Direct injection spark ignition engine

A stratified charge is formed in a direct injection engine by injecting fuel at a predetermined velocity, a predetermined droplet size, and forming the injected fuel with a hollow cone having a predetermined initial cone angle. The injected fuel thereby shallowly penetrates the combustion chamber so as to float therein to reduce wall-wetting and subsequent soot formation. A substantially flat top piston forms the injected fuel into a ball-shaped kernel during a compression stroke of the engine. The ball-shaped kernel remains substantially unmixed with the inducted air, thereby producing the stratified charge. The continued motion of the piston causes the ball-shaped kernel to move toward the spark plug for ignition.

FIELD OF THE INVENTION 
The present invention relates to direct injection engines and more 
particularly to, forming a stratified charge in such engines. 
BACKGROUND OF THE INVENTION 
Direct injection engines are aimed at improving fuel economy at low engine 
loads by providing a stratified charge in the combustion chamber. A 
stratified charge engine is one in which the combustion chamber contains 
stratified layers of different air/fuel mixtures. The strata closest to 
the spark plug contains a mixture slightly rich of stoichiometry, and 
subsequent strata contain progressively leaner mixtures. The overall 
air/fuel mixture within the combustion chamber is lean of stoichiometry, 
thereby improving overall fuel economy at low loads. At high engine loads, 
typically greater than 50% of full engine load, a homogeneous air-fuel 
mixture is provided in the combustion chamber. 
Conventional direct injection engines typically include a piston having a 
depression in the top face thereof (typically referred to as a bowl) and a 
swirl or tumble control valve located in the intake port to produce a 
swirl or tumble of the air entering the combustion chamber. As fuel is 
injected into the combustion chamber, the fuel impinges against the bottom 
of the bowl and cooperates with the motion of the air in the chamber to 
produce the stratified charge, with the richest portion of the charge 
moving toward the ignition source. 
The inventors of the present invention have recognized certain 
disadvantages with these prior art engines. For example, because the fuel 
sprayed from the fuel injector is directed toward the piston bowl, it is 
likely that a portion of the fuel will stick to the piston surface causing 
an undesirable wall-wetting condition. As the remainder of the fuel is 
burned, the flame propagating toward the piston surface is unable to 
completely burn the liquid fuel film on the piston surface. This results 
in undesirable soot formation during combustion. 
In addition, because the design of these engines relies on the fuel 
impinging against the bowl and subsequently directed toward the spark 
plug, fuel injection timing is of a major concern. In direct injection 
engines, fuel injection is a function of time whereas the motion of the 
piston is a function of crank angle. In port injected engines, fuel 
entering the chamber is a function of crank angle because the opening of 
the intake valve is a function of crank angle. As a result, it is 
imperative to control the timing of fuel injection in a direct injection 
engine so that the injected fuel may impinge on the bowl at the proper 
time and the fuel cloud may move toward the spark plug. In other words, if 
the fuel is injected too early, the spray may miss the bowl entirely, 
thereby not deflecting toward the spark plug. If the fuel is injected too 
late, then excess wall-wetting may occur. 
Further, the inventors of the present invention have found that with 
bowl-in-piston engines, switching between a stratified charge and a 
homogeneous charge occurs at part loads ranging between 30% to 40% of full 
engine load. As the engine load increases, more fuel is required. However, 
because of the physical limitations of the bowl (i.e. the size of the bowl 
relative to the size of the combustion chamber), the amount of fuel that 
can be placed in the bowl and still attain a stratified charge is limited. 
Otherwise, the potential for wall wetting and subsequent soot formation 
may increase. As a result, above about 40% of full engine load, fuel 
economy is compromised. 
Other disadvantages with prior art engines results in a heavier piston, 
increased engine height to accommodate the larger piston, a larger 
combustion chamber surface to volume ratio, more heat loss, and increased 
charge heating during the intake and compression strokes, which increases 
the tendency for engine knocking. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a direct injection spark 
ignition engine which overcomes the disadvantages of prior technology. 
