SUPER GT

Engine Development under Fuel Flow Regulations – 2014 to Early 2016 [SUPER GT]

Engine Development under Fuel Flow Regulations – 2014 to Early 2016 [SUPER GT]

New engine regulations were introduced to the GT500 class of SUPER GT in 2014. Until 2013, 3.4-liter V8 naturally aspirated port injection engines were used in this class, with installation of air intake restrictors required to limit air intake flow rates and regulate engine output.

Installation of the intake restrictors created a physical restriction in the path of the air flow, thereby causing the intake air flow rate to peak at a certain engine speed (revolutions per minute, rpm). In this way, even if efforts were made to improve output, it was not possible to do so by increasing rpms to increase the intake air flow rate per unit of time and then improving output by injecting a corresponding level of fuel into the engine. As a result, instead of operating the engines at the theoretical air-fuel ratio of lambda (λ) =1 at which fuel and air are combusted in the ideal quantities, called output air-fuel ratio or power air-fuel ratio, it became normal to operate the engines with a rich fuel mixture (λ<1).

Since 2014, however, 2.0-liter inline four-cylinder direct injection turbocharged engines have been required for this class of car. Fuel restrictors have also been used to regulate the maximum fuel flow rate to 100 kilograms per hour at 7500 rpm (changed to 95 kg/h from 2016).

With the change from intake air flow regulations to fuel flow regulations, the direction for engine development changed dramatically with the focus of development turning to thermal efficiency. The fuel flow regulations placed an upper limit on the rate of fuel injection per unit of time. In other words, the regulations determined the amount of energy possible from fuel used per unit of time. The competition then became how to most efficiently convert that limited energy into engine output.

The formula for finding the theoretical thermal efficiency of the Otto cycle, used in gasoline engines for example, shows that thermal efficiency can be improved by increasing the compression ratio and specific heat ratio. The specific heat ratio can be increased by creating a leaner air-fuel ratio (λ>1) than the theoretical air-fuel ratio. Therefore, engine development for GT500 class cars since 2014 has focused on increasing the compression ratio and promoting leaner air-fuel mixtures at the same time as working to improve combustion and reduce loss associated with cooling, friction, and other factors.

Relationship between air-fuel ratio and engine output

Relationship between air-fuel ratio and engine output

Relationship between air-fuel ratio and thermal efficiency

Relationship between air-fuel ratio and thermal efficiency

GT500 class engines were required to have cylinder bores of 88±2 millimeters, maximum length of 500 millimeters, and minimum weight of 85 kilograms. As with the body, the regulations specified many common engine parts to help reduce engine development costs. The turbocharger was one of those parts. However, room was left to choose from the various options available for turbine A/R ratio* and compressor trim (inlet) diameter.

* A/R ratio: The ratio of the turbocharger turbine nozzle area (A) and the distance from the turbine inlet to the middle of the nozzle (R).

With maximum injection pressure for the direct-injection injectors set at 200 bar, a number of fuel delivery parts were also specified as common parts, including the injectors and high-pressure fuel pumps. However, developers are able to freely choose the number, diameter, and arrangement of jet orifices in the injectors.

Turbocharger, one of the specified common parts

Turbocharger, one of the specified common parts

HR-414E engine throttle and injectors, with the injectors arranged at the side

HR-414E engine throttle and injectors, with the injectors arranged at the side

Honda worked to reduce gas (air-fuel mixture) temperature in the HR-414E engine introduced in 2014. Trying to create a leaner air-fuel mixture to improve thermal efficiency, it had to increase air flow into the cylinders, meaning increased boost pressure. However, the higher boost pressure also increased pressure inside the cylinders, which in turn increased the temperature of the air-fuel mixture and increased the likelihood of engine knocking.

