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Monday, January 18, 2016

Fiat 500X All Wheel Drive System Explored

Fiat 500X at FCA Proving Grounds in Sweden

The Fiat 500X uses an advanced all-wheel drive system that guarantees excellent traction in all weather conditions. Heart of the system is a lightweight and compact front Power Transfer Unit (PTU) that selectively engages all wheel drive when needed and a fully disconnecting Rear Differential Unit (RDU).

Conventional all-wheel drive systems that are engaged all the time typically lose efficiency and fuel economy because axles and the propeller shafts are needlessly turning all the time when not needed. This decrease in fuel economy can be significant for an AWD vehicle. The accepted industry standard is that 80% of losses in AWD are frictional losses, and just 20% are from the additional weight of the components.* A disconnecting AWD system like the one used in the 500X can reduce these frictional losses around 80%. On top of that, the 500X uses the lightest, most compact disconnect AWD system available in the market further improving the 500X's efficiency.**

Fiat 500X Rear Differential Unit

The end result is the 500X AWD system has all the efficiency of a front-wheel drive car while having all-wheel drive capability available within milliseconds. The EPA rates the 500X as getting 30 mpg on the highway, which I can confirm in my own car, achieving a high of 32.7 mpg during a vacation trip.

This also helps the 500X drive like a European grand touring car, and not a clunky, truck-like SUV. That means safe and predictable handling, excellent high-speed stability, and great steering turn in for a nimble feel on mountain roads.

The added benefit of the AWD system in the 500X includes:
  • Superior straight line acceleration, and cornering on all surfaces.
  • Better traction and handling under adverse conditions.
  • Improved hill climbing ability and safer driving.

Here is a look at how the 500X AWD system achieves these benefits and its various components.

Fiat 500X Power Transfer Unit

Power Transfer Unit (PTU)

The All-Wheel-Drive 500X has a compact Power Transfer Unit (PTU) attached to the right side of the transmission. The PTU engages and disengages the propeller shaft via a dog-clutch system controlled by the Drivetrain Control Module (DTCM) and operated by an actuator that is bolted to the PTU.

Compared to a conventional PTU, the 500X PTU is a monoblock housing that fully integrates the propeller shaft's constant velocity joint making it extremely compact. This design also yields superior noise and comfort characteristics, benefiting the overall refinement of the vehicle.

The DTCM uses information received from sensors including engine load, vehicle speed, gear selection and drive mode selected to determine when to engage the propeller shaft. The system is very intelligent and even takes in account driving style, outside temperature, and even wiper operation when making its calculations.

In addition, the DTCM uses the following to determine propeller shaft engagement:
  • ESC button position
  • Environmental conditions such as surface grade
  • Launch characteristics
  • Related ESC events
  • Dynamic factors including wheel slip; detection of understeer/oversteer; steering angle rate; and acceleration/deceleration rates.

Based on these inputs and others, the system is capable of anticipating and then delivering, within 300 milliseconds or less, infinitely variable amounts of torque combinations between the front and rear driven wheels. Up to 2000 Nm of engine torque can be sent to the rear of the car when conditions demand.

Fiat 500X All-Wheel Drive Comparison Tests Videos

Watch this Fiat 500X tackle this graphic test of its AWD system.  Then compare it to the two cars below.

Propeller Shaft

The split propeller shaft runs front to back and carries the power from the PTU to the Rear Differential Unit.

When the Drive Selector is set to Auto Mode, the PTU always keeps the propeller shaft engaged when starting from a standstill or speeds close to 0 km/h (0 mph).

The propeller shaft disengages when the car exceeds a set speed; the engine load is not too high and the road conditions are ideal.

The propeller shaft will re-engages during the following conditions:
  • When traveling over irregular surfaces that can affect the safety and stability of the car.
  • When the car speed returns close to 0 km/h (0 mph).

The propeller shaft is always engaged when the Drive Selector is set to "Traction" or "Sport" mode.

