Electrification revolutionizes All Wheel Drive

Subaru, a division of Fuji Heavy Industries, is one of the smallest Japanese motor manufacturers that produce mostly four-wheel-drive vehicles principally for road use. The VIZIV-2 plug-in hybrid crossover concept unveiled at Geneva demonstrated the potential of AWD when applied to a hybrid platform. The internal-combustion-engine (ICE) power comes from Subaru’s 1998-cm³ flat-four DOHC turbo-diesel, rated at 108 kW and designed principally for the company’s European product range. But in the VIZIV the "boxer" engine drives the front wheels only through a CVT transmission named Lineartronic. [eventPDF] An electric motor is integrated with the boxer engine and CVT transmission in a conventional front drive parallel hybrid drivetrain layout. Two other inboard electric motors are fitted at the rear, providing drive to each rear wheel independently. The hybrid system is a plug-in type, while the diesel engine provides charging on the move to the underfloor nickel-metal hydride battery and inverter/converter. Image Credit: www.greencarcongress.com Subaru refers to this as Hybrid SI Drive. The motors are used to power the car from rest and provide drive at low speeds, with the diesel engine taking over for highway use. Subaru also hinted that this layout might be adopted for future Subaru all-wheel-drive models, with rear drive provided by electric motors. Subaru refers to this as "Independent-rear-motor-driven symmetrical AWD." This is effectively a torque vectoring system where the control ECU determines which power sources to use according to needs. Turning the steering wheel increases the torque delivery to the rear wheels, while turning it back to the straight ahead again increases the front drive bias. The drive system management system assumes the functions of a rear differential and distributes torque between the rear wheels accordingly. So in negotiating a turn, more torque is fed to the outside wheel and less to the inner wheel. Image Credit: www.subaru-global.com This system automatically selects the best time to use each power unit according to its strengths and the driving conditions, achieving greater fuel efficiency and maximum stability and safety. The generous torque generated by the diesel engine and the electric motors ensures strong, linear acceleration and responsive performance. During low-speed or city driving, the front and rear motors supply much of the vehicle’s propulsion, while the diesel engine and high-efficiency Lineartronic take over at higher speeds. The VIZIV-2’s auto start/stop system and its multiple charging methods—engine power generation, regenerative braking and plug-in charging—further improve efficiency. VIZIV is also integrated with Subaru’s Eyesight system; an Advanced Driver Assist System that feeds data to the drive control system. Eyesight uses a forward facing stereoscopic camera. Input from the camera is used to modify engine and motor power output or initiate braking, according to conditions sensed by the camera, with the aim of improving safety and reducing environmental impact. Subaru is well known for its four wheel drive technology, if not for EV or hybrid engineering. At the other end of the AWD scale there’s Tesla Motors, synonymous with EV’s, but not normally associated with AWD. During the launch of the dual-motor version of the P85 performance sedan at the municipal airport near Los Angeles, Tesla CEO Errol Musk explained: "The thing that makes this unique and special, and better than all-wheel drive in the past, is that you can dynamically shift the power from front to rear within milliseconds, much quicker than is possible with a mechanically linked system. This is a digital system, with two motors - one powering the front wheels and one powering the rear wheels." The technologically advanced BMW i8 also employs AWD to enhance performance: The rear wheels are driven by a 266 kW, 320 Newton metres gasoline engine via a six-speed automatic transmission, while the front wheels receive their power from a 96 kW/ 250 Nm electric motor via a two-stage automatic transmission. This provides all-wheel-drive performance which is as dynamic as it is efficient. The BMW i8's intelligent powertrain control system ensures perfect coordination of both power sources. The variable power-sharing between the internal combustion engine and the electric motor makes the driver aware of the sporty temperament of the BMW i8 at all times, while at the same time maximising the energy efficiency of the overall system. Using the two power sources in unison enables the car to sprint to 100 km/h from stationary in 4.4 seconds and reach an electronically controlled top speed of 250 km/h, which can be reached and maintained when the vehicle