The Selection of an Energy-Saving Engine Mode Based on the Power Delivery and Fuel Consumption of a 95 kW Tractor during Rotary Tillage

Abstract

The objective of this study was to estimate power delivery efficiency and fuel consumption based on engine modes. In this study, a 95 kW power-shift tractor was used to analyze power delivery and estimate fuel consumption during rotary tillage. Rotary tillage was conducted in a field experiment with the conventional, APS (auto power shift) power, and APS ECO engine modes. To analyze the field conditions, the soil hardness and soil water content were measured, and soil samples were collected from the experimental site to analyze the soil texture by using the USDA soil texture triangle. Finally, an efficient and suitable engine mode was selected for rotary tillage based on the working load. It was observed that the power delivery and tractive efficiencies when using the APS power mode were the highest among other engine modes, accounting for around 89.23 and 73.45%, respectively. However, the fuel consumption when using the APS power mode was approximately 23.02 L/h, which was also comparatively higher than that of the other engine modes. Additionally, the tractive efficiencies of each engine mode were compared using the Brixius prediction model. The statistical analysis of the predicted tractive efficiencies and those in the tests showed that there were no significant differences among the engine modes; this indicates that the APS controller could perform with high accuracy. In the conventional mode, the power delivery, tractive efficiency, and fuel consumption were approximately 66.48%, 55.89%, and 17.04 L/h, respectively, which were comparatively low. However, the slip ratio in the conventional mode was 18.80%, which was higher than that in the APS power and APS ECO modes. On the other hand, PDE, TE, and fuel consumption when using APS ECO were around 77.57%, 58.44%, and 19.39 L/h, respectively, which were higher than those of the conventional mode, but lower than those of the APS power mode. Furthermore, the comparative analysis showed that the working loads in the APS ECO mode were located in the ungoverned region and were very close to the engine’s maximum torque, which could allow sudden and unwanted engine turn-off due to the fluctuations in working loads, which is to be avoided. The fuel consumption was also comparatively low. However, the working loads in the conventional and APS power modes were located in the governed region, which was outside the engine’s operating range. Therefore, we recommend that users operate tractors in the APS ECO engine mode for rotary tillage, considering fuel economics and high working loads.

1. Introduction

Tractors are mainly off-road vehicles that are used to conduct agricultural operations such as plow tillage [1], rotary tillage [2], baler operation [3], transportation with a trailer [4], construction works with a loader [5], and so on. For these operations, it is important to operate tractors efficiently.

A tractor’s performance includes appropriate power delivery and low fuel consumption. Power delivery depends on the working loads [6]. It was observed that approximately 84% of engine-rated power was consumed during rotary tillage, and that minimal power was consumed during loader operation. Lee et al. [7] investigated the ballast, tire inflation pressure, transmission gear, engine speed, and workload as control variables of fuel efficiency. They developed a linear regression model that could estimate fuel efficiency. Lee et al. [8] developed an engine throttle control system, which was a load-sensitive control system for a tractor. In their control system, the proportional–integral (PI) was applied to maximize the fuel efficiency of the tractor during plow tillage.

Globally, scholars have attempted to determine parameters that can reduce the fuel consumption of tractors during agricultural operations. Mileusnić et al. [9] conducted various field operations to compare the fuel consumption of tractors, but the performance of these tractors was not analyzed based on the working load. Moinfar et al. [10] analyzed the rolling resistance, slip ratio, tractive efficiency, and fuel consumption considering the driving system, ballast weight, and tire pressure. In their study, the factors investigated could improve a tractor’s performance. They stated that the ballast weight, tire inflation pressure, and type of driving system were the most significant factors. Additionally, the highest traction efficiency was obtained for a 4WD system, as the working width was increased. Finally, they suggested that the 4WD system was better than other driving systems when considering traction efficiency and minimal fuel consumption. Shafaei et al. [11] stated that tillage depth is more highly prioritized than speed for power delivery efficiency, because tillage depth indicates the working load. Greater tillage depth occurred with greater working loads. In their study, an automatic slip control system that would be effective at a slip lower than 0.2 was developed while considering the tractive efficiency. Finally, they highlighted that the slip efficiency, as well as the overall power efficiency, highly depended on tillage depth rather than speed. Han et al. [12] developed a cooperative control method that considered engine load and tillage depth to overcome the traditional draft position. They mentioned that this control method aimed to provide high power and low fuel consumption based on tillage depth, because they believed that tillage depth represented the working loads. The control method developed in their study ensured the tillage quality and ideal engine power. In addition, Kim et al. [13] reported that tillage depth had a wide range of effects on the working load of a tractor. They conducted plow tillage in various soil layers and predicted the soil–tool interactions. Kim et al. [14] conducted plow tillage to analyze power transmission efficiency based on the tillage depth of a 42 kW tractor. They also reported that the overall power requirement was significantly increased due to the working load.

