Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
l V 8
BFN 7163 -1-
MEI~HOD AND APPARATUS FOR A
LOW EMMISSION DIESEL EN~INE
Prior Art
This invention relates to compression
ignition engines and in particular to the operation
10 of medium to high-speed compression ignition engines
in such manner as to reduce the amounts of oxides of
nitrogen in the exhaust gases.
As result of increasingly stringent federal
standards with respect to emmissions from automobile
15 and light duty truck exhausts, alternative power
plants for automobiles and light duty trucks are
being investigated. One popular alternative power
plant is the compression ignition engine, commonly
known as the Diesel engine.
The Diesel engine has several advantages
over conventional spark ignition engines. In
particular, Diesel engines burn heavier fuel which
is cheaper than gasoline, they have a higher thermal
efficiency than spark ignition engines, and they
25 have significantly lower emmissions in some respects
than comparable spark ignition engines. While
carbon monoxide emmissions are low because the
Diesel engine operates with excess airl and
hydrocarbons are normally a small constituent of
30 Diesel exhaust, Diesel engines characteristically
produce unacceptably high amounts of oxides of
nitrogen (NOX) and therefore are presently unable
BFN 7163 -2-
to meet govern~ent standards with respect to NOX
emmissions for automobiles and light duty trucks.
The standard Diesel engine used in some
automobiles and most trucks today is a four-stroke
5 or four cycle engine. In the first or intake
stroke, ~he intake valve opens and the piston
decends to draw fresh air into the cylinder. I~ the
second or compression stroke, the intake valve
closes and the piston rises to compress the air
10 which becomes heated. Near the end OL the
compression stroke, fuel is injected into the
cylinder and burns.
In the third or expansion stroke, the
burning mixture expands and forces the piston down.
15 At this time both the intake and the exhaust valves
are closed.
In the fourth or exhaust stroke, the
exhaust valve opens and the burned gases are forced
out of the cylinder by the rising piston.
Slnce the working fluid, namely air, is a
compressible gas th~t enters and leaves the c~linder
in more than an instantaneous period of time, the
closing of the exhaust valve at the end of the
exhaust stroke typically occurs subsequent to the
opening of the intake valve at the beginning of the
air intake stroke. In other words, the exhaust
valve re~ains open until after the piston reaches
top dead center, and the intake valve opens before
the piston reaches top center. The reason for this
"valve overlap" is to effect a more thorough
scavenging of the exhaust gases from the cylinder,
which brings about an increase in power out of
proportion to the amount of air involved.
~hen the exhaust stroke begins and the
exhaust valve opens, the mot on of the exhaust gases
is started by the cylinder pressure exiting when the
BFN 7163 -3-
exhaust valve is opened and is promoted by the
piston motion during the exhaust stroke. The
scavenging of exhaust gases tends to continue during
and after the top center period. Therefore, the
5 intake valve is opened to allow fresh air to enter
the cylinder to displace the last traces of exhaust
gases in the cylinder, and a necessary result of
this procedure is that a certain amount of fresh air
is drawn through the cylinder and out past the
lO exhaust valve where it mixes with the exhaust
gases.
It is believed that the occurrence of this
valve overlap, during which fresh air is drawn in
through the inta~e valve and out through the exhaust
15 valve, is a major cause of the formation of
unacceptable amounts of NOX in the exhaust gas of
a Diesel engine.
Summary of the Invention
The present invention provides an improved
method and apparatus for operating a medium to
high-speed, four cycle, compression ignition engine
in which the valve timing is ad~usted so that the
exhaust valve is completely closed prior to the time
25 the piston reaches top dead center, and the intake
valve opens after the piston passes top dead center
so that no fresh air is permitted to pass out the
exhaust valve. Some exhaust gases may remain in the
cylinder at the beginning of the next cycle. In
30 this fashion, the conditions which create
unacceptably high amounts of NOX in the exhaust
gases are reduced without a significant reduction in
the effective horsepower or mileage.
~ According to one aspect of the present
35 invention, a method of operating a medium to
high-speed four-cycle compression ignition engine of
~1~&10~
BFN 7163 -4-
the type wherein fresh air is introduced through an
intake port, the air is compressed, fuel is injected
and burns to expand the air, and the air is
scavenged through an exhaust port, is improved by so
timing the opening of the intake valve and the
closing of the exhaust valve that no fresh air is
permitted to pass out through the exhaust port. The
apparatus of the present invention includes a
camshaft having cams so shaped and positioned that
10 during operation of the engine, the exhaust valve of
each cylinder is fully closed before its respective
intake valve is opened.
