TABLE OF CONTENTS It should be understood that hybrid vehicles require electrically augmented steering, braking and climate control systems. The vehicle steering system must be full electric assist, or electric over hydraulic, as a minimum to ensure that steering boost is available even with the engine off, regardless of the vehicle at rest or in motion, and similarly for the brakes since engine vacuum is not available during idle-off mode. In fact, some mild hybrid implementations use separate electrically driven vacuum pumps for the brakes during engine off periods. Cabin climate control is the most energy intensive engine off load. The following subsections elaborate on each of these topics.
As a general rule of thumb, when a vehicle steering mechanism rack load exceeds about 8 kN, a low voltage, dc brush motor, electric assist may be inadequate for acceptable steering boost performance. The range of rack loads from 8 kN to roughly 12 kN defines a transition during which 14 V electric assist must give way to 42 V PowerNet systems. The low voltage 14 V power supply is not adequate to source the instantaneous power demanded by steering systems having high rack loading. Above 12 kN of rack load, regardless of vehicle type, the electric assist steering is best served from a 42 V PowerNet vehicle power supply.
Battery EVs will generally operate their electric assist steering from the traction battery. However, this requires attention to high voltage cabling and proper circuit protection. For distribution voltages...
