft and 'b' rpm. At stall, for example, the engine may be inputting power at 16-1800rpm, but the converter output will be zero rpm: the torque however, on a Twin Disc converter, will be around 5.5 times the torque the engine is putting in. Of course, 'you don't get 'owt for nowt' - the converter will get hot and must be cooled. The typical best efficiency on a Twin Disc is about 80% at full power, or put it another way if you put 300hp into the converter, you will get out around 240 to the wheels and 60 as heat.
In the above diagram of a Twin Disc converter, oil is flung from the impeller (A) to the first turnine blades (B1). Guide vanes (C1) direct the fluid onto the second turbines blades (B2), repeated by the second stage guides (C2) and on to the third ring of turbine blades (B3)
So that's the basic difference between a fluid coupling drive and a torque converter - but I said things get fudged. Take a Voith transmission. Clearly a hydraulic, and within its casing you might find a couple of torque converters (seperately filled and emptied and connected by different gearing to the output stage) but also a fluid coupling, which comes in for the upper speed range to give a direct (well, 1:1 fixed) drive to the wheels. So a fluid coupling can form part of a hydraulic transmission but on its own is regarded as a mechanical.