d	the diameter of the piston (m)
C d	the drag coefficient
k z	the reduction coefficient
l 0	the length of the piston out of the cylinder block at the outer dead point (m)
l c	the length of the cylinder block (m)
M cc	the experimental churning losses torque due to the rotation of the cylinder (N·m)
M cp	the experimental churning losses torque due to the circling pistons (N·m)
M dc	the experimental torque acting on the shaft because of the rotation of the cylinder without oil in the case (N·m)
M dcp	the experimental torque acting on the shaft because of the circling pistons and the rotating cylinder without oil in the case (N·m)
M wc	the experimental torque acting on the shaft because of the rotation of the cylinder with oil in the case (N·m)
M wcp	the experimental torque acting on the shaft because of the circling pistons and the rotating cylinder with oil in the case (N·m)
n	the test speed (rpm)
P cc	the experimental churning losses power due to the rotation of the cylinder (W)
P cm	the calculated churning losses power due to the rotation of the cylinder (W)
P’ cm	the calculated churning losses power due to the rotation of the cylinder after optimization (W)
P cp	the experimental churning losses power due to circling pistons (W)
P pm	the calculated churning losses power due to circling pistons (W)
P’ pm	the calculated churning losses power due to circling pistons after optimization (W)
R	the pitch circle radius of piston bores (m)
Rc	the radius of the cylinder block (m)
Re	the Reynolds number
t	the gap between the cylinder block and the housing internal surface(m)
V	the volume of the stirring oil (m3)
z	the number of pistons
ω	the rotation angular velocity (rad/s)
μ	the dynamic viscosity (Pa·s)
ρ	the density of test fluid (kg/m3)
γ	the swash plate angle (rad)