Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TWO STROKE ENGINE CONVERSION
FIELD OF THE INVENTION
This application relates to a method and apparatus for converting an
internal combustion engine from four stroke operation to two stroke operation
which
is applicable to various sizes and configurations of engines having either
spark or
compression ignition.
BACKGROUND
Four stroke internal combustion engines having varying numbers of
cylinders are commonly used in various types of vehicles as they are known to
make
relatively efficient use of fuel as opposed to a two stroke internal
combustion engine.
For certain applications however, it is desirable to make use of the improved
torque
characteristics associated with two stroke engines. Adapting a conventional
four
stroke engine however, into a conventional two stroke engine is generally a
costly
and time consuming procedure as adjustment of the relative orientation of the
pistons as well as the location and timing of the valves comprises a
substantial
replacement of parts.
US Patent 5,154,141 to McWhorter describes an internal combustion
engine which operates as a four stroke engine at low speeds and as a two
stroke
engine at higher speeds. The cycle frequency of the valve operation is doubled
for
two stroke operation by gearing the camshaft to rotate at crankshaft speed
rather
than at half crankshaft speed as in four stroke operation. In order to
successfully
convert from four stroke operation to two stroke operation the engine requires
a
complex arrangement of a gas ejector and an electronic timing circuit which
controls
the rate of fuel injection and spark ignition. The complex arrangement of
numerous
parts results in a costly and high maintenance engine design in order to make
use of
the benefits of two stroke operation.
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SUMMARY
According to one aspect of the present invention there is provided a
method of converting to two stroke operation a four stroke internal combustion
engine having a plurality of pistons arranged for reciprocating movement
within
respective cylinders, an inlet valve and an exhaust valve associated with each
cylinder, a crankshaft coupled to the pistons, the crankshaft being driven to
rotate by
reciprocation of the pistons, an original camshaft assembly coupled to the
crankshaft, the camshaft assembly comprising at least one original camshaft
carrying a plurality of original cams for actuating respective inlet and
exhaust valves,
an original drive assembly coupling the camshaft to the crankshaft such that
the
camshaft is rotated at half a speed of the crankshaft, and electronic fuel
injection for
injecting fuel into the respective cylinders; said method comprising:
providing a replacement camshaft assembly for actuating the
respective inlet and exhaust valves, the replacement camshaft assembly being
arranged to open each of the valves once per revolution of the crankshaft;
replacing the original camshaft assembly with the replacement
camshaft assembly; and
programming the electronic fuel injection to inject fuel in each cylinder
once per revolution of the crankshaft.
A conventional four stroke, four cylinder engine is thus converted to a
two stroke engine wherein two pistons are fired synchronously every half
revolution
of the crankshaft with a minimal replacement of parts. The resulting engine
exhibits
improved torque characteristics without expensive or time consuming engine
refitting. The camshaft assembly can be replaced using conventional tooling
without
requiring extensive work to the engine.
The replacement camshaft assembly may comprise at least one
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replacement camshaft carrying a plurality of replacement cams, each
replacement
cam having a pair of lobes such that a corresponding one of the valves is
opened
twice per revolution of said at least one replacement camshaft. The pair of
lobes of
each replacement cam are preferably 180 degrees out of phase from each other
such that the respective valve is opened twice per revolution of said at least
one
replacement camshaft.
There may also be provided a replacement drive assembly comprising
a driven sprocket on said at least one replacement camshaft driven by a
driving
sprocket on the crankshaft, the driven sprocket having twice a number of teeth
of the
driving sprocket such that said at least one replacement camshaft is rotated
at half
crankshaft speed.
Alternatively, the replacement camshaft assembly may comprise a
replacement drive assembly having a driven sprocket on at least one
replacement
camshaft driven by a driving sprocket on the crankshaft, the driven sprocket
having
a number of teeth equal to a number of teeth on the driving sprocket such that
said
at least one replacement camshaft is rotated at crankshaft speed.
Preferably the replacement camshaft carries a plurality of replacement
cams, each replacement cam having a single lobe such that a corresponding one
of
the valves is opened once per revolution of the replacement camshaft.