Furthermore, Zhang et al. [15] proposed a joint control method for reducing the energy consumption of an electric tractor during plow tillage. In their study, low traction efficiency and significant energy consumption due to the excessive wheel slip were addressed. They also reported that the drive torque distribution was one of the most crucial factors for higher energy consumption. Finally, they suggested that the slip, traction efficiency, and energy consumption were the best in torque distribution mode. However, the energy consumption was analyzed as the target tractor was an electric tractor. In research conducted on hybrid tractors to save energy, an energy-saving control strategy based on instantaneous optimization was proposed, focusing on minimizing the fuel consumption of the hybrid tractor in plowing and rotary tillage. It was found that the fuel consumption for plowing and rotary tillage decreased by approximately 6.31% and 4.70%, respectively [16,17].

Apart from what was described above, the working load is the most important parameter for predicting the fuel consumption, performance, and the power delivery efficiency of a tractor. However, engine modes were not used to predict the tractors’ performance. The engine mode operates as the engine is loaded. Siddique et al. [18] estimated fuel consumption based on the engine load levels by using a PTO (power take-off) dynamometer test and developed the simulation model using AMESim software (version 16, SIEMENS AG, Munich, Germany); the correlations were statistically analyzed and compared with those in the previous literature. Siddique et al. [19] also stated that tractor fuel consumption highly depends on the load that occurs during agricultural operations and the gear stages used for the operations. In their study, fuel consumption was analyzed in both a simulation model and an experimental field test, and the results were statistically correlated. However, power delivery efficiencies were not analyzed during rotary tillage operations based on engine modes.

Therefore, this study focused on analyzing power delivery efficiencies in various engine modes. The tractive efficiencies of all engine modes were compared; finally, the most effective energy-saving engine mode for use during rotary tillage was selected. The specific objectives were as follows:

(i)

To conduct rotary tillage operation in various engine modes.

(ii)

To analyze power delivery efficiencies.

(iii)

To compare tractive performances using the Brixius method [20].

(iv)

To suggest an energy-saving engine mode for use during rotary tillage.

2. Materials and Methods

2.1. Tractor Configurations

A 95 kW partial PST tractor (T130, TYM Co., Ltd., Gongju, Republic of Korea) was used to conduct the field experiment. The tractor powertrain was composed of 2 high and low power shifts, 3 power-shift-type main clutches, 3 mechanical-type range shifts, and 2 creeps. The PST for forward and reverse was made of 36 × 36 gear stage combinations. The engine’s rated power was 95 kW, and the rated torque of the engine was 415 Nm at a rated speed of 2200 rpm. A map of the characteristics of the 95 kW used in this study is shown in Figure 1. The specifications of the tractor in this study are listed in

Table 1. The tractor specifications used in this study.

Parameters		Specifications
		Front Axle	Rear Axle
Model		T130, TYM Co. Ltd., Korea
Weight	Overall weight (N)	44,587
	Distribution (%)	40.30	59.70
Tire	Model	380/85R24	460/85R38
	Diameter (m)	1.256	1.770
	Section width (m)	0.380	0.496
	Section height (m)	0.323	0.496

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2.2. Data Acquisition System for Load Measurement

The tractor was configured to measure the field load, and the sensor installation locations are shown in Figure 2. The engine’s rotational speed and its load were measured using the CAN (controller area network) protocol (J1939) with an ECU (engine control unit). The fuel consumption was also measured in real-time from ECU and logged using the CAN protocol during the rotary tillage. In addition, four-wheel torque meters with radio-telemetry type were installed on the four wheels of the tractor to measure the load generated during rotary tillage and were configured with a torque of 20 kNm and a rotational speed of 120 rpm. Proximity sensors (MP-981, Ono Sokki, Tech. Inc., Yokohama, Japan) with a measurement of up to 20,000 rpm were used to measure the axle rotational speeds. The data monitoring system was capable of displaying real-time data to users during the field experiments, as well as storing the measured data in separate storage devices for further analysis and evaluation of the tractor’s performance. The overall configuration of the experimental tractor is shown in Figure 2.

