DMU v Loco Hauled

[Harbornite]

I don't think that a modern loco-hauled trainset has this restriction either. Most European loco-hauled trainsets use driving trailers, rather than DVTs. A driving trailer is essentially a normal passenger carriage with a cab built into one end. An example can be seen here

The use of MKIII and MKIV DVTs is inefficient, as they do not carry passengers. At the time they were built, the Polmont rail crash was a recent memory, so I suspect that this may have played a factor in BR building DVTs, rather than DTs.

Indeed, one of the advantages of the DBSO is that it can carry passengers.

As an aside, the restriction on having passengers in the leading vehicles of 125 mph trains was dropped for the Voyagers and Pendolinos, but the seats directly behind the cabs have to face away from them.

 

[coppercapped]

They're not designed to, is the short answer. DMUs usually use Voith hydrodynamic transmissions which transmit power hydraulically and are designed to freewheel when power is cut. The 172s use conventional ZF automatic gearboxes (they're the same type that were used on city buses) which don't do this. They're more efficient under acceleration because they lock the torque converter and transmit power mechanically for much of the time, but the lack of coasting rather negates this.
There's no technical reason why you couldn't incorporate a freewheel though.

I don't quite understand. According to published information the ZF Ecomat transmission uses a torque converter followed by a six-speed gearbox. This means that the torque converter can operate effectively at constant input shaft speed, during acceleration this will be the speed of the engine at the top of its torque/revs curve. By design this input shaft speed will be the same as the torque converter's most efficient speed while the gearbox matches the output shaft speed to rail speed. There is no direct mechanical connection through the torque converter in this phase of operation so the torque converter can function as a free wheel if the input shaft speed is reduced.

As the train approaches maximum speed a clutch locks the input and output shafts in the torque converter together so the losses due to stirring the oil in the torque converter are avoided.

Is the argument then, that on closing the throttle when the clutch is engaged the clutch does not release or that on overrun the driven turbine of the torque converter now drives the impeller? In either case, for a rail vehicle, this seems very odd.

Which model transmission is used in the 172s? In any event the Ecomats tried in the Class 158 experiment showed considerable fuel economies but at the cost of having an extra external reversing gearbox. The newer ZF Ecolife transmission now has an integrated internal reversing gear.

 

[Marklund]

The use of MKIII and MKIV DVTs is inefficient, as they do not carry passengers. At the time they were built, the Polmont rail crash was a recent memory, so I suspect that this may have played a factor in BR building DVTs, rather than DTs.

For quite some time, propelling at over 100mph with passengers in the leading vehicle was prohibited.

I just find it amusing that DVT is often used for every proposal for a trailer with a driving cab.
It's like they don't register what the letters actually stand for.

 

[daikilo]

For quite some time, propelling at over 100mph with passengers in the leading vehicle was prohibited.

Then someone with a beard questioned it and a new rule was made, or something like that. If I wanted to run an IC225 at 125mph I would certainly ask to be able to use a DTS.

 

[robbeech]

I don't quite understand. According to published information the ZF Ecomat transmission uses a torque converter followed by a six-speed gearbox. This means that the torque converter can operate effectively at constant input shaft speed, during acceleration this will be the speed of the engine at the top of its torque/revs curve. By design this input shaft speed will be the same as the torque converter's most efficient speed while the gearbox matches the output shaft speed to rail speed. There is no direct mechanical connection through the torque converter in this phase of operation so the torque converter can function as a free wheel if the input shaft speed is reduced.

As the train approaches maximum speed a clutch locks the input and output shafts in the torque converter together so the losses due to stirring the oil in the torque converter are avoided.

Is the argument then, that on closing the throttle when the clutch is engaged the clutch does not release or that on overrun the driven turbine of the torque converter now drives the impeller? In either case, for a rail vehicle, this seems very odd.

Which model transmission is used in the 172s? In any event the Ecomats tried in the Class 158 experiment showed considerable fuel economies but at the cost of having an extra external reversing gearbox. The newer ZF Ecolife transmission now has an integrated internal reversing gear.

But you could still get the negative effects of overrun even if the lock up is disengaged, the drive train will turn the fluid which in turn turns the engine, all be it at a lower speed, but it is still connected by a torque converter and will still provide engine braking. I was under the impression that most units had the ability to engage a neutral position when not taking power.

If we take the 185 as an example, under load the engine drives the torque converter which goes to a gearbox with X number of gears (not sure how the reverser works on them, or indeed any train) which drives the wheels, when you select P0 (whatever it's called on local language) I am fairly confident the engine selects a neutral gear and coasts fully. However, when the service brake is applied the train finds a suitable gear ratio for the speed to assist in braking with the engine, a little bit like you would in a car with a manual gear box and like some cars do themselves with automatic gearboxes.

I think what is being implied here is that the 172 doesn't have this function to engage a neutral gear and as such is inefficient at coasting. Lock up solenoid or not, they will only fully coast if they can be in neutral.

 

[ac6000cw]

Then someone with a beard questioned it and a new rule was made, or something like that. If I wanted to run an IC225 at 125mph I would certainly ask to be able to use a DTS.

