Cylinder head assembly

A small four cycle internal combustion engine is disclosed having a cylinder head assembly which comprises a cylinder head cooperating with a cylinder block of the engine, and a rocker box connected to the cylinder head so as to define an air passage therebetween through which air may pass. The cylinder head has cooling fins projecting into the air passage between the cylinder head and the rocker box and aligned generally transversely to a line extending between the axes of the intake and exhaust valves. The air passage preferably extends between an intake port and an exhaust port of the engine, and above an exhaust gas recirculation port extending between the intake and exhaust ports. A pair of push rod tubes are integral with the rocker box and extend between the rocker box and a crankcase of the engine externally of the cylinder block. The engine of the present invention also includes a cam tower assembly comprising a base member and a pair of parallel shafts extending from the base member. One of the shafts functions as a camshaft and has a unitary cam gear and cam rotatably supported thereon. The other shaft is a follower shaft and has a pair of cam followers rotatably supported thereon.

TECHNICAL FIELD 
This invention relates to internal combustion engines, and more 
particularly to a two-piece rocker box and cylinder head assembly and a 
cam tower assembly for a small four cycle engine. 
BACKGROUND ART 
Small internal combustion four cycle engines are known which have a rocker 
box and a separately formed cylinder head. U.S. Pat. No. 4,601,267 to 
Kronich, for example, discloses a rocker box base fastened on a cylinder 
head integrally formed with a cylinder block. Similarly, U.S. Pat. No. 
5,058,542 to Grayson et al. discloses a thermoplastic or die cast aluminum 
inner rocker box cover bolted on a cylinder head. 
It is also known to make the cylinder head and rocker box as a one piece, 
integrally formed cylinder head assembly. The manufacture of such a 
cylinder head assembly typically involves a complex casting which may 
require four or more slides to form the intricate air passages and cooling 
fins by which heat is dissipated from the hottest part of the engine. 
Because the casting process is so difficult, however, and particularly 
when it is performed on a very small scale for single cylinder engines, 
the resulting wall thicknesses are frequently not uniform. Additionally, 
it is difficult or impossible to cast the rocker box with integral push 
rod tubes. 
Furthermore, the casting process may permit only a single slot on the order 
of a millimeter wide to be formed on either side of the spark plug and 
between the cylinder head and the rocker box. Since engine cooling is a 
function of the air flow through this passage, such a limited air path 
restricts cooling efficiency. The presence of disuniformly thick walls 
compounds this problem. 
Additionally, non-overhead camshaft small four cycle engines which use cam 
followers in the valve train typically have a camshaft on which are 
mounted the camgear and one or more cams, and a follower shaft on which 
are mounted the cam followers. Conventionally, the follower shaft is 
mounted to the cylinder block, and the camshaft is mounted to the 
crankcase. However, this construction introduces variances into the 
desired operation of the valve train for several reasons. Initially, there 
is often a variation in the center distance between the shafts because of 
manufacturing tolerances in the formation of the cylinder block, crankcase 
and valve train components. Additionally, there is some variance in the 
width of the gasket which typically separates the cylinder block and the 
crankcase. 
These variances result in a deviation from the optimal functioning of the 
valves, which can diminish the efficient operation and/or emissions 
performance of the engine. These variances are magnified in smaller 
engines such as single cylinder engines used for lawn and garden work. 
Further reductions in emissions output are known to be obtainable by 
engine exhaust gas recirculation. However, a simple and cost effective 
system for exhaust gas recirculation in small engines is not readily 
available. 
SUMMARY OF THE INVENTION 
The present invention is a small four cycle internal combustion engine 
having a cylinder head assembly which comprises a cylinder head 
cooperating with a cylinder block of the engine, and a rocker box 
connected to the cylinder head so as to define an air passage therebetween 
through which air may pass. The cylinder head has cooling fins projecting 
into the air passage between the cylinder head and the rocker box and 
aligned generally transversely to a line extending between the axes of the 
intake and exhaust valves. The air passage preferably extends between an 
intake port and an exhaust port of the engine, and above an exhaust gas 
recirculation port extending between the intake and exhaust ports. A pair 
of push rod tubes are integral with the rocker box and extend between the 
rocker box and a crankcase of the engine externally of the cylinder block. 
