The Sedan PI and Utility PI from Ford come standard as AWD. Under normal conditions, both vehicles are driven by the front wheels. If the front wheels lose traction, even on dry pavement, the AWD system reacts within 0.1 milliseconds to distribute up to 100 percent of the torque to any one wheel or combination of wheels. The AWD paired with traction control can distribute up to 100 percent of the available torque to the wheel (or wheels) with the most traction.
In a FWD-priority system, if both rear wheels are deemed to have the same traction, AWD will distribute the available power evenly between the rear wheels. If the AWD detects that only one rear wheel has traction, all the rear torque will go to that wheel. If one rear wheel has more traction than the other rear wheel, the AWD will determine the right amount of torque to split between each wheel.
The AWD may distribute up to 100 percent of the torque to any one wheel, front or rear, left or right. For example, the left front tire may be the only one of the four with traction, so it will get all of the torque. Or it may be the right rear.
Importantly, the on-demand AWD systems also improve driving dynamics. Only stability control will limit understeer (or throttle-off oversteer) as you enter a corner or cross a median strip. However, both throttle-on understeer and throttle-on oversteer are greatly reduced by AWD.
The AWD system continuously monitors vehicle speed, throttle position, steering wheel angle and wheel slip to seamlessly deliver torque to the appropriate wheel. This helps to eliminate torque steer from the 305 hp 3.7L V6 in the Sedan PI, the 304 hp 3.7L V6 in the Utility PI and the 365 hp 3.5L EcoBoost V6 optional in the Sedan PI. In fact, this also helps to balance the Sedan PI and the Utility PI during hard cornering because in a curve, the front wheels are asked to both steer the vehicle and propel the vehicle.
AWD System Operation
How does the AWD system operate on various surfaces such as dry pavement and slippery roads, i.e., wet, gravel, sand, grass, snow, ice?
The operation of the AWD system on any surface is basically the same. The AWD system has two basic modes of torque transfer: pre-emptive torque (before wheel slip) and slip control torque (after wheel slip).
When the vehicle accelerates from a stop on any surface, dry pavement, wet pavement, sand, gravel, snow, ice, mud... there is some pre-emptive torque transferred from the front wheels to the rear wheels. This pre-emptive torque is transferred because the AWD system anticipates there could be wheel slip in this situation.
Depending on how quickly the driver wants to accelerate and how slippery the surface, this pre-emptive torque might not be enough torque to control front wheel slip. In this case, slip control torque will be added to equalize front and rear wheel speeds.
On dry pavement, usually pre-emptive torque is sufficient to control wheel slip. On slippery surfaces (wet pavement, gravel, snow, ice), various amounts of slip control torque are added depending on the driving situation to control wheel slip. During driving on any surface, the AWD commanded torque is constantly varying based on driving conditions.
How does AWD operate on snow or ice when three of the four wheels are without traction?
Police Interceptors have brake traction control standard when equipped with AWD. During a situation where only one wheel has traction, the AWD system works together with the brake traction control system to send torque to the wheel with traction.
For example, if a driver is trying to accelerate with three wheels on ice and only the left rear wheel on pavement, the AWD system would send torque to the rear axle. Then, the brake traction control system would apply braking force to the spinning right rear wheel, this then causes torque to be transferred to the left rear wheel. The vehicle now is sending increased torque to the one wheel with traction.
Dry Pavement
How does AWD operate when cornering on dry pavement?
During a very mild cornering event, the AWD system may not send any torque to the rear axle. However, if the cornering gets more aggressive, the AWD system will start to send pre-emptive torque to the rear axle in the anticipation of wheel slip to improve vehicle handling. If the driver is very aggressive, pre-emptive torque and slip control torque will be sent to the rear axle to control wheel slip and improve vehicle handling.
Police Interceptors also have yaw feedback as an input to the AWD system. For example, if the vehicle is in an aggressive handling event, AWD-sent torque will be adjusted based on the lateral acceleration and yaw of the vehicle to help keep the vehicle on the driver's intended path.
During very tight corner turns, such as parking lot maneuvers, the AWD system detects the vehicle is in a tight turn and reduces torque to the rear axle so there is no binding or “crow-hop” felt in the vehicle. However, the system has the ability to increase torque to the rear axle if any wheel slip did occur in this situation.