My understanding is that above a certain maximum speed (I don't know the exact figure, but I think it's a bit more than 110mph) leading passenger-carrying vehicles require a higher standard of crashworthiness, hence the extended non-passenger space at the outer ends of e.g. class 390, 220, 221, 222 and 180 (compared to a 110mph 350 for example).

 

[coppercapped]

But you could still get the negative effects of overrun even if the lock up is disengaged, the drive train will turn the fluid which in turn turns the engine, all be it at a lower speed, but it is still connected by a torque converter and will still provide engine braking. I was under the impression that most units had the ability to engage a neutral position when not taking power.

If we take the 185 as an example, under load the engine drives the torque converter which goes to a gearbox with X number of gears (not sure how the reverser works on them, or indeed any train) which drives the wheels, when you select P0 (whatever it's called on local language) I am fairly confident the engine selects a neutral gear and coasts fully. However, when the service brake is applied the train finds a suitable gear ratio for the speed to assist in braking with the engine, a little bit like you would in a car with a manual gear box and like some cars do themselves with automatic gearboxes.

I think what is being implied here is that the 172 doesn't have this function to engage a neutral gear and as such is inefficient at coasting. Lock up solenoid or not, they will only fully coast if they can be in neutral.

There are significant differences between the ZF transmissions used in the 172s and the Voith transmissions used in the Class 185s.

In the first case the transmission consists of a torque converter followed by a mechanical gearbox with six ratios. The Voith T312bre used under the Class 185s uses a torque converter followed by two fluid couplings - in this case the ‘gear changes’ are made by filling each of the hydraulic circuits in turn with oil. Which of the three is chosen depends on load and rail speed. The Voith numbering system defines the transmission as being for railcars (’T’), for power group 3 (max 870bhp at 2400rpm) with 1 torque converter and 2 fluid couplings. The ‘b’ means it includes a hydrodynamic brake, the ‘r’ that it includes the reversing gear and the ‘e’ that it is controlled electronically. (The early Voith transmissions used a hydraulic logic block that was a wonder of precision machining, plumbing and little valves…!)

The hydrodynamic brake in the Voith transmission dissipates the heat into the cooling system - it is not engine braking as understood in cars, it’s more akin to retarders used on buses and coaches. Its use is automatic under the control of the brake demands made by the driver.

Voith transmissions freewheel by the simple expedient of emptying the hydraulic circuits of oil - there is then no link between engine and rail wheels so the train can coast in ‘neutral’. As an aside Voith have recently introduced a hydromechanical transmission similar in concept to the ZF design and also based on a bus transmission called the DIWARail rated at up to 430bhp.

However none of this answers the question of whether or how the ZF box used under the Class 172s can freewheel or not. I would be very surprised if it doesn’t - the issues of moving a train with a dead engine without mechanically separating the engine/transmission from the rail wheels would become insuperable in traffic.

 

[robbeech]

My understanding is that above a certain maximum speed (I don't know the exact figure, but I think it's a bit more than 110mph) leading passenger-carrying vehicles require a higher standard of crashworthiness, hence the extended non-passenger space at the outer ends of e.g. class 390, 220, 221, 222 and 180 (compared to a 110mph 350 for example).

That is how understood it to be currently.

 

[squizzler]

Back to the original topic, and with the Twindexx hauled stock in Germany, Switzerland and Israel in mind as well as the TGV duplex, it occurred that the economics of loco vs DMU might have something to do with whether the infrastructure can cope with double deck stock.

If you have two levels of passenger accommodation, the capacity of the coach is increased but there is no space for diesel traction equipment. Single deck coaches have a lot of wasted space below the sole bar not required for bogies, braking or air-con where traction machinery can go. On balance, a middling length rake of double decker coaches with loco likely has more capacity than the same length of DMU without loco, and fewer mechanical parts to fail.

 

[61653 HTAFC]

But 185's are not able to take advantage of higher speeds for 'Sprinters'. On Scarborough to York, where it looks like the loco hauled stock will end up the are long sections with higher speeds, up to 90mph for 'Sprinters' while 185's are limited to 75mph. If you look at the RPS times all the fastest times between York and Scarborough are for 158's.

They tried to kid us that the better acceleration of the 185's would make up for it, but the power to weight isn't as different 13.3hp per ton for 185 against 10.4hp per ton for the 158, bear in mind 185's have almost twice the power.

There are also differential speeds between Sheffield and Manchester as well as Leeds and Hull

Though none of those are the "core" which I'd say was Manchester to Leeds, maybe York. The SP or MU speeds at the ends of the route are a problem, but as the current 185s are restricted there's no loss in switching to LHCS. If 158s were still running, the case for LHCS might be weaker but then again the same issues were present when the 185s were introduced. This could quite easily descend into a circular argument!

 

[Billy A]

However none of this answers the question of whether or how the ZF box used under the Class 172s can freewheel or not. I would be very surprised if it doesn’t - the issues of moving a train with a dead engine without mechanically separating the engine/transmission from the rail wheels would become insuperable in traffic.

I've seen post by drivers of 172s explaining that their trains don't coast (much) and they have to be driven downhill and so the potential economy gains can't be fully realised.
You can presumably engage neutral if towing an unpowered unit is needed although I don't think this is good for the gearbox.
Interestingly ZF have a newer DMU gearbox, the EcoWorld, which has a specific coasting and towing function.