"Clive" wrote in message
...
In message , Dave Plowman
writes

Showing just how inefficient a petrol engine is.

Perhaps you'd better set about a more efficient way of converting a
high density fuel like petrol to mechanical energy, then. Electric
vehicles driven off storage batteries have been around for nearly as
long. If they were capable of being developed into a viable competitor,
it would have been done years ago.
Try diesel. Or indeed any compression ignition engine.

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?

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

"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.

"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?

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

In message m, Martin
Underwood writes

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.
A butterfly valve controls the flow of air in a petrol engine to keep
the air fuel mixture correct, thus causing a depression on the inlet
side. This is often used to power servo brakes.
I believe 5 star was phased out because of it's high lead content.
--
Clive

In article m,
Martin Underwood 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?

The lack of pumping losses - due to not having a throttle - was one
reason. But petrol engines are appearing with no throttles too now.

--
*Why isn't there mouse-flavoured cat food?

Dave Plowman London SW 12
RIP Acorn

In article ,
Clive wrote:
Showing just how inefficient a petrol engine is.

Perhaps you'd better set about a more efficient way of converting a
high density fuel like petrol to mechanical energy, then. Electric
vehicles driven off storage batteries have been around for nearly as
long. If they were capable of being developed into a viable competitor,
it would have been done years ago.

Try diesel. Or indeed any compression ignition engine.

They're still incredibly inefficient compared to a decent electric motor
- energy in against energy out.

Also, the difference between a petrol and diesel of similar output both
at full power isn't that great.

--
*A picture may be worth a thousand words, but it uses up a thousand times more memory.

Dave Plowman London SW 12
RIP Acorn

"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

In article m,
Martin Underwood wrote:
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?

It's the actual pressure caused by compression that's important, and a
throttle causes a partial vacuum - so compressing air at less than
atmospheric pressure gives a lower final pressure. Similarly, using a
supercharger etc increases 'atmospheric' pressure and the final compressed
pressure. If you could develop a supercharger which was super efficient,
you could negate the effect of the throttle.

--
*It was all so different before everything changed.

Dave Plowman London SW 12
RIP Acorn