• Active AWD Systems: An Introduction

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    Hi all. With there being so much interest in the Focus RS, and with everyone suddenly paying an immense amount of attention to the AWD system I thought I'd try to help folks understand it better. We're going to start with the basics.

    What makes up an AWD system?
    The AWD portion of the driveline starts after the transmission. Depending on FWD based or RWD based layout you will find a couple differences in components. In both cases you have two axles, four halfshafts, and a couple drive units. A FWD based system will use a Power Transfer Unit (PTU). A RWD based system will have some form of center coupling / differential to send torque back to the front axle. In these applications you might also find a front drive unit (FDU). At the rear of the car you have a Rear Drive Unit (RDU) or Rear Drive Module (RDM). These tend to be used interchangeably. Between the PTU or center coupling and the RDU you obviously have the driveshaft or propshaft. In some applications there is a second propshaft connecting the center coupling to the FDU. Beyond the drive units / modules you have halfshafts which connect to the wheel hub.

    What is a Power Transfer Unit?
    The PTU is a component unique to FWD applications. It's purpose is to send torque to the RDU. The PTU is attached to the transmission. It receives input torque from the transmission output shaft. From here the PTU will send power directly through to the front halfshafts. It will also use a ring and pinion to turn that torque supply 90 degrees and supply it to the propshaft. The ring and pinion ratio in the PTU will effectively reduce the torque sent through the propshaft. This reduction in torque becomes an increase in speed (property of gear ratios). The purpose of this is to allow the propshaft to be a reasonable size / weight. If it needed to sustain the same torque coming out of the transmission it would be gigantic. As an example: Imagine an engine making 300lb-ft of torque. Multiply this by a 1st gear ratio of 4.5:1 and you have 1350lb-ft at the final drive. This final drive has a ratio of 3:1. Now you have 4050lb-ft coming out of the transmission and into the PTU. The PTU ratio will effectively negate the final drive ratio and cut that torque back to 1350lb-ft. If you only need to support 1/3 as much torque you can use a thinner walled and lighter propshaft. You simply have higher requirements for balance as that shaft is generally spinning close to 3000rpm at highway speeds and more than double that when you're approaching maximum velocity in a vehicle like the RS.

    What is a Rear Drive Unit?
    The RDU is the big "pumpkin" at the back which supplies torque to the rear halfshafts. Most often people refer to this as a rear diff, but the differential is simply a component which is located inside the drive module. Like the PTU the RDU contains a ring and pinion. This serves to once again rotate the direction of torque and transfer it from the propshaft to the rear halfshafts. As the gear ratio in the PTU reduced torque and increased speed in the propshaft the RDU does the opposite. It reduces the speed and increases the torque value. This gear ratio is usually slightly different from the PTU such that the RDU is rotating a little bit faster (less than 0.5%) to maintain load on the gear teeth and reduce the probability of gear rattle. In some cases the RDU is just the drive module itself. In some cases it includes a torque tube mounted to the front. The advantage of the torque tube is a shorter propshaft making balancing easier. It also changes the front mounting point of the RDU providing a much higher resistance to axle wind-up. The obvious disadvantage is the weight and sacrifice to packaging.

    Types of AWD Systems
    As mentioned before, there are FWD based and RWD based AWD systems. Without complicating things too much there are also passive and active varieties of components or systems. Passive refers to purely mechanical systems. An open differential or limited slip differential are passive couplings. They react based on their mechanical design. Active refers to couplings using an ECU and software to control their reaction. An eLSD, GKN's Twinster product, BorgWarner's 'Haldex' couplings, and so many more from Magna, JTekt, Linamar, American Axle, and a few others. In a FWD based system the rear axle is generally the only place you find an active coupling. Outside of some demonstration vehicles I'm not aware of anyone using a FWD based AWD system that has an active coupling for the front axle (disconnect systems excluded). In a RWD based product you can find active couplings both at the rear axle and at the front of the propshaft (possibly inside the transmission) to send torque to the front axle.

    Passive Systems
    Easier to design
    Easier to service or repair
    Relatively easy to upgrade

    Fixed responses
    Unable to remove / reduce drag

    Active Systems
    Calibrateable responses and reactions
    Reduction of drag (improved fuel economy)

    Complex design
    Software development and calibration required
    Difficult / restricted service or repair
    Unable to upgrade
    This article was originally published in forum thread: Active AWD Systems: An Introduction started by Scooby921 View original post