Hendon Publishing - Article Archive Details

Print Article Rate Comment Reprint Information

Ford Tough AWD in Police Interceptor

Written by Police Fleet Manager Staff

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.

How does AWD operate when accelerating at highway speeds on dry pavement? During mild accelerations on dry pavement at highway speeds, the AWD system may not send any torque to the rear axle. The vehicle will essentially be a front-wheel drive. As the accelerations get more aggressive, more and more pre-emptive torque will be commanded to the rear axle by the AWD system. At highway speeds on dry pavement, Police Interceptors usually do not require slip control torque to control wheel slip.

 

Aggressive Driving

Under aggressive driving, the two tasks expected of the front wheels (steer and propel) cause a certain amount of tire slip, even if the driver does not realize it. When that happens, the AWD system instantly sends power to the rear tires, allowing the rear wheels to help propel the vehicle. This, in turn, allows the front tires to apply more of their traction to steer the vehicle.

Under aggressive driving or low traction road conditions, all four wheels will have some amount of power sent to it. Under heavy throttle around a right hand turn, the left front tire will be running at the greatest slip angle, followed by the left rear tire, also at a significant slip angle.

The right front tire will be unloaded during body roll, as will the right rear tire, and both are likely to spin if given too much power. In this example, all four tires will receive different amount of torque...updated 100 times a second. Eventually, all the torque will transfer back to the front wheels.

 

Snow Sports

The Police Fleet Manager staff was sent one of the first AWD PI Sedans and was lucky enough to immediately get a week of heavy snow. Even with the factory All-Season tires, the traction was still totally unlike anything in any police sedan or crossover. Fresh snow, packed snow, slushy snow…the result was the same.

The PI Sedan found the tire(s) with the most grip and the vehicle was launched. The PI Sedan had so much traction during acceleration, we had to immediately start thinking about prudent braking distances on snow-covered roads.

A previously flooded parking lot showed the AWD system at its extreme best. The lot had large sections of glare ice, some sections of snow-covered ice, and intermittent patches of dry pavement. With the front wheels on ice and the rear wheels on snow, the PI Sedan shifted torque to the rear and just took off. When the front wheels made it to dry pavement, putting the rear wheels now on ice, the torque shift to the front and we lunged ahead even more.

With the left side on dry pavement and the right side on ice, the PI Sedan jumped forward just as if all four tires were on dry pavement. Even when power braking the 300 hp V6 engine, the front wheels did not spin on takeoff. All you get is a lunge forward. We can’t praise this AWD system enough. You can’t spin the tires on wet pavement, grass or gravel. All you get is traction and acceleration.

 

Volvo XC90-based

While new to law enforcement vehicles, the AWD system used on the Sedan Police Interceptor and Utility Police Interceptor is not new. This AWD system has been used on the entire Volvo line since 1998: Volvo S60 sedan, Volvo V70 wagon, Volvo XC90 SUV. The AWD system from Haldex Traction is currently in its Fourth Generation. Other vehicles using this Haldex system are the Cadillac SRX crossover, Audi A3 Quattro, and Land Rover LR2.

This AWD system was introduced on the 2005 Ford Five Hundred, which became the 2008 Taurus, and later the 2010 Taurus, and now the 2013 PI Sedan and 2013 PI Utility. That means this AWD system has been tracked in the Ford warranty claims and service parts systems for eight years.

Significant reliability and durability issues first show up was warranty issues, followed by redesigns, special service bulletins and/or technical service bulletins. According to Ford, this AWD system simply has not been flagged by warranty claims or by high number of service parts—not in retail use and not in 18 months of police service.

All Ford police package vehicles, beginning with the two NextGen Police Interceptors, include a five-year/100K-mile powertrain warranty. The bumper-to-bumper warranty remains at three years/36K miles. The AWD system—in police use—has a 100K-mile warranty.

 

Tire Wear Tolerant

Some AWD drivetrains are so sensitive that you must have exactly the same tread depth/tire wear on all four times. If your times are half worn and you must replace one tire with a new one, you must actually replace all four tires so the trend depth (overall tire height-diameter) stays the same.

Not so with the Ford AWD system. The AWD controllers are tolerant of any tread depth differences. The PI Sedan and PI Utility can accommodate two new tires on the front and two half-worn tries on the rear, or one new one or three new ones.

 

Police-only Cooling

The police version of AWD has three unique features to improve durability during severe use. First, the Power Transfer Unit on the AWD system uses a police-only auxiliary cooler. The retail AWD unit is air cooled, i.e., the aluminum housing dissipates heat to the surrounding air. When the vehicle is stationary, the amount of cooling is obviously less than when the vehicle is moving.

On the police AWD unit, the front housing of the Power Transfer Unit has a special water jacket cast into the assembly. The integral PTU auxiliary cooler is part of the water cooling system. That means the cooling remains “active” since the cooling water is constantly circulated, whether the police vehicle is moving or stationary. The PTU cooler is plumbed into the bottom of the radiator, where the water is cooler than at the top of the radiator.

Second, the Power Transfer Unit uses an internal temperature sensor, not found on the retail unit. A module keeps track of oil temperature in the PTU and the amount of time spent at that temperature. An electric clutch pack engages and disengages as torque is transferred to the rear wheels.

Both the PI Sedan and PI Utility default to FWD under normal conditions. As the front wheels slip, torque is selectively diverted to the rear wheels. A time versus temp logarithm activates an Oil Minder light in the driver message center if and when it is time to change the PTU fluid. The special synthetic oil may last 100K miles in normal police use.

Third, the Rear Differential Unit on the police version uses a fluid-filled clutch pack, while the retail version uses dry clutches. The fluid-filled RDU can withstand more severe duty.

