Abstract:
An internal combustion engine and a method of operation are described involving: supplying intake gas from an intake manifold ( 18 ) to an air intake port of a combustion chamber in a cylinder ( 11 ); selectively operating an air intake valve using a first operating profile ( 30 ) and, on demand, switching selective operation of the air intake valve to a second operating profile ( 31 ) in which the closing of the air intake valve is delayed compared to the closing of the air intake valve using the first operating profile; wherein at the point ( 43 ) of or prior to switching from the first operating profile to the second operating profile the pressure of the intake gas in the intake manifold is increased.

Description:
FIELD 
     The present disclosure relates to an internal combustion engine and a method of operating an internal combustion engine. 
     BACKGROUND 
     Internal combustion engines comprise inlet and exhaust valves to control the flow of gases into and out of the combustion chamber of each engine cylinder. Ordinarily the valves are mechanically controlled by means of a camshaft. Profiled cams on the camshaft are used to control timing of opening and closing of each valve. 
     Since the physical shape and profile of the cams may only be optimised for one particular operating condition of the internal combustion engine, it is known to use a variable valve actuation system where the operation of the valves may by adjusted to suit changing demand. For example, a standard profile may be utilised during periods of medium or high engine demand and a late inlet valve closing (LIVC) profile may be utilised for the inlet valve during steady state conditions when the demand on the internal combustion engine is relatively low. In a LIVC profile the closing of the intake valve at about the end of the intake stroke is delayed, so that the intake valve remains open for a portion of the compression stroke. This results in a lower pressure within the cylinder. Consequently, the cylinder piston does less work during the compression stroke which leads to improved fuel efficiency. 
     A problem with use of an LIVC profile is that when instantaneously switching from the standard profile to the LIVC profile there is a sudden drop in the air-to-fuel ratio within the cylinder. This leads to a rich mixture which tends to produce unwanted soot particulates and smoke. In order to attempt to overcome this problem it is known to employ a control system that gradually switches from the standard profile to the LIVC profile over relatively large number of engine cycles—typically around 20 cycles. However such control systems are complicated and expensive. 
     DISCLOSURE 
     According to the present disclosure there is provided a method of operating an internal combustion engine comprising at least one cylinder, the method comprising: 
     supplying intake gas from an intake manifold to an intake port of a combustion chamber in the cylinder; 
     selectively operating an intake valve using a first operating profile to open and close the intake port to control flow of the intake gas between the intake manifold and the combustion chamber; 
     on demand, switching selective operation of the intake valve to a second operating profile in which the closing of the intake valve is delayed compared to the closing of the intake valve using the first operating profile; 
     wherein at the point, or prior to the point of switching from the first operating profile to the second operating profile the pressure of the intake gas in the intake manifold is increased. 
     There is also provided an internal combustion engine comprising: 
     at least one cylinder; 
     an intake manifold for receiving intake gas; 
     an intake port communicating between the intake manifold and a combustion chamber of the cylinder; 
     an intake valve movable to open and close the intake port to control flow of the intake gas between the intake manifold and the combustion chamber; 
     a turbocharger or supercharger for pressurising at least a portion of the intake gas supplied to the intake manifold; and 
     a controller configured selectively to control operation of the intake valve between a first operating profile and a second operating profile in which the closing of the intake valve is delayed compared to the closing of the intake valve using the first operating profile; 
     the controller being further configured to increase the pressure of the intake gas in the inlet manifold at the point, or prior to the point of switching from the first operating profile to the second operating profile by directly of indirectly controlling operation of the turbocharger or supercharger. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an internal combustion engine according to the present disclosure; 
         FIG. 2  is a diagram of intake valve lift versus crank angle; 
         FIG. 3   a  is a diagram of inlet manifold air pressure versus time; 
         FIG. 3   b  is a diagram of air-to-fuel ratio versus time; 
         FIG. 3   c  is a diagram of wastegate area versus time; 
         FIG. 3   d  is a diagram of exhaust gas recirculation (EGR) equivalence ratio versus time; 
         FIG. 3   e  is a diagram of EGR valve position versus time; 
         FIG. 3   f  is a diagram of smoke level versus time; and 
         FIG. 4  is a schematic engine map plotting engine torque versus engine speed. 
