DMU failures and revving up

[john prytherch]

Please excuse first a simple question: I have often observed DMUs (both first generation and modern ones) revving their engines hard for a minute, or a few minutes, before starting a journey. I have always thought this was to generate enough vacuum, or air pressure, to release the brakes. Is that what the revving is for? Could it also be, in cold weather, to generate some heat for the saloons? At stops during journeys I have occasionally experienced revving, but never for very long.

Second, an incident last night, when 150203 failed near the site of Dam Lane Junction, between Glazebrook and Birchwood, while working the 2045 Oxford Road - LSP service. The journey was normal until Irlam, where we had to do some revving before getting away. Then at the point of failure, a sudden, not quite emergency, brake application. The driver then did a lot of furious revving, alternately from each end of the train, for 20 minutes or so in each case, but the train could not move. All heating systems (which were not very effective) continued working throughout. After about 80 minutes we were rescued by 150206 and 156471 buffering up to the rear, and as soon as they had coupled up we were running entirely normally, the engines and wheels of all six cars of the formation running at normal line speeds. I was pleasantly surprised, because I thought 150203 would be in some sort of "cripple mode". Would the failure have been something to do with the air brake system, or might there be some other explanation?


John Prytherch.

 

[Bletchleyite]

Yes, the revving is to build air for brakes and doors.

 

[Domh245]

AIUI, there is a "compressor speed up" on most DMUs that will rev the engine higher so that the compressors work faster to build up air pressure, to enable brakes to be released, which would tally with your account of it occurring a few minutes before departure.

 

[Silv1983]

Hi John,

Spot on - what you hear is the compressor being maxed out by the driver to get as much air into the system as possible in a short amount of time. They only need to flick a switch called the 'compressor speed up' on and then off when enough air has been built up.

With 150203, it developed an air leak after an air pipe split. The driver was trying to replenish the air to get the brakes off and the train moving.
When it stopped completely the pipe has burst completely and the compressor couldn't keep up with the leak. It's a fail safe system as on all trains: air keeps the brakes off not on so the train stops in an event of failure. With the assisting trains coupled up the air leak will have been isolated as best as possible, or the brakes isolated and released. With additional compressors all mucking in there will be enough pressure to get the unit moving.

 

[brel york]

The engine speeds up because of the gear change , it doesn’t need maximum air pressure to release the brakes as the train supplies approx 7 bar and the brakes operate at about 3.2-3.5
Compressor speed up is to build up the air quickly when the unit has been turned of
Air leaks are common and there is a set amount they are allowed to lose

 

[brel york]

you could isolate one cars brakes by shutting the isolation cock at the intermediate coupler but you would then isolate the brakes on that car , this would involve pulling up the flag switch and control would need to no that one of the vehicles had no brakes

 

[brel york]

Does anyone know what pipe failed as this is only back from works attention on sat morning

 

[brel york]

And the system is slightly more complex than revving up to get brake release
The system is controlled by the Westinghouse brake control unit , this controls the brakes air supply in conjunction with the secondary air suspension
When doing a brake test we plug a gauge into the air suspension test point and take a reading at each brake stage
There is a table which tells you what the reading must be at each stage other wise it has to be recorded as a fail

 

[Taunton]

The drivers' instructions in the first generation dmus specifically cautioned against revving up to charge the brakes, saying it did not reduce the time to create vacuum. This might have been true on brand new exhausters with all the seals tight, but over time it certainly became useful. Of course, what used to happen is the unit was started at the depot after standing overnight, full revs with a cold engine to create vacuum quickly, and it tore the guts out of the engines pretty quickly.

 

[brel york]

And you don’t need to rev up to get brake release, we test the brake operation with the engine running and the drivers direction switch in neutral then go through the brake steps

 

[brel york]

Compressor air supply pipe failed

 

[37057]

Brake cylinders don't use full main res pressure but there are low main res and low BSR governors to initiate emergency brake applications. Safe if any of those have to be used but no doubt a compromise as WSP systems may become inhibited to save air.

Isolating the air supply to a whole car on a multiple unit makes it unserviceable instantly. Service braking is lost and parking brakes need to be pulled, warning horns, couplers, air suspension, doors and toilets all become inoperative.

I don't work on Sprinters but I'd be surprised if they were any different to the above.

To add... There are generally three types of brakes. Service brakes, Emergency brakes and Parking brakes.

Emergency brakes is energise to release. Emergency brake position, AWS/TPWS, DSD, Low MR and BSR, Pass Comms, Door interlock etc all have the ability to de-energise the Emergency brake solenoid. This applies max pressure to brakes cylinders in respect to air suspension pressure (vehicle loading).

Service brakes is energise to apply (and / or release on modern stock). Stepped or progressive to control the amount of pressure to the brake cylinders in respect to air suspension pressure as above.

Parking brakes are spring applied and released by main res pressure. When main res and brake cylinders have bled off spring brake applies. Pull a cable to release.

Life would be easier if all trains had a Train Pipe and distributor in my opinion.

 

[Eccles1983]

Ive had to do this a lot.

Every station on a particular service because there was a hole in the side of the marton air box.

If the main air res presure drops below 4.5 bar the brakes will come on and stay on until the main air is back to over 5.5.

