A Sonic Throttle Body Characterization Flow Bench Adapted for Fluids Laboratory Instruction via Energy Engineering Laboratory Module Pedagogy
Authors: Wisniewski Carl S. , Niemi Sean R. , Traum Matthew J. 
Published: February 2024

Formula SAE (FSAE) is a competition organized by SAE International where student teams design, manufacture, and race formula type racecars under prescribed constraints. One key engine intake system performance limitation stated in the rules is that “a singular circular restrictor of 20 mm diameter must be placed in between the throttle body and engine”. FSAE racecar throttle bodies are typically open to ambient air, and mass flow through the restrictor limits the engine’s power. Therefore, a central FSAE design focus is maximizing mass flow through the restrictor. Typically, these restrictors assume a converging-diverging (C-D) nozzle geometry to minimize losses, and they achieve sonic velocity at the 20 mm throat corresponding to choked flow and maximum mass rate through the available orifice cross section. FSAE competition teams operate through sponsorships and funds are limited. Thus, an easy-to-build, low-cost sonic flow bench apparatus is needed to characterize flow through the 20 mm FSAE circular restrictor.

This paper describes a student-built mass flow rate and velocity measurement system developed using off-the-shelf PVC pipe fittings mated to two machined parts that attach to the restrictor C-D nozzle throttle body. Ambient air is sucked through the system using shop vacuums, and the flow is characterized using three measurement points: 1) throttle body upstream barometric pressure, 2) static pressure immediately downstream of the orifice restriction, and 3) a pitot-static measurement far downstream of the restriction.

The system is designed to account for or eliminate all major sources of flow loss, and signal processing via low-pass filtration enables accurate characterization of the flow through the throttle body and 20 mm orifice. Engaging / disengaging shop vacs in combination with a user-actuated PVC ball valve that restricts flow far downstream of the pitot-static probe enables access to the full flow condition range through the throat of the C-D nozzle from zero flow to sonic flow.

With application of advanced pressure sensors, mass flow rate can be characterized across the entire throat Mach number range from 0 to 1 with departure from theory no greater than ±5%. These measurements enabled identification and remediation of loss-inducing restrictor features including roughness in the connector epoxy and a sharp corner at the orifice exit.

In addition to its FSAE restrictor flow characterization utility, this sonic throttle body flow bench can characterize flow through many C-D nozzle geometries of interest for instruction. It is therefore useful as a low cost and accessible educational instrument that can be built by virtually any institution and used in fluid mechanics laboratory courses for hands-on compressible flow demonstrations and experiments. The educational utility of this system is expandable by pairing it with a 3D printer able to produce different throttle body and C-D nozzle geometries to create open-ended student-driven experiments exploring impacts of nozzle geometry on flow efficiency.