Trimode cl 769 to operate Reading to Oxford and Gatwick.

[whole diary]

[onthecushions]

A train's performance on a gradient does not depend on whether it is diesel or electric but on its adhesion and therefore tractive effort. This in turn depends on axle load, number of driven axles and rail conditions, where friction can vary typically between 12% and 30%. A  diesel might be less inclined to slip as it could not lay down as much power as when under electric drive.

There are a number of c1:100 gradients at the Eastern end, the worst is 1:96, not the 1:40's experienced in the Pennines!

OTC

[RichT54]

Does it have to be a problem due to lack of adhesion, or could it be that the motors would not have sufficient power to actually turn the wheels against the weight of the train trying to roll back down the slope?

[onthecushions]

By analogy, an 08 shunter with three axles and only 400/260hp can move almost anything, although not very quickly!

OTC

[paul7575]

319s coped with a quite significant gradient between City T/L and Blackfriars for years…

[Electric train]

319s coped with a quite significant gradient between City T/L and Blackfriars for years…

And the loadings at peak times far exceeded any loading they will encounter on the North Downs.

It would be interesting to know what the failures are? 

Is down the legacy equipment on the train these failures could be in part due to the age of the equipment, this can be helped by heavy overhaul, and new / refurb equipment
Is due to the knowledge / experience of the maintenance staff; then this should improve over time  is it down to the new systems and the fine tuning of these
Is it the new systems, then fine tuning and experience should improve this.

If it is a combination of all 3 then that will take some concerted effort to resolve

[nickswift99]

319s coped with a quite significant gradient between City T/L and Blackfriars for years…

But they were using electric traction not diesel.

[grahame]

A train's performance on a gradient does not depend on whether it is diesel or electric but on its adhesion and therefore tractive effort. This in turn depends on axle load, number of driven axles and rail conditions, where friction can vary typically between 12% and 30%. A  diesel might be less inclined to slip as it could not lay down as much power as when under electric drive.

319s coped with a quite significant gradient between City T/L and Blackfriars for years…

But they were using electric traction not diesel.

Is the 769 heavier (having extra diesel engines and fuel tanks) so heavier weight on the drive wheels, and thus better traction (though less power) .   Or is the extra weight over unpowered wheels, meaning the same traction on the powered wheels and indeed a reduced performance because it has a heavier base load in the train as a whole?

[RichT54]

319s coped with a quite significant gradient between City T/L and Blackfriars for years…

But they were using electric traction not diesel.

I thought the 769 still used the electric traction motors when running on diesel. The diesel engines just drive alternators which generate the electricity.

[ellendune]

319s coped with a quite significant gradient between City T/L and Blackfriars for years…

But they were using electric traction not diesel.

I thought the 769 still used the electric traction motors when running on diesel. The diesel engines just drive alternators which generate the electricity.

But when using diesel power the traction motors can only provide enough power if the alternators can supply that power.  If the diesel engines or the alternators are not powerful enough it doesn't matter how powerful the traction motors are. 

[RichT54]

319s coped with a quite significant gradient between City T/L and Blackfriars for years…

But they were using electric traction not diesel.

I thought the 769 still used the electric traction motors when running on diesel. The diesel engines just drive alternators which generate the electricity.

But when using diesel power the traction motors can only provide enough power if the alternators can supply that power.  If the diesel engines or the alternators are not powerful enough it doesn't matter how powerful the traction motors are. 

Agreed and, if there was not enough power to get up an incline, would it come to a halt due to wheelspin (loss of adhesion), or because the motors just couldn't turn the wheels?

[stuving]

Here's a comment about the 769s that may be relevant:

“Rebuilding can often be more difficult than building from new. Making the existing and new components work together can be a real challenge.”

Helen Simpson, innovation and development manager, Porterbrook

She's probably now saying to herself "well, that's one thing I got right at least".

That's from an article in May 2019, so when the programme was already running late. Another quote suggests one of the areas of awkward co-operation:

Wabtec’s biggest engineering challenge was the new electronic control cubicle, which controls local power on the driving cars.

