Elemental Study On Superadiabatic Engine Based On The Porous-Medium Combustion Technique

With the rapid development of global economy, the problems of energy crisis and environment pollution have become two focuses of attention. As the main source of petroleum consumption and emissions to atmosphere, automotive engines have received increased attentions, and how to realize high efficiency and clean combustion has become a urgent objective pursued by the entire engine community. Porous medium (PM) engine, as a newborn thing, is characterized with a number of advantages, such as low-pollution, higher-efficiency and extended limits of flammability, and, hence, is receiving more and more attentions.This thesis presents a theoretical analysis of cycle characteristic and numerical study on the ignition and combustion process of PM engines, and presents the investigation on combustion of liquid fuel in prous media based on the mechanism experimental. The principal aim of this work has been to have a primary recognition about the working characteristics of PM engine, and resolve the key problem of liquid fuel evaporation and combustion in porous medium, with the purpose of providing some theoretical foundations for its application.Firstly, thermodynamic models of porous medium engine are set up based on the knowledge gained from previous studies on porous medium burners, and characteristics of work and efficiency as well as the availability analysis of both ideal cycle and irreversible cycle in porous medium engine are discussed.(1) Based on the classical thermodynamics theory, systematic analysis of theworking cycle is conducted for two types of porous medium engine, i.e. one with closed PM chamber and other with open PM chamber. The general performances of the porous medium engines are theoretically analyzed, and the influences of compression ratio, limit temperatures and volume expansion ratio etc on net work and efficiency of the ideal heat regenerative cycle in the PM engines are discussed, and availabilities in the working process are derived. Comparison of the heat regenerative cycle of the PM engine with the Otto cycle and Diesel cycle shows that performance of the PM engine is prior to the conventional engine, the PM engine can improve net work output at a little expense of thermal efficiency.(2) Finite-time thermodynamic analysis is applied to evaluate the thermodynamic performance of the irreversible heat regenerative cycle in the PM-engine with closed PM chamber. The irreversibility of heat transfer between the working fluid and cylinder wall, and the friction loss due to piston movement are taken into account in the real PM heat regenerative cycle. The relationship between the net work output and thermal efficiency, and corresponding limit conditions for the PM engine are derived, in which the effect of heat losses through the cylinder wall is took into account. According to detailed numerical computations, effects of combustion and heat transfer on the cycle are also discussed. The results obtained here could provide significant guidance for the performance evaluation and improvement of practical PM-engines.Secondly, available combustion models, which are suitable for HCCI engines, are modified and applied to investigate the ignition and combustion characteristics of the PM engine. A simulation system including a single-zone model and a two-zone model for porous medium engine is developed, which could rapidly bring accurate information for the prediction of ignition point.(1) Based on the First thermodynamic Law, the SENKIN code of the CHEMKIN chemical kinetics package, combined with a zero-dimensional single-zone model of engine combustion, was used to simulate the combustion process of a PM engines fueled by n-heptane. The code has been modified to incorporate the Woschni heat transfer correlation, a model of heat transfer within porous medium and heat release rate models to build the energy balance equation. A substitutional combustion rate model and a detailed chemical kinetics mechanism with detailed chemistry mechanism of NO_x formation are used to calculate the heat release rate, separately. Evolutions of pressure and temperature in the PM engine are calculated under various working conditions. Influences of operating parameters, e.g. compression ratio, the initial temperature and volumetric heat transfer coefficient of the porous medium, the excess air ratio etc on the combustion process of the PM engine are discussed. Comparison of the PM engine with conventional engines shows that PM can relax the evolution of the in-cylinder temperature and pressure.(2) The combustion processes of both permanent and periodical contact PM engines were simulated by a quasi-dimensional two-zone model, considering the influences of the mass distribution, heat transfer from the cylinder wall, mass exchange between zones and the heat transfer in porous medium. A computer program was developed and coupled with the chemical kinetics package Chemkin III. The mass exchange model is adopted and simplified from the model of Komninos based on the assumption of uniform pressure throughout both zones. Wall heat losses were predicted with an improved Woschni model for the HCCI process by Chang. The e PM enginewas fueled with iso-octane and a skeletal kinetic mechanism for iso-octane oxidation was used for the chemistry simulation. Influences of operating parameters, e.g. intake temperature and pressure, the initial temperature of PM, compression ratio, the excess air ratio etc on the performance of the PM engine were emphatically discussed. It is found out that the porous medium, acting as a heat recuperator, can preheat the mixture and significantly enhance the evaporation of liquid fuel, which promotes the ignition and combustion in the cylinder; and that the initial PM temperature and the compression ratio are critical factors controlling the compression ignition of the mixture.For the permanent contact PM engine, the modes of fuel supply and the progress of fuel evaporation influence the mass distribution in the cylinder greatly. However, for the periodical contact PM engine, the evaporation process of liquid fuel occurs in the PM chamber, which is decoupled from the cylinder and is almost independent of other operating factors such as spray timing, power output etc. The mixing of fuel vapor and air is controlled by the timing of the PM valve opening, which is the critical factor determining ignitiontiming.Finally, the dissertation presents an experimental study of evaporation enhancementand combustion characteristics of liquid fuel spray aided by porous media. A set of experimental system for combustion of liquid fuel in porous media was built up to study the mechanism of the "in-pore spray" and the superadiabatic combustion in porous medium. The combustion system consists of a combustor (quartz glass tube), a gas supply system, a fuel supply system, a measurement system and so on. The expermental system can be employed for studying the combustion characteristics of both gaseous and of liquid fuels.The PM was preheated by combustion of gaseous fuel for a short time, and then liquid fuel (diesel) was sprayed into the porous medium combustor, where evaporation and combustiton happened. Temperature distributions in the combustion zone are measured with thermocouples, furthermore, the influences of mixture speed and equivalence ratio on the combustion wave speed and the maximum combustion temperature in the packed bed are discussed.