Miriam Bergman

CFD Modeling of Diesel Oil and DME Performance in a Two-Stroke Free Piston Engine

This paper presents the CFD KIVA3V, rel. 2 model and numerical results of combustion process simulations in a two-stroke, uniflow scavenging dual free piston engine, FPE, designed for electricity generation. Two fuels, diesel oil and dimethyl ether (DME), were studied in order to achieve HCCI-like combustion. Limited size reaction mechanisms were constructed and used in the modeling. The diesel oil surrogate mechanism consisting of a mixture of n-heptane and toluene includes 70 species participating in 305 reactions, whereas the mechanism for DME consists of 43 species participating in 211 reactions. The combustion development has been simulated in a FPE prototype geometry. It is illustrated that by varying the direct injection timings, a comparably efficient, low-emission operation has been achieved for both fuels. Specific difficulties when using slowly vaporizing diesel oil and rapidly vaporizing DME are outlined.

CFD-Based Optimization of a Diesel-fueled Free Piston Engine Prototype for Conventional and HCCI Combustion

This paper presents results of a parametric CFD modeling study of a prototype Free Piston Engine (FPE), designed for application in a series hybrid electric vehicle. Since the piston motion is governed by Newton’s second law, accounting for the forces acting on the piston/translator, i.e. friction forces, electrical forces, and in-cylinder gas forces, having a high-level control system is vital. The control system changes the electrical force applied during the stroke, thus obtaining the desired compression ratio. Identical control algorithms were implemented in a MATLAB/SIMULINK model to those applied in the prototype engine. The ignition delay and heat release data used in the MATLAB/SIMULINK model are predicted by the KIVA-3V CFD code which incorporates detailed chemical kinetics (305 reactions among 70 species). Since the piston motion and frequency, the rate of heat release and the initial in-cylinder conditions all affect each other, while predicted using different modelling tools with no direct coupling between them, an iterative procedure was used 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) Effects of varying parameters such as compression ratios, power supplied to the compressor, fuel injection timings and injection pressures have been studied in both conventional diesel and HCCI modes, the target being to identify optimal conditions for the combustion process in which the engine can be operated highly efficiently with very low-emissions.

Modification of a Diesel Oil Surrogate Model for 3D CFD Applied to Conventional and HCCI Combustion

This paper describes an analysis of the Diesel Oil Surrogate (DOS) model used at Chalmers University (Sweden), including 70 species participating in 310 reactions, and subsequent improvements prompted by the model’s systematic tendency to under-predict the combustion intensity in simulations of kinetically-driven combustion modes, e.g. Homogeneous Charged Compression Ignition (HCCI). Key bases of the model are the properties of a model Diesel fuel with the molecular formula C14H28. In the vapor phase, a global reaction decomposes the starting fuel, C14H28, into its constituent components; n-heptane (C7H16) and toluene (C7H8). This global reaction was modified to yield a higher n-heptane:toluene ratio, due to the importance of preserving an n-heptane-like cetane number. Three different (composite) versions of this global decomposition reaction were investigated separately and compared: 1. 3C14H28 + O2 => 4C7H16 + 2C7H8 +2H2O 2. 2C14H28 => 3C7H16 + C7H8 3. 3C14H28 + 3.5O2 => 5C7H16 + 0.5C7H8 + 3.5CO2 Reaction 1 is the original formulation and reactions 2 and 3 are novel alternatives. The effects of modifying the activation energies (E), collision frequency factors (A) and oxygen concentration exponents (AEO) of these reactions was examined when integrated into the 3D KIVA-3V CFD code, while leaving the elementary reactions unchanged. The results were compared with data obtained from measurements of a High Speed Direct Injected (HSDI) engine, operating in both conventional and HCCI combustion modes. Finally, the global reaction version and the rate constants giving the best agreement for the examined regimes were identified.