CROSS-REFERENCE 
     The present application is a divisional application of U.S. patent application Ser. No. 12/690,545, filed Jan. 20, 2010, which claims priority to U.S. Provisional Patent Application No. 61/145,876, filed Jan. 20, 2009, the entirety of both of which is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to an air spring system for an internal combustion engine. 
     BACKGROUND OF THE INVENTION 
     Many internal combustion engines, such as engines operating on the four-stroke principle, have intake and exhaust valves provided in the cylinder head of the engine. The intake valves open and close to selectively communicate the air intake passages of the engine with the combustion chambers of the engine. The exhaust valves open and close to selectively communicate the exhaust passages of the engine with the combustion chambers of the engine. 
     To open the valves, many engines are provided with one or more camshafts having one or more cams provided thereon. The rotation of the camshaft(s) causes the cam(s) to move the valves to an opened position. Metallic coil springs are usually provided to bias the valves toward a closed position. 
     Although metallic coil springs effectively bias the valves toward their closed positions for most engine operating conditions, at high engine speeds, the metallic coil springs have a tendency to resonate. When resonating, the metallic coil springs cause the valves to vacillate between their opened and closed positions, which, as would be understood, causes the intake and exhaust passages inside which the valves are connected to be opened when they should be closed. This results in a reduction of operating efficiency of the engine at high engine speeds. 
     One solution to this problem consists in replacing the metallic coil springs with air springs. An air spring typically consists of a cylinder having a piston therein. An air chamber is defined between the cylinder and the piston. The valve (intake or exhaust) is connected to the piston of the air spring. When the cam moves the valve to its opened position, the piston of the air spring moves with the valve, thus reducing the volume of the air chamber and as a result increasing the air pressure therein. When the cam no longer pushes down on the valve, the air pressure inside the air chamber causes the piston of the air spring to return to its initial position and to return the valve to its closed position. 
     Air springs do not resonate at high engine speeds the way metallic coil springs do. Also, for equivalent spring forces, air springs are lighter than metallic coil springs. Furthermore, air springs have progressive spring rates, which means that the spring force of an air spring varies depending on the position of the piston inside the cylinder of the air spring, which may also be advantageous for certain engines. 
     Although air springs offer many advantages over metallic coil springs, they also have some deficiencies that need to be addressed. 
     One of these deficiencies is that during operation, some of the air inside the air chamber of the air spring blows by the piston as the piston moves to reduce the volume of the air chamber. As a result, the air pressure inside the air spring is reduced, thus reducing the spring force of the air spring. This results in the valve not returning to its closed position as fast as it should, thus reducing the efficiency of the engine. In extreme cases, it is possible that the air pressure inside the air spring is insufficient to return the valve to its closed position. Since the valve remains in its opened position, the engine no longer operates properly, and the piston of the engine can come into contact with the valve, potentially damaging the valve. 
     One solution consists in providing a reservoir of pressurized air in fluid communication with the air springs that replenishes the air inside the air springs as it leaks out of the air springs. However, the pressurized air inside the reservoir is eventually depleted and the reservoir needs to be refilled or replaced. This can prove to be inconvenient for the users of the vehicle or device inside which the engine is provided. 
     Therefore, there is a need for a system for replenishing air inside an air spring used to bias a valve of an engine that does not require frequent replacement or refilling. 
     Another of the deficiencies associated with air springs is that even when the engine is not is use, air can leak out of the air springs. When the air pressure inside the air springs becomes too low, this causes the valves to move to their opened positions. When this occurs and the engine is started, the pistons of the engine can come into contact with the valves, potentially damaging the valves, and as a result preventing operation of the engine. 
     One possible solution consists in providing metallic coil springs having a relatively low spring constant in addition to the air springs. The metallic coil springs are strong enough to bias the valves towards their closed position even when the air pressure inside the air springs is no longer sufficient to do so on its own. However, these metallic coil springs do not provide enough biasing force to return the valves to their closed position fast enough while the engine is in operation. Although the addition of these metallic coil springs will prevent the pistons of the engine from coming into contact with the valves when the engine is started, they add weight and complexity to the air spring system. The additional metallic coil springs can also lead to some resonance as the speed of the engine increases. 
     Therefore, there is a need for a system for replenishing air inside an air spring used to bias a valve of an engine before the engine is started. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to ameliorate at least some of the inconveniences present in the prior art. 
