Secondary air supply for engine exhaust system

Abstract

A chain chamber of an engine, which accommodates a cam chain for driving a camshaft, is formed between a first cylinder and a second cylinder aligned in the direction along the camshaft. A first-cylinder valve chamber and a second-cylinder valve chamber each accommodate a corresponding valve for supplying secondary air to an exhaust system. The first and second chambers overlap the chain chamber and lie adjacent to each other above the chain chamber. The first-cylinder and second-cylinder valve chambers have first and second valve chamber openings, respectively. A first and a second valve chamber cover for opening and closing the respective valve chamber openings are integrally formed together. This construction reduces the number of engine parts associated with a secondary air system and simplifies the sealing structure of secondary air introduction hoses and cover members.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority under 35 U.S.C. § 119 (a)-(d) to Japanese Patent Application No. 2004-237338, filed Aug. 17, 2004, the entire contents of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an engine having a secondary air supply system for supplying secondary air to an exhaust system.

2. Description of the Related Art

There are proposed devices for supplying secondary air to an exhaust system to burn HC and CO contained in exhaust gas and thus to purify the exhaust gas. Conventional devices for supplying secondary air to an exhaust system of this type include the one in which a reed valve device is integrally provided on a head cover mounted on a cylinder head of the engine. See Japanese Patent Publication No. JP-A-2003-56647 as an example of such device.

SUMMARY OF THE INVENTION

An aspect of the present invention involves the recognition that this enhanced-combustion technology can be difficult to employ on certain types of engine. For example, in the case of a center-chain-type engine, in which a timing chain for rotationally driving a camshaft is disposed between first and second cylinders, the first and second cylinders are spaced apart from each other by the width of a chain chamber. Accordingly, a first-cylinder valve chamber and a second-cylinder valve chamber for the corresponding reed valve must also be spaced apart from each other. This results in an increased in the number of parts, such as secondary air introduction hoses connected to the valve chambers, cover members for opening and closing openings of the valve chambers, and bolts for securing the cover members. Also, since there are a number of seal mating faces, the sealing structure may become complex.

Accordingly, there is provided an engine comprising a cylinder block that defines at least two adjacent cylinders. A cylinder head is fastened to the cylinder block and supports at least one camshaft. A head cover is mounted to the cylinder head, and a cam chain drives the camshaft. A chain chamber houses the cam chain and is formed generally between the adjacent cylinders. The chain chamber also has a longitudinal axis that lies substantially parallel to axes of the adjacent cylinders. A first-cylinder valve chamber and a second-cylinder valve chamber each are sized to accommodate a valve for supplying secondary air to an exhaust system of the engine. The first-cylinder and second-cylinder valve chambers are disposed in a manner whereby at least portions of the first-cylinder valve chamber and the second-cylinder valve chamber lie to one side of the chain chamber as viewed in a direction along the longitudinal axis of the chain chamber.

This construction reduces the number of engine parts associated with a secondary air system and simplifies the sealing structure of secondary air introduction hoses and cover members. Additionally, while the cylinders are located on opposite sides of the chain chamber, the first-cylinder and second-cylinder valve chambers can adjoin each other, which allows the first and second valve chamber covers to be integrally formed. Therefore, only a single secondary air hose for introducing air to the first and second valve chambers is required, and the number of fastening bolts can be reduced, so that the structure is simplified.

In a preferred mode, the head cover has a breather chamber for separating oil mist from blow-by gas. The breather chamber is disposed such that at least a portion of the breather chamber overlies the chain chamber as seen in the direction along the longitudinal axis of the chain chamber. The breather chamber also preferably lies adjacent to at least one of the first-cylinder and second-cylinder valve chambers.

The inclusion of the breather chamber at this location provides an ample size without increasing the side of the engine as at least a portion of the breather chamber overlaps the chain chamber. In other words, when the adjacent cylinders are located on opposite sides of the chain chamber, the capacity and space for the breather chamber can be obtained without difficulty. Further, the breather chamber can lie adjacent to the first-cylinder and second-cylinder valve chambers. Thus, the breather chamber cover can be integrally formed with the first and second valve chamber covers. Therefore, the mating faces (seal faces) of the openings are flush with each other, so that the number of fastening bolts is reduced and the structure is simplified.

