Optimal exergy-based control of internal combustion engines

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© 2016 Elsevier Ltd Exergy or availability is defined as the maximum useful work during a process. This metric has been used to analyze and understand loss mechanisms of Internal Combustion Engines (ICEs). In this paper, an optimal control method based on exergy is introduced for transient and steady state operation of ICEs. First, an exergy model is developed for a single cylinder Ricardo engine. The ICE exergy model is based on the Second Law of Thermodynamics (SLT) and characterizes irreversibilities. Such quantifications are not identified in the First Law of Thermodynamics (FLT) analysis. For steady-state operation of the ICE, a set of 175 different operating conditions is used to construct the SLT efficiency maps. Two different SLT efficiency maps are generated depending on the applications whether work, or Combined Power and Exhaust Exergy (CPEX) is the desired output. To include transient ICE operation, a model to predict exergy loss/destruction during engine transients is developed. The sources of exergy destruction/loss are identified for a Homogeneous Charge Compression Ignition (HCCI) engine. Based on the engine operating conditions (i.e., steady-state or transient) SLT efficiency contour maps or predicted exergy losses are determined at every given engine load. An optimization algorithm is proposed to find the optimum combustion phasing to maximize the SLT efficiency. Application of the optimization algorithm is illustrated for combustion phasing control. The results show that using the exergy-based optimal control strategy leads to an average of 6.7% fuel saving and 8.3% exergy saving compared to commonly used FLT based combustion control in which a fixed combustion phasing (e.g., 8°aTD) is used.

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Applied Energy