Abstract:
A valve train for a twin cam engine has three valves per cylinder, including a pair of side-by-side intake valves and a single exhaust valve on an opposite, exhaust side of the combustion chamber. Dual camshafts in a V-type engine separately drive the intake and exhaust cams through a single dual arm intake rocker arm and a single exhaust rocker arm per cylinder, which provides much improved inlet port airflow for improved specific output compared to a single inlet valve engine. The single exhaust valve provides better catalytic converter performance due to lower exhaust heat loss than in four-valve engines of the same output capability. The intake and exhaust valve trains provide coplanar action of the push rods, rocker arms and valve stem axes, yielding an efficient and simplified valve train with a high flow combustion chamber. Additional features and advantages are disclosed.

Description:
TECHNICAL FIELD 
     This invention relates to engine combustion chamber and valve train arrangements and, more particularly, to a valve train arrangement for a twin cam three-valve engine. 
     BACKGROUND OF THE INVENTION 
     It is known in the engine art to provide engine combustion chambers with one or more intake valves and one or more exhaust valves, each arrangement having various advantages and disadvantages. Dual valve arrangements are generally provided for engines where simplicity or economy of manufacture is preferred. Three or four valve arrangements are often provided where the intention is to provide higher horse power output for the same size engine cylinder displacement. Four valve engines are commonly provided with dual overhead camshafts. 
     U.S. Pat. No. 5,560,329 issued Oct. 1, 1996 to the assignee of the present invention provides a two-valve combustion chamber arrangement in a V-type engine having dual in-block camshafts, one of which actuates the intake valves of the engine and the other of which actuates the exhaust valves. Both camshafts are driven by the engine crankshaft. A cam phasing device is provided for varying the phase angle of one of the camshafts with respect to the other camshaft and the crankshaft. This arrangement allows variable valve timing of the intake or exhaust valves in order to provide improved engine performance at varying speeds and loads. A push rod actuated valve train allows cam phasing with a single cam phaser acting on one of the camshafts as opposed to multiple cam phasers required for accomplishing the same purpose in a dual overhead camshaft engine with multiple cylinder banks. 
     SUMMARY OF THE INVENTION 
     The present invention provides a valve train somewhat similar to that disclosed in the previously-mentioned patent combined with a three-valve combustion chamber arrangement. The combustion chamber has a pair of side-by-side intake valves on an intake side of a cylinder and a single exhaust valve on an opposite exhaust side of the combustion chamber or cylinder. The dual camshaft arrangement in a V-type or multi-bank engine provides the same advantages that the arrangement does in the above-noted patent. However, the use of dual intake valves with a single exhaust valve provides much improved intake port airflow for improved specific output compared to a single intake valve engine, while the retention of a single exhaust valve provides better catalytic converter performance due to lower exhaust heat loss than in four-valve engines of the same output capability. Use of a single exhaust valve also provides for better placement of a direct cylinder injector with improved injector targeting over other overhead valve and four-valve engine designs for improved emissions and fuel economy. 
     The simplified valve train still provides the capability of hydraulic lash adjustment for each valve in the layout and the two camshaft drive provides the advantages of cam phasing for one or both camshafts with one or two cam phasers, respectively. The single exhaust valve may be conventionally actuated by a pushrod to one side of the cylinder, allowing a coplanar arrangement of the exhaust valve and pushrod with a rocker arm acting in the same plane and the valve head placed flush with the combustion chamber surface surrounding the valve seat. 
     These and other features and advantages of the invention will be more fully understood from the following description of certain specific embodiments of the invention taken together with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a fragmentary cross-sectional view showing a portion of a V-type engine having a valve train in accordance with the invention; 
     FIG. 2 is a lower plan view of the combustion chamber of one cylinder of the engine viewed from the lower face of the cylinder head; and 
     FIG. 3 is a view similar to FIG. 2 but showing an alternative embodiment in which the exhaust valve train is coplanar with the lateral centerline or lateral plane of the cylinder. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring first to FIG. 1 of the drawings in detail, numeral  10  generally indicates a V-type internal combustion engine having a cylinder block  12  defining a pair of cylinder banks  14 , only one of which is shown. Each cylinder bank  14  normally includes a plurality of longitudinally-spaced cylinders  16 , each cylinder carrying a reciprocating piston connected with a crankshaft, neither of which is shown. 