     
    
    
     DETAILED DESCRIPTION 
     The internal combustion engine of the present disclosure, as shown in  FIG. 1 , may comprise one or more cylinders  11  in an engine block  10 . For example, the engine may contain four or six cylinders  11 . An intake manifold  18  may supply intake gas to each cylinder  11 . The intake gas may be a mixture of air and exhaust gases that are recirculated by an exhaust gas recirculation (EGR) system  12 . The EGR system  12  may comprise a cooler for cooling the recirculated exhaust gases. An EGR valve  13  may be provided operatively to control the amount of exhaust gas recirculated to the intake manifold  18 . 
     Non-recirculated exhaust gases may be conveyed to an exhaust line  16  via a turbine section of a turbocharger  14 . The exhaust gases may cause the turbine section to rotate thus rotating the compressor section of the turbocharger  14 . The compressor section may be configured to pressurise a flow of air supplied to the intake manifold  18  from an air inlet  17 . Ambient air may enter air inlet  17 . 
     A wastegate  15  may be provided to allow exhaust gases to bypass the turbocharger  14 . 
     Each cylinder  11  may contain a piston slidably movable in the cylinder  11 . A crankshaft may be rotatably disposed within the engine. A connecting rod may couple the piston to the crankshaft so that sliding motion of the piston within the cylinder  11  results in rotation of the crankshaft. Similarly, rotation of the crankshaft results in a sliding motion of the piston. For example, an uppermost position of the piston in the cylinder  11  corresponds to a top dead centre position of the crankshaft, and a lowermost position of the piston in the cylinder  11  corresponds to a bottom dead centre position of the crankshaft. 
     As one skilled in the art will recognize, the piston in a conventional, four-stroke engine cycle reciprocates between the uppermost position and the lowermost position during a combustion (or expansion) stroke, an exhaust stroke, an intake stroke, and a compression stroke. Meanwhile, the crankshaft rotates from the top dead centre position to the bottom dead centre position during the combustion stroke, from the bottom dead centre to the top dead centre during the exhaust stroke, from top dead centre to bottom dead centre during the intake stroke, and from bottom dead centre to top dead centre during the compression stroke. Then, the four-stroke cycle begins again. Each piston stroke correlates to about 180° of crankshaft rotation, or crank angle. Thus, the combustion stroke may begin at about 0° crank angle, the exhaust stroke at about 180°, the intake stroke at about 360°, and the compression stroke at about 540°. 
     The cylinder  11  may include at least one intake port and at least one exhaust port, each opening to a combustion chamber within the cylinder  11 . The intake port may be opened and closed by an intake valve, and the exhaust port may be opened and closed by an exhaust valve. The intake valve may be movable between a first, open position in which flow of gas from an intake manifold  18  is permitted to enter the combustion chamber and a second, closed position which substantially blocks flow from the intake manifold  18  into the combustion chamber. The intake valve may be sprung-biased to the second, closed position. 
     A camshaft carrying a cam with one or more lobes may be arranged to operate the intake valve cyclically based on the configuration of the cam, the lobes, and the rotation of the camshaft to achieve a desired intake valve timing. The exhaust valve may be configured in a manner similar to the intake valve and may be operated by one of the lobes of the cam. Alternatively, the intake valve and/or the exhaust valve may be operated hydraulically, pneumatically, electronically, or by any combination of mechanics, hydraulics, pneumatics, and/or electronics. 
     In a first, standard, operating profile of the intake valve, the cam profile may cause the intake valve to open at about the start of the intake stroke (about 360° crank angle) and to close at about the start of the compression stroke or shortly thereafter (about 540° crank angle or shortly thereafter). This first operating profile is shown by the solid line  30  in  FIG. 2 . The first operating profile may be most suitable for medium to heavy engine loading conditions. 
