Four-wheel Drive - Unusual Systems

Unusual Systems

Prompted by a perceived need for a simple, inexpensive all-terrain vehicle for oil exploration in North Africa, the French motor manufacturer Citroën developed the 2CV Sahara. Unlike other 4×4 vehicles which use a conventional transfer case to drive the front and rear axle, the Sahara had two engines, each independently driving a separate axle, with the rear engine facing backwards. The two throttles, clutches and gear change mechanisms could be linked, so the two 12 hp (9 kW) 425 cc (26 cu in) engines could run together, or they could be split and the car driven solely by either engine. Combined with twin fuel tanks and twin batteries (which could be set up to run either or both engines), the redundancy of two separate drive trains meant that they could make it back to civilization even after major mechanical failures. Only around 700 of these cars were built, and only 27 are known to exist today.

BMC experimented with a twin-engine Mini Moke (dubbed the "Twini Moke") in the mid-1960s, but never put it into production. This made advantage of the Mini's 'power pack' layout, with a transverse engine and the gearbox in the engine sump. Simply by fitting a second engine/gearbox unit across the rear, a rudimentary 4×4 system could be produced. Early prototypes had separate gear levers and clutch systems for each engine. Later versions sent for evaluation by the British Army had more user-friendly linked systems.

In 1965, A. J. M. Chadwick, patented, GB 1113068, a 4WD system that used hemispherical wheels for an all-terrain vehicle. Twenty years later, B. T. E. Warne, patented, GB 2172558, an improvement on Chadwick's design that did not use differential gear assemblies. By using near-spherical wheels with provision to tilt and turn each wheel co-ordinatively, the driven wheels maintain constant traction. Furthermore, all driven wheels steer and, as pairing of wheels is not necessary, vehicles with an odd number of wheels are possible without affecting the system's integrity. Progressive deceleration is made possible by dynamically changing the front-to-rear effective wheel diameter ratios.

Suzuki Motors introduced the Suzuki Escudo Pikes Peak Edition in 1996. Earlier Suzuki versions were twin engined, from 1996 on the engine is a twin-turbocharged 2.0 L V6, mated to a sequential 6-speed manual transmission.

Nissan Motors has developed a system called E4WD. It designed for cars that are normally front-wheel drive, however the rear wheels are powered by electric motors. This system was introduced in some variants of the Nissan Cube and Tiida. (This is similar to the system used on the Ford Escape Hybrid AWD.)

Chrysler's Jeep Division debuted the twin engine, 670 hp (500 kW) Jeep Hurricane concept at the 2005 North American International Auto Show in Detroit. This vehicle has a unique "crab crawl" capability, which allows it to rotate 360° in place. This is accomplished by driving the left wheels as a pair and right wheels as a pair, as opposed to driving the front and rear pairs. A central gearbox allows one side to drive in the opposite direction to the other. It also has dual Hemi V8s.

Some hybrid vehicles such as the Lexus RX400h provide power to an AWD system through a pair of electric motors, one to the front wheels and one to the rear. In the case of the AWD model version of the Lexus RX400h (and its Toyota-branded counterpart, the Harrier hybrid), the front wheels can also receive drive power directly from the vehicle's gasoline engine as well as via the electric motors, whereas the rear wheels derive power only from the second electric motor. Transfer of power is managed automatically by internal electronics based on traction conditions and need, making this an all-wheel-drive system.

The 4RM system used in the Ferrari FF is unique in that it uses two gearboxes, in addition to the traditional Ferrari rear transaxle behind the engine, the system also uses a smaller transaxle drawing power from the front of the engine, sending it to the front wheels as needed. To save space and weight the front transaxle only has three forward speeds (and no reverse). Drive to the front wheels is transmitted through two infinitely-variable clutch packs which are allowed to 'slip' to give the required road wheel speeds. The system usually operates in the rear-drive only mode and only engages the clutches for the front transaxle when the rear wheels start losing grip.