HONDA The Power of Dreams

To develop a vehicle conforming to the new regulations introduced for SUPER GT’s GT500 class in 2014, Honda opted to use the second-generation NSX as its base car. The carbon monocoque specified as a common part was intended for front-engine, rear-wheel drive (FR) cars, but Honda was determined to use the same mid-engine, rear-wheel drive (MR) layout as the production NSX. With the consent of the rules governing body and rival automakers, and on the condition that changes to common parts would be kept to a minimum and performance adjustments would be made, Honda was given permission to compete using an MR car.

Honda was committed to one other feature—the inclusion of a hybrid system. After all, the production NSX had a hybrid system and Honda placed importance on its GT500 car being the same. As was the case for the MR layout, inclusion of a hybrid system was approved after consent was given by the governing body and rival automakers.

Under the new regulations, vehicles included a large number of common parts, including the monocoque. It goes without saying that the new regulations were not designed with the inclusion of a hybrid system in mind. A hybrid system consists of an electric motor, a battery, and an inverter for converting direct current from the battery into alternating current for the motor, as well as two cooling systems (each comprising an electric water pump and radiator)—one to cool just the battery, the other to cool the inverter and motor. Working out where to locate all these components was a huge challenge.

Honda’s CR-Z GT, its car competing in the GT300 class between 2012 and 2015, had a hybrid system. It was this system that was adopted, with modifications, for the 2014 NSX CONCEPT-GT, the car developed to comply with the new regulations. One example of the changes made was to increase the motor’s output by 10 kilowatts, giving it the ability to produce a maximum 60 kilowatts. The motor was mounted on the left-hand side of the gearbox. The inverter was located in the engine compartment, behind the monocoque, with the radiator for the inverter and motor nearby.

The CR-Z GT competed in the GT300 class from 2012

To ensure safety in a collision, and wanting, from a dynamic performance perspective, to locate heavy parts close to the center of gravity, the CR-Z GT’s battery—a heavy item—was mounted to the floor on the passenger’s side. This was not possible with the GT500 common monocoque because the fuel tank protruded into the passenger’s side. There was no alternative but to put the battery in front of the monocoque.

NSX CONCEPT-GT Hybrid system layout

An NSX CONCEPT-GT advanced development vehicle was completed in the summer of 2013, but it was discovered that it did not achieve the minimum weight stipulated for mid-engine hybrids (1090 kilograms to round four, 1077 kilograms from round five), therefore necessitating a redesign.

The cause of the weight increase was the structure used to mount the battery. The advanced development vehicle used a dual structure consisting of a battery box inside a metal subframe. This was to provide enough strength to withstand the crush load, though it ended up making the car too heavy.

A different structure was adopted for the actual race car. A carbon fiber reinforced plastic (CFRP) battery box was used, designed to allow the front crushable structure, a specified common part, to function properly. This also functioned as a front frame, supporting suspension input. The aim was to switch from the dual structure—subframe and battery box—to a singular structure to reduce weight while maintaining enough strength.

The radiator for cooling the battery was mounted behind the opening on the right-hand side of the front bumper. Like the radiator for the inverter and motor, it circulated cooling water using a water pump. A longer length of high-pressure cabling was needed because the battery and inverter were separated. This contributed to the end weight of the hybrid system—around 70 kilograms—which included the cooling water.

The hybrid system actually used in the NSX CONCEPT-GT weighed around 70 kilograms, but the governing body deemed the additional weight created by the hybrid system to be 28 kilograms. Hybrid system usage was restricted to compensating for the 28 kilograms. In other words, the remaining 42 kilograms was effectively a weight handicap. Although the electric motor had the ability to produce up to 60 kilowatts, the maximum assistance it could provide was restricted to 21 kilowatts (with no restriction on the regenerative system) and the total energy assistance was restricted to 880 kilojoules per lap. At the regulated maximum output, this worked out to be around 42 seconds of assistance permitted each lap.

However, a condition was introduced, limiting assistance to when the car was being driven at full throttle at an engine speed above 7,500 revs per minute. This meant the hybrid system could not be used, even at full throttle, when coming out of corners, which was when assistance was more effective, because engine speed was too low.

On long racing courses like Fuji Speedway (4.563 kilometers) and Suzuka Circuit (5.807 kilometers), it was found that the total assistance provided was not enough—hybrid system assistance was not available for all full-throttle sections. Honda experimented with how to manage assistance to allow energy to be fully and effectively utilized throughout each lap, pre-determining in which zones assistance should be applied.

Some mass-produced hybrid systems make use of cooperative regenerative braking, which automatically regulates the balance between regenerative brakes and hydraulic brakes to match the driver’s required braking force. The production NSX also uses this technology. In the GT500, however, cooperative regenerative braking was not allowed. So, when regenerative brakes and hydraulic brakes were activated at the rear, the balance between them could not be adjusted. When braking, the load became front-heavy and decreased at the rear, increasing the likelihood of rear wheel lock-up. The only way to deal with this was for the driver to adjust their driving so the rear wheels did not lock up.

Honda competed in the 2014 and 2015 seasons with the hybrid system in place but removed it in 2016 under inevitable circumstances—the Germany-based battery cell manufacturer had been taken over and was pulling out of the battery cell business. Honda looked around but was unable to find another company that could supply the same size cells. In-house development was explored, but the cost could not be accommodated. Continued use of the hybrid system was abandoned.

Honda therefore used for 2016 the same base specifications as it did for 2015, which were designed for inclusion of a hybrid system, but competed with the hybrid system removed. Part of the side air intake on the vehicle’s left-hand side had been allocated to cooling for the radiator assigned to the inverter and motor. Although this opening was no longer needed, given the removal of the hybrid system, changes could not be made for 2016 as aerodynamic development had been suspended. The car competed as it was. Optimization of side air intake assignment for a car without a hybrid system had to wait for the development of 2017 specifications.

In the lead-up to the 2014 season, development resources were allocated to determining how to efficiently incorporate a hybrid system into a chassis intended for an FR layout. Through to the end of the following season, 2015, data was extensively collected and examined to ensure proper functioning of the hybrid system. During the activity over these two years, lessons were learned. The team learned the hard way that the car would be unable to perform to the best of its ability if even just one cell making up the battery was in poor condition. They also realized that for racing, a lighter, more powerful hybrid system was needed.