Gas exchange processes in two-stroke internal combustion engines, i.e. scavenging, remove exhaust gases from the combustion chamber and prepare the fuel-oxidizer mixture that undergoes combustion. A non-negligible fraction of the mixture 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 vital 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. Several simple-scavenging models can be found in the literature that — based on a variety of idealized interaction modes between incoming and cylinder gases — calculate the state of the trapped mixture. In this study, boundary conditions extracted from a validated 1-D predictive model of a single-cylinder two-stroke engine are used to gauge the performance of four simple scavenging models. It is discovered that the assumption of thermal homogeneity of the incoming and exiting gases is a major source of inaccuracy. A new non-isothermal multi-stage single-zone scavenging model is thus, proposed to address some of the shortcomings of the four models. The proposed model assumes that gas-exchange in cross-scavenged two-stroke engines takes place in three stages; an isentropic blowdown stage, followed by perfect-displacement and perfect-mixing stages. Significant improvements in the trapped mixture state estimates were observed as a result.

In internal combustion engines, the chemical composition of the trapped fuel-air-residual gas mixture controls the nature of combustion, which, in turn, determines the characteristics of the ensuing emissions and work production processes. Therefore, knowledge of the trapped mixture’s composition is critical for reliably predicting and controlling engine performance, emissions, and efficiency. A good index of the overall trapped mixture composition is the trapped equivalence ratio. Unfortunately, in two-stroke engines, it is unfeasible to accurately determine the trapped equivalence ratio using traditional intake flow measurements and exhaust emissions data. This limitation arises from the simultaneous occurrence of intake and exhaust processes in two-stroke engines, which causes: (1) exhaust emissions to be diluted by excess fresh air that was supplied for achieving effective gas exchange, that is trapping inefficiencies and (2) a significant fraction of combustion products to stay back in the cylinder as residual gas, that is scavenging inefficiencies. The current paper presents an experimental study carried out on a cross-scavenged, lean-burn, natural-gas, two-stroke engine to characterize its scavenging performance, thus paving the way for trapped equivalence ratio computation. CO2 is used as a tracer for combustion products, and its concentration is tracked in the combustion chamber and exhaust manifold on a crank-angle-resolved basis using high-speed nondispersive infrared sensors. The changes in cylinder CO2 concentration before and after gas exchange are used to determine the trapped residual fraction and various features of the exhaust CO2“wave” are used to explain the temporal progression of the gas exchange process. The presented results show the effects of changes in engine operation (speed, load, and spark-timing) on the engine’s scavenging efficiency. Speed and load changes are found to have the most pronounced effects, which result from changes in port open duration and phasing of reflected waves in the exhaust.

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It is vital to have accurate predictions of the gas exchange behavior of an engine in order to reliably study engine performance and emissions using engine simulation models. There are a multitude of factors, both upstream and downstream of the engine cylinder, which influence its gas exchange characteristics. Quite often these influences are interconnected in a non-linear manner that results in complicated feedback loops, which can introduce significant errors in the computed thermodynamic state of the post-breathing cylinder mixture. The effects of such bi-directional movement of pressure pulses are particularly pronounced in two-stroke engines. This study investigates the importance of exhaust system design on the scavenging characteristics of a piston-scavenged, cross-flow, two-stroke engine. A validated one-dimensional predictive model is used to study the effects of changing the exhaust port timing, exhaust system length, and exhaust port efficiency on the breathing performance of the engine, along with the consequent effects on the thermal efficiency and NOx emissions. Exhaust pressure waves and mass flows across ports are used to understand and explain the observed changes. The results show that while making design changes, thermodynamic efficiency considerations can act as a barrier to improving the scavenging efficiency of the engine; in addition, a trade-off between the two has to be considered in the design process to meet engine performance targets. The effects of such a trade-off on the NOx production are analyzed and two exhaust system modifications are discussed.