The new Panda 4x4 goes on sale in the UK powered by Fiat’s spirited FIRE 1.2 litre 8-valve, 60 bhp engine, which is mated, via a 5-speed manual gearbox, to a variable torque distribution four-wheel drive transmission system.
Four-Wheel Drive Transmission
Fiat’s new four-wheel drive transmission, developed in conjunction with GKN Driveline, is a simple, robust, yet highly effective system capable of mastering severe winter roads, challenging off-road conditions and gradients of more than 50%, with minimal compromise to the Panda 4x4’s on-road refinement.
The system features a front differential allied to a power transfer unit (PTU), a two-piece propshaft, a viscous coupling in front of the rear differential, a rear differential module, and front and rear driveshafts.
The key to the system’s efficiency is the viscous coupling.
How it works
In normal driving conditions, the Panda 4x4 is basically front-wheel drive, with the resultant benefits of fuel economy, tyre wear, etc. The automatic operation of the Panda 4x4 means that without any intervention by the driver, the system will automatically transfer power to the rear wheels when necessary. This is done by the viscous coupling, physically a small cylinder just in front of the rear axle into which the propshaft connects.
Basically, the viscous coupling consists of a sealed container within which there are a large number of metal discs, fixed alternately to the outer casing and the inner shaft. In order to increase the viscous drag, the discs fixed to the outer casing are perforated, whilst those on the shaft, which are free to slide along it, are slotted. The container is mostly filled with a silicone based fluid.
When there is little difference between the speed of the two sets of discs (ie when all the wheels are turning at the same speed) the viscous coupling has little effect, but if one of the front wheels begins to spin, then the set of discs attached to the casing will begin to rotate much faster than the other discs. This relative motion causes viscous drag to force the other discs to rotate, thus transmitting some of the torque to the output shaft. This is known as the viscous mode.
If the front wheels should spin considerably faster than the rear, the difference in the relative speeds of the discs will cause the temperature of the fluid inside the coupling to rise. This will cause it to expand, which will increase the area of discs in contact with the fluid, increasing the viscous drag and hence the transmission of torque.
Once the fluid expands to fill the coupling, further increases of temperature cause the pressure to build up inside the casing. The presence of the perforations and slots causes pressure differences to act on the discs, and those which can slide along the shaft will do so until they are in contact with the discs fixed to the casing. At this point the coupling acts as a friction clutch, giving maximum torque to the gripping wheels.
When the relative motion of the discs reduces, the temperature will drop, and the coupling reverts to its viscous mode until the wheels all rotate at the same speeds again.
Hence the viscous coupling provides a locking action proportional to the difference in speed between its input and output shafts; the higher the speed difference, the greater the torque transference, up to a maximum of a 50:50 split between front and rear wheels.
In addition to its simplicity, the new Panda 4x4’s durable, low maintenance drivetrain has several further advantages: the system is entirely automatic, requiring no driver input. It is entirely mechanical, requiring no electronic control. It has a benign impact on day-to-day driving dynamics, and provides excellent compatibility with the car’s ABS systems.The 1.2 8v FIRE has a 0-62 mph time of 20 seconds and a top speed of 90 mph. This tried and tested unit comes to the new Panda 4x4 with a number of recent refinements to further improve fuel economy without affecting its performance .Fuel consumption around town is 35.8 mpg, out of town 48.7 mpg and a combined figure of 42.8 mpg.
The engine is fitted with a sophisticated, Marelli I.A.W. phased sequential multipoint injection. New, convergent / divergent intake ports and new camshaft profiles optimise power and efficiency, and a new active knock sensor manages advance effectively, under all conditions.
Lighter componentry has also improved fuel consumption. In particular, a glassfibre reinforced polymer intake manifold has been developed with U-shaped branches and a built-in plenum chamber; while an auxiliary drive circuit features built-in mountings in aluminium squeeze castings, and an automatic low torque tensioner has also been developed for improved reliability. The mating clearance between crankshaft and crankcase has also been optimised through computerised selection of main bearings.
Quality of life on board has been improved through a special engine installation system which minimises the transfer of engine vibration to the body – a barycentric power unit mounting system consisting of two blocks plus a reaction rod that acts as a link. The new mounts are aligned along an axis that passes through the engine’s centre of gravity to obtain reaction forces with zero offset.
Additionally, the catalytic converter is welded to the side of the exhaust manifold within the engine bay, allowing it to reach high temperatures more rapidly, thus reducing emissions before the engine is at optimum running temperature.Published 31 January 2005