U.S. patent application number 10/035271 was filed with the patent office on 2003-07-03 for continuously variable valve timing, lift and duration for internal combustion engine.
Invention is credited to Wang, Yushu.
Application Number | 20030121484 10/035271 |
Document ID | / |
Family ID | 21881640 |
Filed Date | 2003-07-03 |
United States Patent
Application |
20030121484 |
Kind Code |
A1 |
Wang, Yushu |
July 3, 2003 |
Continuously variable valve timing, lift and duration for internal
combustion engine
Abstract
Continuously variable valve timing, lift and duration in an
engine are achieved by altering the location of the pivot of the
rocker arm and by using a variable rocker arm assembly. The pivot
of the rocker arm assembly alters when a variable valve lift disc
rotates. The variable rocker arm assembly is composed of two sets
of arms. One end of both arms rotates around the pivot and the
other ends of both arms lay over on each other and are separated by
a spring. The separation gap is used to compensate the amount of
valve lift. When the separation gap between the two arms changes
due to the pivot shaft position change, valve lift changes
accordingly. The variable lift disc is used to alter the position
of the rocker arm pivot and it has engaging teeth that mate with a
helical gear. The rocker arm shaft is mounted on the variable lift
disc, when the disc rotates, it alters the rocker arm pivot
position. The disc rotates under the longitudinal movement of the
helical gear controlled by the hydraulic oil pressure from
lubricant. The longitudinal travel of the helical gear is directly
related to the engine speed and the hydraulic pressure. Under
higher engine RPM, higher hydraulic pressure pushes helical gear
longitudinally and rotates the variable lift disc, thus achieve
higher valve lift by eliminating the separation gap between two
arms. Under low engine RPM or low hydraulic oil pressure, a spring
attached to the helical gear assembly forces the helical gear
subsequently the pivot of rocker arm back to its original position.
Therefore, the separation gap in the variable rocker arm assembly
increases, consequently the valve lift decreases. The valve lift,
depending on engine speed, varies continuously from zero to maximum
lift.
Inventors: |
Wang, Yushu; (Marshall,
MI) |
Correspondence
Address: |
Yushu Wang
6 Friendship Lane
Marshall
MI
49068
US
|
Family ID: |
21881640 |
Appl. No.: |
10/035271 |
Filed: |
January 3, 2002 |
Current U.S.
Class: |
123/90.16 ;
123/90.27; 123/90.43; 123/90.44 |
Current CPC
Class: |
F01L 2820/01 20130101;
F01L 13/0021 20130101; F01L 1/18 20130101; F01L 1/34 20130101; F01L
13/0026 20130101 |
Class at
Publication: |
123/90.16 ;
123/90.27; 123/90.43; 123/90.44 |
International
Class: |
F01L 001/34; F01L
001/02; F01L 001/18 |
Claims
I claim:
1. A continuously variable valve timing, lift and duration system
for an internal combustion engine comprising a variable rocker arm
assembly, a variable lift disc, and a driving device.
2. The system as claimed in claim 1 wherein the said rocker arm
assembly is end pivoted and has two sets of arms. Both arms have a
pivot rocker shaft at one end. The other ends of the arms lay over
on each other and are separated by a spring.
3. The system as claimed in claim 1 wherein the said rocker arm is
center pivoted and has two sets of arms. Both arms share a pivot
rocker shaft. The other ends of the arms lay over on each other and
separated by a spring.
4. The system as claimed in claim 1 wherein the pivot of rocker arm
is mounted to the said variable lift discs. The said disc is
toothed on inside diameter. The position of the said rocker arm
pivot alters when the said disc rotates.
5. The system as claimed in claim 1 wherein the said pivot point of
rocker arm is mounted to the said variable lift discs. The said
disc has engaging gear teeth on outside diameter. The position of
the said rocker arm pivot alters when the said disc rotates.
6. The system as claimed in claim 1 wherein the said variable lift
disc is a section of a circular plate. It alters the rocker arm
pivot position when rotates.
7. The system as claimed in claim 1 wherein the said variable lift
disc is an eccentric shaft. The said shaft alters the rocker arm
pivot potion when it rotates.
8. The system as claimed in claim 1 wherein the said driving device
comprises a gear, a spring, and a hydraulic system. The said gear
makes the said variable lift disc rotate when the said gear moving
by the said hydraulic system. The said spring pushes the said
helical gear back towards its original position when the said
pressure of hydraulic system drops. And the pressure of said
hydraulic system is a function of engine speed.
