On a Diesel/hydraulic DMU train what exactly does the “go faster” lever control?

Question

Let's say that you are driving a Diesel Multiple Unit^† with either mechanical^* (i.e. an actual gearbox with multiple ratios) or hydraulic (i.e. two or more torque converters) transmission. When you move the “go faster” lever to a higher notch, what exactly is being commanded?

Possibilities that I can think of:

• Higher mass flow of fuel into the engine cylinders, whatever happens after that, happens
• Engine governor set to a higher rpm, whatever happens after that, happens
• Something else

This weirdly specific question does relate to a question presented by a client, but I'm not being paid to solve it.

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^† My question is specifically about DMUs — and not locos — examples of the kind of train I'm interested in include: Class 150 Sprinter and Class 172 TurboStar

^* Although unusual, and for a long time considered obsolete, there are still mechanical transmissions for this application in production. For example, the Voith DIWARail pack for small railcars, derived from a bus power pack, and I believe that ZF offer a similar solution.

Answer 1

Depends on the exact system. The first Danish diesel mechanical multiple units used 5-speed ZF Ecomat gearboxes adapted from buses, for example the IC3 trains. They then had a small hydraulic torque converter to start on, which locked at around 30km/h after which the train was in first gear. This locking felt like a shift, so the converter plus 5 gears felt like 6. Very smooth.

In these trains I expect the commands mostly increased fuel to the engines, the gearboxes were mostly autonomous, apart from a few exceptions.

Cruise control.

I think they tuned the gearboxes to shift as close together as possible (different wheels may have different wear/diameter).

Maybe they somehow locked them in 1st gear until the startup torque converter was locked.

They did also have an anti-wheelspin system of some sort - certainly in 1st gear full power (or torque of a 400HP V8 or 450HP V6 on one axle) would otherwise easily spin and wear the wheels.

I think something similar was used in Germany, Norway and a bit in Sweden and a few in England.

I use past tense as the Danes were so fond of the idea that these trains from the nineties were refurbished with 12-speed unsynchronised gearboxes from ZF Friedrichshafen and dry clutches in the 2000s. And they are still going. In this case the engines, clutches and gearboxes are controlled together. For instance lots of power requested when cruising at low RPM, may result in disengaging the clutches, blipping/revving the engines to match the cogwheel speeds like a truck, downshifting 2 gears, reengaging the clutches and then giving the engines fuel.

These were less smooth, but an interesting experience with a few fast shifts when starting (a train is relatively heavy compared to a car, so that is one reason for the many gears, a low gear is required to start even a lightweight train on a dry clutch). Then it would really rev through gear 3 or 4 or so to accelerate quite notably, before behaving more like the original version in the higher gears.

I will update with a couple of videos if they still exist.

Answer 2

Whilst I am not an expert or remotely educated in this field of engineering, I am aware that the increase in speed is down to increase in electromotive power to each carriage. The diesel engines provide electricity to electric motors which drive the individual carriages. The "go faster" lever probably sets a lower resistance to the motors of each carriage and the dynamic power outout of the diesel motors adjusts automatically to reduced resistance to current flow. This in turn draws higher current and their greater energy consumption from the motors. This is why trains do not seem to "change gear" with acceleration. The engines kick up to the power output demand immediately and continue to deliver the load required until the electric motors bring equilibrium between power demand and kinetic motion.

Answer 3

Large diesels function best at a constant shaft speed that represents their optimum operating point. When unloaded, the engines turn at idle speed and from that point, pushing the speed lever forward switches in the motors, increases engine speed, and feeds electricity into the motors with their armature and field windings in parallel. This draws high current at relatively low voltage and hence produces maximum starting torque.

Pushing the speed lever further forward increases engine speed more and eventually switches the motor wiring into series, which reduces torque while increasing wheel speed.

Once the engine(s) are running at their design point, pushing the speed lever further feeds more fuel to the cylinders and increases power output. That power is then managed by further changes to the motor wiring so as to absorb the full output of the diesel generator while holding the diesel engine speed more or less constant.

These instructions are communicated between the control unit and the slave units by electropneumatic servos.