Thumper engine noise when pulling away

[JohnElliott]

In videos of 1001 moving off (for example, this one), as soon as the train starts moving the engine revs drop and it seems to hesitate, before recovering. I remember the same thing happening when trains of multiple 205s/207s left East Croydon. But I haven't seen/heard it with preserved 205s or 207s, 73s running on diesel, or (for that matter) any other type of diesel-electric. Does it only happen when there's more than one motor car in the train, and does anyone know what causes it?

 

[John Webb]

In videos of 1001 moving off (for example, this one), as soon as the train starts moving the engine revs drop and it seems to hesitate, before recovering. I remember the same thing happening when trains of multiple 205s/207s left East Croydon. But I haven't seen/heard it with preserved 205s or 207s, 73s running on diesel, or (for that matter) any other type of diesel-electric. Does it only happen when there's more than one motor car in the train, and does anyone know what causes it?

Is it possible that to prevent too high a current through the traction motors when starting to move off, the engine revs are dropped back to reduce generator output until the train starts moving?

Curiously this is the exact opposite of what I remember them doing when I first saw them on the Charing Cross-Hastings services when on starting to move off the engine would be noticeably reved up, followed by a further increase in revs after the train had moved a short distance. Perhaps this different technique is to avoid stressing the ancient components in this unit?

 

[Taunton]

Being 55 years old, they had mechanical centrifugal engine governors to set the rpm when idling. When you apply power you are applying a load from the generator onto the engine. This causes the rpm to fall until the governor detects this and opens up the fuel flow to come back to the desired governed rpm.

When you get wear in the governor you may get the engine note rising and falling every few seconds while idling. Class 08 (same basic engine) and old buses are notable for this.

 

[physics34]

In videos of 1001 moving off (for example, this one), as soon as the train starts moving the engine revs drop and it seems to hesitate, before recovering. I remember the same thing happening when trains of multiple 205s/207s left East Croydon. But I haven't seen/heard it with preserved 205s or 207s, 73s running on diesel, or (for that matter) any other type of diesel-electric. Does it only happen when there's more than one motor car in the train, and does anyone know what causes it?

wonderful noise on that video

 

[bramling]

In videos of 1001 moving off (for example, this one), as soon as the train starts moving the engine revs drop and it seems to hesitate, before recovering. I remember the same thing happening when trains of multiple 205s/207s left East Croydon. But I haven't seen/heard it with preserved 205s or 207s, 73s running on diesel, or (for that matter) any other type of diesel-electric. Does it only happen when there's more than one motor car in the train, and does anyone know what causes it?

If the train heating is on, the engine idles at a slight faster rate than normal. The control circuits are designed to fall back to normal to avoid too higher a startup load, which is why you hear the engine slow down prior to starting. This doesn't occur if the heating is off.