This study aims to investigate the impact of circulation injection quantity on the cold start and emission performance of small agricultural diesel engines. The optimal quantity of circulation injection was also determined at critical temperatures. The study focuses on an inline twin-bar high-pressure common rail diesel engine. Using the environment simulation cabin whole engine test platform, several pre-tests were conducted. The results showed that the diesel engine started successfully without opening the preheating device at an altitude of 2 000 m and a critical temperature of -5 ℃. Three types of injections were used in this case. Specifically, the injection time of pre-injection 2, pre-injection 1 and main injection was -22, -14.4 and -7.7 °CA ATDC (crank angle after top dead center), respectively, and the injection duration of pre-injection 2 was 2.5 °CA, and the circulation injection quantity was fixed at 2.5 mg. The pre-injection 1 injection duration was 3 °CA, and the pre-injection amount was fixed at 3 mg. The circulation injection quantity was varied in the main injection volume, with the values of 8, 15, 17, 20, 25, 30, 34, and 40 mg. The target speed was set as 1 050 r/min. The cold start time and emissions of HC, CO, and NOx were measured at various circulation injection quantities during the test. The results indicate that the cold start time of the diesel engine increased or even resulted in the start failure with the larger or smaller amount of circulation fuel injection. As the circulation injection quantity increased, the cold start time initially decreased and then increased again. The shortest start time was 266 s at the 20 mg of circulation injection quantity. The longest start time was 365 s at the 15 mg of circulation injection quantity, with a difference of nearly 100 s. Additionally, the lowest peaks of HC and CO emissions were also observed as 12 984 and 3 008 mg/L, respectively, at 20 mg. The variation in the circulation injection quantity also resulted in an increase in the peak of HC and CO emissions. Furthermore, the peak value of CO emission increased significantly with the larger circulation injection quantity, where the peak value appeared earlier. The circulation injection quantity had a greater effect on the NOx emission peak. The lowest peak of NOx emission was 368 mg/L at the circulation injection quantity of 30 mg. Therefore, there was a significant impact of circulation injection quantity on the cold start and emission performance of diesel engines. The optimal quantity of circulation injection was also beneficial to reducing the start time and emissions for higher generating efficiency and a friendly environment. Furthermore, 20 mg was the optimal circulation injection quantity for the cold start in this small diesel engine at an altitude of 2 000 m and an ambient temperature of -5 ℃. These findings can also provide experimental data to optimize the cold start and emissions of engines. The circulation injection quantity with after-treatment devices can further reduce the cold start emissions.

A diesel engine is the most energy-efficient powerplant in agricultural machinery. However, the performance of cold start has been limited to the diesel fuel viscosity in the low-temperature environment. Fluidity deterioration has also inhibited diesel spray atomization and evaporation, leading to the low reliability of diesel engines. In this study, the backlight test was conducted to compare the variation in the spray macrostructure of high and low-temperature diesel under different working conditions. The digital image processing of the spray images was then carried out using MATLAB to obtain the macroscopic characteristic parameters of the spray. The test results showed that the much more outstanding impact on the injection volume was obtained in the diameter of the spray hole 0.12 mm with the various injection pressures at diesel temperature. The continuous viscous force slowed down the flow velocity of the fluid layer near the wall and then suppressed the diesel emission. Once the diesel temperature was lower than 0 ℃, the diesel temperature was reduced by the amount of injection. The diesel temperature decreased to enhance the viscous force among droplets at high injection pressure. The continuous ejection of diesel was then promoted under the action of kinetic energy and inertial force. The fuel injection amount increased with the decrease in diesel temperature. When the injection pressure was 75 MPa, the diesel temperature dropped to -20 ℃, compared with 38 ℃ diesel injection quantity increased by nearly 23.87%. The diesel temperature decreased under small pore size, leading to the increase of viscous force among droplets. There was an increase in the droplet size and the greater momentum of droplets. Thus, the radial development trend of diesel spray was