"Neil Williams" wrote in message
...
On Mon, 29 Aug 2005 09:07:59 -0400, "David Spiro"
wrote:

I agree that diesel is more fuel efficient, but, IIRC, it also tends to be
more of a pollutant that regular gasoline. Has there been any improvement
in
Europe in combating this problem? As for automatic transmissions, they are
better at being fuel efficient on the highway, if there is a c"cruise
control" feature built in.

Cruise control is provided on most executive level manual-transmission
cars in the UK. If you think about it, it's not necessary to change
gear to maintain a constant speed on the motorway, certainly not in
the types of car that tend to have it, which tend to be those with
larger engines. It is fitted to more and more cars these days because
it's something that can be provided using a few lines of code in the
ECU and no additional physical hardware bar an on/off switch. It has
nothing at all to do with the transmission.

Cruise control is only really useful in very light traffic. As an experiment
I tried driving at a constant 60 and then 70 on a motorway /
dual-carriageway journey today. At 60, you have to remain in the left lane,
otherwise you get in the way of traffic wanting to go faster. But in the
left lane you fall foul of HGVs and Sunday drivers trundling along at 40 or
50, so you have to either reduce to their speed or else accelerate to a
suitable speed to overtake them. At 70, you can use the middle lane without
getting V signs from people who want to get past you but you still have to
keep coming off the power to re-create a safe distance from the car in front
as cars overtake and then cut in far too close in front of you. Late at
night, I'm sure a cruise control can be quite useful; likewise I'm sure it's
invaluable in making sure you don't exceed 30 and 40 limits, especially
where these feel exceptionally slow and there is a constant tendency to
speed up to a speed that feels right for the road conditions.

Automatics are probably less fuel efficent in city driving though,
and of that, I have no doubt.

Automatic transmissions in general are less fuel-efficient because
there are losses from the torque converter. If a converter lock-up
feature is provided, as it is with many such transmissions, the losses
can be reduced at motorway speeds, but when not engaged the losses
remain. The loss is fairly obvious in how gutless small-engined
automatic cars seem to be compared with the equivalent manual.

Part of this is due to the algorithm that is used to determine when the
gearbox changes down. My experience is that automatics are far too ready to
change to a low gear in situations where I would hold onto the higher gear
but floor the accelerator - for example, when accelerating out of a
roundabout, many automatics approach in top, stay in top while going round
and then (or this is how it feels) drop into first as soon as you apply
power on the far side: you either have sluggish acceleration with no power
or else they flip into giving kick-in-the-back acceleration if you apply
just slightly more power - there's no happy medium. I had an unfortunate
experience in a hire car (for a business trip): this automatic Ford Focus,
probably a 1.6, had a pathological aversion to accelerating and would change
down as you applied more and more power, so you always ended up going at 50,
but with a choice of gears and engine speeds! Not very easy when you're
trying to overtake cars on the motorway. When I reached my destination, I
reported it to the hire company and asked for a replacement car for the
return journey.

There are, of course, other types of automatic transmission that use
hydraulics to operate a conventional clutch and gearbox. These are
rather more efficient, but most people find that the driving
experience is uncomfortable, especially because you can lose power for
a couple of seconds at the "wrong" time while the gearbox shifts,
which can be downright dangerous at worst. CVTs are very efficient
indeed, but also don't last long enough so are uneconomic.

I've heard that the automatically-controlled manual gearboxes fitted in some
Citroen C3s and VW Golfs are the bext of both worlds: the efficiency of a
manual geabox because there's no wastage in the fluid flywheel and the
convenience of a geabox that can either be totally automatic or else
sequential-manual according to preference. A colleague was very impressed
with his C3 and demonstrated that he could change up and down at will, when
accelerating, decelerating or at constant speed, with virtually no jerkiness
or loss of power.