The Next-Generation e:HEV Refined by Sensory-Rich Technologies
Honda’s e:HEV original two-motor hybrid system realizes elevated levels of both environmental and driving performance, reflecting a commitment to motor and engine efficiency. The system continues to evolve as a core technology in the electrification of Honda vehicles.
The next generation of e:HEV systems will further enhance environmental performance and the sophisticated and exhilarating driving experience through heightened synchronization with driver controls, use of electric all-wheel drive (AWD), and other technologies that stimulate the senses. This will involve an overhaul of components, such as the engine and drive unit, and control technology for both small- and medium-sized models.
An AWD System Suitable for the Next-Generation e:HEV
A sophisticated and exhilarating driving experience is achievable only when there is outstanding driving stability, the vehicle turning and accelerating as the driver intends. All-wheel drive (AWD) is one means of accomplishing this.
Honda’s AWD systems have previously been mechanical AWD systems (e.g. Real Time AWD). These distribute driving torque generated by a single power source, such as an engine, to front and rear wheels via a propeller shaft. Next-generation e:HEV models will have an electric AWD system with a newly developed rear drive unit that drives the rear wheels using an electric motor. The additional driving force from the rear drive unit complements the driving force from the front drive unit, enabling more powerful initial acceleration. Furthermore, an electric motor delivers an excellent response, allowing finer control of front and rear driving force according to the grip of each tire. This contributes to even greater stability on slippery surfaces when starting from standstill or while turning.
The electric AWD systems to be integrated into next-generation e:HEV models employ a 50-kilowatt-class unit compatible with up to medium-sized models. The aim is to realize the sophisticated and exhilarating driving experience characteristic of an e:HEV by fully leveraging the high precision and high response advantages of electric systems.
In terms of the packaging, a flatter floor structure can be realized as it is now possible to reduce the size of the center tunnel along the cabin floor that housed the propeller shaft in a mechanical AWD system. Besides enhancing comfort for rear seat occupants, this allows the battery pack, previously located under the floor of the cargo space, to be moved under a seat. Cargo space can be increased as a result.
Leveraging Motor Characteristics for AWD Control
Motor characteristics were leveraged to further advance front and rear driving force distribution control technology previously amassed through mechanical AWD system development. The electric AWD units for next-generation e:HEV systems realize more precise and flexible driving force distribution with adjustments made according to changes in tire ground contact load. When starting from standstill, or during acceleration, distribution of driving force to the rear wheels increases, adjusting to the increase in load as the load shifts to the rear wheels. This maximizes tire grip on all four wheels to efficiently transfer driving force to the road surface. During deceleration, when the nose of the vehicle is more likely to dip forward, regenerative braking of the rear wheels complements front-wheel braking to both maximize regenerative power and suppress pitching, stabilizing the posture of the vehicle. When turning, driving force distribution to the rear wheels is increased while determining the grip limit of the rear inside wheel. This reduces the driving force burden on the front wheels, freeing up grip and improving the tire load balance to stabilize vehicle behavior.
As well as using high-precision and highly responsive motor torque control to achieve precisely optimized front-rear driving force distribution, a newly developed Traction Control System (TCS), which controls front and rear motors independently, suppresses tire slip with high precision and at high speed. In the existing TCS, Vehicle Stability Assist (VSA) would detect slipping and control it via management and motor ECUs. In the newly developed TCS, independent motor ECUs at front and rear detect slipping of front and rear wheels separately and directly control the power output of each motor. By controlling slippage quickly and maximizing the driving force in each of the front and rear wheels, the maximum tire grip can be achieved with all four wheels even when ascending hills on slippery surfaces. When only the front wheel is slipping, for example, a mechanical AWD system might suppress output to both front and rear wheels. The new system only suppresses output to the wheel that is slipping and retains enough driving force to realize a highly reassuring driving performance.
By combining the electric AWD system and the newly developed TCS, Honda wants to give drivers greater reassurance, allowing them to drive at will. When cornering on winding roads, for example, slipping is controlled during deceleration on entry to turns by applying regenerative braking of the rear wheels in addition to the front-wheel braking. The resulting posture of the vehicle during deceleration is more likely to elicit a high steering response. That posture is retained as the vehicle enters the turn. By optimizing driving force in all four wheels, with driving force distributed also to the rear wheels, vehicle behavior is stabilized. Acceleration on exiting a corner is smooth as driving force distribution to the rear wheels is increased according to the increase in load on the rear wheels to ensure maximum traction while preventing erratic vehicle behavior due to slipping.
This series of control procedures elicits an outstanding ability to follow a desired driving line and excellent handling stability, regardless of road surface conditions. Tire grip is also maximized when driving on slippery snowy roads, realizing a highly reassuring driving performance with slip-induced instability suppressed.