The method may include mounting a turbocharger on the engine in
communication with the inlet valves for supplying combustion air above
atmospheric
pressure to the cylinders, mounting a supercharger on the engine in
communication
with the inlet valves for supplying combustion air above atmospheric pressure
to the
cylinders, or both.
When the engine includes a plurality of pairs of pistons, the method
preferably comprises firing each pair of pistons synchronously once per
revolution of
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the crankshaft.
The method also preferably includes providing a pressure feed
lubrication system within a sump of the engine for lubricating the pistons.
According to a second aspect of the present invention there is provided
a two stroke internal combustion engine comprising:
a plurality of pairs of pistons arranged for sliding movement within
respective cylinders, a first and second piston of each pair of pistons being
located
in a same position relative to the respective cylinders as the first and
second pistons
are displaced together within the respective cylinders such that the first and
second
pistons are fired synchronously;
an inlet valve and an exhaust valve associated with each cylinder;
a crankshaft coupled to the pistons, the crankshaft being driven to
rotate by the synchronous firing of the first and second pistons of each of
the pairs of
pistons;
a camshaft assembly coupled to the crankshaft, the camshaft
assembly comprising:
at least one camshaft;
a plurality of inlet and exhaust cams mounted on said at least one
camshaft for engaging the respective inlet and exhaust valves; and
a drive assembly coupling said at least one camshaft to the crankshaft
such that the inlet and exhaust cams open the respective valves once per
revolution
of the crankshaft;
the inlet and exhaust cams being arranged such that the exhaust valve
of each cylinder opens before the respective inlet valve opens and closes
before the
respective inlet valve closes;
the inlet valve and the exhaust valve of each cylinder both being open
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together for a portion of each piston stroke in an overlapping configuration;
and
the inlet valve and the exhaust valve of the first and second pistons of
each pair of pistons being arranged to be opened and closed synchronously by
the
inlet and exhaust cams;
and air supply mechanism mounted in communication with the inlet
valves arranged to supply combustion air above atmospheric pressure to the
cylinders.
Each cam may include a pair of lobes spaced 180 degrees apart about
the respective camshaft and the drive assembly couples the camshaft to the
crankshaft to rotate the camshaft at half crankshaft speed such that each of
the
valves is opened twice per revolution of the camshaft.
The engine preferably includes two pairs of pistons, the first and
second pistons of each pair of pistons being arranged to be fired
synchronously
once per revolution of the crankshaft. The two pairs of pistons are preferably
arranged to be fired 180 degrees apart in a four cylinder engine.
The air supply mechanism may comprise a turbocharger mounted in
communication with the inlet valves for supplying combustion air above
atmospheric
pressure to the cylinders.
According to a further aspect of the present invention there is provided
a two stroke internal combustion engine comprising:
a plurality of pairs of pistons arranged for sliding movement within
respective cylinders, a first and second piston of each pair of pistons being
located
in a same position relative to the respective cylinders as the first and
second pistons
are displaced within the respective cylinders such that the first and second
pistons
are fired synchronously;
an inlet valve and an exhaust valve associated with each cylinder;
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a crankshaft coupled to the pistons, the crankshaft being driven to
rotate by reciprocation of the pistons;
a camshaft assembly coupled to the crankshaft, the camshaft
assembly comprising:
a plurality of cams mounted on a camshaft for engaging the respective
inlet and exhaust valves; and
a drive assembly coupling the camshaft to the crankshaft such that the
cams open the respective valves once per revolution of the crankshaft;
a cam actuated fuel injector for injecting fuel into the respective
cylinders;
and dual lobed injector cams driven at half crankshaft speed for
injecting fuel into the respective cylinders once per revolution of the
crankshaft.
