A new clean diesel combustion concept has been proposed and its excellent performance with respect to gas emissions and fuel economy were demonstrated using a single cylinder diesel engine. It features the following three items: (1) low-penetrating and highly dispersed spray using a specially designed injector with very small and numerous orifices, (2) a lower compression ratio, and (3) drastically restricted in-cylinder flow by means of very low swirl ports and a lip-less shallow dish type piston cavity.Item (1) creates a more homogeneous air-fuel mixture with early fuel injection timings, while preventing wall wetting, i.e., impingement of the spray onto the wall. In other words, this spray is suitable for premixed charge compression ignition (PCCI) operation, and can decrease both nitrogen oxides (NOx) and soot considerably when the utilization range of PCCI is maximized.However, in diffusive combustion, especially at full load, a low-penetrating spray potentially causes higher soot emissions and results in lower maximum torque. In this case, item (2) is applied to recover full-load performance. The lower compression ratio enables diffusive combustion phasing to be advanced more with an earlier injection timing because of a larger margin between the compression-end pressure and the allowable maximum in-cylinder pressure. This results in lower soot emissions because enough time is created to oxidize soot before the end of the combustion period.A lower compression ratio often leads to worse cold-condition engine performance aspects, such as cold startability, unburned hydrocarbons, and white smoke. Item (3) is applied to compensate for such practical problems. Drastically weakened in-cylinder flow keeps the compression-end temperature to the same level as a conventional engine with an ordinary compression ratio by decreasing heat-flux escaping through the chamber wall (i.e., heat-loss).Although a weak in-cylinder gas motion might lead to higher soot emissions due to slower fuel-air mixing, it should be noted that the highly dispersed spray of item (1) enables PCCI-dominant combustion in which the fuel-air mixing process is less dependent on in-cylinder flow.In this way, these three items act mutually to compensate for each other's drawbacks, while maximizing their advantages. Consequently, NOx emissions in the New European Driving Cycle (NEDC) can be reduced drastically to less than 1/4 of the level of a conventional engine, or less than half of the Euro 6 standard without deteriorating fuel consumption, full-load torque, or cold-condition performance.