A Theory of Cyclic Variations in Small Two-Stroke Cycle Spark Ignited Engines - An Analytical Validation of Experimentally Observed Behaviour

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.