Type three locos are a bit of a dark eve's obsession for me now, as much as chasing 37s about in the 1980s was then.
The one I regret not being in service when I was a nipper, was the BR Class 35 Hymek. With its' tractory, thrashy v16 single engine and petite and pretty looks the loco design seemed to punch over its weight, and many a Hymek nut who worked on them in service life, will tell you they were the best Diesel-Hyrdaulic Locos in the UK.
I was delving deeper into the Hymek story when i actually fell into a great disappointment or rather a hole in my knowledge due to being a presumptious of understanding, runs in the family, son #1, 5.6 y.o. bullshits away to me about how things work or are too. I happened upon a set of GA drawings (general arrangement) for the whole loco, and the bogies. It struck me that for a large part of the last four years I have been going over GA drawings being a technical outsourcing purchaser , and that I had actually bought some diesel hydraulic off the shelf equipment: a rotary table with turbine motor run from the "main circuit" in turn probably run directly from a diesel unit like a v16 cat or a rustons v12.
So I decided to cast my eye more critically upon the drawings and saw a "whole bunch" of cardan shafts. The Hymek type 3 has of course only one power unit so must distribute the drive to both bogies unlike most other DH locos of the time.
It dawned on me that I had gone along in a little myth about DH locos which was a self told fallacy : I had thought that the locos used a gear box and torque convertor to pump oil into what we call a hydraulic motor or turbine motor at ever higher pressures as speed went up, with hosing going to each driven axle and a final drive turbine motor arranged around the axle. Well the name is Diesel Hydraulic ???
In fact though I now discover to my ignorance, that the term Mekydro coined in good old Deutscheland is far more accurate: Diesel hydraulic loco is a misnomer!!! These locos are mechanical with hydraulic torque transmission.
Given a single torque convertor in fact, the amount of oil in the system would be probably no more than in an equivalent big mid speed engine like a 12 CSVT or 8LDA of around the same output. You of course run the fan and you need to have a volume and cooling heat exchangers to suit the thermodynamics of the oil so that adds, but the point is that a single torque convertor transmission for a v16 MTU today would be relatively small, with a matter of tens of litres of circuit volume for fluid in action so to speak.
So anyway, self made mad myth busted ( unless someone tells me the final drives are cardan shaft driven turbines or fluid mechanical gear joints), I started thinking more about how these things work in practice and what the issues are with them and what the advantages over diesel-electric are or other tranmission forms?
Why are DE and DH Favoured over Mechanical Geared Direct Drive in almost All Locos over 1000kw ?
There are several shunters, light self driven rail cars or sets (DMUs) and of course the whacky Fell locomotive of much more horse power, which use gearing and clutches to achieve transmission. In 1987 a pal of mine who was technically minded asked me why there were not more mechanical direct drive locos and why they resorted to DE or DH drive....
The main issues in using direct mechanical drive in a loco, from the type you would find in a car with a standard clutch or a motor bike:
1) Wear and tear on gears and clutches, due to high starting loads, variable speeds, and uneven travel
2) heat in the gears and oil
3) requirement for a great many gears for higher speed, higher horse power to reduce and apply the power successfully
3) difficulty in starting heavier trains
The last two matters really hit the head of the nail, whereas the first two are actually problems partly shared in DH locos because they are mechanical-hydraulic drive with basically the clutching of main engine drive being the hydraulic bitty.
In order to start heavier trains , just as in slipping the clutch on your car on a hillstart, you need to be able to exact a stationary starting force on the axles in order for them to have enough torque to start to rotate.
A bit like getting a nut opened with a long bendy spanner, you need to ease in the power without it :
1) on the one hand resulting in instant slip as the right pressure is met and then quickly exceeded by the leverage of gears on the axle,
2) on the other hand that there is deformation of the gears or excessive wear on the clutch plates.
3) Pull a third hand out from paddling kids at Sellafield: You need to change gear immediately after you start as the gearing is so low to achieve this "bendy torque wrench" effect, thus making progress awkward and potentially stalling the train anyway.
Here is where DE and DH locos excel and why the first series of uk diesel multiple units were erm, a bit crappy. They can both start a much heavier train and retain reliability and sensible construction of the transmissions.