9. The system as claimed in claim 1 wherein the said driving device
is a stepping motor. The motor drives the variable disc and alters
the rocker arm pivot position, thus alters the valve lift.
10. The system as claimed in claim 8 wherein the said spring is
replaced with a hydraulic unit. The pressure difference of said
hydraulic system drives the gear and rotates the variable disc. The
pressure difference of said hydraulic system is a function of
engine speed.
11. The system as claimed in claim 2 wherein the amount of said
separation between two arms in the said variable rocker arm
assembly changes when the said rocker arm pivot alters its
position.
12. The system as claimed in claim 3 wherein the amount of said
separation between two arms in the said variable lift rocker arm
assembly changes when the said rocker arm pivot alters its
position.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to an internal combustion
engine using poppet type valves to direct gases into and out of one
or more cylinders. More specifically, the present invention
addresses the variable valve timing, lift and duration mechanism of
internal combustion engines. The degree of valve lift and duration
may be continuously altered depending on the engine speed, along
with the opening and closing times of the valves, so that engine
torque at different engine speeds, fuel efficiency, emission, idle
stability, and valvetrain wear characteristics are all
improved.
BACKGROUND OF THE INVENTION
[0002] The flow dynamics of air or air/gas mixture entering and
exiting internal combustion engines is one of the controlling
factors of engine performance. Most engines must work over a wide
speed and load range, making it difficult to achieve optimum
efficiency over more than a narrow part of that range. For
simplicity, economy and durability, most conventional four stroke
engines use the tried-and-true fixed camshaft systems that have
constant phase (when the valves are opened), duration (how long the
valves are held open) and lift (how far the valves are lifted off
their seats). This leads to certain design compromises to achieve
acceptable performance. An engine that produces high torque for its
capacity at low engine speeds usually gives poor torque at higher
engine speeds, and vice versa. In a paper published by the Society
of Automotive Engineers (Hara, Kumagai and Matsumoto, 1989, SAE
paper 890681), the authors present experimental results on an
engine in which the timing and lift were varied. Torque was
improved by 7% at 1600 rpm by variation of lift, and the
improvement at 6000 rpm was 14%. Alteration of lift of the intake
valve produced most of the effects seen.
[0003] Many approaches have been proposed and tried in attempts to
optimize the flow processes. Improvements to the flow dynamics are
achieved by three separate but interrelated processes: variable
phase, variable duration, and variable lift. It is well known that
engines that produce high torque at low speeds have lower overlap
between the closing of the exhaust valve and opening of the intake
valve. Small overlap allows for little communication between the
exhaust gases and the incoming fresh charge, limiting the amount of
uncontrolled mixing. This leads to stable operation. However, at
high speeds the inertia of the gases requires a greater period of
overlap to allow for gas exchange. The simplest way of achieving
the change in overlap is to alter the relative timing, or phasing,
of the intake camshaft to the crankshaft and exhaust camshaft.
[0004] If the phase of a valve event is altered, say advancing the
valve opening to an earlier crankshaft angle, then the closure of
that valve is also advanced. In many cases this causes a reduction
of the amount of combustible gas that can enter the engine. To
overcome this situation, the duration of the valve event may be
altered. In the example above, as the engine speed is increased and
the valve overlap is increased (opening the intake valve earlier),
the period that the intake valve stays open is extended to delay
the closing.
[0005] The peak lift of valves is designed to accommodate gas flow
at maximum engine speeds without significant pressure drops. This
is more important for the intake process than the exhaust process,
where the piston pushes the gases out. At engine speeds below
maximum, the velocity of incoming gases through the valve curtain
will produce less turbulence, and may lead to lower torque than
would be achieved with a smaller valve opening. By varying valve
lift with engine speed, torque may be enhanced over the entire
operating range of the engine. Additionally, reduced valve lifts at
lower speeds reduce the frictional losses of the valve train and
reduce valvetrain wear as well, depending on the design.
