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BR 1st Generation DMU Speed vs RPM


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As I recall the 1st generation DMU's had a top speed of 70mph.  If I use the maximum engine rpm of 1800 it equates to around 66-67 mph with new 36inch wheels. Did the engine operate beyond 1800rpm (my docs indicate it was governed to 1800) or are my assumptions wrong?  I definitely remember them achieving 71-72 between Swindon and Kemble in the 1970's

DMUCalc.png

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2 hours ago, Stevebr said:

As I recall the 1st generation DMU's had a top speed of 70mph.  If I use the maximum engine rpm of 1800 it equates to around 66-67 mph with new 36inch wheels. Did the engine operate beyond 1800rpm (my docs indicate it was governed to 1800) or are my assumptions wrong?  I definitely remember them achieving 71-72 between Swindon and Kemble in the 1970's

DMUCalc.png

Hello Steve

 

If it's any help, I used to operate Leyland 6** engines on buses and they were governed to 2,100-2,200. I believe these are very closely related to those used on first generation DMUs. Of course, changing the governor maximum is a case of adjusting a screw (at least it was on the engines we had).

 

Certainly the engines would be capable of running to those RPMs.

 

The other thing is your mention of fluid coupling slip. I thought gear two onwards were direct drive (for others not familiar, that means that the impellor and turbine mechanically lock together to reduce the losses of a fluid coupling).

** I am going based on my experience with road vehicles and the understanding that the same technology- especially from Leyland- was used in various DMU builds. I am not stating a fact re rail.

Edited by Derekstuart
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The class 123 Swindon 4-car DMUs were rated to run at 75mph, and IIRC so were the 5-car Trans Pennine sets.  I believe this was effected by different gear ratios and the 123s had B4 bogies with smaller diameter wheels.  They were perceptibly slower in acceleration despite the higher top speed. 
 

DMU performance in traffic differed enormously; a ‘strong’ set would manage the timings easily but OTOH I’ve been on a 120 with all 4 engines working that could not manage more than 48mph on level track due to headwinds.  

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1 hour ago, keefer said:

There's a section about the DMU engines & drivetrains at railcar.co.uk

https://www.railcar.co.uk/technology/

 

 

From there, my bold:

“The gear ratios for the Self Changing Gears Ltd R14 gearboxes are:

1st gear 4.28:1
2nd gear 2.43:1
3rd gear 1.59:1
4th gear 1.1:1

which is different to the value in the table in the first post. 
(Bottom of the page at https://www.railcar.co.uk/technology/gearboxes/?page=r14)

 

And yet at 

https://www.railcar.co.uk/technology/gearboxes/?page=gears

it says “...This arrangement gives a 'straight through' drive in top gear.”

Edited by eastwestdivide
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5 hours ago, Stevebr said:

As I recall the 1st generation DMU's had a top speed of 70mph.  If I use the maximum engine rpm of 1800 it equates to around 66-67 mph with new 36inch wheels. Did the engine operate beyond 1800rpm (my docs indicate it was governed to 1800) or are my assumptions wrong?  I definitely remember them achieving 71-72 between Swindon and Kemble in the 1970's

DMUCalc.png


Remember seeing 75 on the clock of a 120 between Droitwich and Kidderminster. Surely the engine governor only controls fuel supply? If other factors apply ... gravity, following wind... then engine power may not be the limiting factor for top speed.

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Gravity plays a part definitely but the RPS archive shows many class117 units exceeding 70mph out of Paddington on the level. I do have a vague recollection that the rev counter was marked to change up at 1800rpm but was calibrated further (1850? or so)

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4 hours ago, The Johnster said:

The class 123 Swindon 4-car DMUs were rated to run at 75mph, and IIRC so were the 5-car Trans Pennine sets.  I believe this was effected by different gear ratios and the 123s had B4 bogies with smaller diameter wheels.  They were perceptibly slower in acceleration despite the higher top speed. 
 

DMU performance in traffic differed enormously; a ‘strong’ set would manage the timings easily but OTOH I’ve been on a 120 with all 4 engines working that could not manage more than 48mph on level track due to headwinds.  

Both classes 123 and 124s official top speed was 70 mph. However have been on test runs with a Trans Pennine and 88 mph was achieved.

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3 hours ago, Phil Bullock said:


Remember seeing 75 on the clock of a 120 between Droitwich and Kidderminster. Surely the engine governor only controls fuel supply? If other factors apply ... gravity, following wind... then engine power may not be the limiting factor for top speed.

The governors have two functions regulate the speed and manage the fuel delivery. The top revs can de adjusted by the large adjusting nut on the front of the fuel pump governor top revs was usually set at 1900 rpm., there is also a max fuel stop on the side of the governor which was usually wire locked.

