Assessment of controlling effectiveness of a combustion engine’s utility parameters in relation to ecological and utility criteria

Compression ignition engine is very often used as a power unit in any type of vehicles, as well as in stationary installations. This is mainly due to high efficiency of such engines and resulting low fuel consumption. There exists clear relation between the utility parameters of the engine and emission level of air pollutants. Obtainment of satisfactory relation between the utility parameters of the engine and the emissions level creates a difficult task, requiring great knowledge and experience. To support decision-making process, a method how to assess results of controlling of the utility parameters of compression ignition engine has been depicted in the presented study. It has been proposed how to perform multicriteria assessment in relation to the utility and ecological criteria. Within group of the ecological criteria, emission levels of individual harmful components of exhaust gases were taken as the partial criteria to the assessment. In turn, parameters such as value of mean indicating pressure, thermal efficiency, maximal temperature and pressure of the cycle were considered within group of the utility criteria. Within framework of the study it has been performed evaluation of a few sets of the utility parameters, obtained in course of numerical calculations of partial engine loads. The calculations have comprised resolving of minimization task of nitrogen oxides emissions with limitations imposed on the utility parameters and other harmful compounds of exhaust gases. The method of analytic hierarchy process was applied to accomplishment of the multicriteria assessment.


Introduction
In case of a piston-type combustion engine, substances causing pollution of the natural environment can be emitted from three sources: from crankcase, from fuel supply system, and from exhaust system [5]. The most crucial are emissions from the exhaust system, emitting products of incomplete combustion of liquid fuel. In the compression ignition engines (CI), in spite of supplied excess of air, combustion is incomplete, what can lead to generation in exhaust gases with substances containing carbon. In turn, presence of nitrogen oxide is an effect of secondary reactions in process of the combustion. Emission of nitrogen oxides mentioned here, being effect of combustion, is significantly connected with the utility parameters of the engine. An attempt was made in the study to determine minimal level of nitrogen oxide emission at assumed values of the utility parameters and emission levels of other components of exhaust gases. A h n

Subject of the study
Model of the working cycle presented in the previous chapter constitutes the tool enabling selection of adjusting parameters of the engine (fuel dose, injection advance angle, degree of exhaust gas recirculation) due to any objective function. Considerations performed in the present study comprise selected issue of assessment of the controlling efficiency through selection of adjusting parameters in relation to minimization task of nitrogen oxides emission from the CI engine. Model tests were performed for previously calibrated model of the working cycle of the engine, which main technical parameters are specified in table 1. As starting point to the considerations undertaken in this study were used results of the controlling obtained for some selected points of engine operation and presented within framework of the previous studies performed by the authors [6,7], in which the objective function has taken form: where: x NO F -emission of nitrogen oxides in the exhaust gases, F NO x C -coefficient defined with reference to emission of nitrogen oxides for factory settings of the engine, together with limitations relating to allowable emission levels of other harmful components of exhaust gases and changes in characteristic parameters of the working cycle. Increase in emissions of carbon monoxide, hydrocarbons and smokiness with 30%, 20% and 50% max respectively, with regard to values obtained for the factory settings, was allowed in discussed in this paper task of selection of the adjusting parameters. Moreover, it has been allowed reduction of thermal efficiency of the engine with 5% max, and increase of maximal pressure and maximal temperature of the cycle with 10%. Objective of the presented study is multicriteria assessment of results obtained from the task in form as described in (5), which enables evaluation of controlling effectiveness of the adjusting parameters of compression ignition engine in ecological and utility aspects. These results will be analyzed for a few operational points of the engine, corresponding to selected conditions of partial engine loads, as shown in table 2. To perform the multicriteria assessment of controlling efficiency it has been implemented method of the analytic hierarchy process [9]. This method enables decomposition of the assessment issue against both cumulated categories, as well as factors being part of individual categories. Within framework of individual cumulated categories the following specific criteria were taken to the assessment: Results of the applied controlling, referenced to parameters comprising specific criteria of the assessment and juxtaposed in form of table 3 have constituted the basis to the analysis discussed in the present study.   Method of decomposition of analysed in this study issue of the assessment was presented in figure 1. It can be seen that controlling effectiveness for each from the operational points is analyzed separately because of each from factors constituting specific criterion of the assessment. In turn, elements from the higher level include the cumulated categories. Significance of individual specific factors, constituting a given cumulated category, is evaluated at this level. Final assessment is possible after evaluation of mutual significance level of analyzed cumulated categories. Pairwise comparison is the basis of performed relative assessment at all stages of the analysis. Classic Saaty scale was used to quantification of the relative assessments [9].

