2015-16 Intake Design

Requirements

- Supply fresh air to each cylinder - Fuel injector placement

Constraints

- Rules o IC1.4.1 Air Intake System Location All parts of the engine air and fuel control systems

(including the throttle or carburetor, and the complete air intake system, including the air cleaner and any air boxes) must lie within the surface defined by the top of the roll bar and the outside edge of the four tires.

o IC1.4.2 Any portion of the air intake system that is less than 350 mm (13.8 inches) above the ground must be shielded from side or rear impact collisions by structure built to Rule T3.24 or T3.33 as applicable.

o IC1.4.3 Intake Manifold – The intake manifold must be securely attached to the engine block or cylinder head with brackets and mechanical fasteners. This precludes the use of hose clamps, plastic ties, or safety wires. The use of rubber bushings or hose is acceptable for creating and sealing air passages, but is not considered a structural attachment. The threaded fasteners used to secure the intake manifold are considered critical fasteners and must comply with ARTICLE 11.

T11.2.1 All critical bolt, nuts, and other fasteners on the steering, braking, driver’s harness, and suspension must be secured from unintentional loosening by the use of positive locking mechanisms. Positive locking mechanisms are defined as those that: a. The Technical Inspectors (and the team members) are able to see that the device/system is in place, i.e. it is visible. b. The “positive locking mechanism” does not rely on the clamping force to apply the “locking” or anti-vibration feature. In other words, if it loosens a bit, it still prevents the nut or bolt coming completely loose. Positive locking mechanisms include: a. Correctly installed safety wiring b. Cotter pins c. Nylon lock nuts (Except in high temperature locations where nylon could fail approximately 80 degrees Celsius or above) d. Prevailing torque lock nuts NOTE: Lock washers, bolts with nylon patches and thread locking compounds, e.g. Loctite®, DO NOT meet the positive locking requirement.

o IC1.4.4 Intake systems with significant mass or cantilever from the cylinder head must be supported to prevent stress to the intake system. Supports to the engine must be rigid. Supports to the frame or chassis must incorporate some isolation to allow for engine movement and chassis flex.

Goals

- Even cylinder filling o Improves power

- Maximize volumetric efficiency o Improves power

- Reduce system weight o Usually a good idea

- Smooth the power band

o Increased drivability Design Selection

- Two main types o Center-fed

Easier to design for even cylinder filling o Log-style

Easier to package Better access to clean air

- Log style chosen for easy packaging and access to clean air, trick design using CFD mitigates tendency to unevenly fill cylinders

o Used steady-state CFD sim with constant and equal cylinder pressures Inherently less accurate than a transient model, but shows natural characteristic

of intake o Two stage plenum

Upper Stage: decreasing cross-sectional area to force more air out at cylinders close to the elbow

Lower stage: acts as an air reservoir - Bellmouths used to reduce the vena-contracta effect in runners

o Iterated through several combinations of outer diameter, length and radius to find which resulted in the highest mass flow of air

- Runner length chosen to produce a resonance at 9500RPM to help smooth out the power band o Ohata-Ishida Intake Resonance Model used (we have the most data on this method (the

Helmholtz model is an alternative)) Inputs

Throttle body area

Runner area

Plenum volume

Cylinder volume

Restrictor length Is a 1-D model, so used previous season’s data to create a scaling factor to apply

to result to improve accuracy - Structural Design

o Hybrid structure 0.050” 3D printed layer with external isogrid ribs to increase stiffness

3D print allows for more freedom in internal geometries Outer carbon fiber layer for structure

4 layers of sock on runners and elbow

5 layers of twill on the plenum o ANSYS sim performed to validate design for a backfire

Intake Spec Sheet

Plenum Data

Plenum Volume 4.65 L (296 in3)

Runner Length (not inc engine or bell) 23.9 cm (9 in)

Intake Port Length 7.62cm (3in)

Initial Runner Diameter 4.27cm (1.68 in)

End Runner Diameter 3.58cm (1.41 in)

Runner Taper Angle 0.885 deg

Restrictor Length 33.02 cm (13 in)

Throttle Body Diameter 3.94 cm (1.55 in)

Cylinder Volume 0.15 L (9.15 in3)

Bellmouth Data

Length 4.27 cm (1.68 in)

Outer Diameter 6.10 cm (2.40 in)

Inner Diameter 4.27 cm (1.68 in)

Radius 0.49 cm (0.19 in)

Intake Geometry The subsequent four images are cross sectional views of the plenum at four stations along its length to demonstrate the decreasing cross sectional area of the upper plenum stage. The last two images are views of the entire plenum, one of which is a cross section along the length to display the flange dividing the upper and lower plenum stages.

Validation of Intake Design The following two images are the results of two CFD simulations, one of an earlier plenum design without the internal flange and the other of the current design. As shown by the color scale, the earlier design has a larger pressure gradient across the length of the plenum than the current design. This validates that utilizing a decreasing cross sectional area upper plenum stage results in a more even pressure distribution across all cylinders, which will result in more even cylinder filling than previous iterations. Earlier iteration

Current design

Intake Manufacturing After receiving the 3D printed components (plenum and four runners), the plenum was prepared for the layup by sanding the outer surface and bonding the runners to the plenum with location pins to ensure proper placement. The layup schedule consisted of five layers of carbon twill on the plenum and four layers of carbon sock on the runners. A vacuum bag that allowed the interior of the plenum to vent to atmosphere ensured that the plenum would not deform under vacuum.

Testing To demonstrate that the carbon fiber would not delaminate from the 3D printed layer after being subjected to heat cycles, a representative test sample was manufactured and tested. After several heat cycles representative of what the plenum would experience, the test sample did not experience any delamination, qualifying the full plenum for use on the vehicle.

Design Validation The effectiveness of the addition of the internal flange was determined through the use of Exhaust Gas Temperature (EGT) sensors placed at the beginning of each exhaust runner. All other things held constant, the more evenly the cylinders are filled with air, the closer the EGT readings for each cylinder should be to each other. As shown in the first figure below for the plenum without the flange, the spread of EGTs was about 40C compared to about 15C for the current design, a 25C improvement. This result verifies the effectiveness of the internal flange in even cylinder filling. Please note that for each figure, one of the EGT amplifiers was malfunctioning, resulting in faulty readings.

The Power Curve The following figure is the power curve associated with the current intake. As shown, there is a peak in power at approximately 8900rpm, and not at 9500rpm as targeted by the runner length selection mentioned above. I believe this is due to presence of the internal flange changing the manner in which waves are reflected back within the plenum. Since the internal flange effectively decreases the volume of the plenum, the results from the Ohata-Ishida model will be invalidated.

Potential Improvements To improve this design, more research needs to be performed on the effect that runner length, plenum volume, and bellmouth geometry play on intake resonance. This would allow design decisions to be based more heavily on empirical data rather than the results of a 1D model. While the composite carbon and 3D printed structure did result in a one pound weight reduction when compared to a solid 3D printed structure, further weight reduction may be found in utilizing a purely 3D printed structure with external ribbing to improve plenum strength and stiffness. Moving away from incorporating carbon fiber in the structure of the plenum will also improve its manufacturability, since complex layups will not be needed and post processing will be vastly reduced. Other improvements may be found in: placing the air filter in the plenum to increase the surface area of the filter and further improve even cylinder filling, incorporating sharped-edged orifices within the plenum to increase wave reflection back into the plenum, and using an instrumented plenum to further validate CFD analysis.