2.3. Field Conditions for Rotary Tillage

The soil samples from the selected experimental site were collected, and the soil texture was analyzed using the USDA soil texture triangle. The hardness of the soil (cone index, CI), which is an important factor, can increase the fuel consumption by around 3% with an increase in soil hardness [21]. The soil hardness of the experimental site was determined to be around 3948 kPa. The electrical conductivity (EC) was also measured to determine the field conditions. Soil samples were randomly collected at 5 points at each experiment site, and their physical and mechanical properties were measured. The results of the soil analysis results for the field are listed in

Table 2. The soil analysis of the experimental site for all engine modes.

Parameter	Values
Soil component (%)	Sand: 27.83, Silt: 56.67, Clay: 15.50
Soil texture	Silt loam
EC (ds/m)	1.18
Shear stress (kgf.cm)	155.67
Cone index (kPa)	3948
Water content (%)	18.66
Temperature (°C)	23.82

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2.4. Operational Conditions for Rotary Tillage

The field experiment was conducted with the conventional, APS power, and APS ECO engine modes for rotary tillage. Each operation was replicated 3 times to analyze the power delivery efficiency and fuel consumption of the tractor in order to evaluate its performance. The theoretical velocity of the tractor for each gear stage was calculated according to Equation (1) [13]:

$$V_t=\frac{\pi \times D_w\times N_a\times GR\times 60}{1000},$$

(1)

where $V_t$ is the theoretical velocity of the tractor (km/h), $N_a$ is the rear axle rotational speed (rpm), $D_w$ is the diameter of the rear wheel (m), and $GR$ is the final gear ratio for target gear stages (L3 and L4), which are listed in

Table 3. The operational conditions for rotary tillage.

Parameter	Gear Stages	Gear Ratio
Conventional	L3	0.009
APS power	L4	0.007
APS ECO

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2.5. Control Algorithm of the Engine Modes for Rotary Tillage

In this study, the engine mode acts as an engine load that can maintain the tractor’s velocity. During rotary tillage, the sub-shift L was selected. Then, the working speed and gear stage needed to be set; this could be obtained by using the following Equations (2) and (3):

$$X_O={\alpha }_O,$$

(2)

$$Y_O={\mathrm{m}}_O\mathrm{Z}+{\alpha }_O{X}_O,$$

(3)

where $X_O$ is the dial for adjusting the ECO or power mode during operation (%), ${\alpha }_O$ is the rotational speed (rpm) of the ECO or power mode during operation, $Y_O$ is the action of the accelerator pedal (km/h) during operation, ${\mathrm{m}}_O$ is the engine load setting (%), and $\mathrm{Z}$ is the dial for adjusting the speed of the ECO or power mode during operation (%). An APS controller is required to adjust the speed and gear stage, which can be confirmed with Equations (4) and (5):

$$Y_O>k_{e}Z,$$

(4)

$$\mathrm{M}>\mathrm{N},$$

(5)

where $k_e$ is the engine load (%). If $k_{e}Z$ is not higher than the accelerator speed, the APS controller should shift the gear stage down. If $k_{e}Z$ is greater than the accelerator speed, the APS controller should follow Equation (5). $\mathrm{M}$ is the main clutch setting and $\mathrm{N}$ is the current main clutch. If $\mathrm{M}$ is greater than $\mathrm{N}$, the APS controller should shift the gear stage up. If not, the controller should maintain the current main clutch. In addition, the APS controller can be manually adjusted, as shown in Figure 3.

