Effects of diesel fuel temperature on fuel flow and spray characteristics
Introduction
Diesel engines are recently receiving more attention in the passenger car market due to their high fuel efficiency and torque characteristics compared with gasoline engines. However, the cold start issue has made trouble in diesel engines especially, below-zero temperatures because combustion processes of diesel engines are governed by the air–fuel mixing process and compression heat [1], [2]. Under very low ambient temperatures, the cylinder head and block absorb most of compression heat, thus preventing ignition [3]. Also, the low compression temperature suppresses fuel vaporization process, leading to incomplete combustion [4]. Changes in diesel fuel properties, such as density and viscosity, according to the lowered ambient temperature also affect the cold start [5]. In addition, emissions regulation, which is becoming increasingly stricter on nitrogen oxides (NOx) and particulate matter (PM), is forcing the compression ratio of diesel engine to be lowered [6], [7], [8], [9]. This trends deteriorate the cold start problem. The cold startability or start delay is defined as the ability of an engine to start within a specified time and continue to run without any malfunction [3]. If the cold startability is aggravated, unburned hydrocarbon (HC), PM and carbon monoxide (CO) would be increased during the cold start due to the incomplete combustion [3], [10], [11]. Stability in idling will also be worsened. With regard to the cold startability issues, several studies related to the fuel injection of diesel engine during the cold start have been conducted. A spilt injection strategy during the cold start is known to be an effective way to improve the cold startability [12], [13], [14]. Chartier et al. reported that the cold startability under the ambient temperature of 244 K could be improved by the application of three pilot injections [12]. Zhong et al. revealed that split-main injection strategies could improve the cold startability and, at the same time, reduce the injected fuel mass and unburned HC emissions by almost 50% [13]. Payri et al. also showed that multiple injection strategies could improve the idling stability after the cold start [14]. However, inaccurate injection quantity and timing may lead to a lower load and unstable combustion during the cold start. Research about spray characteristics of biodiesel fuel in various fuel and ambient temperature have been conducted [15]. The results showed that evaporation of injected fuel was suppressed as the fuel temperature decreased from 360 K to 300 K, which resulted in lengthened liquid fuel penetration and decreased vapor fuel mass.
From previous studies, it is known that the injection control according to the fuel/ambient temperature affected the cold startability of diesel engines significantly, indicating that precise injection control during the cold start is required. Thus, in this study, the effects of diesel fuel temperature on fuel flow and macroscopic spray characteristics with a wide range of fuel temperature from 243 K to 313 K were investigated for more precise injection control. The experiments were conducted using a common-rail injection system and a constant-volume combustion chamber (CVCC) under the cold ambient condition. The correction factors for the injection quantity and timing were also derived according to the fuel temperature variation.
Section snippets
Experimental setup
Fig. 1 shows a schematics of fuel injection system. A fuel tank was designed to control the fuel temperature with a range from 243 K to 313 K. The fuel tank has a refrigerator and an electric heater, thus the diesel fuel temperature can be controlled. A Bosch common-rail fuel injection system including a high pressure fuel pump driven by an electric motor and a solenoid type injector with seven-hole was utilized. To maintain the fuel temperature in the injector and the rail-to-injector fuel tube,
The fuel temperature effect on injection quantity
Fig. 4(a) and (b) shows the injection quantity according to the fuel temperature with the injection pressures of 30 and 100 MPa, respectively. The ‘difference to 313 K’ was calculated with the following equation.where
The difference means that relative ratio of injection quantities based on fuel temperature of 313 K case. The results showed
Conclusions
In the present work, the effects of diesel fuel temperature on fuel flow and macroscopic image characteristics were investigated to provide insight for improving cold startability of diesel engines using a common-rail system and a CVCC. The major findings from this research are summarized as follows:
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The injection quantity decreased gradually as the fuel temperature decreased from 313 K to 243 K under the same energizing time. It was also found that the difference in the injection quantity at a
Acknowledgements
The authors would like to appreciate Hyundai Motor Company for financial and technical support of this research.
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