1991-02-01

Development of an Infrared Method for Ignition Delay Measurements 910847

The paper describes experimental validation of a laboratory flow apparatus used to measure the ignition delay times of diesel fuels at atmospheric pressure in near quiescent air. To validate the proposed method the experimental data were compared with the results from the studies performend on non-engine combustion chambers with continuous air flow at atmospheric pressure and various temperatures.
The proposed flow apparatus, described in an earlier paper, has the means to provide air temperatures in the range between 650 and 730°C. An infrared radiation detector monitors the evolution of the temperature inside the combustion chamber. Ignition delay is measured as the time interval between the beginning of the needle lift and the beginning of increase in infrared radiation detected by the sensor.
Six test fuels were used. Five were mixtures of n-hexadecane (cetane) and heptamethylnonane with cetane numbers of 100, 95, 90, 85, and 80, and one was #2 diesel fuel with a cetane number of 50.
The comparison of the relation between the apparent activation energies and cetane number determined using the infrared method in a flow apparatus at atmospheric pressure and the tests performed on engines at elevated pressure give similar trends but the magnitudes are different, as expected. With the help of the calibration curves obtained in this study, it is possible to determine the actual or apparent cetane numbers of fuels from their infrared ignition delays (IDIR).
CRUDE OIL is expected to play the major role in the world's energy scene during the next few decades [1]. Diesel engines are facing the prospect of running on fuels based on heavy and/or low cetane crude and renewable fuels. Among the fuel combustion characteristics which should be considered in assessing the suitability of a fuel for diesel engines, the most important single property is ignition delay.
Ignition delay has been historically an area of intense analytical and experimental investigation. During the ignition delay period, a complex set of physical and chemical phenomena must occur before self-sustained chemical reaction occurs. These include liquid fuel injection, atomization, penetration, vaporization, mixing with air, localized, heterogeneous heat release by chemical pre-flame reactions, etc. At the present time, the cetane number is the indicator universally accepted for rating diesel fuels for their compression ignition characteristics in an engine. Fuel evaluation is performed according to ASTM D-613 and uses a reference scale based on the two primary reference fuels, n-hexadecane and heptamethylnonane. Since high cetane fuels ignite at lower temperatures than low cetane fuels, the method allows the ranking of fuels with respect to their ability to self-ignite. Some recent research work indicated that there are some problems associated with the use of this procedure to rate the autoignition quality of some petroleum derived fuels and particularly alternative fuels [2, 3, 4, 5, 6 and 7].
It was shown that the cetane number is a fair measure of ignition quality only for fuels which have the same type of ignition processes and the same temperature dependence of ignition delay as the reference fuel mixtures. For instance, plant oils do not fit this category. At low temperatures, such as given during idle and low load conditions, the ignition quality of a plant oil is generally very much worse than that of diesel fuel, while at high temperatures, such as given during high load conditions, ignition quality may be equal to or better than that of diesel fuel.
Generally, the suitability of an alternative fuel is determined in presently available diesel engine types. The performance of an engine, however, reflects the fuel characteristics as well as the engine design characteristics. Thus the data generated in such tests include the interactions between the engine and the fuel and not necessarily describe the fuel itself.
It appeared desirable to develop a test facility for the basic study of the combustion properties of diesel engine fuels without being subject to the influences imposed by the configurational and operational combinations of an actual engine or an engine-like apparatus. A constant flow apparatus has been built which measures ignition delay of fuels injected into a high temperature air stream. Of particular appeal are low cost, simplicity of design, ease of operation and control, and the possibility to vary one parameter at a time while keeping all others constant. It has been described in detail in a previous paper [8], and an initial evaluation of its performance was presented.
The main objective of this companion paper is the validation of the infrared flow apparatus for effective screening of fuels for diesel engines. To validate the proposed method, the experimental data were compared with the results of other studies. Recently there have been several studies using a large variety of methods for fuel evaluation, such as [9], [10], [11], [12], [13], or [14]. From the multitude of existing studies, only those having similar conditions to this method were selected. Among these are the experimental studies using non-engine combustion chambers with continuous air flow at atmospheric pressure and various temperatures. Only some of the results from engine studies have been included in the comparative analysis because of the major differences in operating conditions.
The results obtained were compared with those by Hiroyasu, et. al. [15], Iinuma [16], Mullins [17], Koizumi and Kitaoka [18], Henein and Bolt [19], and Hardenberg and Hase [20], and discussed chiefly in conjunction with the relation between activation energy and cetane number.

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