They call it stock car racing, but it’s anything but stock. By the time a NASCAR racing car hits the track, only the outline of the car’s body conforms to the cor- responding model on the showroom floor.
Some of the biggest changes are reserved for the engine block. For teams such as Richard Childress Racing–home of Nextel Cup drivers, Kevin Harvick, Jeff Burton, and Clint Bowyer–the challenge is to take the basic SB2 General Motors engine and squeeze out the most horsepower and torque possible within NASCAR rules.
The original SB2 design dates back 50 years, and until recently, there was no 3-D digital CAD model of the engine block. GM wanted an accurate digital model of the engine block for finite element analysis (FEA) and computational fluid dynamics (CFD) studies and to create a complete digital engine assembly in the future.
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Richard Childress Racing (RCR), well known for its innovative use of reverse engineering technology, won the bid to create the first 3-D digital model of the SB2 engine block. The team has extensive experience using scanners and reverse engineering software to fine-tune cylinder head ports for its cars. The process enables RCR to generate highly accurate ports in about one-sixth the time it took to do the work by hand.
The GM project embodied five of the six main reasons that automotive engi- neers use reverse engineering:
1. to create free-form shapes that are difficult to model in CAD software;
2. to overcome obstacles in data exchange and data integrity;
3. to create complex geometries that might not have a CAD model;
4. to resolve and correct problems arising from discrepancies between the CAD master model and the actual tooling or as-built part; and
5. to ensure quality and performance through computer-aided inspection and engineering analysis.
The typical starting point for a reverse-engineering project without 3-D CAD data would be the 2-D drawings. In this case, however, the 2-D prints did not contain the design GM wanted to re-create. When 3-D molds and cores were constructed in the 1950s and later years, changes were made and they were not detailed in the 2-D prints. RCR wanted to capture these details as they appear in the as-built part. Complicating matters further, the cast-feature shapes are diffi- cult to reproduce using direct CAD geometry, even after precise measurement.
Radius rounds from an actual casting, for example, might be spline-shaped, not spherical with a single radius as depicted in CAD models. “We wanted a model of what we have, rather than an idealization of what we think we have”, said Clifton Kiziah, the RCR engineer who managed the project. “The 3-D model needed to match the physical block as accurately as possible.”
Accuracy for GM requires casting tolerances of ± 0.030 of an inch. Anything less might compromise the credibility of FEA and CFD test results. Meeting the Figure 7.2. The General Motors SB2 racing car engine block and its digital duplication. Copyright © Richard Childress Racing Enterprises, Inc., Welcome, NC, USA, www.rcrracing.com. Reproduced with permission.
project’s goals required a reverse engineering system that would capture data from an actual SB2 engine block and turn it into a highly accurate 3-D CAD model. The software needed to bridge the gap between the physical and digital worlds.
The project was particularly challenging for the following reasons:
1. The cylinder block of a racing car engine contains very complex topology.
2. Interior cavities cannot be captured by a range-image scanner (also called a line-of-sight scanner).
3. GM wanted a true CAD model in which parts of the assembly could move during simulation.
RCR was ready for the challenge. To capture data in cavities, urethane was poured into the actual sand core molds used in the block-casting process.
A structured-light scanner was used to capture the cores and engine block as eight separate point cloud files. The engine block was a 20-MB file and the cores were about 15 MB each.
The point cloud files were brought into reverse engineering software and processed to reduce noise and delete outliers. The point clouds were already in the same coordinate space, so no alignment was needed. The software then automatically created a triangulated surface from the point cloud, and holes were filled to create a watertight mesh surface. Because of the need to produce CAD models with moving parts, the mesh surfaces were parameterized to create a nonuniform rational B-spline (NURBS) surface, a mathematical representation used by most CAD software. The reverse engineering software computed the data for aligning the water jacket surface with cores to create cavities, making the model ready to be used by CAD software.
The eight surface files–complete with datum planes, axes, and references–
were imported individually into CAD software, and the cores were assembled as they would be to cast an actual water jacket in an engine block. A cut-out feature was used to subtract cores from the block.
The engine was reverse engineered to achieve three times the accuracy re- quired by casting, tightening the tolerance to 0.010 of an inch, and in some cases
Figure 7.3. Reverse-engineered model of urethane molds of the internal cavities from the SB2 engine.
Copyright © Richard Childress Racing Enterprises, Inc., Welcome, NC, USA, www.rcrracing.com. Reproduced with permission.
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to within 0.005. “It would probably have taken several months if we had to model it directly, and I’m not sure we would have been able to capture the com- plexity of the actual cast surfaces”, Kiziah said. “The finished model looks and measures the same as the real block.”
GM and its racing teams now have an accurate digital model of the SB2 en- gine for CFD tests to optimize cooling, and secondary machining simulations to check for clearances and fit of new parts. FEA simulations will be used to deter- mine where material can be removed in secondary machining without affecting the strength of the block.
Reverse engineering of the SB2 engine block could be just the start in RCR’s quest to digitize parts that do not have 3-D CAD models. The team is working on a digital model of a total car assembly, complete with surface models of the chas- sis and the entire engine.