HR-414E engine introduced in 2014

HR-414E engine introduced in 2014

With the HR-414E, Honda adopted the Miller cycle as a technology for reducing the temperature of the air-fuel mixture within the cylinders. The Miller cycle, in which the expansion stroke is longer than the compression stroke, is a high expansion ratio cycle created by adjusting the timing for closure of the intake valve. In this cycle, the intake valve can be closed at a point just before the piston reaches bottom dead center in the intake stroke (“early closing”), or it can be closed just after entering the compression stroke (“late closing”).

Honda chose the early closing method for the HR-414E. When gases are compressed, their temperature increases. Conversely, expanding the gases has a cooling effect. In the early closing method, adiabatic expansion occurs after the intake valve is closed, which reduces the temperature compared to air intake at the bottom dead center. This results in a lower temperature at compression top dead center than in a normal cycle, which subsequently raises the knocking limit.

Adopting the Miller cycle not only improved the combustion cycle efficiency, but also promoted leaner operation through the cooling effect of the air-fuel mixture, raised the geometric compression ratio, and enabled optimal ignition advance, which led to improved thermal efficiency.

In engine development for the 2015 specifications, Honda worked on improved premixing of the air-fuel mixture. This approach sought to increase combustion speed and improve thermal efficiency by ensuring the air and fuel were well mixed. Specifically, this meant high-tumble combustion. In a tumble flow, the air-fuel mixture flows into the cylinders through the intake ports and then flows around the walls of the cylinders to create a vortex with its axis in the horizontal direction (vertical vortex).

Port injection engines, where fuel is injected into the cylinders via the intake ports, are suited to premixing because of the distance and time required to reach the cylinders. In contrast, the distance and time required to reach the cylinders in direct injection engines is shorter, making them less favorable to premixing. Honda, therefore, decided to use the power of tumble flow to facilitate premixing of the air-fuel mixture. Flame propagation after ignition is faster in a high-tumble flow as well, which improves the efficiency of converting energy to pressure and leads to improved thermal efficiency. This is another reason that Honda focused on achieving high-tumble combustion.

Improved thermal efficiency through the Miller cycle, a high-tumble flow, and other efforts increases maximum cylinder pressure (P-Max). Because this also increases thrust force of the piston against the cylinder walls, durability became an issue (with increased friction also leading to reduced efficiency). For the 2015 engine specifications, Honda applied an offset to the crankshaft to reduce the increased thrust force resulting from increased P-Max. This technology adopted a layout with the crankshaft axis shifted from the center of the cylinders to reduce connecting rod inclination during the expansion stroke, in which the load is greatest, and reduce thrust force.

To improve reliability as well, the two piston ring design used in the 2014 engine specifications was changed to three piston rings. Three rings are the norm in mass production engines, with the top and second rings mainly acting as gas seals. The bottom ring is an oil ring mainly acting to scrape oil from the cylinder walls to prevent it reaching the combustion chamber side of the cylinders. Racing engines, on the other hand, mostly use a two piston ring design, comprising a top ring and an oil ring, to help reduce friction.

The 2015 HR-414E engine specifications were changed to a three piston ring design with the aim of not only improving reliability, but improving sealing performance as well. The intention was to improve engine output by reducing gas escaping to the crankcase side via the piston rings in the expansion stroke.

Honda’s 2016 engine specifications in the first half of the year were very different to those in the second half. In the first half of the season, the focus was on improving the compression ratio and reducing friction. Honda achieved friction reductions by reducing the amount of lubricants used in the oil system. It was able to identify the exact amount of lubricants required through operation of the engines during the 2014 and 2015 seasons. By removing excess amounts, it reduced oil agitation resistance, which led to improved engine output.

Honda dramatically improved engine output in its 2014 engine by adopting the Miller cycle. From 2015 through the first half of 2016, it continued to constantly improve engine output by predominantly focusing development on improvements to combustion speed.

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TechnologyMotorsports TechnologySUPER GTEngine Development under Fuel Flow Regulations – 2014 to Early 2016 [SUPER GT]