Power delivery efficiency of a wheeled tractor at oblique drawbar force

•Power delivery efficiency (PDE) of tractor was calculated based on the wheel slippage.•Wheel slippage was expressed as a function of the drawbar force and its angle.•Mathematical equations and an algorithm were formed for the calculation of PDE.•It was examined the influence of oblique drawbar force at two soil surfaces on PDE.•The importance of properly attaching the implement to the tractor was confirmed. The offset position of an implement creates an oblique drawbar force. This force occurs when the resistance direction of the implement is at a certain angle with respect to the longitudinal axis of the tractor (tractor traction direction). The focus of this study was to demonstrate the dependence of wheeled tractor power delivery efficiency on wheel slippage for central and oblique drawbar forces, both on plowed and unplowed stubble. For this purpose, the travel speed of tractor, wheel slippage, drawbar force and engine speed were measured. Mathematical models were formed using a nonlinear regression analysis, and after that, an algorithm for calculating the power delivery efficiency of wheeled tractors depending on the drawbar force and the angle of the drawbar force was developed. This method allowed us to determine the dependence of the power delivery efficiency on the wheel slippage for different angles of the drawbar force. The highest power delivery efficiency of a tractor is achieved at a central drawbar force (angle of the oblique drawbar force equals zero). Results showed the influence of oblique drawbar force on the power delivery efficiency of the wheeled tractor on soft and loose types of soil. Power delivery efficiency of the tractor on unplowed stubble decreased by 6.7% when the angle of the oblique drawbar force increased to 10 degrees, and by 15.0% when the angle of the oblique drawbar force increased to 20 degrees. On plowed stubble, power delivery efficiency decreased by 10.4% at a 10 degree angle of oblique drawbar force and by 21.9% at an angle of oblique drawbar force of 20 degrees. The importance of properly attaching the implement to the tractor was also confirmed.