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The role of transfer port geometry in improving scavenging near the spark plug with consequent improvement in cyclic variations and irregular combustion have been studied through engine tests by recording the pressure-time data of many cycles with the original and modified ports on a 150cc crankcase compressed shnurle loop scavenged two stroke engine under various operating conditions. The results show that there is an optimum upsweep angle which gives reduction in cyclic variations and lowering of delivery ratio at which irregular combustion starts. The side sweep angle does not seem to affect much. The improvement seems to be closely related to the reduction in the duration of the initial phase of combustion and its variations.

Causes leading to cyclic variations and resulting in misfiring in a two-stroke cycle spark-ignited engine are explained through analytical validation of experimentally observed behaviour. Experimental observation of pressure-time (P-t) histories of individual cycles suggests that cyclic variations are dependent on combustion delay and initial flame development and that local factors near the spark plug are mainly responsible for this. This has been used to build a hypothesis to explain the behaviour of poor cycles and to predict theoretically, using a quasi-dimensional thermodynamic model, the P-t histories of the individual cycles for comparison with the experimental results. It is observed that the gross mixture quality of each cycle is not an important factor for initial flame development, which depends mainly on the local conditions around the spark plug. For the engine under study, the reasons for the behaviour of poor cycles was attributed to poor scavenging near the spark plug.

Two-stroke cycle engines have excessive speed fluctuations and poor part-throttle fuel economy as a consequence of cyclic combustion variations and irregular combustion, leading to misfiring. This study is to identify the nature of cyclic variations and the regime of irregular combustion in order to understand the possible causes for this phenomena. Studies were carried out on a 150 cc two-stroke cycle S.I. engine. Statistical evaluations of peak pressure(Pmax) and indicated mean effective pressure (imep) were used to characterize cyclic variations and irregular combustion. A parametric study on the effect of air fuel ratios, spark timing, engine speed, spark plug locations, throttle setting, and cylinder head shape on cyclic variations were carried out. The study has identified the regimes of stable combustion, irregular combustion and misfiring.