operates solely on the gasoline engine. Variable front-rear power splitting in line with changing driving conditions makes for exceptional dynamic cornering. On entering the corner, the power split is biased towards the rear wheels to improve turning precision. For optimal acceleration out of the corner, the powertrain controller returns to the default split as soon as the steering angle reduces. Although multi motor EV and hybrid platforms are ideally suited to AWD, most modern ICE powered all-wheel-drive cars and SUVs already offer some type of computer-controlled, part-time engagement that enhances performance and saves fuel. These electro-mechanical systems use the speed sensors usually used for ABS to detect if one or more wheels are rotating faster than the vehicle’s speed, or if the vehicle is yawing off its intended path of travel, at which point the system steps in. First, it engages the other drive axle and applies a proportion of the vehicle’s torque to it. If the wheels continue to spin, the computer reduces engine torque or even brakes one of the wheels, if necessary. However more modern systems, such as those from Ricardo in Britain and ZF in Germany, have taken a fairly radical step forward, to the point where an engine’s torque can be passed around, or vectored, to each corner of the car. In other words, the torque can be directed from front to rear as in traditional all-wheel-drive setups and be distributed from left to right on a given axle; in fractions of a second. This significantly improves traction on slippery surfaces but also improves handling on dry roads. The Ricardo Cross-Axle Torque-Vectoring system uses wet clutches and planetary gearsets in both the front and rear differentials that are controlled by electrical, electromechanical and electrohydraulic control systems. Ricardo claims the system’s response time, from the push of the accelerator to the delivery of up to 90 percent of available torque, is only about 0.1 seconds. If Ricardo’s vectoring is used only in an all-wheel-drive vehicle’s center differential, the engine torque effectively gets directed front-to-rear and side-to-side with split-second accuracy, under all driving conditions. German transmission and driveline company ZF has also developed a torque-vectoring system, called Vector Drive which distributes drive torque individually to each of the rear wheels, generating a yaw movement around the vertical axis. This improves both cornering performance and vehicle stability in less-than-ideal road conditions. Image Credit: www.zf.com When driving straight, the torque vectoring rear axle behaves like an ordinary open differential, distributing torque equally to the wheels. The gears of the planetary gearset don’t turn when driving straight, thereby saving fuel. However during cornering torque is distributed individually along both halfshafts on an axle, controlled by an electromechanically actuated multi-disc brake. The ZF system also generates wheel differential torque independently of the drive torque. When cornering through a downhill section on trailing throttle, the outer wheel receives more drive torque than the inner wheel, allowing more accurate turn-in. The torque-vectoring drive also acts like a positive-traction or locking differential on dry or uneven traction startups, with torque going to the wheel with the highest friction potential. Notwithstanding the performance gains to be made with AWD; on non-EVs the Achilles heel will always manifest in fuel consumption and emissions penalties as a result of the additional drag. In order to address this, Schaeffler has developed an AWD disconnect clutch which, depending on the driving conditions, decouples the unused drive axle from the powertrain. At the 2014 North American International Auto Show Schaeffler claimed the system returned savings of up to 6% on the highway with a similar potential for reducing CO2 emissions. Image Credit: wot.motortrend.com While AWD may not be directly related to fuel efficiency and emissions, engine downspeeding certainly is included in most manufacturer’s strategies aiming to squeeze as much as 25% brake thermal efficiency (BTE) out of next-generation engines, using a combination of downspeeding, further downsizing and aggressive cooled EGR. However the challenge presented by downspeeding is that, to attain similar levels of tractability and peak performance over a reduced engine speed range, transmissions need to be adapted to match the torque curve. The most common solution to this is to increase the number of speeds. Author: Peter Els Related Stories: Transmission Update: Imroving Fuel Consumption and Reducing Emissions

Electrification revolutionizes All Wheel Drive

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