In a further preferred mode, a secondary air hose connects the air cleaner and the first-cylinder and second-cylinder valve chambers, and a breather hose connects the breather chamber with the air cleaner. Because of the close proximity of the valve chambers and the breather changers, thee hoses preferably are bundled together. Thus, the routing structure of the hoses can be simplified, and hence the vicinity around the head covers can be simplified.

BRIEF DESCRIPTION OF THE DRAWINGS

The above-mentioned and other features of the invention disclosed herein are described below with reference to the drawings of a preferred embodiment. The illustrated embodiment is intended to illustrate, but not to limit, the invention. The drawings include the following six figures.

FIG. 1 is a sectional side view of a chain chamber of an engine in accordance with a preferred embodiment of the present invention.

FIG. 2 is a partially sectional side view of the engine in FIG. 1.

FIG. 3 is a plan view of a forward cylinder head of the engine in FIG. 1, shown with a head cover removed from the cylinder head.

FIG. 4 is a plan view of the forward head cover of the engine in FIG. 1, shown with the valve chamber covers removed.

FIG. 5 is a plan view of the forward head cover of the engine in FIG. 1.

FIG. 6 is a sectional side view, showing reed valves of the engine in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described below with reference to accompanying drawings. FIGS. 1 through 5 are explanatory views of the engine configured in accordance with a preferred embodiment of the present invention. The following description of the engine and the present secondary air supply will describe the components of the engine, including the secondary air supply, in the orientation shown in the accompanying drawings. The engine, however, can be used in other orientations depending upon the particular application of the engine.

In the drawings, reference numeral 1 denotes a water-cooled, four-stroke, V-type, four-cylinder engine, which has the following construction. A cylinder block 2 has forward bank of cylinders 2c and rearward bank of cylinders 2d arranged to form a V-shape. A crankcase 3 is connected to a lower mating face 2a of the cylinder block 2. Forward and rearward cylinder heads 4, 5 are connected to corresponding upper mating faces 2b, 2b of the forward and rearward cylinders 2c, 2d, respectively. Forward and rearward head covers 6, 7 are mounted on the forward and rearward cylinder heads 4, 5, respectively. Those skilled in the art will appreciate, however, that various aspects and features of the present secondary air supply can be used with engines having other constructions, having other number of cylinders, and having other cylinder orientations (e.g., in-line).

In the illustrated embodiment, the forward and rearward banks of cylinders 2c, 2d each have left and right cylinder bores (first and second cylinders) 2e, 2f formed in alignment with each other in the direction along camshafts. Pistons 31 are reciprocally disposed in the left and right cylinder bores 2e, 2f, and are connected to a crank pin of a crankshaft 8 through respective connecting rods 32. The crankshaft 8 has left and right journal portions and two other journal portions at its central portion with respect to its axis, which are supported by journal bearings preferably formed between the lower mating face 2a of the cylinder block 2 and an upper mating face 3a of the crankcase 3.

An axially central portion of the crankshaft 8 is integrally formed with crank sprockets 9, 9 for the forward and rearward cylinders. An intake camshaft 10 and an exhaust camshaft 11 are supported by each cylinder head 4, 5. In the illustrated embodiment, the intake and exhaust camshafts 10, 11 are located on respective upper mating faces 4a, 5a of the corresponding cylinder head 4, 5 and lie generally parallel to the crankshaft 8. The intake camshaft 10 preferably is positioned on the V-bank “A” side. An axially central portion of each intake camshaft 10 is formed with a cam sprocket 10a, which is connected to the crank sprocket 9 through a timing chain 12. In another variation, the engine can employ a single camshaft to actuate both the intake and exhaust valves of the cylinder bank.

The engine also includes an intake-side timing gear 10b that is disposed generally at the axially central portion of the intake camshaft 10. The intake-side timing gear is fastened to one side of the cam sprocket 10a preferably by bolts. The intake-side timing gear 10b is engaged with an exhaust-side timing gear 11a disposed on the exhaust camshaft 11. In this manner, rotation of the crankshaft 8 is transmitted to the intake camshaft 10 via the timing chain 12, and the rotation of the intake camshaft 10 is transmitted to the exhaust camshaft 11 via the timing gears 10b, 11a. Of course, other types of transmissions mechanism can be used to drive the camshafts 10, 11 off the crankshaft 8.

