Evaluating the energy efficiency of tractors under actual operating conditions

Agricultural machinery is ever-increasingly either displacing or augmenting human labor with the development of the economy and technology. Among them, fuel efficiency has a significant impact on energy conservation and emission reduction, indicating high economic and social significance. Particularly, a tractor is one of the most typical agricultural machinery. In this study, the overall energy efficiency of tractors was evaluated under actual operating conditions. The performance of tractors was also measured comprehensively. The actual energy efficiency of tractors was clearly defined and calculated as well. The common operating points of tractor engines were extracted by K-means clustering and pairwise comparison matrix, according to the shortest distance partition. The energy efficiency of 186 162 kW tractors was analyzed to propose the classification standard using tractor energy efficiency. Energy efficiency limits and energy efficiency ratio limits were determined at all levels. The average energy efficiency was compared using different operating links. The results showed that there was a significant difference in the distribution and weight between the actual eight operating points and the ISO steady-state eight operating points. The main reason was that the ISO steady-state eight operating point was targeted at non-road diesel engines, covering a wide range of engine models, power, and applications. However, the actual eight operating points were more specific, and refined for the actual operating conditions of multiple 162 kW tractor engines. The error range of tractor energy efficiency was −0.483-0.487 (kW·h)/kg using two datasets of working conditions. Therefore, the energy efficiency calculation was necessary and meaningful using the actual eight working conditions. In addition, there was a significant difference in the tractor energy efficiency using actual operating conditions, where 50% of tractor energy efficiency values were distributed in the range of 3.20 to 3.65 (kW·h)/kg, with an average value of 3.42 (kW·h)/kg. This difference was attributed to the driving habits of agricultural machinery operators, the tractor maintenance and repair levels, as well as the tractor age, resulting in the varying degrees of reduction in engine performance indicators. Then, the grading was constructed using the tractor energy efficiency, where 3.07 (kW·h)/kg was determined as the qualification limit of actual energy efficiency in the tractor. The limit values were also determined for the tractor energy efficiency ratio for levels 1-4. As such, the classification of energy efficiency levels was achieved for tractors. There was a significant variation in the average energy efficiency at different operation stages of each tractor. The rotary tillage mode presented the highest value of average energy efficiency, whereas, the walking mode was the lowest. The trend was attributed to the different engine loads of tractors at different operating stages. Therefore, there was great potential for the real-world energy efficiency performance of tractors from a user perspective. The actual operating performance of tractors was explored to establish an evaluation system applicable to the energy efficiency performance of tractors in the real world. The findings can provide the basic data for energy-saving and emission reduction in tractor engines. A strong reference can be also offered to assess the energy efficiency of agricultural machinery on application subsidy suitable for the level of green operation.