Rear Differential Unit (RDU)

The Fiat 500X Rear Differential Unit (RDU) contains an electronically controlled multi-disc wet clutch that can vary the torque transmitted to the rear wheels based on patented algorithms programmed into the Drivetrain Control Module (DTCM). When AWD is not needed, the axle disconnects and the rear wheels can free-wheel. While some other AWD systems can disconnect drive to the rear axle, the Fiat 500X system can disconnect it fully*** allowing the wheels to effectively free-wheel, and the propeller shaft and gears in the rear differential not to turn.

All in all, the 500X's disconnect system can improve fuel economy by up to 4 percent compared to permanent AWD systems.

Fiat 500X in Snow

Electronic Traction Control

The Fiat 500X uses its braking system to enhance its AWD capabilities. For example, the Brake-Lock Differential System (BLDS) offers similar performance as a locking differential. If one wheel starts slipping, the Anti-lock Brake System (ABS) slows the spinning wheel. This automatically transfers more drive torque to the wheel with traction. The advantages of BLDS over a mechanical locking differential are faster intervention and lighter weight.

The Brake Traction-Control System (BTCS) helps to keep driving wheels from spinning during acceleration from a stop or during slow speeds by applying individual brakes to the slipping wheel(s). This sends more drive torque to the wheels with traction and results in better and safer performance.

Electronic Stability Control (ESC) enhances driver control and helps maintain directional stability under all conditions including turns, and when driving on snow, ice or gravel. If there is a discernible difference between driver input through the steering wheel and the vehicle’s path, ESC applies selective braking and throttle input to guide the vehicle back onto the driver’s intended path.

Fiat 500X Dynamic Drive Mode Selector

Fiat 500X Dynamic Drive Mode Selector

The Fiat 500X Drive Mode Selector allows drivers to select 3 different drive modes that tailor how the vehicle drives and responds according to their needs and road and traffic conditions. The drive mode selector alters engine and steering response, transmission and AWD system characteristics, as well as the ESC system. To select a drive mode, the driver twists and holds the selector located on the center console to the appropriate setting for at least half a second and releases the knob. The mode selected is displayed on the instrument panel and the corresponding LED on the selector knob.

"Auto" MODE
This mode is the default setting and emphasizes comfort and safety in normal grip and driving conditions. On AWD versions, this mode also reduces fuel consumption, as it automatically disconnects the transmission from the rear wheels when road and driving conditions allow it. When AWD is called for, drive torque is automatically distributed between the front and rear axles.

When starting from a standstill or speeds close to 0 km/h (0 mph) the propeller shaft is always engaged. This allows the system to activate all-wheel drive quickly if the front wheels lose traction. The system will engage all-wheel drive in the event of rapid acceleration from a stop in anticipation of wheel spin. When the vehicle speed exceeds a set threshold, the engine load is not too high, and the road conditions are ideal, the system will disengage the propeller shaft to save fuel.

The propeller shaft will re-engage when:
  • Rough road surfaces compromise the dynamics/safety of the car.
  • When the car speed returns to close to 0 km/h (0 mph).

"Traction+"/" All Weather" MODE
This mode is used for increased safety in wet or slippery road surfaces in rain and snow. In vehicles with AWD, use is also recommended on unsurfaced roads or off-road.

The propeller shaft is always engaged in Traction+ mode. The DTCM tells the rear differential unit's multi-disc clutch to transmit torque to the rear wheels with a control logic that favors off-road driving. To help keep wheels from spinning, throttle response is slowed down, and the transmission will start off in second gear. The transmission shifts are also more drawn out to keep shifting from upsetting stability.

"Sport" MODE
Sport mode highlights the sporting attributes of the 500X. Traction is optimized for sport driving and road holding throughout turns and straights.

Sport mode keeps the propeller shaft always engaged. The DTCM controls the Rear Differential Unit's clutch to increase the torque reaching the rear wheels as quickly as possible.

Using Sport mode sharpens throttle response and recalibrates transmission shift points so gears are held longer and downshifts are quicker. Steering assist is also reduced for better feel.

Read more:

Images and Video: Courtesy of FCA LLC
Video: AWD Tests CarQuestion Youtube Channel
Sources: FCA, GKN
With thanks to FCA US LLC


1 comment: said...

Make sure you visit the Fiat 500X Forum :