A chain chamber “c” is formed between the left and right cylinder bores 2e, 2f by the crankcase 3, the cylinder block 2 and the cylinder heads 4, 5 to place the crank chamber “d” in communication with the cam chamber “e.” The exhaust-side and intake-side timing gears 11a, 10b, cam sprocket 10a, timing chain 12 and crank sprocket 9 are disposed in the chain chamber “c.”

Reference numerals 24, 25 denote tensioner members for tension adjustment of the respective timing chains 12, 12. Lower ends 24a, 25a of the tensioner members 24, 25 are swingably supported by a retaining member 40. The retaining member 40 preferably is attached to the lower mating face 2a of the cylinder block 2. Tensioners 26, 27 urge respective upper parts of the tensioner members 24, 25 in the direction of application of tension to the timing chains 12.

The engine also includes hydraulic switching valves 28, 29 for switching a valve timing variable mechanism (not shown), which are located within the V-bank “A” with their axes preferably parallel to the crankshaft 8.

Lower mating faces 4b, 5b of the cylinder heads 4, 5 are formed with recessed combustion chambers “b,” each of which has at least one opening to an exhaust port 36 and at least one opening to an intake port 37. Each opening of the exhaust and intake ports 36, 37 on the combustion chamber side is provided with either an exhaust valve 33 or an intake valve 34, which are driven to open and close the openings by the exhaust camshaft 11 and the intake camshaft 10, respectively. Reference numeral 51 denotes a spark plug disposed in generally central portions of the cylinder heads 4, 5. As so disposed, an electrode of each spark plug 51 is positioned in a generally central portion of the corresponding combustion chamber “b.” Reference numeral 50 denotes a plug hole to allow removal of the spark plug 51.

An exhaust system (not shown) is connected to an outside connection opening of the exhaust port 36. The intake port 37 extends generally vertically upward within the V-bank “A,” and an outside connection opening 37a of the upper end of the intake port 37 is connected to an induction system that preferably includes a throttle body 35. In the illustrated embodiment, the throttle body 35 includes a downstream-side throttle valve 35a and an upstream-side throttle valve 35b. A fuel injection valve 38a is provided at the downstream end of the throttle body 35 in a manner such that the fuel injection valve 38a is positioned within the V-bank “A” and fuel is injected to the opening of the intake port 37 on the combustion chamber side. The present secondary air supply system and blow-by gas system can be used with throttle-less inductions systems as well.

The engine 1 in accordance with the present embodiment has a secondary air supply system for supplying secondary air downstream from the exhaust valve opening of the exhaust port 36, and a blow-by gas discharge system for separating mixed oil mist from blow-by gas and discharging such gas to an intake system. The blow-by gas is mainly the burned gas which leaked through the gap between a piston ring and the cylinder bore and entered into the crank chamber, in which atomized engine oil is mixed with these gases. However, such blow-by gas must not be discharged to the ambient air without treatment. Therefore, the blow-by gas, after the oil mist has been separated therefrom and collected, is discharged to the intake system and burned again in the engine. The following will describe the air supply system and the blow-by gas discharge system in connection with the front cylinder bank (left) cylinder bank, as seen in FIG. 1. It should be understood, however, that these systems can be employed on the rear (right) cylinder bank in the alternative or in addition to being employed on the front cylinder bank.

The secondary air supply system has a left valve chamber (first-cylinder valve chamber) 41 associated with the exhaust port from the left cylinder bore 2e and a right valve chamber (second-cylinder valve chamber) 42 associated with the exhaust portion from the right cylinder bore 2f. The left and right valve chambers 41, 42 preferably are recesses of a rectangular shape in plan view formed in top surfaces of the head covers 6, 7. As seen in the direction along an axis that lies generally parallel to the corresponding cylinder axes, the left and right valve chambers 41, 42 overlap the chain chamber “C,” at least at their portions that lie closer to the axially center portion of the crankshaft. In the illustrated embodiment, the left and right valve chambers 41, 41 also lie adjacent to each other above the chain chamber “C,” with their outer portions (with respect to the crankshaft axial center) overlapping the left and right cylinder bores 2e, 2f, respectively (see FIG. 4).

