'By-wire' boosts hybrid vehicle dynamics

  • 13-Mar-2009 08:19 EDT
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By-wire technology is a key enabler for safe and comfortable hybrid-electric vehicles, according to BMW engineers. Shown are the company’s X6 and 7 Series ActiveHybrid concept vehicles.

Due to consumers’ increased environmental awareness and rising gas prices, the market for hybrid-electric vehicles (HEVs) has grown immensely over the past 10 years. Hybrid technology is viewed as an important step toward a more sustainable use of the automobile, but so far it has mostly been understood as a powertrain topic that mainly touches the engine, transmission, and energy storage.

Other vehicle systems require some fundamental adaptations as well. Chassis systems and vehicle dynamics, for example, are directly influenced by the powertrain concept and need to be considered carefully as safety and comfort characteristics can change significantly compared to a conventional vehicle.

Specific braking and steering systems need to be integrated into a hybrid concept to maximize the benefit of features such as electric driving or regenerative braking. Accomplishing this goal basically requires electrically assisted systems that use “by-wire” technology.

Electric driving and engine start/stop have a significant impact on chassis systems. Usually steering and braking systems are power-assisted on all vehicles exceeding a curb weight of 1200 kg (2645 lb), which is true for virtually all hybrids in production today. As the combustion engine typically powers braking and steering systems through hydraulics or vacuum in conventional vehicles, they would not work when the engine stops. This would be the case when an HEV comes to a stop, or more importantly, when it’s propelled only by the electric motors.

Therefore, alternative power assistance is required that works independently from the combustion engine. Electrically assisted chassis systems are ideal, because the hybrid system already requires a powerful electric infrastructure.

The most fundamental change for chassis systems is necessary because regenerative braking needs to be integrated into the conventional friction braking system. A hybrid vehicle does not have regenerative braking instead of, but rather in addition to, friction brakes. Depending on the situation, regenerative braking is applied exclusively, together with friction brakes, or not at all.

A decoupled braking system becomes necessary to apply substantial braking torque (i.e., when regenerative braking alone can yield deceleration of more than 0.1 g). In that way, the driver does not notice any difference whether regenerative braking is applied or not, and the recovery of motion energy can be maximized. Since there is no fixed connection between brake pedal and friction brakes in this case, the brake controller needs to ensure that the total brake torque (friction plus regenerative braking) always equals the driver’s input.

Such a system layout uses many aspects of by-wire technology. For example, electrohydraulic and electromechanic systems include solutions that feature the driver input (i.e., brake pedal) not being directly connected to the wheel brakes and a controller applying the respective brake output.

Regenerative braking also has some interaction with the suspension. While longitudinal tire forces from friction braking are compensated by the suspension (anti-dive), regenerative braking forces are compensated by the drivetrain (anti-squat). If anti-dive and anti-squat characteristics differ too much, the resulting change in body pitch might compromise ride comfort as the brake controller shifts between regenerative and friction braking.

To avoid these implications, the suspension should be designed with similar anti-dive and anti-squat characteristics, or the shifting between friction and regenerative braking should be performed slowly enough so change in pitch happens without jerkiness. This example shows that purely mechanical chassis components also need to be considered for an HEV to accomplish the ride quality of a conventional vehicle.

System complexity usually does not have a direct impact on vehicle dynamics or chassis systems, but there are some indirect implications. As the powertrain becomes more complex with additional electric machines, energy storage, and controls, weight is added to the vehicle.

Increased weight impacts safety and comfort aspects such as braking performance, handling characteristics, and vehicle body motion. As a result, stopping distance might increase because the tires carry more vertical load without increasing maximum longitudinal forces by the same rate. The same applies to lateral tire forces so that impeded lateral tire performance limits maximum lateral acceleration and handling qualities. Additionally, a higher body (sprung) mass increases body displacements such as roll, pitch, and heave, which has a negative impact on safety and comfort.

Adapting tires, springs, dampers, and antiroll bars according to the increased vehicle weight can easily mitigate some of those implications; but, increased weight cannot be compensated for completely. That is why some compromises in vehicle dynamics are conceptual for a hybrid compared to a conventional vehicle.

There are three main aspects in vehicle dynamics that need to be considered: safety, comfort, and controls. When an HEV is derived through redesign of a conventional vehicle in particular, these aspects need to be regarded carefully:

• Safety—yaw stability, handling, and braking performance should not be affected by the interactions of the hybrid system with powertrain and braking systems

• Comfort—roll, pitch, and vertical motion should not be affected by the changes due to the hybrid system

• Controls—driver/passengers should not notice any changes in steering or braking characteristics when hybrid controls interact with powertrain or braking systems.

Because the most important features of an HEV are electric driving, engine start/stop, boost, and regenerative braking, these aspects need to be observed more closely to evaluate their impact on vehicle dynamics. And regenerative braking, in particular, is unprecedented in the field of vehicle dynamics given the characteristic of using two relatively diverse braking options in one vehicle.

To ensure yaw and braking characteristics similar to a conventional vehicle, the respective controller needs to limit regenerative braking to low and moderate lateral acceleration, suspend regenerative braking at the latest upon the intervention of vehicle dynamics control systems, and control the center clutch or variable transmission of an all-wheel-drive system depending on the actual use of regenerative braking.

The implications for safety, comfort, and controls show that a detailed integration of hybrid architecture, driveline concept, vehicle dynamics controls, and suspension is necessary. A decoupled braking system that controls friction and regenerative braking simultaneously is a key component for such a concept.

It appears that hybrid and by-wire technologies complement each other perfectly as they use the same electric power architecture and benefit equally from electric chassis systems. Hence, an integrated approach of those two advanced technologies increases powertrain efficiency while maintaining vehicle safety on a high level.

This article is based on SAE International technical paper 2009-01-0442 by Sven A. Beiker, formerly of BMW Hybrid Technology Corp., and Renate C. Vachenauer, BMW Group. The paper was presented as part of the Vehicle Dynamics and Simulation session at the SAE 2009 World Congress.

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