Variable camshaft drive system for internal combustion engine

A variable camshaft drive system for an internal combustion engine includes a primary camshaft phaser interposed between the engine crankshaft and a first camshaft. A secondary camshaft phaser determines the phase relationship between the first camshaft and a second camshaft driven by the first camshaft and which is located on the same bank of cylinders as the first camshaft. The operation of each phaser is controlled by an electronic controller which samples one or more engine operating parameters.

FIELD OF THE INVENTION 
The present invention is related to a variable drive system for operating 
the cylinder valves of a reciprocating internal combustion engine with two 
camshafts for each cylinder bank, with the phase relationships of the 
camshafts being variable with respect to each other and with respect to 
the crankshaft. 
BACKGROUND OF THE INVENTION 
It is well known that spark ignition internal combustion engines will 
exhibit improved power and torque output, as well as reduced fuel 
consumption and exhaust emissions if such engines are equipped with 
variable valve timing operated with an optimized strategy. 
With dual overhead camshaft engines, a first camshaft on each bank is 
usually operated either by single flexible, inextensible drive member such 
as a chain or belt or, alternatively, by gears from the crankshaft. With 
such a system, it is commonly known to shift the phase relationship 
between the crankshaft and both camshafts simultaneously by means of a 
belt-timing control system such as that shown in U.S. Pat. No. 5,088,456 
to Suga, which is hereby incorporated by reference. The '456 patent 
discloses a type of so-called "dual-equal" system in which the intake and 
exhaust camshafts are phase shifted with respect to the crankshaft of the 
engine by the same amount. Although the dual-equal system will achieve 
certain benefits, it is generally incapable of changing the phase 
relationship between the camshafts of an individual cylinder bank. As will 
be explained below, it is desirable to have the capability to change the 
phase relationship between the dual camshafts on a cylinder bank. 
U.S. Pat. No. 4,726,331 to Oyaizu discloses a system for changing the phase 
relationship between the intake and exhaust valves on a single cylinder 
bank of an engine. However, this system is incapable of changing the gross 
camshaft timing with respect to the crankshaft. More specifically, with 
the system of the '331 patent, the relationship between the exhaust 
camshafts and the crankshaft does not change. U.S. Pat. No. 5,109,813 to 
Trzmiel et al. discloses another system for altering the overlap between 
dual camshafts, with the system being responsive only to engine oil 
pressure. This system suffers from the deficiency that its dependency upon 
engine oil pressure renders it unable to control the camshaft phase 
relationship independently of engine speed. Although the concept of 
independently controlling the phase relationship between the crankshaft 
and both camshafts of a dual overhead camshaft setup is known, such a 
system would be very expensive and complex. 
A system according to the present invention offers the advantage that the 
gross timing between both camshafts and the crankshaft can be controllably 
changed but, more importantly, the phase relationship between the two 
camshafts on a given bank of cylinders may also be controlled. This offers 
the important benefit of being able to change the valve overlap occurring 
between the opening of the intake valve and the closing of the exhaust 
valve. During operation at idle and very light loads, it is desirable to 
have little overlap between the opening of the intake valve and the 
closing of the exhaust valve. A small overlap is desired to minimize the 
fraction of the incoming intake charge which comprises exhaust gas 
remaining from the previous cycle because an excessive exhaust gas 
fraction will cause combustion instability at idle and, therefore, higher 
exhaust gas hydrocarbon emissions. On the other hand, at part loads, it is 
desirable to use exhaust gas recirculation ("EGR"); for this reason, 
external EGR valves and hardware have been installed on engines. Of 
course, such hardware is expensive and requires extra maintenance to keep 
the EGR system in order. Accordingly, if the valve overlap could be 
increased such that the number of degrees between the opening of the 
intake valve and the closing of the exhaust valve is increased, internal 
EGR could be increased, thereby obviating the need for an external EGR 
valve and its associated plumbing. A system according to the present 
invention uses variable valve overlap so as to provide variable internal 
EGR while also allowing a form of dual-equal phaseshifting. 
