CN105711581B - A kind of control method of stepless speed changing tractor control system - Google Patents

Abstract

A kind of control method of stepless speed changing tractor control system, the control system include engine, hydraulic mechanical stepless gearbox, vehicle speed sensor, drawbar load sensing mechanism, engine electronic control unit, transmission electronic control unit, transmission pressure control unit；The present invention usesIt is optimal that maximum characterizes tractor productivity ratio economy synthesis, by the way that the optimal gear ratio and engine speed that meet that tractor productivity ratio economy synthesis is optimal are calculated than optimization to speed changer, it can ensure that tractor on the premise of larger productivity ratio is obtained, still there is good fuel economy.

Description

A control method for a continuously variable speed tractor control system

technical field

The invention relates to a tractor control system, in particular to a control method for a continuously variable speed tractor control system.

Background technique

Tractor is an important tool for farmland operations. The complex working environment makes the traction load and driving speed of the tractor fluctuate greatly. The hydraulic mechanical continuously variable transmission can adapt to complex working conditions and greatly improve the performance of the vehicle. Generally, the working modes of continuously variable transmission tractors are divided into the working mode of maximum productivity and the best working mode of economy. Among them, in the working mode of maximum productivity, the fuel economy of the tractor is poor; It is the lowest point of consumption. At this time, if the traction load of the vehicle is suddenly increased, it is easy to cause the engine to stall.

At present, in order to obtain greater productivity while ensuring the economy of the tractor, weight coefficients are introduced in the tractor control method. For example, Hao Yunzhi, Sun Dongye, etc. proposed in the article "Overall Optimal Control Strategy of Continuously Variable Transmission System" that the overall optimum of the vehicle transmission system under certain conditions can be obtained by selecting appropriate weight coefficients based on the driver's judgment. However, after actual operation, the proficiency of each driver's operation and the judgment of complex road conditions are different, and good results cannot be obtained.

Contents of the invention

In order to overcome the problem that the prior art method is difficult to solve the contradiction between tractor economy and productivity well, the present invention provides a control method for the control system of the continuously variable tractor, which can ensure that the tractor can still achieve high productivity under the premise of obtaining high productivity. Has good fuel economy.

The technical solution adopted by the present invention to solve the above problems is: a continuously variable tractor control system, including an engine, a hydraulic mechanical continuously variable transmission, a vehicle speed sensor, a traction sensor, an engine electronic control unit, a transmission electronic control unit, and a transmission hydraulic control unit. unit, the hydromechanical continuously variable transmission includes a mechanical speed change mechanism, a hydraulic speed regulating mechanism, and a planetary gear mechanism that combines mechanical and hydraulic power. Type hydraulic speed regulating mechanism, the electro-hydraulic proportional variable pump includes electro-hydraulic proportional solenoid valve, oil cylinder plunger and swash plate; wherein, the output ends of the traction sensor and the vehicle speed sensor are connected with the input end of the transmission electronic control unit, and the transmission electronic control unit The output end is connected with the input end of the engine electronic control unit and the transmission hydraulic control unit. The transmission hydraulic control unit outputs the control signal of the swash plate inclination according to the optimal gear ratio. The electro-hydraulic proportional solenoid valve receives the control signal and passes through the amplifier to control the oil cylinder. The plunger moves to push the swash plate to deflect, changing the displacement ratio of the variable displacement pump and realizing stepless speed change.

In the present invention, a control method of a continuously variable tractor control system includes the following steps:

Step 1. Respectively set the input speed of the engine The range is set to 1300~2300 r/min, the input torque of the engine It is set to 0~1200 N·m, where, the formula (1) is the gear ratio of the transmission , , is the input angular velocity of the engine in rad/s, is the output angular velocity of the transmission, in rad/s; is the output torque of the transmission, the unit is N m; set the traction force of the tractor The range is 0.01~80kN, the increment of each step =1 kN;

Step 2: Use the vehicle speed sensor and the force sensor to measure the tractor's driving speed and traction resistance, according to the formula (2) and formula (3) , respectively obtain the output angular velocity of the transmission and output torque , where the slip rate ; rolling friction , the unit is N, where, is the coefficient of friction, =0.05~0.75, m is the mass of the tractor, the unit is kg; is the driving speed of the tractor, in m/s; is the radius of the drive wheel of the tractor, in m; is the main transmission ratio of the tractor; ; ;set up =0.01~40km/h, the increment of each step ;

