An experimental study was carried out to characterize the scavenging behavior of a cross-scavenged, piston-aspirated, two-stroke, natural gas engine to aid in the development of computationally inexpensive simple scavenging models for onboard engine control by (1) studying the effects of changing operational parameters on the engine’s scavenging performance, and (2) identifying the underlying phenomena driving the observed effects. Tracer based methods were used to quantify the scavenging and trapping performance of the engine - CO2 was used as a tracer for combustion products and pre-mixed...
An experimental study was carried out to characterize the scavenging behavior of a cross-scavenged, piston-aspirated, two-stroke, natural gas engine to aid in the development of computationally inexpensive simple scavenging models for onboard engine control by (1) studying the effects of changing operational parameters on the engine’s scavenging performance, and (2) identifying the underlying phenomena driving the observed effects. Tracer based methods were used to quantify the scavenging and trapping performance of the engine - CO2 was used as a tracer for combustion products and pre-mixed fuel was used as a fresh charge tracer. CO2 concentration was measured on a crank angle resolved basis both in the engine cylinder and exhaust using portable NDIR sensors, while unburned fuel concentration was measured in the exhaust using the FID module of a standard five gas analyzer.
It was found that scavenging took place in three stages, an initial perfect displacement type stage, followed by a short-circuiting, and a perfect mixing type stage. Engine speed and load changes were found to have the strongest effects on the trapping and scavenging performance of the engine; spark timing effects were less significant. Changes in measured scavenging and trapping efficiencies at different operating points resulted from a combination of influences, namely (1) reduced time for gas exchange at high speeds, (2) higher expansion and scavenging pressures at high loads and retarded spark timings, and (3) phasing of the reflected ‘scavenging’ and ‘plugging’ pulses in the exhaust pipe relative to BDC and EPC, respectively. Increasing engine load made the engine scavenge significantly better and increasing engine speed resulted in a larger fraction of the delivered air being trapped. The combined effect of these scavenging changes and changes in the engine’s fuel conversion efficiency resulted in the engine running leaner at high speeds (more air delivered and higher trapping efficiency) and at low loads (higher trapped residuals).
The results were then used to gauge the performance of the simple scavenging model (the hybrid model) developed in phase II of the project. While encouraging results were obtained at high speed, the trapped air mass was overestimated at medium speed; suggesting the need for adding a low scavenging efficiency sub-model. Recommendations have been made about adding a short-circuiting zone to address this limitation of the model.