The left and right valve chambers 41, 42 have left and right valve chamber openings (first and second valve chamber openings) 41a, 42a, respectively, which preferably open to the exterior of the engine. The left and right valve chamber openings 41a, 42a preferably have a rectangular shape and are each located with its longer sides extending in the direction along the camshafts. The left and right valve chamber openings 41a, 42a lie adjacent to each other above the chain chamber “C” with a common partition wall separating the openings 41a, 42a.

Valves 44 regulate air flow through the chambers 41, 42. In the illustrated embodiment, the valves take the form of reed valves 44 that preferably have the same structure in the left and right valve chambers 41, 42, respectively. Each reed valve 44 divides its respective valve chambers 41, 42 into lower and upper sections. A base plate 44b of each reed valve 44 preferably has a rectangular valve opening 44a. The lower surface of the base plate 44b is provided with a valve plate 44c including a rectangular, thin plate, for opening and closing the valve opening 44a. The opening of the valve plate 44c is regulated by a stopper plate 44d. The valve plate 44c is located with its longer sides extending in the direction along the camshafts. An outside end of each valve plate 44c, which is positioned opposite the chain chamber “C,” is fastened to the base plate 44b together with the stopper plate 44d with a bolt. The reed valves, however, can have other shapes and orientations within the valve chambers and the valves need not be of the same type.

In the bottom surfaces of the left and right valve chambers 41, 42, air introduction passages 41b, 42b have openings, and run in the cylinder head and lead downstream from the exhaust valve 33 of the exhaust port 36. Reference numeral 44e denotes a shield plate for restricting backflow of exhaust gas to the reed valve 44 side. The shield plate 44e comprises a plate member having a number of small holes formed therein.

The blow-by gas discharge system has a breather chamber 45 for separating oil mist from blow-by gas. The breather chamber 45 is formed in the shape of a recess in the top surfaces of the head covers 6, 7 in a manner where the breather chamber 45 generally overlies the chain chamber “C,” as seen in a direction lying generally parallel to the cylinder axes. The breather chamber 45 preferably has a generally rectangular shape and is located adjacent to the longer sides, closer to the cylinder axes, of the left and right valve chambers 41, 42. The bottom surface of the breather chamber 45 has breather gas introduction openings 45a, 45a formed in communication with the chain chamber “C.” Blow-by gas is thereby introduced into the breather chamber 45 through the chain chamber “C” from the crank chamber “d.”

The breather chamber 45 has a breather chamber opening 45b which allows the breather chamber 45 to access outside. The breather chamber opening 45b and the left and right valve chamber openings 41a, 42a preferably are provided with a single removable common cover 46 with bolts 49.

The cover 46 preferably is formed by integrally forming left and right valve chamber cover portions 46a, 46b, which have generally the same shape as that of seal faces of the left and right valve chamber openings 41a, 42a, and a breather chamber cover portion 46c, which has generally the same shape as that of a seal face of the breather chamber opening 45b. The seal faces of the left and right valve chamber openings 41a, 42a and the seal face of the breather chamber opening 45b are formed to be at least generally flush with one other.

The boundary area between the left and right valve chamber cover portions 46a, 46b is formed with a raised portion 46c protruding upward for placing the left and right valve chambers 41, 42 in communication with each other. The swell portion 46c is integrally formed with a tubular air inlet 46d. Also, the breather chamber cover portion 46c′ is formed with a tubular air exhaust portion 46e, generally at its central portion between the respective camshaft axial centers. As seen in FIG. 6, reference numeral 46e′ denotes a rib for contacting the left and right base plates 44b, 44b. The rib 46e′ is integrally formed with the cover 46. In this manner, the cover 46 can be used to affix the base plates 44b, 44b to the front cylinder head cover 6.

The air inlet 46d is connected to the downstream end of an air introduction hose 47 for common use between the left and right valve chambers 41, 42. The upstream end of the air introduction hose 47 is connected to the secondary side of an air cleaner (not shown). The air exhaust portion 46e is connected to the upstream end of an exhaust hose 48, and the downstream end of the exhaust hose 48 is connected to the secondary side of the air cleaner. The exhaust hose 48 and the air introduction hose 47 are routed to run the same path and bundled with each other.