Assessment of controlling results with respect to individual specific criteria
Matrix of preferences is determined at the first stage in order to make assessment of the controlling effectiveness of each from analysed specific criteria. Assuming that for the specific criterion k, the ) (k ij r is the relative assessment of the controlling obtained for the operating point i P , in comparison to that one obtained for the operating point j P , created matrixes of preferences are the square matrixes in form: In this study the values of ) (k ij r were obtained calculating quotients of the values presented in table 3, obtained for the operating points i P and j P respectively for the criterion k, and next rescaling linearly obtained result to range of values used on the Saaty scale. Next, it has been evaluated vectors of priorities representing assessment of the controlling effectiveness due to isolated specific criterion k by solving the equation: where:  -maximal eigenvalue of the matrix Vectors of priorities were determined with use of the iterative method [10], assuming that allowable evaluation error of the vector component's value is not bigger than 10 -5 . In figure 2   In successive stage of the proceeding it has been determined vectors of relative assessments within scope of analysed cumulated categories.

Assessment of significance of the specific criteria
Investigating the cumulated category, which comprises the ecological properties, significance level of the emission of individual harmful components of exhaust gases was determined on the basis of the following presumptions: -with reference to the objective function implemented during resolving of the selection task of the adjusting parameters, in the ecology context, the emission of nitrogen oxides was assumed as the most important criterion to assessment of the obtained results, -smokiness of the exhaust gases was recognized as more significant factor than emission of hydrocarbons and carbon monoxide, due to greater harmfulness of the sooth, resulting from strong tendencies to binding of hydrocarbons generated in course of indirect processes, especially having ring structure [2], -emission of hydrocarbons and emission of carbon monoxide are the issues of a similar significance for the analysed type of the engine, with a light indication on preference in area of restriction of hydrocarbons emission, because of lower tendency to oxidation.
Matrix of preference for the ecological criteria, obtained in result of pairwise comparisons has the following form: r r r r r Vector of priorities calculated in the next step, representing in the assessment of the controlling efficiency the significance of emissions of individual harmful compounds of exhaust gases has form: Next, considering cumulated category comprising the utility parameters, significance level of individual parameters analysed within framework this category has been determined on the basis of the following presumptions: -limitation of maximal temperature was considered as more important than limitation of maximal pressure in the cylinder; because higher temperature of the cycle has important effect on durability of exhaust valves, on durability of supercharging system with turbocharger, durability of recirculation system of the exhaust gases, and on durability of after-treatment system of exhaust gases comprising catalytic converter and DPF [1], -higher temperature of working medium in the cylinder promotes increased emission of nitrogen oxides.
The preference matrix obtained in result of pairwise comparisons of the utility parameters has form: Hence, calculated vector of priorities, representing significance of the cumulated categories has form:  Analysis of the obtained results from performed multicriteria assessment indicates that among analysed points of engine operation, the best controlling results were received for two conditions of lowest engine loads, and hence corresponding to the points P5 and P1 of the engine operation. Wherein, it is worth noting that in assessment of the effectiveness due to the utility criterion, the best results were obtained for the point P1. In general, from the viewpoint of multicriteria assessment for the operating points P7 and P8, it is also possible to highlight comparability of obtained controlling effectiveness. It is also easy to notice, that solely usage of single cumulated category to the assessment of the effectiveness can lead to separate arrangements of the analysed set of results.

Summary
Controlling effectiveness of the adjusting parameters in selected points of engine operation varies, while assessment of this effectiveness can be performed due to many criteria. Usage of the multicriteria assessment in evaluation task of the effectiveness enables to take collectively into account a number of the partial criteria, having varied significance. Such assessment of a set of solutions requires determination of values of suitable local priorities. As the effectiveness of cumulated categories varies, values of the global priority decide about final result of the assessment.
Attainment of acceptable overall efficiency of the combustion engine with simultaneous compliance with emission standards is a difficult and labour consuming task, requiring a lot of knowledge and experience. Method of the proceeding presented in this paper is aimed at supporting of decision-making process, needed during partial assessment of results of the controlling parameters of a compression