Hornet Racing, a student engineering organisation at California State University, USA, prides itself on engineering highly competitive race cars. Each year, the team designs, builds and tests a Formula-style, open-wheel, single-seat car in preparation for an international Formula Society of Automotive Engineers (SAE) competition with other universities. Competition guidelines are intended to challenge participants and encourage creativity in tackling difficult design and engineering problems.
In 2017, Hornet Racing used Carbon 3D’s end-to-end digital design and 3D manufacturing capabilities to revolutionise the engine’s intake manifold, optimising performance in ways that were previously impossible. An intake manifold is essential to an engine’s function and car performance. The manifold supplies an air/fuel mixture to the engine’s cylinders.
The results of the redesign were staggering. Enabled by Carbon’s Digital Light Synthesis technology, the team achieved: a new engine intake manifold design that is impossible to produce with other technologies, reducing weight by 50%; a substantial reduction in the number of weld joints through part consolidation; smoother airflow to the cylinder head, enabling a more consistent power delivery - a drastic upgrade from prior years; and significantly improved overall engine performance.
DESIGN CHALLENGES: THE ENGINE INTAKE MANIFOLD
Hornet Racing’s car uses a Honda CBR600RR series, four-cylinder engine, which comes with four individual throttle bodies (one for each cylinder) that are each 44mm in diameter, placed very close to the cylinder head. Throttle bodies control airflow to an engine based on a driver’s acceleration pedal input. Having four throttle bodies maximises throttle response and performance of the engine, with revolutions up to 14,000rpm. However, per Formula SAE design challenge guidelines, stock throttle bodies must be removed and replaced with a single throttle for all four cylinders. Additionally, guidelines dictate that a 20mm diameter restrictor must be placed behind the single throttle, creating a performance constraint akin to someone needing to breathe through a narrow straw or coffee stirrer. This restrictor significantly limits the engine’s power output, and challenges students to rethink the design and engineering of the engine structure.
The design of Hornet Racing engines also resulted in drivability difficulties related to throttle response and smooth power delivery. When the driver attempted to accelerate by pressing the throttle to the floor, poor airflow would result in nonlinear power delivery and cause a delay that made it difficult to drive smoothly and consistently.
The team’s prior challenges, combined with the Formula SAE throttle body competition restrictions, set the stage for Hornet Racing to take a fresh approach when designing the intake manifold for its 2017 race car. Carbon’s Digital Light Synthesis technology enabled unique design advantages and production capabilities, enabling Hornet Racing to overcome previous challenges rooted in design restrictions associated with conventional manufacturing.
IDENTIFYING REDESIGN OPPORTUNITIES
Hornet Racing’s legacy intake manifold had been used for several years prior to the team’s 2017 redesign effort. The majority of its components were aluminium and needed to be welded together after machining steps. Additional components were made separately using carbon fibre moulds. These conventional manufacturing methods imposed numerous design limitations. The team was restricted to using basic part geometries and could not quickly iterate on designs. Such limitations contributed to substantial engine performance problems, including considerable boundary layer formation and uneven air distribution across the four cylinders. Additionally, assembling the legacy intake manifold involved many small components and intricate steps, creating considerable room for error.
Setting its sights on simplifying the intake manifold design, Hornet Racing identified the following goals for redesigning the part: optimise the airflow for better engine performance; create components that promote minimal boundary layer formation to allow for smooth airflow; integrate the fuel injector ports into the base of the intake runners (tubes that connect the plenum with the cylinder heads) to achieve minimal flow turbulence; and reduce overall manifold weight to promote improved handling characteristics.
As the team moved forward to perfect its legacy manifold, it became clear that with traditional manufacturing methods, the desired improvements would be impossible to manufacture or prohibitively expensive.
To overcome the design constraints and high costs associated with traditional manufacturing approaches, Hornet Racing turned to Carbon’s Digital Light Synthesis technology and Carbon’s RPU 70 material to execute its vision for an improved intake manifold.
OPTIMISING DESIGN THROUGH TO PRODUCTION
With Carbon’s technology, the team was able to access previously impossible geometries, reimagining an entirely new manifold design unconstrained by conventional manufacturability. The part could be produced rapidly, with no lead-time constraints and tooling costs. The result was an isotropic, durable, engine-ready intake manifold manufactured to optimise vehicle performance.
Central to the new design is a ‘bulb’ only 7in in length that replaced the 2ft long diffuser and the large plenum (over a half-gallon in volume). Inspired by supersonic jet engine shock cones, which regulate air intake based on shape, the team combined the functionalities of the diffuser and plenum by designing a spike-like flow split within the bulb structure. The spike feature enables airflow optimisation in a diffuser that is only 30% of the length of a traditional diverging nozzle diffuser. As a result, the team was able to eliminate the traditional plenum entirely.
Moreover, taking design inspiration from the dimples on golf balls, the shock cone-inspired spike structure contained a dimpled pattern on its main body, which helps air flow directly into the intake runners without losing velocity. With this design, the team exceeded its own expectations and achieved an intake that allows the engine to rev to the original redline of 14,000rpm. This 33% increase in performance was impossible with the team’s legacy manifold intake, which at best enabled the engine to reach 10,500rpm.
Beyond the spike structure that allows for optimised airflow between the diffuser and upper intake, the new intake manifold also possesses the following advantages enabled by Carbon’s technology: seamless integration of fuel-injector ports into the base of the intake runners; customised intake runner tubes with a precisely tapered diameter that minimise boundary-layer formation for smooth airflow; and major weight savings due to a simplified, more compact design and the use of Carbon’s RPU material. The new intake weighs approximately 50% less than the legacy intake. Since the intake is positioned high in the vehicle, a heavier intake can destabilise the car’s roll centre and vehicle dynamics. The lighter intake contributes to improved vehicle handling and a better overall driving experience.
Ultimately, Hornet Racing optimised its intake manifold design in ways that were either physically impossible or prohibitively expensive using traditional manufacturing methods. The vehicle performance results were astounding. The 2017 race car - the HR2017 - had its best competitive finish in the history of CSU Sacramento Hornet Racing. The team placed 16th overall out of 80 university teams from all over the world.