     The intake valve may include a variable valve actuation system comprising an intake valve closing mechanism structured and arranged selectively to interrupt cyclical movement of and extend the closing timing of the intake valve to provide a second operating profile for the intake valve. For example, closure of the intake valve may be delayed by about 60° crank angle compared to the first operating profile. This second operating profile is shown by the dashed line  31  in  FIG. 2  and represents a LIVC profile. The second operating profile may be most suitable for steady-state and/or low engine loading conditions. 
     The intake valve closing mechanism may be operated hydraulically, pneumatically, electronically, mechanically, or any combination thereof. For example, the intake valve closing mechanism may be selectively operated to supply hydraulic fluid, for example, at a low pressure or a high pressure, in a manner to resist closing of the intake valve by spring-bias. That is, after the intake valve is lifted, i.e., opened, by the cam, and when the cam is no longer holding the intake valve open, the hydraulic fluid may hold the intake valve open for a desired period. The desired period may change depending on the desired performance of the engine. 
     A controller  19  may be provided for controlling operation of the internal combustion engine. The controller  19  may be operatively connected to the EGR valve  13 , the wastegate  15  and the intake valve closing mechanism of each cylinder  11 . 
     The controller  19  may also be operatively connected to one or more sensors  20  which may provide the controller  19  with indications of one or more engine conditions or other data from which the loading of the internal combustion engine can be determined. The sensors  20  may include sensors detecting engine speed, engine torque, or detecting the work state of a vehicle in which the internal combustion engine is incorporated. For example, sensors detecting heavy digging, hill climbing or fast digging may be utilised. 
     The operation of the intake valve may be switched from the first operating profile  30  to the second operating profile  31  instantaneously. By ‘instantaneously’ is meant that the switching of operation from the first operating profile to the second operating profile is not phased gradually over many engine cycles. Rather, switching may take place between one engine cycle and the next engine cycle, for example within one revolution of a cam shaft of the internal combustion engine. 
     In order to reduce or avoid a drop in the air-to-fuel ratio within the cylinder  11  when switching from the first operating profile to the second operating profiles, the pressure of the intake gas in the intake manifold  18  may be increased at the point, or preferably, prior to switching. This may be achieved by temporarily closing the wastegate  15  to avoid exhaust gases bypassing the turbocharger  14 . This results in an increased mass flow of gas through the turbocharger  14  resulting in an increased boost level of the pressurisation of the inlet air fed to the inlet manifold  18 . 
     Movement of the wastegate may be relatively slow. Therefore, in addition or instead of closing the wastegate  15 , the EGR valve  13  may be partially or fully closed prior to switching from the first operating profile  30  to the second operating profile  31  more quickly to increase the quantity of exhaust gas fed to the turbocharger  14 . Movement of the EGR valve  13  may be relatively fast compared to movement of the wastegate  15 . Closing the EGR valve  13  may also have the effect of increasing the proportion of fresh air entering the cylinders  11 . 
       FIG. 4  is a schematic engine map plotting engine torque versus engine speed of the type which may be suitably incorporated into the programming of the controller  19  to control switching between the first operating profile  30  and the second operating profile  31 . Region  50  signifies a steady state operating zone where the first operating profile  30  for the intake valve is used. Region  51  signifies a steady state operating zone where the second operating profile  31  for the intake valve is used so as to delay closing of the intake valve. The regions between boundaries  55  and  56  represent a transitional zone, the effect of which will be described below. 
     A non-limiting example of switching from the first operating profile  30  to the second operating profile  31  will now be described. The engine condition may start in region  50 . Under changing engine conditions boundary  56  may first be encountered. At this point the wastegate  15  and/or EGR valve  13  may be closed. As the engine conditions cross a switching boundary  52  the intake valve profile may be instantaneously switched to the second operating profile  31 . As the engine conditions cross a debounce boundary  53  the EGR valve  13  may be returned to its prior, open position. However, the wastegate  15  may remain closed. The region between switching boundary  52  and debounce boundary  53  may be used for hysteresis control of the switching process. 
