Impact of fuel spray angles and injection timing on the combustion and emission characteristics of a high-speed diesel engine
Introduction
The demands for diesel engines has been increasing because of their higher thermal efficiency compared to gasoline engines. However, NOx (nitrogen oxides) and PM (particulate matter) from the combustion process of diesel engines have raised concerns related to environmental pollution and conservation. In order to reduce the atmospheric contamination due to exhaust gases from automobiles, international emission regulations have increasingly become more stringent. Therefore, various eco-friendly combustion technologies, such as the use of alternative fuels (biodiesel [1], [2], dimethyl-ether [3], [4], [5], bioethanol [6], [7] etc.), RCCI (reactivity controlled compression ignition) [8], and LTC (low temperature combustion) [9], [10], in a diesel engine have been actively studied by many researchers to reduce exhaust emissions while maintaining combustion performance.
In high-speed direct injection diesel engines, the atomization characteristics of fuel spray play an important role in the combustion characteristics because the formation process of a mixture between the atomized spray and the entrained air have direct influence on the emission formation and combustion performance. The common-rail injection system, consisting of a solenoid type injector and a high pressure chamber, show good performance in regard to the fast response time and fuel atomization. In addition, the injection timing and spray angle have a great influence on the combustion and emission characteristics in a common-rail diesel engine because the targeting points at the surface of the piston bowl are determined by the spray angle and the injection timing. Mobasheri and Peng [11] revealed that the narrow-spray-angle injector can reduce the NOx and soot emission without the deterioration of fuel consumption because of the improvement of the air-fuel mixture quality. Yoon et al. [12] researched the effect of the spray angle and injection strategy on the combustion of DME (dimethyl ether), its emissions, and particle size distribution characteristics in a common-rail diesel engine. The results revealed that the combustion pressure from single combustion for narrow-angle injectors (60° and 70°) increased, with advanced injection timing compared to the results of the wide-angle injector (156°). In the diesel engine system, the spray targeting point is very important because wall wetting of the injected fuel causes UHC (unburned hydrocarbon), and the degree of utilization of oxygen in the combustion process highly depends on the targeting point [13], [14], [15]. In addition, combustion performance deteriorates, and knocking and large amounts of NOx emissions are caused by rapid combustion in the fuel-rich regions. Therefore, investigations have been carried out by various researchers [16], [17], [18], [19] that aimed the determination of the optimal spray angle. Fang et al. [19] reported that narrow angle injector with a 70° spray angle resulted in higher soot emissions due to the fuel film deposition on the piston wall, and in lower NOx emission due to the rich air/fuel mixture near the piston bowl wall. The ignition timing and combustion characteristics in the diesel combustion process are also significantly affected by the injection timing. Recently, researches [20], [21], [22], [23], [24] examined various injection timings as well as very early- and late-injection timings, and have studied the reduction of exhaust emissions using methods such as the HCCI (homogenous charge compression ignition) and LTC. Saravanan et al. [21] revealed that a decrease in NOx emission without deterioration of smoke emissions can be achieved by the retardation of injection timing. However, they reported that the brake thermal efficiency was slightly decreased by the retardation of the injection timing. In the research study of Kannan and Anand [22], it was revealed that the brake thermal efficiency was improved, and the reduction of NOx and smoke emissions could be achieved by employing advanced injection timing with a high injection pressure.
In this paper, the effects of spray angle and injection timing on combustion and exhaust emissions characteristics are experimentally investigated in a single cylinder diesel engine. In particular, broad test ranges are used for the injection timing, with values of 40° BTDC (before top dead center) to TDC (top dead center). In addition, the spray behavior under different injection and ambient pressures is studied to obtain an accurate understanding of combustion and emission characteristics.
Section snippets
Spray visualization system and image analysis procedure
The spray visualization system consists of a high speed camera (Fastcam APX-RS, Photron), metal-halide lamps (HVC-SL, Photron), a fuel injection system, and the spray image storing devices, as illustrated in Fig. 1. These were needed in order to measure the spray characteristics from two diesel injectors at different spray angle. In this study, two injectors were utilized with spray angles of 156° and 60°. Based on the 156° spray angle injector (conventional injector), the characteristics of
Spray behavior of two different spray angle injectors
Fig. 3 shows the comparison of the spray development process of conventional (156°) and narrow (60°) spray angle injectors. Test conditions include a 1200 bar injection pressure, a 6 mg injection quantity, and a 25 bar ambient gas pressure. As mentioned in previous reports, the increase of the ambient gas pressure, which caused an increase of the ambient gas density, led to a slow spray development [25], [26], [27]. Fig. 3 shows the spray images from the narrow angle injector, which seem to be
Conclusions
We performed an experimental study on combustion, emission, spray behavior analysis and performance characteristics, for a diesel engine with two spray angles and various injection timings. The conclusions are summarized as follows:
- (1)
Regarding the spray behaviors of the two spray angles, the increase in the ambient gas pressure led to a decrease in the spray development. The development of the spray from the injector with the narrow spray angle was longer than that from the injector with a
Acknowledgment
This study was financially supported by Basic Science Research Program (2014R1A1A2057805) and Basic Research Laboratory Program (2015R1A4A1041746) through the National Research Foundation of Korea (NRF) funded by the Ministry of Education.
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These authors contributed equally to this work.