Ettore Musu

Computational study of a two-stroke direct-injection reactivity-controlled compression ignition engine:

Reactivity-controlled compression ignition combustion has proved to be effective in reducing the pollutant emissions and the fuel consumption in four-stroke internal-combustion engines. The application of this combustion mode to port-controlled crankcase-scavenged two-stroke engines also seems promising to avoid short-circuiting of fresh charge and to take advantage of the intrinsic residual exhaust gas. Accordingly, a computational study of a small-bore two-stroke dual-fuel direct-injection reactivity-controlled compression ignition engine was made including computational fluid dynamics simulations and zero-dimensional modeling. The zero-dimensional model is used to supply suitable initial conditions for the computational fluid dynamics simulations and to generate useful operating maps. These maps predict the engine behavior, highlighting the conditions where combustion would be controllable by means of in-cylinder reactivity stratification. The computational fluid dynamics simulations were validated against experimental data under motored and fired conditions, and the spray model was calibrated against dedicated bench tests. The in-cylinder behavior was explored to understand the effect and the importance on the engine operation of several types of stratification, including thermal stratification and reactivity stratification caused by the scavenging process and fuel injections. The models emphasize the importance of the exhaust gas thermal content which can promote combustion. Furthermore, its stratification in the combustion chamber due to the scavenging process, together with the reactivity stratification caused by the dual-fuel injection is able to change both the combustion phasing and the combution duration, thereby increasing the efficiency and reducing the combustion roughness.

Examination of Initialization and Geometric Details on the Results of CFD Simulations of Diesel Engines

Computational fluid dynamic simulations using the AVL FIRE and KIVA 3V codes were performed to examine commonly accepted techniques and assumptions used when simulating direct injection diesel engines. Simulations of a steady-state impulse swirl meter validated the commonly used practice of evaluating the swirl ratio of diesel engines by integrating the valve flow and torque history over discrete valve lift values. The results indicate the simulations capture the complex interactions occurring in the ports, cylinder, and honeycomb cell impulse swirl meter. Geometric details of engines due to valve recesses in the cylinder head and piston cannot be reproduced axisymmetrically. The commonly adopted axisymmetric assumption for an engine with a centrally located injector was tested by comparing the swirl and emissions history for a noncombusting and a double injection low temperature combustion case with varying geometric fidelity. Consideration of the detailed engine geometry including valve recesses in the piston altered the swirl history such that the peak swirl ratio at TDC decreased by approximately 10% compared with the simplified no-recess geometry. An analog to the detailed geometry of the full 3D geometry was included in the axisymmetric geometry by including a groove in the cylinder head of the mesh. The corresponding emissions predictions of the combusting cases showed greater sensitivity to the altered swirl history as the air-fuel ratio was decreased.

Direct Injection and Charge Stratification in a 50 cc Two-Stroke Engine: CFD Studies and Test Bench Results

Direct fuel injection has become necessary in two-stroke S.I. engines, since it prevents one of the major problems of these engines, that is fuel loss from the exhaust port. Another important problem is combustion irregularity at light loads, due to excessive residual gas in the charge, and can be solved by charge stratification. High-pressure liquid fuel injection is able to control the mixing process inside the cylinder for getting either stratified charge at partial loads or quasi-stoichiometric conditions, as it is required at full load. The feasibility of this solution for a small engine for light motorcycles has been studied using CFD tools. An exhaustive investigation carried out by the KIVA3v code allowed to design a 50 cm3 engine prototype with a satisfactory behaviour even at light loads in unthrottled condition, as proved by good fuel economy and engine stability in dynamometric bench tests. Exhaust gas analysis and indicated pressure behaviour confirm stratification and combustion correctness. For the final part of the research the adoption of the AVL-Fire code has been considered: the possibility to take into account any combustion chamber and transfer duct geometric details and the accuracy of spray breakup and wall film models allow to better understand the engine behaviour throughout the operating range, obtaining useful information in order to efficiently shorten the experimental time required for the EU map-setting.© 2006 ASME

Four Stroke Engine Geometry for Stratified Charge Combustion

In four-stroke engines direct injection increases power and fuel economy, which is further improved by charge stratification, due to pumping loss reduction and better combustion efficiency at partial loads. Charge stratification can be obtained by different techniques and injector designs. In every case late injection is necessary for stratification, which however is impaired by fuel dilution and spreading in consequence of burnt gas expansion, leading to incomplete combustion at very light loads. A numerical study has been carried out modifying KIVA code to handle new piston shapes. An innovative combustion chamber that is split in two volumes and allows fuel confinement during combustion has been conceived. CFD comparison has been made between a conventional combustion chamber and the proposed new one in term of combustion efficiency. Combustion is enhanced by the new design and unburnt emissions are reduced.Copyright

