Not exactly true. The more powerful engine will only save fuel
if the less powerful engine is running beyond it's optimum range.
However, the converse also applies. If the more powerful engine is
running under it's optimal range, it would also use up more fuel
(i.e. most of the time at low rpms due to too little a load). The
key is to find a optimal balance where the drivetrain would be most
efficient for the type of terrain and service (city traffic or
short distance high turnover, rural, highway) that the bus would
operate on.
The powerful engines scenario applies especially to large
displacement engines, where the crankshaft of the engine would have
to travel a longer distance at every cycle, thus being very
inefficient, and space consuming due to the larger engine. Advanced
systems such as intercoolers and variable geometry turbocharges do
provide the ability to have a smaller displacement engine while
maintaining similar power and torque levels.
Last but not least, the more powerful the engine, the
maintaience and consumerables costs is generally higher as there
would be more stress on components. In such cases, oil and filter
changes would have to be done more often. The engine could then be
derated from it's original output to save costs and extend it's
lifespan.
thank you for your very valuable insight. i think the best
comprimise would be an 8/9 litre engine with an output of around
280 to 300hp or so, considering that buses seem to be heavier
nowadays