[0006] There are many examples in the U.S. patent literature of
methods of varying either or all of phase, duration and lift. Many
authors have recognized that engine performance over a wide speed
range may be improved by providing a means of switching between two
independent cam profiles for low and high speed operation. Such an
"on or off" type controller will provide different values of phase,
duration and lift between the two (or possibly more) different
engine speed ranges, resulting in improved performance and
efficiency for each speed range. However, within each speed range,
there is no means or varying phase, duration and/or lift. Examples
of such mechanisms are given in U.S. Pat. No. 4,151,817 by Mueller,
U.S. Pat. No. 4,205,634 by Tourtelot, U.S. Pat. No. 4,970,997 by
Inoue, et al., and U.S. Pat. No. 5,113,813 by Rosa.
[0007] Variable valve timing, lift and duration have found many
commercial applications in the last decade. Honda's latest 3-stage
VTEC (Variable Valve Timing and Lift Electronic Control) system has
three cam lobes with different timing and lift profile, one has
fast timing and high lift, the second has slow timing and medium
lift, and the third has slow timing and low lift. During low rpm
operations, the rocker arms riding the low rpm lobes push directly
on the top of the valves. In most of the cam profiles of the two
intakes valves will be slightly different, promoting swirl in the
combustion chamber for better driveability. At high rpm, the ECU
sends a signal to an oil control valve that allows pressure to flow
into the low rpm rocker arm. A third, high rpm rocker arm sits
between two low rpm arms and follows a much more aggressive lobe.
When oil pressure arrives, hardened steel pins pop out of the sides
of the low rpm rocker arms and slide into sockets in the high rpm
arm, and the valves start following the larger cam profile. Nissan
Neo VVL has very similar 3-stage lift mechanism. Toyota's VVTi
system uses slide pin to latch and unlatch the hydraulic lift to
vary the valve lift for the direct acting type valvetrain. For
rocker arm type valvetrain, it uses a sliding pad to latch and
unlatch, so achieve the 2-stage valve lift. There are many more
manufacturers using the similar discrete lift mechanism for
variable valve lift and duration and using separate cam phasing for
variable valve timing. BMW however has a mechanical system called
Valvetronic that uses conventional lobes. It also uses a secondary
eccentric shaft with a series of levers and roller followers,
activated by a stepper motor. The stepper motor changes the phase
of the eccentric cam, modifying the rocker ratio of the rocker
system to achieve a continuous variable valve lift and duration.
There are many components added to the Valvetronic valvetrain
system and the stiffness of valvetrain is significantly reduced.
This is expected to pose problem for valvetrain dynamics at high
speeds.
[0008] Another means of achieving variation in all three parameters
is to use an axially moveable camshaft, with a variable profile in
the axial direction. In this case there may be a smooth transition
between different values of phase, duration and lift, although the
relationship between all three is again fixed for a particular
axial position of the camshaft. U.S. Pat. No. 5,080,055 by Komatsu,
et al., describes such devices.
[0009] An alternative approach to varying all three parameters
involves the use of multi-part rocker arms, with one or more of the
arms pivoted eccentrically. In U. S. Pat. No. 4,297,270 by Aoyama
two interacting rocker arms function to vary phase, duration and
lift.
[0010] In U.S. Pat. No. 4,714,057 by Wichart, the author discloses
control over all three parameters by using a multi-part rocker arm,
and a control cam as well as the lift cam. A major purpose of their
invention is to be able to control engine load without a throttle
plate.
[0011] An innovative scheme is disclosed in U.S. Pat. No. 4,898,130
by Parsons, to vary the phase, duration and lift of the valves,
with an eccentrically mounted oscillating drive.
[0012] Variable valve lift is achieved by yet another means in U.S.
Pat. No. 5,031,584 by Frost. Two fixed pivot rocker arms are
combined with a movable interposed member to alter the mechanical
advantage of the camshaft to valve movement. Another means
achieving variable valve lift by moving the pivot point is given by
Hoffman in U.S. Pat. No. 5,205,247. A rotatable pivot shaft locates
a pivot point for a circular rocker arm. The centers of the
circular arms of the rocker arm are located on the same side of the
rocker arm as the pivot. As the pivot point is varied, the circular
shape of the rocker arm offers the same geometry to the cam and
valve at each location of the pivot. The valve timing is altered by
using different radii and/or offset centers for the arc segments
either side of the pivot, combined with cam profiles that differ
from standard profiles.
[0013] Entzminger offers a simple concept for varying valve lift in
U.S. Pat. No. 4,721,007. A toothed pivot shaft mates with a toothed
rack embedded in an elongate rectangular slot in the rocker arm.
The pivot shaft translates and rotates simultaneously, following a
linear path defined by another stationary toothed rack.