2.6. Estimated Power Delivery and Fuel Consumption in Field Tests

The engine is the tractor’s main power source. Engine power is traditionally divided into the driving power (axle power), PTO power, and hydraulic power. In this study, the axle power and PTO power of the tractor were considered. Therefore, the engine power of the tractor can be obtained by using Equation (6) [22,23], and it was measured during the rotary tillage operation:

$$P_e=\frac{2\times \mathsf{\pi }\times T_e\times N_e}{\mathrm{60,000}},$$

(6)

where $P_e$ is the engine power (kW), $T_e$ is the engine torque (Nm), and $N_e$ is the engine rotational speed (rpm). The axle and PTO power were calculated by using Equations (7) and (8):

$$P_a=\frac{2\times \mathsf{\pi }\times T_a\times N_a}{\mathrm{60,000}},$$

(7)

$$P_{PTO}=\frac{2\times \pi \times T_{PTO}\times N_{PTO}}{\mathrm{60,000}},$$

(8)

$$P_{db}=\frac{D\times V_a}{3.6},$$

(9)

$$D=F_a+F_b+F_c,$$

(10)

where $P_a$, $P_{PTO}$, and $P_{db}$ are the axle, PTO, and drawbar power (kW), respectively; $T_a$ and $T_{PTO}$ are the axle and PTO torque (Nm), respectively; $N_a$ and $N_{PTO}$ are the axle and PTO rotational speed (rpm), respectively; $V_a$ is the actual velocity of the tractor (km/h); $D$ is the draft force (kN) applied to the six-component load cell; $F_a$, $F_b$, and $F_c$ are the forces (kN) measured by the load cell for each position. Therefore, the tractive efficiencies (TEs) and power delivery efficiencies (PDEs) for the conventional, APS power, and APS ECO modes were obtained via Equations (11) and (12):

$$TE=\frac{P_{db}}{P_a}\times 100,$$

(11)

$$PDE=\frac{P_e}{P_{rated}}\times 100,$$

(12)

where $TE$ is the tractive efficiency (%) for each engine mode, and $PDE$ is the power delivery efficiency (%) for each engine mode.

In addition, it is noted that the fuel consumption was directly measured with the ECU and logged by using the CAN protocol during rotary tillage in each engine mode. Finally, the fuel consumption measured in each engine mode was analyzed.

2.7. Predicted Traction Model

There are several mathematical expressions for predicting the traction of a tractor, such as those of Wismer and Luth [24], Brixius [24], Upadhyaya et al. [25], and Zoz and Brixius [20], which were developed for the interaction of the traction device and the soil parameters. Among them, the Brixius model is the most commonly used to predict the traction of a tractor. In this study, the Brixius model was used to evaluate the traction of the tractor, and is shown in Equations (13)–(15) [26]:

$$B_n=(\frac{bdCI}{F_w})(\frac{1+5\frac{\delta }{h}}{1+3\frac{b}{d}}),$$

(13)

$$\mathrm{k}=0.88\left(1-e^{-0.1B_n}\right)\left(1-e^{-7.5S}\right)-[\frac{1}{B_n}+\frac{0.5S}{\sqrt{B_n}}],$$

(14)

$$\rho =\frac{1}{B_n}+\frac{0.5S}{\sqrt{B_n}}+0.04,$$

(15)

where $B_n$ is the mobility number; $b$ is the unloaded tire width (m); $d$ is the unloaded tire diameter (m); $CI$ is the cone index (kPa); $\delta$ is the tire deflection under load (m); $h$ is the tire section height (m); $F_w$ is the tire load (N); $\mathrm{k}$ is the traction coefficient; $S$ is the slip; and $\rho$ is the rolling resistance coefficient.

2.8. Statistical Analysis

In this study, statistical approaches were used to analyze the power delivery measured for various engine modes during rotary tillage. A one-way ANOVA (analysis of variance) test and Duncan’s multiple-range test (DMRT) were performed to analyze the significance of the power delivery efficiency, fuel consumption, and tractive efficiency. The software used for the statistical analysis was IBM SPSS Statistics (SPSS 25, SPSS Inc., New York, NY, USA).

3. Results

3.1. Required Engine Power for Rotary Tillage

Figure 4 shows the engine power required to conduct rotary tillage in the conventional, APS power, and APS ECO modes. There were two operational cycles of the tractor during the field experiment. Therefore, the test data were divided into three sections: (a) preparation, (b) operation, and (c) lifting the PTO and turning the tractor. It was clearly observed that the highest and lowest powers were required in the APS power and conventional modes, respectively, to conduct rotary tillage. The average engine powers were approximately 63.17 ± 2.82, 73.67 ± 3.92, and 84.76 ± 2.99 kW for the conventional, APS ECO, and APS power modes, respectively. The statistical analysis showed that there were significant differences among the engine power required in each engine mode, which are listed in

Table 4. Analysis of engine power required for rotary tillage.

