L.M.T. Somers

Premixed charge compression ignition combustion modelling with a multi-zone approach including inter-zonal mixing:

Premixed charge compression ignition combustion is a clean and efficient alternative to classical diesel combustion. The concept of premixed charge compression ignition combustion is associated with early injection of the fuel while applying high exhaust gas recirculation levels and operation with a highly lean mixture such that ignition takes place (well) after the injection event. Thus, it is possible to reduce the soot emissions and the nitrogen oxide emissions simultaneously. Premixed charge compression ignition combustion is analysed using a multi-zone model. In the multi-zone model, chemical mechanisms which are much more detailed than those used in the computational fluid dynamics approaches can be introduced directly. The computational fluid dynamics model is still used to predict the initial fuel stratification in the cylinder, which is important to improve the quality of the model. For the analysis, dedicated experiments with n-heptane are used to evaluate the results of the model. In such a multi-zone model, 10 zones prove to be sufficient to describe the stratification with adequate resolution. It is observed that different fuel distributions have a large influence on the emissions when there is no mixing between the zones. To overcome this dependence, basic inter-zonal diffusive mixing is applied. The level of mixing is estimated with a sensitivity study. When the inter-zonal mixing is included, the emission results become much less sensitive to the crank angle at which the charge stratification is sampled and the simulation is initialized.

Correlating Flame Location and Ignition Delay in Partially Premixed Combustion

Controlling ignition delay is the key to successfully enable partially premixed combustion in diesel engines. This paper presents experimental results of partially premixed combustion in an optically accessible engine, using primary reference fuels in combination with artificial exhaust gas recirculation. By changing the fuel composition and oxygen concentration, the ignition delay is changed. To determine the position of the flame front, high-speed visualization of OH-chemiluminescence is used, enabling a cycle resolved analysis of OH formation. A clear correlation is observed between ignition delay and flame location. The mixing of fuel and air during the ignition delay period defines the local equivalence ratio, which is estimated based on a spherical combustion volume for each spray. The corresponding emission measurements using fast-response analyzers of CO, HC and NOX confirm the decrease in local equivalence ratio as a function of ignition delay. Furthermore multiple injection strategies are investigated, applying pilot as well as post injections, in combination with a main injection at constant load. From these results it is concluded that both pilot and post injections result in an increase of unburned hydrocarbon and CO emission and a slight decrease of nitric oxide emissions.

Combustion and Emission Characteristics of a Heavy Duty Engine Fueled with Two Ternary Blends of N-Heptane/Iso-Octane and Toluene or Benzaldehyde

In this work, the influences of aromatics on combustion and emission characteristics from a heavy-duty diesel engine under various loads and exhaust gas recirculation (EGR) conditions are investigated. Tests were performed on a modified single-cylinder, constant-speed and direct-injection diesel engine. An engine exhaust particle sizer (EEPS) was used in the experiments to measure the size distribution of engine-exhaust particle emissions in the range from 5.6 to 560 nm. Two ternary blends of n-heptane, iso-octane with either toluene or benzaldehyde denoted as TRF and CRF, were tested, diesel was also tested as a reference. Test results showed that TRF has the longest ignition delay, thus providing the largest premixed fraction which is beneficial to reduce soot. However, as the load increases, higher incylinder pressure and temperature make all test fuels burn easily, leading to shorter ignition delays and more diffusion combustion. For each test fuel, the particulate number concentration in nucleation mode decreased with the increase of EGR rate, while more particles in accumulation mode were generated. Moreover, compared with TRF and CRF, diesel produces the most mass and number concentrations of particulate matter.

A comprehensive methodology for computational fluid dynamics combustion modeling of industrial diesel engines

Combustion control and optimization is of great importance to meet future emission standards in diesel engines: increase in break mean effective pressure at high loads and extension of the operating range of advanced combustion modes seem to be the most promising solutions to reduce fuel consumption and pollutant emissions at the same time. Within this context, detailed computational fluid dynamics tools are required to predict the different involved phenomena such as fuel–air mixing, unsteady diffusion combustion and formation of noxious species. Detailed kinetics, consistent spray models and high quality grids are necessary to perform predictive simulations which can be used either for design or diagnostic purposes. In this work, the authors present a comprehensive approach which was developed using an open-source computational fluid dynamics code. To minimize the pre-processing time and preserve results’ accuracy, algorithms for automatic mesh generation of spray-oriented grids were developed and successfully applied to different combustion chamber geometries. The Lagrangian approach was used to describe the spray evolution while the combustion process is modeled employing detailed chemistry and, eventually, considering turbulence–chemistry interaction. The proposed computational fluid dynamics methodology was first assessed considering inert and reacting experiments in a constant-volume vessel, where operating conditions typical of heavy-duty diesel engines were reproduced. Afterward, engine simulations were performed considering two different load points and two piston bowl geometries, respectively. Experimental validation was carried out by comparing computed and experimental data of in-cylinder pressure, heat release rate and pollutant emissions (NO x , CO and soot).

Comparing Different Turbulence Modelling Approaches for Square Duct Simulations Using the Kiva Code

In internal-combustion engines (ICE) the fluid dynamics is characterized by strong anisotropy. The standard two equation k–e model is well known to be not appropriate in this case. A detailed study of the numerical modelling properties of the well known Kiva-3V code has been performed for two different approaches to anisotropic turbulence modelling. In the first approach a Smagorinsky-type LES model is evaluated. In the second approach a Time-Dependent RANS model has been adopted, using the Explicit Algebraic Stress Model of Gatski and Speziale [1]. For validation of both approaches numerical simulations of a turbulent flow in a square duct geometry are compared to DNS data. It is concluded from this work that the applied RANS approach is the best available practise to model the anisotropic properties of the fluid flow for ICE simulations as long as the computational resources to perform real LES simulations remain limited.Copyright