The Mazda 787b is a car that won the 1991 24 hours of Le Mans. An aspect of this car that made it completely unique to every other car on the grid was its rotary engine, also called a Wankel engine. This project aimed to emulate this engine as an ode to its impressive automotive history.
Aiman, Olisaneme, and Summan decided to replicate a Wankel engine for our mechanical mechanism. A Wankel engine is a “type of internal-combustion rotary engine distinguished by an orbiting triangular rotor that functions as a piston [1].” See an image of the Wankel engine mechanism below.
Figure 1: Wankel engine mechanism [2]
To begin constructing this mechanism, the team found a CAD model of the Wankel engine from GRABCAD [3]. We downloaded the file, opening it in SOLDWORKS, and then converted the parts we needed into .dxf files. Because we created our parts out of a 3D model, we used adobe illustrator to remove unnecessary layers that appeared when we “flattened” the 3D model into 2D pieces. When this gear was flattened into a 2D .dxf file, it appeared as the gear teeth, along with layers of circles within it that needed to be removed. Converting the 3D files into 2D drawings was tedious due to removing these extra lines.
Figure 2: Initial gear dxf vs post processed dxf
We repeated this process for all the parts we needed to laser cut. The image below shows which essential components of the engine we chose to cut out.
Figure 3: Pieces for assembly
Our initial prototype included the spur gear, housing, and the rotor (which was combined with the internal gears for ease). This comprised our low fidelity prototype. At this point we did not have a CAM shaft to provide a mechanism for rotation, but we did test the mechanism by manually rotating the rotor with out hands. See our low fidelity prototype!
Figure 4: Low fidelity prototype
Next, we worked to scale down our prototype. When we did this, we needed to increase the number of gear teeth as the tiny teeth were getting charred off in the laser cutter. To generate gears with larger teeth, we used gear generator, opting for a pitch diameter that matched the initial diameter of the model. Additionally, we added in our CAM shaft components as a way to actuate the rotary motion. There are duplicates of some parts, partly to serve as spares, but mostly, because we decided to make our model multi-layer at this point to accommodate the CAM shaft. See out modified .ai files below. Notice how we placed parts within each other to conserve material.
Figure 5: Files for Medium Fidelity Prototype
During the process of assembling these pieces together, we noticed the rotor getting stuck against the housing as we rotated the mechanism. To quickly fix this, we eliminated the housing, and instead used clear acrylic pieces in the shape of the housing, to allow for fixation of our spur gear while still giving the look of the Wankel engine for our medium fidelity prototype. See our medium fidelity prototype below!
Figure 6: Medium Fidelity prototype
For our high-fidelity prototype, we assembled everything with wood glue, or super glue where applicable (acrylic to wood). We used additional layers of housing here for support instead of cardboard. We overcame issues with the housing and rotor getting stuck by adjusting inner dimensions of the housing and by sanding the inside of the housing layers using a dremel to ensure smooth rotation. These resulted in scratches on the acrylic but were essential to the functionality of the Wankel engine.
Figure 7: Dremel modification
To break down the model at this point, we’ve included a list of the layers:
- Layer 1 +2: Acrylic housing layer attached to wood housing layer. Spur gear fixed on acrylic. Rotor with internal gear.
- Layer 3: Wood housing layer. Rotor without internal gear. CAM shaft component.
- Layer 4: Wood housing layer. CAM shaft component.
- Layer 5: Acrylic layer
All wood pieces were post-processed by sanding with P400 grit sandpaper.
Additionally we added a racing flag shaped handle to the dowel of the CAM shaft components as shown below. We also added our stickers and name plate. The stickers helped to minimize the aesthetic impact of the scratching of the acrylic.
Figure 8: Complete Assembly
Lastly, to ensure the mechanism doesn’t move around, we added rubber pads to the bottom that we found in the consumables area of the OEDK as shown in figure .
Figure 9: Complete Assembly
In conclusion, this project was more complex than expected partially because of issues with creating initial models. My personal advice for facing the septic projects is to try to get a physical replica as soon as possible, creating an initial full build that can be manipulated by hand. If I were to remake the device, I would make the whole of the casing wider to prevent the scratching on the acrylic that I caused when modifying the engine.
Access to the a video of a working mechanism: IMG_7702
Figure 10: Clean work space
The total cost of this project was estimated to be $214.42. While this seems quite costly for the final product I think the cost could be significantly decreased if we repeated the project. A major source of the costs were associated with plywood board usage and laser cutter time. Given the experiences the team has gained, recreating the Wankel Engine would likely only take one hour of laser cutting time and one plywood board, decreasing our cost by as much as $75.
FINAL COST SUMMARY
| Cost Type | Cost | Price | Source | Quantity | Total |
| Materials | 2 x 4 ft plywood board (¼”) | $14.94/board | homedepot.com | 4 boards* | $59.76 |
| 16” x 20” acrylic sheet | $12.94/sheet | dickblick.com | 1 sheet | $12.94 | |
| 400 grit Sandpaper | $5.98/package | homedepot.com | 1 sheet | $1.99 | |
| Masking tape | $2.99/roll | amazon.com | 1 roll | $2.99 | |
| Vinyl | $0.62 / Foot | Amazon.com | 2 ft | $1.24 | |
| Labor | Prototyping Engineer (You!) | Free | Us | 3 engineers,
6 hours each |
$0, 18 total hours spent |
| R & D(decal and cut out design) | $17/hr | ziprecuiter.com | 1.5 hour of designing and assembling | $42.5 | |
| Vinyl cutter Operator | $18/ hr | ziprecuiter.com | 1 hour | $18 | |
| Overhead | Laser Cutter Machine Time | $15/hour | accurl.com | 5 hours | $75 |
| Total | $214.42 |
* Accounts for extra wood used in our iterative process. Many of these pieces were scrapped and not used in the final prototype.
References
[1]https://www.britannica.com/technology/Wankel-engine
[2]https://www.howacarworks.com/technology/how-a-rotary-wankel-engine-works
[3]https://grabcad.com/library/wankel-rotary-engine-model-2
