A premixed compression-ignited (PCI) combustion system, which realizes lean combustion with high efficiency and low emissions, was investigated and its effects and problems were ascertained. With PCI combustion, fuel was injected early on the compression stroke and a premixed lean mixture was formed over a long mixing period. The test engine was operated with self-ignition of this premixed lean mixture. From the results of combustion observation and numerical simulation, a need to prevent the fuel spray from adhering to the cylinder liner and combustion-chamber wall was identified. Consequently, an impinged-spray nozzle with low penetration was made and tested. As a result, an extremely low nitrogen-oxide (NOx) emission level was realized but fuel efficiency was detracted slightly. Also, the engine operating range possible with PCI combustion was found to be limited to partial-load conditions and PCI combustion was found to cause an increase in hydrocarbon (HC) emission. Since it offers ideal combustion characteristics with a high degree of constant volume of heat release, however, PCI combustion potentially represents a more efficient, cleaner combustion (ultra-low NOx and low smoke) than those of conventional diesel engines.