The aforementioned timing of the valv2s is
achieved by adjusting the relative positions of the
15 cams actuating the intake and exhaust valves
relative to one another as well as the contour of
the flank and nose portions of the cam. Although
there is a virtually infinite number of possible
combinations of cam contours and relative cam
20 combinations, the desired effect is to time the
closing of the exhaust valve zt the end of the
exhaust stroke so that the air entering the cylinder
does not pass through the exhaust port without being
burned. In some instances, this requires that the
25 closing of the exhaust valve occur before the
opening of the inlet valve, thus eliminating valve
overlap. Since the method of the invention can be
performed using a standard compression ignition
engine on which only relatively minor adjustments
30 have been made, the invention is ideally suited for
retrofit applications. By substituting a camshaft
ground in the manner of the invention for the
standard camshaft of a conventional compression
ignition engine in a vehicle, that vehicle will have
35 significantly reduced emissions, regardless of its
vintage.
V 8
BFN 7163 -5-
Although the method of the present
invention will reduce significantly the presence of
NOX in the exhaust gases of all medium to
high-speed compression ignition engines, the results
5 are most noticeable in those compression ignition
engines equipped with a turbocharger. If an engine
is turbocharged, a greater differential exists
between the pressure of the fresh or unburned air
flowing into the combustion chamber and the pressure
lO Of the exhaust gas or burned air in the combustion
chamber than is the case with a non-turbocharged
engine. As a result, air enters the combustion
chamber during the intake stroke at a faster rate
than with a non-turbocharged engine, and a greater
15 amount of air enters the combustion chamber, even
though the intake valve is opened for a shorter
period of time.
Similarly, with the turbocharged engine,
there exists a greater differential in pressure
20 between the exhaust gases or burned air in the
ccmbustion chamber and those in the exhaust manifold
than exists with a non-turbocharged engine. This
increased pressure differential causes the exhaust
gases within the combustion chamber to scavenge more
25 rapidly than would a non-turbocharged cylinder.
The overall result is that 2 sufficient
volume of fresh air enters the cylinder to impart a
powerful thrust to the piston upon burning, and
subsequently the cylinder is scavenged without the
30 "blow by" that occurs in prior art compression
ignition engines and causes excessive NOX in the
exhaust gases.
In adaition, it is believed that a
turbocharged compression ignition engine is
35 particularly suitable for the method of the present
invention. With such an engine, the valves are
1 ~5~108
BFN 7163 -6-
timed in the manner of the prior art so that thereis valve overlap at the end of the exhaust stroke
and the beginning of the air intake stroke, when an
even greater amount of fresh air passes out the
5 exhaust port.
Accordingly, it is an object of this
invention to provide an improved method of operating
a medium to high-speed four stroke compression
ignition engine in which the amount of NOX present
10 in the exhaust gases is at an acceptable level
without an appreciable decrease in horsepower
generated or fuel efficiency.
Other objects and advantages of the
invention will be apparent from the following
15 description, the accompanying drawings and the
appended claims.
Brief Description of the Drawings
Fig. 1 is a side elevation in section of
20 the invention during the intake stroke;
Fig. 2 is a side elevation in section of
the invention during the compression stroke;
Fig. 3 is a side elevation in section of
the invention during the combustion or expansion
25 stroke;
Fig. 4 is a side elevation in sec~ion of
the invention during the scavenging or exhaust
stroke;
Fig. 5 is a partial side elevation in
30 section of the cam and valve assembly of the
invention;
Fig. 6 is a side elevation in section of a
prior art compression ignition engine at the end of
th~e exhaust stroke and the beginning of the intake
35 stroke;
~ 15~108
BFN 7163 -7-
Fig. 7 is a valve timing diagram of the
present invention;
Fig. 8 is a valve timing diagram o~ a prior
art compression ignition engine;
Fig. 9 is a side elevation in section
showing a turbocharger schematically; and
Fig. 10 is a partial side elevation in
section of a compression ignition engine of the open
chamber type also showing a cam and valve assembly
lOof the invention.
