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...)