Applying greater driving torque to an outer wheel than to an inner wheel when accelerating in a turn (orbiting motion) generates a yaw moment (rotating motion) around the center of gravity. At the start of the 1990s, this effect was confirmed to improve turning performance. From this, Honda developed the Active Torque Transfer System (ATTS), which distributes different amounts of driving torque to the left and right wheels. It was used for the first time in 1996 on the Prelude, thereby achieving practical application of the technology to improve turning performance with front-wheel-drive (FWD) cars.
In 2004, Honda developed the world’s first Super Handling All-Wheel Drive (SH-AWD) system with the ability to freely control torque distribution, and it debuted the technology on the Legend. SH-AWD optimally distributes driving torque between the front and rear wheels, and between the left and right rear wheels, depending on the situation. This further improved turning performance by maximizing the performance of each of the four tires in a range of driving conditions.
The SH-AWD system transmits driving torque from the engine, via the transfer mechanism, to the rear drive unit, which distributes that driving torque between the rear left and right wheels. In 2014, Sport Hybrid SH-AWD, a three-motor hybrid system, was used on the Legend for the first time. With two rear-mounted motors for control, the system achieves highly responsive driving torque distribution between the rear left and right wheels, regardless of the level of driving torque generated by the engine.
This evolved motor-based control enables distribution of positive torque (driving torque) and negative torque (deceleration torque) between the left and right wheels, thereby greatly expanding the range of control to not only accelerating in turns, but to decelerating in turns as well.
In 2016, the NSX underwent the first complete redesign of the supercar in 26 years, while inheriting the “human-centered supercar” concept of previous generations of the NSX, making it a car that anyone can drive at ease. In pursuit of ideal dynamic performance, the Sport Hybrid SH-AWD system used on the Legend was adapted to a midship layout in the NSX.
The configuration and layout of the Sport Hybrid SH-AWD system used on the NSX are explained below.
Sport Hybrid SH-AWD system configuration diagram (second-generation NXS)
● 3.5-liter V6 DOHC twin-turbo engine, with dry sump lubrication system, (max. output of 373 kW at 6,500–7,500 rpm, and max. torque of 550 Nm at 2,000–6,000 rpm) mounted longitudinally midship, with a direct drive motor (max. output of 35 kW at 3,000 rpm, and max. torque of 148 Nm at 500–2,000 rpm) connected directly to the crankshaft, and an exclusively designed 9-Speed dual clutch transmission (DCT) mounted to the rear
● Power Drive Unit (PDU), located below the center console, to optimally control the three motors
● Intelligent Power Unit (IPU), with integrated high-capacity lithium-ion battery, DC-DC converter, and ECU, mounted behind the seat
● Twin Motor Unit (TMU), enabling torque vectoring to the left and right front wheels, mounted within the front sub frame
TMU system configuration diagram
The direct drive motor helps drive the rear wheels with motor-assist in all DCT steps. It includes a direct water-cooling system, working in conjunction with the engine cooling water system, to achieve effective stator cooling and maintain stable high performance even when driving at the limits of performance at the circuit.
The basic structure of the TMU is the same as that used in the Sport Hybrid SH-AWD system used on the Legend. In other words, it comprises two symmetrically-mounted motors (max. per-motor output of 27 kW at 4,000 rpm, and max. per-motor torque of 73 Nm at 0–2,000 rpm), planetary reduction gear mechanisms, a one-way clutch, and brake.
Planetary reduction gear mechanism configuration diagram
The double pinion type planetary gears increase torque from the front motors. When decelerating, the ring gears are fixed so that half the energy from the wheels rotates the motors to recover energy.
The planetary gear-based reduction gear mechanisms built in to the motors comprise a sun gear directly connected to the motor shaft, three double-pinion gears that mesh with the sun gear, a planetary carrier that supports the double-pinion gears, and an outer ring gear. They reduce motor speed while increasing torque to drive the wheels.
Like the Sport Hybrid SH-AWD system used on the Legend, torque is transmitted in the following manner.
1. The motor rotates the sun gear.
2. The sun gear rotates the double-pinion gears.
3. The double-pinion gears rotate the planetary carrier because the ring gear is fixed.
4. The planetary carrier rotates the wheel via the axle shaft and drive shaft.
To enable generation of a yaw moment toward the inside of turns using a motor assist function worthy of a supercar, and torque vectoring creating a torque differential between the front left and right wheels, allowable motor speed on the NSX was increased to 15,000 rpm from 12,000 rpm on the Legend. The gear reduction ratio was also lowered to 8.505, from 10.382 on the Legend, to achieve gearing of about 18% higher, which has increased assisted speeds to 200 km/h when driving on circuits and other high-speed locations.