Detailed DescriPtion of the Preferred Embodiment
As shown in Figs. 1 through 4, the method
and apparatus of the present invention can be
15 integrated into a standard, high-speed, four stroke,
compression ignition engine. The power generating
portion of such engines typically consists of a
piston 10 which is pivotally connected to a piston
rod 12 mounted on a cranksha~t 14 which transmits
20 the piston move~ent to a drive train (not shown).
The piston 10 reciprocates within a cylinder 16 that
defines a combustion chamber 18 which communicates
with an intake manifold 20 by means of an inlet port
22 and with an exhaust manif old 24 through an
25 exhaust port 26. The inlet and exhaust ports 22, 26
are shaped to receive inta~e and exhaust valves 28,
respectively, which can be moved to open and
close passages in the inlet and exhaust ports.
A fuel injection nozzle 32, which is
30 connected to a ~uel source (not shown), communicates
with a pre-combustion chamber 34. The
pre-combustion chamber 34 in turn communicates with
the combustion chamber 18.
~ As shown in Fig. 5, a typical valve 38 in a
35 compression ignition engine pivots against a rocker
arm 40 in which is pivotally journaled a push rod
1 15~1~8
BFN 7163 -8-
42. The push rod 42 terminates in a cam follower 44
which rolls against a cam 46 fixedly journaled to
the camshaft 48. The camshaft 48 is turned by the
crankshaft 14 by means of a linkage (not shown)
5 well-known in the art. As the camshaft 48 rotates,
the eccentricity of the cam shape causes the cam
follower 44 to rise and ~all thereby causing the
valve 38 to engage and disengage a typical port 50
defining a port. The valve 38 is urged against its
lO valve seat by ~eans of a spring 52 which operates
between the cylinder head 54 and the retainer
portion 56 of the valve 38.
The timing of the opening and closing of
the intake and exhaust valves 28, 30 is a function
15 not only of the positions of their respective cams
46 in relation to one another on the camshaft 48 but
also of the cam contour. The cam contour is
comprised of a base circle portion 58, a nose 60,
and two flanks 62. The shapes of the flanks 62 and
20 the nose 60 of a cam 46 determine the rate at which
each valve is opened and the duration that it
remains open.
The method of operating the ~iesel engine
of the present invention is as follows. As shown in
25 Fig. l, the crankshaft 14 may turn in a clockwise
direction, drawing the piston lO downward within the
cylinder 16, and at the same time, the intake valve
28 is moved away from the inlet port 22, thus
allowing fresh air 64 from the intake manifold 20 to
30 be drawn into the cylinder. This process begins
when the piston is approxlmately l~ to 3 past top
dead center, that is, when the crankshaft 14 has
turned l to 3 beyond the position it was in at the
time the piston lO reached its maximum ascent within
35 the cylinder 16. The intake valve 28 remains open
until the piston lO has reached approximately 30
1 15~08
BFN 7163 9
past bottom dead center, that is, the crankshaft 14
has turned 30 beyond the position it was in at the
time the piston 10 reached its furthest decent
within the cylinder 16.
As shown in Fig. 2, the compression stroke
begins with the closing of the intake valve 28 and
the travel o the piston 10 upward within the
cylinder 16. As the air 64 is compressed within the
cylinder 16, it becomes hotter.
When the piston 10 is near top dead center
a charge of fuel 6S is injected through the nozzle
32 as a fine spray into the hot air 64, and ignition
takes place. As shown in Fig. 3, the expanding
gases 66 force the piston 10 downward on the third
15 stroke of the cycle, and the movement of the piston
is transmitted to the cranksha~t 14 by the piston
rod 12.
As shown in Fig. 4, the exhaust valve 30
opens when the piston 10 is approximately 30 before
20 bottom dead center, and the scavenging or exhaust
stroke begins. The piston 10 reaches bottom dead
center and begins its ascent up the cylinder 16 to
orce the exhaust gases 68 out through the exhaust
port 26 and the exhaust manifold 24. When the
25 piston 10 is near top dead center, the exhaust valve
30 closes the exhaust port 26 completely, thereby
cutting off the flow of exhaust gases 68 through the
port and trapping a small amount of exhaust gas
within the cylinder 16. As the piston 10 passes top
30 dead center and begins the first or intake stro~e,
the inta~e valve 28 opens the inlet port 22, and
fresh air 64 is admitted. Thus, in the method of
the present invention, a small amount of exhaust gas
68 may remain in the cylinder, and no fresh air 64
35 is permitted to "blow by" and mix with the exhaust
gases in the exhaust manifold 24.