     It is an object of the present invention to provide a method of supplying air to an air spring biasing one of an intake valve and an exhaust valve of an internal combustion engine to a closed position. The engine has an air compressor for supplying air to the air spring. The air compressor can be driven by a motor and by a rotating shaft of the engine. The motor drives the air compressor prior to engine start-up, and once the engine has started, the rotating shaft of the engine drives the air compressor. 
     In one aspect, the invention provides a method of supplying air to an air spring biasing one of an intake valve and an exhaust valve of an internal combustion engine to a closed position. The method comprises: driving an air compressor with a motor prior to starting of the internal combustion engine, the air compressor fluidly communicating with the air spring to supply air to the air spring; determining that a predetermined condition has been reached; starting the engine once the predetermined condition has been reached; and driving the air compressor with a rotating shaft of the engine once the engine has started. 
     In a further aspect, the predetermined condition is a predetermined amount of time for which the air compressor is driven by the motor. 
     In an additional aspect, the predetermined condition is a predetermined air pressure indicative of an air pressure inside the air spring. 
     Embodiments of the present invention each have at least one of the above-mentioned objects and/or aspects, but do not necessarily have all of them. It should be understood that some aspects of the present invention that have resulted from attempting to attain the above-mentioned objects may not satisfy these objects and/or may satisfy other objects not specifically recited herein. 
     Additional and/or alternative features, aspects, and advantages of embodiments of the present invention will become apparent from the following description, the accompanying drawings, and the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a better understanding of the present invention, as well as other aspects and further features thereof, reference is made to the following description which is to be used in conjunction with the accompanying drawings, where: 
         FIG. 1  is a side elevation view of an internal combustion engine according to the present invention; 
         FIG. 2  is an end elevation view of the engine of  FIG. 1 ; 
         FIG. 3  is a perspective view of a first embodiment of internal components of a cylinder head of the engine of  FIG. 1 ; 
         FIG. 4  is a partial cross-sectional view of a valve, air spring, and camshaft assembly of the engine of  FIG. 1 ; 
         FIG. 5  is a perspective view of an air compressor of the engine of  FIG. 1 ; 
         FIG. 6  is a cross-sectional view of the air compressor of  FIG. 5  taken along line  6 - 6  in  FIG. 5 ; 
         FIG. 7  is a perspective view of a second embodiment of some of the internal components of the cylinder head of the engine of  FIG. 1 ; 
         FIG. 8  is a partial cross-sectional view of the components of  FIG. 7 ; 
         FIG. 9  is a perspective view of the cylinder head of the engine of  FIG. 1  containing the internal components of  FIG. 7 , with the cylinder head cover removed; 
         FIG. 10  is another perspective view of the cylinder head of the engine of  FIG. 1  containing the internal components of  FIG. 7 , with the cylinder head cover removed; 
         FIG. 11  is a perspective view of covers of the cylinder head of  FIG. 9 ; 
         FIG. 12  is a logic diagram illustrating a method of supplying air to an air spring of the embodiment show in  FIG. 7 ; 
         FIG. 13  is a schematic diagram of an alternative embodiment of an air spring system of the engine of  FIG. 1 ; and 
         FIG. 14  is a logic diagram illustrating a method of supplying air to an air spring of the embodiment show in  FIG. 13 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An internal combustion engine  10  in accordance with the present invention will be described with reference to  FIGS. 1 to 3 . The engine  10  operates on the four-stroke principle, however it is contemplated that aspects of the present invention could be used on engines operating on other principles and having intake and/or exhaust valves. The engine  10  has a crankcase  12 . The crankcase  12  houses a crankshaft  14  and an output shaft  16 . The output shaft  16  is operatively connected to the crankshaft  14  via a transmission (not shown) also housed in the crankcase  12 . The output shaft  16  extends out of the crankcase  12  to transmit power from the engine  10  to an element operatively connected to the output shaft  16 . In the case where the engine  10  is provided in a wheeled vehicle, such as a motorcycle, the output shaft  16  is operatively connected to the wheels of the vehicle to transmit power from the engine  10  to the wheels. It is contemplated that the engine  10  could be used in other types of vehicles, such as a snowmobile, or in other types of applications. 
     A cylinder block  18  is connected to the crankcase  12 . The cylinder block  18  defines a cylinder  20 . A piston  22  is disposed inside the cylinder  20 . The piston  22  is connected by a connecting rod  24  to the crankshaft  14 . During operation of the engine  10 , the piston  22  reciprocates inside the cylinder  20  along a cylinder axis  26  defined by the cylinder  20 , thus driving the crankshaft  14 , which drives the output shaft  16  via the transmission. It is contemplated that the cylinder block  18  could define more than one cylinder  20 , and, as a result, the engine  10  would have a corresponding number of pistons  22  and associated parts. It is also contemplated that the engine could be a V-type engine having two cylinder blocks  18 . 
