I fear the 769s may be worse wimps on wheels than OTC foretells.
There is more information in a
Rail Engineer article last year by Maclolm Dobell, based on going to visit Porterbrook. They are reported thus:
The rated output of two diesel engines at 780kW is the maximum they can produce. By the time alternator efficiency, auxiliary supplies, and traction control efficiency are taken into account, the electrical input to the traction motors will be approximately 550kW, little more than half the maximum rated output of the traction motors.
That's only 70% of prime power, and there's still motor losses to come (and these are still the old
DC▸ motors) - far worse than OTC's already rather miserable assumption. They are going to struggle to get rid of all that heat!
On performance, they resort to what sounds like waffle.
Simon and Helen are well aware of this deficit. They explained that simple headline figures do not tell the whole story and that the required performance on diesel was likely to be as good as, if not better than the Class 150.
...
Compared with a Class 150, the Class 769 has a higher tractive effort on starting, but the tractive effort falls away more steeply. This difference in tractive effort curves makes it difficult simply to predict performance on any particular route, illustrating the importance and value of modelling. Modelling has shown the gradient balancing speed on a flat gradient when powered by the diesel engines to be approximately 87mph and the trains will retain the 100mph capability when powered by electricity. The modelling has also shown that two 1000-litre fuel tanks should be ample for the expected duty.
If it can only do 87 mph on the flat, an envelope flip suggests it might just do 52 mph up 1 in 100.
Incidentally, the minimum tare weight of a 319 (based on equal axle loads in each vehicle) is 140.3 t and for a 769 the DTSOs put on 7.5 t each. Presumably the other two gain only a little, but the total must be over 156t. Note I used tare weights, I don't know if that 87 mph did - adding passengers will obviously make it even slower.
If you want an example that goes to the other extreme, look at the Stadler class 755/4 just come into service with
GA▸ . Here the weight is a bit of a guess, but based on
this it's only 114 t. For some reason these 4-cars have four engines (the 755/3 has two) with an output of 1920 kW! Now, Stadler do say one is a spare - so they can run the rest at 75%, or give each a rest in turn. But even in that case you're looking at 12.6 kW/t (prime) - compared with 5.0 for the 769, before allowing for the very high losses. Must be for all those famous East Anglian hills.
