Effect of Injection Strategies and Air Injection on Combustion Characteristics in CNG-DI Engine

Abstract

In the present study, the performance of Compressed Natural Gas (CNG) engine has been investigated under various fuel injection strategies and Air-injected combustion conditions. A single-cylinder optical engine was used as a test engine, and two outward-opening poppet-valve injectors were applied on Direct Injection (DI) and Port Fuel Injection (PFI), respectively. The in-cylinder pressure, intake air flow rate, and flame images were acquired from the engine experiment. Indicated Mean Effective Pressure (IMEP) was used to compare the engine performance, and the fuel conversion efficiency was calculated from the IMEP and volumetric efficiency. The high-speed sequential flame images revealed the difference in the flame propagation speeds under various test conditions. An image processing program was used to brighten the flame images up and detect the flame boundary. 1-D simulation program, AMESim, was also used to further investigate the relation between the CNG injection and the volumetric efficiency. The experimental and numerical results showed that the CNG-DI and CNG-PFI engine had the similar engine performance under the early fuel injection during the intake stroke. For the early fuel injection before the Intake Valve Close (IVC), the volumetric efficiency of the DI and PFI were both about 38.62% and the standard deviations were less than 0.2%. However, the combustion of CNG-DI was strongly affected by the injection timing. As the injection timing was retarded toward the IVC, the combustion speed and the volumetric efficiency of the CNG-DI were increased while the CNG-PFI was not changed. The late fuel injection increased the combustion speed because the short time interval between the injection and ignition induced strong turbulence. The flame propagation speed was directly identified in the flame images. The volumetric efficiency was increased by about 4% under the late fuel injection around the IVC compared to the early fuel injection during the intake stroke. The early fuel injection cases had the lower volumetric efficiency because the fuel volume in the cylinder interfered the intake process. Conversely, the late fuel injection cases had the higher volumetric efficiency because the fuel injected at around the IVC could not affect the intake process. This result implies that a CNG-DI engine can achieve higher engine torque under the late fuel injection condition. On the other hand, the fuel injection pressure was not an important factor for the combustion characteristics. The three different injection pressure conditions of 0.5, 0.8, and 1.1 MPa were tested, and showed the similar combustion procedure under the same injection timing condition. However, this result showed that the End of Injection (EOI) timing rather than the Start of Injection (SOI) timing was more critical factor for the combustion characteristics. When the SOI was fixed, the EOI depended on the injection pressure, and caused the difference in the combustion characteristics. However, if the combustion characteristics were reorganized based on the EOI, the combustion characteristics did not depend on the injection pressure. Even though the turbulent flow induced by the fuel jet increased the combustion speed and engine efficiency, too late fuel injection caused poor combustion due to the lack of time to form homogeneous air–fuel mixture. Therefore, in the present study, the air and N2 was injected instead of fuel to supply the additional turbulence. The fuel was injected by the PFI injector. The air injection greatly increased the IMEP and engine efficiency. There were four factors that improved the combustion characteristics in the Air-injected combustion: faster combustion speed induced by the strong turbulence and high O2 concentration, increase of in-cylinder pressure by the assistance gas injection, and higher combustion efficiency under the lean combustion condition. The effect of turbulence was already identified by comparing the various fuel injection timing conditions. The effect of O2 concentration on the combustion speed was also identified by comparing the Air- and N2-injected combustion. Because the N2 injection reduced the O2 concentration whereas the air injection increased it, the air injection yielded the faster combustion. According to the experimental results, the air injection at the Before Top Dead Center (BTDC) 70° increased the fuel conversion efficiency by about 3.5% compared to the normal combustion conditions without the air injection. Therefore, it was concluded that the Air-injected combustion is one of effective combustion strategies that increase engine efficiency.