"Clive" wrote in message
...
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.

Why should the compression of a petrol engine vary with throttle position?
Surely compression ratio is determined simply by the ratio of the volume of
the cylinder with the piston at the top of its travel to the volume with the
piston at the bottom of its travel. Or am I missing something very obvious?

Why were high-compression-ratio petrol engines and 5-Star petrol phased out?
I can remember in the 1970s, many makes of car had a top-of-the-range model
with a high-compression engine. Was more energy/pollution generated in
refining higher-octane petrol - was that what spelled the death-knell of
those engines? From what you are saying, high-compression engines would
presumably have been more efficient to run. However I presume
higher-compression petrol engines had a greater tendency for compression
ignition (pinking) to occur accidentally.

Is there a trend to run modern petrol engines at higher rpm (ie
lower-geared) that there used to be? My Mark 2 Golf (1800 non-injected
engine) was certainly higher-geared (mph/rpm) than my Mark 3 Golf (1800
injected engine) and had a greater acceleration, especially 50-70, though it
may have been a lighter car so its power or torque to weight ratio would
have been higher.