The engine of the present invention also includes a cam tower assembly 
comprising a base member and a pair of parallel shafts extending from the 
base member. One of the shafts functions as a camshaft and has a unitary 
cam gear and cam rotatably supported thereon. The other shaft is a 
follower shaft and has a pair of nested cam followers rotatably supported 
thereon. The cam tower assembly is adapted to be attached to the engine 
such that the rotation of the cam actuates the followers, which in turn 
operate the remainder of the valve train. 
Accordingly, it is an object of the present invention to provide a cylinder 
head assembly of the type described above having a die cast aluminum 
cylinder head and a discrete, die cast aluminum or magnesium rocker box 
with integral push rod tubes. 
Another object of the present invention is to provide a cylinder head 
assembly of the type described above having improved cooling 
characteristics. 
Another object of the present invention is to provide a cylinder head 
assembly of the type described above which can be simply and inexpensively 
manufactured. 
Still another object of the present invention is to provide a cam tower 
assembly of the type described above in which the distance between the 
camshaft and the follower shaft can be closely controlled. 
Still another object of the present invention is to provide a small 
internal combustion engine of the type described above having an exhaust 
gas recirculation port extending between an intake port and an exhaust 
port of the engine. 
These and other objects, features, and advantages of the present invention 
are readily apparent from the following detailed description of the best 
mode for carrying out the invention when taken in conjunction with the 
accompanying drawings.

BEST MODE FOR CARRYING OUT THE INVENTION 
With reference to the drawings, the preferred embodiments of the present 
invention will be described. FIGS. 1 through 7 show a small one-cylinder, 
four cycle engine 10 according to the present invention preferably having 
a displacement of between about 20 and 80 cubic centimeters. The engine 10 
comprises a cylinder head assembly 12 and a piston 14 reciprocable in a 
cylinder block 16. 
The piston 14 is operatively connected to actuate an intake valve 18 and an 
exhaust valve 20. As shown in FIG. 2, reciprocation of the piston 14 
imparts rotation to a cantilevered crankshaft 22 disposed in a crankcase 
24 through a connecting rod 26, as is well known in the art. A crankgear 
28 mounted on the crankshaft 22 in turn meshes with a camgear 30 mounted 
on a camshaft 32 in a valve drive chamber 34 of the crankcase 24 to drive 
a single lobe cam 36 at one-half engine speed. Rotation of the cam 36 is 
translated to reciprocable motion to reciprocate a pair of pushrods 38 and 
40 by a pair of frog-leg shaped followers 42 and 43, as disclosed in 
pending U.S. patent application Ser. No. 08/021,496. The push rods 38 and 
40 operate through rocker arms 44 and 46 to respectively actuate the 
exhaust valve 20 and the intake valve 18. Of course, one skilled in the 
art will appreciate that a conventional construction including tappets can 
be provided to perform the function of the followers 42 and 43. 
As shown in FIGS. 3 through 7, the cylinder head assembly 12 includes a 
unitary, die cast aluminum cylinder head 48 adapted to cooperate with the 
cylinder block 16, and a rocker box 50. The rocker box 50 is also a 
unitary die cast aluminum or magnesium part, and is adapted to at least 
partially house the rocker arms 44 and 46. The rocker box 50 has a first 
pair of holes 52 therethrough adapted to receive means, such as bolts or 
rocker studs, for connecting the rocker box to the cylinder head 48. In a 
preferred embodiment shown in FIG. 1, rocker studs 54 extend through the 
cylinder head 48 to secure the entire cylinder head assembly 12 to the 
cylinder block 16. The rocker box 50 also has a second pair of holes 56 
therethrough adapted to respectively receive a pair of valve guides 58 and 
60 projecting from the cylinder head 48. A third pair of holes 62 are 
formed in the rocker box 50 for receiving the push rods 38 and 40. If the 
rocker box is formed from aluminum, the holes 62 can be cast in a 
generally oval shape, as shown in FIG. 3, to act as push rod guides. If 
the rocker box is formed from magnesium, stamped steel guide plates having 
generally oval holes therethrough are preferably added to act as push rod 
guides. 
The rocker box 50 is connected to the cylinder head 48 so as to define an 
air passage 64 therebetween through which cooling air may flow. The air 
passage 64 preferably extends between cross flow intake and exhaust ports 
66 and 68, respectively, formed in the cylinder head 48. The cylinder head 
48 has a plurality of cooling fins 70 which project into the air passage 
64 between the cylinder head and the rocker box 50. In particular, a main 
cooling fin 72 projects rearwardly from a spark plug boss 74 and up into 
an expanding groove 76 formed in the bottom of the rocker box 50. All the 
cooling fins 70, including the main cooling fin 72, are aligned generally 
transversely to an imaginary line extending between the axes of the intake 
and exhaust valves 18 and 20. 