     Each cylinder bank carries a cylinder head  18  defining a combustion chamber  20  closing the ends of the cylinders  16 . A lower view of the single illustrated combustion chamber  20  is shown in FIG.  2 . Each cylinder bank defines a longitudinal plane  22  which passes through the aligned central axes of the cylinders of that bank. Each cylinder also defines a lateral plane  24  intersecting the longitudinal plane at the central axis of each of the cylinders. 
     Each cylinder head defines a pair of longitudinally-spaced intake ports  26  for each cylinder, the ports connecting an associated intake manifold  28  with valve seats  30  formed adjacent the inner surface  32  of the combustion chamber  20 . The cylinder heads further define single exhaust ports  34  for each cylinder connecting exhaust valve seats  36  with externally mounted exhaust manifolds  38 . 
     The intake ports  26  are controlled by dual intake valves  40  having heads  42  seatable on the valve seats  30  and preferably longitudinally spaced equally on opposite sides of the lateral planes  26  of their respective cylinder. Valve stems  44 , connected with the heads  42 , preferably extend along parallel valve axes  46 , canted with respect to the cylinder axis  24  but extending parallel to the lateral plane  26  of the cylinder. 
     Similarly, each cylinder has a single exhaust valve  48  controlling its respective exhaust port  34  and including an exhaust valve head  50  seatable on its valve seat  36  and a valve stem  52  extending on a canted axis  54 . The axis  54  forms a compound angle relative to the lower cylinder head surface  56  and thus lies in a plane which is not parallel with either of the longitudinal and transverse planes through its respective cylinder. This is caused by the placement of the exhaust valve  48  on the outboard side of the cylinder and longitudinally offset to one side of the lateral plane  24 , while the inner surface of the exhaust valve head  50  is located generally flush with the adjacent surface of the combustion chamber. 
     With this placement of the exhaust valve, the space longitudinally opposite the exhaust valve may be used for mounting a spark plug  60  and, if desired, a direct injection fuel injector  62 . Optional alternative arrangements for the exhaust valve and spark plugs will be subsequently discussed. 
     Referring again to FIG. 1, the intake and exhaust valves  40 ,  48  of the engine are actuated by a valve train which includes separate intake and exhaust camshafts  64 ,  66 , respectively. The camshafts are rotatably carried within the engine block  12  and extend longitudinally therein. Both camshafts are driven by chain or other conventional drive means from the engine crankshaft, not shown. As illustrated, the exhaust camshaft  66  is located above the intake camshaft  64 , both being aligned on a central plane  68  of the engine block  12 . The camshafts are mounted within the “V” or valley of the engine between the cylinder banks  12 . 
     The intake camshaft  64  includes intake cams  70  which actuate reciprocable followers  72 . The followers drive pushrods  74 , each connecting with a single rocker arm  76  having dual output arms  78 , each carrying a hydraulic lash adjuster  80 . If desired, mechanical lash adjusters could be provided instead of the hydraulic type. The lash adjusters  80  are positioned to engage the ends of the intake valve stems  44  so as to open the valves when the rocker arm is actuated by the cam follower  72  and pushrod  74  of the respective cylinder. Preferably, the single pushrod  74  is aligned with the cam follower on the lateral plane  24  of the cylinder and the dual arms  76  and lash adjusters  80  are aligned with the valve stems  44  in planes passing through the respective valve axes  46 . Conventional valve springs  82  are provided to return the valves to their seated positions when the cam follower travels down the backside of the cam  70 . 