     A cylinder head  28  is connected to the cylinder block  18 . The cylinder head  28  has two side walls  30 , two end walls  32 , and a cylinder head cover  34 . The cylinder head  28 , the cylinder  20 , and the piston  22  define a variable volume combustion chamber  36  of the engine  10  therebetween. 
     As seen in  FIG. 3 , two air intake passages  38  are provided in the cylinder head  28 . One end of each air intake passage  38  is connected to the combustion chamber  36 , and the other end of each air intake passage  38  is connected to a corresponding outlet of an air intake manifold  40  ( FIG. 1 ) having a single inlet. A carburetor  42  ( FIG. 1 ) is connected to the inlet of the air intake manifold  40 . The carburetor  42  controls the flow of air and fuel that enters the combustion chamber  36  via the air intake passages  38 . It is contemplated that the carburetor  42  could be replaced by a throttle body that only controls the flow of air to the combustion chamber  36 , in which case a fuel injector in communication with the combustion chamber  36  would be provided in the engine  10 . Each air intake passage  38  is provided with an intake valve  44  that is movable between an opened position and a closed position to allow or prevent, respectively, air and fuel to enter the combustion chamber  36  as described in greater detail below. Each intake valve  44  is provided with an air spring  45  that biases the intake valve  44  toward its closed position. 
     Two exhaust passages  46  are provided in the cylinder head  28 . One end of each exhaust passage  46  is connected to the combustion chamber  36 , and the other end of each exhaust passage  46  is connected to a corresponding inlet of an exhaust manifold (not shown) having a single outlet. The outlet of the exhaust manifold is connected to an exhaust system of the engine  10  which releases the exhaust gases from the engine  10  to the atmosphere. Each exhaust passage  46  is provided with an exhaust valve  48  that is movable between an opened position and a closed position to allow or prevent, respectively, exhaust gases to exit the combustion chamber  36  as described in greater detail below. Each exhaust valve  48  is provided with an air spring  49  that biases the exhaust valve  48  toward its closed position. 
     It is contemplated that there may be only one, or more than two, of each of the air intake and exhaust passages  38 ,  46  with a corresponding number of intake and exhaust valves  44 ,  48  and associated elements. It is also contemplated that there may be a different number of air intake and exhaust passages  38 ,  46 . For example, it is contemplated that there could be two air intake passages  38  and a single exhaust passage  46 . Also, although it is preferred that each of the valves  44 ,  48  be provided with an air spring  45  or  49 , it is contemplated that only some of the valves  44 ,  48  (or only one of the valves  44 ,  48  should there be only one intake valve  44  and/or one exhaust valve  48 ) could be provided with an air spring  45  or  49 . 
     An intake camshaft  50  is disposed in the cylinder head  28  generally parallel to a rotation axis of the crankshaft  14 . A sprocket  52  is disposed at one end of the intake camshaft  50 . A chain (not shown) operatively connects the sprocket  52  to a sprocket (not shown) operatively connected to the crankshaft  14 , such that the intake camshaft  50  is driven by the crankshaft  14 . Two intake cams  54  (one per intake valve  44 ) are disposed on the intake camshaft  50 . Each intake cam  54  engages a corresponding intake cam follower  56  rotatably disposed on an intake cam follower shaft  58 . Each air spring  45  is biased against its corresponding intake cam follower  56 , such that, as the intake camshaft  50  rotates, each intake cam  54  pushes on its corresponding intake cam follower  56 , which in turn pushes on its corresponding air spring  45  and moves the corresponding intake valve  44  to the opened position. As the intake camshaft  50  continues to rotate, each air spring  45  returns the corresponding intake valve  44  to its closed position. 
