Publication Date




Embargo Period


Degree Type


Degree Name

Doctor of Philosophy (PHD)


Mechanical Engineering (Engineering)

Date of Defense


First Committee Member

Michael Swain

Second Committee Member

Ryan Karkkainen

Third Committee Member

Landon Grace

Fourth Committee Member

Matthew Swain


The exhaust process in an internal combustion engine directly affects the ability to produce power. Currently, experimental testing is used to test and perfect the exhaust process using a steady state low pressure flow bench. The flow through the exhaust port is initially critical which is not captured by the conditions of testing set on a flow bench. The goal of this study was to understand critical “blowdown” flow in exhaust ports to ultimately extract more performance out of an engine. A sonic flow bench was used in order to study the critical flow phase of an exhaust stroke. Blowdown testing using a sonic flow bench alongside CFD testing and low pressure flow bench testing was performed on three cylinder heads and a sonic nozzle. The three methods combined allowed to come to conclusions on how the blowdown phase works and how it can be optimized. The overall results revealed that the blowdown event can be split into two phases in which the nature of the flow is characterized differently. At high pressures, immediately following the opening of the valve, a development phase occurs where the mass flow rate starts from zero and eventually reaches a theoretical mass flow rate. This zone is very responsive to changes in geometry in the flow region affecting the overall mass flow rate during blowdown by more than 10%. This phase is characterized by the flow accelerating through the valve opening thus meaning it is highly transient. The second phase of the flow is called the fully developed region where the flow reaches a quasi-steady state matching theoretical values for mass flow rate. The high pressure development phase is most important for port design. By using a low pressure flow bench this is not captured as this is the result of a transient effect. Blowdown in an engine occurs extremely fast in time. This means that the blowdown flow never reaches a quasi-steady state and is always in the transient development phase. Valve and seat design has a large influence on performance. The results from testing valves and seat at different angles and with different design ideas allowed to conclude that a shape that resembles a venturi works best at low pressures however the opposite is needed for good blowdown. Abrupt transitions in shape promote the transient phase to last longer and go above the theoretical maximum. Using the right combination of blowdown performance and low pressure performance is therefore the goal to increase exhaust performance as a whole. This study shows the tools and designs needed to increase blowdown performance and in turn increase the overall performance of an exhaust stroke.


internal combustion; engine; exhaust port; critical flow; choked flow