According to yet another aspect of the present invention there is
provided a method of converting to two stroke operation a four stroke internal
combustion engine having a plurality of pistons arranged for reciprocating
movement
within respective cylinders, an inlet valve and an exhaust valve associated
with each
cylinder, a crankshaft coupled to the pistons, the crankshaft being driven to
rotate by
reciprocation of the pistons, an original camshaft assembly coupled to the
crankshaft, the valve camshaft assembly comprising at least one original valve
camshaft carrying a plurality of original valve cams for actuating respective
inlet and
exhaust valves, an original valve drive assembly coupling the valve camshaft
to the
crankshaft such that the valve camshaft is rotated at half a speed of the
crankshaft,
cam actuated fuel injectors, and an original injector drive assembly including
at least
one original injector camshaft carrying a plurality of original injector cams
for
actuating the fuel injectors, the original injector drive assembly coupling
the injector
camshaft to the crankshaft for injecting fuel into each cylinder once for
every two
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rotations of the crankshaft; said method comprising:
providing a replacement valve camshaft assembly for actuating the
respective inlet and exhaust valves, the replacement valve camshaft assembly
being
arranged to open each of the valves once per revolution of the crankshaft;
replacing the original valve camshaft assembly with the replacement
valve camshaft assembly;
providing a replacement injector camshaft assembly for actuating the
fuel injectors, the replacement injector camshaft assembly being arranged to
inject
fuel into each cylinder once for every rotation of the crankshaft; and
replacing the original injector camshaft assembly with the replacement
injector camshaft assembly.
The method may include providing dual lobed fuel injector cams driven
at half crankshaft speed for injecting fuel into the respective cylinders once
per
revolution of the crankshaft.
Alternatively, the method may include providing single lobed fuel
injector cams driven at crankshaft speed for injecting fuel into the
respective
cylinders once per revolution of the crankshaft.
BRIEF DESCRIPTION OF THE DRAWINGS
In the accompanying drawings, which illustrate an exemplary
embodiment of the present invention:
Figure 1 is a cross sectional view of a piston cylinder arrangement
within a spark ignition variant of the engine.
Figure 2 is a longitudinal cross sectional view of the engine showing
four inline cylinders arranged for two stroke operation.
Figure 3 is an end view of a camshaft having single lobed cams
thereon.
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Figure 4 is an end view of a camshaft having dual lobed cams thereon.
Figure 5 is a cross sectional view of a piston cylinder arrangement
within a compression ignition variant of the engine.
Figures 6 and 7 are respective side elevational and end views of an
injection pump camshaft for rotation at crankshaft speed in the compression
ignition
variant of the engine.
Figures 8 and 9 are respective side elevational and end views of an
injection pump camshaft for rotation at half crankshaft speed in the
compression
ignition variant of the engine.
Figure 10 is a valve timing diagram for both the compression ignition
and spark ignition variants of the present invention.
DETAILED DESCRIPTION
Referring to the accompanying drawings, there is illustrated an engine
generally indicated by reference numeral 10. The engine 10 is a conventional
inline
four cylinder engine which has been modified from four stroke operation to two
stroke operation such that two pistons fire simultaneously with each half
rotation of
the crankshaft.
The engine 10 includes an engine block 12 having four cylindrical
bores 13 extending therethrough from a top end to a bottom end. The
cylindrical
bores are spaced one beside the other in a row. The block 12 includes an oil
pan 14
which encloses the bottom end of the bores 13 and a cylinder head 16 which
encloses the top end. Each bore forms a cylinder 17 which houses a piston
assembly 15 therein.
A cooling jacket 18 extends through the block 12 and cylinder head 16
for passing cooling fluid therethrough such that the engine block is cooled in
a
conventional manner.
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The oil pan 14 acts as a sump for collecting lubricating oil 19 which is
drained from the engine. The oil 19 is drawn from the oil pan 14 by a
regulated
constant pressure, continuous feed oil pump 20 and fed by the pump through an
oil
filter 22 to a recirculating lubricating system of the engine.
A crankshaft 24 is mounted on the engine block 12 to extend across
the bottom end of each cylinder 17. The crankshaft 24 is supported on bearings
for
rotation about a longitudinal axis 25. The bearings are mounted on the engine
block
in a conventional manner.
Each piston assembly 15 includes a connecting rod 26 which is
pivotally mounted at a bottom end on the crankshaft, spaced radially from the
longitudinal axis 25, diametrically opposed from a corresponding counterweight
27
mounted on the crankshaft. A top end of the connecting rod 26 is pivotally
mounted
on a piston 28 which is arranged for sliding movement within the corresponding
cylinder 17.
