| dc.description.abstract | Gas exchange processes in two-stroke internal combustion engines, commonly referred to as scavenging, are responsible for removing exhaust gases from the combustion chamber and preparing the fuel-oxidizer mixture that undergoes combustion and thus converts the chemical energy of the fuel into mechanical work. Scavenging is a complicated phenomenon because of the simultaneous introduction of fresh gases into the engine cylinder through the intake ports and the expulsion of combustion products from the previous cycles through the exhaust ports. A non-negligible fraction of the gaseous mixture that is trapped in the cylinder at the conclusion of scavenging is composed of residual gases from the previous cycle. This can cause significant changes to the combustion characteristics of the mixture by changing its composition and temperature, i.e. its thermodynamic state. Thus, it is essential to have accurate knowledge of the thermodynamic state of the post-scavenging mixture to be able to reliably predict and control engine performance, efficiency, and emissions. Of particular import is the trapped equivalence ratio, as it is a good predictor of engine-out NOx emissions and can be used as a control parameter for ensuring emissions compliance.
Unfortunately, it is not practical to directly measure the trapped residual fraction (which is needed to calculate the trapped equivalence ratio) outside of the laboratory setting. To overcome this handicap, simple scavenging models that estimate this fraction based on some economically measurable engine parameters can be used. This project studies various simple scavenging models using a one-dimensional GT-Power model of a large-bore, natural gas fueled, two-stroke engine; highlights their shortcomings, and proposes an improved multi-stage, two-zone simple scavenging model that can produce accurate estimates of the trapped mixture composition for such engines. The simulation results are directly validated by conducting scavenging experiments to calculate the trapped mixture composition. CO2 is used as a tracer for combustion products to study scavenging performance of the engine, and fuel - as a part of a pre-mixed fuel-air mixture - is used as a tracer for fresh charge to experimentally study the trapping performance of the engine. | en |