Step 3, use the output speed of the transmission obtained in step 2 and output torque ,like , proceed to the next step; otherwise, the optimal gear ratio , after storing the optimal gear ratio, return to step 2 to recalculate with the increment set by the optimization calculation; among them, is the rated torque of the engine, in N m; is the maximum output power of the engine, in W;

Step 4. Change the gear ratio of the transmission to Set to 0.01~2.24, the increment of each step , using formula (1), the gear ratio set by and the output speed of the transmission ,calculate , and judge Whether meet 1300≤ ≤2300, if satisfied, proceed to the next step; otherwise, according to the increment of each step Reselect the gear ratio ;

(1), if At the end of the cycle, according to the increment set in step 2 reselect , otherwise, return to step 3;

(2), if At the end of the cycle, according to the increment set in step 1 =1 reselect , otherwise, return to step 2;

(3), if When the loop ends, all ends, otherwise, return to step 1;

Step 5. Use formula (4) , calculated according to step 4 Find the limit torque corresponding to the engine , and set the increment per step ; using formula (5) and formula (6) Find the specific fuel consumption of the tractor ,in, is the fuel consumption rate of the engine, , is the transmission efficiency of the transmission; ; ; using formula (7) Calculating Tractor Pulling Power , the unit is W; judgment Is it the largest, if is the largest, then the corresponding variable speed ratio is the optimal variable speed ratio; otherwise, return to step 4.

The present invention utilizes tractor traction power ( ) maximum characterizes the maximum productivity, using the specific fuel consumption of the tractor ( ) minimum represents the best economy, with the optimal productivity-economic combination as the control target, using The maximum characterizes the best combination of tractor productivity-economy. Guarantees that the tractor is The maximum is the basic requirement to realize the optimal combination of tractor productivity and economy. When the tractor is working under the optimum conditions of productivity and economy and the traction load changes, in order to meet the work needs, the tractor can run stably at the set target speed to ensure the work quality.

The shift rule formulated by the present invention is to determine the transmission operating point in the transmission output characteristic field in the load characteristic field according to the driving speed of the tractor and the traction load, and determine the engine operating speed according to the optimal speed ratio.

Any vehicle speed and traction load during tractor running correspond to a speed and torque in the output characteristic field of the transmission. By continuously adjusting the gear ratio of the transmission, countless engine speeds and torques can be obtained corresponding to them. Through optimization calculation, find out the The maximum corresponding transmission gear ratio is the optimum gear ratio. The working speed of the engine can be calculated according to the speed of the transmission and the optimal gear ratio.

Beneficial effect: the present invention adopts The maximum characteristic of tractor productivity-economical combination is the best; the optimal transmission ratio and engine speed that satisfy the tractor's productivity-economical combination are obtained by optimizing the transmission ratio; using vehicle speed, traction resistance, optimal transmission ratio and engine speed As a control parameter, the transmission electronic control unit determines the optimal transmission ratio and engine speed according to the vehicle speed and traction load, and adjusts the engine speed and transmission transmission ratio through the engine electronic control unit and transmission hydraulic control unit, so that the tractor's productivity and economy can reach the comprehensive optimum , to ensure that the tractor has better traction performance, fuel economy and driving comfort, and improve the automatic operation level of the tractor.

Description of drawings

Fig. 1 is the flow chart of the control method of the variable speed control system of the continuously variable tractor of the present invention;

Figure 2 is a schematic diagram of the synergistic control of the engine and the transmission when the tractor with continuously variable transmission is optimally shifted in terms of productivity and economy;

Figure 3 is a schematic diagram of the optimal control system for the continuously variable tractor productivity-economic efficiency;

Fig. 4 is the control schematic diagram of the engine, variable pump-quantitative motor hydraulic speed regulating mechanism when the productivity-economic efficiency of the continuously variable tractor is optimal.

Reference signs: in accompanying drawing 4, 1, electro-hydraulic proportional solenoid valve, 2, cylinder plunger, 3, relief valve, 4, high-pressure relief valve, 5, pressure-stabilizing relief valve, 6, filter, 7 ,tank.

detailed description

The present invention will be further described below in conjunction with specific examples, so that those skilled in the art can better understand the present invention and implement it, but the given examples are not intended to limit the present invention.

