"Pooh Bear" wrote in message
...

carneyke wrote:

Grahm,
Hot is hot, no matter how you "analyze" it. Today's cars generate much
more heat and there is many more cars than 30 years ago. No science
involved Sherlock, just plain old common sense, I was blessed with and
you.........

I was simply pointing out that the heat generated is in direct relation to
the quantity of fuel used, as opposed to internal working temperatures
where higher temps often means improved efficiency which actually reduces
the overal heat output, like for like.

Of course if certain countries in particular would consider using smaller
engined vehicles then this problem could be vastly ameliorated. Many of
today's cars have power well beyond what is actually reasonably *required*
for personal transportation and as such, this profiligate use of energy
can be considered to be a luxury use.

Graham

Compared to some others in this thread, Graham, you're a physics genius.
However, putting relative efficiency aside, engine size is not as great a
factor as some think, and is far less of a factor today than it was 20 years
ago.

If it requires 50 hp to drive a car at a certain speed, it doesn't matter
(in theory) whether that comes from an engine that produces 50 hp or one
that can produce 200. However, the 200 hp engine would have to be geared
down (lower rpm for a given speed) to produce comparable efficiency, because
the big factor here is effective compression ratio. The larger engine is
running at a lower compression ratio, if it's producing only a fraction of
its potential torque at a given number of rpm, and that hurts it thermal
efficiency. (We're assuming spark-ignition engines here; diesels are another
kettle of fish.)

The peak-efficiency range of an engine is fairly narrow: too high, and you
have too much internal friction. Too low, and you have too much heat loss to
cylinder walls, piston crown, and combustion chamber.

But modern fuel injection and ignition timing can result in similar
*combustion* efficiency over quite a range of rpm. When the engine gets too
big relative to its normal operating demands, "gearing it down" will put it
in the too-low range of rpm. If you gear it for somewhat higher rpm (and
this is the normal condition), the engine is operating at a fraction of its
design compression ratio, and thermal efficiency goes to hell.

Overall, the mpg of cars with larger engines vs. smaller ones falls in a
narrower range than it used to. But there is still a spread, for the reasons
above. The big factors are total car weight and total drag (air drag plus
rolling resistance) and internal friction in the total driveline. Bigger
cars suck up more gas, even with relatively small engines.

Here's a sidelight that you may find interesting: When direct-port injection
had reached a fairly good level of sophistication, BMW ran tests on how much
fuel was required to reach a given speed. Unlike the conventional wisdom,
best efficiency in reaching speed was achieved at FULL THROTTLE, but also by
shifting at relatively low rpm. Effective compression ratio was near its
max; heat loss to cylinders was balanced by internal friction; and there was
nothing to be gained by accelerating more slowly.

So much for conventional wisdom. It goes in the crapper, along with "common
sense."

--
Ed Huntress