The cooling ability of the police-only auxiliary radiator-water jacket combination was tested in 110 deg. F weather. Hard driving—where power is constantly shifting from front to rear, back and forth—can take many different forms.

Think aggressive driving during a city pursuit: hard throttle for a city block, 90-degree turn, hard throttle, where torque definitely shifts from front to rear. Also think getting unstuck in desert sand. Even though the vehicle and wheel speeds are not high, torque is constantly shifting between the front and rear spinning wheels.

Under the worst conditions of high-torque, clutch engagement and disengagement, the synthetic oil reached temperatures around 195 deg F. The special lube is rated for temps just over 500 deg F. The air-to-water cooler works.

 

EVOC Instructor Input 

In addition to durability testing, the AWD system on Police Interceptors has also conducted a great deal of development testing. Extensive AWD calibration work has been conducted at Ford’s winter testing facility to confirm the AWD system on Police vehicles has optimal traction and handling on snow and ice surfaces. The goal was to provide traction capabilities of a part-time 4x4 system while also having handling benefits that are above and beyond a conventional 2WD vehicle.

The AWD system was also optimized with direct feedback from EVOC instructors at MSP and LASD for limited handling performance. This co-development generated an AWD calibration that met Ford’s internal requirements and provided a vehicle that officers could easily transition to from their current 2WD vehicles.

The AWD system can be tuned to feel like under-steer, or over-steer, or any combination of mid-neutral steering. It can be tuned to feel like a true FWD vehicle or a true RWD vehicle by putting more of a percentage of the power to the front or to the rear.

 

Tested On the Track

During development, these AWD drivetrains accumulated 5500 laps around a Ford test tracks, i.e., 9000 miles of racetrack driving to simulate high-speed pursuits. Finally, in production trim, the AWD Sedan PI and AWD Utility PI completed 170 32-lap certification tests. In addition, the AWD system has endured 450 hours of testing on the Michigan State Police and Los Angeles County Sheriff test tracks.

The AWD system on the Police Interceptors undergoes durability testing double that of retail vehicle testing. Basically, the same battery of durability tests conducted for retail vehicles are performed twice for police vehicles, along with some police-specific tests. Run the Boston Marathon once, turn around, and run it again.

However, Ford’s durability tests for police vehicles involve more than retail tests run twice. This includes some one-off, police-specific tests. One of the added, police-specific tests is a 40 mph curb test. The curb is slightly ramped, like an aggressive speed bump, as opposed to a square, 90-degree curb. The vehicle approaches the curb at an angle, as opposed to a head-on impact. However, it is indeed a 40 mph impact.

The formal durability testing also includes the grueling LASD and MSP vehicles tests, which are brutal on brakes, tires, powertrain and drivetrain components and overall cooling: road racing, drag racing, top speed tests, panic stops, and a pass-fail subjective handling evaluation. 

To further prove out the robustness of the AWD system, extensive hot weather testing was conducted on Police Interceptor vehicles in the Arizona summer desert. Vehicles were tested at temperatures near 100 F and required to travel at their maximum speed until driveline temperatures stabilized or they used an entire tank of fuel. This testing assured that the AWD system would have no issues with high-speed operation in hot weather climates.

Ford’s Vehicle Durability Tests

AWD Stonewash: This event, conducted in pea gravel, is intended to simulate deep sand driving and exercises the AWD system by placing constant high loads at all four wheels.

AWD Gravel Hill Route: The gravel hill route includes various grades for the vehicle to climb that require the AWD system to transfer torque to the appropriate wheels to prevent and control wheel slip but not bind in tight turns.

17 Percent Roller Grade: This event simulates a worse-case traction scenario. In one condition, the vehicle climbs the 17 percent grade and positions the front wheels on rollers and the rear wheels on dry pavement. In this condition, all torque to drive the vehicle up the grade must be transferred to the rear wheels, since the front wheels on the rollers have no traction. 

The next part of the 17 Percent Roller Grade test involves placing one side of the vehicle’s wheels on rollers and the other side on dry pavement. In this condition, AWD works in conjunction with brake traction control to send torque to the appropriate wheels to drive the vehicle up the grade. This is a very severe test involving high loads on the components.

Split Mu & Homogeneous Traction Control Events: Wet tiles are used on level ground to simulate ice for the split mu and homogeneous traction control events. Like the 17 percent roller grade, one part of the test involves placing one side of the vehicle’s wheels on wet tiles and the other side on pavement. The next part of the test involves placing all four wheels on the wet tiles. In both cases, AWD works in conjunction with traction control to send torque to the appropriate wheels to drive through the event.

Gravel Roads: During the gravel road event, the vehicle conducts wide-open throttle accelerations that require the AWD system to prevent and control wheel slip.

High-Speed Track: The AWD system is tested on the high-speed track conducting aggressive accelerations to verify the system can prevent and control wheel slip while still being robust to high-speed operation.


Published in Police Fleet Manager, May/Jun 2013

Rating : Not Yet Rated


Comments

Comment on This Article

No Comments


Related Companies

Ford
 

Related Products

AWDBrake Traction Control SystemCalibrationCrossoverEVOC (Emergency Vehicle Oper Course)EVOC InstructorsFord NextGen Police Interceptor ConceptFord PI SedanFord PI UtilityFord Police InterceptorFord Police Interceptor SedanFord Police Interceptor Utility Front Wheel DriveFWDLASD Vehicle TestsMSP Vehicle TestsPowertrainRear Differential UnitSUVTechnical Service Bulletin (TSB)Tire TractionTractionTraction Control SystemVehicle Durability TestsWarrantyWheel Slip
 

Article Images

Click to enlarge images.

 
 
Events and Tradeshows: LAOPFMTRPSIT
Latest News: LAOPFMTRPSIT
 
Close ...