Method of controlling vehicular drivelines including a continuously variable transmission

A method of controlling a vehicle driveline including an engine, a continuously variable ratio transmission (CVT) and a final drive in which over a working range of engine speed, the engine is constrained to work to a performance map correlating each value of engine speed to a particular value of engine torque wherein a demand by a driver for a increase in vehicle speed initiates a quick increase in engine speed to a predetermined engage speed without any substantial rise in the final drive speed and then increasing both engine speed and drive speed by a predetermined function until a selected engine speed limit is reached which is commensurate with the driver's demand, and finally maintaining the selected engine speed limit despite further changes in the final drive speed whereby the ratio range of the CVT is extended by providing two regimes of operation with a synchronous ratio between those regimes and the predetermined function is graphically represented as a straight line slope which corresponds to the slope the synchronous ratio. The invention also relates to a driveline controlled by the method.

FIELD OF THE INVENTION 
This invention relates to vehicular drivelines including a 
continuously-variable-ratio transmission, or CVT, the input of which is 
connected to an engine or other prime mover, and the rotary output of 
which will be referred to as the final drive. 
While the invention is applicable to boats, aircraft, railed and other 
vehicles in general, it applies particularly to drivelines for automobile 
vehicles, in which the final drive is connected to the driven wheel or 
wheels. And while the invention is also applicable to vehicular drivelines 
including other types of CVT, for instance those of belt-driven, Kopp, or 
Beier type, and in which the ratio-varying element may be directly and 
mechanically controlled to determine the instantaneous ratio. 
The invention applies particularly to CVTs of the toroidal-race 
rolling-traction type in which there is no such direct mechanical control 
upon the rollers. Instead the angular setting to which they settle, and 
thus the ratio that they transmit, is determined by a balance between the 
resultant torque to which they are subjected, and the operating force 
applied to the carriages in which they are mounted. Such toroidal-race 
CVTs are now known in the art as being of "torque-controlled" type, and 
recent examples are described for instance in Patents GB-B-2227287 and 
GB-B-0356102. 
It is also now well understood in the art to control such drivelines so 
that the engine performs at all times in conformity with an electronic 
"map" constructed to optimise efficiency in some form. FIG. 1 of the 
accompanying drawings is a typical graph in which the y-axis represents 
engine torque T and the x-axis engine speed N, with the hyperbolae 1 
indicating lines of constant power. Ordinates 2 and 3 indicate idling and 
maximum engine speeds respectively. Function 4 represents the maximum 
torque of which the engine is capable over its full range of working 
speeds. Function 5 represents the preferred and predetermined "control 
line", to which the engine is programmed to work when under power demand, 
to achieve optimum performance according to a chosen criterion. That 
criterion may for instance be optimum emission characteristics, or least 
knock. With sophisticated control systems, an engine can be programmed to 
work to alternative control lines from which the driver can select. 
Typically, and as shown in FIG. 1, function 5 is chosen to achieve maximum 
power per unit of fuel consumed. Function 8 indicates the typical 
correlation of T and N when demand is withdrawn and the engine is 
over-running.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
The graph of FIG. 2 illustrates the typical correlation of engine speed N 
(y-axis) and vehicle speed V (x-axis) in an automotive vehicle driveline 
where the engine is controlled as indicated in FIG. 1, and where the 
driven wheels of the vehicle are the final drive. Lines 9 and 10 
correspond to ordinates 2 and 3 of FIG. 1, and the sloping linear function 
11 (which passes through the origin 0) corresponds with first gear ratio 
in a typical fixed-ratio transmission and thus defines the effective 
minimum values of N and V at which it is judged desirable for the 
driveline to work in practice. Ordinate 12 marks the maximum safe value of 
V. The effective "area" within which the driveline is capable of working 
is therefore bounded by lines 9, 11, 10 and 12. 