How the MEK is Attenuated Nicely by the Hydro
As I realise to my own chagrin, DH is an overstatement: we are talking an automatic ford granada is technically spot on compared to the turbine breaks of the APT-P. Now there happened to be a lot of old Granada Automatics in the UK in the 80s and 90s and of course Americania lasts for ever from the fifties and sixties (before they got too fancy and all gay with their trann'ies pun, pun) so in fact the system would seem to have some longevity benefits.
Usually DH locos have a transmission shaft "stator" array which passes through several turbines , ie the donut shaped torque convertors themselves. The torque convertor looks a bit like a triallabite but it is actually a marvel of simple, wonderful engineering. It allows basically for a slipping clutch without any mechanical wear on its internal exchange surfaces unlike a standard car or oil encased multiplate oil clutch, the motor bike type, which are incidentally common in machinery. This slipping of the clutch is effectively a form of variable gearing allowing for both
a) a standing force to be built up progressively while the drive shaft is loaded up with torque smoothly
b) the engine to carry on to apply more power from its own torque output , the curve of which may be a lot higher than a direct mechanical system could take, thus burning out a clutch slipping.
c) Effectively this is a continuosly variable gearing until the rotor speed matches the stator speed at which the application of torque becomes linear - until one or the other slips -
d) coming back on that point in c- this also means that there is an inbuilt resistance to slipping in the convertor as the blades do not want to pump oil backwards but rather the system will slip if the speed differential becomes greater than the designed flow.
Now here is a great claim in C and D we must come back to
The best and worst is really to be heard in the sprinter units which have fairly small engines which have to be revved hard to get the starting force going, and then there is either a gear change or the engine backs down to allow for synchroyny in the torque convertor, which it sounds like it does and feels like from the speed being fairly un -sprinty.
We have looked at a single torque convertor which would be fine in itself: this would in effect mean that a locomotive with an engine rpm range effective of 700 rmp engaged to max would run a single mechanical gear much longer than if it was directly connected to the output drive shaft, ie the clutch was let out and stayed on. You would need more gears to cope with the torque differential. Here also as you go from a starting torque and reach maybe synchrony in stator -rotor speed which should in theory be linear, a peaky v12 say can continue to exploit the slip of the clutch in applying its own progression of rpm and torque which the wheels catch up at the end of the day.
Back to sprinters then: horrid things, they rev far too much for the progress the rather light train should be making. The 170s and after comers, are a bit better at applying power though but still irritating compared to a loco up the front. The point being here that if you have a revvy, low torque engine you have to rev a lot to get the starting force out, noisy, vibrat'ey, and then you have to change gears a lot at lower speeds to continue to overcome the inertia of the train until the resistance is then more friction: 170s are pretty quiet when powering along at "high" speed.
The irony of these horrid plastic rail-bus thingies, ie modern DMUs, is that the majority of non electrified routes are now DH driven! How a few long gone GWR traction engineers must have sniggered in oily-sloshy heaven when that day came in the late 1990s.
On a more powerful loco then you have a lot more torque to play with from a big engine but you have then a lot more torque to control and apply and more potential friction in both mechanical components and in deed on the hydraulic oil.
You need gearing but by in large that is limited to three forward and a reverse ( I wonder if they have a heavier system to change direction of travel so that the gear direction is fully reversed? ) In effect your torque convertor is also giving you an extra low first gear without a gear change to second slow speed acceleration gear, and then onto a third.
However the clever Gerry Clogs at Voith a very long time ago now, came up with the idea of multiple stator-rotor turbines arranged along the same shaft. You then engage these by hydraulic actuation of each circuit with it being essentially on low pressure, lubrication when the circuit does not pump oil in. This means that you can probably then (?) rev the engine down less to change up a gear because
a) you can slip out of gear and into the new without major issues in just engaging the new donut down the line and phasing out the current gear
b) the new gear will be able to catch up by the very principle of torque conversion.
The sequential convertor gearing, three stages of them on a Western Voith transmission, are then based on delivering progressively higher pressures to the rotor drive shaft or would that be higher flow at lower pressure? looks like they go down in size like mechanical gears and that also the power relies on a longer smoother application of torque /rpm from the main mover Power Unit.
Now anyone who has driven a modern lorry over 10 tonnes will know that they have a lot of gears, and you use most at low speed, as above noted for DH drive. Another approach is to have more gears on a single torque convertor.