Each piston 28 is a cylindrical member having a domed crown 30 on a
top face and a short skirt 32 extending downward from a bottom end.
Diametrically
opposed bosses 34 in the skirt 32 pivotally support respective ends of a wrist
pin
therein. The wrist pin pivotally mounts the top end of the connecting rod 26
thereon
for coupling the connecting rod to the piston 28. The wrist pin may be press
fit into
the connecting rod or of a full floating type. Retainers can be mounted on the
bosses 34 as required for retaining the respective ends of the wrist pin
therein.
A pair of annular compression ring seals 38 are mounted spaced apart
about an outer face of the skirt 32 for sealing the piston against the side
walls of the
cylinder 17. An oil control ring 40 is formed below the ring seals 38 for
controlling
lubrication to the side walls of the cylinder 17 as the piston reciprocates
within the
cylinder.
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A combustion chamber 41 is defined within the cylinder 17 between
the piston crown 30 and the cylinder head 16. The oil control ring 40
restricts the
lubricating oil from seeping into the combustion chamber 41 from the bottom
end of
the cylinder.
An inlet and scavenger port 42 is mounted in the cylinder head 16 for
communicating with the combustion chamber and for supplying fresh air into the
combustion chamber 41 when an inlet poppet valve 44 mounted within port 42 is
opened. An exhaust port 46 is mounted in the cylinder head for communicating
with
the combustion chamber and for removing exhaust gases from the combustion
chamber when an exhaust poppet valve 48 mounted within the port 46 is opened.
The exhaust gases are subsequently expelled through an exhaust manifold 49.
The inlet poppet valve 44 is controlled by an inlet valve cam 50 having
a steep cam profile providing the required short opening duration. The exhaust
poppet valve 48 is similarly controlled by an exhaust valve cam 52 which also
has a
steep cam profile. The inlet and exhaust valves are illustrated laterally
spaced apart
in Figure 1, however their operation is similar when the valves of all the
cylinders are
mounted along a common longitudinal axis as shown in Figure 2.
Scavenging air above atmospheric pressure is pumped into the
combustion chamber 41 through the inlet port 42 by a supercharger 54. The
supercharger 54 is a conventional type which delivers a controlled amount of
pressurised air with each pumping cycle. The supercharger 54 further
pressurises
air which is already pressurised which the supercharger receives from a duct
56
connected to a turbocharger 58. The turbocharger 58 is coupled to the exhaust
port
and is driven by the exhaust gases exiting the cylinder 17.
The use of a supercharger allows the pump for the lubricating oil to
provide lubrication under pressure to the components of the engine because the
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sump is not used to pump air through the inlet valves in this arrangement.
A spark ignition system is mounted in the cylinder head 16 and
includes a spark plug 60 mounted at the top end of each cylinder 17. A fuel
injector
62 is mounted in the cylinder head 16 adjacent to each spark plug 60. The fuel
injector 62 is an electronically controlled direct injection type.
Alternatively to fuel
injector 62, a computerised electronically controlled multi-point, multi-port
fuel
injector 64 can be mounted upstream within the inlet port 42 for injecting
fuel directly
into the inlet manifold. In either case, the injector 62 or 64 is a known
programmable
type injector having an adjustable quantity of fuel delivered and an
adjustable timing
for varying engine speed and power output.
In operation, pressurised air is admitted into the cylinder and creates a
turbulent swirling motion to scavenge the burnt combustion gases, to drive
them
from the exhaust port 46. This provides a relatively clean combustion with a
low
level of exhaust gas emissions to the atmosphere.
The supercharger 54 is equipped with an air cleaner which is not
illustrated. The supercharger is operatively connected to the crankshaft by
either a
V-belt and pulleys or a timing chain and sprockets in a conventional manner.
The oil
pump 20 is similarly connected to the crankshaft. The turbocharger 58 also has
an
air cleaner which is not illustrated, the air cleaner being bolted to the
exhaust
manifold 49. The supercharger and turbocharger can be connected in parallel or
in
series while still operating effectively.
