overcome the increased resistance to draft. In order not to overload
the engine and let it operate normally, work is usually done by not
utilizing the full load. When draft increases and engine speed becomes
slower, it works in a range of higher torque, so greater traction
is ensured even without switching gears. Power reserve in the
conditions of agricultural production varies in the range from 6% to
18% of total engine power. During their whole lifecycle most of the
tractors are running at 60–70% of max. load [16–18]. Many
researchers of tractors argue that in the field works, when tractors
are loaded partially, work should be done at a reduced engine
speed. When it is necessary to reach full engine force, it is recommended
to run the engine in constant power mode. Rakopoulos
and Giakoumis [19] made a review of factors having influence on
sudden changes to engine load and speed. In modern tractors, reserve
for engine torque reaches 40–50%, but anyway it is not enough,
because resistance to traction varies more. Therefore, bigger
variation of torque shall be compensated by using transmission.
Stepless and automatic speed adjustment allows engine to run in
steady mode or close to it. During tractor operation in stepless
speed adjustment mode, maximum load can be applied on engine
in order to develop highest or close to it output. [15,16,20–22].
However, during soil cultivation and other field operations, the
choice of gear by the driver plays a rather important part in fuel
consumption and emission amounts [15,23].