Abstract:
An exhaust manifold for attaching to exhaust ports of an engine, the manifold having a plenum and a plurality of exhaust runners each associated with a respective exhaust port, the plenum having a progressively tapering cross sectional area along its length as defined by the direction along which successive exhaust runners connect to the plenum and a connector at its larger downstream end, for connecting to further components of the exhaust system, characterized in that the lengths of the gas flow paths in all the exhaust runners, as measured from the end of each runner adjacent the exhaust port to an interior wall of the plenum opposite the other end of the runner, are substantially equal to one another.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to an exhaust manifold for connecting exhaust ports of an internal combustion engine to an exhaust system. 
       BACKGROUND 
       [0002]    Recent environmental concerns have led to ever more stringent regulations requiring motor manufacturers to seek out methods of reducing vehicle emissions. The most obvious of these is to downsize engine capacity, but the reduction in output power this leads to has been rejected by the motorist. To provide the best of both worlds, a typical solution is to utilize turbocharging. 
         [0003]    While the addition of a turbo alone can yield environmental and performance benefits, optimizing its installation will yield further improvements in the volumetric efficiency of the engine. Notably, especially in high performance vehicles, as the exhaust flow is used to drive the turbine of a turbocharger, it is important to balance the resistance to flow of the exhaust gases from different engine cylinders. Balancing can be adjusted by altering the design of the exhaust manifold which is used to contain exhaust gases exiting the cylinder head. 
         [0004]    In one design of manifold, the runners or branches of the manifold leading from the different exhaust ports are separated until they are all joined together at an exhaust collector immediately before flowing into the downstream components of the exhaust system, such as a turbocharger. To achieve the desired matching of the resistance to flow in the individual branches, they are typically made of the same length and diameter pipework, bent differently to accommodate their equal lengths into the available space. In such a design, the convoluted pipes are very bulky and may present difficult to package within the confines of a crowded modern engine bay. Such manifolds are referred to as equal length tubular manifolds and are typically associated with high performance engines due to the time and cost involved in production and the perceived performance advantages. In “V” configuration engines, there may be two such tubular manifolds, one associated with each bank of cylinders. 
         [0005]    Typically, mass produced cars, whether normally aspirated or turbo charged utilize a log manifold. This is a cast steel manifold having a single chamber or plenum chamber in fluid communication with several runners each connected to an exhaust port of a cylinder of an engine. The exhaust stream from each cylinder flows along the runner and into the plenum chamber mixing with streams from other cylinders and eventually exiting the plenum chamber into a further exhaust component. The significant difference between a log and an equal length manifold is that the exhaust streams from each cylinder add together into a single exhaust stream at different points along the plenum chamber before exiting into the rest of the exhaust system. In an equal length manifold, each stream is separated with a predetermined volume of pipework through which the exhaust may flow allowing it to remain laminar and uninterrupted by the turbulent feed from successive streams joining. The different streams are then smooth joined to one another at a collector intended to introduce them smoothly to one another into a larger volume pipe capable of maintaining the properties of the exhaust gas stream that are engineered into its design. This may be the retention of heat (energy) in order to promote turbine spool up or accelerate catalytic converter light-off, or simply to reduce noise. 
         [0006]    The advantages of an equal length manifold are additional power which may be tuned to deliver increased gas scavenging ability at a specific engine speed, typically this is chosen to increase top end power but may come at the expense of power at lower engine speeds. Even without performance tuning, the equal length tubular manifold tend to result in lower back pressure which improves the volumetric efficiency of the engine resulting in greater efficiency or if desired, power. 
         [0007]    The log manifold by contrast is intended to simply join the exhaust streams up within the minimum of wasted space time, money and complexity by simply joining the streams together and feeding them to the next component in the exhaust chain. That component will vary depending on the design and configuration of the engine, for example it may proceed to an up-pipe through a catalytic converter and then on to a turbocharger, or directly into a catalytic converter then turbo and then downpipe but the construction of the remainder of the exhaust is not relevant to the invention. 
         [0008]    The advantages of a log manifold are much lower cost and speed of mass production, greater sound deadening capability due to greater mass of metal, better retention of heat within the exhaust gases for the same reason, increased longevity due to no welded joins between pipes that may be prone to cracking, but most importantly, simpler design meaning much smaller and more easily fitted within tight engine bays. In practice, when compared with equal length tuned tubular manifolds, cast log manifolds have been shown to provide good exhaust gas scavenging at lower engine speeds. This is even more beneficial in turbocharged engine since the more energetic gas stream at low engine speeds aids in the earlier spool of the turbo. This further compounds the low engine speed performance. 
