792
accuracy. Also, time is in the first assumption infinitely
determinable. In this case, the relative position of the
crank is determined by a fully deterministic
relationship: the speed multiplied by time gives the
angle of rotation of the shaft. In order to know the
position of a given throw at any time, we still need to
know the initial conditions: the angle (Fig. 9) at the
t=0 moment. Let us assume, that the initial position of
the crankshaft was determined extremely precisely:
0=36.001°. Such a record means that the initial angle of
rotation of the throw has been determined to an
accuracy of ±0.001°. The actual angular position may be
=36.00149...° or =36.00050...°. The initial angular
position of the shaft can be written in a form:
0=36.001nnnnn...°. Where "n" stands for an unknown
number. After ten ignitions (in each cylinder) the throw
will rotate by 10x36°, it is enough to multiply the
number 0 by ten. In the decimal system it is reduced
to moving the point one position to the right. As a
result of this operation we will obtain:
10=360.01nnnnn...° = 0.01nnnnn...°. The precise
knowledge of the throw position is still high (±0,01°)
but has decreased tenfold! After a thousand ignitions
our knowledge of the crankpin position is reduced to
±1° because: 1000=36001.nnnnn...° = 1.nnnnn...°.
It can be observed that after 10 thousand ignitions
the accuracy of the crank position drops to the value of
±100°, i.e. in practice it is unknown (theoretically a
completely undetermined position occurs after 100
thousand ignitions). It should be stressed that having
ideally deterministic dependencies (with infinitely
accurate parameters of time and rotational speed) we
are not able to determine the searched value after a
sufficient number of cycles of work. If the rotational
speed of the considered engine is n=100 rpm, then for a
10-cylinder engine 100 thousand ignitions will occur
after 10 thousand revolutions, i.e. after 100 minutes.
After about an hour and a half it is impossible to
determine the position of the crank despite the
determination of its initial conditions with a
remarkable accuracy of 0.001. In practice, the situation
of the researcher is even more difficult, because the
engine speed can be determined only with a finite
accuracy, and what is more, the assumption of a
constant speed is also a certain (often coarse)
approximation. Only the error of time determination
can be ignored in engineering practice, because of the
possibility of obtaining a very high accuracy of its
determination.
In order to graphically illustrate the described
phenomenon, a simple procedure of crankshaft crank
position calculation was performed. It was assumed
that the starting throw position, equal to 10°, is
determined with the accuracy of ±0.01°. The next
position is obtained by multiplying the previous
angular position by 10 (due to the ease of operation in
the decimal system) - the subsequent cycles of work are
a tenfold increase in the angle describing the throw
position. Of course, the results obtained above 360° are
reduced to the range of 0-360°. As already shown in the
previous paragraph, each new cycle is a tenfold
reduction in the accuracy of the throw position
determination. The subsequent numerical values of the
angular positions are calculated deterministically, but
their final values have a random scattering equal to the
accuracy of the position determination for the given
calculation cycle. For example, in the second
calculation cycle we have the angular position 100°
with the dispersion of ±0.1°; so for further calculations
the number 99.9° or e.g. the number 100.05° can be
used. Fig. 10 shows an exemplary (due to randomness,
each subsequent calculation will give a different
course) course of changes in the position of throw of
the crankshaft. In order to make the drawing easier to
read, the crank position (navy blue line) is given for
every tenth calculation cycle, while the size of the
position error (red line) is given for every hundredth
calculation cycle. It can be observed that after 2,000
cycles the error rate covers the entire 0-360° range. This
means that the throw position is undetermined.
Figure 10. Position of the crank and its dispersion in
subsequent calculation cycles
The sensitivity of the system to initial conditions is
shown in Fig. 11. The position of the throw of a shaft of
the crankshaft for three starting values has been
calculated. The initial throw position was determined
as 10.00° (continuous blue line), 9.99° (dotted green
line) and 10.01° (dotted brown line). It should be noted
that all lines start from practically the same point, while
the course of throw position changes is random and
drastically different for each initial condition. For
example, for cycle no. 1801 the throw can be deflected
from the vertical by 56° or 286° (for 0=9,99°)) or 114°
(for 0=10,01°).
Figure 11. Sensitivity to the determination of the crank
position to initial conditions
The determination of the throw of a shaft position
with any finite accuracy will not permit the
determination of its angular position after a sufficiently
long time. It should be noted that the above examples
have been prepared for the simplest way of modeling -
linear models. In case of the occurrence of non-
linearities (e.g. geometrical, material), describing
physical phenomena becomes very complicated. In the
descriptions there are bifurcations (jumping change of
properties of a mathematical model with a small
change of its parameters), which lead to the formation
of completely different families of solutions passing
through bifurcation points. The whole of these
phenomena is defined by the term deterministic chaos
[7]. Generally, hypersensitivity to initial conditions