Multiple injection and rate shaping Part 2: Emissions reduction in passenger car Diesel engines Computational investigation 2009-24-0012

In modern passenger car diesel engines, multiple injection, MI, and rate shaping are measures, which in conjunction with others help to achieve the emissions legislation EU6 and US Tier2 Bin5. However, where hitherto mainly pollutant emissions where considered, CO₂ output - i.e. fuel consumption - becomes increasingly important. Also, off cycle emissions may have to be regarded in the future. Additionally engine noise and drivability need consideration.

The complexity and effect of an applied injection strategy is defined by the overall engine concept including the after treatment system, and also by the vehicle inertia. Additionally a modern fuel injection system not only has to allow for the necessary injection strategies but at the same time needs to offer robust performance over life time.

Although a complex injection system may entail a penalty in cost, this has to be regarded in the context of the complete engine system including a possibly needed exhaust gas after-treatment, since a more expensive injection system can contribute to an overall reduction in cost. A well applied and stable injection system may allow for a simple and cheap after treatment system to be used, optimised for low engine exhaust back pressure.

In the present work, the commercial 3D CFD code “AVL Fire” was used to analyse the functioning of the so-called “ramp injection”, in particular the opening ramp. The validation data originated from a 500 cm³ single cylinder research engine, run with the directly actuated Continental piezo injector, which allows for application of variable opening velocities of the nozzle needle for injection rate shaping. The physics of mixture formation and their effects on combustion and, hence, on emissions generation was elucidated. It was established in previous work, that the quantity and the rate shaping of the pre-main injected fuel mass had a significant effect on soot emissions. Therefore the emphasis of the present analysis was on the processes affecting soot generation and soot oxidation, in order to minimise the overall soot emission. The analysis was based on the quantification and localisation of the volumina of rich mixture above an equivalence ratio of 2. The computed results of in-cylinder pressure, heat release rate and engine out soot emission trends, were validated against test results from the same single cylinder engine, which lend its geometry and combustion system configuration for the generation of the computational grid and the initial and boundary conditions of the computation.

Agreement of the results from experiments and simulation was excellent, giving confidence in the validity of the analysis of local phenomena, which explain the differences in mixture formation and combustion of various injection strategies.

A detailed treatise on the experimental investigation is provided in Part 1 of this paper, SAE 2009-24-0004. It also includes a brief review on previous work on the subject on injection rate shaping, including the boot injection.