30-08-2011, 09:28 AM
abstract
An opposed-piston hydraulic free piston engine operating with homogenous charge compression ignition (HCCI) combustion, has been proposed by State Key Laboratory of Engines as a means of significantly improving the IC engine’s cycle thermal efficiency and lowering exhaust emissions. Single and multi-zone Chemkin model with detailed chemical kinetics, and unique piston dynamics extracted from one dimensional gas dynamic model, have been used to analyze the combustion characteristics and engine performance. Intake heating, variable compression ratio and internal EGR are utilized to control the combustion phasing and duration in the cycle simulations, revealing the critical factors and possible limits of performance improvement relative to conventional crank engines. Furthermore, real engine effects such as heat transfer with air swirl, residual mass fraction, thermal stratification, and heat loss fraction between zones are considered in the sequential CFD/multi-zone method to approach the realistic engine performance at an acceptable knock level.
1. Introduction
Free piston engine concept can be traced to original gasifiers for single stage turbines as well as gas compression engines in the mid-20th century [1]. From then on, more novel applications such as hydraulic pumps and linear generators have been studied. Free piston engines are linear, ‘crankless’ engines, in which output power is extracted by a linear load device directly coupled to the moving piston. The absence of the crank mechanism allows the compression ratio to be varied. Compared to conventional engines, the free piston engine has potential advantages of simplicity of the units, giving a compact engine with low frictional losses, and the operational flexibility through the variable compression ratio, potentially offering extensive multi-fuel and operation optimization possibilities [2,3]. Because of its advantages, in recent years, the free piston engine concept attracts more attention among research engineers. The homogenous charge combustion ignition (HCCI), as an alternative to spark-ignition and compression–ignition combustion in internal combustion engines, has the advantages of high efficiency and low emissions [4,5]. Mikalsen and Roskilly [6] presented the free piston engine is well suited for HCCI operation, since the piston motion is not controlled by a crankshaft, making it have low ignition timing control requirements. Recent studies of HCCI combustion on a free piston engine using a rapid compression expansion machine have shown that very rapid combustion is possible with certain fuels, and that ideal Otto cycle performance can be closely approached, while low NOX emissions are possible (<10 PPM) [7]. A zero dimensional, thermodynamic model with detailed chemical kinetics, empirical scavenging, heat transfer, and friction component models has been used to analyze the steady state operating characteristics of this engine using hydrogen as the fuel [8]. The successful HCCI combustion process employed by the free piston engine is demonstrated by Sandia National Laboratories [7], operating at high compression ratio (_30:1) and low fuel/air equivalence ratio (_0.35). Free piston engine combustion was studied in the mid-20th century and differences when compared to conventional engines were reported by a number of authors. Golovitchev et al. [9] presented numerical results of combustion process simulations in a two-stroke, uniflow scavenging dual free piston engine designed for electricity generation. Wherein two fuels, diesel oil and dimethyl ether (DME), were studied in order to achieve HCCI-like combustion. Li et al. [10] investigated the performance of a twostroke free piston engine under HCCI combustion for electrical power generation, using Matlab/Simulink, Chemkin, as well as the finite element method (FEM). Mikalsen and Roskilly [11] investigated the in-cylinder gas motion, combustion process and nitrogen oxide formation in a free piston diesel engine and compared the results to those of a conventional engine, using a computational fluid dynamics engine model
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