     Similarly, the exhaust camshaft  66  is provided with exhaust cams  84  which actuate exhaust cam followers  86 . Followers  86  actuate pushrods  88  which in turn pivot exhaust rocker arms  90  on canted axes  92 . The rocker arms  90  directly engage the stems  52  of the respective exhaust valves  48  to open the valves upon actuation by the respective exhaust cams  70 . Valve springs  94  again provide for closing of the exhaust valves when the followers  72  move down the closing side of the exhaust cams  70 . To adjust lash in the valve trains of the single exhaust valve for each cylinder, the exhaust cam followers  86  are provided with internal hydraulic lash adjusters, not shown, which operate in known manner. If desired, lash adjusters could instead be mounted on the rocker arms and mechanical adjusters could be provided, if desired. 
     In operation of the engine, the timings of the intake and the exhaust valves are controlled separately by the individual intake and exhaust camshafts. Accordingly, either or both of the camshafts may be provided with cam phaser devices which are operable to vary the phase rotation of the respective camshaft relative to the crankshaft. In this way, the intake cams may be varied as to their opening and closing timing relative to the exhaust cams and vice versa. 
     The particular three-valve arrangement of the cylinder, shown in FIGS. 1 and 2, provides a number of specific advantages. For one, asymmetrical rocker arms are not required. Further, the splayed exhaust valve arrangement with compound valve angles on the exhaust side allows for a larger exhaust valve for a given bore size than is possible with dual exhaust valves. Packaging of the splayed valve allows for an improved spark plug location in the space longitudinally opposite the valve on the outside (exhaust side) of the cylinder. The arrangement also allows a vertical or near vertical fuel injector orientation for direct injection fuel systems. Additionally, the water jacket geometry surrounding the exhaust valve provides for more optimum coolant flow around the valve seat. 
     In the case of a cam phased three-valve engine, the disclosed overhead valve configuration with a single exhaust valve allows for a valve event of much longer duration than a non-phased configuration. This longer event permits operation with lower valve lift for a given level of exhaust port performance, which reduces valve train stress and allows for an increase of valve train operating speed which may increase specific power output. Additionally, the three-valve arrangement greatly simplifies the valve train for the engine and the offset exhaust valve permits arrangement of the exhaust pushrod and valve stem axes in a common plane in which the rocker arm  90  pivots on an axis normal to the plane of the exhaust valve and pushrod. Note, however, that the exhaust cam follower  86  has an axis that lies normal to the exhaust camshaft  66 . Accordingly, the roller follower  86  is rotated relative to the exhaust valve plane to rotate on an axis parallel to that of the exhaust camshaft. Thus, a co-planar arrangement of the exhaust valve axis  54 , exhaust pushrod  88 , and the exhaust rocker arm  90  is maintained without requiring canting of the exhaust cam follower relative to the camshaft. To accomplish this, the cam follower  86  is located to one side of the cylinder and the pushrod  88  extends longitudinally outboard of the pushrods of the dual intake valves. 
     Referring now to FIG. 3, there is shown one of numerous possible alternative arrangements for a three-valve engine having a valve train in accordance with the invention. In FIG. 3, wherein like numerals indicate like parts, the position of the intake valves  40  is maintained but the location of the exhaust valve  48  is changed so that the valve is centered on the transverse plane  24  of the cylinder with the valve axis  54  lying in the plane  24 . While such an arrangement is practical, it may require some modification of the valve train itself. For example, separate cam followers for the intake valves could be used so that a centered exhaust cam follower  86 , push rod  88 , rocker arm  90 , and the exhaust valve axis  54  all lie in the lateral plane  24  through the axis of the cylinder. Other arrangements for actuating the valves could also be provided. With such an arrangement, it would be possible to provide dual spark plugs, one on either side of the exhaust valve as shown. However, a more conventional side entry point for a direct fuel injector might have to be resorted to. Nevertheless, the advantages of the larger exhaust valve and the generally flush position of the valve head in the cylinder are advantages which would remain in the modified three-valve arrangement with their accompanying benefits. Of course, it would alternatively be possible to operate the engine with a single spark plug if desired, depending upon the arrangement of the combustion chamber on the piston side of the cylinder. 
     While the invention has been described by reference to certain preferred embodiments, it should be understood that numerous changes could be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the disclosed embodiments, but that it have the full scope permitted by the language of the following claims.