Zhang, Qi Xian (1995) Modelling the scavenging process in a two-stroke I.C. engine. Coursework Master thesis, Victoria University of Technology.

Abstract

The primary concern associated with a two-stroke engine performance is its gas exchange process (scavenging process). The success of the scavenging process greatly affects the thermodynamic properties of cylinder content at the trapping conditions and hence the combustion and power output. The unsteady gas flow in engine pipes has a marked influence on the scavenging process. In the current study, the two-stroke engine simulation model has been developed to predict the steady state performance characteristics of a crankcase compressed, piston port-timed, two-stroke engine equipped with expansion chamber. The characteristics include parameters such as engine torque and power, BSFC, scavenging efficiency and charging efficiency. The model also has the ability to predict the unsteady gas dynamic behaviour in various engine pipes. The instantaneous pressure fluctuation and mass flow rates at inlet port, transfer port and exhaust port were calculated and analysed. The model has several advantages compared with other one-dimensional isentropic model. It includes an improved procedure to account for the variation in geometry of pipe and to determine the thermodynamic states in cylinder/crankcase. The model also considered the temperature discontinuity at port/pipe interface. All these efforts increase the accuracy and numerical stability of the prediction. A single cylinder two-stroke engine dynamometer rig and dedicated fast data acquisition hardware and software have been developed in the project. Engine torque, speed and fuel consumption can be measured on the dynamometer rig. The dynamic pressure signals in engine cylinder, transfer port and exhaust port, together with crankshaft position signal, can be acquired at a speed of 50,000 sample/second per channel, sufficient for accurate acquisition of experimental data. Substantial simulations and experiments were performed and the computer model was validated. A variable exhaust system, as described in this study, was used to improve a twostroke engine's performance under off-design engine speeds. This was realised by making the mid-parallel section of a conventional expansion chamber extendable, while the other dimensions of that chamber remained unchanged. In the study it was found that with the adjustable expansion chamber, the pressure wave timing at the exhaust port was under control within the test speed range (2200 - 4400 RPM) so that such wave timing matched the engine port timing, leading to a relative optimum in performance. The numerical approach proposed could satisfactorily handle the complicated pipe flow calculation and predict both engine performance and dynamic wave variations in the test engine. Thus the program can be used for improving the design and development of naturally aspired, port-controlled crankcase compressed two-stroke engines.

Search Google Scholar

Repository staff login