         [0009]    It would be advantageous to be able to combine the benefits of both types of manifold into one. 
       SUMMARY 
       [0010]    With a view meeting the foregoing aim, the present invention provides an engine as set forth below. 
         [0011]    Log manifolds exist which taper along their length to provide additional space for the additional gases as they are introduced. Such manifolds are typically conical in design and have not been designed to maximize efficiency of a turbo charger. As the space available between the engine block and the adjacent wall of the engine compartment is often relatively narrow, the uniform width of the manifold is dimensioned to fit within this space. The dimensions of the manifold are however less critical in a transverse plane and in the invention the plenum chamber is designed to taper only in this transverse plane to match the resistance to exhaust flow experienced by the different engine cylinders as each additional cylinder joins the plenum chamber. 
         [0012]    The preferred embodiment provides the benefits of a successively increasing cross sectional area in the same way as a conical log manifold, but introduces that additional area, by increasing the dimensions of the manifold in one dimension, leaving another substantially perpendicular dimension to be constant or constrained by the dimensions of the engine bay. 
         [0013]    In this way, the tapered log manifold can deliver most of the performance benefits of an equal length manifold whilst still being relatively inexpensive to make quickly and in large volume, resistant to cracking since it remains cast, but crucially, still being small enough to fit in a tightly packaged engine bay. 
         [0014]    Perhaps more important than the packaging of the manifold are the fluid dynamic considerations of its design, notably the handling of pulses. Pulses are the individual pressure waves of exhaust gases created at each exhaust port resulting from each cylinder&#39;s exhaust stroke. Due to the requirement for balance of the engine, different cylinders fire at different angles within the revolution of the crank shaft. In a four stroke engine, the crank shaft must rotate through 720 degrees for all four strokes of each cylinder to be completed. To provide the smoothest possible rotation of the engine total number of cylinders fire at evenly spaced angular intervals across the entire 720 degrees. This is known as the firing order of the engine. 
         [0015]    For a given cylinder following each combustion stroke, there is an exhaust stroke as the exhaust valve opens and the rising piston forces out the exhaust gases. As the piston pushes the exhaust gases out it creates a pressure wave or pulse that travels out of the exhaust port, through the respective exhaust runner and into the plenum of the of the exhaust manifold. 
         [0016]    Performance tuning of the exhaust manifold design ensures that pulses are evenly spaced, time wise, in the combined exhaust stream from all the cylinders that flow into a single plenum. This is even more important in engines having a turbo charger as evenly spaced pulses ensure efficient, consistent and smooth rotation of the turbine wheel. 
         [0017]    When designing the plenum, or the main chamber of the exhaust manifold, engineers have to pay specific attention to the combination wave. The combination wave is created when components of the pulses from the exhaust ports combine with pulses generated upstream by other cylinders. The shape of the combination wave is a function of the geometry of the exhaust ports, the runners and plenum of the exhaust manifold. 
         [0018]    For better understanding of the concept of the combination wave, reference can be made to “Introduction to Internal Combustion Engines (4th Edition) by Richard Stone”. 
         [0019]    The principal reflection of the combination wave is created when the pulses from an exhaust port bounce off the wall immediately opposite the exhaust port, or normal to the axis of the exhaust port. 
         [0020]    In existing tapering design manifolds, such as WO02073010, US2010/0018192, FR2727466, EP0731258 and DE102009037505, the wall immediately opposing the exhaust ports is inclined at an oblique angle to the exhaust port gas flow because of the tapering design of the plenum. Theoretically this is expected to promote exhaust flow and deflect pulses downstream into the plenum, towards the collector (and possibly turbine wheel) and therefore away from each exhaust port. 
         [0021]    The inventors of the present invention have found that contrary to the theory of the design, of conventional tapering area manifolds, the reflected combination wave is asymmetrical which leads to unequal reflected pulses travelling back up the exhaust runners and into the exhaust ports. While this is of little consequence in a single exhaust valve engine, modern engines tend to have at least two exhaust valves per cylinder (arranged adjacent one another in a line parallel to the major axis of the plenum), and the asymmetrical reflected combination wave leads to an unequal scavenging of exhaust gases from subsequent exhaust strokes. 
         [0022]    Inefficient exhaust scavenging results in lower volumetric efficiency of the engine and therefore ultimately compromised power and or fuel consumption. 
         [0023]    Since the control parameters for the engine of injection and ignition timing are constant for both valves of the cylinder, there is no simple method of computerized control that can counteract this negative effect. 
