In message m, Martin
Underwood writes

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.
--
Clive