You cant "rev" the engine. You turn the compressor speed up on (which can only be done at a stand with the direction selector in neutral)
Its more common than you think.

 

[brel york]

They must of isolated the one car and moved it with the other units , my contact at nh says it should have had compressor pipe checked at exam after return from works but slipped through the net

 

[ash39]

The sound of 3x 150's all revving their guts off under the roof at York at around 6am each morning after stabling overnight is quite something!

 

[brel york]

Yeah it’s just to build the air up

 

[GW43125]

To add... There are generally three types of brakes. Service brakes, Emergency brakes and Parking brakes.

Emergency brakes is energise to release. Emergency brake position, AWS/TPWS, DSD, Low MR and BSR, Pass Comms, Door interlock etc all have the ability to de-energise the Emergency brake solenoid. This applies max pressure to brakes cylinders in respect to air suspension pressure (vehicle loading).

Service brakes is energise to apply (and / or release on modern stock). Stepped or progressive to control the amount of pressure to the brake cylinders in respect to air suspension pressure as above.

As can be seen on training videos for the old air braked EMUs (such as this one, which had the energise-to-apply EP brake, and the requires-air-to-release "auto brake" (Westinghouse air brake). The two systems were always overlapped such that in a failure of the energise-to-apply brakes, the train pipe could be destroyed to bring the train to a halt.

 

[superkev]

The sound of 3x 150's all revving their guts off under the roof at York at around 6am each morning after stabling overnight is quite something!

All that pollution filling the station. Amazing someone isn't campaigning 're air quality in some cramped stations likr Man Vic, Birmingham etc with those polluting dmu's
K

 

[DarloRich]

Hi John,

Spot on - what you hear is the compressor being maxed out by the driver to get as much air into the system as possible in a short amount of time. They only need to flick a switch called the 'compressor speed up' on and then off when enough air has been built up.

With 150203, it developed an air leak after an air pipe split. The driver was trying to replenish the air to get the brakes off and the train moving.
When it stopped completely the pipe has burst completely and the compressor couldn't keep up with the leak. It's a fail safe system as on all trains: air keeps the brakes off not on so the train stops in an event of failure. With the assisting trains coupled up the air leak will have been isolated as best as possible, or the brakes isolated and released. With additional compressors all mucking in there will be enough pressure to get the unit moving.

Is the authorised repair for a broken brake pipe not brown paper and boot lace?

 

[BestWestern]

Is the authorised repair for a broken brake pipe not brown paper and boot lace?

Hazard tape sir, always!

 

[rich r]

All that pollution filling the station. Amazing someone isn't campaigning 're air quality in some cramped stations likr Man Vic, Birmingham etc with those polluting dmu's
K

It's particularly unpleasant on some of the more enclosed low roofed platforms, for example 7A at Leeds. The exhaust of thousands of DMUs revving up from cold at that platform over the years has made the roof look like steam trains are still in use

Not to mention all the people waiting to get on being choked by the exhaust as it floods the platform area. The wise stand further away until the doors open.

 

[brel york]

Is the authorised repair for a broken brake pipe not brown paper and boot lace?

You could repair vacuum pipes like that but not air lol

 

[Taunton]

You could repair vacuum pipes like that but not air lol

I believe it was actually banned long ago, when such a temporary repair between loco and train was partially sucked into the vacuum pipe, formed a plug with the vacuum at maximum, and the brakes did not apply on the train, resulting in a collision.

When changing locos to a Western Region steam loco, they worked with a higher vacuum than the rest of BR, I believe 25" instead of 21", as the GWR had always done. This used to show up any weak spot in the coaches coming from other regions. The Bristol TM C&W examiner had a strategic stock of vacuum spares positioned along the main westbound platform where loco changes would take place.

 

[DarloRich]

it seems few people here know their Thomas the Tank engine

 

[Controller1701]

Life would be easier if all trains had a Train Pipe and distributor in my opinion.

More like a Main Res, Train Pipe and triple valve

Paul

 

[physics34]

ive noticed on 170, the main res pressure in one cab is different from the other...... i know they all have their own main res goveners, tanks and compressors but should it be the same pressure continuously throughout the train?

 

[sw1ller]

Would an isolation of the CGIC have been sufficient, negating the need to isolate a whole car or any brakes. Sounds like like was the case as it ran at normal line speed afterwards. And like has been said, you can’t use the compressor speed up whilst moving so after the driver has used what presious little air he/she had to stop at the station, they needed to “Rev” it to build up air to get going again. Then the normal revs of the movement would have been sufficient until there was a complete failure of the air line.

 

[whhistle]

Is the authorised repair for a broken brake pipe not brown paper and boot lace?

Nice :P
This forum needs a like/agree/helpful/thanks button!

 

[BestWestern]

ive noticed on 170, the main res pressure in one cab is different from the other...... i know they all have their own main res goveners, tanks and compressors but should it be the same pressure continuously throughout the train?

The Res presumably will vary slightly on each vehicle, depending on compressor efficiency, condition of the overall air system and any leakage, and so on? Just as opening of dump valves happens at different times on each vehicle; they don't all go off in unison. That's assuming each gauge takes an idependent reading from the tank on that specific vehicle?

 

[brel york]

There are numerous res tanks under a 150 , there’s four surge tanks for the secondary suspension