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In 2016, development of a larger 40 hp engine began. Building off of many of the lessons learned from, this engine also needed to be designed for the higher loads of compression ignition. After several concepts, the team selected to proceed with a solid shaft (unlike Xmini) for durability, lubrication, and cooling reasons. Breathing now was done through windows in the side covers. For the initial prototype, I was responsible for design of the crankshaft, gears, and housing. After a proving that the engine could fire and withstand the higher pressures of compression ignition, several of the components needed to be redesigned for steady-state running. One such part was the rotor, which needed to be cooled to withstand extended running and high power.
Oil Cooled Rotor
Oil Cooled Rotor
One my projects for X4 was to lead a small team designing the first generation cooled rotor. The project was involved project management, heat transfer, and structural analysis.
Project Flowchart
PROBLEM STATEMENT:
PROBLEM STATEMENT:
Design a functioning rotor for X4 that will operate at or below the target surface temperature during full load conditions at rated brake power.
Finished Rotor
CONSTRAINTS:
CONSTRAINTS:
Strength: rotor must be able to withstand predicted pressure and temperatures
Weight: lighter rotor = smaller counterweights and higher power density
Oil pressure drop: smaller pressure drop = smaller pump = higher brake efficiency
Breathing area: higher volumetric efficiency = higher power density
Packaging: cooling passages must be able to fit into current engine geometry
Manufacturability/cost: easy transition to higher volume manufacturing
In the initial brainstorming phase, we came up with several methods to potentially cool the outer surface of the rotor. Some of this included oil cooled pin arrays, microchannels, water injection, and heat pipes. Each concept was evaluated against our design criteria above. Through this process, two concepts were selected for further investigation by the team: oil cooling via 3D printed (DMLS) pin arrays or a welded fin assembly.
A fellow engineer analyzed the pin array concept, while I looked at the fin assembly. The pins had a higher surface area for heat transfer at the risk of a higher pressure drop and difficulty in manufacturing. The fins would be wire-EDM and a series of plates would be welded to form inlet and outlet channels.
Heat Pipe
Rotor Fins
Pin Array
Microchannel Array
To analyze the fins, I created a 0-D model of the fin channels so the geometry could be optimized to maximize heat transfer and limit pump power. In parallel, I performed structural FEA on the rotor with the goal of limiting the weight of the array—because the rotor rotates eccentrically, any extra weight must be counterweighted. To quickly iterate on the structural rigidity of the rotor ribs, analysis on the rotor was done on a section of the rotor. With this design, I was able to maintain a thin rotor wall (more effective heat transfer) without adding additional supports and weight. These supports increase manufacturing complexity and cost.
Deflection Plot of Rotor
From results of both our analysis, the fin design showed a lower surface temperature than the pin design for the same power loss. Also, there is less manufacturing risk as the processes and materials are more conventional. The decision was made to proceed in implementing fin arrays into the X4 rotor.
The rotor is composed of a machined “core” and several plates to reduce weight. These plates are welded together and then the critical dimensions and features are post machined. I dynamically balanced the rotor, so vibrational forces limited. One of the challenges of with this rotor was accounting for the mass of the oil while at operating temperature.
Mechanical prints were released for each part and process. We worked with several local shops to manufacture this rotor. The rotor was manufactured and successfully ran at steady state without exceeding our temperature limit.
Project Takeaways:
Project Takeaways:
Managing a complex project and coordinating between engineers
Optimization of heat exchanger
Downselction of concepts
Other Selected X4 Projects:
Other Selected X4 Projects:
Brainstorming and development of initial concepts
Investigated and presented trades off for varying engine geometry
Redesign for cooled hardware: including 0-D model and thermal FEA of housing water jacket
Thrust bearing design and modeling
Oil seal design and material selection
Concepts for improved combustion chambers
Created method of analyzing crankshaft FEA data in MATLAB to determine fatigue life
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