Articulated piston

An articulated piston for use in heavy duty diesel engines including a piston crown having an outer surface, a peripheral pending side wall and an inner surface having a pair of pin bosses extending downwardly from the inner surface with the inner surface and the pin bosses defining a hollow cooling cavity opening downwardly and extending about a circumference of the piston crown and a piston skirt with the piston skirt including a first longitudinal plane containing a pair of diametrically opposed bores for receiving a wrist pin for connection to the bosses of the crown and defining two semi-cylindrical thrust and non thrust surfaces of the skirt with a thickness of the non thrust surface being less than a thickness of the thrust surface and a second longitudinal plane, perpendicular to the first plane thereby dividing the piston skirt into four peripheral quarters. At least one of the quarters containing a cooling medium inlet through which a cooling medium, injected by a nozzle, can pass and impinge against the hollow cavity of the piston crown for cooling the crown. The piston skirt also includes a recessed tray formed in an upper surface of the skirt facing upwardly and opening toward the hollow cavity with the recessed tray extending for at least the peripheral quarter containing the cooling medium inlet and defined by a pair of substantially axially disposed side walls and a bottom wall extending radially inwardly between the side walls. The recessed tray is peripherally inclined towards a quarter diametrically opposed to the cooling medium inlet for conveyance of the cooling medium collected from the hollow cavity of the crown.

TECHNICAL FIELD 
The present invention relates to articulated engine pistons, particularly 
to the shape of a piston skirt which is generally articulately connected 
to a piston crown and a small end of a piston rod by way of a common wrist 
pin. 
BACKGROUND OF THE INVENTION 
Competitive pressures have increased the reliability and durability 
requirements for heavy duty diesel engines. In addition, performance and 
exhaust emission improvements have increased the thermal and mechanical 
loading on critical heavy duty diesel engine components. 
To meet such requirements, the use of articulated pistons has grown in 
recent years. Conventional articulated pistons generally comprise a piston 
crown having an outer combustion chamber and, for cooling the crown, an 
inner hollow cooling cavity between the combustion chamber and a 
peripheral pending leg outwardly receiving piston rings. Such a piston is 
illustrated in U.S. Pat. No. 5,279,268 issued to Brink et al. 
For the purpose of improving the cooling properties at the piston crown, a 
tray or trough is provided at the upper portion of the piston skirt, such 
a tray being open towards the hollow cavity so as to partially close this 
cavity, thus forming what is known as a cooling gallery. When the engine 
is running, a cooling liquid such as lubricating oil is injected by a 
nozzle against the hollow cavity, through an oil inlet provided axially 
along the tray, thus partially removing heat from that region. The oil 
impinged against the hollow cavity flows down and is collected by the 
tray. Due to the reciprocating motion of the piston oil collected from the 
cavity is shacked against and around this cavity thereby increasing the 
removal of heat from that region. 
A like assembly of piston crown and piston skirt are disclosed in many 
prior letters patents granted the assignee, Cummins Engine Company, such 
as U.S. Pat. No. 5,144,884 issued to Kelly, while a variety of piston 
crown portions are disclosed, among others, in U.S. Pat. No. 5,459,922. 
Although the above-noted articulated pistons exhibit good performance even 
when operating at severe conditions, specially with regards to the 
temperature and pressure which can be very high and often times run at a 
very high speed in modern Diesel engines, there has been a need to achieve 
an articulated piston assembly of greater thermal and motional 
equalization. 
Thermal equalization as used here and throughout the specification refers 
to a more uniform cooling of the piston crown while motional equalization 
is intended to include a more appropriate structure in terms of resistance 
and reduced weight to withstand the mechanical loads as well as a more 
controlled tilting motion (lateral and rotational movements) of the skirt 
during the engine operation, rendering thereby an increased service life 
for such a component and the respective engine as well. 
Referring firstly to thermal equalization, applicants noticed that a 
serious concern with respect to the cooling provided on the crown is the 
uneven heat removal around the cooling cavity, what leads some regions of 
the crown to work in temperatures which are higher than desirable thus 
reducing its service life. One of the facts generating this unfavorable 
scenario arises from the baseline of the majority of the engines employing 
articulated pistons which include only one cooling nozzle at the 
crankcase. 
In like engines, the cooling oil is impinged by the nozzle against only a 
minor portion of the hollow cooling cavity of the crown which is covered 
by the oil spray. Therefore, the oil to be collected by the tray for the 
shaking will also be present in only a minor portion of the tray, causing 
the cocktail shaking to be inefficient in the remaining peripheral 
portions of the crown. In this instance, the piston is subject to a 
premature fail, most often in the region of either the peripheral pending 
leg or at the rim of the combustion chamber. Particularly, it was noticed 
that regions of the piston crown diametrically opposed to the cooling oil 
inlet were not cooled to the extent that those onto which oil is directly 
impinged by the nozzle because the oil spray does not reach such portions 
and, in addition, the respective portions of the tray thereunder receive 
only a little amount of oil for the shaking. 
