scott ) gurgled happily, sounding much like they were
saying :

I don't often drive through "fluid",

Really? Mastered the art of driving through solids or in a vacuum
have you? ;-)

How hard IS it raining where you are? It's quite sunny here.

A fairer example would be cycling.

Cycle ten miles, on the flat, at a certain pedal cadence (engine rpm)
in a low gear.

Now cycle ten miles, on the flat, at the same pedal cadence (engine
rpm) in a higher but still comfortable gear.

Yes, if I go fast cycling I get hot and knackered.

As I said, in a *comfortable* gear.

On my bike I can get to about 20mph for a few minutes at a time, if I
drop that to 15mph I can go for *much* longer.

So let's assume a very low gear and walking speed, and 10-15mph.

If you're cruising (low throttle opening) in a car in a highish gear,
at low revs, that's bound to emit less pollution for a given journey
than similar revs in a lower gear at lower speed - because those revs
are being used for far less time.

Yes, but you'll be using less petrol. A lower speed = less power from
the engine. This = less pollution.

But the engine's turning at the same speed for both. I explicitly said
that. Yes, there's a certain amount more load in the higher gear, but I
also explicitly stated that we weren't talking about a high load
situation akin to your 20mph on your bike.

How do you work that one out? If I'm using more power to go faster,
surely I need to be using more petrol? As air resistance increases
with speed squared, the amount of petrol used goes up quite quickly
once you get to higher speeds.

Which is cancelled out by the higher efficiency of being in a higher
gear. Obviously, there's a point where that's not true, but almost any
car will sit at 40mph or so with virtually no throttle. Try it.

"Adrian" wrote in message
. 1.4...
W K ) gurgled happily, sounding much like they were
saying :

As I stated elsewhere, the only place I have seen such things
discusses was by people who loved their monstorous 4x4s

*Bzzzzt*

I can't stand "monstrous 4x4s"

Discussed by people who seemed to be quoting actual figures or research.

"W K" wrote in message
...

"Nick Finnigan" wrote in message
...
"W K" wrote in message
...

london-research speed data used to be, which
showed pollution per km is best around 45mph (except NOx)

Perhaps because in london, any road where you are going less than 40 is
horrendously stop start.

Even supposing that the research was London specific, where
could you average 38mph and be horrendously stop start?

Are there many roads within london where you can do an average 38?

I don't know.

I was going to say 30.
I'd still be interested how they got the information.

Unfortunately, www.london-research.co.uk seems to have
disappeared of the face of the world.

scott wrote:

How do you work that one out? If I'm using more power to go faster,
surely I need to be using more petrol? As air resistance increases
with speed squared, the amount of petrol used goes up quite quickly
once you get to higher speeds.

As I've said before, it depends on the car/engine. My top speed is 130mph
(not had it above 120 - but still plenty of puff left), so at motorway
speeds it's running around 4000rpm (80-ish) with virtually no throttle
applied. A smaller engined car will usually not have much in hand at those
speeds - notice what happens when a small car starts to overtake at the
bottom of a hill. It will usually run out of power and drop back.

Same in town. Smaller engined car drivers are up and down the gearbox to
make progress at speeds varying from 10 to 30mph where I can stay
*comfortably* in 3rd gear through all that range without creating huge gaps
or having to brake sharply.

So. Whose engine is working harder?

--
Phil ,,,^.".^,,,


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Adrian wrote:
scott ) gurgled happily, sounding much like they were
saying :

I don't often drive through "fluid",

Really? Mastered the art of driving through solids or in a vacuum
have you? ;-)

How hard IS it raining where you are? It's quite sunny here.

Eh? I assume you realise that air is a fluid and I'm missing something
obvious here...

A fairer example would be cycling.

Cycle ten miles, on the flat, at a certain pedal cadence (engine
rpm) in a low gear.

Now cycle ten miles, on the flat, at the same pedal cadence
(engine rpm) in a higher but still comfortable gear.

Yes, if I go fast cycling I get hot and knackered.

As I said, in a *comfortable* gear.

On my bike I can get to about 20mph for a few minutes at a time, if
I drop that to 15mph I can go for *much* longer.

So let's assume a very low gear and walking speed, and 10-15mph.

If you're cruising (low throttle opening) in a car in a highish
gear, at low revs, that's bound to emit less pollution for a
given journey than similar revs in a lower gear at lower speed -
because those revs are being used for far less time.

Yes, but you'll be using less petrol. A lower speed = less power
from the engine. This = less pollution.

But the engine's turning at the same speed for both. I explicitly said
that. Yes, there's a certain amount more load in the higher gear, but
I also explicitly stated that we weren't talking about a high load
situation akin to your 20mph on your bike.

Well fair enough, but there will still be more power being generated at
higher speeds. THe power required = force required times speed. The times
speed bit sorts out the "you'll be going for a shorter time" argument, so
it's purely down to the force. If the force were constant for all speeds,
then the pollution for a give journey would be constant no matter how fast
you went. The force however increases with speed so the faster you go the
more energy is used. This is the same for travelling through any fluid (ie
force proportional to speed^2).

How do you work that one out? If I'm using more power to go faster,
surely I need to be using more petrol? As air resistance increases
with speed squared, the amount of petrol used goes up quite quickly
once you get to higher speeds.

Which is cancelled out by the higher efficiency of being in a higher
gear. Obviously, there's a point where that's not true, but almost any
car will sit at 40mph or so with virtually no throttle. Try it.

