In this paper, we aim to develop a comprehensive ignition model for three-dimensional (3D) computational fluid dynamics (CFD) combustion modeling in spark-ignited (SI) engines. In the proposed model, we consider the following aspects separately to model the spark ignition process comprehensively. An electrical circuit is solved for calculation of the energy transferred to the spark plasma channel. The spark itself is represented by computational particles for monitoring its motion and ignitability. Heat diffusion from the spark toward the surrounding mixture is calculated with a one-dimensional (1D) model, resulting in the temperature obtained at the surface of the spark channel. Based on the calculated temperature and interpolated pressure and local mixture composition, an instantaneous ignition delay time is read from tabulated values for every particle representing the spark channel. The final ignitability criterion is defined by a precursor calculated with a zero-dimensional (0D) model, which accounts for the history of changes in spark surface temperature and local mixture properties. As soon as the precursor reaches a threshold value for a given spark channel particle, a flame kernel is introduced at a position of the particle. Flame propagation is generally treated by the G-equation combustion model. Validation is performed by measurements of the spark discharge process in high-velocity flow field and single-cylinder AVL research engine. We demonstrate that the proposed model can correctly reproduce the electrical circuit, spark channel dynamics, and overall engine performance.
Abstract. In many heat devices designers and operators meet the problem of low efficiency of combustion and restricted emission standards. This process should be improved to maximize its efficiency and satisfy additional requirements as, for example, uniform temperature fieldin combustion chamber, low noise level or very low NOx emission. These requirements are satisfied by homogeneous combustion. Such combustion method is particularly attractive for the steel or glass industry or power industry based in particular on natural gas. In this paper factors, which have the biggest influence on performance of flameless combustion, are discussed, among others: momentum of fuel and oxidizer, composition of the mixture, the temperature of the inlet gases. Additionally, blind simulations of combustion in a combustion chamber of a furnace are run to assess how high is the influence of these factors individually. Numerical simulations are performed in a CFD code AVL Fire. The detailed chemical kinetics mechanism GRI-mech 3.0 is used for combustion calculations. Calculations results are correlated with experimental data. Blind simulations and experiment provide similar level of NO X emission (~6-8 ppm). Experiments showed that the effect of the addition of ethylene to fuel on emissions of NO X , CO, THC is not significant. Similarly, numerical simulations predict that influence of ethylene is negligible. CO, THC and CO 2 were on a stable level across all cases. NO X emissions increases when mass flow of air and fuel increases due to higher heat release in the same volume, what results in higher temperature of combustion products. When temperature of fuel increases NO X level decreases.
Extended AbstractThe possibility of achieving high thermal efficiency brings opposed-piston (OP) engines back into interest of research centers. The main advantage of such design is that combustion chamber is formed between moving pistons -there is no additional cylinder head to be cooled which directly results in an increased thermal efficiency. If made as a 2-stroke, the possibility of uniflow scavenging arises together with lower engine cost due to removal of unnecessary parts, like camshafts or poppet valves. Furthermore, engine operation in a homogeneous charge compression ignition (HCCI)/controlled autoignition (CAI) mode at high compression ratios (CR) raises a possibility of reaching even higher efficiencies and very low emissions [1].There are several types of OP engine configurations (cranckless, single crankshaft, multiple crankshaft, rotary or barrel) [2]. The barrel type OP engine is of particular interest for the authors because of its robust design, not many moving parts (high mechanical efficiency) and relatively easy incorporation of a variable compression ratio (VCR) [3]. This kind of experimental 2-stroke engine equipped with the VCR is currently under development at Warsaw University of Technology. The VCR is necessary to enable engine control at CAI operation mode. In addition, the engine will be equipped with a water injection system for charge cooling which is another measure to control the CAI combustion process. The injected water is supposed to cool the charge in the cylinder through the evaporation and retard the auto-ignition (AI) in case of CAI mode or to prevent knock in case of spark ignition (SI) mode allowing engine operation at higher compression ratios. Although the idea of HCCI/CAI combustion is desirable because of possible benefits in terms of efficiency and emissions, it is also very challenging to implement in an internal combustion engine. Therefore, numerical simulations can become a useful tool for better understanding of the combustion process and can shorten the time span for engine development.The aim of this study is to investigate a possibility of CAI control in a 2-stroke barrel type OP direct injection engine fueled with a gasoline. To that end, a 3D CFD numerical simulations of the scavenging and combustion processes were performed with use of the AVL Fire solver that is based on a Finite Volume Method (FVM) discretization and offers a number of tools dedicated to numerical simulations of working processes in internal combustion engines [4]. The VCR and water injection were considered for the ignition timing control. Hence, several cases were calculated with different engine compression ratios and different amount of injected water, varying in a range of 0-100% of the fuel mass. The influence of the VCR and water injection on the ignition timing were determined and their application in the real engine was discussed.
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