Valvetrain for a pushrod engine

An internal combustion engine has a crankcase, a cylinder bank extending from the crankcase and a crankshaft disposed in the crankcase. A cylinder in the cylinder bank is closed by a cylinder head which carries intake and exhaust valves for regulating the flow of gasses through the cylinder. An intake and an exhaust camshaft are rotationally disposed within the crankcase and are driven by the crankshaft. The intake camshaft has a cam lobe which operates the intake valve through a pushrod extending between the cam lobe and the valve and the exhaust camshaft has a cam lobe which operates the exhaust valve through a pushrod extending between the cam lobe and the valve. The use of separate intake and exhaust camshafts separates the intake valve actuation event from that of the exhaust valve and allows the application of camshaft phasing to vary the opening relationship, or timing of the intake and the exhaust valves. In addition, the use of two camshafts facilitates the use of multi-step cam lobe.

TECHNICAL FIELD 
The invention relates to valve trains for internal combustion, pushrod 
engines. 
BACKGROUND 
Variation of valve actuation events in an automotive internal combustion 
engine provides improved low speed combustion processes and resultant 
system power output without sacrificing desired engine operating 
characteristics at high engine speed. Engines having pushrod actuated 
valve trains traditionally exhibit less flexibility with respect to valve 
train variability due to the location of the intake and exhaust valve 
actuators, or cam lobes, on a single crankshaft driven camshaft such as is 
illustrated in FIG. 1. In such an engine, a single rotating camshaft is 
typically located in the valley of the engine block above, and parallel 
to, the engine crankshaft. The camshaft actuates the intake and the 
exhaust valves via cam followers, pushrods and rocker arms. Because the 
inlet and exhaust valve events are fixed, relative to one another by 
placement of the inlet cam lobes 2 and exhaust cam lobes 4 on the same 
shaft, the timing or relationship of the events can not be easily altered. 
A two-step cam follower is known, having two cylinders with a coincident 
longitudinal axis. The outer cylinder may be used for "high-lift" valve 
events and is operated on by a first pair of spaced cam lobes 6 of shaft 
5' as is illustrated in FIG. 2, and the inner cylinder may be utilized for 
"low-lift" valve events and is acted on by a second cam lobe 7 between the 
first pair of lobes 6. As is evident from the illustrations of FIGS. 1 and 
2, implementation of a two-step cam follower requires a greater investment 
in axial camshaft length, per valve, than in a non-variable system. In an 
internal combustion engine having a pushrod actuated valve train with 
specific, fixed cylinder bore center distances, the axial shaft distance 
between camshaft bearings may not permit the packaging of a two-step 
follower system. 
SUMMARY OF THE INVENTION 
The present invention relates to an internal combustion engine having a 
pushrod actuated valve train. The subject engine has improved valve 
actuation which facilitates variation of valve lift and timing during 
engine operation. The subject engine has a crankcase, housing a piston 
driven crankshaft. First and second cylinder banks extend from the 
crankcase to each define a cylinder bank, and associated plane. The 
cylinder bank planes intersect at the crankshaft axis and are inclined at 
an angle, relative to one another, to thereby establish the crankcase 
valley therebetween. A first camshaft is driven by the crankshaft and 
includes a first series of valve actuating cam lobes, and a second 
camshaft, preferably driven from the first camshaft by a chain or gear, 
has a second set of valve actuating lobes. The first camshaft controls the 
actuation of either the intake or the exhaust valves and the second 
camshaft controls the actuation of the other set of valves. In the present 
invention, the first and second camshafts extend in parallel to the 
crankshaft axis and are located along a plane which extends through the 
crankshaft axis and bisects the crankcase valley. 
The application of multiple camshafts to a pushrod engine facilitates the 
application of physically larger, two-step cam followers by eliminating 
the requirement of locating both intake and exhaust cam lobes on a single 
shaft of limited axial length. 
In addition, the separation of the intake and exhaust actuators in the 
pushrod engine allows the use of conventional camshaft phasing mechanisms 
which are not applicable in engines having the intake and exhaust 
actuation events controlled by a single shaft. Camshaft phasing allows the 
timing of the intake and exhaust valve events to be varied, relative to 
one another by varying the relative rotations of the two camshafts. Such a 
variation changes the flow characteristics through the engine cylinders. 
