Induction System Development
The RAETECH Corporation has been racing in the SCCA's GT2 class since 1982. Despite the fact that most road going cars are fuel injected, SCCA dictated that GT2 cars must run using Weber carburetors. Although this arrangement has been a standard in sportscar racing since the fifties, it has fundamental limitations that keep the cars from realizing their full potential. For example, Weber carbs atomize fuel into the intake tract well upstream leading to difficulty keeping fuel suspended all the way to the combustion chamber. Structure in the venturis causes flow losses. Tuning for optimal air/fuel mixture at the RPM that produces peak torque results in compromised mixture under light load leading to poor wet track performance. And lastly, the only way to optimize for changes in barometric pressure or temperature is to re-jet the carbs.
In 2003 the SCCA enacted a rules change that would permit the use of fuel injection in GT2. While performance of the RAETECH Porsche outstripped the capabilities the OEM fuel injection arrangement, the creation of a specialized fuel injection system would allow the RAETECH team to address the shortcomings of carburetion and provide additional enhancements. The switch to electronic fuel injection requires an engine control unit (ECU) to monitor and control fuel and ignition. Concurrent benefits would come from the ECU's capacity to support a comprehensive suite of sophisticated engine and chassis sensors reporting to an integral data acquisition module. Improvements in combustion efficiency combined with enhanced adjustability produce a significant performance increase. Exactly how significant an increase remained to be seen.
The RAETECH Corporation maintains technical partnerships with Optimum Power Technologies, ANSYS and Bosch Motorsports. Bosch provides the Engine Management system, sensors and data acquisition. Optimum supplies automated engine design (AED) software that models the entire combustion cycle and Distributed Network Processing that allows calculations to be distributed amongst all the PCs in a local network. ANSYS provides structural analysis and computational fluid dynamics software.
Flow analysis is conducted through all elements of the intake and exhaust tract. RAETECH engineers saw the opportunity to use ANSYS' CFX package in tandem with Optimums' 4-Stroke engine modeling suite to accurately model a myriad of potential system designs. These models could then be "run" in simulation and performance could be evaluated.
Creating the model begins with dimensional inputs describing the intake and exhaust passages in the cylinder head, the exhaust system, a theoretical induction system and camshaft profiles. Optimum's 4-Stroke AED package then allows the user to set variation parameters to represent changes in intake runner length, diameter, cam profiles or any other aspect of the engine running cycle.
The Optimum AED software also provides RAETECH engineers with a valuable tool for acoustical tuning. Once the realm of "cut and try", now extensive combinations can be simulated and assessed before making actual parts. The pulses created in the intake runner by the opening and closing of the valve, precisely matched to cam characteristics allow the cylinder to continue to fill during the compression stroke. This "pulse charging" can result in volumetric efficiencies in excess of 130%. Optimum's Automated Design prompted refinements to intake, cam and exhaust features for peak performance.
ANSYS' CFX software yielded significant results when used to study flow through the throttle bodies, manifolds and ports. As initially designed, the throttle plates, or butterflies, mirrored standard practice in that they were constructed from a piece of flat sheet metal that is attached to a round shaft using screws. CFD analysis revealed significant wake turbulence caused by the unstreamlined round throttle shaft and the screws attaching the butterfly. Based on this information, RAETECH redesigned the throttle components using a new trapezoidal section butterfly and carefully designed shaft attachment to provide much less disruption to the airflow in the runner.
Until recently, the sheer volume of calculation required to run these simulations would have required either super-computer access or an unmanageable amount of time. Optimums' distributed network processing enables companies on RAETECH's scale to access unutilized processing capability in a typical desktop PC/server network providing resources previously unavailable.
RAETECH put the model's performance forecast to the test by fabricating an induction system, cams and exhaust to the specifications indicated as most advantageous by the simulation. When dyno results were overlaid on simulation prediction, the match served as compelling validation of the method. The performance that was expected was the performance that was attained.
The result is an engine that increases maximum power output by optimizing volumetric efficiency, provides a broader power band by optimizing the air/fuel mixture across the RPM band, corrects mixture to compensate for atmospheric conditions and allows the driver to select from different fuel mapping profiles on the fly.
The performance provided by ANSYS, Optimum Power Technologies and Bosch Motorsports products were synergized by the RAETECH team to create a complete induction, cam and exhaust system in only eight weeks. The resulting engine provided the RAETECH Porsche with the performance needed to win the national championship.
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» Example of Engine Modeling Results