Assume that the vehicle has been travelling in steady state at a vehicle 
speed V.sub.1 and engine speed N.sub.1 indicated by the intersection of 
the two lines at point 13. Assume now that while other variables remain 
unchanged, the driver wishes to call for greater road speed. He does so by 
depressing the accelerator pedal and so demanding greater engine speed, 
say N.sub.2. The typical response of a known driveline, programmed to 
operate the engine to a control line 5 as shown in FIG. 1, has been to 
reach a final stable state of engine speed N.sub.2 and vehicle speed 
V.sub.2 (point 14) in two stages. Firstly by "kickdown", that is to say by 
causing engine speed to rise very quickly to N.sub.2 at point 15. Then 
secondly by "upshift"--that is to say, change of transmission ratio at 
constant engine speed--to move from point 15 to 14. While efficient, such 
a change can be disconcerting to drivers, especially those who are used to 
the response of typical manual and automatic transmissions containing a 
number of fixed ratios. With such known transmissions, the driver knows 
that rises in engine and road speeds go together, in any single ratio. The 
note of the engine is a reassuring guide to vehicle speed, and to the rate 
of change of that speed However, when the performance of the vehicle of 
FIG. 2 moves between points 13 and 14, the message to the driver is less 
clear. Between points 13 and 14 the engine note rises sharply, but vehicle 
speed V does not change. Between points 15 and 14 the vehicle speed 
changes, but the constant engine note suggests otherwise. At no time, 
during the transition from 13 to 14, do acceleration and engine note 
correlate in a way familiar to a conventional driver. 
The present invention aims to modify the mutual control of the engine and 
CVT so that although the engine can still operate to an optimum-efficiency 
control line like item 5 in FIG. 1, during at least part of the 
acceleration following a pedal depression (and in reverse, the 
deceleration following a relaxation) the engine note and the acceleration 
are correlated in a manner more familiar to a driver used to conventional 
manuals and automatics. The invention is defined by the claims, the 
contents of which are to be read as included within the disclosure of this 
specification. In particular the invention includes methods and apparatus 
as described by way of example with reference to the following further 
figures of the accompanying drawings, in which: 
FIG. 3 is generally similar to FIG. 2, but illustrates two modes of 
operating according to the invention, and 
FIG. 4 is a further graph correlating engine speed N with accelerator pedal 
position P. 
FIG. 5 is a block diagram of the driveline apparatus 43 comprising a prime 
mover 40, a CVT 41 and a final drive 42. 
FIG. 2 included line 11, indicative of the ratio (e.g. 5 mph vehicle speed 
per 1000 rpm engine revolution) of a typical first forward gear in a 
conventional fixed-ratio automobile transmission. Because zero vehicle 
speed correlates with zero engine rpm in any such fixed ratio, line 11 
passes through origin 0. Line 11 is repeated in FIG. 3, as is line 16 
indicative of the ratio (e.g. 10 mph/1000 rpm) both of a typical fixed 
ratio second gear, and also of a typical synchronous ratio--that is to 
say, the ratio where change takes place between first and second 
regimes--in a two regime CVT of the toroidal race or other type. FIG. 3 
also includes further lines 17-19, indicative of typical fixed 
ratios--e.g. 20, 30 and 40 mph per 1000 rpm respectively--of third, fourth 
and fifth forward gears in fixed-ratio transmissions. 
Assume that a vehicle, fitted with a driveline according to the invention, 
is travelling in steady state as indicated by point 20. That is to say, 
vehicle speed is V.sub.1 (as at 13 and 15 in FIG. 2) and the engine is 
running at its minimum permitted speed (say 1500 rpm) as indicated by line 
9. The driver now depresses the accelerator pedal, to a degree which the 
driveline is programmed to interpret as demanding a engine speed 
corresponding to N.sub.2 (as at points 15 and 14 in FIG. 2). According to 
one mode of operation of the present invention the driveline responds to 
the driver's demand in three stages, as follows. Firstly a rapid rise 
("kickdown") in engine speed to a value (referred to in the claims as 
"engage speed"), predetermined and built into the control system as the 
lowest speed at which the engine should be running at the beginning of a 
period of substantial acceleration. In a typical engine as shown in FIGS. 
2 and 3, capable of running between an idling speed of 1500 rpm. and a 
maximum speed of say 5000 rpm, this speed could for example be 2500 rpm. 
In FIG. 3 the "engage speed" is the same speed N.sub.1 as in FIG. 2 and is 
represented by line 6, and at the end of the first of the three stages of 
response point 13 has therefore been reached. 
