Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02540729 2006-03-22
TIMING ADVANCEMENT FOR EXHAUST VALVE IN
INTERNAL COMBUSTION ENGINE
Background of the Invention
This invention relates to internal combustion engines and, more
particularly, to the timing of an exhaust valve of an engine to promote
efficiency
of fuel consumption.
A form of the internal combustion engine, generally used for powering
automobiles, operates in accordance with the Otto cycle, and may be referred
to
herein as a gasoline engine, as distinguished from a diesel engine. The
gasoline
engine, as well as the diesel engine, employs one or more cylinders, each
cylinder having a piston movable along an axis of the cylinder with
reciprocating
motion, and being connected by a connecting rod to the engine crankshaft for
driving the crankshaft with rotary motion. Output power of the engine for the
driving of a load is obtained from the rotating crankshaft. In the case
wherein the
load is a vehicle driven by the engine, the engine crankshaft normally is
connected via a transmission with selectable gear ratios to a propeller shaft
of
the vehicle for imparting rotation to the drive wheels of the vehicle.
In the four-stroke form of the gasoline engine, the movement of a piston in
its cylinder is characterized by four strokes, which occur in a repeating
sequence,
the sequence of the four strokes being: an induction stroke, a compression
stroke, a power (or expansion) stroke, and an exhaust stroke. During the
Induction stroke, the piston moves away from the head of the cylinder to
produce
a vacuum that draws in a mixture of air and fuel vapors via an Intake valve
generally located in the head of the cylinder. During the compression stroke,
the
piston moves towards the cylinder head to compress the air-fuel mixture.
Approximately at the beginning of the power stroke, there is ignition of the
air-
fuel mixture and, during the power stroke, the expanding gases produced by the
combustion of the fuel drive the piston away from the cylinder head. During
the
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exhaust stroke, the piston moves towards the cylinder head to drive the
exhaust
gases out of the cylinder via an exhaust valve generally located in the
cylinder
head. In the usual construction of such an engine, an intake manifold is
provided
for bringing air and fuel from a carburetor or fuel-injection assembly to the
intake
ports of the cylinders, and an exhaust manifold is provided for removal of
combustion gases via exhaust ports of the cylinders.
It is useful to compare operation of the gasoline engine with the diesel
engine. In the case of the gasoline engine, both fuel and air are present in
the
cylinder during the compression stroke. The temperature produced in the gases
within the cylinder is below the ignition temperature of the air-fuel mixture
so as
to avoid premature ignition of the air-fuel mixture. Ignition is produced by
an
electric spark of a spark plug, mounted within the cylinder head. In a modem
engine, activation of the spark plug is provided by a computer at an optimum
moment, relative to the time of occurrence of the power stroke. In the case of
the diesel engine, only the air is present in the cylinder during the
compression
stroke. The geometry of the piston within the cylinder of the diesel engine
differs
somewhat from the corresponding geometry of the gasoline engine such that the
compression stroke of the diesel engine provides significantly more
compression
of the gases within the cylinder (a compression ratio of approximately 15:1)
than
occurs in the gasoline engine (a compression ratio of approximately 8:1). As a
result, in the diesel engine, the temperature of the air is raised by the
compression stroke to a temperature high enough to ignite fuel. Accordingly,
in
the diesel engine, the fuel is injected into the cylinder at approximately the
beginning of the power stroke, and is ignited by the high air temperature.
It is observed furthermore, that in the usual construction of a gasoline
engine and of a diesel engine, the ratio of the expansion of the volume of
cylinder gases in the power stroke, namely the final volume divided by initial
volume, is equal to the ratio of the compression of the volume of the cylinder
gases in the compression stroke, namely the initial volume divided by final
volume. The expansion of the cylinder gases in the power stroke is
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accompanied by a reduction in the temperature of the cylinder gases. Well-
known theoretical considerations show that an important consideration in
determining the efficiency of the engine is the ratio of the gas temperature
at the
beginning of the power stroke to the gas temperature at the end of the power
stroke. A greater temperature ratio is obtained in the case of the diesel
engine
than for the gasoline engine. This is one of the reasons that the diesel
engine
can operate more efficiently than the gasoline engine.