     An exhaust camshaft  60  is disposed in the cylinder head  28  generally parallel to the intake camshaft  50 . A sprocket  62  is disposed at one end of the exhaust camshaft  60 . A chain (not shown) operatively connects the sprocket  62  to a sprocket (not shown) operatively connected to the crankshaft  14 , such that the exhaust camshaft  60  is driven by the crankshaft  14 . Two exhaust cams  64  (one per exhaust valve  48 ) are disposed on the exhaust camshaft  60 . Each exhaust cam  64  engages a corresponding exhaust cam follower  66  rotatably disposed on an exhaust cam follower shaft  68 . Each air spring  49  is biased against its corresponding exhaust cam follower  66 , such that, as the exhaust camshaft  60  rotates, each exhaust cam  64  pushes on its corresponding exhaust cam follower  66 , which in turn pushes on its corresponding air spring  49  and moves the corresponding exhaust valve  48  to the opened position. As the exhaust camshaft  60  continues to rotate, each air spring  49  returns the corresponding exhaust valve  48  to its closed position. 
     It is contemplated that the cam followers  56 ,  66 , and the cam follower shafts  58 ,  68  could be omitted and that the cams  54 ,  64  could engage the air springs  45 ,  49  and valves  44 ,  48  directly. It is also contemplated that the cam followers  56 ,  66  could be replaced by rocker arms. It is also contemplated that each cam  54 ,  64  could engage more than one valve  44 ,  48 . It is also contemplated that there could be only one camshaft having both the intake and exhaust cams  54 ,  64  disposed thereon. It is also contemplated that the shape of the cams  54 ,  64  could be different from the one illustrated in the figures depending on the type of engine performance that is desired. 
     A spark plug  70  ( FIG. 1 ) is disposed between the camshafts  50  and  60  and extends inside the combustion chamber  36  to ignite the air fuel mixture inside the combustion chamber  36 . 
     Turning now to  FIG. 4 , one of the air springs  49  will be described in more detail. The other air spring  49  and the air springs  45  have the same construction and as such will not be described in detail herein. The air spring  49  includes a cylinder  72  and a piston  74  disposed inside the cylinder  72  to reciprocally move therein. The top of the piston  74  is the portion of the air spring  49  which comes into contact with the exhaust cam follower  66 . An air chamber  76  is defined between the cylinder  72  and the piston  74 . A cotter  78  disposed around the end of the exhaust valve  48  connects the exhaust valve  48  to the piston  74  such that the piston  74  and the exhaust valve  74  reciprocate together. A shim  80  is disposed between the end of the exhaust valve  48  and the piston  74 . The thickness of the shim  80  is selected such that the exhaust valve  48  will properly sit in the inlet of the exhaust passage  46  when the valve  48  is in its closed position and will extend to the desired position when the valve  48  is in its opened position. A valve stem guide  82  is integrally formed with the cylinder  72  and, as the name suggests, guides the stem  84  of the exhaust valve  48  to ensure that the exhaust valve  48  only moves along a straight line. An air port  86  is formed in the bottom  88  of the cylinder  72 . The air port  86  is connected to an air supply line  90  used to supply air to the air chamber  76  of the air spring  49  as described in greater detail below. The air port  86  is dimensioned such that, as the piston  74  moves toward the bottom  88  of the cylinder  72 , the air pressure inside the air chamber  76  will increase and the piston  74  (and the exhaust valve  48 ) will return to its initial position (due to the air pressure) before enough air drains out via the air port  86  to adversely affect the performance of the air spring  49 . 
     Turning back to  FIGS. 1 to 3 , a first embodiment of the air spring system of the engine  10  will be described. A compressor  100 , described in greater detail below, is disposed inside the cylinder head  28 . During operation of the engine  10 , the compressor  100  supplies air to the air springs  45 ,  49  via the air supply line  90  so as to maintain the air pressure inside the air springs  45 ,  49 . 
     The compressor  100  is held inside a compressor cover  102  ( FIGS. 1 and 2 ) that is fastened over an aperture (not shown) formed in one of the side walls  30  of the cylinder head  28 . When the compressor cover  102  is in place as shown in  FIGS. 1 and 2 , the air compressor  100  is disposed just below the intake camshaft  50  so as to be driven by the intake camshaft  50 , as will be described in more detail below. It is contemplated that the air compressor  100  could alternatively be driven by another rotating shaft of the engine  10 , such as the exhaust camshaft  60  or the crankshaft  14 . By supporting the air compressor  100  in the compressor cover  102 , the air compressor  100  can be removed from the cylinder head  28  without having to remove the cylinder head cover  34  and the intake camshaft  50 . Also, by locating the air compressor  100  inside the cylinder head  100  below the intake camshaft  50 , the packaging of the cylinder head  28  and its components remains compact and maintenance on the camshafts  50 ,  60 , air springs  45 ,  49 , and valves  44 ,  48  can be done without having to remove the air compressor  100 . 