         [0024]    The present solution to this is to revise the design of the tapered cross sectional manifold such that the main reflecting wall (opposite the exhaust ports) is perpendicular to the primary direction of exhaust pulses through the exhaust runners. This is commonplace in conventional log manifolds. Unfortunately these do not provide the advantages of a cross sectional area that increases downstream as respective exhaust ports join the manifold. 
         [0025]    To solve both problems, concurrently the cross sectional area of the manifold increases by increasing the “vertical” height of the plenum in the downstream direction whilst limiting the width so as to produce a cross section that becomes increasingly more oval along the length of the manifold plenum. 
         [0026]    Another option is to maintain the reflecting wall perpendicular to the exhaust port gas flow direction but to increase the vertical height in steps as each associated exhaust runner joins the plenum. This is less desirable as the stepped increase in area itself creates pulse reflections and reduces laminar flow, potentially reducing the flow performance of the manifold. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The invention will now be described further by way of example with reference to the accompanying drawings in which: 
           [0028]      FIG. 1  is a block diagram representation of a front end view of an engine bay showing a V-opposed engine; 
           [0029]      FIG. 2  is a side view of the engine of  FIG. 1  including a prior art manifold, attached to one of the cylinder head banks of the V-opposed engine; 
           [0030]      FIG. 3  is a front end view similar to that of  FIG. 1 , showing the engine and prior art manifold of  FIG. 2  attached to one of the cylinder head banks, installed within the engine bay; 
           [0031]      FIG. 4  is a block diagram representation of a front end view of an engine bay showing an inline engine; 
           [0032]      FIG. 5  is a top view of the engine of  FIG. 4  connected to a manifold according to the present invention; and 
           [0033]      FIG. 6  is a side view of the engine and manifold of  FIG. 5  as seen looking through a wall of the engine compartment. 
       
    
    
       [0034]    The various figures described below relate to different embodiments of the invention relating to different engine configurations. Common reference numerals are used for common components throughout. 
       DETAILED DESCRIPTION 
       [0035]      FIG. 1 , shows an engine  10  located within an engine compartment  12 . The engine compartment here is represented by four bounding walls forming the outer square of the diagram. In practice, there will most likely be no lower wall on which the engine sits, more likely a sub-frame allowing access to the engine  10  from underneath, but for the purposes of describing this invention, such detail is not necessary. The side walls of the engine compartment  12 , are more representative of what is actually provided within a vehicle engine bay. These are typically in the form of wheel arches or bulkhead walls, and form a large flat typically metal surface bounding the engine between typically 3 vertical walls—the front space being open to receive airflow to heat exchangers which are typically mounted there. 
         [0036]    The view shown in  FIG. 1  is a front end view and would therefore be looking through the engine&#39;s radiator at the engine  10 . The engine is formed from a crank case  14  and due to the V-configuration of this example, two cylinder heads  16  arranged to form a V with the crank shaft axis (not shown). 
         [0037]    Conventional V-configuration engines include an inlet manifold arranged between the cylinder heads above the engine block  14  to allow air into the intake ports (not shown). After combustion has occurred the exhaust gases exit the cylinder heads  16  through the exhaust ports. 
         [0038]    The multiple exhaust ports, corresponding to each cylinder are joined together by a manifold  20  (not shown in  FIG. 1 , see  FIGS. 2 and 3 ) that provides a single flow path through connector  22  to the exhaust system. 
         [0039]    The figures utilize arrows to indicate the dimension of the manifold which tapers with respect to the orientation of the engine. Arrows labelled  1  show the dimension of the manifold which is approximately constant, whereas arrows labelled  2  show the dimension of the manifold that tapers. 
         [0040]    As seen in the prior art reference of  FIG. 2  which shows the right most cylinder head  16  of  FIG. 1  viewed from the side, as if looking through the boundary wall of the engine compartment  12 , the manifold  20  is formed from a plenum chamber connected to three branch, exhaust runner or header pipes  18 . Each of these is connected to a respective exhaust port opening in the side face of the cylinder head  16 . In terms of the exhaust gas flow of the manifold, the left most branch pipe  18  in  FIG. 2  joins the plenum chamber of the manifold first. The gases flow along the plenum of the manifold (to the right in the diagram) until the next runner  18  from the middle exhaust port of the cylinder head  16  connects to the plenum chamber. The exhaust gas from both these exhaust runners  18  then continues to flow through the plenum until the right most branch pipe  18  connects to the gas stream inside the plenum. All three combined exhaust gas streams then flow out of the connector  22  (sometimes called a collector). 