In spite of the foregoing theoretical considerations for the efficiency of a
power stroke of the engine, there appears to be other factors in the operation
of
an engine that result in a needless wasting of the energy in the fuel with a
resultant reduction in the fuel efficiency of the engine. In a test conducted
by the
present Inventor, an automobile powered by a gasoline engine was driven on the
highway over a period of time, such as one minute, at an engine speed of 3000
RPM (revolutions per minute) corresponding to approximately 60 mph (miles per
hour). The test was then conducted again by driving the automobile In the
opposite direction of the highway so as to cancel out effects of any hills and
the
presence of any winds. The average amount of the fuel consumed was
determined, which amount for illustrative purposes may be considered to be 100
cc (cubic centimeters). The engine was also allowed to idle at an engine speed
of 3000 RPM, with the transmission in neutral to provide a no-load condition
for
the engine. Again, the fuel consumption over the foregoing testing interval of
one minute was measured, and determined to be 75 cc, only 25 percent less
than the amount of fuel consumed under loading, namely the foregoing amount
of 100 cc. The difference of only 25 percent shows that the major source of
wasted fuel lies directly within the operation of the engine. There is a need
for
obtaining better fuel economy from an engine, and in view of the foregoing
testing, it appears to be necessary to alter the engine itself in order. to
obtain
Improved fuel economy.
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Summary of the Invention
In accordance with the invention, it has been found that the withdrawal of
the exhaust gases at the end of the power stroke of the engine provides a
critical
role in the improvement of engine efficiency, particularly with respect to the
timing of the withdrawal of the exhaust gases in the interval of time
extending
from an end portion of the power stroke through an initial portion of the
exhaust
stroke. It is noted that in the conventional engine, wherein the exhaust valve
is
opened by a cam on a camshaft, the valve is opened gradually from a state of
full closure to a state of complete opening, and that an interval of time in a
range
of 1-3 milliseconds, more particularly approximately 1.5 milliseconds, is
required
for removal of the bulk of the exhaust gases. In order to provide a reference
point for the beginning of the open interval of the exhaust valve, the
starting point
for the open interval is taken at the instant when the exhaust valve has risen
0.050 inches from the valve seat.
By way of a possible explanation to the dependency of engine efficiency
on the timing of the exhaust interval, is noted that the velocity of the
piston head
along the axis of a cylinder is sinusoidal with respect to time, and undergoes
a
maximum deceleration both at top dead center (TDC) and at bottom dead center
(BDC) of the path of the piston travel. With respect to the bottom dead
center,
both the piston with the exhaust gases must be decelerated to bring the piston
to
a state of zero speed before the direction of piston movement reverses at the
beginning of the exhaust stroke. If the exhaust interval, initiated by the
opening
of the exhaust valve, begins at a time prior to bottom. dead center, then much
of
the exhaust gases can be removed prior to the maximum deceleration of the
piston. This results in reduced forces on the piston journal and the journals
holding the crankshaft within the engine, and thus avoids dissipation of
energy
associated with any distortion of the engine in resisting the journal forces.
With respect to the matter of advancing the inception of the exhaust
interval, in the conventional engine, intended to operate most often at a rate
of
rotation of 3000 RPM (a lower value of RPM being present in highway driving
for
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the case of a vehicle having over-drive in the transmission), the opening of
the
exhaust interval is advanced in time, so as to occur prior to bottom dead
center
by 1.5 milliseconds, this being accomplished by timing the opening of the
exhaust valve to begin at a crankshaft angle of 30 degrees prior to bottom
dead
center. The intent of the advancement of the exhaust interval, in the
conventional engine, has been to maximize power output of the engine by
removal of most of the exhaust gases prior to the exhaust stroke so that
engine
power need not be wasted by having the piston push more than a minimal
amount of the exhaust gases out through the exhaust valve during the exhaust
stroke. For ease of reference, this advancement interval at the end portion of
the power stroke may be referred to hereinafter as the power advancement
interval. In the case of a conventional engine intended to operate at a higher
crankshaft rotation rate, such as a rate of rotation of 8000 RPM in a racing
engine, the opening of the exhaust valve at the inception of the exhaust
interval
may be as early as approximately 70 degrees before bottom dead center to
provide a suitable interval of time for exhausting the exhaust gases. Here
too, in
the case of the racing engine, the intent of the advancement of the exhaust
interval is to maximize available power output from the engine. There is no
suggestion in any of these engine designs to advance the exhaust interval any
more than that which maximizes the output power available from the engine.
