Impact of Diesel Nozzle with Micro-Orifice on Performance and Emission Characteristics of Diesel-Fuel and Natural-Gas Compression-Ignition Engine

Abstract

The diesel fuel/natural gas compression ignition engine is an essential approach to achieving low-carbon goals. The injection parameters of the diesel assume a critical role in the combustion process of mixture formation and ignition of methane. The micro-orifice diesel fuel nozzle was developed based on the spray mixture theory of diesel injection in a modified engine. The combustion and emissions of injection strategy of rail pressure, multiple injections and injection timing, and different rate of substitution (Rs) under two loads are discussed and compared with the original nozzle in the compression ignition engine. The results show that the combustion center of the micro-orifice nozzle moves forward with increasing Rs. In contrast, the original nozzle presents an opposite trend. The combustion and emissions of the two nozzles are almost identical at high Rs. The atomization improvement of reduced orifice size does not significantly affect the performance and emissions with a single injection strategy. The thermal efficiency of the micro-orifice nozzle reached 43.2% at high Rs, and the total hydrocarbons (THC) was reduced by 85% compared to the original nozzle, which is a main reason for the increased efficiency at high load. The effect of improved atomization by higher pressure in multiple injections is weaker than the effect of the insufficient wet wall caused by excessive penetration. The micro-orifice nozzle with early injection and low pressure of 70 MPa can improve the atomization and shorten the diesel spray penetration, reducing unburned THC emissions and achieving higher thermal efficiency. The closer the pre-injection timing is to top dead center (TDC), the earlier the combustion starts and the higher the maximum heat release rate (HRR). The definition of the combustion delay in a traditional diesel engine is not easy to clearly describe the fuel and oxygen mixing process in the cylinder during diesel pre-injection and natural gas (NG) premixing. The shape of the HRR of the micro-orifice nozzle at different main injection timing has always maintained a similar shape of single-peak HRR, which only shifts back and forth with changes of the main injection timing. The micro-orifice nozzle with pre-injection strategy forms a homogeneous mixture in the cylinder. At the same time, the main injection of diesel is only used to stabilize the ignition. The THC emissions and thermal efficiency are not sensitive to the main injection timing. They are maintained at an ideal level, freeing the diesel engine efficiency from the limitation of the diesel injection timing. Hence, the micro-orifice nozzle combined with the two-injection strategies achieves ideal premixed combustion.