This object is achieved, and disadvantages of prior art approaches 
overcome, by providing a method of forming a stratified charge mixture for 
a direct injection spark ignition internal combustion engine. The engine 
has a cylinder block with a cylinder bore formed therein. The cylinder 
bore defines a longitudinal axis. A flat top piston is reciprocally housed 
within the cylinder bore and a cylinder head is attached to the block to 
close the top end of the bore to form a combustion chamber. An intake port 
is formed in the cylinder head and communicates with the combustion 
chamber via an intake valve for introducing air into the chamber. A fuel 
injector, which defines an axis and communicates with the combustion 
chamber, supplies fuel directly into the combustion chamber. An ignition 
source communicates with the combustion chamber and ignites fuel within 
the chamber. In one particular aspect of the invention, the method 
includes the steps of injecting fuel from the fuel injector into the 
combustion chamber at a predetermined velocity and with a predetermined 
droplet size. The injected fuel is formed with a hollow fuel cone having a 
predetermined initial cone angle. The injected fuel thereby shallowly 
penetrates into the combustion chamber so as to float therein to reduce 
wall-wetting. The method also includes the step of forming the fuel cone 
into a substantially ball-shaped kernel with the substantially flat top 
piston during a compression stroke of the engine. Thus, the fuel remains 
substantially unmixed with the inducted air, thereby producing the 
stratified charge. The ball-shaped kernel is then moved toward the 
ignition source for ignition. 
An advantage of the present invention is that wall-wetting on the piston 
surface is reduced. 
Another, more specific, advantage of the present invention is that a near 
complete combustion occurs with little or no soot formation. 
Yet another advantage of the present invention is that a less complex 
engine is provided in that no bowl is required for the piston. 
Another advantage of the present invention is that little or no swirl or 
tumble motion of intake air is required. 
Still another advantage of the present invention is that regulated 
emissions may be reduced. 
Another advantage of the present invention is that the engine load range in 
which a stratified charge may be produced is extended. 
Other objects, features and advantages of the present invention will be 
readily appreciated by the reader of this specification.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Internal combustion engine 10, shown in FIGS. 1 and 2, includes cylinder 
block 12, having cylinder bore 14 formed therein and piston 16 
reciprocally housed within bore 14. Piston 16 has a substantially flat top 
18. Cylinder head 20 is attached to block 12 and encloses top end 22 of 
bore 14 to form combustion chamber 24. Engine 10 is a multi-valve engine 
having, for example, two intake ports and two exhaust ports. For the sake 
of clarity, only one intake port 26 is shown and is formed within cylinder 
head 20 and communicates with combustion chamber 24 through intake valve 
28. According to the present invention, intake port 26 is a conventional 
intake port providing substantially no swirl or tumble motion of intake 
air within combustion chamber 24, although deactivation of one of the 
intake valves may produce some swirl motion. Engine 10 also includes spark 
plug 30 communicating with combustion chamber 24 for igniting an air/fuel 
mixture within combustion chamber 24. Engine 10 further includes fuel 
injector 32 defining axis 34 for injecting fuel directly into combustion 
chamber 24. In the example described herein, injector 32 is generally 
located along axis 36 of cylinder 14. However, injector 32 need not be 
coincident with axis 36. In fact, injector 32 may be mounted on the side 
of cylinder 14, typically referred to as a side mounted injector, whereas 
in the example shown, injector 32 is a centrally mounted injector. 
Injector 32 includes tip 36 having an orifice 38 for injecting fuel from a 
fuel system (not shown) to combustion chamber 24. 
Engine 10 further includes controller 40 (see FIG. 1) having memory storage 
device 42. A plurality of sensors 44 sense numerous engine operating 
parameters such as engine speed, engine load, spark timing, EGR rate, fuel 
delivery rate, engine air charge temperature, engine coolant temperature, 
intake manifold absolute pressure, the operating position of the throttle, 
vehicle gear selection, vehicle speed, intake manifold air mass flow rate, 
accelerator position, and other parameters known to those skilled in the 
art and suggested by this disclosure. 
According to the present invention, fuel injector 32 injects fuel into the 
combustion chamber 24 at a predetermined velocity along axis 34 of 
injector 32 and at a predetermined droplet size. Fuel is injected during 
the compression stroke at about 80.degree. before top dead center. The 
injected fuel is formed into a hollow cone 50 (see FIG. 1) having a 
predetermined initial cone angle .theta.. Accordingly, the injected fuel 
shallowly penetrates into combustion chamber 24 so as to float therein to 
reduce wall-wetting. As piston 16 progressively compresses the air within 
combustion chamber 24 during the compression stroke, fuel cone 50 is 
progressively formed into a substantially ball-shaped kernel 52 (see FIG. 
2) by the action of the substantially flat top piston 16. The fuel thereby 
remains substantially unmixed with air inducted through intake port 26, 
thereby producing the stratified charge in combustion chamber 24. Further, 
the piston motion causes the ball-shaped kernel 52 to engulf the spark 
plug 30 so that the fuel may be ignited. 