[0014] Another class of actuation mechanisms that can vary lift and
duration is that of hydraulic actuation, with lost motion. In this
method, the cam follower allows enclosed hydraulic fluid to leak
out either through a fixed orifice, or through a controlled
orifice. For the passive mechanism, the result is that the valve
will not open as far or as long at low engine speeds, while at high
speeds the leakage is insufficient to significantly alter the valve
movement from a conventional system. The active control approach
allows lift and duration to be controlled more closely. The result
is that conventional throttling may be discarded, as valve motion
may be enough alone to control the intake charge. Such a system is
described in SAE Paper 930820 (Urata, et al., 1993).
[0015] It has been recognized that non-variable valve duration is
no more acceptable, from the point of view of engine efficiency.
Variable valve duration and lift, even limited at only two or three
stages of the speed spectrum of an engine, demonstrates, among
other advantages, the superior capability of torque and emissions
control. Obviously, valve duration and lift that is optimal at
every point on the engine speed scale, and for all conditions of
engine operation, would be proportionately superior to a two or
three stage system as discussed. Of the many systems proposed to
achieve variable valve duration, the proposed system offers a
predictable baseline of induction and exhaust control throughout
the engine speed range, it offers opportunities to maximize fuel
usage, and minimize polluting emissions, factors of crucial
importance today and into the future.
SUMMARY OF THE INVENTION
[0016] An object of the present invention is to provide a
continuously variable valve timing, lift and duration in an
internal combustion engine.
[0017] To achieve this object, it is proposed that the system shall
comprise: a variable rocker arm assembly, a variable disc and a
driving device.
[0018] The rocker arm assembly has two sets of arms. Both arms
rotate around the same pivot and the other ends of both arms lay
over on each other and are separated by a spring. The separation
gap between two arms varies and compensates the amount of valve
lift when the rocker arm pivot point alters, therefore leads to the
change of valve timing, lift and duration accordingly.
[0019] The variable lift disc is toothed either on inside or on
outside diameter, connects with the variable rocker arm pivot. The
variable rocker arm pivot alters when the disc rotates through
mating gear of a driving device. The driving device can be either a
hydraulic pressure system or a stepping motor system. The
combination of the rocker arm pivot position change and the
separation gap between the two arms controls the degree of valve
timing, lift and duration.
[0020] This continuously variable valve timing, lift and duration
system is expected to achieve the improved fuel economy, emission,
engine performance, and valvetrain wear characteristics throughout
the engine speed range.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is a schematic of continuously variable valve timing,
lift, and duration (CVVTLD) device with end pivot rocker arm
assembly
[0022] FIG. 2 is a schematic of variable rocker arm assembly (end
pivot) with low valve lift, note the separation gap h between two
arms is large and separated by a spring
[0023] FIG. 3 is a schematic of variable rocker arm assembly (end
pivot) with high valve lift, note the gap h between two arms
separated by a spring is diminished due to rocker arm pivot
movement
[0024] FIG. 4 is a schematic of variable disc with engaging gear
teeth on inside diameter. The disc rotates when the helical gear
moves longitudinally through the inside diameter gear teeth. The
helical gear is driven by a hydraulic system. The spring returns
the gear and disc to its original position when the hydraulic
pressure drops.
[0025] FIG. 5 is a schematic of CVVTLD device with center pivot
rocker arm
[0026] FIG. 6 is a schematic of variable rocker arm assembly
(center pivot) with low valve lift, note the separation gap h
between two arms is large and separated by a spring
[0027] FIG. 7 is a schematic of variable rocker arm assembly
(center pivot) with high valve lift, note the gap h between two
arms separated by a spring is diminished due to rocker arm pivot
movement
[0028] FIG. 8 is a schematic of variable disc toothed on outside
diameter and driven by a helical gear
[0029] FIG. 9 is a schematic of another variable disc example with
gear teeth on outside diameter
[0030] FIG. 10 is a schematic of yet another variable disc example
with gear teeth in outside diameter
[0031] FIG. 11 is a normal valve lift diagram, neglecting the
ramping at opening and closing
[0032] FIG. 12 is a conventional phasing diagram when the intake
advanced
[0033] FIG. 13 is a conventional phasing diagram when the intake
advanced and the exhaust retarded
[0034] FIG. 14 is the lift diagram of CVVTLD system when cam
phasing is set at peak valve lift, i.e. intake valve opens at top
dead center (TDC) at peak valve lift. Note that timing retarded and
duration reduced as valve lift decreases
[0035] FIG. 15 is the lift diagram of CVVTLD system when cam
phasing is set at minimum valve lift, i.e., intake valve opens at
top dead center (TDC) at minimum valve lift. Note that there is no
overlap at minimum lift, but the overlap and duration increase as
the valve lift increases.