In accordance with the invention, it has been discovered that still further
advancement of the exhaust interval relative to bottom dead center has the
effect of reducing the average output torque available for driving the wheels
of
the vehicle, this being an expected result, and also has the effect of
improving
the engine efficiency, this being an unexpected result. In the case of too
much
advancement of the exhaust interval relative to bottom dead center, so much
energy of the burned fuel is lost that there is little torque available for
driving the
vehicle, and there is no improvement in the engine efficiency.
For example, in a test for the driving of a car over a prescribed course by
an engine having a camshaft modified to provide for the advancement of the
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opening of the exhaust valve at a crankshaft angle of 90 degrees before bottom
dead center, it was observed that the fuel consumption was the same (7.0
gallons in the test vehicle) as in a car having a conventional camshaft with
advancement by 30 degrees before bottom dead center. However, the car with
the modified camshaft had greatly reduced available torque such that a long
time
was required to accelerate to a reasonable vehicular speed. Other tests,
conducted with camshafts providing advancement of 50 degrees and 70 degrees
provided equal values of fuel consumption (6.0 gallons in the test vehicle)
for
improved efficiency. Optimum efficiency appears to occur for advancement at a
crankshaft angle of approximately 60 degrees prior to bottom dead center, at
highway speed in a test vehicle having over-drive transmission. This situation
provided minimal consumption of fuel (5.5 gallons) while providing adequate
(but
reduced) available torque for driving a vehicle. Thus, utilization of the
present
invention provided a 21 percent improvement in fuel efficiency over a
conventional vehicle having a camshaft advancing the exhaust-valve opening by
30 degrees of crankshaft rotation.
The benefits of the invention are believed to apply both to a gasoline
engine and to a diesel engine. Operation of the exhaust valve may be by a
valve-driving device that is mechanical (as with the aforementioned camshaft),
hydraulic or electromagnetic. In engines wherein the valve timing is
adjustable
by a computer, the timing of the exhaust valve advancement may be adjusted in
accordance with engine speed so as to obtain improved energy efficiency while
minimizing power loss.
Brief Description of the Drawing
The aforementioned aspects and other features of the invention are
explained in the following description, taken in connection with the
accompanying drawing figures wherein:
Fig. 1 shows a stylized view of an internal combustion engine constructed
in accordance with the invention;
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Fig. 2 shows diagrammatically the connection of a piston to a section of
crankshaft in a viewing direction along an axis of the crankshaft;
Fig. 3 is a diagram showing positions of a crankshaft for different values of
torque produced by piston forces on a connecting rod; and
Fig. 4 is a timing diagram showing operation of valves and a piston
associated with a cylinder of the engine.
Identically labeled elements appearing in different ones of the figures refer
to the same element but may not be referenced in the description for all
figures.
Detailed Description of the Invention
Fig. 1 shows a diagrammatic view of an engine 10 having a plurality of
cylinders 12 with pistons 13 therein. One of the cylinders 12 is sectioned to
show its piston 13, and the remaining cylinders 12 are shown in phantom view.