~lS~1~8
BFN 7163 -lO-
The foregoing explanation of the method and
apparatus of the present invention is contrasted
with the operation of a conventional Diesel engine
of the prior art as shown in Fig. 6. Fig. 6 depicts
the position of the piston 10, intake valve 28 and
exhaust valve 30 at the end of the exhaust stroke
and the beginning of the intake stroke.
In the operation of Diesel engines of the
prior art, both valves 28, 30 are open at this time
to allow fresh air 64 to enter the combustion
chamber 18, thereby completely scavenging the
exhaust gases 68 from the combustion chamber.
~owever, a certain amount of "blow by" occurs
wherein fresh air 64 passes into the combustion
chamber 18 and out the exhaust port 26 without
supporting the combustion of the fuel. In order to
reduce significantly the presence of unacceptable
levels of NOX in the exhaust gases of the engine
of the present invention, the prior art
20 configuration depicted in Fig. 6 does not occur at
any time during the operation of the Diesel engine
o~ the present invention.
Fig~ 7 is a valve timing diagram for the
operation of a Diesel engine of the present
25 invention. The circle generally designated A can be
considered as the path traced by a point positioned
on the crankshaft 14 o~ the present invention. The
line segment TDC represents the postion of the
crankshaft 14 -- and hence the piston 10 -- at top
30 dead center, that is, when the piston has risen to
its highest point in the cylinder 16. The line
segment BDC represents the position of the
crankshaft 14 and piston 10 at bottom dead center,
that is, the point at which the piston has reached
35 its furthest descent within the cylinder 16.
1 15~1~8
BFN 7163
Thus to depict the valve se~uence for a
Diesel engine of the present invention, the piston
begins at a point TDC on the valve diagram and
begins to descend as the crankshaft turns in a
5 clockwise manner. The inlet valve opens at line
segment W, which represents a cylinder position
approximately 3 after top dead center, and remains
open to line segment X approximately 30~ after
bottom dead center. The area bounded by lines W and
10 X represents the period of time during the first
cycle when the intake valve 28 is open.
Line X also designates the beginning of the
second or compression stroke. This stroke continues
to a point near top dead center at which time the
15 fuel is sprayed into the combustion chamber 18
through the nozzle 32 and the expansion stroke
begins. During the expansion stroke, the crankshaft
14 is turning from line TDC to line Y, located
within circle A. Line Y denotes tha opening of the
20 exhaust valve 30 and the beginning of the exhaust
stroke shown in ~ig. 4.
The exhaust stroke begins at approximately
before bottom dead center and ccntinues to a
point denoted by line Z which is approximately 3 4
25 before top dead center. Line segment Z denotes the
point at which the exhaust valve is completely
closed. The segment of the tlming cycle between
lines Z and W represents a period of crankshaft
rotation during which both the intake valve 28 and
30 the exhaust valve 30 are closed~ It is crucial to
the operation of a Diesel engine according to the
present invention that this segment appear on the
valve timing sequence.
~ In contrast, a valve timing diagram of a
35 Diesel engine operated according ~o the ~ethod of
prior art is shown as circle A' in Fig. 8. The
1 0 8
BFN 7163 -12-
start of the first or intake stroke is shown by line
segm~nt W' which occurs before top dead center. The
intake valve 28 remains open until line segment X',
typically about 25 past bottom dead center. The
5 compression stroke begins at line X' with the
closing of the intake valve 28 and continues through
to a point near top dead center, at which time the
fuel is sprayed into the combustion chamber 18 from
the nozzle 32 and the third or expansion stroke
10 begins.
The expansion stroke continues through to
line segment W', located within the circle A'. Line
Y' denotes the opening of the exhaust valve 30 and
the beginning of the exhaust stroke. The exhaust
15 stroke continues through to a point Z', typically
after top dead center.
Thus, the segment of the valve timing
diagram of Fig. 8 denoted by the double
cross-hatching represents the time during the
20 four-stroke cycle of the prior art in which both the
int~ke and the exhaust valves 28, 30 are open, as
shown in Fig. 6. It is at this time that fresh air
64 enters the combustion chamber 18 as the exhaust
gases 68 are leaving the combustion chamber 18, and
2S some fraction of the fresh air 6~ leaves the
cylinder along with the exhaust gases 68. By
eliminating the time during which both the intake
valve 28 and the exhaust valve 30 are open, "blow
by" of fresh air 64 entering the combustion chamber
30 18 is prevented, and the amount of NOX formed in
the exhaust gases 68 is reduced.