A control system for a continuously variable tractor, comprising an engine, a hydromechanical continuously variable transmission, a vehicle speed sensor, a traction force sensor, an engine electronic control unit, a transmission electronic control unit and a transmission hydraulic control unit, and the hydromechanical continuously variable transmission includes a mechanical transmission Mechanism, hydraulic speed regulating mechanism and planetary gear mechanism that combines mechanical and hydraulic power. The hydraulic speed regulating mechanism is a closed hydraulic speed regulating mechanism composed of electro-hydraulic proportional variable pump and quantitative motor. Hydraulic proportional solenoid valve, cylinder plunger and swash plate. Among them, the output terminals of the traction force sensor and the vehicle speed sensor are connected with the input terminals of the transmission electronic control unit, the output terminals of the transmission electronic control unit are connected with the input terminals of the engine electronic control unit and the transmission hydraulic control unit, and the transmission hydraulic control unit according to the optimal transmission Ratio output control signal of swash plate inclination angle, electro-hydraulic proportional solenoid valve receives the control signal, after passing through the amplifier, controls the movement of cylinder plunger, pushes the swash plate to deflect, changes the displacement ratio of the variable pump, and realizes stepless speed change.

Wherein, the composition of transmission electronic control unit is as prior art, comprises central processing module, signal input module, signal output module, communication module and emergency treatment module, and wherein, central processing module comprises power supply module, reset module, clock module, JTAG interface ;The signal input module is used to input to the central processing module to reflect the driver's intention and the status information of the whole system; the signal output module is used to drive the solenoid valve of the transmission hydraulic control unit to realize the speed change of the transmission; the communication module includes CAN communication module, SCI communication module, SPI Display modules.

A control method for a control system of a continuously variable speed tractor, comprising the following steps: Step 1, input speed of the engine respectively The range is set to 1300~2300 r/min, the input torque of the engine It is set to 0~1200 N·m, where, the formula (1) is the gear ratio of the transmission , , is the input angular velocity of the engine in rad/s, is the output angular velocity of the transmission, in rad/s; is the output torque of the transmission, the unit is N m; set the traction force of the tractor The range is 0.01~80kN, the increment of each step =1 kN;

Step 2: Use the vehicle speed sensor and the force sensor to measure the tractor's driving speed and traction resistance, according to the formula (2) and formula (3) , respectively obtain the output angular velocity of the transmission and output torque , where the slip rate ; rolling friction , the unit is N, where, is the coefficient of friction, =0.05~0.75, m is the mass of the tractor, the unit is kg; is the driving speed of the tractor, in m/s; is the radius of the drive wheel of the tractor, in m; is the main transmission ratio of the tractor; ; ;set up =0.01~40km/h, the increment of each step ;

Step 3, use the output speed of the transmission obtained in step 2 and output torque ,like , proceed to the next step; otherwise, the optimal gear ratio , after storing the optimal gear ratio, return to step 2 to recalculate with the increment set by the optimization calculation; among them, is the rated torque of the engine, in N m; is the maximum output power of the engine, in W;

Step 4. Change the gear ratio of the transmission to Set to 0.01~2.24, the increment of each step , using formula (1), the gear ratio set by and the output speed of the transmission ,calculate , and judge Whether meet 1300≤ ≤2300, if satisfied, proceed to the next step; otherwise, according to the increment of each step Reselect the gear ratio ;

(1), if At the end of the cycle, according to the increment set in step 2 reselect , otherwise, return to step 3;

(2), if At the end of the cycle, according to the increment set in step 1 =1 reselect , otherwise, return to step 2;

(3), if When the loop ends, all ends, otherwise, return to step 1;

Step 5. Use formula (4) , calculated according to step 4 Find the limit torque corresponding to the engine , and set the increment per step ; using formula (5) and formula (6) Find the specific fuel consumption of the tractor ,in, is the fuel consumption rate of the engine, , is the transmission efficiency of the transmission; ; ; using formula (7) Calculating Tractor Pulling Power , the unit is W; judgment Is it the largest, if is the largest, then the corresponding variable speed ratio is the optimal variable speed ratio; otherwise, return to step 4.

