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The future is a another country – we may have to do things differently there
This column's on-going analysis of the IC125 replacement, otherwise known as HST2, has generated a large number of questions and suggestions on traction power. My current view is that IC125 is irreplaceable and that we will end up with different horses for courses on the Great Western, East Coast and Midland main lines.
But equally we have to get from here to there – there, hopefully, being an electric railway, and dual mode operation is a recurring topic in readers' correspondence.
A dual mode power car would have both a diesel engine and a pantograph and transformer so that it could run as an electric train under the wires. My old company, of course, provided this facility with the Class 73 electro-diesel on the third rail.
In that case you had a small electric locomotive which could also run as a lower-powered diesel when the 750V direct current third rail was isolated. In a dual mode HST2. the respective ratings would depend on the proportion of miles under the wires and the performance required under diesel power.
For example, today's Class 43 IC125 power car has a rating of 2,250hp (1.7MW) on a 17.5 tonne axle-load. It also has a large luggage compartment.
In a similarly configured HST2, you could transfer the motored bogies to the adjacent passenger coach, representing 6-7 tonnes of traction motors and transmissions, and you could fit a transformer in the luggage compartment to produce an electro diesel. Ratings of 2MW diesel/4MW electric would be feasible.
On the other hand, whether any manufacturers would sign up to such a loco on the Class 43 axle load is debatable. More likely, we would be looking at a Class 91 sized power unit.
Of course, if some ROSCO had some development cash, I am sure the Loughborough Class 43 masters could produce an electro diesel version for evaluation.
I am indebted to reader Alistair Smith for reminding me that the Class 22X, with its Alstom Onix three phase transmission is the obvious basis for a dual mode multiple unit. With a 750hp Cummins QSK19 engine, these 50 tonnes-ish vehicles are grossly overpowered at 15hp/tonne. This gives EMU acceleration capability.
As an engineering informed source, now having fun in Scotland , pointed out, the choice for traction power on outer-suburban style intercity routes, such as Greater Western, is between a pantograph or a QSK19. Well, electrification every time, but a dual-mode multiple unit would give you both while the wires were going up.
And it would be simple to achieve. Class 22X units already have a 1000 Volt power bus line down the train, allowing each coach can cross feed others in the rake in the event of an engine failure. So all that would be needed is an extra trailer car with transformer and a pantograph. With the wiring in place, the drive could be configured to take power from either the diesel-engine driven alternator or the ‘mains'.
Performance would remain the same under both modes. Thus a Hull Trains-type operation could run electric under the wires for most of the journey, then fire up the engines for the remaining sector, or, during electrification, where the wires had yet to reach. And as the diesel powered sectors diminished you could ‘simplicate and add lightness' by removing one of more engine rafts and fuel tanks.
This may sound convoluted, but the 1981 Electrification Review (last month) spent some time analysing the costs of converting diesel powered units with a long residual life, such as IC125 power cars, to electric traction. And it does emphasise that the key issue with HST2 is going to be coping with change and, if you believe the future is electric, getting from here to there economically will be the clever bit.
Experience tells me that one of the biggest snares and delusions is the belief that car technology can be scaled up for rail traction. Currently, exhibit ‘A' is the Toyota Prius ‘hybrid' car beloved of the greens.
There are many misconceptions about the way in which hybrid cars reduce fuel consumption, including references to electric ‘engines' rather than ‘motors'. Engines generate power, motors use it.
Thus in a hybrid vehicle, the wheels can be driven by the engine, or an electric motor taking power from a battery or both for maximum performance. This means that the engine can be rated at the power needed for constant speed running. In urban traffic you run off the battery; on the open road the engine provides motive power. But when more urge is required, for example when accelerating, both the engine and the motor drive the wheels.
Let's put some numbers on this for the Prius. In the latest version the engine is rated at 57kW (76hp) and the motor at 50kW. The 200 Volt Nickel Metal Hydride (NiMH) battery has 168 cells and supplies an Integrated Gate Bipolar Transistor (IGBT) inverter, which trains have used for ages. The inverter and the ac motor also provide regenerative braking – again, just like a three-phase drive traction package.
With the electric motor on full power, the inverter is drawing 250 Amps. But what counts for a hybrid vehicle is not the current but how long it is drawn.
In the case of a Prius, with both engine and motor on full whack you get up to 60 mile/h in a shade under 11 seconds. This is barely an Ampere hour (Ah), compared with the 50Ah lead acid battery in a conventional car. Despite also running on electric power at low speed, the Prius gets by with a relatively small battery rated at 28.5Ah, NiMH batteries being expensive.
Now, this sounds ideal for a diesel train with electric transmission. And it is in use on urban light rail vehicles. But high speed trains run into scale problems.
First acceleration, which is a matter of power and mass. As a rule of thumb, an intercity train needs around 10hp (7.5kW) per tonne. Think Deltic + eight and IC125.
Inter-urban DMUs (Class 22X/Class 185) have around 15 hp/tonne and are appallingly environmentally hostile because they are both overweight and over-powered.
Now various forms of legislation suggest that HST2 coaches will be heavier than the Mk 3 – say 25% - unless we have some brave engineers and managers. Similarly with power cars. So 10 40 tonne coaches plus two 75 tonne power cars totals 550tonne which gives 4.1MW (5500hp) if it is to keep up with a Deltic+8
When this train is cruising on the level at 125mile/h it will need around 2.5MW (3350hp) to balance largely aerodynamic resistance to motion.
So a hybrid HST2 could cruise at 125 mile/h with a 1.25MW (1700hp) diesel engine in each power car. Battery power would provide the extra 1.6MW when accelerating. Say 800kW per battery.
Battery capacity can be expressed in two ways – the amount of energy stored and the amount of power which can be provided. Specific energy is measured in KWh/kg and specific power in W/kg.
In our accelerating hybrid high speed train, we are looking for a rating of 800kW. Taking a specific power of 100W/kg, roughly between lead acid and NiMH, you get a battery weight of 8 tonnes per power car
Specific energy varies widely between battery types and there appears to be much wishful thinking for the newer technologies. Typical ratings are 25-35Wh/kg for lead acid and 50-80 Wh/kg for NiMH. Call it 50 Wh/kg and the capacity of an 8 tonne battery is 400kWh – 30 minutes at our acceleration rate.
So, as always with rail traction, it's not how much energy you have, but how quickly you can get it too the wheels, in other words battery size is determined by Watts, not Watt hours.
So a hybrid HST2 could match today's performance and even drive in and out of stations under electric power to minimise pollution. But, a commercially available 800kW NiMH battery doesn't exist and if it did would be very expensive.
On top of which hauling around two diesel engines, their fuel and 16 tonnes of batteries would add to the train's total energy consumption – even with regenerative braking topping up the batteries. In contrast, an electric train requires only a pantograph and transformer, doesn't have engines and batteries which wear out and can run on whatever energy sources are available.
Which is not to say hybrid rail traction is a blind alley. Alstom is producing a Citadis hybrid tram and. if and when hybrid buses and commercial vehicles are in series production, a hybrid 21 st Century Pacer could be an option. (Is that a threat or a promise – Ed)