QOTD: What Police Car Did It Best?

Corey I’m almost 100% sure you’re right.

It has the standard Chevy Tahoe system which includes a “4wd Auto” that gives the functionality HDC is talking about

“The pursuit-capable Tahoe PPV and the special-service package Tahoe 5W4 use exactly the same 4WD drivetrain, hardware and software. The 4WD system on the police package Tahoe PPV is not new. It has been used on the special-service package 4WD Tahoe 5W4 for a decade. It has also been used on hundreds of thousands (some company officials estimate over a million) of retail Chevrolet Tahoes and Silverados and GMC Yukons and Sierras.

The police-package and special-service package Tahoe both use a truck-based 4WD system. The transfer case in AUTO mode works very similar to a car-based AWD system. They both activate a clutch to send torque to a second axle either when slip occurs or pre-emptively when slip is anticipated.

The main distinction between this truck-based system and a car-based system is in the transfer case operation. This truck transfer case has a 2HI, AUTO, 4HI and 4LO. The 4HI and 4LO are available modes that allow the Tahoe to navigate off-road conditions that a standard AWD system cannot.

The 4WD system manages the clutch pressure to divide the torque between the front and rear axles. A clutch pack in the transfer case applies progressively more or less pressure to send progressively more or less torque to the front wheels. In AUTO, it is always partly ON (engaged) and never fully OFF (disengaged).

In AUTO mode, the front axle is always engaged so the clutch in the transfer case is ready to immediately send torque to the front axle. It is easier on the driveline to remain partially engaged rather than to have the mechanical and hydraulic stress of a sudden engagement or a sudden disengagement throughout the life of the system.

Driving in AUTO mode affects the fuel economy because the front axle is engaged all the time. It is the drag from the spinning front axle that reduces the fuel economy in AUTO mode by about ½-mpg. In 2HI mode, the 4WD system disconnects, or completely disengages, the front axle for improved fuel economy. With the option of selecting 2HI, the Tahoe achieves the maximum fuel economy since the front axle and driveshaft are disconnected from the drivetrain. Otherwise, the system is always engaged, i.e., the driveshafts to both axles are always spinning.

Driver’s Actions

The AUTO system sends a constantly varying amount of torque to the front axle, as dictated by the driver’s actions, and based on many inputs. The system’s primary inputs are throttle position, throttle acceleration, engine rpm, trans gear state, lateral acceleration, and steering wheel angle.

Simply put, the sensors and software for the ABS, traction control, and stability control are all involved in the AUTO system. During lower speed maneuvers with high steering input, the system also provides slip correction during aggressive cornering.

For all practical purposes, if the rear wheels ever turn faster than the front wheels, the AUTO system increases transfer clutch pressure to send more torque to the front axle. As the rear wheels begin to turn at speeds closer the front wheels, the clutch pressure is reduced to send less torque to the front axle.

At full throttle and lower speeds, about one-half of the engine’s torque can be sent to the front wheels, while at full throttle and higher speeds, about one quarter of the engine’s torque can be sent to the front wheels. That is, the AUTO mode defaults nearly all the power to the rear axle, but can put vary up to one-half the torque to the front axle or any combination in between, monitored many times a second.”

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