Search Results

Lean burn and EGR concepts are being used increasingly in spark-ignition engines. Compared with a stoichiometric petrol engine, however, these dilution concepts are said to give a certain reduction in power output, in particular with higher EGR ratios or under lean mixtures near the minimum flammability limit. Some of the reduction can be recovered by correctly optimizing the MBT, and increasing the intake pressure which in addition reduces the pumping losses. In this work, experimental and theoretical studies of engine performance and emissions have been carried out with special emphasis on the combustion process. These have been analyzed from both combustion images in the early stages of the flame kernel development, and from the pressure-time history. Simultaneous measurements of engine operating conditions, pressure traces and sequences of combustion images have been made in a single-cylinder four-stroke engine.

A physical model for flame kernel development in spark ignition engines has been developed. It is based on a coupled experimental and theoretical analysis of early stages of the flame kernel development. The experimental work involves high-speed imaging of chemi-luminescent combustion light during the very early stages of combustion just after ignition. The resulting sequences of flame kernel images were analyzed to yield the time dependencies of quantities such as: the total kernel growth, the thermal expansion part of growth, the local translational velocity of the centroid, the stretching of the flame kernel surface and its roughness. The theoretical part of the model is one-dimensional and derived from the conservation equations and well-known thermodynamic relations. It considers input of electrical energy, combustion energy release, heat transfer to the spark plug and to the unburned mixture.

This study concerns experimental and theoretical analysis of early stages of the flame kernel development and subsequent stages of combustion and peak pressure in a spark-ignition engine. The simultaneous measurement of engine operating conditions, pressure traces and sequences of combustion images have been made in a single cylinder four stroke engine, in particular, at part load and under lean burn conditions. The early stages of the combustion have been analyzed using a new image analysis methods. These techniques can measure the total kernel growth, the thermal expansion part, the local translational velocity of the centroid, stretching of the flame kernel surface and its roughness as well as the flame contact area with the electrodes. In addition, the fraction of the flame surface area supporting propagation has been determined from the directional variation of flame propagation between successive image frames.

This article reports on the effects of various types of high field electrical phenomena imposed on low load lean combustion in a spark ignition petrol engine. The first part of the work deals with purely electrical field effects with no induced ionization. An electric field was applied via an insulated static high voltage DC electrode (up to 15 kv) which does induce excess charge effects, near the spark source. This was found to have very modest effects on the imep and its cyclic variability, which are unlikely to be significant enough to warrant commercial development. Further work reported here concerns the testing of an electrohydrodynamic device formed from a modified spark plug. This was operated in a test chamber under typical pre-combustion pressures where it was possible to obtain corona flows up to 10 m/s.

This article sets out to indicate the type of description that can be obtained for early combustion in a spark ignition petrol engine using a combination of multi-variate data obtainable from modern multi-processor controlled engine test systems and recent developments in image analysis techniques. The image processing is applied to sequences of pictures of the chemi-luminescent combustion light captured electronically at 0.1ms intervals during each cycle. This approach is well suited to studies of the genesis of cyclic variability with the conventional engine variables being used to identify sub-classes of combustion. By combining straightforward object size analysis with the new technique of Dilation Correlation the paper shows how it is possible to separate out various elements of the early growth of the flame kernel. This separation allows the comparison of the purely thermal expansion aspect of flame kernel growth with the total flame growth.

An investigation into the effect of arc parameters on cycle-by-cycle variation in the cylinder pressure development (CBCV) has revealed that under certain conditions there is a strong dependency. Both arc duration and gap can have a significant influence on combustion quality and stability in lean mixtures and the effects become more pronounced as ignition timing is advanced and load reduced. Ionisation probes have shown a strong arc-duration dependent component in their output, suggesting the arc has a fundamental effect on the fully developed flame.

Practical aspects of the measurement of cycle-by-cycle variability (CBCV) are discussed. That there is a strong ‘memory’ element in the process under certain conditions is revealed by autocorrelation techniques. The system is shown to be well represented by a statistical model involving two distinct processes, with the parameter values of this model significantly dependent on engine operating conditions. Arc duration has been found to be particularly significant in this respect. Misfiring has the effect of destroying the chain of cause and effect between cycles, but the modelling process described here allows the combustion under extreme conditions to be investigated independently of this complication. The degree of memory in the system is shown to be inversely related to the degree of cyclic variability.