Intake Plenum FEA
About This Project
This project was a part of the University of Waterloo Formula Motorsports' 2023 vehicle. The 2022 vehicle had incorporated a 3D printed (FDM) intake with a very similar geometry although it had broken during testing. Due to this, the team was required to use an aluminum intake. To optimize air flow and increase horsepower, I decided to revisit the intake and perform FEA on the model to identify high stress concentrations and alleviate them. In addition to this, the team had access to a selective laser sintering (SLS) printer which greatly reduced the anisotropic properties of FDM-produced parts. Although this had considerable benefits, there were some drawbacks as outlined below.
Summary
Objective
Perform FEA on intake plenum to adapt it to withstand vacuum and backfire pressure
Select material and manufacturing method from existing intake pressure data
Engineering Decisions
Selected nylon 12 due to its high stiffness - ideal for an intake as it cannot alter its geometry during operation
Selected SLS printing due to its high layer bonding in Z axis and ability to print complex geometries
Ensured 2-3 elements throughout model thickness in mesh
Results
Altered geometry to reach a max deformation of 0.32mm
Removed stress concentrations in flange mounts to provide a safety factor of 6
Design Preparation & Material Consideration
The previous FDM-printed intake had split at a layer line during a backfire. As the new intake would be SLS-printed, it was imperative that this did not occur. We had access to multiple materials such as nylon 11, nylon 11 carbon fiber, nylon 12 and nylon 12 glass fiber. Although the added fibers would increase the strength of the material in the X-Y plane, adding them would decrease the bonding of each layer in the Z plane. In addition to this, the ductility of nylon 11 ruled it out as the intake was required to be extremely stiff and resistive to any strains.
Following this, I researched properties of SLS-printed polyamide (nylon) 12 and found this article which highlights the virtually isotropic properties of SLS prints as well as this article which identifies the fatigue properties of SLS prints. This is crucial to material consideration as the intake will undergo countless cycles from low pressure (almost vacuum) to high pressure (backfires). The exact max/min values were gathered from intake pressure data analyzed in MoTeC i2 and were found to be 17.2kPa/103.9kPa.
Original Model Failure, MoTeC i2
Updated Model, Original Model
CAD
The model for the intake had already been designed by a previous student, however, due to changes in the vehicle's chassis, mounting points had to be updated. The overall volume of the intake also had to be slightly reduced as the original model was larger than the SLS printer's build volume. In addition to this, another flange was added to split the plenum into three parts due to build volume restrictions.
After I made the required changes in SolidWorks, I was then able to bring the model into ANSYS Mechanical to simulate.
FEA Setup & Mesh
The model's mesh was created using tetrahedrals with a 3mm element size. This was done to ensure that there were 2-3 elements throughout the thickness of the part.
To accurately model the stress the intake would undergo, I added several different forces. These forces consisted of standard Earth gravity, 1G acceleration, vacuum pressure (17.2kPa) and ambient pressure (101kPa). The simulation was then run two more times, once with backfire pressure (103.9kPa) and once with cycle pressure (17.2kPa-103.9kPa). Each flange face was assumed to be a bonded contact to simplify the model as the previous intake had no issues at the flanges. The model was also fixed at the engine and chassis mounts.
Mesh Element Quality
FEA Results & Printed Intake
Although the material's UTS of 50MPa was provided, it's yield strength was not. A conservative estimate of 30 MPa for the yield strength was found based off of this article outlining the stress/strain properties of powder bed fusion (PBF) additive manufacturing methods such as SLS.
ANSYS displayed the highest von-Mises stress (10.9 MPa) during vacuum pressure as seen below. This was at a stress concentration at the restrictor mount which was then alleviated with an added radius. The bulk of the part was under even less stress (~3.5 MPa - ~1MPa) which was significantly under the material's yield strength.
Performing a fatigue cycle analysis also showed that the intake would not fail as the stresses never exceed the endurance limit according to the S-N curve.
von-Mises Stress (MPa), Deformation (mm), Printed Intake