The present invention adopts TMS320C24X type DSP micro-controller, and the main interface signals include: input signals include analog signals such as vehicle forward, reverse, gear switch, rack displacement, traction resistance, driving speed, clutch pressure, etc. Output signal Clutch electromagnetic directional valve switch control signal, variable pump control proportional solenoid valve control signal, engine command speed signal. The man-machine interface displays vehicle speed, gear ratio, engine speed, displacement ratio and other values.

DSP has built-in microprocessor, program and data Flash memory, RAM, PWM control circuit, A/D and D/A, parallel I/O interface, CAN interface, etc. The photoelectric isolation circuit can protect the microcontroller and improve the anti-interference ability of the TCU. The signal conditioning circuit amplifies and filters the analog signal to ensure reliable sampling input. The power amplifier circuit amplifies the power of the signal to reach the required value, and short-circuits and back pressure protection. The emergency circuit is used to force the low-speed forward or reverse clutch of the transmission through the external switch circuit, so that the engine power can be transmitted to the driving wheels of the tractor through the transmission, so that the driver can transfer the tractor from the fault point to the maintenance station. .

The engine adopts electronic speed regulation. The electronic governor receives the command speed of the engine, outputs the driving signal of the rack displacement, controls the displacement of the oil pump rack, adjusts the fuel supply, and then changes the engine speed to ensure that the actual speed of the engine is consistent with the command speed. Measure the engine speed and rack displacement in real time, and control the rack displacement and speed in a closed-loop manner.

In the present invention, the process of solving the optimal gear ratio of the transmission and the engine speed from the traction load and vehicle speed is a reverse solution, and the efficiency calculation of the hydromechanical continuously variable transmission (HMCVT) is complicated. In order to improve the calculation efficiency, the optimal gear ratio obtained from the optimization calculation is Number table form is stored in memory unit; Slip rate is as an important factor affecting vehicle performance, and the relational expression of slip rate and driving force under different road conditions is fitted out according to the tractor experiment data of different road surfaces under the standard working condition in the present invention, To meet the needs of non-standard working conditions through modification.

The present invention proposes to use the target vehicle speed, traction resistance, optimal gear ratio and engine speed as control parameters to realize the engineering application of the optimal continuously variable speed law; Adjust the transmission ratio and engine speed to achieve the optimal combination of tractor productivity and economy. The control input is the target vehicle speed and traction; the control output is the optimal transmission ratio and engine speed. The tractor productivity-economy comprehensive optimal transmission control system formulated according to the law of continuously variable transmission, including: vehicle speed sensor, used to measure the speed of the tractor; traction force sensor, used to measure the traction resistance of the tractor; transmission electronic control unit, according to Vehicle speed and traction load determine the engine load, determine the optimal gear ratio of the transmission and the corresponding engine speed; the engine electronic control unit adjusts the engine speed according to the engine speed signal; the HMCVT hydraulic control unit adjusts the variable pump swash plate inclination according to the gear ratio signal, Change the displacement ratio of the pump to realize stepless speed change. Among them, the vehicle speed sensor is installed on the non-driving wheel of the tractor, and the traction force sensor is installed at the connection between the hydraulic suspension mechanism of the tractor and the agricultural implement.

The present invention utilizes the electronic control technology to realize the comprehensive optimal stepless speed change and control of the hydraulic mechanical stepless speed change tractor in terms of productivity and economy. By monitoring tractor traction resistance and vehicle speed, adjusting the gear ratio of the transmission and the engine speed, the tractor can work in the optimal working condition of productivity-economy.

Specific embodiment: the present invention takes a certain type of Dongfanghong 400-horsepower wheeled tractor equipped with a hydraulic-mechanical continuously variable transmission as the object, and the main parameters of the system are: tractor vehicle mass m=7760kg, tractor main transmission ratio =36.348, transmission ratio The range is 0~2.24, the tractor drive wheel radius r _d =0.9046m, the engine rated power N _e0 =228kw, the maximum output power of the engine =294kW, engine rated speed =2100(r/min), the maximum engine speed =2300(r/min), maximum engine torque =1200N·m, maximum traction force =80kN.

