Automotive basics
The need for a transmission in an automobile
is a consequence of the characteristics of the internal
combustion engine. Engines typically operate over a range of 600
to about 6000 revolutions per minute (though this varies from
design to design and is typically less for diesel engines),
while the car's wheels rotate between 0 rpm and around 2500 rpm.
Furthermore, the engine provides its highest
torque outputs approximately in the middle of its range, while
often the greatest torque is required when the vehicle is moving
from rest or travelling slowly. Therefore, a system that
transforms the engine's output so that it can supply high torque
at low speeds, but also operate at highway speeds with the motor
still operating within its limits, is required. Transmissions
perform this transformation.
Most transmissions and gears used in
automotive and truck applications are contained in a cast iron
case, though sometimes aluminum is used for lower weight. There
are three shafts: a mainshaft, a countershaft, and an idler
shaft.
The mainshaft extends outside the case in both
directions: the input shaft towards the engine, and the output
shaft towards the rear axle (on rear wheel drive cars). The
shaft is suspended by the main bearings, and is split towards
the input end. At the point of the split, a pilot bearing holds
the shafts together. The gears and clutches ride on the
mainshaft, the gears being free to turn relative to the
mainshaft except when engaged by the clutches.
Manual transmission
Manual transmissions come in two basic types:
a simple unsynchronized system where gears are spinning freely
and must be synchronized by the operator to avoid noisy and
damaging "gear clash", and synchronized systems that will
automatically "mesh" while changing gears. The former type is
only used on some rally cars nowadays.
Manual transmissions dominate the car market
outside of North America. They are cheaper, lighter, usually
give better performance and fuel efficiency (although the latest
sophisticated automatic transmissions may yeld results slightly
closer to the ones yelded by Manual transmissions) and it is
customary for new drivers to learn, and be tested, on a car with
a manual gearchange. In Germany, the UK, and France at least, a
test pass using an automatic car does not entitle the driver to
use a manual car on the public road unless a second manual test
is taken. In most of the other European nations like Italy and
The Netherlands, obtaining a drivers licence is only possible by
passing a drivers test driving a car with manual transmission.
Automatic transmission
Most modern North American cars have an
automatic transmission that will select an appropriate gear
ratio without any operator intervention. They are primarily
using hydraulics to select gears, depending on pressure exerted
by fluid within the transmission assembly. Rather than using a
clutch to engage the transmission, a torque converter is put in
between the engine and transmission. It is possible for the
driver to control the number of gears in use or select reverse,
though precise control of which gear is in use is usually not
possible.
Automatic transmissions are easy to use. In
the past, automatic transmissions of this type have had a number
of problems, they were complex and expensive, and sometimes had
reliability problems (which sometimes caused more expense in
repair), and often have been less fuel-efficient than their
manual counterparts. With the advancement of modern automatic
transmissions this has changed. With computer technology,
considerable effort has been put into designing gearboxes based
on the simpler manual systems that use electronically-controlled
actuators to shift gears and manipulate the clutch, resolving
many of the drawbacks of a hydraulic automatic transmission.
Automatic transmissions have always been
extremely popular in the United States, where perhaps 19 of 20
new cars are sold with them (many vehicles are not available
with manual gearboxes anymore). In Europe automatic
transmissions are gaining popularity as well.
Attempts to improve the fuel efficiency of
automatic transmissions include the use of torque converters
which lock-up beyond a certain speed eliminating power loss, and
overdrive gears which automatically actuate above certain
speeds; in older transmissions both technologies could sometimes
become intrusive, when conditions are such that they constantly
cut in and out as speed and such load factors as grade or wind
vary slightly. Current computerized transmissions possess very
complex programming to both maximize fuel efficiency and
eliminate any intrusiveness.
For certain applications, the slippage
inherent in automatic transmissions can be advantageous; for
instance, in drag racing, the automatic transmission allows the
car to be stopped with the engine at a high rpm (the "stall
speed") to allow for a very quick launch when the brakes are
released; in fact, a common modification is to increase the
stall speed of the transmission. This is even more advantageous
for turbocharged engines, where the turbocharger needs to be
kept spinning at high rpm by a large flow of exhaust in order to
keep the boost pressure up and eliminate the turbo lag that
occurs when the engine is idling and the throttle is suddenly
opened.
Semi-automatic transmission
The creation of computer control also allowed
for a sort of half-breed transmission where the car handles
manipulation of the clutch automatically, but the driver can
still select the gear manually if desired. This is sometimes
called "clutchless manual". Many of these transmissions allow
the driver to give full control to the computer.
There are some specific types of this
transmission, including Tiptronic, Geartronic and Direct Shift
Gearbox.
There are also sequential transmission which
use the rotation of a drum to switch gears. A great example of
this is the 7-speed sequential transmission on the Bugatti
Veyron, a supercar that puts out 1,001 horsepower (746 kW) and
goes 254 miles per hour (409 km/h). |
