Boltar ) gurgled happily, sounding much like they
were saying :

"The battery pack is kept at optimum power by a 1.9 litre diesel Euro
IV engine. In other words, an engine which would normally be found in
a family car keeps the battery"

1.9 sounds a bit too small. I'm sure in start and stop traffic it
might be ok but ultimately the energy all comes from that engine.
Seems to me the bus would be hopeless for any long or even middle
distance runs especially if any hills are involved, since once the
battery has been used up its relying on the engine only and a 10 ton
(probably more) bus won't be going anywhere fast with an engine that
size.

Don't forget that the engine is basically just being used as a generator,
therefore can run at optimum revs for the specific output - there's no
requirement for any kind of useful rev range or torque curve.