Andreas M. Lippert

Multidimensional simulation of diesel engine cold start with advanced physical submodels

Abstract The complex physical processes occurring during cold starting of diesel engines mandate the use of advanced physical submodels in computations. The present study utilizes a continuous probability density function to represent more fully the range of compositions of commercial fuels. The model was applied to singledroplet calculations to validate the predictions against experimental results. Analysis of a high-pressure diesel spray showed axial composition gradients within the spray. Previous wall-film modelling was extended to include the continuous multicomponent fuel representation. Using these models, the cold-start behaviour of a heavy-duty diesel engine was analysed. The predictions show that multicomponent fuel modelling is critical to capturing realistic vaporization trends. In addition, the spray-film interaction modelling is crucial to capturing the spray impingement and subsequent secondary atomization. Heating the intake air temperature was shown to result in reduced ignition delay and accelerated vaporization. Increasing the fuel temperature increased vaporization prior to and away from the initial heat release. Increasing the injection pressure increased vaporization without much change in the ignition delay. Split injections, with 75 per cent of the fuel contained in the second pulse, displayed a substantial reduction in ignition delay due to ignition of the first pulse. The timing of the first injection was found to be an important parameter due to differences in the spray impingement behaviour with different timings.

Development and Optimization of a Small-Displacement Spark-Ignition Direct-Injection Engine - Stratified Operation

Superior fuel economy was achieved for a small-displacement spark-ignition direct-injection (SIDI) engine by optimizing the stratified combustion operation. The optimization was performed using computational analyses and subsequently testing the most promising configurations experimentally. The fuel economy savings are achieved by the use of a multihole injector with novel spray shape, which allows ultra-lean stratification for a wide range of part-load operating conditions without compromising smoke and hydrocarbon emissions. In this regard, a key challenge for wall-controlled SIDI engines is the minimization of wall wetting to prevent smoke, which may require advanced injection timings, while at the same time minimizing hydrocarbon emissions, which may require retarding injection and thereby preventing over-mixing of the fuel vapor. These conflicting requirements are heightened in a small-displacement engine, which has short path lengths between the injector and piston and is therefore prone to increased wall wetting. However, a side-injection system also requires sufficient spray penetration to reliably transport fuel to the centrally mounted spark plug at suitable injection timings. The multihole injector can simultaneously satisfy these different requirements because of enhanced vaporization, resulting in a shortened liquid length. This is attributable to increased air entrainment available because the spray does not collapse under elevated cylinder pressures typical of late injection. The piston bowl was optimized with respect to bowl depth and bowl volume to ensure sufficient mixing and air utilization at higher part-loads, minimization of wall wetting, and containment of the fuel-air mixture at light loads. The use of a variable swirl-control valve allows the air-motion to be optimized depending on the condition. It was found that the multihole injector configuration requires higher swirl compared to a fan-type injector. The combination of in-cylinder swirl, a suitable piston bowl shape and multi-hole injectors made it possible to obtain stable stratified combustion throughout a wide operating range with reduced NOX emissions under a large EGR rate.

Application of non-linear turbulence models in an engine-type flow configuration

Abstract Three non-linear k-ε models were implemented into the multi-dimensional computational fluid dynamics code GMTEC with the purpose of comparing them with existing linear k-ε models including renormalization group variations. The primary focus of the present study is to evaluate the potential of these non-linear models in engineering applications such as the internal combustion engine. The square duct flow and the backwards-facing step flow were two simple test cases chosen for which experimental data are available for comparison. Successful simulations for these cases were followed by simulations of an engine-type intake flow to evaluate the performance of the non-linear models in comparison with experimental data and the standard linear k-ε models as well as two renormalization group types. All the non-linear models are found to be an improvement over the standard linear model, but mostly in simple flows. For more complex flows, such as the engine-type case, only the cubic non-linear models appear to make a modest improvement in the mean flow but without any improvement in the root-mean-square values. These improvements are overshadowed by the stiffness of the cubic models and the requirements for smaller time steps. The contributions of each non-linear term to the Reynolds stress tensor are analysed in detail in order to identify the different characteristics of the different non-linear models for engine intake flows.