3.1. Transmission efficiencies
Results obtained from the model with the acceleration test
data show that the full-powershift estimated TE (A) had an
average value of 0.646 and a standard deviation of 0.039
(coefficient of variation-CV: 6.1%). This result appears to be in
line with that of Lindgren and Hansson (2002) with regard to
the study by Reiter (1990): the TE of a tractor with a fixed-ratio
mechanical transmission dropped to 65% at 30 km h1 and
generally decreased with increasing speed. The same paper
also mentions that Reiter found even lower coefficients for
powershift transmission tractors, probably due to the friction
generated in the electro-clutches.
The two tested CVTs results appear to have very different
TEs’ average values (0.868; 0.655), standard deviations (0.047;
0.025) and CVs (5.4%; 3.8%), and they are not necessarily better
than those of tractor A (the TE of tractor C was not statistically
different from tractor A; Fig. 4).
The observation of average values suggests that there
probably exists a certain influence of constructional choices
(unknown to the experimenters) that takes precedence over
the general type of transmission; thus, the overall efficiency is
significantly influenced by how the power is divided between
the mechanical and the hydrostatic part and, therefore, by the
degree of prevalence of one over the other. Nevertheless,
these considerations about CVTs do not affect the obvious
advantages that tractors equipped with this type of gearboxes
have over tractors with fixed-ratio powershift gears, i.e. the
possibility to operate the engine at constant speed corresponding
to the maximum torque, with obvious advantages in
the execution of work and consumption.
The observed variability is due to the measurement errors
involving all the quantities used in the model; a lower variance
of TE values and therefore a greater precision of estimations
can be obtained by using more precise instruments.
However, the obtained CVs (