The combustion process in the diesel engine

Abstract

The Diesel engine is characterized by injection of fuel in the combustion chamber and by selfignition. Both characteristics give rise to typical problems. The former characteristic, i.e., injection of the fuel with subsequent formation of a combustible mixture, is the fundamental one. This mixture formation immediately preceding combustion and continuing during combustion introduces the element of heterogeneity of the mixture to an extent unknown in gasoline engines. As a matter of fact, one is faced in the Diesel engine with mixture elements varying from pure liquid globulae and deposits to fuel vapors and pure air, the condition of this heterogeneous complex changing with tremendous rapidity, due to agitation, evaporation, and combustion. As a result, conditions are always elusive; for instance, the strength of the mixture and the temperature of the flame vary continually throughout the charge. This heterogeneity has its advantage, for it allows one to vary the load down to zero solely by regulating the fuel input, which is the cause of the excellent economy of the Diesel engine under varying load. That, however, is the only advantage. Most drawbacks of the Diesel engine do come from this very heterogeneity of the charge, the most obvious one being the impossibility of burning e~cient[y a quantity of fuel corresponding to the full amount of oxygen available in the cylinder, so that the power output stays behind that of an efficient gasoline engine. A second drawback, but in practice often the more important one, is the great tendency of the Diesel engine towards incomplete combustion, causing internal troubles as well as dirty and foulsmelling exhaust gases. Figure 1 represents schematically the charge in a Diesel engine at a phase of its combustion process. Figure 2 represents schematically and comparably the charge in a gasoline engine; the difference in simplicity of conditions as compared with Fig. 1 is obvious. This heterogeneity may be put forward as an excuse for the still existing lack of precise knowledge concerning the Diesel process, now that the Diesel engine has reached its fortieth anniversary. Whereas today the knowledge of the combustion process of the gasoline engine has developed to a point where quantitative results of general validity can be foretold, that of the Diesel engine allows only an accept'tble qualitative analysis. For the first two decades the process of the Diesel engine was only a confused picture, in which chemical and physical considerations, ignition and combustion phenomena, space and time were all mixed up. RiedleP was one of the first to try to disentangle the various elements of the problem. He suggested the following picture of the stages of the combustion process: (1) introduction of the fuel, (2) atomization, (3) evaporation, (4) mixture formation, (5) decomposition, and (6) combustion. I t is interesting to note that these stages, which Riedler visualized as being subsequent in time, actually do exist but overlap each other considerably.