[0036] FIG. 16 is the CVVTLD phasing diagram when the intake
advanced and the exhaust retarded at minimum valve lift setting.
The cam phasing is set for both exhaust valve closes and intake
valve opens at top dead center (TDC) at the minimum lift setting.
Note also that there is no overlap at minimum lift, but the overlap
and duration increase as the lift increases.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0037] FIG. 1 shows a preferred embodiment of the present invention
depicting the end pivot rocker assembly. In practice, camshaft 2
rotates in synchronism with the engine crankshaft so as to displace
cam follower 3 which actuates the valve 1. Both arms 5 and 6 in the
said rocker arm assembly have the pivot shaft 4 at one end. The
other ends of arms 5 and 6 lay over each other. There is a spring
between arm 5 and arm 6 to separate the two arms, FIG. 2 and FIG.
3. The spring can either winds around the rocker shaft 13 or
directly placed between the ends of two arms 12. Or both springs 12
and 13 can be used together. In FIG. 2, the pivot 4 of rocker arm
moves downwards, spring 12 and/or spring 13 push the arm 5 and arm
6 apart. When camshaft 2 rotates, cam lobe pushes arm 5 down to
close the gap h between arm 5 and arm 6 before it can lift the
valve 1. Therefore, when the rocker arm shaft moves down, the valve
lift is getting smaller, duration getting shorter and timing
retarded i.e., opens late but close earlier. When the pivot 4 of
rocker arm assembly moves up, the gap h between arm 5 and arm 6 is
closing up. The gap h fully closes when the rocker shaft moves up
to a preset limit, the valve lift reaches its peak lift under a
specific cam profile, FIG. 3. The pivot 4 of rocker arm assembly
can be altered using a device rotatable within predetermined limits
by any suitable control system including hydraulic, electrical and
mechanical means and the like. FIG. 4 shows details of an example
of such mechanism with a variable lift disc 7 and helical gear 8,
both are used together to alter the rocker arm pivot 4 position.
Under hydraulic pressure 15, the helical gear moves longitudinally
and rotates the variable lift disc. It therefore alters the pivot
of rocker arm assembly. The spring 10 or 11 is used to push back
the helical gear back, thus changes the rocker arm pivot position
back to low valve lift. The spring 10 and 11 can be replaced with a
hydraulic chamber. The amount of disc rotation, rocker arm pivot
motion, helical gear travelling is a function of hydraulic
pressure, or engine speed.
[0038] FIG. 5 shows a second preferred embodiment of the present
invention depicting the center pivot rocker assembly. In practice,
camshaft 2 rotates in synchronism with the engine crankshaft so as
to displace cam follower 3 which actuates the valve 1. Both arms 5
and 6 in the said rocker arm assembly have the pivot shaft 4 at the
middle of arm 6 and one end of arm 5. One end of arm 5 contacts cam
lobe 2 and the other ends of both arm 5 and arm 6 lay over each
other. There is a spring between arm 5 and arm 6 to separate the
two arms, FIG. 6 and FIG. 7. The spring can either winds around the
rocker shaft 13 or directly placed between the ends of two arms 12.
Or both springs 12 and 13 can be used together. In FIG. 6, the
pivot 4 of rocker arm moves upwards, spring 12 and/or spring 13
push the arm 5 and arm 6 apart. When camshaft 2 rotates, cam lobe
pushes arm 5 down to close the gap h between arm 5 and arm 6 before
it can lift the valve 1. Therefore, when the rocker arm pivot moves
up, the valve lift is getting smaller, duration getting shorter and
timing retarded i.e., opens late but close earlier. When the pivot
4 of rocker arm assembly moves downwards, the gap h between arm 5
and arm 6 is closing up. The gap h fully closes when the rocker
pivot moves up to a preset limit, the valve lift reaches its peak
lift under a specific cam profile, FIG. 7. The pivot 4 of rocker
arm assembly can be altered using a device rotatable within
predetermined limits by any suitable control system including
hydraulic, electrical and mechanical means and the like. FIG. 4
shows details of an example of such mechanism with a variable lift
disc 7 and helical gear 8, both are used together to alter the
rocker arm pivot 4 position. Under hydraulic pressure 15, the
helical gear moves longitudinally and rotates the variable lift
disc. It therefore alters the pivot of rocker arm assembly. The
amount of disc rotation, rocker arm pivot motion, helical gear
travelling is a function of hydraulic pressure, or engine
speed.