One possible solution for equalizing the cooling could be increasing the 
number of cooling nozzles so that a greater amount of cooling oil could be 
impinged against a greater surface of the hollow cooling cavity and hence 
collected by the tray for the shaking action. However, this solution is 
disadvantageous in that the cost of the engine by the incorporation of the 
additional nozzle would be increased. Further, this solution would require 
the use of a more powerful oil pump which decreases the power made 
available by the engine and likewise increase costs. With respect to the 
equalization of the piston motion, when a piston of the articulated type 
is travelling in pendular motion in the associated cylinder, the piston is 
guided substantially by the skirt at surfaces known as thrust and non 
thrust surfaces, each of these defined by the skirt length perpendicularly 
to the piston pin axis. It was noticed that the loads received by skirt at 
the non thrust surface are substantially less than the loads received by 
thrust surface. Therefore, known articulated piston skirts which are 
generally symmetrical in relation to the pin axis have shown as being 
quite over dimensioned at the non thrust surface with regards to 
structural resistance and consequently over weight. The inappropriate 
shape of the skirt makes the articulated piston heavy and creates 
secondary undesirable movements of the skirt, which tends to increase the 
power loss of the engine. In this regard, the majority of the skirts for 
articulated pistons are obtained by gravity casting a light alloy, e.g., 
an aluminum alloy. Despite being a relatively inexpensive method for the 
production of an articulated piston skirt, as broadly known, the skirts 
made according to this method may present undesirable porosity at portions 
with great thickness which ultimately reduces its service life. 
Accordingly, there is a pressing need for an articulated piston having a 
piston skirt which leads itself to increasing the service life of the 
articulated piston as well as the engine as a whole. This is achieved by 
providing an articulated piston assembly and more specifically a piston 
skirt which exhibits greater thermal and motional equalization. 
SUMMARY OF THE INVENTION 
A primary object of the present invention is to overcome the aforementioned 
shortcomings associated with prior art articulated piston assemblies. 
A further object of the present invention to provide an articulated piston 
having an associated skirt including an effective design solution which 
increases reliability and durability of the assembly without sacrificing 
cost, thus overcoming all the aforementioned insufficiencies. 
It is a further object of the present invention to provide an articulated 
piston having improved thermal equalization. 
Yet another object of the present invention to provide an articulated 
piston which exhibits improved motional equalization. 
A still further object of the present invention is to provide an 
articulated piston which exhibits improved thermal equalization and 
improved motional equalization while being produced by a gravity die 
casting process. 
These, as well as additional objectives of the present invention, are 
attained by an articulated piston comprising a piston crown of a 
cylindrical shape having a combustion chamber and an inner surrounding 
hollow cavity between the combustion chamber and an outer peripheral 
pending leg outwardly bearing the piston rings. The cooling of the piston 
crown is improved by providing trays at the upper portion of the piston 
skirt, such trays being open towards the hollow cavity so as to partially 
close the cavity forming thereby a cooling gallery into which a cooling 
liquid such as oil is injected by a cooling nozzle suitably arranged at 
the engine crankcase as detailed above. The thermal equalization of the 
articulated piston, particularly at the piston crown is achieved by 
including a piston skirt defined by an upper recessed tray facing upwardly 
and open to the hollow cavity, extending for at least the peripheral 
quarter containing the cooling liquid inlet and defined by a pair of side 
walls substantially axially disposed with lower ends that meet a bottom 
radially inwardly and peripherally inclined towards the surface 
diametrically opposed to the cooling liquid inlet for conveyance of oil 
collected from the hollow cavity of the crown to that uncooled portion of 
the crown. 
Still according to the present invention, the skirt thickness at the thrust 
surfaces is varied thus providing the motional equalization of an 
articulated piston which is shaped in harmonious fashion with respect to 
the loads received along the piston travel. This leads to the reduction of 
undesirable secondary movements due to better weight balance and reduced 
overall weight and is achieved by reducing the thickness of the non-thrust 
surface as compared to that of the thrust surface. 
Even with these modifications, it is still possible to produce piston 
skirts by gravity die casting having substantially reduced porosity. 
The aforementioned features of the invention will be more fully understood 
from the following description of a preferred embodiment of the invention 
appreciated together with the accompanying drawing figures which is not 
intended to unduly limit the scope of the invention.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to the several figures, a piston according to a preferred 
embodiment of the present invention is generally comprised of a piston 
crown 10 and a piston skirt 20 whose assembly results in an articulated 
piston 30. 