The benefit here is that you reduce the complexity of the hyrdaulic transmission and control, while also you can have the gears in a smaller volume per gear because torque convertors and their associated hydraulic control and cooling equipment take up more space than gears with multi plate clutches.
I think if I remember right, the warship class had each mechanical gear case on the engine side of a single torque convertor, whereas the Hymek have their single gear case after the convertor. You introduce then the need for mechanical - hydraulic feedback control for speed in order to shift gear automatically and not have the driver doing it (crunch, screeach, kangeroo starts and so on....safer to give them a simple couple of levers to pull on and let t' loco look after hesself)
Also in having three or more gears in a mechanical box, you introduce more points of maintenance and more parts which wear out. Torque convertors of this type cannot run in reverse and in fact the stator is locked in a single rotational direction to ensure this! Otherwise pressure differentials could brake and reverse the stator potentially
Surely a better compromise would have been to have a twin gear hydraulic torque transmission and two more gears in a gear box with also the main direction of travel ? Or maybe an inboard gear box, two gears, then the double convertor system and then a "reverser" ? Essentially then you get a very broad range of operation speed-tractive effort with a combination smooth progress when you most want it, over then efficiency when you switch the big mechanical gears? Also in that arrangement, you reduce the opportunity for heavy mechanical forces being applied to the main gear box, while also making that gear changing box simpler by taking out the reverser? If you do get some kind of damage caused by a nasty sheering force ("negative torque" torquing back to you so to speak LOL ) or when the loco crashes or jolts its bogies, the simpler reverser gear box takes the hit and is designed to break somehow limiting damage "up river".
Advantages DH vs DE
If you could have had greater reliability in the higher speed power units necessary for DH in the 1960s then you could have a big advantage over DE straight away because then your service interval is longer for major out of traffic overhauls, and given there is no damage, the worn parts can be changed out quickly. The parts list is allegedly cheaper than DE of the time of big dieselisation.
The other big advantage which is alleged is that DH designs are very resistant to wheel slip. I guess this is for two reasons- firstly there is a lot of built in inertia and friction up line in the carden shafts and bevel gearing, and secondly because the torque convertor is both locked in forward direction and the rotor will encounter resistance if it tries to go faster than the stator. Unlike in a car, the engine will not be dragged into the wheel slip by suddenly being free to rev a lot faster.
In older DE designs, wheel slip had occured before anything was done about it, and in the lighter designs or those like the 58 with poor bogie design, the rate of damage by wheel slip on steeper routes must have been frightening for the depots accountants.
The jury is a little out on the wheel slip thing. It seems maybe that electric traction motors can apply a far higher starting force and a higher / faster application of torque through the speed range than hydraulic locos, and that a lot less power is lost to heat ie they are more efficient at converting power even though they go through a tortuous mech-electric-mech route rather than the oil sloshers more direct path.
The wheel slip advantage is something the jury is out on with the new Voith loco 3.2 kw jobbie DH, erm , gaining traction in the market. However given you are allowed to run bloody heavy stuff like 66s and 60s and the new electric creep control and wheel slip avoidance / detection control and SEPEX not in the least, then the benefits of DH are diminished.
Disadvantages of DHs as built in the 1950s / 60s in the uk are
1) engines can be thristy relatively on idle
2) both the engine (power unit) , gear box and especially the hydraulic transmission are /were prone to overheating
3) General Heavy Repair can be very costly requiring many new gear parts
If the fleet of DH locomotives for the GWR were for some reason, out to tender today and not in 1958, then we would see of course the use of computers and a lot more experience in building and operating them in Germany. Then you could optimise the whole system for a given use, tonnage and speed range and keep it within its boundaries, while having very very efficient gear changes due to computer monitoring and control. Also as has been recently obvious, 1500 rpm engines in the 1 - 2.5 kw range are seemingly necessary in order to meet the new emissions standards, and GE / Catipillar boast long service intervals on these units. Finally materials, machining tolerances, lubricants, and of course hydraulic oils have all become far more advanced since 1958.
So today the GWR would get an interesting bunch of locos, probably including Voith's own monster and some Vosloh "dog bone" locos which would no doubt compare favourably to the class 70 in particular for freight work, and be a far nicer way of applying oily mediated progress to passenger trains than those rather horrid 3rd gen DMUs.