With respect to an individual one of the cylinders 12, the piston 13 is driven
by a
crankshaft 14 of the engine 10, and connects by a connecting rod 16 with the
crankshaft 14 for reciprocating motion of the piston 13 during rotation of the
crankshaft 14. Motion of the piston 13 is characterized by a repeating
sequence
of four strokes, as described above. During the induction stroke and during
the
power (or expansion) stroke, the distance between the piston 13 and a head 18
of the cylinder 12 increases to provide for an increase in the volume of
cylinder
available for containing gases within the cylinder. During the compression
stroke
and during the exhaust stroke, the distance between the piston 13 and the head
18 decreases to provide for a decrease in the volume of the cylinder available
for
the containment of gases within the cylinder. Typically, in the construction
of the
cylinder head 18, the interior of the head 18 may be provided with a complex
shape to enhance combustion within the cylinder 12; however, for an
understanding of the present invention, the interior of the cylinder head 18
may
be represented by the more simple shape of a right circular cylinder as shown
in
Fig. 1.
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The engine 10 further comprises an intake valve 20, and an exhaust valve
22 located in the cylinder head 18, these two valves being present in both the
gasoline and the diesel forms of the engine 10. The valves 20 and 22 are
operated, respectively, by cams 24 and 26 of camshafts 28 and 30. It is
understood that the two camshafts are provided by way of example, and that, by
way of further example, a single camshaft with two cams thereon may be
employed for operation of the foregoing valves. The intake valve 20 is
operative
to close and to open an intake port 34 of the head 18. The exhaust valve 22 is
operative to close and to open an exhaust port 36 of the head 18. Also shown
in
Fig. 1 is a spark plug 40 for ignition of gases in the cylinder 12 in the case
of the
gasoline engine and, as an alternative form of construction, Fig. I also shows
a
fuel injector 42 for injecting fuel into the heated air of the cylinder 12 at
the
beginning of the power stroke for the case of the diesel engine.
The engine 10 also includes a timing device 44 for synchronizing rotation
of the crankshaft 14 with rotations of the camshafts 28 and 30. Lines 46 and
48
represent, respectively, connections of the timing device 44 to the camshafts
28
and 30. Line 50 represents connection of the timing device 44 to the
crankshaft
14. In the practice of the invention, the driving of the valve 20 and the
valve 22
may be accomplished by well-known mechanical, hydraulic or electromagnetic
apparatus synchronized to the crankshaft 14, which apparatus is represented
diagrammatically by the camshafts 28 and 30 and the timing device 44. By way
of example, in the case of a mechanical driving of the valves 20 and 22, the
timing device 44 with its connecting lines 46, 48 and 50 may be provided by
means of gearing and a timing belt (not shown) which interconnects gears on
the
crankshaft 14 and on the camshafts 28 and 30 to provide desired rates of
rotation and timing of the rotations of the camshafts 28 and 30 relative to
the
rotation of the crankshaft 14.
By way of further example, in the case of an electromagnetic driving of the
valves 20 and 22, the timing device 44 may be provided with a computer 52, the
line 50 represents. a shaft angle encoder providing instantaneous values of
the
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angle of the crankshaft 14 to the computer 52, and the lines 46 and 48
represent
electric motors for rotating the camshafts 28 and 30 in response to drive
signals
provided by the computer 52. The computer 52 may include a read-only memory
storing optimum camshaft angles for opening and closing both the intake valve
20 and the exhaust valve 22 as a function of various engine operating
conditions
such as crankshaft angle and rate of rotation, as well as possibly intake air
mass
flow rate and accelerator pedal position, by way of example. Based on data
stored in the memory as well as data provided to the computer 52 by engine
sensors, as are well-known, the computer 52 outputs the drive signals to the
electric motors for rotating the camshafts 28 and 30, thereby to operate the
valves 20 and 22 at the optimum times, respectively, for accomplishing the
induction and the exhaust functions. Information stored in the memory of the
computer 52, with respect to the optimum timing of each of the valves 20 and
22,
may be obtained by experimentation. The functions provided by the computer
52 may be provided by the engine-control computer found in a modem-day
engine, which computer may be provided, in accordance with the invention, with
programming designed to optimize the timing of the operation of the exhaust
valve 22 for best fuel efficiency of the engine. The computer 52 may be
provided
with further programming to momentarily cancel the additional advancement of
the exhaust interval to obtain maximum power, such as in the situation wherein
the driver of the vehicle fully depresses the accelerator pedal for entry into
a line
of traffic.