Figure 2 shows the four inline cylinders 17 arranged for two stroke
operation. A first piston 114 and a fourth piston 120 on opposing ends of the
block
are shown in a bottom dead centre position. In the bottom dead centre position
both
the inlet and scavenger valve 44 and the exhaust valve 48 are in a fully open
position. A second piston 116 and a third piston 118 which are spaced
befinreen the
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first and fourth pistons, are both shown in a top dead centre position. In the
top
dead centre position both the inlet and scavenger valve 44 and the exhaust
valves
48 are in a fully closed position. The first and fourth pistons 114, 120 are
thus
shown 180 degrees out of phase of the second and third pistons 116, 118.
In this arrangement, the first and fourth pistons are fired
simultaneously while the second and third pistons fire simultaneously 180
degrees
out of phase from the first and fourth pistons such that two pistons are fired
simultaneously for each half rotation of the crankshaft in a four cylinder
engine.
A camshaft 122 is mounted in the cylinder head for opening and
closing the inlet and scavenger valves 44 and the exhaust valves 48. The
camshaft
includes the inlet valve cams 50 and the exhaust valve cams 52. The cams 50
and
52 of the first and fourth pistons 114 and 120 are oriented in the same
respective
radial directions relative to the camshaft while the cams 50 and 52 of the
second and
third pistons 116 and 118 are also oriented in the same respective radial
directions
relative to the camshaft.
The camshaft 122 is coupled to rotate with the crankshaft 24 by a
timing chain 124, a driven sprocket 126 and a driving sprocket 128 mounted on
the
camshaft and crankshaft respectively. The sprockets 126 and 128 have equal
dimensions and an equal number of teeth such that the camshaft and the
crankshaft
rotate at the same speed. The cams 50 and 52 are thus arranged to open the
respective valve once for each revolution of the camshaft.
An end view of the camshaft 122 is illustrated in Figure 3 which shows
the relationship between the inlet and exhaust valve cams 50 and 52 for one
cylinder. The exhaust valve opens before the inlet valve opens and closes
before
the inlet valve closes. The inlet and exhaust valve cams are shown to overlap
for a
portion of the rotation at bottom dead centre when both valves are open.
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In a further embodiment of Figure 4, a camshaft 150 is shown in an
end view with cams for operating a single cylinder thereon. The camshaft 150
includes dual lobed cams. A first lobe 151 of each of the inlet valve cam 152
and
the exhaust valve cam 154 are oriented relative to each other to extend
radially in
the same direction as the cams of the camshaft 122, however a second lobe 156
is
located diametrically opposite from each first lobe 151. In this arrangement
the
second lobes are 180 degrees out of phase from the first lobes such that the
camshaft is only required to tum at half the crankshaft speed to effectively
open and
close the valve once for each revolution of the crankshaft. The camshaft 150
has a
driven sprocket mounted thereon having twice as many teeth as the driving
sprocket
128 for rotation at half crankshaft speed in use.
The camshaft 122 is particularly useful for converting a four stroke
engine into a two stroke engine with a minimal replacement of parts as the
gearing
for the camshaft does not need readjusting. In a four stroke engine, the
camshaft
operating the valves is generally geared to rotate at half crankshaft speed
such that
each valve is opened once for every two rotations of the crankshaft. In
converting a
four stroke to a two stroke engine, the gearing between the camshaft and
crankshaft
can be preserved by replacing the camshaft to one having dual lobed cams such
that the valves are opened once for each revolution of the crankshaft as
desired for
two stroke operation.
In Figure 5 a compression ignition variant of the engine 10 is shown.
The engine 10 is similarly arranged to the spark ignition variant, with the
exception of
the valves and ignition system and the lack of a turbocharger. The engine 10
of
Figure 5 includes a pre-combustion chamber 166 mounted in the cylinder head 16
in
communication with the combustion chamber 41 through a torch passage 167. A
glow plug 168 and a fuel injector 170 are installed in the pre-combustion
chamber
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166 by conventional means.
