Fuel efficient vehicles have a choice of transmission type
- manual, automatic or CVT. So which of these would provide the greatest
fuel efficiency for a vehicle?
Manuals have a better fuel efficiency than auto versions
of an otherwise identical vehicle the vast majority of the time. A manual
transmission system is fundamentally a more efficient system because there
is a completely mechanical linking of engine, gearing and wheels. Manual
transmissions consist of different diameter cogs, a metal chain and a clutch.
The various cogs provide the different gears - first gear being the widest
cog, second gear the second widest, and so on. The clutch uncouples the
cogs from the engine in order that you can bring the vehicle to a halt or
change gear without having to switch off the engine. The clutch can also
be used to change the amount of torque applied by the engine to the wheels.
This simple coupling of the engine to the wheels in manual vehicles gives
high fuel efficiency.
Automatics are less fuel-efficient by design - they don't
maintain a continuous mechanical link between engine and wheels since the
system always includes a fluid filled torque converter. A torque converter
is a hydraulic fluid coupling between engine and gearing. Automatic transmission
is less fuel efficient than manual transmission primarily due to pumping
losses in the torque converter and hydraulic actuators. In addition a hydraulic
control system demands power from the engine. The system can be improved
to give better automatic fuel efficient vehicles by the inclusion of an
economy function. This results in upshifts at lower engine revs - something
which cannot be enforced in manuals!
CVT (continuously variable transmission) is not well understood
but offers fuel efficiency benefits over both manuals and automatics. This
type of transmission is becoming more widespread because of its advantages.
Like a manual it has a direct mechanical link between engine, gearing and
wheels. Unlike a manual it requires no clutch, no gear changing but infinite
gearing ratios. This is how CVT works. At the heart of the design is a single
cone shaped gear replacing the various different cogs in a manual gearbox.
If the vehicle is traveling slowly the wider end of the cone is used and
at fast speeds the smaller end is used. In effect the cone offers you an
unlimited number of gears. While true CVT adjusts the gearing ratio constantly
as you drive, many car manufacturers define gearing points on the cone which
when used with gear shifters give the feel of a manual without a clutch.
The direct link between engine and wheels makes CVT vehicles more fuel efficient
than an automatic. The fact that true CVT systems constantly change engine
rev speed as you drive also leads to more fuel efficient vehicles than manuals.
Engine revolutions in a manual for a specific gear are solely determined
by how fast the car is going. CVT vehicles can vary the gear ratio to keep
the engine revs at their most efficient.
Continuously variable transmission sounds very modern but in fact its been
around ages - around 60 years. For the vast majority of this time the CVT
transmission system has only been implemented in tandem with small engines
(scooters use CVT) because it wasn't strong enough to handle powerful engines.
Due to an improvement in the strength of materials and CVT technology, the
transmission system vastly improved around 10 years ago and is now able
to be reliably implemented in much more powerful vehicles. The use of continuously
variable transmission is now fairly widespread with manufacturers such as
Honda, Audi, Subaru and Nissan all using it.
All else about a car being equal, CVT definitely has the capability to be
the most fuel efficient transmission type, followed by manual gearboxes,
and bringing up the rear is automatic.