This paper describes knocking combustion in a diesel homogeneous charge compression ignition (HCCI) engine. The knocking combustion was investigated by a harmonic analysis, and the complex logarithmic spectrum (CLS), calculated by Fast Fourier Transformation (FFT) based on the acquired in-cylinder pressure, was defined to evaluate the knocking intensity. The consistencies among maximum in-cylinder pressure, maximum pressure rise rate and knocking parameters of harmonic analysis were proved. The effects of engine load, speed, internal and external exhaust gas recirculation (EGR) on the knocking combustion in this HCCI diesel engine were experimentally studied. The results showed that the CLS in the range of 180th- to 350th-order harmonics was more sensitive to the knocking intensity, and the CLS increased with the increase in knocking intensity. The increasing of load, speed and internal EGR would increase the occurrence probability and intensity of the knocking combustion, but the increasing of the external EGR would decrease the occurrence probability.Highlights► The harmonic characteristics of knocking combustion were analyzed. ► The amplitude of peak-domain harmonic can characterize knocking intensity. ► Increasing engine load and speed will increase HCCI knocking intensity. ► Internal and external EGR have different effects on knocking combustion.

The variable geometry exhaust manifold (VGEM) turbocharging system can realize the switch between two charging modes by the switching valve, and it can give a good performance both at the high load operation and the low load operation. When the switching valve is closed during the low load or transient response operation, the VGEM turbocharging system works as a pulse turbocharging system. When the switching valve is opened during the high load operation, the VGEM turbocharging system works as a semi-constant pressure turbocharging system. This paper puts forward a newly designed VGEM turbocharging system for an 8-cylinder marine diesel engine. The original turbocharging system for this marine diesel engine is a modular pulse converter (MPC). The VGEM turbocharging system simulation model is modified based on the original MPC model; the difference between them is only the exhaust manifold model. The GT-POWER simulations on both steady state and transient state have been done. The results show that in all four loads of 25, 50, 75 and 100%, the average scavenging coefficient of the VGEM turbocharging system is greater than that of the original MPC turbocharging system, while the brake specific fuel consumption (BSFC) is less than that of the original MPC turbocharging system. In the 25% load case, the BSFC could be reduced by 15 g/kW h. The transient analysis shows that the performance of the VGEM turbocharging system is also better than the original.

This paper presents a dual-injection strategy to achieve diesel partial homogeneous charge compression ignition (p-HCCI), which is the combination of HCCI-like combustion and traditional diesel combustion. The dual-injection strategy involves a negative valve overlap (NVO) fuel injection and a traditional fuel injection. The NVO injection occurs during the negative valve overlap period to prepare the homogeneous mixture for HCCI combustion. The traditional injection occurs at the end of the compression stroke to achieve diffused combustion. The results indicate that the NVO injection affects the combustion and emission characteristics in a different way compared to the traditional injection. The increase of NVO injection turns the combustion into HCCI-like combustion and advances the combustion phase. The increase of traditional injection makes the maximum heat release rate (HRR) higher. The emissions results indicate that p-HCCI can greatly reduce the NOx emissions compared to the baseline engine and maintain the same thermal efficiency. However, the NOx emissions increase with the increase of total injection. More traditional injection deteriorates the smoke emissions easily because of little time for fuel to mix with the intake air. The NVO injection ratio has been optimized at medium load according to the emissions and the thermal efficiency, and the optimum result is in the range of 30−40%, which can reduce the NOx emissions by about 40% and the smoke emissions by about 30%. However, the NVO injection should not be used at low load to stabilize the combustion.

•LTC combustion based on two-stage injection is studied.•Trigger injection can control the process of LTC combustion.•Two-stage injection strategy can achieve better fuel economy and lower emissions.Low temperature combustion (LTC) based on two-stage injection is the combustion mode which uses main injection to generate homogeneous charge and trigger injection to ignite. In this paper, the effect of injection ratio and starting-point of injection (SOI) of main injection on LTC were studied. The result shows that the trigger injection can control the process of combustion and make LTC more stable. Using trigger injection can effectively broaden the limits of SOI of the main injection. And the SOI of main injection is quite important for LTC. Inappropriate injection time will deteriorate the performance of LTC. By optimizing the SOI of main injection for specific injection ratios, effective control of the combustion phase can be realized and emissions can be reduced.