SUMMARY OF THE INVENTION 
A variable drive system for operating the cylinder valves of a 
reciprocating internal combustion engine includes a first camshaft driven 
by a crankshaft of the engine, a primary phaser means for coupling the 
first camshaft to the crankshaft and for varying the phase relationship 
between the crankshaft and the first camshaft, and a second camshaft 
driven by the first camshaft through a secondary phaser means which 
couples the first camshaft to the second camshaft and which also varies 
the phase relationship between the first and second camshafts. The first 
and second camshafts are preferably mounted to the cylinder head of an 
engine with the first camshaft operating one or more exhaust valves and 
the second camshaft operating one or more intake valves for each cylinder 
of the engine. The phase relationship between the crankshaft and the first 
camshaft may be infinitely varied by the primary phaser means whereas the 
phase relationship between the first camshaft and the second camshaft may 
be variable in finite steps by the secondary phaser means. 
A system according to the present invention may further comprise an 
electronic control means for operating the primary and secondary phaser 
means, with the electronic control means sensing one or more engine 
operating parameters, such as engine speed and load, and adjusting the 
primary and secondary phase relationships according to the values of 
measured speed and load. 
According to yet another aspect of the present invention, a first camshaft 
drives a second camshaft by means of a flexible, generally inextensible 
drive element which is trained over sprockets affixed to the camshafts, 
such that two chords of the drive element extend between said sprockets, 
and with said secondary phaser means comprising a variable position 
tensioner controlled by the electronic control means. The tensioner has 
elements bearing upon both drive element chords extending between the 
camshaft sprockets. The variable position tensioner comprising the 
secondary phaser preferably includes upper and lower tensioning elements 
with one of the tensioning elements being in contact with each chord of 
the drive element and with each of the two tensioning elements being 
carried upon a separate plunger mounted in a separate oil-filled barrel 
mounted to the cylinder head between the chords of the drive element such 
that the amount of oil within each barrel determines the position of both 
the plunger and the associated tensioning element mounted thereto. The 
amount of oil in the barrels is determined by the position of solenoid 
equipped check valves operated by the electronic control means such that 
oil will be caused to flow into one of the barrels from the other barrel 
when the tension of the chord of the drive element associated with the 
selected barrel is less than the tension of the chord associated with the 
non-selected barrel. In this manner, torque reversals imposed upon the 
camshafts by lift mechanisms interposed between the camshafts and the 
engine valves, will cause the actual movement of the tensioner elements 
and plungers into or away from their respective barrels. 
According to yet another aspect of the present invention, the secondary 
phaser means has two stable positions corresponding to a position of 
lesser overlap between the opening of the intake valve and the closing of 
the exhaust valve, with this lesser overlap being applied when the engine 
is idling or is otherwise operating at low load, and a position of greater 
overlap between the opening of the intake valve and the closing of the 
exhaust valve which is applied when the engine is operated at medium 
loads. Other combinations of the position of the primary phaser and the 
secondary phaser are available according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As shown in FIG. 1, a V-block engine has a crankshaft 12 driving first 
camshaft 10 of the cylinder bank which is on the right side of FIG. 1. 
Those skilled in the art will appreciate that a system according to the 
present invention could be used with an engine having but a single bank of 
cylinders or multiple banks of cylinders, it being understood that the 
present system is merely replicated for additional banks of cylinders. 
Crankshaft 12 drives first camshaft 10 by means of primary timing chain 22, 
which may comprise a silent chain type of drive or a synchronous belt made 
of fiber reinforced elastomeric materials or other types of generally 
inextensible drive elements known to those skilled in the art and 
suggested by this disclosure. The engine shown in FIG. 1 is a dual 
overhead cam type of engine. Accordingly, not only first camshaft 10 but 
also a second camshaft 16 are mounted to cylinder head 19. As noted above, 
first camshaft 10 is driven directly by crankshaft 12 via primary timing 
chain 22 and primary driven sprocket 23. In turn, second camshaft 18 is 
driven by first camshaft 10. As shown in FIG. 2, drive sprocket 26, which 
is non-rotatably affixed to first camshaft 10 drives sprocket 28, which is 
non-rotatably affixed to second camshaft 16. In similar fashion to the 
primary drive of the engine, secondary timing chain 24, which is also a 
generally inextensible but flexible drive element, is trained over 
sprockets 26 and 28 such that two chords of drive element 24 extend 
between sprockets 26 and 28. 
In conventional fashion, first camshaft 10 drives a plurality of exhaust 
valves 20 via a lift mechanism (in this case a plurality of bucket tappets 
21, it being understood that only one poppet valve 20 and one tappet 21 is 
shown). Second camshaft 16 will accordingly operate the intake poppet 
valves (not shown) of each cylinder bank. The present invention takes 
advantage of torque reversals which operate upon the camshafts because of 
the forces imposed on the cam lobes by the bucket tappet lift mechanism. 
