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Modern diesel cars, fitted with state-of-the-art aftertreatment systems, have the capability to emit extremely low levels of pollutant species at the tailpipe. However, diesel aftertreatment systems can represent a significant cost, packaging and maintenance requirement. Reducing engine-out emissions in order to reduce the scale of the aftertreatment system is therefore a high priority research topic. Engine-out emissions from diesel engines are, to a significant degree, dependent on the detail of fuel/air interactions that occur in-cylinder, both during the injection and combustion events and also due to the induced air motion in and around the bowl prior to injection.In this paper the effect of two different piston bowl shapes are investigated. Experiments are performed in a single-cylinder engine fitted with a production cylinder head and fuel injector in order to quantify the effect of the bowl shape and spray targeting-varied by varying the injector nozzle tip protrusion-on emissions and fuel consumption. Multi-dimensional CFD modelling is used to detail the effect of these geometry changes on the in-cylinder flow and fuel/air mixing processes thereby guiding the interpretation and understanding of the experimental results. The results suggest that improvements in engine-out emissions, as well as fuel consumption, may be obtained from current diesel engines by the careful matching of combustion system geometry with fuel injection hardware and strategy, and that an integrated approach with experimental and numerical studies working in parallel is essential to maximise these benefits.

It is known that low-temperature combustion (LTC) strategies can help simultaneously reduce nitrogen oxides (NOx) and particulate matter (PM) emissions from diesel engines to very low levels. However, it is also known that LTC may cause emissions of unburned hydrocarbons (UHC) to rise — especially in low load operating conditions. Recent studies indicate that end-of-injection (EOI) processes may support ignition recession back to injector nozzle thereby helping to reduce these emissions. This paper contributes to the physical understanding of this EOIphe-nomenon, combustion recession, using computational fluid dynamics studies at LTC conditions. Simulations are performed on a single-hole injection of n-dodecane under a range of Engine Combustion Network’s “Spray A” conditions. The primary objective of this paper is to assess the ability of a Flamelet Generated Manifold (FGM) combustion model to predict and characterize combustion recession. First, a baseline condition FGM simulation is compared with two other combustion models namely the Well Stirred model (WSR), the Representative Interactive Flamelet model (RIF) using the commercially-available CFD solver, CONVERGE. Further studies were carried out for FGM model alone including: varying ambient temperature conditions and chemical mechanisms. Two chemical kinetics mechanisms with low temperature chemistry for n-dodecane are employed to help to predict the occurrence of combustion recession. All simulations are performed under the Reynolds-Averaged Navier-Stokes (RANS) framework in a grid-converged Lagrangian spray scenario. The simulation of combustion recession is qualitatively validated against experimental data from literature and the efficacy of each model in predicting combustion recession is evaluated. Overall, it was found that the FGM model was able to capture the combustion recession phenomenon well — showing particular strength in predicting distinct auto-ignition events in the near nozzle region.

A novel combustion modelling approach is proposed here to study the transient effects of diesel spray. Conditional Source-term Estimation (CSE) is a combustion model which invokes the Conditional Moment Closure (CMC) hypothesis to provide an approximation of the mean chemical source term in an averaged transport equation. Unlike CMC, where transport equations are solved for conditional moments, CSE recovers these conditional moments through the solution of an inverse problem. Integral equations are inverted for the conditional moments, by assuming spatial homogeneity in the conditional averages where Tikhonov regularization is applied. Previous CSE studies have shown that the model is able to predict the flame characteristics successfully for both premixed and non-premixed combustion modes. However, most of these investigations were based on methane flames. This study will be the first successful application of CSE to a complex hydrocarbon fuel, n-dodecane, under the Engine Combustion Network's (ECN) "Spray A" conditions. Detailed chemistry is included in tabulated form using the Flamelet Generated Manifold (FGM) methodology. The predictions of this study include both the Favre averaged conditional mass fraction of reactive species and temperature. The results are compared with available experimental data and previous numerical results. Both RANS and LES simulations are performed under the same condition. The objectives of the paper are (i) assessment of the application of CSE on igniting diesel spray (ii) comparison of the CSE numerical results with available experimental results and previous numerical simulations. Overall, the combination of a chemical mechanism that has been tuned to predict "Spray A" conditions with the CSE-FGM model is able to successfully predict autoignition delay time and lift-off length of n-dodecane spray within the scatter of the experimental data. CSE-FGM offers a feasible tool for detailed combustion analysis of diesel spray flames. Both RANS and LES can give reasonably good global predictions of the flame.