     A non-limiting example of switching from the second operating profile  31  to the first operating profile  30  will now be described where the engine may be under high speed, high load conditions such that the engine components may be operating near their design limits. The engine condition may start in region  51 . Under changing engine conditions, for example during a transient loading event, boundary  55  may first be encountered. At this point the wastegate  15  may start to be opened. As the engine conditions cross the switching boundary  52  the intake valve profile may be instantaneously switched to the first operating profile  30  while at the same time the EGR valve  13  may be briefly opened (if not already open) or further opened which may help to reduce any temporary increase in the speed of the turbocharger  14  and temporary increase in the pressure in the intake manifold  18  which might be detrimental to engine components operating near their design limits. As the engine conditions cross a debounce boundary  54  the wastegate  15  may be opened and control of the EGR valve  13  may return to the steady state control regime for region  50 . The region between switching boundary  52  and debounce boundary  54  may be used for hysteresis control of the switching process. 
     The beneficial effect of boosting the pressure in the intake manifold  18  prior to switching from the first operating profile  30  to the second operating profile  31  may be seen in  FIG. 3 . In  FIG. 3  the point in time of switching from the first operating profile  30  to the second operating profile  31  is depicted by line  43 . In each of  FIGS. 3   a  to  3   f , line  40  indicates the effect of instantaneous switching from the first to the second operating profile without changing the position of the wastegate  15  or EGR valve  13 . Line  41  indicates the effect of closing the EGR valve  13  and wastegate  15  at the point of switching  43 . Line  42  indicates the effect of closing the EGR valve  13  and wastegate  15  prior to the point of switching  43 . 
       FIG. 3   c  shows the wastegate area and illustrates that the wastegate area may be reduced prior to switching for line  42 . 
       FIG. 3   e  shows the position of the EGR valve  13  and illustrates that the valve may be closed or partially closed prior to the point of switching  43  before being reopened following switching. 
       FIG. 3   d  shows the effect of the EGR valve position on the EGR equivalence ratio and illustrates that the ratio decreases before the point of switching  43  before recovering as the EGR valve  13  is reopened. 
       FIG. 3   a  shows the effect of altering the operation of the wastegate  15  and EGR valve  13  as described above. In particular the graph illustrates that the pressure in the intake manifold  18  increases prior to the point of switching  43  for line  42 . 
       FIGS. 3   b  and  3   f  show the effect of the increased intake manifold pressure on the air-to-fuel ratio (AFR) and smoke levels respectively. It can be seen that the AFR may undergo a much-reduced dip after the point of switching  43  for line  42  where the intake manifold pressure is increased prior to switching. As shown in  FIG. 3   f  this may have the effect of significantly reducing smoke and soot levels—which may be determined using an AVL meter, for example. 
     It can therefore be seen that whilst closing the EGR valve  13  and wastegate  15  at the point of switching  43  (line  41 ) may have some beneficial effect, the most beneficial effect may be achieved by closing the EGR valve  13  and wastegate  15  prior to the point of switching  43  (line  42 ). 
     INDUSTRIAL APPLICABILITY 
     The present disclosure finds application in the design and operation of internal combustion engines and leads to improvements in the control of air-to-fuel ratios when utilising variable valve actuation systems. 
     REFERENCE NUMERALS 
     
         
           10  Engine block 
           11  Cylinder 
           12  EGR system 
           13  EGR valve 
           14  Turbocharger 
           15  Wastegate 
           16  Exhaust line 
           17  Air inlet 
           18  Inlet manifold 
           19  Controller 
           20  Sensor(s) 
           30  First operating profile 
           31  Second operating profile 
           40  Switching from the first to the second operating profile without changing the position of the wastegate  15  or EGR valve  13   
           41  Switching from the first to the second operating profile and closing the EGR valve  13  and wastegate  15  at the point of switching 
           42  Switching from the first to the second operating profile and closing the EGR valve  13  and wastegate  15  prior to the point of switching 
           43  Point of switching 
           50  Steady state region for first operating profile 
           51  Steady state region for second operating profile 
           52  Switching boundary 
           53  Debounce boundary 
           54  Debounce boundary 
           55  Transitional zone boundary 
           56  Transitional zone boundary