The essential characteristics of the internal combustion engine are ultimately determined by the processes that take place in the combustion chamber. It is these processes that generate the power output, the heat losses, and the formation of pollutants and therefore determine the trade-off between efficiency and emissions. The need thus arises to identify and qualify the significant physical processes that take place in the combustion chamber. Only after this accomplishment is it possible to establish a control of the major parameters that influence the combustion process so desired design goals can be achieved. In the past an acceptable level of understanding of the phenomenon involved has been reached through an analysis of experimental evidence and the pursuit of theoretical investigations. The processes inside the combustion chamber of an internal combustion engine involve a broad range of different subjects. The characterization includes chemistry, thermodynamics, fluid mechanics, and heat transfer, to mention a few of the most important fields as outlined in Fig. 1. Furthermore, the combustion process takes place in such a highly complex environment that an accurate and detailed description on a fundamental level cannot be done with present-day knowledge. Through numerous experiments a large amount of information has been acquired allowing the key processes to be identified and investigated in more detail. Guided by these experiments and the theoretical analysis, semiempirical methods have been used to develop the internal combustion engine, and such two-sided investigations are necessary for the successful development of the internal combustion engine. Though today’s engine is highly sophisticated, continued research efforts produce an increased understanding of the different processes leading to an improvement of the overall engine performance.

Flame Propagation and Heat-Transfer Effects in Spark Ignition Engines

In the present study, a flow field developed in an opposed jet burner and its turbulence characteristics were measured in detail using a hot-wire anemometer. The flame structure was clarified by schlieren photographs and instantaneous temperature measurements with a thermocouple. Extremely strong turbulence of small scale is generated by impingement of two mixture flows. For the case of a low mixture supply velocity, a flame within the transition regime from a wrinkled laminar flame to a distributed reaction zone is produced in the opposed jet burner. With increasing the mixture supply velocity, the structure of the flame zone changes to the distributed reaction zone. These two types of structure correspond to those of region 2 and region 3 of the 3-region model of the turbulent premixed flame structure.

Characteristics of Mixture Turbulence and Structure of Opposed Jet Burner Flames.

Recent Advances in Experimental Studies of Turbulent Premixed Flames

Studies on flame characteristics of swirled-turbulent open premixed nozzle flames

The present investigation highlights a new approach to the correlation of turbulent burning velocity data, which basically stems from the fact that detailed measurements of turbulence, using wire meshes and perforated discs as generators of isotropic turbulence, show a unique trend when the ratio of Kolmogorov microscale η to lateral macroscale L is plotted against the r.m.s. turbulent velocity u ′. For weak turbulence, the data obtained for different generators telescoped into a single curve, whereas for strong turbulence distinct demarcations could be detected for different grids. This cardinal finding was further pursued apropos of turbulent burning velocity data. Systematic measurements were carried out in cold flows using a hot-wire anemometer and a real time wave analyzer from which the requisite intensity and scales of turbulence were deduced. Subsequently burning velocity data for methane-air mixtures burning in an open burner were obtained by analyzing densitometrically the direct photographs of flames. Analysis reveals that two correlations are adequate for interpreting the data obtained. When turbulence intensity is low, the ratio of turbulent to laminar burning velocity S T /S L depends only on η/L in much the same fashion as u ′/S L while for large intensities, u ′/S L turns out to be an additional parameter besides η/L. When this analysis is extended to the data of other investigators, the forms of the correlating equations remain the same as in the present work but require different empirical constants. To sum up, it may be said that for weak turbulence η/L alone is adequate as a correlating parameter, while for strong turbulence both u ′/S L and η/L must be considered.

V. Sriramulu

Papers

A new approach to the correlation of turbulent burning velocity data