HONDA The Power of Dreams

New engine regulations introduced to the GT500 class of SUPER GT in 2014 specified that large cast parts, including the cylinder head and cylinder block, could only be redesigned once every three years. On the other hand, machining was allowed, so specification changes could be made when introducing a new engine in the middle of a season.

Honda adopted a new combustion technology when introducing a new engine in the second half of the 2016 season. That technology was called pre-chamber ignition (PCI). Although intending to introduce this technology in the opening round of the season, adaptations during development took time, so its introduction was delayed until mid-season.

The structural characteristic of PCI, which is also known under other names such as pre-chamber jet combustion, is a pre-chamber located at the tip of each spark plug. The pre-chamber contains several small orifices through which the air-fuel mixture, created from the intake stoke to the compression stroke, is injected into the pre-chamber (with adaptations required to facilitate injection).

When ignited by the spark plug, flames are injected with force into the main chamber through the pre-chamber orifices and the air-fuel mixture within the main chamber combusts all at once. This is the main characteristic of PCI combustion. In addition to PCI increasing the flammability of lean mixtures, the technology has increased the speed of combustion and reduced the time of combustion (especially in the latter stages of combustion). With the significant impact of PCI, an additional 10 kilowatts or more of power was achieved for the HR-414E engine in 2016. The faster and shorter combustion period improved the efficiency of converting combustion energy to pressure and led to improved thermal efficiency, and therefore improved engine output.

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Honda’s SUPER GT engine changed name to the HR-417E in 2017. Over three years of ongoing development from 2014, Honda increased maximum cylinder pressure (P-Max) considerably, resulting in a corresponding improvement in engine output. In anticipation of further increases in P-Max thereafter, it strengthened the structure of the HR-417E engine and continued to use PCI. While an existing cylinder head adapted for PCI was used in 2016, a new cylinder head optimized for PCI was designed for the 2017 HR-417E.

Another significant change was the positioning of the injector. The direct-injection injector, which was on the intake valve side in the HR-414E, was moved to the exhaust valve side with the aim of improving premixing during mid-range engine speeds. Piston movement increases at higher revolutions, so the air-fuel flow within the cylinder naturally increases to promote premixing. However, premixing was an issue during mid-range speeds because of the slower piston movement.

By moving the injector to the exhaust valve side, it became possible to inject the fuel in the opposite direction to the flow of air from the intake valve side, which dramatically increased homogeneity of the air-fuel mixture and improved mid-range torque. As a result, Honda was able to achieve significant improvements, including an expanded power band and improved peak output. The exhaust valve side injector also had the effect of reducing fuel adhesion to the cylinder head and pistons, which also contributed to improved engine output.

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The 2014 HR-414E specifications achieved a dramatic improvement in thermal efficiency, and subsequent improvement in engine output, with the adoption of the early closing Miller cycle. However, the extent of early closing was reduced for the 2017 HR-417E. After realizing that drivability was sacrificed when too much emphasis was placed on efficiency, Honda decided to concentrate on improving drivability.

Although adopting the Miller cycle led to improved thermal efficiency, closing the intake valve before bottom dead center meant that the full capacity of the cylinder was not being utilized. In naturally aspirated engines, intake air volume is reduced by early closing, but this air volume can be supplemented in turbocharged engines by increasing boost pressure.

Turbo lag under throttle, such as when coming out of corners, can be eliminated and drivability improved through an anti-lag system. The lag occurs because taking the foot off the gas pedal during deceleration has the effect of greatly reducing exhaust energy, which causes turbine speed to fall. Then, when accelerating again, boost pressure does not increase sufficiently until the turbine speed can recover, which feels like poor acceleration to the driver.

Although allowed in the FIA World Rally Championship (WRC) and other races, anti-lag systems that introduce fresh air into the exhaust pipe by externally attaching a device to the exhaust system are not allowed in the SUPER GT GT500 class. For this reason, the throttle valve is left slightly open even under off-throttle conditions in GT500 engines. This mechanism maintains turbine speed by using heat from the exhaust pipe to ignite the air-fuel mixture blown out of the combustion chamber.

When adopting the Miller cycle, additional work is done by the turbo to achieve higher boost pressure, so durability is an issue. Relying on this anti-lag system to eliminate turbo lag, which tends to increase as a consequence of higher boost pressures, increases fuel consumption and increases pit stop times for refueling during a race, which has a detrimental effect on competitiveness. This was another issue during development.

Volumetric efficiency prior to applying boost pressure is important for reducing turbo lag. When developing the HR-417E therefore, Honda selected the most appropriate valve timing for reducing the extent of early closing in the Miller cycle and thereby improving volumetric efficiency. As a result, it also improved both responsiveness and drivability. The improved volumetric efficiency also reduced dependence on turbo boost, which also reduced the effect of anti-lag measures and had a positive effect on fuel efficiency as well.

Honda also changed the exhaust system in the HR-417E to improve engine output. In the exhaust system, the four pipes coming from the exhaust ports are called the primary pipes, and where they converge to form a single pipe is called the collector. The part after the collector is called the tailpipe. Adjusting primary pipe diameter and length can improve engine output through utilization of the pulsation effect of the exhaust, but the shape of the collector should not be ignored either. Honda conducted one-dimensional simulations of the primary pipes and three-dimensional simulations of the collector to improve the exhaust system, with the final collector shape being optimized through coupled analysis of the 1D and 3D simulations.

The newly designed collector was manufactured with a metal lamination 3D printer, using high-strength, high heat-resistant Inconel, to help reduce the length of the development cycle. Used throughout the 2017 season, Honda was able to confirm that the part had no problems with durability.

The 2018 HR-417E specifications increased the compression ratio, which had already been increased for the engine specifications used in early 2016, and achieved improved engine output through improved intake system efficiency and other efforts. The 2019 specifications changed the exhaust system collection method to reduce exhaust pressure, which improved engine knocking. Although minor, this change was also able to achieve an increase in engine output.

Over six development seasons from 2014 to 2019, the change that made the greatest contribution to improved thermal efficiency, and therefore engine output, was the introduction of the PCI system in the second half of 2016. The improved mid-range torque characteristics in 2017 also improved drivability. Throughout 2018 and 2019, Honda continued to constantly improve engine output through ongoing development based on PCI.