Function 22, drawn through point 13 and the origin 0, is now pre-chosen by 
the control system as an ideal, equivalent fixed-ratio for the CVT and 
engine of the driveline to follow in the second stage of the response, 
during which the driveline moves from point 13 on line 6 to point 23 when 
engine speed N.sub.2 is attained. During this second stage of the 
acceleration response the driveline is therefore programmed to respond as 
if in fixed ratio, so that the driver "feels" the acceleration of the 
vehicle just as he would if the driveline had the fixed ratio represented 
by function 22. Once the demanded engine speed N.sub.2 is reached at point 
23, the third stage of the response begins. During this the engine speed 
remains at V.sub.2, so that the engine note does not change. Any further 
rise in vehicle speed to a final value (e.g. to point 24, value V.sub.2 as 
in FIG. 2), at which the force exerted on the vehicle by the engine is 
matched by the forces resisting the vehicle's motion will be accommodated 
by ratio change within the CVT. 
A possible drawback of the embodiment of the invention shown in FIG. 3 and 
so far described is that for the complete acceleration (between points 20 
and 24) shown in the figure the "fixed-ratio" second stage, between points 
13 and 23, may be unduly prolonged because of the relatively gentle slope 
of function 22. Conversely, if acceleration of vehicle speed had been 
demanded from velocity V.sub.3 (point 26) to V.sub.4 (point 29) at the 
left-hand side of the graph, by way of a "fixed-ratio" second stage 27, 28 
coinciding with the "first ratio" line 11, that stage would have been 
relatively brief because of the steepness of the ratio line. In the 
alternative embodiment of the invention also shown in FIG. 3, such 
extremes are avoided by programming the driveline so that during the 
second, "fixed ratio" stage of the response to a driver's demand for 
acceleration, vehicle and engine speeds rise together at a rate parallel 
to a chosen fixed ratio line--in this case, the line 16 representing the 
typical synchronous ratio of a two-regime toroidal-race CVT, or the second 
forward gear of a typical fixed-ratio transmission. Now, as shown in 
broken lines, the acceleration of the vehicle from speed V.sub.1 at point 
20, to speed V.sub.2 at point 24, is by way of points 13 and 31 instead of 
13 and 23, so that the second stage of the response is briefer than before 
but the third stage longer. Conversely for the acceleration from the 
V.sub.3 to V.sub.4 the second and third stages are now 27-32 and 32-29 
instead of 27-28 and 28-29, so that the second stage is longer than before 
and the third stage briefer. 
In practice the three-stage response to a demand for acceleration, of a 
driveline according to the invention, will probably be fractionally slower 
than the known two-stage response described with reference to FIG. 2. 
However that possible small increase in total time will be offset by the 
driver being able to sense the response to his demand in a way that has 
not been customary hitherto in CVT drivelines. With relation to FIG. 3, it 
should be noted that while such parameters as "engage speed", "limit 
speed" and the various notional ratios have all be shown for convenience 
as straight-line functions, it is within the scope of the invention and 
obviously within the capabilities of programmed driveline control that 
non-straight-line functions could be substituted. In particular, the value 
of "engage speed" could easily vary over the total range of vehicle speed. 
FIG. 4 interprets the operation of the invention, as already described with 
reference to FIG. 3, in relation to accelerator pedal depression P 
(x-axis). As before, the y-axis represents demanded engine speed N. As the 
left-hand section of FIG. 4 indicates, a typical driveline according to 
the invention may be programmed so that the three-stage reaction to a 
driver's demand for acceleration does not begin until the pedal is 
depressed at least 30%. Thereafter a "limit speed" 35, of any value 
between idling speed 9 and maximum engine speed 10 may be demanded. In 
FIGS. 2 and 3 N.sub.2 was the single example shown of such a "limit 
speed". As FIG. 4 indicates, the "engage speed" value may for instance 
rise steadily (slope 36) to a range or value which it holds over the 
majority of the working range of the driveline--e.g. the value of 2500 
rpm, line 6 in FIG. 3. However, as shown at the right-hand side of FIG. 4, 
the driveline may conveniently be controlled so as to raise the engage 
speed to a much higher value 37, so that the second, "fixed ratio" stage 
of the response starts on a much higher engine speed if the accelerator 
pedal is depressed almost fully, for instance in response to an emergency. 
In FIG. 4 the hatched part 38 of the graph indicates the "area" over which 
the second stage of the response can take place, and the hatched part 39 
the area over which the first "kickdown" stage takes place, during which 
engine speed rises rapidly, but there is no substantial change in vehicle 
speed. As already described, for each acceleration demand, the third stage 
of the response begins when engine speed reaches "limit speed".