The authors applied one-dimensional (1-D) simulation and 3-D Computational Fluid Dynamics (CFD) simulation to evaluate the potential of in-cylinder control methods on a low-speed 2-stroke marine engine to reach the International Maritime Organization (IMO) Tier 3 NO_x emissions standards. Reducing the combustion temperature is an important in-cylinder measure to decrease NO_x emissions of marine diesel engines. Miller-cycle and Exhaust Gas Recirculation (EGR) are effective methods to reduce the maximum combustion temperature and accordingly decrease NO_x emissions. The authors’ calculation results indicate that with a combination of 2-stage turbocharging, a mild Miller-cycle and 10% EGR rate, the NO_x emissions can be decreased by 48% without the increased Specific Fuel Oil Consumption (SFOC) penalties; with a medium Miller-cycle and 10% EGR, NO_x can be decreased by 56% with a slight increase of SFOC; with a medium Miller-cycle and 20% EGR, NO_x can be decreased by 77% and meet IMO Tier 3 standards, but with the high price of a considerable increase of SFOC. The first two schemes are promising to meet IMO Tier 3 standards with good fuel economy if other techniques are combined.

Using low price heavy fuel oil (HFO) in high-speed diesel engines is a practical way to reduce running costs. However, most high-speed diesel engines’ fuel systems and combustion systems cannot adapt to HFO. This causes the problem of deterioration of combustion performance. In order to solve this problem, the authors have modified the fuel injection system and combustion system of a high-speed diesel engine to use HFO. In addition, reducing the viscosity of HFO is necessary before it is fed into the engine. Because heating apparatus are not feasible for high-speed engine users, light diesel was blended with HFO to reduce the fuel viscosity. The blend is called HFO-L. Meanwhile, for the purpose of further reducing NO_x emissions and soot emissions, water-emulsified HFO-L, named HLW, was used on the research engine. When fueled with 10% water content HLW, the engine presented the same power performance and thermal efficiency as the baseline engine fueled with light diesel. Due to the low price of HFO, the fuel economy of the engine was greatly improved. In addition, fueling HLW led to a considerable reduction of the engine’s NO_x emissions compared with the baseline engine.

An improved multicomponent evaporation model was developed to study the evaporation characteristics of the pure diesel and alcohol (C₂–C₉) as well as their blended droplets. In this model, the Predictive Soave–Redlich–Kwong (PSRK) equation of state (EOS) was employed to evaluate the vapor–liquid equilibrium for pure and blended droplets. The results indicated that the model predictions agree very well with the experimental measurements. The evaporation characteristics of different pure alcohol droplets were analyzed, and the influence of the addition of alcohols in diesel on the evaporation behavior of droplets under various conditions was discussed.

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.

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.

The possibility of lube oil droplets’ existence in cylinders for two-stroke low-speed gas engines is higher because of the much higher lube oil consumption rate. Some droplets are directly injected into cylinders by lube oil injectors, and some are blown into cylinders through the scavenging ports. Autoignition of cylinder oil droplets is the main cause of preignition. This research study indicates that under in-cylinder conditions, overlarge single droplets cannot autoignite due to the long evaporation time, and overly small single droplets cannot autoignite because of the low vapor concentration. To find out what kinds of oil droplet groups could autoignite and cause preignition, 3-D computational fluid dynamics simulation in OpenFOAM was carried out. The model predictions were validated against the experimental results, including the evaporation rate of n-heptane droplets and the ignition delay of lube oil droplets. Also, the simulation was used to investigate the characteristics of multiple droplets under different ambient temperatures and pressures. The evaporation lifetime and the ignition delay of a large single droplet are dozens of times longer than that of multiple droplets, which confirms that the droplet group is more dangerous than a large single droplet. The evaporation rate and ignition delay are affected by the distance and number of droplets. A larger number causes a lower average evaporation rate. A smaller distance causes a shorter ignition delay. The local vapor concentration and temperature could be greatly reduced due to the existence of multiple evaporation and ignition cores. Additionally, these findings of multiple droplets were confirmed under nonconstant cylinder conditions of a natural gas engine. This research provides a guide to design the lube oil injectors and scavenging ports so as to reduce the preignition caused by lube oil autoignition.