The valves 44 and 48 are opened and closed by respective valve cams
50 and 52 with the use of rocker arms 172 mounted on rocker shafts 173. The
cams
50 and 52 deflect the respective rocker arms 172 about the respective rocker
shafts
173 for effectively deflecting the valves and opening and closing the valves
as
prescribed by the cams. The cam profiles are similar to those of the first
embodiment.
In Figures 6 and 7, a camshaft 180 is illustrated which operates an
inline fuel injector pump for use with the engine of Figure 5. The fuel
injector pump
is a conventional type having a multi-pumping element for use in four cylinder
diesel
engines. The camshaft is geared to the crankshaft to rotate therewith at the
same
speed.
The camshaft 180 includes a pair of first cams 182 which are arranged
to engage fuel injectors in the respective first and fourth cylinders. The
first cams
182 extend from the shaft in identical radial directions for simultaneously
injecting
fuel into the first and fourth cylinders with each full rotation of the
crankshaft.
The camshaft also includes a pair of second cams 184 which are
arranged to engage fuel injectors in the respective second and third
cylinders. The
second cams 184 extend from the shaft in identical radial directions for
simultaneously injecting fuel into the second and third cylinders with each
full
rotation of the crankshaft. The second cams are 180 degrees out of phase from
the
first cams such that the one pair of cylinders are injected with fuel with
each half
rotation of the crankshaft.
In an alternate arrangement shown in Figures 8 and 9, a camshaft 190
is provided which operates an inline fuel injector pump for use with the
engine of
Figure 5 similarly to the camshaft 180. The camshaft 190 is geared to the
crankshaft
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to rotate at half the crankshaft speed. This is similar to the gearing of a
four stroke
engine, such that the camshaft 190 is useful for converting a four stroke
engine into
a two stroke engine with minimal replacement of parts as replacement of the
existing
camshaft for the camshaft 190 does not require replacement of any gearing
between
the camshaft and the crankshaft.
The camshaft 190 includes a pair of first cams 192 which are arranged
to engage fuel injectors in the respective first and fourth cylinders. The
first cams
192 extend from the shaft in identical radial directions for simultaneously
injecting
fuel into the first and fourth cylinders as the crankshaft is rotated.
The camshaft also includes a pair of second cams 194 which are
arranged to engage fuel injectors in the respective second and third
cylinders. The
second cams 194 extend from the shaft in identical radial directions for
simultaneously injecting fuel into the second and third cylinders with each
full
rotation of the crankshaft.
The first and second cams each includes a first lobe 196 and an
identical second lobe 198 which is diametrically opposite from the first lobe
such that
the second lobe injects fuel 180 degrees out of phase from the first lobe.
The lobes of the second cams are 90 degrees out of phase from the
corresponding lobes on the first cams such that the one pair of cylinders are
injected
with fuel with each quarter rotation of the camshaft corresponding to half
rotation of
the crankshaft. The engine thus fires one pair of the cylinders simultaneously
with
each half rotation of the crankshaft.
In Figure 10, the valve timing for every cylinder in both the spark
ignition and compression ignition variants of the engine are shown
schematically, as
well as the timing of the fuel injection for the compression ignition variant.
The
power stroke of the engine extends from top dead centre indicated by reference
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numeral 200 to between 105 and 120 degrees after top dead centre indicated by
reference numeral 202.
The exhaust of spent combustion gases begins between 105 and 120
degrees after top dead centre when the exhaust valve opens indicated by
reference
numeral 202. It ends between 207 and 230 degrees after top dead centre of
crankshaft travel when the exhaust valve is closed, indicated by reference
numeral
204.
The scavenging of burnt combustion gases with a fresh charge of air
begins between 130 and 145 degrees after top dead centre with the opening of
the
inlet and scavenge valves, indicated by reference numeral 206. It ends between
230 and 240 degrees after top dead centre of crankshaft travel when the intake
and
scavenge valve is closed, indicated by reference numeral 208.
Compression of the fresh air charge commences with closing of the
intake and scavenge valve at reference numeral 208 and ends at top dead
centre.