Ah, the efficiency of the engine, I'm glad you mention that! What's the
efficiency in the two situations you describe then? How does that compare
with the difference in power/force at two different speeds? (Hint: the
efficiency of petrol engines varies by very little across their working
range, eg compared to humans...)

Redonda wrote:
scott wrote:

How do you work that one out? If I'm using more power to go faster,
surely I need to be using more petrol? As air resistance increases
with speed squared, the amount of petrol used goes up quite quickly
once you get to higher speeds.

As I've said before, it depends on the car/engine. My top speed is
130mph (not had it above 120 - but still plenty of puff left), so at
motorway speeds it's running around 4000rpm (80-ish) with virtually
no throttle applied. A smaller engined car will usually not have
much in hand at those speeds - notice what happens when a small car
starts to overtake at the bottom of a hill. It will usually run out
of power and drop back.

Same in town. Smaller engined car drivers are up and down the
gearbox to make progress at speeds varying from 10 to 30mph where I
can stay *comfortably* in 3rd gear through all that range without
creating huge gaps or having to brake sharply.

So. Whose engine is working harder?

Well assuming both drive the same speed, the bigger car is probably
generating more power as it has to shift more weight around. Assuming both
engines are equally efficient, the bigger car is therefore generating more
pollution. At constant speeds the power will be closer matched, although
the bigger car probably has more drag, so again, bigger car makes more
pollution.

scott ) gurgled happily, sounding much like they were
saying :

So. Whose engine is working harder?

Well assuming both drive the same speed, the bigger car is probably
generating more power as it has to shift more weight around.

Yet the bigger engine may well be running at much lower load.

Assuming
both engines are equally efficient, the bigger car is therefore
generating more pollution. At constant speeds the power will be
closer matched, although the bigger car probably has more drag, so
again, bigger car makes more pollution.

Not so. It's far easier to make a large car more aerodynamic than a short
one, both in terms of pure aerodynamics and in terms of packaging. Frontal
area comes into play via CdA, but there's not that huge a difference
between the frontal area of a short car and a long one where both are
required to offer similar interior space, as the long car doesn't have to
be as tall.

In short, it's impossible to say.

On 6 Apr 2004 08:23:45 GMT, Adrian
wrote (more or less):

scott ) gurgled happily, sounding much like they were
saying :

So. Whose engine is working harder?

Well assuming both drive the same speed, the bigger car is probably
generating more power as it has to shift more weight around.

Yet the bigger engine may well be running at much lower load.

Assuming
both engines are equally efficient, the bigger car is therefore
generating more pollution. At constant speeds the power will be
closer matched, although the bigger car probably has more drag, so
again, bigger car makes more pollution.

Not so. It's far easier to make a large car more aerodynamic than a short
one, both in terms of pure aerodynamics and in terms of packaging. Frontal
area comes into play via CdA, but there's not that huge a difference
between the frontal area of a short car and a long one where both are
required to offer similar interior space, as the long car doesn't have to
be as tall.

In short, it's impossible to say.

Of course, aerodynamics don't come into play until high speeds are
reached. (IIRC air resistance dominates over rolling resistance from
about 60mph)

At low and medium speeds, rooling resistance is more significant. In
which weight /does/ play a significant part.


Cheers,
Euan
Gawnsoft: http://www.gawnsoft.co.sr
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Smalltalk links (harvested from comp.lang.smalltalk) http://html.dnsalias.net/gawnsoft/smalltalk

Adrian wrote:
scott ) gurgled happily, sounding much like they were
saying :

So. Whose engine is working harder?

Well assuming both drive the same speed, the bigger car is probably
generating more power as it has to shift more weight around.

Yet the bigger engine may well be running at much lower load.

What do you mean by "load"? In absolute terms, the heavier car's engine
will always be developing more power to make it move the same as a lighter
car. If you mean what % of maximum power though, that is going to depend on
the power to weight ratio of the car. I thought this was about pollution
and emissions, in which case I think the raw amount of fuel burnt would be
the main factor. Just look at the fuel economy factors for little cars and
big cars!

Assuming
both engines are equally efficient, the bigger car is therefore
generating more pollution. At constant speeds the power will be
closer matched, although the bigger car probably has more drag, so
again, bigger car makes more pollution.

Not so. It's far easier to make a large car more aerodynamic than a
short one, both in terms of pure aerodynamics and in terms of
packaging. Frontal area comes into play via CdA, but there's not that
huge a difference between the frontal area of a short car and a long
one where both are required to offer similar interior space, as the
long car doesn't have to be as tall.

In short, it's impossible to say.

I agree, you'd have to measure the drag on two cars, you can't say that all
big cars create more drag than smaller cars or vice versa. Longer cars do
tend to have a bigger frontal area though IMO, otherwise they'd look silly
like a stretch-mini or something! More importantly though, anytime you want
to accelerate the lighter car will always use less power.

scott wrote:

How do you work that one out? If I'm using more power to go faster, surely
I need to be using more petrol?

That depends how much more power you're using and how much faster you're
going.

As air resistance increases with speed squared, the amount of petrol used
goes up quite quickly once you get to higher speeds.

Assuming you're measuring by distance, the amount of energy used to
overcome air resistance is proportional to speed squared. However there
are other factors to consider. The amount needed to accelerate a car is
proportional to the speed you're accelerating to (minus the speed you're
accelerating from) and IIRC rolling resistance is proportional to the
distance travelled. Then there's the energy needed to keep the engine
turning (which is proportional to the time taken, so you use more if you
go more slowly.

However, the characteristics of engines vary widely - for each design,
efficiency varies differently according to speed, load, and how hard you
accelerate them. Until you know how they do, you won't be able to
determine anything much.