In the present embodiment, the left valve chamber 41 and the right valve chamber 42 overlie the chain chamber “C,” at least at their portions that lie closer to the axially center portion of the crankshaft, and lie adjacent to each other above the chain chamber “C.” Thus, while the left cylinder bore 2e and the right cylinder bore 2f are located on opposite sides of the chain chamber “C,” an increase of the distance between the left and right valve chambers is avoided, so that the left and right valve chambers 41, 42 can adjoin each other. This allows the left and right valve chamber cover portions 46a, 46b to be integrally formed. Therefore, only a single secondary air hose 47 for introducing air to the left and right valve chambers 41, 42 is required, and the number of fastening bolts 49 can be reduced, so that the structure is simplified.

Further, the left and right valve chambers 41, 42 preferably have a rectangular shape extending longer in the direction along the camshafts. The reed valves 44 in the left and right valve chambers 41, 42 also preferably have a rectangular shape and are each located with its longer sides extending in the direction along the camshafts. Thus, when the left and right cylinder bores 2e, 2f are located on opposite sides of the chain chamber “C,” the left and right valve chambers 41, 42 can be located adjacent to each other without difficulty, and the capacity, area and space for the valve chambers 41, 42 and the reed valves 44 can be provided in a center-chain-type engine.

Further, the breather chamber 45 generally overlaps the chain chamber “C.” Thus, when the left and right cylinder bores 2e, 2f are located on opposite sides of the chain chamber “C,” the capacity and space for the breather chamber 45 can be obtained without difficulty by effectively using a ceiling wall of the chain chamber “C.”

Further, the breather chamber 45 lies adjacent to the left and right valve chambers 41, 42. Thus, the breather chamber cover portion 46c′ can be integrally formed with the left and right valve chamber cover portions 46a, 46b. Therefore, the mating faces (seal faces) of the openings 41a, 42a and 45b are flush with each other. Consequently, the number of fastening bolts 49 can be reduced and the structure can be simplified.

Further, the secondary air hose 47 for communicating the air cleaner and the left and right valve chambers 41, 42, and the breather hose 48 for communicating the air cleaner and the breather chamber 45 are bundled together. Thus, the routing structure of the hoses 47, 48 can be simplified, and hence the vicinity of the head covers can be simplified.

Although this invention has been disclosed in the context of a certain preferred embodiment and variations thereof, it will be understood by those skilled in the art that the present invention extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. In addition, while a number of variations of the invention have been described, other modifications, which are within the scope of this invention, will be readily apparent to those of skill in the art based upon this disclosure. It is also contemplated that various combinations or sub-combinations of the specific features and aspects of the embodiments may be made and still fall within the scope of the invention. Accordingly, it should be understood that various features and aspects of the disclosed embodiments can be combine with or substituted for one another in order to form varying modes of the disclosed invention. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims.

Claims

1. An engine comprises a cylinder block defining at least two adjacent cylinders, a cylinder head fastened on the cylinder block and supporting at least one camshaft, a head cover mounted on the cylinder head, a cam chain driving the camshaft, a chain chamber housing the cam chain and being formed generally between the adjacent cylinders, and a first-cylinder valve chamber and a second-cylinder valve chamber each being sized to accommodate a reed valve for supplying secondary air to an exhaust system of the engine, the first-cylinder and second-cylinder valve chambers being disposed in a manner such that at least portions of the first-cylinder valve chamber and the second-cylinder valve chamber overlie the chain chamber as viewed in the direction generally parallel to the axes of the cylinders, the first-cylinder and second-cylinder valve chambers also lying adjacent to each other to one side of the chain chamber and each having a valve chamber opening that opens to an exterior side of the engine, and a cover having a portion that closes the valve chamber openings of both the first-cylinder valve chamber and the second-cylinder valve chamber.

2. The engine of claim 1, wherein the first and second valve chambers each have a rectangular shape extending longer in the direction generally parallel to an axis of the camshaft, and each reed valve in the first and second valve chambers includes a rectangular, thin plate that is located with its longer sides extending in the direction generally parallel to the axis of the camshaft.

