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This paper describes the optimization process of a small single-cylinder research HSDI diesel engine, starting from a conventional combustion towards split-injection low-temperature combustion. Targets for emissions, fuel consumption and combustion noise are defined with the characteristics of low temperature combustion in mind, in other words, high CO, HC and combustion noise but low soot and NOX. In this investigation the targets are defined for a medium-load working modes of a typical small four-cylinder turbo-charged diesel engine, equipped with a particulate trap and oxidation catalyst. They are introduced into an objective target function which is a guide for the optimization process. The statistical optimization procedure used is the method of consecutive screenings. With this methodology, six factors are optimized: mass distribution of the fuel injection pattern, injection pressure, combustion phasing, EGR rate, boost pressure and dwell time between injection events.

Among the various homogeneous combustion concepts, the “late injection strategy” shows potential to put NOx and particulate emissions within the Euro 5 box at low loads. However, the corresponding retarded injection timings lead to increased fuel consumption. This article gives an overview of techniques which improve fuel consumption by enabling the combustion to be phased closer to top dead center. Primarily, injection duration can be shorten using an adapted Common Rail and high flow tips. Secondly, the ignition delay can be increased through lowered compression ratio or retarded inlet valve closing. Lastly, the mixing of air and fuel can be enhanced as a result of additional nozzle tip holes, optimized A/F and swirl level. The end result for this combination of improvements is a defined combustion system that yields the same NOx/BSFC trade-off as conventional combustion at low loads, but with very low soot emissions.