In some cases it may be useful to utilize a two-step cam follower with cam 
phasing and such an application is made possible by the present invention. 
Other objects and features of the invention will become apparent by 
reference to the following description and to the drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring first to FIG. 3, a V-configured, internal combustion engine, 
designated generally as 10, is shown. The engine 10 includes crankcase 12 
having a crankshaft 14 mounted for rotation therein along a longitudinal 
crankshaft axis 16. Cylinder banks 18,19 extend from the crankcase 12 and 
define cylinder bank planes 20,21 which intersect one another at the 
crankshaft axis 16. The cylinder banks 18,19 and their respective planes 
20,21, are inclined with respect to one another at an angle "A" which 
defines a crankcase valley 22 therebetween. A vertical plane 23, which 
extends through crankshaft longitudinal axis 16, bisects the crankcase 
valley 22. In FIG. 3, the cylinder banks are, for example, inclined at a 
ninety degree angle to one another. The cylinder banks 18 include piston 
cylinders 25 housing pistons (not shown) which drive the crankshaft 
through a mechanical linkage. The cylinders 25 are closed at their upper 
or outer ends by cylinder heads 27 which, in cooperation with the pistons, 
establish combustion chambers in the cylinders 25. A fuel air mixture 
enters each of the cylinders 25 through an associated intake runner 24 in 
the cylinder heads 27, with the timing of the charge entry controlled by 
an intake poppet valve 26 disposed in a valve seat 28 situated between the 
intake runner 24 and the piston cylinder 25. In a similar fashion, the 
products of the combustion of the fuel/air mixture exit the piston 
cylinders 25 through exhaust valves 30 and corresponding exhaust runners 
32 in the heads 27. 
The opening and closing of the intake 26 and exhaust valves 30, referred to 
as valve events, are controlled, through mechanical means, by the rotation 
of first and second camshafts 34 and 36, respectively. In the preferred 
embodiment of the invention disclosed herein, the intake camshaft is 
represented by the camshaft 34 and the exhaust camshaft is represented by 
the camshaft 36, however, it should be noted, the assignment of a 
particular camshaft to actuate the intake or exhaust valves may be varied 
with design application. The intake and exhaust camshafts 34,36 extend in 
longitudinal, parallel orientation to the longitudinal crankshaft axis 16 
and are both located along vertical plane 23, bisecting valley 22 of the 
engine crankcase 12 with the intake camshaft 34 located above (as viewed 
in the Figures) and parallel to the exhaust camshaft 36. Rotational drive 
from the crankshaft 14 is through a chain, a belt, or a gearset which 
engages the camshaft 36 and typically reduces the camshaft rotation by 
one-half of the crankshaft rotation for a four cycle engine. FIGS. 4, 5 
and 7 illustrate a preferred apparatus for driving exhaust camshaft 36 in 
which crankshaft 14 includes a sprocket 38 which, through chain 40, drives 
exhaust camshaft 36 through a corresponding sprocket 42 on the camshaft. 
The exhaust camshaft 36 has a series of axially spaced cam lobes 44 
disposed thereon which act on a valve actuation mechanism to open the 
exhaust valves 30 at the appropriate time and for the proper duration as 
is determined by the cam lobe profile and its location in the rotation of 
the shaft 36. The valve actuating mechanism for a pushrod engine of the 
type described herein may typically include a cam follower 46, a pushrod 
48, extending from the follower through the crankcase 12 and cylinder head 
27, and a rocker arm 50 which translates the movement of the pushrod 48 
into an opening and closing force on the end of the exhaust valve 30. The 
opening force on the valve 30 is resisted by a closing force exerted by 
the compression spring 52. 
