A STUDY OF IRREGULAR COMBUSTION, MISFIRING, AND CYCLE-BY-CYCLE VARIATION IN SMALL TWO-STROKE SPARK-IGNITED ENGINES

Abstract

Two- stroke engines have a major disadvantage of excessiv, engine speed fluctuations, surging and poor fuel economy at part throttle settings. Some earlier studies (145) have identified the causes of these problems. They occur as a consequence of cyclic combustion variations and irregular combustion leading to misfiring in a certain range of part throttle settings. Considerable studies (3) have been carried out on four stroke 'engines .concerning the -cyclic variations and misfiring. However, the information pertaining to two-stroke engines is still lacking. In a two-stroke engine, the mixture formation is guided by the scavenging process which consists of the simultaneous charging of the fresh charge and the expulsion of the burned products towards the end of the expansion and the beginning of the compression. As a consequence, the residuals present in the charge is about 10 times higher than those in a four- stroke engine. Moreover, the scavenging process results in the fluid motion which is significantly different from that in a four stroke engine. The objective of the present study is to understand the reasons for the cyclic variations leading to misfiring in two-stroke engines. An experimental study was undertaken to identify the nature of cyclic variations and the regime of irregular combustion. Also ii an analytical simulation for the closed period of the engine cycle was done, mainly toweplip4velfte the experimental pressure-time observations. The experimental study and the analytical model,haVe served to build a hypothesis to explain the behaviour of poor cycles and to understand the causes for the cyclic variation and the irregular combustion at part loads. The hypothesis that the poor scavenging flow pattern near the spark plug is responsible for the cyclic variations and irregular combustion was tested by a preliminary experimental cold flow study done to understand this flow pattern. The engine studies were carried out on a 150cc two-stroke S.I. engine. The cyclic variations and the misfiring, were characterized by a statistical analysis of 1000 individual cycles for peak pressure and 90 cycles were used for indicated mean effective pressure (imep). The regime at which, the irregular combustion starts was identified as the delivery ratio of 0.3 at all the speeds. However, the cyclic variation and irregular combustion are largely dependent on throttle settings. The factors which increase the combustion delay appear to have the major effect on cyclic variations. In order to investigate the phenomena of cyclic variations and misfiring in more detail, the calculation of mass burnt fraction L41;1:11 m.4o-rcis of with respect to crank angle 90 individual cycles was made. Relationships of combustion delay, combustion duration and total heat release for each cycle with corresponding FmaX and imep were examined. From such an analysis it was established that gross mixture quality of each cycle is not an important factor for initial flame development and consequently for Pmax and imep. It appears that the initial flame development depends mainly on the local condition around the spark plug. The experimental findings were further substantiated by a quasi-dimensional thermodynamic model by assuming chemical equilibrium for the burned products and a cylindrical flame propagation model for the mass burned rate calculations. The model has been used to predict average P-t histories and the mass burned rate (MBR) at average operating conditions. From a parametric study using the model, it has been concluded that the cyclic variations are mainly dependent upon the variations in combustion delay and that the local factors near the spark plug are mainly responsible for these variations and misfiring. This has been further explained by an experimental cold flow study done to obtain the velocity mapping, which suggests that the flow pattern near the spark plug during scavenging is poor and would lead to uneven distribution inside the cylinder. These variations and the scavenging in this region lead to cyclic 6:utorh& variations and irregular combustion.