Designing a combustion engine can take a lot of time, like three to five years. What if you could cut that time in half? For Lumenium LLC, a Virginia-based startup developing an innovative family of internal combustion engines, 3D printing with a Desktop Metal system enabled them to reach such a goal.

The company’s Inverse Displacement Asymmetrical Rotational (IDAR) engine is a novel design for producing powerful, efficient, internal combustion. Its unique engine geometry permits dramatic yet efficient work output from a small, light engine consuming less fuel and producing lower emissions. Key to the development of this engine was the ability to quickly iterate on part features and designs during prototyping.

Lumenium’s parts must withstand the extreme heat and stress inherent to internal combustion engine operation. Each engine component must meet specific requirements—including high dimensional accuracy, strength under dynamic loads, and low thermal expansion—and the weight of each part is an important consideration for overall power density and efficiency.

Additive manufacturing helped the design team meet these requirements and tackle complex part geometries—like internal cooling channels to improve engine performance. With the Studio System from Desktop Metal, the team brought this technology into their existing workspace for faster design iteration and functional prototyping.

A $350 billion market

Internal combustion engines represent a $350 billion market with three categories of engines: the traditional piston engine, novel opposed piston engines, and rotary engines. Lumenium’s IDAR engine technology adds a fourth category sitting in the middle of traditional engine technology, representing a paradigm shift in the power production. The engine components must withstand dynamic loading conditions, combustion forces of 1500 psi, and combustion temperatures of 1500 °C.

The ability to perform frequent design iterations can benefit final engine performance. The full engine development cycle for each generation of the IDAR engine takes between three to five years. Identifying a faster, more cost–effective approach to prototyping was critical.

Why 3D printing rather than CNC machining

Lumenium makes approximately 20 prototype parts per month. The majority (about 95%) are manufactured in-house using 5-axis CNC machining and wire EDM. Machining complex geometries with CNC involves complicated tool paths and sometimes more than 80 machining operations. Each operation requires re-programming, which often involves custom fixturing and an operator to realign the part. Even if post machining is required on the printed part, the number of overall machining operations is significantly less. Programming the CNC machine requires a trained, dedicated operator and can take weeks for a single, complex job. Some parts require post processing by outside vendors adding up to three weeks to the fabrication.

The remaining 5% of prototype parts—typically conventional, round parts—are sent to an outside machine shop where lead times average about three weeks.

In addition to extended lead-time and high costs, machining offers limited options for light-weighting parts. Weight is critical to engine performance where a 50% reduction in engine weight can potentially double the rated engine speed (RPM) and power output. To reduce weight with machining methods, engineers can do little beyond selecting a lightweight material. Without changing material, reducing weight with machining typically requires altering the part geometry—adding time and complexity—which can result in weak points within the part structure.

The Studio System prints parts with closed-cell infill—an internal lattice structure printed throughout the part. Users can adjust infill spacing to meet strength and weight requirements. Parts printed with infill will have significantly lower thermal transfer. This reduces part weight while maintaining strength, which enabled the design team to pursue steel as part of their solution.

In most extrusion-based 3D printing methods, including Bound Metal Deposition, horizontal holes require internal support structures to retain the shape. However, adapting the shape of the hole can eliminate the need for supports. The design team modified the design, changing the round holes to a more angular shape (i.e. a teardrop), which does not require support structures during fabrication.

The saddle design consists of serrations along the top and bottom edges that mate with the swing arms. The serrations help the component withstand engine forces, and machining these critical features allows for a smooth and accurate mating surface than cannot be achieved by printing alone.

In Fabricate, users can adjust shell thickness selectively. It is important to note that this does not change the dimensions of the part. Instead, it thickens the solid shell around the part to prevent exposing the part’s infill during machining. The design team increased the shell thickness of only the top- and bottom-facing features to 5.2 mm to account for material that would be removed during machining of the serrations.

Once fabricated, the parts underwent post-processing. This included CNC machining and wire EDM. The saddle’s critical surfaces were machined, serrations were added to the top and bottom, and holes drilled and threaded. Once complete, the design team bolted together the saddle and swing arm.

For Lumenium, rapid prototyping is critical to product development and maturation as the IDAR engine approaches commercialization. The design and function of each part within the assembly is critical, so the ability to refine and iterate quickly has a direct impact on the overall engine performance. The printed saddle, swing arm, and connecting rod demonstrate the time and cost savings that the Studio System delivers.

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