"Nick Finnigan" wrote in message
...
"Martin Underwood" wrote in message
s.com...
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?
It is quite possible to design an 'inefficient' diesel engine, but
no-one would buy a car with one instead of a petrol engine.
Do you have examples of greater torque at lower revs?
Yes: a quick web search on
www.ford.co.uk shows the following:
1.8i 16V Zetec
1796 cc
Cylinders – 4 in-line
Electronic fuel injection with EEC V engine management system
Electronic breakerless ignition
Catalyst
Power 85 kW (115 PS) at 5500 rpm
Torque 160 Nm at 4400 rpm
1.8 Duratorq TDCi
1753 cc
Cylinders – 4 in-line
Direct fuel injection with EEC V engine management system
Turbo intercooler
Oxidation catalyst
Exhaust gas recirculation
High-pressure common-rail fuel injection
Dual-mass flywheel
Variable nozzle turbocharger with intercooler
Power 85 KW (115 PS) at 3800 rpm
Torque 250 Nm at 1850 rpm
I'm comparing two engines with the same rated power and almost identical
capacities. The diesel has a torque that peaks to a higher level than the
petrol and at a lower engine speed. That sounds like a good example of
higher torque at lower engine revs - a common feature of diesel engines.
Similarly from the Peugeot 306 brochure (July 1999):
1.8 16v petrol
4 cylinders
Power 112 bhp @ 5500 rpm
Torque 16.1 mkg @ 4250 rpm
2.0 HDi diesel
4 cylinders
Turbo
Power 90 bhp @ 4000 rpm
Torque 21.5 mkg @ 1900 rpm
I presume to convert torque in "mkg" into torque in the more sensible units
of "Nm", you multiply by g (9.81 m/s^2).
Maybe the difference is partly due to the diesels being turbo-charged and
the petrols not being. Could this be a fairer comparison?
VW Golf brochure (2002):
1.4 16v petrol
4 cylinders
non-turbo
75 bhp @ 5000 rpm
126 Nm @ 3300
1.9 SDI diesel
4 cylinders
non-turbo (naturally aspirated)
68 bhp @ 4200 rpm
133 Nm @ 2200-2600 rpm