weakened to enhance the axial movement. Furthermore, the spray penetration increased with the decrease of diesel temperature, whereas, the spray cone angle decreased. The diesel spray penetration increased by 39.89 mm at 0.70 ms when the diesel temperature decreased from 38 ℃ to -20 ℃. When the nozzle diameter was 0.28 mm, the injection pressure was less than 75 MPa, and the injection quantity decreased with the decrease of diesel temperature. More momentum of diesel was consumed to overcome the greater internal friction under the same injection pressure. The farthest distance to reach was weakened. Diesel temperature was reduced to decrease the spray penetration and spray cone angle. When the injection pressure was 75 MPa, the diesel spray penetration at -20 ℃ at 0.70 ms was 30.86 mm shorter than that at 38 ℃. The spray penetration of -20 ℃ diesel decreased with the increasing nozzle diameter at the same injection pressure, whereas, the spray cone angle increased. Secondly, there was a more prominent influence of diesel temperature on the macroscopic characteristics of the spray with the increase in injection pressure. The higher the injection pressure was, the more outstanding the shortening trend of spray penetration caused by the decrease in diesel temperature was. Therefore, the injection pressure and nozzle diameter can be expected to improve for better evaporation and gasification of diesel liquid phase spray at low temperatures.

Cold-start performance has been one of the most restricting factors in the internal combustion power equipment at in extreme cold conditions in plateau areas. The aim of this study was to explore the cold-start characteristics of diesel engines at different ambient temperatures in a plateau. Different strategies of fuel injection were then evaluated for the environmental adaptation. The research object was taken as the diesel engine with direct injection, high-pressure, and common rail. A test chamber with a low-temperature environment and a cold-start test bench were constructed on at the an altitude of 2 000 m. Then, the environmental adaptation experiments were carried out using 3 injection strategies at extreme cold conditions (the lowest ambient temperature was -45 ℃) in the plateau. The cold-start performances of diesel engines were evaluated with the different advanced angles of fuel injection. The results showed that the double injection (DI) and triple injection (TI) strategies were effectively improved the speed stability and reliability of the diesel engine cold-start in extreme conditions, compared with the single injection (SI) strategy. Furthermore, the strategies of fuel injection were ranked in the ascending order of TI < DI < SI, according to the delay timing of the combustion mean ignition. While the mean maximum combustion pressure was ranked in the opposite direction. Specifically, the longest delay of combustion ignition was observed in the SI strategy under cold-start conditions without the assistance of a glow plug, where the ignition delay was 4.5 ℃A after the top dead center. The ignition delay of the TI strategy was advanced by 5.6 ℃A, compared with the SI strategy, indicating the enhanced proportion of pre-mixed combustion. And tThen the uniform, stable, and high performance was achieved in the cold-start combustion of the diesel engine with the TI strategy. The optimal advance angle of fuel injection appropriately increased for the combustion of cold-start, which was 30 °CA BTDC. The largest peak of HC emission was generated by the diesel combustion using the SI strategy, while the middle and smallest peaks were found using the DI and TI strategy, respectively. Among them, the peak of HC emissions that generated by diesel combustion with the TI strategy decreased by 58.6% and 64.8% at 20 and -5 °C, respectively, compared with the SI strategy. The engine was started unsuccessfully at the temperatures of -25 °C and below for the SI strategy and -45 °C for the DI strategy. By contrast, the TI strategy was enabled on the engine cold-start performance in at one time in a wide temperature range of -45~20 °C. Particularly, the cold-start success rate was significantly improved for the average acceleration, in order to shorten the cold-start duration. Therefore, the diesel engine cold-start time of the SI strategy was the longest, the DI strategy was in the middle, and the TI strategy was the shortest under the cold-start conditions of consistent temperature. TI strategy was greatly improved the cold-start performance and environmental adaptation of the diesel engine under extreme cold conditions in the plateau. The research findings can also provide a strong reference to improve the cold-start performance and environmental adaptation of internal combustion power equipment under the a wide range of ambient temperatures in multiple regions.