By
EWAN KENNEDY
29 January 2007
I've had a couple of recent conversations with people who
have just bought new cars and there seems to be some misunderstanding
about the latest-generation automatic transmissions. With these
mega-ratio autos being introduced seemingly by the week, this may be as
good a time to discuss how they differ from the older transmissions.
Many automatics these days run either five or six
forward ratios (speeds), with Mercedes trumping the others by having a
seven-speed unit in many of its models. Lexus is soon to join the ‘mine
is bigger than yours’ game with an eight-speed automatic transmission
in its upcoming LS460.
The extra ratios aren't just there to give the marketing
gurus something to boast about. Their prime function is to reduce fuel
consumption and therefore exhaust emissions. As a real bonus they also
provide better acceleration and hillclimbing ability.
Everything else being equal, the lower the engine revs
at a steady speed the less fuel is used. And the higher the revs, again
at a fixed speed, the more power is generated. The more forward gears
that are provided, the easier it is for the engine to be in the correct
one at the right time for circumstances that can change moment by
moment.
As well as having more speeds, today’s autos also have
more intelligence. That is electronic control by way of information
being provided by various sensors around the car. Info on speed,
throttle position, road gradient, even outside temperature. The best
automatics can even communicate electronically with the engine so know
how hard, or otherwise, the engine is working at the time.
Information flows the other way as well. The
transmission can send a message to the engine to let it know it’s
about to change gears. The engine responds by backing off its torque
momentarily to smooth out the shift.
Clever stuff, but I've had people coming to me in some
concern, saying that their new car is changing gears too often. For
example, one friend actually chose not to buy a car because on the test
drive he could feel it change down from sixth to fifth on even slight
hills.
I explained that the sixth gear was a tall one designed
purely for easy cruising and that fifth was on hand as soon as a bit
more oomph was needed. But having learned to drive in the era where
drivers boasted that their car could be left in top gear almost all the
time, he really took a dislike to the gearchange programming of the
six-speed.
At the other end of the auto ratios, I've also had
people say that their new car takes off too quickly. That’s because
the extra ratios in the box allow the fitment of a very low first gear.
This gives it plenty of what road testers call ‘launch feel’, but
should you not like that, simply use less accelerator off the line. You
have almost certainly been unconsciously using plenty of pedal in your
older car because the higher-geared first demanded extra power from the
engine.
May I suggest that the next time you’re test driving a
new car with more gears than you have ever had before, you give it a
longer run than normal to let your mind acclimatise to the new way of
doing things?
You certainly won’t be sorry in the long run,
especially if a long run in that just-purchased car sees you putting
significantly less petrol in the tank than you had anticipated.