Engine Modes	Required Engine Power (kW)
	Max	Min	Avg. ± SD. *
Conventional	70.81	48.43	63.17 ± 2.82 a
APS ECO	79.90	56.48	73.67 ± 3.92 b
APS power	90.32	73.98	84.76 ± 2.99 c

^a,b,c Means within each row for all engine modes with the same lettering are not significantly different at p < 0.05 according to Duncan’s multiple-range test. * Avg. ± SD. is the average ± standard deviation.

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3.2. Axle Power for Rotary Tillage

Figure 5 shows the axle power in each engine mode for rotary tillage. It was observed that the highest and lowest axle powers were consumed in the APS ECO and conventional modes, accounting for averages of around 53.45 ± 4.47 and 31.74 ± 3.49 kW, respectively. However, the axle power in the APS power mode was around 45.29 ± 4.95 kW. The statistical analysis also showed that the axle powers were significantly different, and they are listed in

Table 5. Analysis of the axle power for rotary tillage.

Engine Modes	Axle Power (kW) for Rotary Tillage
	Max	Min	Avg. ± SD. *
Conventional	38.88	14.17	31.74 ± 3.49 a
APS ECO	63.77	32.82	53.45 ± 4.47 b
APS power	58.89	33.56	45.29 ± 4.95 c

^a,b,c Means within each row for all engine modes with the same lettering are not significantly different at p < 0.05 according to Duncan’s multiple-range test. * Avg. ± SD. is the average ± standard deviation.

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3.3. PTO Power for Rotary Tillage

Figure 6 shows the PTO power in the conventional, APS power, and APS ECO modes during rotary tillage. Considering the operation time, the highest and lowest PTO power was calculated for the APS power and APS ECO modes, respectively. The average PTO power was about 31.43 ± 1.54, 20.24 ± 2.02, and 39.48 ± 4.21 kW in the conventional, APS ECO, and APS power modes, respectively. The statistical analysis is listed in

Table 6. Analysis of the PTO power for rotary tillage.

Engine Modes	PTO Power (kW) for Rotary Tillage
	Max	Min	Avg. ± SD. *
Conventional	42.52	29.21	31.43 ± 1.54 a
APS ECO	24.42	14.57	20.24 ± 2.02 b
APS power	51.07	29.34	39.48 ± 4.21 c

^a,b,c Means within each row for all engine modes with the same lettering are not significantly different at p < 0.05 according to Duncan’s multiple-range test. * Avg. ± SD. is the average ± standard deviation.

.

3.4. Power Delivery Efficiency (PDE) for Rotary Tillage

Figure 7 shows that the total engine power required in each engine mode was equal to the sum of the axle and PTO powers. In addition, for a comparison with the rated power of the engine, the highest power delivery efficiency was calculated in the APS power mode, which had an average of around 89.23%. The minimum power delivery was calculated to be approximately 66.48% in the conventional mode of the engine during rotary tillage. The power delivery efficiency in the APS ECO mode was around 77.57%, as shown in Figure 8.

3.5. Fuel Consumption for Rotary Tillage

According to the working load, the fuel consumptions in each engine mode had a similar tendency. The fuel consumption in the conventional, APS power, and APS ECO modes is shown in Figure 9. The average fuel consumption was approximately 17.04 ± 0.84, 19.39 ± 1.08, and 23.02 ± 1.21 L/h in the conventional, APS ECO, and APS power modes, respectively. The statistical analysis showed that there was a significant difference in the fuel consumption in the APS power and APS ECO modes. The statistical analysis is shown in

Table 7. Analysis of the fuel consumption for rotary tillage.

Engine Modes	Fuel Consumption (L/h) of Rotary Tillage
	Max	Min	Avg. ± SD. *
Conventional	18.77	13.01	17.04 ± 0.84 a
APS ECO	21.14	15.02	19.39 ± 1.08 b
APS power	25.74	20.54	23.02 ± 1.21 c

^a,b,c Means within each row for all engine modes with the same lettering are not significantly different at p < 0.05 according to Duncan’s multiple-range test. * Avg. ± SD. is the average ± standard deviation.