         [0041]    The tapering manifold clearly shown in  FIGS. 2 and 3 , reflect the designs of the manifolds disclosed in the prior art references cited in the introduction to this patent application. This is particularly visible in  FIG. 2 , which shows the shortest path length of the exhaust flow from the left most exhaust runner  18  before hitting the opposing wall of the manifold  20  at an oblique angle. The path length of the middle runner  18  is longer, and the right most runner, adjacent the collector  22 , longer still. The disadvantages of this flow path geometry have already been described in detail above. 
         [0042]      FIG. 3  is intended to show this by viewing the engine block and manifold  20  from the front of the engine or left most end of the manifold  20  shown in  FIG. 2 . In this diagram, the front most face of the manifold  20  is circular and obscures some of the cross sectional shape of the manifold  20  as it becomes progressively more oval as the further two (in  FIG. 2 , corresponding to the middle and right most) branch pipes  18  connect into the plenum chamber. 
         [0043]    The gases then travel along a combined exhaust path to a further exhaust system component. This may be a catalytic converter, a turbine wheel of a turbocharger or directly to a silencer of the exhaust system. In engine configurations having two or more cylinder heads  16 , the exhaust streams from multiple manifolds (each one corresponding with a cylinder head) may be joined together and then exit the exhaust or collectively drive a single turbocharger. 
         [0044]    Typical log manifolds have a plenum of constant cross sectional flow area. This is due to ease of design and manufacture and historical lack of requirement to optimize the flow therethrough. CFD or computational fluid dynamics teaches us that as additional exhaust gas is fed into a common or log manifold due to the successive additional volume of gases, it is preferable to provide a plenum capable of accommodating an increasing volume of gas. For this reason, tapered manifolds exist which grow in internal cross sectional flow area as successive branch pipes join into the plenum chamber of the manifold  20 . 
         [0045]    The internal flow path may smoothly increase in cross sectional area or may step up at the junction of each successive exhaust runner  18 . 
         [0046]    The present invention recognizes that while it is known to taper the design of the manifold to produce this flow efficiency benefit, there are particular advantages which may be empirically demonstrated which result from the choice of which dimension of the manifold is tapered. The resulting invention is shown in  FIGS. 4 to 7 . 
         [0047]    For reasons of flow efficiency, the shape of the internal flow path of a manifold is typically round. In the preferred embodiment, the internal flow path would start off substantially round but become progressively more oval with the major axis of the oval increasing in length as the minor axis, or diameter of the original circle remained constant. This is the same as the above mentioned prior art references but for the orientation of the increasing axis relative to the gas stream through the exhaust runners  18 . 
         [0048]      FIG. 4  shows a similar view to  FIG. 1  but utilizing an engine having an inline configuration, in this case with six cylinders and therefore six exhaust ports attached to one cylinder head  16 . 
         [0049]    The available space for the manifold  20  is constrained by the distance from the cylinder head  16  to the side wall of the engine compartment  12  (right hand side of the square shown in  FIG. 4 ). The “constant” dimension of the manifold is again labelled by an arrow  1  but note in the invention shown in  FIG. 4 , the arrow  1  points directly towards the exhaust ports rather than normal to as shown in the prior art depiction for  FIGS. 1 to 3 . 
         [0050]      FIG. 5  shows the manifold  20  of the present invention when viewed from above. The line at the bottom of the diagram represents a dimension restricting side wall of the engine compartment. In this view it is clear that the path lengths of the exhaust gases along all the runners  18  to the opposing wall of the plenum, is constant in the direction of arrow  1 . 
         [0051]      FIG. 6 , which is a side view similar to  FIG. 2 , shows that in this depiction of the invention, the collector for allowing the exit of exhaust gases is located in the center of the manifold  20  and so the manifold tapers in two directions from its axial ends corresponding to the first and sixth cylinders towards its widest point in the middle of the manifold between cylinders  3  and  4 . In the side view of  FIG. 6 , the manifold  20  obscures the runners  18  (depicted in dotted lines). This aids in showing that tapering dimension of the manifold is orthogonal to the flow of exhaust gas through the runners  18 . The “constant” dimension of arrow  1  cannot easily be depicted here as it points normal to the plane of the page. 
         [0052]    In the example of the present invention the central collector  22  exits from underneath the center of manifold again directing the flow of exhaust gases toward either a turbo, or the remainder of the exhaust system. Here along the axis of the manifold, in simplified terms, the internal cross sectional shape would start as a circle, around cylinder  1 , then stretch into an oval increasing is length until a largest area point between cylinders  3  and  4 , then contract again towards a circle in line with cylinder  6 , at all times the minor axis of the oval (corresponding to the diameter of the initial circle) being substantially constant. The example of a central collector is not essential to the invention. It is equally possible for the overall design to be similar to that of the prior art example except for the dimensions in which the manifold tapers, as defined by the present invention.