The cylinder head 48 also has drilled therein an exhaust gas recirculation 
(EGR) port 77. The EGR port extends between the innermost sections of the 
intake port 66 and the exhaust port 68, and generally below the air 
passage 64. The EGR port 77 is preferably generally coaxial with the 
exhaust port and offset slightly from the axis of the intake port as 
viewed from above, although this arrangement may be reversed. The EGR port 
77 preferably has a constant circular cross-section with a diameter of 
about 1.25 millimeters. Throughout the range of engine operation, and in 
particular at the normal operational speed of about 7-8000 rpm, 
approximately 10% of the total exhaust gases produced by the engine are 
drawn back through the EGR port 77 to the intake port 66 for mixing with 
the incoming fuel-air mixture. 
A pair of elongated push rod tubes 78 and 80 in which push rods 38 and 40 
are respectively reciprocable are integrally formed with the rocker box 
50. The push rod tubes 78 and 80 extend, externally of the cylinder block 
16, from the rocker box 50 to sealingly cooperate with the valve drive 
chamber 34 of the crankcase 24. Both the cylinder head 48 and the rocker 
box 50 also have a plurality of horizontal cooling fins 82 disposed at 
least partially around their perimeters, preferably proximate the intake 
and exhaust ports 66 and 68 and adjacent the push rod tubes 78 and 80. 
Because the cylinder head and the rocker box of the present invention are 
discrete components that can be separately cast, the cylinder head 
assembly has relatively uniform wall thicknesses throughout, which 
facilitates engine cooling. 
As best shown in FIG. 2, the pin forming the camshaft 32 and a generally 
parallel pin forming a follower shaft 84 extend from a bracket or base 
member 86 to comprise a carrier 88. The base member 86 is preferably 
formed as an alloy steel powdered metal part. The cam gear 30 and the cam 
36, which are preferably formed as a unitary powdered metal part, are 
rotatably supported on the camshaft 32. Likewise, the followers 42 and 43 
are rotatably supported on the follower shaft 84. Together, the carrier 
88, cam gear 30 and cam 36, and the followers 42 and 43 comprise a cam 
tower assembly. Because the shafts 32 and 84 extend from a common base 
member 86, rather than being secured separately to the cylinder block 
and/or the crankcase, the distance between the shafts is more closely 
controllable. This eliminates the assembly and tolerance problems involved 
in the conventional method of assembly where the cam and the follower are 
assembled as individual components on separate pins on the crankcase and 
the cylinder assembly, respectively. The present construction also 
eliminates potential oil leak areas often found in conventional designs 
where the walls were drilled to accept the pins therethrough. 
The cam tower assembly is preferably attached to the engine by means of two 
bolts or socket head screws 90 extending through open grooves in the base 
member 86 and into the crankcase 24. The crankcase can be either cored or 
drilled and tapped to accept the screws. This structure makes the cam 
tower assembly easily serviceable because with the removal of only the 
screws 90, the entire assembly can be removed from the cranckcase. While 
the cam tower assembly is shown in FIG. 2 with the base member 86 
proximate the flywheel and the cam gear 30 proximate the cylinder block, 
it should be appreciated that a mirror image of this arrangement with the 
base member proximate the cylinder block and the cam gear proximate the 
flywheel is equally feasible. 
FIG. 8 shows that the axis 92 of the horizontal portion 94 of the intake 
port 66 is not aligned with the axis 96 of the vertical portion 98 of the 
intake port. Because of this offset intake port feature, the incoming 
fuel-air mixture is deflected or pre-swirled by the wall of the vertical 
portion 98 of the intake port around the stem of the intake valve, and 
continues to swirl as it is introduced into the combustion chamber. The 
swirling mixture thus created burns more quickly and/or completely when 
subsequently ignited by the spark plug. As an alternative to offsetting 
the intake port to induce a clockwise swirl in the cylinder as viewed from 
above, the intake port can be offset below the horizontal axis of the 
intake port so as to create a counterclockwise swirl. 
It should be understood that while the forms of the invention herein shown 
and described constitute preferred embodiments of the invention, they are 
not intended to illustrate all possible forms thereof. It should also be 
understood that the words used are words of description rather than 
limitation, and various changes may be made without departing from the 
spirit and scope of the invention disclosed.