In order to clarify the potential of internal purification methods on medium speed diesel engines to meet the IMO Tier III nitrogen oxide (NO_x) emission regulations, combined 1-D engine working cycle simulation and 3-D CFD simulation were conducted to predict the performance and emissions of the engine under different valve close timings, geometric compression ratios, injection timings, and Exhaust Gas Recirculation (EGR) rates. The numerical results show that, as the inlet valve close timing is advanced, NO_x is reduced by as much as 27%, but the peak of premixed combustion heat release rate is increased; this can weaken the ability to reduce NO_x with the Miller cycle. Moreover, the peak of premixed combustion heat release rate is reduced when the geometric compression ratio is increased to 15.4, and linking with injection timing by delaying 6°CA can further reduce NO_x by 55.3% from the baseline. Finally, over 80% NO_x reduction can be achieved when the above schemes are combined with over 15% EGR. The NO_x and soot can be reduced simultaneously by using moderate Miller cycle combination with moderate EGR, and the results show a large reduction of NO_x and moderate reduction of soot. This can be a feasible technical solution to meet Tier III regulations.

Low-temperature combustions (LTCs), such as homogeneous charge compression ignition (HCCI), could achieve high thermal efficiency and low engine emissions by combining the advantages of spark-ignited (SI) engines and compression-ignited (CI) engines. Robust control of the ignition timing, however, still remains a hurdle to practical use. A novel technology of jet-controlled compression ignition (JCCI) was proposed to solve the issue. JCCI combustion phasing was controlled by hot jet formed from pre-chamber spark-ignited combustion. Experiments were done on a modified high-speed marine engine for JCCI characteristics research. The JCCI principle was verified by operating the engine individually in the mode of JCCI and in the mode of no pre-chamber jet under low- and medium-load working conditions. Effects of pre-chamber spark timing and intake charge temperature on JCCI process were tested. It was proven that the combustion phasing of the JCCI engine was closely related to the pre-chamber spark timing. A 20 °C temperature change of intake charge only caused a 2° crank angle change of the start of combustion. Extremely low nitrogen oxides (NOx) emission was achieved by JCCI combustion while keeping high thermal efficiency. The JCCI could be a promising technology for dual-fuel marine engines.

The double-layer diverging combustion chamber (DLDC chamber) aims to improve the fuel–air mixing formation and promote in-cylinder air utilization by changing fuel spray spreading characteristics. In order to investigate how the DLDC chamber profile and injection parameters affect the fuel spray spreading, visualization of fuel injection and impingement tests were carried out on two different DLDC chambers with different fuel injection parameters. The visualization test results showed that double-layer fuel spray spreading was obtained in the two DLDC chambers and the peripheral top clearance of each chamber was utilized efficiently. The DLDC chamber with a 50% upper layer volume provided a larger fuel spray distribution region after the start of injection. The DLDC chamber with a 70% upper layer volume obtained a larger fuel spray distribution region with better top clearance utilization at the later stage of injection. The injection parameters mentioned in this research showed significant effects on the fuel spray spreading in the DLDC chamber. Increasing the injection pressure provided a larger fuel spray distribution area at the beginning of injection. Decreasing the nozzle hole diameter had a positive influence on obtaining a larger fuel spray distribution. Advancing the injection timing enabled the enlarging of the fuel distribution region.

Combustion chamber is a main component for Diesel, as it organizes the fuel-air mixing and combustion processes and eventually determines Diesel performance. In the past decades, the chamber turned into the higher radius-to-depth ratio and lower area-to-volume ratio type to increase the in-cylinder air utilization and promote the fuel spray spreading. The Double-Layer Diverging Combustion chamber (DLDC chamber) was designed in this background. In order to obtain the DLDC chamber performance characteristics comprehensively, a 135-type Diesel with a DLDC chamber, a re-entrant chamber and a ω chamber was selected. The results suggested that the DLDC chamber decreased the BSFC and soot emission, but increased the NO_x with higher p_max and heat release rates during the premixed combustion phase, as it could divide the fuel into two layers and promote the fuel spreading and combustion; adjusting the injection parameters made the BSFC and exhaust emissions of the DLDC chamber change quite differently, because the different layers of the DLDC chamber had their own chamber-spray-charge motion matchings. Finally, the investigation indicated that the DLDC chamber provided lower exhaust emissions compared with the other two chambers by keeping the same fuel consumption; moreover, the DLDC chamber provided the lowest BSFC and soot emission in the three chambers under the same NO_x emissions level.