Dave Plowman (News) ) gurgled happily, sounding
much like they were saying :

The power output of a diesel is controlled by the amount of fuel
injected.

Yes, as a petrol engine.

No. A petrol engine is controlled by the amount of air admitted via
the throttle. Petrol is then injected to give the correct mixture.
This is one major reason why petrol engines are less efficient at part
throttle - pumping losses. There are developments to change this like
using variable valve timing rather than a throttle, but IIRC still
only in development.

Don't forget that BMW have had a throttle-less petrol in production for a
few years.

In principle is there any reason, other than the technical difficulty
in implementation, why a petrol engine couldn't have a fuel demand
valve connected to the pedal with the engine management determining
the necessary throttle setting?

Sounds like fairly conventional fuel injection to me...

Because petrol can only burn satisfactorily within a small range of
fuel/air ratio.

Using conventional stochiometry, yes - around 14:1. Which isn't actually
the best range for either economy or power, but is the limited range around
which a conventional three-way cat works for best emissions - hence
lambdas.

LeanBurn is somewhere around 25:1 - which brings NOx back down again
according to http://www.designnews.com/article/CA159977.html