[0039] FIG. 8 shows the details of third embodiment of the
invention depicting the engaging teeth on outside diameter of the
variable lift disc. The disc is not necessarily circular as
illustrated, it can be a sector gear or with teeth on part of the
disc. As shown in FIG. 4, the mechanism of the variable lift disc 7
and helical gear 8, is similar, both are used together to alter the
rocker arm pivot 4 position. Under hydraulic pressure 15, the
helical gear moves longitudinally and rotates the variable lift
disc from outside engaging teeth. It also achieves the object of
altering the pivot of rocker arm assembly. The spring 10 or 11 is
used to push back the helical gear back, thus changes the rocker
arm pivot position back to low valve lift. The spring 10 and 11 can
be replaced with a hydraulic chamber. The amount of disc rotation,
rocker arm pivot motion, helical gear travelling is also a function
of hydraulic pressure, or engine speed.
[0040] FIG. 9 and FIG. 10 are two more embodiments showing the
driving mechanisms that alter the rocker arm pivot position when
engaging teeth are located outside diameter of the disc.
[0041] FIG. 11 shows a normal valve lift diagram without any
overlap in timing. Exhaust valve opens at bottom dead center (BDC)
and closes at top dead center (TDC) after 180.degree. cam angle,
thereafter intake valve opens at TDC and closes at BDC after
180.degree.. All lift diagrams in FIG. 9 and subsequent figures
have been simplified by eliminating the ramps at opening and
closing.
[0042] FIG. 12 is a conventional phasing diagram when the intake is
advanced. Basically, it varies the valve timing by shifting the
phase angle of camshafts. This movement is controlled by engine
management system according to need, and actuated by hydraulic
valve gears. Note that cam-phasing VVT cannot vary the duration of
valve opening. It just allows earlier or later valve opening.
Earlier opening results in earlier close, of course. It also cannot
vary the valve lift.
[0043] FIG. 13 is a conventional phasing diagram when the intake
advanced and the exhaust retarded. This enables more overlapping,
hence higher efficiency.
[0044] FIG. 14 is the continuously variable valve timing lift and
duration diagram when the CAM phasing set at the peak lift. This is
not a good setting since the intake valve opening delayed after TDC
when the valve lift reduced.
[0045] FIG. 15 is the CVVTLD phasing diagram when the CAM phasing
set at the minimum lift. The intake valve opening advances before
TDC when the valve lift increases. The changes in timing, lift and
duration can be continuous depending on engine speed.
[0046] FIG. 16 the CVVTLD phasing diagram when the intake advanced
and the exhaust retarded. The setting is based on minimum lift,
i.e., exhaust valve closes and intake valve opens at TDC at the
minimum lift setting. Therefore, the overlap and valve lift
duration increase when the valve lift increases. The change in
timing, lift and duration is continuous and a function of engine
speed. At low engine speed, such as at idle, valve lift is low and
lift duration is short. Since the valve lift and duration control
the air/charge flow, the result is that conventional throttling may
be discarded, as valve motion may be enough alone to control the
intake charge and the pump lost is then minimized. Valvetrain wear
is also minimized, at low lift at low engine speed, the valve seat
seating velocity is low, the reciprocating speed at stem/guide
interface is low, and the contact stress at cam and cam follower is
also low. There is a possibility that the lash compensating device
such as hydraulic lifter, lash adjuster and hydraulic capsule can
be eliminated since there is no lash existing in the CVVTLD system.
Another benefit of the CVVTLD system is, as in all variable valve
timing devices, the increased overlap as the engine speed increases
resulting in the internal exhaust gas recirculating (EGR) effect.
As illustrated in FIG. 16, the variable valve timing or increased
overlapping can be achieved by the CVVTLD system without using the
cam phasing device. Therefore, the CVVTLD system can improve fuel
economy, emission, engine performance, and valvetrain wear
characteristics throughout the engine speed range without
significantly compromising the design, structure, and cost.
* * * * *