Because the poppet valves are conventionally urged into the closed 
position by coil springs (not shown), the tappets are also urged into the 
closed position by the valve springs. This force also acts upon the lobes 
of the camshaft and will cause the imposition of bi-directional torque 
pulses upon the camshafts. 
Those skilled in the art will appreciate that the phase relationship 
between crankshaft 12 and first camshaft 10 is governed solely by primary 
phaser 14, which comprises any suitable device drawn from the class of 
known devices for changing the phase relationship between a crankshaft and 
the camshaft of an engine. Primary phaser 14 has an effect on the phase 
relationship of second camshaft 16 with respect to crankshaft 12, which 
relationship may be at least partially overridden by the action of 
secondary phaser 18. Primary phaser 14 may be of the infinitely variable 
type, whereas secondary phaser 18 may vary the phase relationship between 
the first and second camshafts in finite steps. In the present case, 
phaser 18 varies the phase relationship between first camshaft 10 and 
second camshaft 16 in two finite steps. 
As shown in FIG. 3, electronic controller 70 receives information from 
speed sensor 72 and load sensor 74, which may comprise any of the commonly 
used speed and load sensors such as a throttle position sensor or manifold 
pressure sensor, or yet other sensors known to those skilled in the art of 
electronic engine control design and suggested by this disclosure. 
Controller 70 uses the speed and load information and, if desired, 
information from other engine operating parameters as well to determine 
the proper phase relationship between crankshaft 12 and the camshafts 10 
and 16, as well as the proper phase relationship between camshafts 10 and 
16 themselves. Set forth below is a table showing various engine operating 
modes and the position of the primary and secondary phasers, and also the 
timing of intake valve opening, intake valve closing, exhaust valve 
opening, and exhaust valve closing, and also the overlap between intake 
valve opening and exhaust valve closing for an 8-cylinder engine having an 
intake and exhaust duration of events of 250.degree.. 
TABLE 
__________________________________________________________________________ 
Operating 
Primary 
Secondary 
Mode Phaser 
Phaser 
IVO IVC EVO EVC 
__________________________________________________________________________ 
Overlap 
Idle Full Ad 
Ret 12.degree. BTDC 
58.degree. ABDC 
62.degree. BBDC 
8.degree. ATDC 
20.degree. 
Light 36.degree. Ret 
Ret 24.degree. ATDC 
94.degree. ABDC 
26.degree. BBDC 
44.degree. ATDC 
20.degree. 
Load 
Med 40.degree. Ret 
Adv 8.degree. ATDC 
78.degree. ABDC 
22.degree. BBDC 
48.degree. ATDC 
40.degree. 
Load 
Full L 
14.degree. Ret 
Adv 18.degree. BTDC 
52.degree. ABDC 
48.degree. BBDC 
22.degree. ATDC 
40.degree. 
Low Sp 
Full L 
Full Ad 
Ret 12.degree. BTDC 
58.degree. ABDC 
62.degree. BBDC 
8.degree. ATDC 
20.degree. 
Md Sp 
Full L 
6.degree. Ret 
Ret 6.degree. BTDC 
64.degree. ABDC 
56.degree. BBDC 
14.degree. ATDC 
20.degree. 
High Sp 
__________________________________________________________________________ 
As shown in the table, when the engine is at idle, primary phaser 14 is in 
the fully advanced position, whereas secondary phaser 18 is in the 
retarded position. This means that the intake valve opens at 12.degree. 
before top dead center and closes at 58.degree. after bottom dead center, 
whereas the exhaust valve opens at 62.degree. before bottom dead center 
and closes at 8.degree. after top dead center. It should be noted that the 
intake valve opens before top dead center on the exhaust stroke of the 
engine. It is also shown in the first line of the Table that the overlap 
between the intake valve opening and the exhaust valve closing is 
20.degree.. The reader's attention is now directed to line 3 of the table, 
wherein at medium load, primary phaser 14 is operated in the 40.degree. 
retard position, whereas secondary phaser 18 is operated in the advanced 
position. This means that the intake valve opens at 8.degree. after top 
dead center, and closes at 78.degree. after bottom dead center. Exhaust 
valve 20 opens at 22.degree. before bottom dead center and closes at 
48.degree. after top dead center. Secondary phaser 18 produces an overlap 
between the opening of the intake valve and the closing of the exhaust 
valve which is 40.degree.. Different overlaps are used because at idle, it 
is desirable to have minimal EGR so as to promote maximum combustion 
stability and minimum exhaust hydrocarbon emissions. At medium load, 
however, it is desirable to apply EGR to control the formation of oxides 
of nitrogen, and the 40.degree. overlap between the intake valve opening 
and the exhaust valve closing allows increased internal EGR during medium 
load operation of line three of the Table. Note that in lines 5 and 6 of 
the Table which correspond to full load, medium speed operation, and full 
load, high speed operation, 20.degree. of overlap is used. 