With reference also to Fig. 2, which presents a fragmentary view of the
engine 10 taken in a direction parallel to an axis of the crankshaft 14,
connection
of the piston 13 to the connecting rod 16 is made by way of a pin 54 that
enables
the connecting rod 16 to pivot relative to the piston 13. The opposite end of
the
connecting rod 16 connects with the crankshaft 14 via a journal 56 located in
a
crank arm 58 of the crankshaft 14, the journal 56 permitting the crankshaft 14
to
rotate about its axis 60 relative to the connecting rod 16. The crankshaft 14
is
supported by a set of bearings 62, two of which are shown in Fig. 1, located
in a
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housing 64 of the engine 10. The bearings 62 enable the crankshaft 14 to
rotate
relative to the housing 64.
In Fig. 3, the schematic representation of the connecting rod 16 and the
crank arm 58 corresponds to the presentation of Fig. 2, and shows various
positions of the crank arm 58 assumed prior to the reaching of bottom dead
center. Four positions of the crank arm 58 are shown, namely, BDC, 30 degrees
before BDC, 60 degrees before BDC, and 90 degrees before BDC.
Fig. 4 presents a timing diagram showing the various strokes of the piston
travel with the reciprocating motion in the cylinder. Also shown are the open
and
closed positions of the valves with reference to the piston travel. Five
graphs are
presented, each graph having a horizontal axis representing the time. In the
first
graph at the top of the diagram, the piston travel is shown as a sinusoidal
movement between the top of the stroke and the bottom of the stroke,
identified
in the figure. The midpoint of a stroke is also identified. The successive
strokes
are identified as: (1) the induction stroke wherein the piston travels from
the top
dead center position, adjacent the cylinder head, to the bottom dead center
position; (2) the compression stroke wherein the piston travels from the
bottom
dead center position to the top dead center position; (3) the expansion (or
power)
stroke wherein the piston travels from the top dead center position to the
bottom
dead center position; and (4) the exhaust stroke wherein the piston travels
from
the bottom dead center position to the top dead center position.
The second graph shows that the intake valve is open during the induction
stroke and closed during the other three strokes. The next three graphs show
operation of the exhaust valve for three separate situations. The first of the
exhaust graphs depicts an ideal situation wherein the crankshaft is rotating
very
slowly relative to the time required for exhausting the gases, in which case
the
exhaust valve is open during the exhaust stroke and closed during the other
three strokes. The second of the exhaust graphs depicts the situation for
conventional engine design, generally found in engines for automobiles,
wherein
the exhaust valve is opened before the conclusion of the power stroke by an
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interval (the power advancement interval) identified as A in Figs. 3 and 4. In
the
case of the conventional engine operation depicted in Fig. 4, for the power
advancement interval A, the opening of the exhaust valve is advanced by 30
degrees prior to bottom dead center in the rotation of the crankshaft. The
third
of the exhaust graphs depicts the situation for implementation of the present
invention in gasoline and diesel engines suitable for use in an automobile
wherein the opening of the exhaust valve occurs before the conclusion of the
power stroke by advancement of the opening of the exhaust valve by 60 degrees
prior to bottom dead center (30 degrees prior to the power advancement
interval
A) in the rotation of the crankshaft, this being optimum for the crankshaft
rotational speed of 3000 RPM. After the opening of the exhaust valve at the
aforementioned 60 degrees prior to bottom dead center, the exhaust valve
remains open during the remainder of the power stroke and into the period of
the
exhaust stroke.
In the practice of the invention, it is understood that the advancement of
the exhaust-valve opening, corresponding to the 60 degrees presented in the
last graph of Fig. 4, is a fixed amount provided in one embodiment of the
invention wherein the engine has a relatively simple design such that the
advancement is fixed for all values of rotational speed of the crankshaft.