A Preliminary Study Towards an Innovative Diesel HCCI Combustion

Homogeneous-charge, compression-ignition (HCCI) combustion is triggered by spontaneous ignition in diluted homogeneous mixtures and has a gradual trend thanks to suitable solutions. It is considered a very effective way to reduce engine pollutant emissions, however only experimental prototypes have been based on this concept, except for a few small two-stroke engines. HCCI combustion is feasible with fuels both for S.I. and for C.I. engines, but currently it does not cover the whole engine operating field, thus the engine must be built to operate also as a conventional engine. In order to obtain a gradual combustion and not a simultaneous reaction (as it would be in spontaneously ignited homogeneous mixture), lean mixture is used and appropriate solutions, as Exhaust Gas Recirculation (EGR), are necessary. However, the admission of exhaust gas into the cylinder goes to detriment of engine maximum mean effective pressure. This paper concerns a preliminary study of an innovative concept to control HCCI combustion in Diesel-fuelled engines, apart from exhaust gas presence, the function of which is limited to NOx emission control. The main purpose of the research is the obtaining of Diesel HCCI combustion also with high mean effective pressures rendering the combustion behaviour more controllable as well. The concept consists in forming a pre-compressed homogenous charge outside the cylinder and in gradually admitting it into the cylinder during the combustion process. In this way, combustion can be controlled by the flow rate transfer and high pressure gradients, typical of common HCCI combustion, can be limited as well. A first analysis has been done, considering a cylinder filled with a perfectly stirred mixture of air and diesel fuel through a transfer duct, only to test the validity of the concept, regardless of which effective solution will be adopted. Both Two and Four Stroke operations have been considered to realize the concept. Results in terms of pressure, heat release rate, temperature and emission production have pointed out the validity of the concept. Especially the Two Stroke solution produces more soot than the conventional Diesel, pointing out that the air-fuel mixing is probably not optimized. Regarding NOx emissions, both the proposed solutions give better results than the conventional Diesel engine.Copyright

Improvements in Efficiency and Mixture Formation for an Innovative Diesel HCCI Concept

Homogeneous-charge, compression-ignition (HCCI) combustion is triggered by spontaneous ignition in dilute homogeneous mixtures. The combustion rate must be reduced by suitable solutions such as high rates of Exhaust Gas Recirculation (EGR) and/or lean mixtures. HCCI is considered to be a very effective way to reduce engine pollutant emissions, however only a few production engines have been built. HCCI combustion currently cannot be extended to the whole engine operating range, especially to high loads, since the use of EGR displaces air from the cylinder, limiting engine mean effective pressure, thus the engine must be able to operate also in conventional mode. This paper concerns a study of an innovative concept to control HCCI combustion in diesel-fueled engines. The concept consists in forming a pre-compressed homogeneous charge outside the cylinder and in gradually admitting it into the cylinder during the combustion process. In this way, combustion can be controlled by the transfer flow rate, and high pressure rise rates, typical of standard HCCI combustion, can be avoided. This new combustion concept has been called Homogenous Charge Progressive Combustion (HCPC). This paper concerns CFD analysis focused on improving efficiency and reducing pollutant emissions considering a new HCPC engine configuration. Results show an indicated efficiency around 45% and a consistent reduction of soot emission compared to conventional diesel engine.Copyright

A General Rezoning Technique for KIVA3V Internal Combustion Engines CFD Simulations

A computational mesh is required when performing CFD-combustion modeling of internal combustion engines. For combustion chambers with moving pistons and valves, like those in typical cars and trucks, the combustion chamber shape changes continually in response to piston and valve motion. The combustion chamber mesh must then also change at each time step to reflect that change in geometry. The method of changing the mesh from one computational time step to the next is called rezoning. This paper introduces a new method of mesh rezoning for the KIVA3V CFD-combustion program. The standard KIVA3V code from Los Alamos National Laboratory comes with standard rezoners that very nicely handle mesh motion for combustion chambers whose mesh does not include valves and for those with flat heads employing vertical valves. For pent-roof and wedge-roof designs KIVA3V offers three rezoners to choose from, the choice depending on how similar a combustion chamber is to the sample combustion chambers that come with KIVA3V. Often, the rezoners must be modified for meshes of new combustion chamber geometries to allow the mesh to successfully capture change in geometry during the full engine cycle without errors. There is no formal way to approach these modifications; typically this requires a long trial and error process to get a mesh to work for a full engine cycle. The benefit of the new rezoner is that it replaces the three existing rezoners for canted valve configurations with a single rezoner and has much greater stability, so the need for ad hoc modifications of the rezoner is greatly reduced. This paper explains how the new rezoner works and gives examples of its use.Copyright