The "bus" isn't an infinite busbar, or anything like. The two alternators and the four motor/converter loads are looking at only each other, and that close coupling offers a lot of scope for instability, surges, etc. And if that damages the motors, they may even fail when not on diesel power. Also, if the alternators can't meet the motors' current demands, that limits the power and torque available.

I know the GTO converters do what the big series resistors do in a tram: allow the starting volts across the motor to be low and then to rise as the train speeds up. That should mean that the starting current from the alternators is quite low, while in the motors it's high. I can't find out whether the controllers still use field switching or weakening for higher speeds.

Neither have I found out anything about the ABB alternators, though this is a key component. Vivarail had a lot of problems with their light-weight modern TSA^▸ (Ticketing and Settlement Agreement.) ones (similar to the ones in 800s, I think) in the 230s, and ended up replacing them all with much heavier but more robust old-style ones.

Making the train accelerate depends on the adhesion (at low speed) and power (in the middle and higher speed ranges). Adhesion depends on the fraction of the weight on driven wheels, which for a 319 is 0.36 (4/16 axles motored, but a lot of weight in the MSO^▸ (Motor Standard Opens)). Adding the two diesel generators adds 15 t (over 10%) to two trailers, bringing that fraction down to 0.33. That's similar to an HST^▸ (High Speed Train), and much lower than most modern units (typically 0.5-0.6).

Here is a more explicit statement from Porterbrook about the available power (specially for OTC):

Simon explains: “The MAN engines have a rail pedigree and are compliant with Stage IIIb emissions rules. Their maximum rating is 390kW, but we decided to have them working at less than that to give ourselves some ‘headroom’. In service, the two engines will deliver around 720kW at the DC^▸ (Direct Current) Link (just under 1,000hp), which is somewhat less than the 1.2MW available in electric mode, but in general speeds are expected to be lower when working on diesel, and this arrangement gives good power in the mid-range. Performance should be comparable to a Class 150.”

An unusual usage of "somewhat"?

[nickswift99]

That’s a useful post.

So if I have read that right the Diesel engines have added 10% to the weight while the available power has dropped 20%?

I appreciate there are other factors at play but that does suggest that performance is never going to be anywhere near that of a 319.

Does anyone know if a 150 has ever done the North Downs route?

[ellendune]

That’s a useful post.

So if I have read that right the Diesel engines have added 10% to the weight while the available power has dropped 20%?

No you fell into the trap as I did initially comparing 1000hp to 1.2MW

"In service, the two engines will deliver around 720kW at the DC^▸ (Direct Current) Link (just under 1,000hp), which is somewhat less than the 1.2MW available in electric mode"

1.2MW to 720kW is a 40% drop in power!

Hence stuving's comment about an unusual usage of "somewhat".

[Fourbee]

Does anyone know if a 150 has ever done the North Downs route?

Not sure, but when pairs of ex-Southern 456s were used on the Guildford-Ascot runs they really struggled to get out of Guildford and that was on lightly loaded trains, without extra engines and equipment added. Wikipedia quotes the power output as 373kW (so about 750kW as a pair of 2-cars).

[onthecushions]

Again, by analogy, the  unlamented "Tadpole"units that once inhabited this route weighed in at 113t with a 500hp diesel and only one motor bogie.

Slipping was a minor feature of the Southern's EE507, big traction motor, single motor car policy - part of the reason the 508's were banished to Merseyside.

The other problem is the perennial Diesel electric problem, that the generator/alternator produces more power at "right away" than the TM^‡(resolve)'s can use, leading to excessive windings' temperature rise. I think the 47's had a minimum continuous speed of 27mph to avoid TM damage, which limited their freight use.

What it does mean is that the makers need time to get even an adaptation right before launching anything new into service and making the public or at least the TOC^▸ (Train Operating Company)'s driver managers put up with an unfinished job.

OTC