The piston crown 10 comprises a combustion chamber 11 formed in an upper 
surface thereof. Pending from the crown periphery there is a leg 12 
outwardly surrounded by at least one and preferably a plurality of ring 
grooves 13 for receiving piston rings (not shown) in a known manner. At 
the crown inner surface there is a pair of pin bosses 14 formed thereon 
defining with the leg 12 a hollow cavity 15 opening downwardly and 
extending around the circumference of the crown 10 between the bosses 14 
and the leg 12. This feature being illustrated in FIG. 5. 
The piston skirt 20 includes a first longitudinal plane F containing a pair 
of opposed bores 21 for receiving a wrist pin (not shown) for connection 
to bores 16, the bosses 14 of the crown 10 resulting in the articulated 
piston 30 shown in FIG. 5. As is conventional, the bores 21 include ring 
grooves 22 which aid in positioning and securing the wrist pin in place. 
For the purpose of improving the cooling properties at the piston crown 10, 
a peripheral tray 25 is produced, e.g. by a corresponding casting die, at 
the upper portion of the piston skirt 20, such tray 25 being open towards 
the hollow cavity 15 so as to partially enclose the same, thus forming a 
cooling gallery. The tray 25 being best illustrated in FIGS. 4 and 6. 
As mentioned hereinabove, the skirt 20 has a first longitudinal plane 
indicated by F containing the pair of opposed bores 21 for receiving the 
wrist pin (not shown) for connection to the bosses 14, thus laterally 
dividing the skirt 20 in two semi cylindrical sections, one portion 
including a thrust surface 22 and the other including non thrust surface 
23. Dynamics of an articulated piston 30 running in a cylinder (not shown) 
leads the thrust surface 22 to receive a greater load than does the non 
thrust surface 23. Accordingly, the non thrust surface 23 has a radial 
thickness t.sub.23 that is less than the corresponding radial thickness 
t.sub.22 of the thrust surface 22. In this regard, the non thrust surface 
is reduced by tapering the inner wall 35 of the skirt below the tray 25 
for the length of the skirt between a bends 37 and 39 tangent to the 
opposing bores. Further to an equalization in terms of load, this 
difference in thickness provides better motional equalization of the skirt 
20. In fact, considering all the parameters involving piston dynamics such 
as angular velocity, the weight of piston etc., the aforementioned effects 
are best realized and equalized when the thickness t.sub.23 of the non 
thrust surface 23 is from 0.5*t.sub.22 to 0.95*t.sub.22, and more 
preferably when the thickness t.sub.23 of the non thrust surface 23 is 
0.75*t.sub.22, the symbol "*" standing for multiplication. 
Accordingly, it is possible to manufacture the piston skirt 20, using 
conventional manufacturing processes such as gravity casting, having a 
reduced side wall thickness thereby reducing the costs of the overall 
piston while still providing a skirt which is capable of withstanding the 
forces exerted on the skirt during operation. That is, the thrust surface 
22 would be of a thickness suitable for operating under conditions 
dictated by the internal combustion engine in which the piston is to be 
placed while the thickness of the non thrust surface 23 may be much less 
resulting in material cost savings without diminishing the performance of 
the skirt. Further, with the reduced thickness of the non thrust surface 
23, the overall weight of the engine is similarly reduced. 
Intersecting the first plane F of the skirt 20 is a second imaginary 
longitudinal plane S positioned perpendicular to plane F thereby defining 
four peripheral quarters of the skirt 20, at least one of which contains a 
cooling medium inlet 24. To improve the cooling properties of the crown 
10, the tray 25 is inclined radially inwardly from the outer surface of 
the tray 25 and circumferentially toward the quarter diametrically opposed 
to the cooling medium inlet 24 as shown by the arrows A and B of FIG. 4A. 
When the engine (not shown) is running a cooling medium such as lubricating 
oil is injected in a conventional manner by a nozzle (not shown) against 
the hollow cavity 15 formed in the piston crown 12, through the oil inlet 
24 provided axially along the skirt 20, thus partially removing heat from 
that region. At least a portion of the oil impinged against the hollow 
cavity 15 flows down and is collected by the tray 25. Due to the 
inclination of tray 25 and reciprocating motion of the piston 30, 
respectively, oil collected from the cavity 15 is better distributed 
around the tray 25. Further, when impacting against and around the cavity 
15 in a shaker type fashion, the removal of heat from the crown 10 is 
increased leading to the thermal equalization of the articulated piston 
30. 
Owing to characteristics such as cooling oil viscosity, tilting motion 
intervals and others pertaining to the piston dynamics, it has been found 
that an improved thermal equalization of the piston 30 is reached when the 
bottom of the tray 25 is inclined 1 degree to 10 degrees toward the 
quadrant opposed to the oil inlet 24 and preferably 2 degrees to 6 degrees 
toward such quadrant. More specifically, it has been found that an 
improved thermal equalization of the piston 30 is reached when the bottom 
of the tray 25 is inclined 4 degrees toward the quadrant opposed to the 
oil inlet 24. 
