Quantification of Energy Pathways and Gas Exchange of a Small Port Injection SI Two-Stroke Natural Gas Engine Operating on Different Exhaust Configurations

This paper examines the energy pathways of a 29cc air-cooled two-stroke engine operating on natural gas with different exhaust geometries. The engine was operated at wide-open-throttle at a constant speed of 5400 RPM with ignition adjusted to yield maximum brake torque while the fueling was adjusted to examine both rich and lean combustion. The exhaust configurations examined included an off-the-shelf (OTS) model and two other custom models designed on Helmholtz resonance theory. The custom designs included both single and multi-cone features. Out of the three exhaust systems tested, the model with maximum trapping efficiency showed a higher overall efficiency due to lower fuel short-circuiting and heat transfer. The heat transfer rate was shown to be 10% lower on the new designs relative to OTS model. The fuel slip rate was in the range of 20-30% with custom designs showing 15% higher fuel slip rates on average, whereas the exhaust thermal energy was in the range of 12-18% of total input fuel energy and found to be 20% lower on custom designs relative to OTS model. In addition to energy pathways, various exhausts impacted the scavenging efficiency and trapped mass during the gas exchange process. Even though the delivery ratio and trapping efficiency varied between the new exhaust designs, they showed a similar increase in volumetric efficiency relative to the OTS model - 60% on average.