The method and apparatus of the present
invention are particularly effective when used in
c~njunction with a turbocharged Diesel engine as
35 shown in Fiy. 9. An exhaust turbine 70 located in
the exhaust manifold 24 is driven by the exhaust
i 1~6 1~
BFN 7163 -13-
gases 68 leaving the combustion chamber 18 during
the exhaust stroke. The exhaust turbine 70 is
coupled to an inlet turbine 72 by a drive shaft 74,
and the inlet turbine is rotated by the exhaust
5 turbine 70 to force fresh air 64 into the combustion
chamber 18 during the air intake stroke. The result
is that a much greater amount of fresh air 64 is
present in the combustion chamber 18 during the
operation of the engine, and consequently more fuel
10 can be injected and a greater horsepower generated
for a given cylinder.
Since higher pressures are involved, there
is a greater amount of blow by of fresh air 64 in
the operation of a prior art Diesel. The
15 elimination of valve overlap eliminates all blow by
and thereby reduces significantly the amount of
~x in the exhaust gases 68.
Although the invention has been discussed
previously as used in connection with a compression
20 ignition engine which includes a precombustion
chamber, the invention has been successfully tested
in combination with an engine of the open chamber
type, as shown in Fig. 10. In an open chamber type
engine, the cylinder head 54' is designed so that
25 the fuel in~ection nozzle 32' injects fuel directly
into the combustion chamber 18'.
The piston 10' has an upper surface 76
which defines a recess 78 to receive a charge 65' of
fuel. However, the configuration and ope~ation of
30 the cam and lifter assembly 79 are the same as that
shown in Fig. 5. A typical valve 38' in a
compression ignition engine pivots against a rocker
arm 40' in which is pivotally journalled push rod
42'. Push rod 42' terminates in a cam follower 44'
35 which rolls against a cam 46' fixedly journalled to
camshaft 48'.
1~5~08
BFN 7163 -14-
As discussed previously, rotation of the
camshaft 48' causes cam follower 44' to rise and
fall in response to the eccentrici~ies of the shape
and contours of cam 46'. This cam is ground to the
5 proper contour to time the opening and closing of
valve 38' to eliminate blow by of unburned air 64'.
The open chamber engine shown in Fig. 10
may be turbocharged, and is shown schematically with
turbocharging apparatus. As was discussed in
10 connection with Fig. 9, the turbocharger 80 of Fig.
is preferably OL the exhaust gas type, and
includes an exhaust turbine 70 which is rotated by
the force o~ escaping exhaust gases 68', an inlet
turbine 72', and a drive shaft 74' which joins the
15 inlet turbine to the exhaust turbine. The rotation
of the exhaust turbine 70' causes the drive shaft
74', and hence the inlet turbine 72', to rotate,
thereby compressing the fresh air 64' entering the
combustion chamber 18'. This compressed fresh air
20 64' permits a greater amount of fuel to be injected
into and burned in the combustion chamber 18',
resulting in greater horsepower for that engine
configuration ~han without tur~ocharging.
In accordance with the above discussion,
25 Tables 1 and 2 show the effect of variations in
valve overlap on the amount of NOX present in the
exhaust gases of a medium speed turbocharged Diesel
engine of the open chamber type. By "medium speed"
is meant a Diesel engine which is designed for a
30 maximum operating speed of from 2400 to 2600 rpm. at
full load, and as compared with high-speed engines
~hich operate in a speed range in excess of 2600
rpm. The testing equipment and procedures used in
gènerating this data were capable of duplicating the
35 City and ~ighway Modes of the Federal Test
Procedures as outlined in Part 86 of Chapter 1,
1 0 8
BFN 7163 -15-
Title 40 of the Code of Federal Regulations as
applicable to light-duty vehicles. The testing
facility at which the tests were performed was one
of ten such facilities in the country listed by the
5 U.S. Environmental Protection Agency as beinq
equipped to per~orm emmission tests in accordance
with the aforementioned federal procedures.