.

According to the fuel consumption, the conventional mode was the most economical mode for rotary tillage. It was necessary to analyze the tractive efficiency in order to select an energy-saving engine mode for rotary tillage in this study.

3.6. Tractive Efficiency (TE) in Rotary Tillage

To compare the tractive performance in the conventional, APS power, and APS ECO modes during rotary tillage, the tested and predicted tractive performance was analyzed with the slip ratio, as shown in Figure 10. It was observed that the APS power mode had the highest tractive efficiency, whereas the conventional mode had the lowest tractive efficiency. In addition, the slip ratio in the APS power mode was the lowest.

The average tested TEs in the conventional, APS ECO, and APS power modes were approximately 55.89 ± 9.52, 58.44 ± 8.68, and 73.45 ± 9.84%. The predicted tractive efficiencies were approximately 54.63 ± 9.48, 57.45 ± 8.80, and 74.32 ± 9.12% in the conventional, APS ECO, and APS power modes, respectively. The slip ratios in the conventional, APS ECO, and APS power modes were around 18.80 ± 3.45, 14.04 ± 2.01, and 10.35 ± 0.77%, respectively. It was also observed that there were no significant differences between the tested and predicted tractive efficiencies. The statistical analysis showed that the TE and slip ratio had highly significant differences among the engine modes, as shown in

Table 8. Analysis of the TE and slip ratio during rotary tillage.

Engine Modes	TE (%) *		Slip Ratio (%) *
	Test	Predicted
Conventional	55.89 ± 9.52 a	54.63 ± 9.48 a	18.80 ± 3.45 d
APS ECO	58.44 ± 8.68 b	57.45 ± 8.80 b	14.04 ± 2.01 e
APS power	73.45 ± 9.84 c	74.32 ± 9.12 c	10.35 ± 0.77 f

^a,b,c,d,e,f Means within each row for all engine modes with the same lettering are not significantly different at p < 0.05 according to Duncan’s multiple-range test. * The average ± standard deviation.

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According to the tractive efficiency, the APS power mode was the most efficient engine mode for rotary tillage, but fuel consumption was comparatively higher. To confirm the selection of the energy-saving engine mode, a comparative analysis of the working load with the map of engine characteristics was required. Therefore, the working load in each engine mode during rotary tillage was analyzed, as shown in Figure 11. It was observed that the working load in the APS ECO mode was in the ungoverned region and was very close to the engine’s maximum torque. This indicated that the engine could work at a high load, and fuel consumption was comparatively low.

On the other hand, the working load in the conventional and APS power modes was in the governed region, where the engine torque was low but the fuel consumption was higher. As the torque was low, the engine would be suddenly turned off due to the sudden occurrence of greater working loads. Therefore, it was confirmed that the APS ECO mode is a highly efficient and suitable engine mode for rotary tillage based on the working load and fuel consumption determined in this study.

4. Discussion

In this study, rotary tillage was used to analyze the power delivery efficiency and fuel consumption of a tractor based on its engine modes. The results of this study are discussed below.

(i)

It was observed that the maximum engine power, which was around 84.76 kW, was consumed in the APS power mode, and the minimum required engine power was 63.17 kW in the conventional mode during rotary tillage. In the case of the axle power, the highest and lowest axle power was consumed in the APS ECO and conventional modes, accounting for 53.45 and 31.74 kW, respectively. This indicated that the conventional, APS ECO, and APS power modes, respectively, delivered around 31.43, 72.55, and 53.43% of the engine power to both axles. After analyzing the PTO power, it was observed that the maximum and minimum PTO power was 39.48 and 20.24 kW in the APS power and APS ECO modes. This meant that approximately 49.75, 27.47, and 46.56% of the engine power was consumed as PTO power in the conventional, APS ECO, and APS power modes, respectively. Hensh et al. [27] reported that the PTO power consumption was in the range of 20.11% to 71.77% for a depth increase of 50 mm to 150 mm. Upadhyay and Raheman [28] also stated that the PTO power consumption was in the range of 26.90% to 63.74% for an average working depth of 120 mm. These studies agreed that the APS ECO mode consumed sufficient PTO power for rotary tillage.