Operation of the secondary phaser means will now be explained with 
reference to FIG. 2. Keeping in mind that sprocket 26 is the drive 
sprocket for chain 24, the chord running on the top of FIG. 2 will 
generally tend to be in tension, whereas the chord of chain 24 running on 
the bottom or lower part of FIG. 2 will generally tend to be slack. 
However, because of the previously explained torque reversals which are 
imposed upon the camshafts, the two chords of chain 24 will alternatively 
be placed in tension and allowed to become slack. This phenomenon assists 
secondary phaser 18 in accomplishing the phase change between camshafts 10 
and 16. 
The positions of upper tensioning element 30 and lower tensioning element 
32 determine the phase relationship between camshafts 10 and 16. The upper 
and lower tensioning elements are carried upon separate plungers mounted 
in separate oil filled barrels mounted upon cylinder head 19 between the 
chords of drive element or chain 24, such that the amount of oil within 
each barrel determines the position of the plunger and its associated 
tensioning element. Upper plunger 34 is mounted in barrel 38, whereas 
lower plunger 36 is mounted in barrel 40. Barrels 38 and 40 are 
interconnected by means of passages such that the total amount of oil 
contained in the barrels remains relatively constant with make-up being 
provided by valve 54 from the engine's oil supply. Because the amount of 
oil within both barrels remains relatively constant, any movement of one 
plunger must be accompanied by a corresponding movement in the same linear 
direction by the other plunger. When controller 70 decides according to 
the control scheme shown in the Table that more overlap is needed, and 
increases the overlap from 20.degree. to 40.degree. , both upper 
tensioning element 30 and lower tensioning element 32 must move up so that 
upper plunger 34 extends into barrel 38 to an extent less than lower 
plunger 36 extends into barrel 40. For this to happen, controller 70 must 
appropriately control upper barrel solenoid valve 42 and lower barrel 
solenoid valve 52. The solenoid valves are constructed identically. Upper 
barrel solenoid valve 42 includes ball check 44 which is spring loaded 
against a seat by means of spring 46. Solenoid plunger 48 is selectively 
engageable by means of coil 50, which is operated by controller 70 by 
means of a suitable electronic driver of the type known to those skilled 
in the art and suggested by this disclosure. When either solenoid is 
energized, the associated plunger will maintain its check ball against the 
seat and prevent the flow of oil past the check ball. In this case, the 
solenoid of lower barrel solenoid valve 52 is energized and thereby keeps 
check ball 53 on its seat. Then as the torque reversals cause the upper 
and lower tensioning elements to press their respective plungers into the 
barrels, oil leaving barrel 38, will flow into barrel 40, causing upper 
plunger 34 to move downward. Lower plunger 36 will also move downward, 
with the result that the overlap between first camshaft 10 and second 
camshaft 16 will be reduced. Conversely, if it is desired to increase the 
overlap between first camshaft 10 and second camshaft 16, controller 70 
will energize solenoid 50 associated with upper barrel solenoid valve 42. 
When solenoid 50 is energized, oil will not be allowed to pass around 
check ball 44, and as a result, the torque reversals imposed upon the 
camshafts which cause the chords above and below the camshafts to 
alternatively become tight and to go slack, will cause oil to flow from 
barrel 40 past check ball 53 and into barrel 38, thereby causing the 
overlap between the camshafts to increase as the length of the upper chord 
of chain 24 increases and the length of the lower chord decreases. As 
noted in the Table above, reduced or increased overlap may be employed at 
a variety of settings of the primary phaser. 
Those skilled in the art will appreciate that the phase relationships 
described herein are merely exemplary and that the particular values of 
overlap produced by the secondary phaser, as well as the phase 
relationship between the camshafts and the crankshaft will be dictated by 
the requirements of the engine to which the present invention is being 
applied.