The above-noted feature being best illustrated in FIGS. 5-12. Initially, 
FIGS. 5 and 6 illustrate the cross-sectional view taken along line V--V of 
FIG. 4B wherein it can be readily seen that the tray 25 is inclined 
downwardly from an outer periphery or side wall 26 toward an inner 
periphery or side wall 27 of the tray 25. As discussed hereinabove, this 
allows for greater and more efficient distribution of the coolant medium 
which is collected in the tray 25. 
Referring to FIGS. 7 and 8, which is viewed in accordance with the 
cross-sectional lines VII--VII of FIG. 4B, the tray 25 is inclined in a 
direction away from the oil inlet 24. As discussed hereinabove, this 
inclination is in the range of 1.degree. to 10.degree., more specifically 
in the range 2.degree. to 6.degree. and preferably 4.degree.. As is 
readily apparent from FIG. 8, the initial point 29 of the tray 25 is of a 
depth significantly less than that of the point 31 illustrated therein. 
Accordingly, the coolant medium which is caught in the tray 25 will be 
displaced about the perimeter of the skirt 20 by way of the inclination of 
the tray 25. 
Referring now to FIGS. 9 and 10 which are taken along cross-sectional line 
IX--IX of FIG. 4B, it is readily apparent that the depth of the tray 25 
has increased significantly as compared to that illustrated in FIGS. 5 and 
6. Likewise, the tray 25 is inclined downwardly toward the central axis of 
the piston skirt 20 as exemplified by the depth at the inner periphery or 
side wall 27 of the tray 25 being greater than the depth at the outer 
periphery or side wall 26 of the tray 25. Again, such inclination aids in 
the dispersion of the coolant medium within the tray 25. 
As is apparent from FIG. 4A, the coolant tray 25 is actually divided in to 
two diametrically opposed coolant trays or reservoirs 25 and 25B, each of 
which are inclined away from the oil inlet 24 as well as being inclined 
inwardly toward the central axis of the piston skirt 20. The coolant trays 
or reservoirs 25 and 25B being separated by hubs projecting upwardly from 
the skirt 20. The inclination of the tray 25B away from the oil inlet 24 
is best illustrated in FIGS. 11 and 12. Similar to that illustrated in 
FIG. 5, the tray 25B is inclined from an initial point 33 away from the 
oil inlet 24. As with the tray 25, the tray 25B is likewise inclined in a 
range of 1.degree. to 10.degree., more specifically in a range of 
2.degree. to 6.degree. toward the quadrant opposed from the inlet 24 and 
preferably, the tray 25B is inclined approximately 4.degree. toward the 
quadrant opposed to the oil inlet 24. Accordingly, like the tray 25, the 
tray 25B aids in the distribution of the coolant medium about the 
periphery of the skirt 20 thus better distributing the coolant medium and 
increasing the thermal equalization of the articulated piston 30 by 
providing more coolant medium in the quadrant opposed to the quadrant 
including the oil inlet 24. 
Accordingly, by manufacturing an articulated piston in accordance with the 
foregoing, the articulated piston will exhibit improved thermal 
equalization as well as improved motional equalization. As noted 
hereinabove, the piston skirt serves two central functions, the first 
being to guide the piston assembly in the cylinder and the second being to 
aid in the cooling of the piston crown. The piston skirt is subjected to 
thrust loading as it guides the piston crown in the cylinder and as such 
is carried out, the thrust loading is higher on the major thrust side of 
the piston. Typically, the piston skirt design utilizes a uniform thrust 
wall of thickness, however, as mentioned hereinabove, applicants achieved 
greater success when the skirt design incorporates a major thrust side 
wall of a thickness which is capable of withstanding thrust loads and a 
minor or non thrust side wall of a thickness which is 5% to 50% thinner 
than that of the major thrust side wall. This feature attributes 
significantly to the motional equalization of the articulated piston. 
Similarly, in order to aid in the shaker effect of the cooling medium 
entrained in the tray provided in the piston skirt, the tray is inclined 
downwardly circumferentially away from the point at which the cooling 
medium is sprayed into the cooling gallery. Likewise, the tray is sloped 
radially inwardly toward the axis of the piston skirt to further aid in 
the dispersion of the coolant medium within the tray. Each of the 
foregoing aspects resulting in an articulated piston exhibiting improved 
reliability and durability which is suitable for use in heavy duty diesel 
engines. 
While the present invention has been described with reference to a 
preferred embodiment, it will be appreciated by those skilled in the art 
that the invention may be practiced otherwise than as specifically 
described herein without departing from the spirit and scope of the 
invention. It is, therefore, to be understood that the spirit and scope of 
the invention be limited only by the appended claims.