Clive wrote:

I've always wondered: is it the fact that diesel engines use
compression ignition or the fact that they use different fuel which
gives rise their greater efficiency and their greater torque at lower
engine revs?
Higher compression makes them more efficient, further as a petrol engine
throttles back the compression drops even more making them more
inefficient. I think the only thing in a petrol engines favour is it's
quietness at idle, caused by low compression and therefore
correspondingly lower flame propagation speed.

Sorry , I disagree. The only thing in diesels favour is its lower
fuel consumption. Thats it. Diesels max rpm are limited by the physical
characterstics of the fuel , which results in a lower max power for a given
engine size than a petrol engine (when was the last time you saw 15,000
rpm diesel motorbike engine or a diesel F1 car?) , their throttle response
frankly is rubbish in comparison , they're heavier and the combustion process
at full power is a mess (how many times have you seen even a new diesel vehicle
belching out black soot) and because of this they can't use catalytic
converters and to get any reasonable power out of a diesel you have to shove
on an expensive turbo. Yes diesels put out more torque than a petrol engine of
equiv size but thats easily solved by different gearing on the petrol. The lack
of power however can't be solved by gearing.

To be honest diesel engines are a prehistoric bit of kit which belong alongside
steam engines in a museum and which only still exist because of their
*slightly* higher fuel efficiency than petrol (and if
you measure it by weight of fuel used and not volume the story isn't so rosy
for diesel as you'd imagine) and slightly less maintence. If diesel prices were
to rise relative to petrol the diesel car (and possibly other vehicles) would
soon vanish off the roads and the sooner the better IMO.

B2003