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.

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.

  • $\begingroup$ Terrific. I have a DMU named after me - "Named units The following Class 150 units are currently or were previously named: 150105 - Hutchie/Bernie 150108 - "Phil" 150120 - Gospel Oak - Barking 2000 (denamed) 150121 - Willesden Eight (denamed) 150121 - Silver Star (denamed) ..... $\endgroup$ – Phil Sweet Jun 16 '20 at 1:16
  • $\begingroup$ Oh, It appears these things run on a can buss these days. So based on what I have gathered, the prime movers will have ecus with can buss connections, and there will be a control module in each of the cars that has controls. The can buss connects all the cars and controllers, which talk to the ecus. So the "go faster" lever is a polite request relayed to the master controller, which decides how to handle the request and issues commands to the ecus over the buss. The systems appear to not be plug and play. Each new design needs new software. $\endgroup$ – Phil Sweet Jun 16 '20 at 1:26

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.

  • $\begingroup$ This is all true for diesel-electric transmissions, but the questions specifically refers to mechanical and hydraulic transmissions $\endgroup$ – Joe Malt Dec 17 '19 at 21:13
  • $\begingroup$ Whilst I am not an expert or remotely educated in this field... $\endgroup$ – Rhodie Dec 18 '19 at 11:46

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.

  • 1
    $\begingroup$ "Let's say that you are driving a Diesel Multiple Unit with either mechanical ... or hydraulic ... transmission." $\endgroup$ – Transistor May 15 '20 at 21:25

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