Three different cam designs yielding three
different amounts of valve overlap were tested in a
lO standard turbocharged Diesel engine mounted in one
of two light-duty vehicles. All tests were run in
accordance with the 1975 Federal Test Procedure. In
this Federal Test ~rocedure, the vehicle to ~e
tested was placed on a dynamometer set at
lS predetermined resistance to simulate wind and
rolling friction, and its exhaust gases were sampled
while the vehicle was put through a series of
accelerations, decelerati~ons and idle periods in a
way designed to simulate actual driving conditions.
20 The results for the entire test were reported in
terms o grams of a particular pollutant per mile of
vehicle operation on the dynamometer.
1~5~108
BFN7163 -16-
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1~61~8
BFN 7163 -18~
Table 1 shows the data generated by the
vehicles which were put through a total o three
Federal City Mode tests, each time with a cam design
yielding a different degree of valve overlap.
In Test 1, a van having a standard,
unmodiied, turbocharged Diesel of a type
exemplifying a prior art engine was tested. The
engine displacement was 3.7 liters (226 in.3) and
the dynamometer was set to simulate resistance for a
10 1818.2 kg (4000 lbs.) vehicle. The amount of valve
overlap, that is, the range of crankshaft ansles
during which both the inlet valve and the outlet
valve were open (see Figs. 6 and 8), was
approximately 30. The amount of NOX generated
15 for the entire Federal City Mode was 9.65 gm/mi.
In Test 2, a pick-up truck having a
turbocharged Diesel engine whose cams had been
modified so that the valve overlap was reduced to
approximately 1 to 3 was tested. The amoun~ of
20 NOX present in the exhaust gases for the City Mode
was ~.35 gm/mi.
In Test 3, a pick-up truck having the same
type ~f turbocharged Diesel engine whose cam had
been modiied in accordance with the present
2~ invention was tested. The amount of valve overlap
in this test was approximately -1 to -3 . The
amount of NOX generated was approximately 1.85
gm/mi. Clearly, a turbocharged Diesel engine whose
cam has been modified in accordance with the present
invention displays a significant decrease in the
amount of NOX generated in the exhaust gas during
normal use.
Similarly, Table 2 depicts the same three
vehicle and engine combinations subjected to the
Federal Highway Mode on the same test acilities
described above. The data from tests 4, 5 and 6
I156108
BFN 7153 -19-
show that a modification of the engine to effect a
negative valve overlap results in a significant
decrease in the amount of NOx in the exhaust
gases.
Table 3 shows the data generated by the
testing of a light duty truck having a four-cylinder
turbocharged compression ignition engine of the open
chamber type at the aforementioned facilities and
under the same types of tests. The engine had a
lO displacement of 3.7 liters (226 in.3) and a
compression ratio of 18:1. The tr~ck underwent the
test on a dynomometer set at 1818.2 kg (4000 pounds).
In tests 7 and 8, the subject was the
aforementioned vehicle whose engine included a cam
15 shaft modified in the manner of the invention to
eliminate valve overlap and fresh air blow by. The
amoun~ of negative overlap was approximately 2~. In
test 7, the vehicle was driven according to the
Federal City Mode and generated 1.35 gm/mi of NOx
20 and l.Ol gm/mi of hydrocarbons. In test 8, the same
vehicle was driven according to the Federal Highway
~ode. The vehicle generated 1.68 gm/mi of NOx and
0.52 gm/mi of hydrocarbons.
In tests 9 and lO, the same vehicle was
25 retested according to the Federal City and Highway
Modes respectively, but this time the engine was
fitted with a standard cam shaft which allows
approximately 30 of overlap. The results showed
that significantly higher amounts of NOx were
30 generated. In particular, in test 9, the vehicle
generated 3.19 gm/mi of Nx while driven according
to the City Mode and in test lO generated 4.07 gm/mi
of NOx when driven in the Highway Mode.
~ It should be noted that the amounts of
35 hydrocarbons (HC) that were generated by the vehicle
and measured during the tests were greater when the
11$~108
BFN 7163 -20-
engine was modified according to the invention.
~owever, this increase is believed to be relatively
insignificant when compared to the relatively large
reduction of oxides of nitrogen.
S While the methods and forms of apparatus
herein described constitute preferred embodiments of
this invention, it is to be understood that the
invention is not limited to these precise methods
and forms of apparatus, and that changes may be made
lO therein without departing from the scope of the
invention.