 

Edited by 45125
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I don't think 1800 rpm was the rev limit, but was the recommended/suggested change-up speed.

Certainly some units accelerated to the top speeds better than others. Having ridden classes 110 and 111 from Leeds to York and back, they had no difficulty sitting on 70 mph (looking through the screens behind the driver). Similarly, two power twin class 104 units from Manchester Victoria via the Oldham loop were racing along at top speed within a short time of leaving the station stops.

On the other hand, a class 120 unit on the Ayr line was extremely ponderous, even allowing for speed limits, it was very slow accelerating to the speeds. I haven't checked, but weren't the 120s quite heavy units but still fitted with the AEC/Leyland 150 bhp engines? As an aside, one of these units I rode on had all sorts of masking tape around the inside corners of the cab, and someone had scrawled in black marker pen, "Very cold and draughty cab". :D

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The 120s were on 63' underframes as opposed to the 57' of the 110/111 units, so the extra 6' of underframe and bodywork, plust the extra seating bays, would account for extra weight and lower the power/weight ratio of the longer framed vehicles. 

 

I twice saw dmu speedometer needles up againsts the stop with the units travelling at what must have been considerably more than 70mph assuming the speeds were being correctly recorded, both 116s.  One occasion was collecting a solitary DMS from Swindon Works to return to Canton dmu, which means that a single unit 121 or 122 must have been capable of the same performance as well, and the other was an empty stock working on a New Year's Eve when both myself and the driver were keen to get back to the pub before last orders.  The train was the ecs of the 'last Aber' which terminiated at Aber Jc just north of Caerphilly and we pretty much flew down the bank under power; I would guess at a speed in the mid to high 80s past Heath Jc.  The driver shut off here and made a full service brake application at Monthermer Road bridge, to bring speed down to the required 40mph for Crockherbtonw Jc and the 15mph for the bridge over the SWML south of Queen Street, then he opened up again to pass Cardiff Central, shutting off at the west end of Platform 7 and observing the 15mph through the junctions between there and the Carriage Shed reception; we were a little liberal with the 5mph through the carriage washer which was not working anyway, but made last orders at the Craddock with 5 minutes to spare, 23.40 off Aber, and 23.52 coming to a stand at Canton.

 

Neither of these exploits could be described as normal working, though the driver was a man I had complete confidence in, and a maestro on a dmu vacuum brake.  The Rhymney Railway main line between Cefn Onn and Crockherbtown Jc has a line speed of 90mph.

Edited by The Johnster
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Hi,

 

Thank you everyone for the great insight.  The issue around the 4th gear ratio appears to be 1:1 as it it shown as a direct drive with no reduction being shown (source railcar.co.uk/Technology/Gearboxes/Gears) and a document from the Western Region CMEE available at Silvermoon consulting.  I did notice when looking at a picture of the DMU rev-counter that it was marked between 1100 rpm and 1800 rpm (as expected) but that it was calibrated to 1950 rpm which would give a theoretical speed of around 73mph on new wheels and 70 mph on worn which looks about right.  I can definitely remember the needle being beyond this marked zone (1800) from observations behind the driver in the mid 1970's. I suspect that the power output drops rapidly after 1800 rpm but another doc indicates that fuel cut-off only occurs between 1900 and 2000 rpm.  Happy if anyone can give any clarity on the above.  Going forward I will update the info to include the higher powered (Class 123etc) units which had different final drives and a higher rpm (1900) but the same 70 mph top speed.  I will then generate Tractive Effort vs Speed graphs for both which I will upload here and on my website.

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4 hours ago, Stevebr said:

Hi,

 

Thank you everyone for the great insight.  The issue around the 4th gear ratio appears to be 1:1 as it it shown as a direct drive with no reduction being shown (source railcar.co.uk/Technology/Gearboxes/Gears) and a document from the Western Region CMEE available at Silvermoon consulting.  I did notice when looking at a picture of the DMU rev-counter that it was marked between 1100 rpm and 1800 rpm (as expected) but that it was calibrated to 1950 rpm which would give a theoretical speed of around 73mph on new wheels and 70 mph on worn which looks about right.  I can definitely remember the needle being beyond this marked zone (1800) from observations behind the driver in the mid 1970's. I suspect that the power output drops rapidly after 1800 rpm but another doc indicates that fuel cut-off only occurs between 1900 and 2000 rpm.  Happy if anyone can give any clarity on the above.  Going forward I will update the info to include the higher powered (Class 123etc) units which had different final drives and a higher rpm (1900) but the same 70 mph top speed.  I will then generate Tractive Effort vs Speed graphs for both which I will upload here and on my website.

I believe the gear ratios are:

1st - 4.28:1
2nd - 2.43:1
3rd - 1.59:1
4th - 1.1:1

 

This gives a nominal maximum of 65.5 to 65.75mph in 4th gear

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