Fundamental Study of Diesel-Piloted Natural Gas Direct Injection Under Different Operating Conditions

Abstract

Natural gas as an alternative fuel in engine applications substantially reduces both pollutant and greenhouse gas emissions. High pressure dual fuel (HPDF) direct injection of natural gas and diesel pilot has the potential to minimize methane slip from gas engines and increase the fuel flexibility, while retaining the high efficiency of a diesel engine. Speed and load variations as well as various strategies for emission reduction entail a wide range of different operating conditions. The influence of these operating conditions on the ignition and combustion process is investigated on a rapid compression expansion machine (RCEM). By combining simultaneous shadowgraphy (SG) and OH* imaging with heat release rate analysis, an improved understanding of the ignition and combustion process is established. At high temperatures and pressures, the reduced pilot ignition delay and lift-off length minimize the effect of natural gas jet entrainment on pilot mixture formation. A simple geometrical constraint was found to reflect the susceptibility for misfiring. At the same time, natural gas ignition is delayed by the early pilot ignition close to the injector tip. The shape of heat release is only marginally affected by the operating conditions and mainly determined by the degree of premixing at the time of gas jet ignition. Luminescence from the sooting natural gas flame is generally only detected after the flame extends across the whole gas jet at peak heat release rate. Termination of gas injection at this time was confirmed to effectively suppress soot formation, while a strongly sooting pilot seems to intensify soot formation within the natural gas jet.

A Review of Hydrogen Direct Injection for Internal Combustion Engines: Towards Carbon-Free Combustion

“…Under an assumption that a maximized overlapping of diesel and gas jets after the auto-ignition of diesel fuel will yield the best performance, a converging injector configuration with small angle (a few degrees) was recommended [88]. The effect of the dual-fuel jet interaction under engine-relevant reactive conditions was studied by Fink et al [89,90] in a RCEM. Different injector orientations were used to change the degree of jet interaction-the gas injection angle was changed by rotating the injector while the diesel jet was fixed.…”

“…Fink et al [90] investigated the effects of ambient pressure and temperature on dual-fuel combustion using shadowgraphy and OH-chemiluminescence in a RCEM. It was found that the ignition of gas jet becomes possible at a wider range of injector orientations and relative SOIs with an increasing ambient pressure and temperature (i.e., from 780 to 920 K).…”

“…The injection pressure limit of the HPDI 2.0 system is around 600 bar for both diesel and CNG. Previous studies [89,90] employed a prototype injector by L'Orange from Germany to inject CNG at high pressure of up to 330 bar. However, no information about the applicability of this prototype for hydrogen injection is available.…”

Heat release rate and emissions regimes of stratified pilot-ignited direct-injection natural gas combustion

“…In addition to the NG stratification, several studies have concluded that complex interactions between the NG jet and pilot jet and ignition are critical to PIDING combustion performance. 27–29 Using a rapid compression-expansion machine (RCEM), Fink et al 28 demonstrated that for positive RIT where pilot ignition occurs undisturbed by the NG jet, a wide range of relative spray angles can be used to produce robust NG ignition. However, when there is temporal overlap of the pilot and NG jets (i.e.…”

Investigation of the Combustion Kinetics Process in a High-Pressure Direct Injection Natural Gas Marine Engine

“…In order to further understand the combustion process for HPDI engines, numerous efforts have been performed to investigate the effects of various injection and air-exchange strategies on the engine performance and emission characteristics. ,− The combustion process of HPDI engines is more complicated than traditional diesel and gasoline engines. − To get better understanding of the combustion process, it is essential to well reveal the detailed in-cylinder combustion process. Optical diagnostics techniques have been playing a significant role in visualizing the engine flow, spray, combustion, and emission formation processes, which contributes to the development of fluid dynamics models (CFD) .…”