At very high speeds where motor allowable speed is exceeded, the planetary reduction gear mechanism’s brake is released to allow the ring gear to rotate, thereby breaking the connection between the wheel and motor to protect the motor. The one-way clutch then acts to restrict reaction force, acting on the ring gear, to the traveling direction only.
When driving, one-way clutch operation enables efficient transmission of motor torque to the wheels. When decelerating, the brake locks the ring gear to transmit reaction force back from the tire and rotate the motor, which enables the deceleration energy to be recovered as electric power. The TMU employs these technologies to independently control driving torque to the left and right rear wheels without relying on rear wheel driving torque. It can then create driving torque and deceleration torque differentials between the left and right wheels to generate a yaw moment toward the inside of turns whether applying or releasing the gas pedal in the turn.
The NSX comes with four selectable driving modes—Quiet, Sport (default), Sport+, and Track—that change vehicle characteristics according to driving conditions to meet the needs of the driver. Quiet and Sport modes employ torque vectoring to deliver agility at low- to mid-range speeds and stability at high speeds. At low- to mid-range speeds, negative torque (regenerative braking mode) is applied to the front left and right wheels during the turn-in phase of the corner (decelerating in turn). The system applies a greater negative torque to the inside wheel than to the outside wheel to generate a yaw moment toward the inside of the turn, which achieves outstanding steering response when braking into the corner.
In the corner, positive torque is then applied to the outside front wheel and negative torque is applied to the inside front wheel to generate a larger yaw moment toward the inside of the turn. This delivers outstanding line tracing performance even in sharp corners.
When exiting the corner, the system applies positive torque to both front wheels to deliver powerful acceleration. With greater torque applied to the outer wheel than to the inner wheel, distribution of torque is optimized to the corner to improve stability.
To ensure stability in high-speed corners, negative torque is applied to the outside front wheel to generate a yaw moment toward the outside of the turn, in the turning suppression direction.
In Sport+ mode, more aggressive torque vectoring is used to draw out even more agile dynamic performance. Finally, Track mode improves steering response to achieve maximum performance from this model.
NSX Type S
Announced in August 2021, the NSX Type S comes with improved driving torque distribution control through the key Sport Hybrid SH-AWD technology, thereby increasing interaction with the car and expanding the range of driving scenarios where drivers can experience the joy of driving.
Including the 16 kW higher maximum output of the engine alone, achieved through higher boost pressure, maximum system output was increased to 449 kW (22 kW higher) and maximum system torque was increased to 667 Nm (21 Nm higher). The TMU has improved responsiveness in standing starts through a 20% lower gear ratio, while the battery has 20% greater usable capacity and 10% higher output. This all contributes to increased system output.
The four driving modes on the NSX have undergone a comprehensive overhaul, including optimization of driving torque distribution. In Sport mode, the driver can enjoy light and responsive steering with minimal steering wheel operation to turn the car faster, while in Sport+ mode, the driver can enjoy a sense of unity as the car reacts to driver operation and grips the ground tightly in the turns. The high output Sport Hybrid SH-AWD system achieves light handling without any feeling of the car’s weight while ensuring stability on the road.
Sport+ mode, in particular, further increases the driver’s sense of unity with the car through optimal control of driving torque distribution between the front left and right wheels, as well as rear driving torque, and through a turning posture that hugs the corner while creating a large vehicle body slip angle (β angle) with a small steering angle. Turning a corner with a large β angle opens up visibility into the corner, giving the driver more confidence to enter the corner at speed.
In this way, full application of driving torque to the front wheels when passing the apex and starting to exit a corner, with less use of the gas pedal compared to the first NSX model, can maximize system potential from the start of acceleration and provide direct acceleration from all four wheels. This model has also been tuned to achieve rhythmical synchronization of gear changes, engine speeds, and acceleration G-forces at this time to make the experience even more comfortable.
In developing the NSX, Honda has optimized the high output Sport Hybrid SH-AWD system in line with its “human-centered supercar” concept. Using motors to deliver an outstanding level of steering response, beyond what was possible with the engine alone, the NSX offers light handling for turning freely in a range of driving scenarios, from everyday driving, to winding roads, and even racing on circuits.
