Exhaust Manifold
Objective
The exhaust manifold is a vital component of the exhaust system. It has to sustain high temperature and high pressure exhaust gases exiting the combustion chamber. A well-designed manifold will have significant impacts on the engine's performance and efficiency.
In this project, the goal is to design and test a custom manifold design compatible with a Third Generation Chrysler 5.7L Hemi V8 engine header gasket and a standard oxygen sensor.
Manifold Configuration
The manifold ideation and design process was conducted in Autodesk's Fusion 360 CAD software. The model consists of three sub-components, the head flange, the runners, and the collector, all intended to be manufactured independently and welded together to realize the complete manifold design. Some intended parameters for the design to adhere to include:
Runners of 1-3/4" to 2" in diameter
4 to 1 collector with a 3" diameter exit pipe
Direct compatibility with the mating parts illustrated below.
Chrysler Specified Header Gasket
Mopar Parts SKU: 5045496AA
Oxygen Sensor
McMaster-Carr SKU: 1245N27
Runner Length Testing
Unequal Length
Runners generated with standard 1/20" thick tubing through minimal straight and angled segments connecting each of the head flange ports to the collector ports.
Equal Length
Runners generated using lofts about specified centerlines and linked to the appropriate receiving collector ports. This configuration achieves equal lengths across all four runners.
CFD Analysis
Both runner configurations were imported into Autodesk CFD, a Computational Fluid Dynamics software, to simulate the conditions expected under standard operational loads.
A velocity study was conducted with the use of the Finite Element Method (FEM) to test runner length impact on the exhaust gas velocity gradient. Nitrogen was used as the material property since it is the most prominent compound in exhaust gases. The boundary conditions for the study were input velocity magnitudes of 100 inches per second at each of the head ports along with a gage pressure set to the exit of the collector.
Since this velocity study simulates Nitrogen, a compressible fluid, calculating the velocity gradient across the runners depends on multiple differential variables. The resulting velocity maps are shown to the right with trace lines showing the velocity gradient across the runners. Upon close visual inspection of the velocity study, a more smooth gradient with smaller magnitudes is apparent along the equal length runners. This is primarily a result of the smooth bends when compared to the unequal length design.
A final design should incorporate smooth bends along the runners to minimize the effect of an unstable velocity gradient. This will result in a less turbulent and more stable exhaust flow through the manifold.
Unequal Length Velocity Map
Equal Length Velocity Map
Unequal Length Pressure Map
Equal Length Pressure Map
A subsequent test was run with input pressure magnitudes of 5 psi set at all head ports to generate a pressure map for both manifold configurations. The resulting maps are shown to the left and illustrate the pressure gradient across each runner. Analyzing the trace lines in both figures, we observe a more balanced distribution across the equal length manifold configuration. This satisfies the Darcy Weisbach formula given by:
Where the runner length (L) is directly proportional to the change in pressure (ΔP) along a pipe of constant diameter (d).
When fixed to an internal combustion engine, the exhaust gases travel in pulses rather than a stream. This principle is a product of both valve timing and the combustion cycle. The pulsing exhaust gases create negative pressure waves that draw additional exhaust gases out of the combustion chamber. A manifold with a balanced distribution across all of its runners permits better maximization of the scavenging effect and in turn improves the engine's performance and efficiency.
Redesign
A redesign of the runner geometry and configuration was done to ensure equal length runners were incorporated while keeping manufacturability in mind. The new runners consist of standard bent and straight segments rather than spline defined lofts. These segments can be adjusted and welded to satisfy the fitment and clearance demands of the adjacent components in the engine bay. This allows the design to achieve the optimal functionality identified with CFD analysis, while the manufacturing process remains fast, efficient, and cost effective.
ASME Y14.5 Technical Drawings for Manufacturing
To better illustrate component interoperability and assembly, ASME standard technical drawings have been provided to aid in manufacturing a prototype for testing. These drawings contain geometric dimensioning and tolerancing as well as additional section views to ensure the design is completely specified.
Renderings of Exhaust Manifold Assembly
Some renders of the driver and passenger side assembly in a brushed stainless steel finish to help visualize how the finished exhaust will look.