As shown in Figure 1, a continuously variable tractor tractor productivity-economic comprehensive optimization calculation method for optimal transmission ratio, the steps are as follows:

(1) Set the limit speed and torque of the engine and the range of the HMCVT transmission ratio. The speed of the wheeled tractor in the field is mainly 0.5~10km/h, and the speed of field and road transportation is mainly 15~30km/h. Optimal calculation The set speed range is 0.01~40km/h. In order to ensure the normal operation of the tractor, the traction load should be less than the maximum traction force of the tractor. In the optimization calculation, the traction load range of the tractor is set to 0.01~80kN. In order to avoid the phenomenon that the divisor is 0, HMCVT The range of gear ratio is 0.01~2.24, in the optimization calculation =1, , , ;

(2) Calculate tractor power and transmission output angular velocity according to tractor traction load and vehicle speed , torque , if the power , corresponding to each gear ratio The corresponding engine speed can be obtained , in order to avoid the engine working on the external characteristics, the actual torque should be limited within the external characteristics to ensure that the engine has a certain torque reserve, and calculate the corresponding limit torque of the engine according to the engine speed , according to the engine speed and torque, the efficiency of HMCVT and the fuel consumption rate of the engine can be calculated. If the power is greater than , , engine rated torque.

(3) judgment Is it the largest, if The maximum, then the corresponding variable speed ratio is the best variable speed ratio, if not the maximum, enter the next cycle of calculation;

(4) If the output power of the transmission is greater than the maximum power of the engine, the optimal transmission ratio is the ratio of the output torque of the transmission to the rated torque of the engine;

(5) End the loop when the speed and torque of the engine, transmission and gear ratio all reach the limit value.

The optimization variables include the gear ratio of the transmission and the operating point of the engine, and the optimization objective is maximum. Constraints include engine and transmission speed, torque constraints, gear ratio constraints and power constraints. The slip rate of the tractor is calculated according to the slip rate characteristic curve, and the slip rate characteristic curve under different road conditions is stored in the memory unit in the form of a mathematical expression.

In order to realize fast real-time control, the optimal transmission ratio and engine speed calculated by optimization are stored in the memory unit of the controller in the form of a table, and the optimal transmission ratio corresponding to any vehicle speed and traction load can be obtained by using an interpolation algorithm and engine speed.

As shown in Fig. 2, the principle of synergistic control of the engine and the transmission at the time of the optimum speed change of the continuously variable tractor’s productivity-economic efficiency, the driving force and the transmission torque are obtained from the traction force of the tractor, and the vehicle slip is calculated according to the characteristic curve of the tractor’s slip rate. Rotation rate; according to the target speed, calculate the theoretical speed of the tractor and the speed of the transmission, calculate the optimal speed ratio of the transmission and the speed of the engine from the table of the optimal speed ratio, and adjust the speed ratios of the engine and the transmission respectively according to the speed of the engine and the best speed ratio. In order to ensure the stability of the vehicle speed, feedback control is carried out on the vehicle speed.

As shown in Figure 3, tractor productivity-economy integrates optimal shifting rules and control system principles. Signals from traction sensors and vehicle speed sensors are input to the signal processing unit of the shifting ECU. After processing, the signals are transmitted to the engine load determination unit and cooperative control unit. After calculation, the engine speed and the optimal gear ratio of the transmission are obtained. The engine speed command is input to the engine ECU, which controls and adjusts the engine speed. Displacement ratio, realize stepless speed change.

As shown in Figure 4, the principle of the engine, pump-motor control system when the tractor productivity-economy is integrated and optimal variable speed control, the engine adopts electronic speed regulation, the electronic governor receives the engine command speed, outputs the rack displacement driving signal, and controls the oil pump The rack displacement adjusts the fuel supply, and then changes the engine speed to ensure that the actual engine speed is consistent with the command speed. The governor measures the engine speed and rack displacement in real time, and controls the rack displacement and speed in a closed-loop manner.

In the present invention, as shown in Figure 4, the variable pump-quantitative motor adopts closed transmission, and the electro-hydraulic proportional solenoid valve 1 receives the electric signal of the inclination angle of the swash plate of the transmission. After passing through the amplifier, it controls the movement of the plunger 2 of the oil cylinder to push the swash plate to deflect , to change the displacement ratio of the variable pump. The electro-hydraulic proportional solenoid valve 1 realizes closed control through the mechanical rigidity feedback of the oil cylinder plunger 2, so that the inclination angle of the swash plate changes in proportion to the control current. In order to prevent the oil circuit pressure from reaching the maximum working pressure caused by the swash plate rotating too fast during acceleration and deceleration, the safety valve releases pressure from the control oil circuit when the oil pressure in the main oil circuit exceeds the set value; When the maximum value is exceeded, the oil circuit overflows to protect the safety of the pump-motor system; the flushing valve overflows a small amount of oil on the low-pressure side of the main oil circuit back to the oil circuit, which plays the role of flushing impurities in the oil circuit and cooling down, preventing oil Excessive temperature affects the performance and leakage of the hydraulic system; the leakage of the pump-motor and the overflow oil of the flushing valve flow into the cooler, and then return to the oil tank for recycling after cooling. The high-pressure relief valve 4 is to protect the main oil circuit when the pressure of the main oil circuit reaches the maximum pressure value; the pressure-stabilizing relief valve 5 stabilizes the pressure of the oil coming out of the fuel tank 7; the oil entering the fuel tank 7 needs to be filtered by the filter 6 .

