CFD-based Analysis of Combustion Processes in a Free-Piston Diesel Engine
Abstract: The Free Piston Engine (FPE) which is capable of operating in a variable compression ratio mode, allows flexible auto-ignition control on a cycle-to-cycle basis without any hardware modifications. In this study combustion processes in an FPE were examined using the KIVA-3V CFD code with detailed chemical kinetics (305 reactions among 70 species) of a Diesel surrogate fuel incorporated. Since the piston motion and frequency, the rate of heat release and the initial in-cylinder conditions all affect each other, but were predicted using different modeling tools that were not directly coupled, an iterative procedure was applied among models describing: 1. Piston dynamics governed by Newton’s second law including a high-level control system (using MATLAB/SIMULINK) 2. Combustion processes (using KIVA-3V) 3. Intake and exhaust system dynamics (using the GT-POWER module of the GT-SUITETM) The control system changes the electrical force applied during the stroke, thus obtaining the desired compression ratio. It is considered the most vital part of this FPE, designed for electricity generation in a series-hybrid electric vehicle. The effects of varying the compression ratio, supplied compressor power, injection pressure and timing were investigated for both conventional diesel and Homogeneous Charge Compression Ignition (HCCI)-like combustion, the target being to identify optimal conditions for the combustion process in which the engine can be operated highly efficiently with low emissions. Identification of the best operating conditions facilitates costly experimental work. In order to ensure proper predictions of liquid sprays in the engine, the spray penetration length was studied in a constant volume vessel and the model constants adjusted. In addition, the Rate of Heat Release (RoHR) predictions for kinetically-driven combustion, i.e. HCCI, deviated unacceptably from experimental values. Therefore, the Diesel Oil Surrogate (DOS) model was revised by introducing new global fuel decomposition reactions, yielding higher n-heptane to toluene ratios. Their activation energies, collision frequency factors and oxygen concentration exponents were carefully selected when integrated into the CFD code. These adjustments resulted in the RoHR being substantially closer to the experimental data obtained from a High Speed Direct Injected (HSDI) crank-shaft controlled engine, and data obtained from the first firing cycles of the free piston engine. In order to acquire knowledge about the temperature vs. equivalence ratio (T-phi) conditions in which key products and intermediates are formed and destroyed T-phi maps were constructed. Engine pressure traces obtained from CFD simulations were discretized in time-pressure intervals and T-phi emission maps were constructed accordingly. The spectrum of engine T-phi conditions were plotted on the maps, visualizing correlations between T-phi conditions and emissions.
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