However, in accordance with a second embodiment of the invention, wherein the
invention is implemented in an engine having a variable timing capability, the
advancement in the time of the opening of the exhaust valve would be varied in
accordance with the rotational speed of the crankshaft. For example, at an
engine speed of 1500 RPM, the advancement in the exhaust-valve opening
would be approximately 50 degrees prior to bottom dead center, and at an
engine speed of 4500 RPM, the advancement in the exhaust-valve opening
would be approximately 70 degrees prior to bottom dead center. Thus, in
accordance with the second embodiment of the invention, a range of values of
advancement of the exhaust-valve opening, namely a range of 50-70 degrees of
crankshaft angle indicated at B in Fig. 3, provides for more efficient
operation of
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the engine over a range of engine speeds, 1500-4500 RPM, generally used in
the driving of an automobile. After the opening of the exhaust valve at the
value
of advance in the aforementioned range of 50-70 degrees prior to bottom dead
center, the exhaust valve remains open during the remainder of the power
stroke
and into the period of the exhaust stroke. Both embodiments of the invention
provide for reduced fuel consumption with improved efficiency of the engine.
With reference to Fig. 3, while the range B is shown encompassing
advancement values of 50-70 degrees, it is understood that the range may be
extended somewhat to both larger and smaller values of the advance. For
example, at an advance of 75 degrees, it is believed that there would be an
improvement in efficiency, but that this improvement would be smaller than the
improvement at the advance of 70 degrees during normal driving conditions.
Similarly, at an advance of 45 degrees, it is believed that there would be an
improvement in efficiency, but that this improvement would be smaller than the
improvement at the advance of 50 degrees except for very low engine speed.
It is observed that at the 90 degrees advance, the crank arm 58 is
approximately perpendicular to the connecting rod 16. Thus, the piston force
is
delivering maximum torque to the crankshaft 14. At the 60 degrees advance, the
crank arm 58 is inclined somewhat to the connecting rod 16 for a reduction in
torque, and also the further travel of the piston 13 provides for a reduction
in the
force of the burnt gases, this resulting in a still further diminution of the
torque.
By use of the same reasoning, the torque is much smaller at the 30 degrees
advance, and drops to zero at bottom dead center at which position the crank
arm 58 is oriented parallel to the connecting rod 16. Therefore, it is
believed that
any benefit, in terms of fuel consumption, in an extension of the advance
range
B close to the 90 degrees advance would be negated by the loss of engine
power in the vicinity of maximum torque. This is consistent with the
aforementioned test results wherein the fuel consumption, in the cases of the
exhaust valve opening at the 30 degrees advance and at the 90 degrees
advance, was found to be the same.
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As can be seen by the geometry of Figs. 2 and 3, a constant rate of
rotation of the crankshaft 14 provides a piston speed which varies in a
sinusoidal
fashion, such that a maximum speed of the piston 13 is obtained at the
midpoint
of its path of travel between the top-dead-center and the bottom-dead-center
positions. Maximum acceleration and maximum deceleration occur,
respectively, at the top-dead-center and the bottom-dead-center positions
wherein the piston reverses direction of motion. With respect to the
deceleration
of the piston in the vicinity of the bottom-dead-center position, a maximum
force
must be exerted by the journal 56 and the bearings 62 against the piston to
accomplish the deceleration. This deceleration force is proportional to the
mass
of the piston and to the mass of the burnt gases, and must also counteract any
remaining propulsive pressure of the burnt gases. Thus, a significant amount
of
work must be done on the piston to decelerate it in the vicinity of bottom
dead
center. This work does not contribute to the rotation of the crankshaft and,
accordingly, is wasted work converted into heat. It is believed that the
invention,
by exhausting a major portion of the exhaust gases before the crank arm enters
into the vicinity of the bottom-dead-center position, reduces the amount of
wasted work to be done in decelerating the piston to zero speed, and thereby
improves the fuel efficiency of the engine.
It is to be understood that the above-described embodiments of the
invention are illustrative only, and that modifications thereof may occur to
those
skilled in the art. Accordingly, this invention is not to be regarded as
limited to
the embodiments disclosed herein, but is to be limited only as defined by the
appended claims.
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