3. The engine of claim 1, wherein the head cover has a breather chamber for separating oil mist from blow-by gas, the breather chamber being disposed such that at least a portion of the breather chamber overlies the chain chamber as seen in the direction generally parallel to the axes of the cylinders, the breather chamber further lying adjacent to at least one of the first-cylinder and second-cylinder valve chambers.

4. The engine of claim 3, wherein the breather chamber has a breather chamber opening that opens to the exterior side of the engine, and the cover includes a portion that extends over the breather chamber opening, said portion being integrally formed with the portion of the cover that closes the first-cylinder and second-cylinder valve chamber openings.

5. The engine of claim 3, wherein a secondary air hose and a breather hose are bundled together, the secondary air hose connects an air cleaner to the first-cylinder and second-cylinder valve chambers, and the breather hose connects the breather chamber to the air cleaner.

6. The engine of claim 3 additionally comprising a second camshaft disposed between the cylinder head and the head cover, the camshafts drivingly engaging each other so as to rotate together.

7. The engine of claim 6, wherein the breather chamber is disposed generally to the side of a space between the camshafts.

8. The engine of claim 6, wherein one of the camshafts is a driving camshaft and the other is a driven camshaft, and wherein the valve chambers are disposed generally to an upper side of the driven camshaft.

9. The engine of claim 8, wherein the driven camshaft opens and closes exhaust valves supported by the cylinder head.

10. The engine of claim 1, wherein the reed valves are orientated so as to deflect in a direction generally parallel to the axes of the cylinders.

11. The engine of claim 1, wherein the cylinder head includes first and second passages, each passage connecting a side of the respective valve chamber, which lies downstream of the corresponding reed valve, with a corresponding exhaust port of the cylinder head.

12. The engine of claim 1, wherein each reed valve includes a base plate that is sandwiched between portions of the cover and the head cover.

13. The engine of claim 1, wherein each valve chamber houses an exhaust gas deflector plate disposed on a downstream side of the corresponding reed valve.

14. An engine comprising a cylinder block defining at least two adjacent cylinders, a cylinder head fastened to the cylinder block and supporting at least one camshaft, a head cover mounted to the cylinder head, a cam chain driving the camshaft, a chain chamber housing the cam chain and being formed generally between the adjacent cylinders, the chain chamber having a longitudinal axis that lies substantially parallel to axes of the adjacent cylinders, and a first-cylinder valve chamber and a second-cylinder valve chamber each being sized to accommodate a valve for supplying secondary air to an exhaust system of the engine, the first-cylinder and second-cylinder valve chambers being disposed in a manner such that at least portions of the first-cylinder valve chamber and the second-cylinder valve chamber lie to one side of the chain chamber as viewed in a direction along the longitudinal axis of the chain chamber.

15. The engine of claim 14, wherein the first-cylinder and second-cylinder valve chambers lying adjacent to each other and each has a valve chamber opening that opens to an exterior side of the engine

16. The engine of claim 15 additionally comprising a cover having a portion that closes the valve chamber openings of both the first-cylinder valve chamber and the second-cylinder valve chamber.

17. The engine of claim 16, wherein the head cover has a breather chamber for separating oil mist from blow-by gas, the breather chamber being disposed such that at least a portion of the breather chamber overlies the chain chamber as seen in the direction generally parallel to the axes of the cylinders, the breather chamber further lying adjacent to at least one of the first-cylinder and second-cylinder valve chambers.

18. The engine of claim 17, wherein the breather chamber has a breather chamber opening that opens to the exterior side of the engine, and the cover includes a portion that extends over the breather chamber opening, which is integrally formed with the portion of the cover that closes the first-cylinder and second-cylinder valve chamber openings.

19. The engine of claim 17, wherein a secondary air hose and a breather hose are bundled together, the secondary air hose connecting an air cleaner to the first-cylinder and second-cylinder valve chambers, and the breather hose connecting the breather chamber to the air cleaner.

20. The engine of claim 17 additionally comprising a second camshaft disposed between the cylinder head and the head cover, one of the camshafts being an intake camshaft actuating at least one intake valve supported by the cylinder head and the other camshaft being an exhaust camshaft actuating at least one exhaust valve supported by the camshafts, and wherein the breather chamber is disposed generally to a side of a space between the camshafts and the valve chambers are disposed generally to a side of the exhaust camshaft.