Well not really, you don't need full power to maintain 125mph - full power provides maximum electricity for when maximum load is placed on the motors that power the unit forward. I accept that tractive effort is just (a big) part of the story, but bhp/tonne isn't a reasonable measure of how well units perform, there's much more to it than that.
Notwithstanding the above, vast sections of Class 800 routes planned to be covered by diesel power alone have much lower speed limits than what you would expect on a mainline - thus you don't require the same amount of Mw applied to the motors as you would on a mainline - this is clearly a strategy being used by Hitachi, in that the Class 800 units are derated to reduce ultimate loads on power packs and reduce time out of service and in the "803" units, power packs are fully rated to ensure they can deal with the topographical issues down on the hilly South East routes - which notably don't have 125mph speed limits.
I meant acceleration, not ultimate speed. Power = (force * velocity). If both power and force available are limited, below a certain speed the force limits acceleration, above it power is the limit. Ultimate speed is determined when tractive effort, which is decreasing with speed, balances the resistance to motion, which increases. A nice document showing the calculations with graphs is
here.
So in addition to lower top speed, the acceleration will be lower at the higher speed ranges on diesel, and balancing speed on gradients will be lower too.
Interestingly, the 'maximum acceleration' graph in the
IEP TTS (which looks suspiciously like a tractive effort curve) seems to indicate that the acceleration becomes power-limited around 30 mph. Assuming that's for electric, It will be lower for diesel (I haven't seen definitive figures, my back-of-envelope calculations are that diesel will have around 60% of the power of electric, but I stand to be corrected on that.)
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Interesting - so for a 5 car 2800 bhp of which some has to power the auxiliaries. Let's say 200 bhp (will probably be more than that) leaving 2600 bhp to power the train. You'll get around 80 percent at rail 2100 rhp for 5 coaches. Its not too dissimilar to what Javelins get on 3rd rail.
Switching back momentarily to a sane system of measurement, your 2.1 MW of installed power will reduce to around 1500 kW at the motors. On electric, assuming the motors are the limit, you would have 2700 kW at the rail.
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Ask sprinterguy
Well yes, sort of - it's a decent approximation though. Leaving improvements in generator/motor/control efficiency aside, the power generated by the engine will determine the power available to the traction motors. Tractive effort will depend on how this power is delivered to the rail - i.e. the number of powered axles, and the weight on each of these (Class 800 will have lower power per axle so ought to have better traction I think?). I can't believe that with more power available, and distributed traction rather than powercars, a Class 800 would perform worse than an HST in either acceleration or top speed.
Tractive effort and adhesion aren't the same thing. Your extra powered axles help getting off the blocks, where maximum force cannot be applied without spinning the wheels. On clean dry rails, you can apply maximum force at relatively low speeds, even with few axles powered. The distributed motors really help in poor rail conditions, and for regenerative braking.
HSTs have around 2.6 MW at the rail, and will be geared lower than the IEPs, so I think the HST clearly beats IEP on diesel. On electric, IEP will win hands down.