Claims (1)

1. A control method of a continuously variable tractor control system, the control system comprises engine, hydraulic mechanical continuously variable transmission, vehicle speed sensor, traction force sensor, engine electronic control unit, transmission electronic control unit, transmission hydraulic control unit, is characterized in that: The hydromechanical continuously variable transmission includes a mechanical speed change mechanism, a hydraulic speed regulating mechanism, and a planetary gear mechanism that combines mechanical and hydraulic power. Speed regulating mechanism, electro-hydraulic proportional variable pump including electro-hydraulic proportional solenoid valve, cylinder plunger and swash plate; Among them, the output terminals of the traction force sensor and the vehicle speed sensor are connected with the input terminals of the transmission electronic control unit, the output terminals of the transmission electronic control unit are connected with the input terminals of the engine electronic control unit and the transmission hydraulic control unit, and the transmission hydraulic control unit according to the optimal transmission Ratio output the control signal of swash plate inclination angle, the electro-hydraulic proportional solenoid valve receives the control signal, after passing through the amplifier, it controls the movement of the plunger of the oil cylinder, pushes the swash plate to deflect, changes the displacement ratio of the variable pump, and realizes stepless speed change; control method Include the following steps: Step 1. Respectively set the input speed of the engine The range is set to 1300~2300 r/min, the input torque of the engine It is set to 0~1200 N·m, where, the formula (1) is the gear ratio of the transmission , , is the input angular velocity of the engine in rad/s, is the output angular velocity of the transmission, in rad/s; is the output torque of the transmission, the unit is N m; set the traction force of the tractor The range is 0.01~80kN, the increment of each step =1 kN; Step 2: Use the vehicle speed sensor and the force sensor to measure the tractor's driving speed and traction resistance, according to the formula (2) and formula (3) , respectively obtain the output angular velocity of the transmission and output torque , where the slip rate ; rolling friction , the unit is N, where, is the coefficient of friction, =0.05~0.75, m is the mass of the tractor, the unit is kg; is the driving speed of the tractor, in m/s; is the radius of the drive wheel of the tractor, in m; is the main transmission ratio of the tractor; ; ;set up =0.01~40km/h, the increment of each step ; Step 3, use the output speed of the transmission obtained in step 2 and output torque ,like , proceed to the next step; otherwise, the optimal gear ratio , after storing the optimal gear ratio, return to step 2 to recalculate with the increment set by the optimization calculation; among them, is the rated torque of the engine, in N m; is the maximum output power of the engine, in W; Step 4. Change the gear ratio of the transmission to Set to 0.01~2.24, the increment of each step , using formula (1), the gear ratio set by and the output speed of the transmission ,calculate , and judge Whether meet 1300≤ ≤2300, if satisfied, proceed to the next step; otherwise, according to the increment of each step Reselect the gear ratio ; (1), if At the end of the cycle, according to the increment set in step 2 reselect , otherwise, return to step 3; (2), if At the end of the cycle, according to the increment set in step 1 =1 reselect , otherwise, return to step 2; (3), if When the loop ends, all ends, otherwise, return to step 1; Step 5. Use formula (4) , calculated according to step 4 Find the limit torque corresponding to the engine , and set the increment per step ; using formula (5) and formula (6) , get the specific fuel consumption of the tractor ,in, is the fuel consumption rate of the engine, , is the transmission efficiency of the transmission; ; ; using formula (7) Calculating Tractor Pulling Power , the unit is W; judgment Is it the largest, if is the largest, then the corresponding variable speed ratio is the optimal variable speed ratio; otherwise, return to step 4.