In a similar manner, the intake camshaft 34 is located in plane 23, above 
(as viewed in the Figures) and parallel to the crankshaft 14 and the 
exhaust camshaft 36. Rotational drive for the camshaft 34 is through a 
chain, a belt, or a gear which engages the exhaust camshaft 36. Referring 
to FIGS. 4, 5 and 7, a preferred apparatus for driving intake camshaft 34 
is shown. Exhaust camshaft 36 includes a second drive sprocket 54 which, 
through chain 56, drives intake camshaft 34 through a corresponding 
sprocket 58 on the intake camshaft 34. Sprockets 54 and 58 are typically 
sized such that the intake and exhaust camshafts are driven at the same 
rotational speed. In the embodiment described herein, the intake camshaft 
34 has a series of axially spaced cam lobes 60 disposed thereon which act 
on a valve actuation mechanism to open the intake valves 26 at the 
appropriate time and for the proper duration as is determined by the cam 
lobe profile and its location in the rotation of the shaft 34. As is 
illustrated in FIG. 5, however, the cam profile for the intake valves of 
the present engine are configured to be used with a two-step cam follower 
62 of the type disclosed in U.S. application Ser. No. 08/251,702 filed May 
31, 1994 and owned by the assignee of the present application. With such a 
follower, an inner cylinder, or low-lift follower, rides on the inner or 
center cam of a set of three closely spaced cams on the camshaft 34. The 
low-lift cam 64 and follower is operable during low speed engine operation 
with the cam having a profile well suited to such operation. Hydraulic 
actuation of the cam follower 62 will subsequently engage the outer 
cylinder, or high-lift follower, which rides on the outer two cams 66 on 
the camshaft which are better suited to high speed engine operation. Such 
a variable lift cam and actuator provide flexibility in the operating 
characteristics of the engine across its entire operating range and, is 
made possible in the pushrod engine due to the application of a second 
camshaft. The valve actuating mechanism for the intake valves 26 includes, 
in addition to two-step cam follower 62, a pushrod 68 which extends from 
the follower through the crankcase 12 and cylinder heads 18, and a rocker 
arm 70 which translates the movement of the pushrod 68 into an opening and 
closing force on the end of the intake valve 26. The opening force on the 
valve 26 is resisted by a closing force exerted by the compression spring 
72. 
In addition to the application of two-step or variable lift cam followers 
to a pushrod engine, facilitated by separate intake and exhaust camshafts 
34 and 36, the dual camshaft arrangement allows the relative rotation of 
the two shafts to be varied during operation through the application of a 
cam phasing device 73 of the type shown in FIGS. 6 and 8. It may be 
desirable to vary the timing of the intake and the exhaust events by 
moving the cam lobes 44 of the exhaust camshaft 36, relative to those of 
the intake camshaft 34 to thereby vary the amount of valve overlap 
occurring, and valve opening and closing points, during different 
operating ranges of the engine. Crankshaft 14 drives the cam phaser 
housing 74 through chain 76 and sprocket 78 on the housing. A second 
sprocket 80 on housing 74 directly drives the intake camshaft 34 through 
chain 82 and sprocket 84. Exhaust camshaft 36 is, however, indirectly 
driven through cam phasing apparatus 73, of the type disclosed in U.S. 
Pat. No. 5,033,327, issued Jul. 23, 1991, in the name of Lichti et al. 
allowing the rotation of the exhaust camshaft 36 to be varied, relative to 
the rotation of the intake camshaft 34. 
As should be evident from the description provided, the multiple camshaft 
pushrod engine 10 is subject to several variations. For example, a pushrod 
engine is now available in which the intake camshaft, the exhaust 
camshaft, or both camshafts may now include two-step cam followers 
allowing both intake and exhaust events to be varied. 
In addition, the application of intake and exhaust cam phasing is possible 
with the disclosed pushrod engine and may be used with or without variable 
valve actuation using two-step followers. In some applications it is now 
possible to vary the operational profile of the valve actuation events 
while simultaneously varying the relative timing of the intake and exhaust 
events. 
While the multiple camshaft pushrod engine of the present invention has 
been disclosed and described for application to a V-configured internal 
combustion engine, it is contemplated that this improvement can be 
utilized on an inline pushrod engine as well. 
The foregoing description of the preferred embodiment of the invention has 
been presented for the purpose of illustration and description. It is not 
intended to be exhaustive nor is it intended to limit the invention to the 
precise form disclosed. It will be apparent to those skilled in the art 
that the disclosed embodiments may be modified in light of the above 
teachings. The embodiments described were chosen to provide an 
illustration of the principles of the invention and its practical 
application to thereby enable one of ordinary skill in the art to utilize 
the invention in various embodiments and with various modifications as are 
suited to the particular use contemplated. Therefore, the foregoing 
description is to be considered exemplary, rather than limiting, and the 
true scope of the invention is that described in the following claims.