(ii)

It was also observed that the PDE in the APS power mode was the highest, accounting for around 89.23%, and the lowest PDE was around 66.48% in the conventional mode. However, the PDE in the APS ECO mode was around 77.57% of the rated engine power. Kim et al. [6] stated that the PDE of a 78 kW tractor was around 84.1% of the rated engine power for rotary tillage. Lee [29] also stated that the maximum PDE of a 30 kW tractor was approximately 85% of the rated engine power during rotary tillage. According to the PDE, the APS power mode was the best engine mode for rotary tillage. To select the most economical engine mode in terms of fuel, the fuel consumption must be analyzed.

(iii)

In the case of fuel consumption, the conventional mode had the lowest fuel consumption, and the APS power mode consumed the most fuel. The fuel consumption in the APS ECO mode was 19.39 L/h. Siddique et al. [19] conducted similar research and stated that the APS ECO mode was the most economical engine mode comparison with the conventional and APS power modes; it was around 42.35% more economical in terms of fuel than the conventional mode. However, it was observed that the conventional mode consumed the least fuel during rotary tillage. In addition, a comparison of the tested and predicted tractive efficiencies showed that they were almost similar. The statistical analysis showed that there were no significant differences between the tested and predicted tractive efficiencies in each engine mode. In the conventional mode, the slip ratio was higher than those in the APS ECO and APS power modes, accounting for 18.80%. Therefore, a comparative analysis was conducted based on the working load in rotary tillage.

(iv)

The comparative analysis showed that the working load in the APS ECO mode was located in the ungoverned region. In addition, it showed that the working load was close to the maximum engine torque. This indicated that the APS ECO mode could be used to perform rotary tillage with high loads that could suddenly occur during operation [8,18]. In addition, the slip ratio in the APS ECO mode was around 14.04% for rotary tillage, which was lower than that in the conventional mode. Zhang et al. [15] suggested that power loss could occur with a higher slip ratio [24,30].

On the other hand, the conventional and APS power modes were located in the governed region, where the working load was comparatively low and the fuel consumption was higher. On the map of engine characteristics, the governed region is outside of the engine operating range, where the fuel efficiency rapidly decreases [8]. Lee [31] conducted research on maximizing the fuel efficiency of agricultural tractors. He suggested that the ungoverned region is the safest and most suitable region of the engine characteristic map for conducting agricultural operations because the engine’s operating range is from the points of maximum torque to the points of maximum power of the engine [32]. In addition, this could allow unexpected engine turn-off due to fluctuations in the working load, which are to be avoided.

In conclusion, the conventional and APS power modes would not be suitable or efficient for rotary tillage due to their being in the governed region and due to the fluctuation in agricultural working loads. For this reason, it could be said that the APS ECO mode is the most suitable and efficient engine mode for the working loads that are in the engine’s operating range. Therefore, we would suggest that users perform rotary tillage using the APS ECO mode efficiently.

5. Conclusions

In this study, the power delivery efficiency and fuel consumption of a 95 kW tractor were analyzed based on the conventional, APS ECO, and APS power engine modes. An APS (auto power shift) controller was used to select the engine modes. Finally, an energy-saving and efficient engine mode was selected for rotary tillage based on a comparative analysis. The power delivery and tractive efficiency in the APS power mode were the highest, but the fuel consumption was also the highest compared to that in the other engine modes. The slip ratio in the APS power mode was around 10%. In addition, a comparison of the tested and predicted tractive efficiencies showed that there were no significant differences for each engine mode. This indicated that the APS control algorithm was accurately performed.

In the case of the conventional mode, both the power delivery and tractive efficiencies were the lowest, as was the fuel consumption. However, the slip ratio in the conventional engine mode was rather high. On the other hand, the PDE and TE in the APS ECO mode were moderate, as was the fuel consumption, whereas the slip ratio was below 15%. The comparative analysis showed that the APS ECO mode was in the engine’s operating range, and the highest engine torque was possible in comparison with the APS power and conventional modes. In addition, the fuel consumption was comparatively lower than that in the APS power mode. However, the conventional and APS power modes were in the governed region, where that the engine could suddenly be turned off due to fluctuations in the working load. For this reason, we can say that APS ECO mode could be the most suitable, efficient, and energy-saving engine mode for rotary tillage. Finally, we recommend that rotary tillage is conducted by efficiently using the APS ECO mode.