This paper presents a comparison of the combustion phenomena of several Diesel fuels in CFR testing engine, by means of hte rate of pressure change-time diagrams. As the combustion performance of a fuel is largely dependent on various operating conditions, it is evident that the general comparison becomes more complicated as the number of variables increases. Thus, during the tests reported herein, certain conditions were kept constant. The fuels were selecting and current use in other test programs. The data reported in this paper are limited to these specified fuels and operating conditions, and the generality of any conclusions that may be drawn from consideration of these data would be also limited. the purpose of this report, therefore, will be illustrate the fundamental differences among fuels of different molecular structures, and the trend of variation due to that change of molecular structures.
The rate of change of pressure in the engine was recorded by a commercial oscillograph in the manner previously used by Mr. R. W. Hurn at the University of Wisconsin (4). In Fig. 1 is shown the rate of pressure change-time curve together with the corresponding pressure-time curve. Both curves can be divided into three stages. The first stage, usually termed "delay period", is the interval of time which elapses between the commencement of injection and the sudden rise in pressure above the normal compression pressure. The second stage starts at the moment when the pressure rises sharply due to combustion of the fuel already present in the cylinder. This combustion is more or less uncontrolled and depends chiefly upon the quantity and condition of the fuel. It can be better detected in the rate of pressure change-time diagram. The third stage is reached when the rate of pressure rise starts to decrease from the peak point, and during which the remainder of the fuel burns under direct mechanical control at the entrance of the nozzle. The rate of pressure rise at the second stage practically depends on the length of the first stage, or, the delay period. It is obvious that the effect of the variation in the weight of fuel injected during varying delay period is to mask the effect of its ignition quality on the uncontrolled combustion phase (3).
Copyright 1948
Engine Research Center
University of Wisconsin-Madison