The duration of the exhaust valve opening is between 85 and 118
degrees of crankshaft rotation. Likewise, the duration of the opening of the
intake
and scavenge valve is between 90 and 120 degrees of crankshaft rotation.
The interval between the opening of the exhaust valve and the intake
and scavenge valve is between 22 and 35 degrees of the crankshaft rotation as
indicated as the difference between reference numerals 202 and 206. The
interval
between the closing of the exhaust valve and the intake and scavenger valve is
between 22 and 25 degrees of crankshaft rotation which is illustrated as the
difference between reference numerals 204 and 208.
In the compression ignition variant of the engine, the injection of
atomised fuel is approximately between 14 and 16 degrees before top dead
centre
as indicated by reference numeral 210.
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The conversion of four stroke operation to two stroke operation can be
accomplished on any size and configuration of engine having any number of
cylinders including 1, 2, 3, 4, 6, 8 or more. The resulting two stroke engine
will be
substantially similar to the original four stroke engine with the exception of
engines
having an even number of cylinders wherein the cylinders are fired in
simultaneous
pairs resulting from a minimal replacement of parts. The replacement of the
valve
cams from single lobe to dual lobe and the adjustment of the fuel injector
timing by
either doubling the lobes of the injector cams or by electronically adjusting
the timing
will effectively convert a four stroke engine into two stroke operation for
improved
power characteristics.
The engines described in the foregoing have been equipped with short
skirt pistons with a wrist pin that is press fit into the wrist pin end of the
connecting
rod. No retainer ring is required at the piston bosses with this arrangement.
Short
skirt pistons of this type are primarily used in short stroke engines, however
with
long stroke engines having larger displacement, long skirt pistons having full
floating
wrist pins with retainers at the bosses are primarily used.
The turbocharger and the supercharger combination which can be
connected in parallel or in series can be utilised in larger displacement
engines. The
engines could also be operated effectively using the supercharger alone.
A multi-cylinder engine could also be produced using the principles of
the above described invention having side mounted valves in a flat head design
with
the camshaft mounted in the block. Also, the addition of valves beyond the two
described above to increase the breathing capacity of the engine could be
employed
while the engine could still be converted from four stroke operation to finro
stroke
operation in a similar manner.
Using the valve timing and the components described above, a
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conventional four stroke internal combustion engine can be converted to two
stroke
operation with a minimal replacement of parts. In order to convert the engine
to two
stroke operation, the camshaft and gearing assembly is replaced by a camshaft
assembly which is arranged to open the inlet and exhaust valves once per
revolution
of the crankshaft. This can be accomplished by doubling the lobes or by
doubling
the speed of rotation of the camshaft by either of the methods noted above.
If the crankshaft is left unchanged in a four cylinder engine, the pistons
will be arranged such that two pistons fire synchronously with each half
revolution of
the crankshaft. Either a supercharger, a turbocharger or both may then be
mounted
on the engine in communication with the inlet ports for supplying combustion
air to
the cylinders in place of the expansion stroke in a four stroke engine. A
positive
pressure oil lubrication pump may then be mounted within the sump of the
engine as
the sump is not required for pumping air through the inlet valves.
In the case of an engine having electronic fuel injection, the fuel
injectors are reprogrammed to inject fuel into the respective cylinders once
per
revolution of the crankshaft as opposed to every second revolution as in four
stroke
operation. In the case of cam actuated fuel injectors, the fuel injector
camshaft can
be replaced by a camshaft having dual lobed cams geared to turn at half
crankshaft
speed or a camshaft having single lobed cams geared to tum at crankshaft speed
for injecting fuel into the respective cylinders once per revolution of the
crankshaft.
In either embodiment, a conventional four stroke, four cylinder engine
is converted to a two stroke engine wherein two pistons are fired
synchronously
every half revolution of the crankshaft with a minimal replacement of parts.
The
resulting engine exhibits improved torque characteristics without expensive or
time
consuming engine refitting.
While some embodiments of the present invention have been
CA 02352616 2001-07-06
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described in the foregoing, it is to be understood that other embodiments are
possible within the scope of the invention. The invention is to be considered
limited
solely by the scope of the appended claims.