In a preferred embodiment, the droplet size, as measured by the Sauter Mean 
Diameter method, is between about 8 .mu.m and about 10 .mu.m. The 
injection velocity of the fuel entering into the combustion chamber is 
between about 9 m/s and about 12.5 m/s, as measured along axis 34 of 
injector 32. Also, the initial cone angle .theta. of fuel cone 50 is 
between about 80.degree. and about 100.degree. , and preferably 
90.degree.. 
The effects of having such a shallowly penetrating fuel injected from fuel 
injector 34 is clearly shown in the graphs of FIGS. 3-5. In FIG. 3, a plot 
of fuel vapor mass versus fuel injection velocity at various cone angles 
and droplet diameters is shown. At a fuel injection velocity of about 12.5 
m/s, it is clearly shown that, with a droplet diameter of about 10 .mu.m 
and a cone angle .theta. of about 90.degree., over 95% of the injected 
fuel is vaporized at the time of ignition. This is a highly desirable 
result to avoid wall-wetting of either the piston surface 18 or the wall 
of cylinder 14. 
Referring now to FIG. 4, it is shown that a sharp boundary 54 between the 
rich region and lean region may be obtained according to the present 
invention. FIG. 4 is a plot of fuel mass in the lean region at the time of 
ignition versus fuel injection velocity, wherein the lean region is 
defined by the fuel-air equivalence ratio phi being less than about 0.4. 
Phi is defined as the stoichiometric air/fuel ratio of 14.6 to 1 divided 
by the desired air/fuel ratio in the lean region. Thus, in this example, 
the lean region has an air/fuel ratio of anything greater than 36.5, which 
is 14.6 divided by 0.4. Continuing with reference to FIG. 4, at a fuel 
injection velocity of about 12.5 m/s and a droplet diameter of about 10 
.mu.m with a cone angle .theta. of about 90.degree., it can be seen that 
the amount of fuel in the lean region is less than about 17%. This is 
highly desirable to create a stratified charge, with the richest portion 
of the charge nearest spark plug 30. 
Referring now to FIG. 5, a graph of the amount of fuel mass in the rich 
region, where phi is greater than 2.5, upon combustion of the air/fuel 
mixture is shown. Fuel mass of phi greater than 2.5 results in a high 
probability of soot formation. At a fuel injection velocity of about 12.5 
m/s and a droplet diameter of about 10 .mu.m with a cone angle .theta. of 
about 90.degree., it is seen there is substantially no fuel in the rich 
region and consequently no soot is formed. 
According to the present invention, controller 40 controls a switch point 
for switching between a stratified charge produced in combustion chamber 
24, as described above, and a homogeneous charge. Those skilled in the art 
will recognize in view of this disclosure that changing between a 
stratified charge and a homogeneous charge may be accomplished by changing 
injection timing from the compression stroke to the intake stroke, for 
example. The switch point occurs at a point greater than about 50% of full 
engine load, and, more desirably, at a point between about 60% and about 
70% of full engine load. This is due to the fact that a stratified charge 
may be produced in combustion chamber 24, as described above, with a 
relatively large amount of fuel being delivered therein without the 
potential for wall wetting and subsequent soot formation because the 
charge is not constrained by a bowl formed in piston surface 18 of a 
limited volume, but rather is constrained by the entire volume of 
combustion chamber 24. 
Those skilled in the art will recognize in view of this disclosure that, to 
form such a low penetration fuel injection as previously described, a 
swirl injector or an injector having a deflector in its tip may be used, 
as shown in FIGS. 6 and 7, respectively. FIG. 6 shows injector tip 38' 
having a swirl chamber 60 causing a swirling motion of fuel, shown as 
arrow F.sub.1. The swirling fuel necessarily forms a hollow cone with the 
size of orifice 40' cooperating with the function of the swirl chamber 60 
to produce the desired droplet size, injection velocity and initial cone 
angle .theta.. 
Turning now to FIG. 7, injector tip 38" is formed with pintle 62 having 
deflector 64 formed thereon. Deflector 64 is designed to properly define a 
hollow cone with an initial cone angle .theta., as previously described. 
In addition, orifice size 40" is sized to produce the previously described 
desired droplet size. 
While the best mode for carrying out the invention has been described in 
detail, those skilled in the art to which this invention relates will 
recognize various alternative designs and embodiments, including those 
mentioned above, in practicing the invention that has been defined by the 
following claims.