     The air compressor  100  is a reciprocating air compressor, and more specifically a piston-type air compressor. In order to reduce the pressure pulses that are inherent from this type of compressor, air from the air compressor  100  flows to an accumulator chamber  104  (schematically shown in  FIG. 3 ) that is formed in the cover  102 . The accumulator chamber  104  is fluidly connected to the air supply line  90  which supplies air, first to the air springs  45 , then to the air springs  49 . The air supply line  90  connects the air springs  45 ,  49  in series, and as a result the air supply line  90  is generally C-shaped. From the last of the air springs  49 , the air supply line  90  connects to a pressure relief valve  106  which prevents pressure inside the system from exceeding a predetermined level. The pressure relief valve  106  is provided since the compressor  100  is constantly running and as a result, supplies air to the air spring system faster than is required to replace the air that escapes the air springs  45 ,  49 . 
     Turning now to  FIGS. 5 and 6 , the air compressor  100  and its operation will be described in more detail. The air compressor  100  has a body  110  defining a main chamber  112  and a sub-chamber  114  that selectively fluidly communicate together via passage  116 . A check valve consisting of a spring  118  and a disk  120  is disposed inside the sub-chamber  114 . The spring  118  biases the disk  120  against the passage  116  so as to selectively prevent air flow from the main chamber  112  to the sub-chamber  114  via the passage  116 . Air inlets  122  formed in the body  110  fluidly communicate the main chamber  112  with the atmosphere. Air outlets  124  formed in the body  110  fluidly communicate the sub-chamber  114  with the accumulator chamber  104 . A piston  126  is disposed inside the main chamber  112 . A wheel  128  having an integrally formed axle  130  is disposed inside the top of the piston  126  with the ends of the axle  130  extending out of the sides of the piston  126 . The axle  130  passes through slots  132  formed in the body  110  of the air compressor  100  so as to guide piston  126  as it reciprocates inside the main chamber  112  as described below. A collar  134  is disposed around the body  110  and abuts the ends of the axle  130 . A spring  136  is disposed between the collar  134  and the portion (not shown) of the cover  102  supporting the air compressor  100  so as to bias the piston  126  toward the position shown in  FIGS. 5 and 6 . 
     As can be seen in  FIG. 3 , a compressor driving cam  138  is disposed on the intake camshaft  50  engages the wheel  128  of the air compressor  100 . As the intake camshaft  50  rotates, the compressor driving cam  138  pushes on the wheel  128 , which in turn moves the piston  126  towards the passage  116 . As it moves, the piston  126  blocks the air inlets  122 , and as result the air pressure inside the main chamber  112  increases as the volume of the main chamber  112  decreases. When the air pressure inside the main chamber  112  becomes high enough to overcome the bias of the spring  118 , the disk  124  moves away from the passage  116 , thus allowing the pressurized air to flow from the main chamber  112  to the sub-chamber  114  via the passage  116 . From the sub-chamber  114 , the pressurized air flows through the outlets  124  to the accumulator chamber  104 , and from there, to the air springs  45 ,  49 , as described above. As the intake camshaft  50  continues to rotate, it no longer pushes on the wheel  128 , and the spring  136  biases the piston  126  back to the position shown in  FIGS. 5 and 6  and the spring  118  biases the disk  120  back against the passage  116 . In this position air can enter the main chamber  112  via the inlets  122 . The air compressor  100  continues to operate as described above for as long as the intake camshaft  50  rotates. 
     Turning now to  FIGS. 7 to 11 , another embodiment of a cylinder head  28 ′ and its corresponding elements will be described. For simplicity, the elements shown in  FIGS. 7 to 11  which are similar to those of  FIGS. 1 to 6  have been labelled with the same reference numerals and will not be described again in detail. 
     In this embodiment, the air spring system is provided with an air compressor  100 ′. The air compressor  100 ′ has the same construction and operates in the same way as the air compressor  100 , except that the spring  136  abuts a shoulder  140  formed by the body  110 ′ of the air compressor  100 ′. 
     The air compressor  100 ′ is disposed inside the cylinder head  28 ′. It is supported inside a holder  150  ( FIG. 11 ) formed on an inner side of the cover  102 ′. As with the cover  102 , the cover  102 ′ is fastened over an aperture  152  ( FIG. 10 ) formed in a side wall  30  of the cylinder head  28 ′. As can be seen in  FIG. 11 , the cover  102 ′ also has an accumulator chamber  104  formed therein. 
     As in the system described above, from the air compressor  100 ′, the air flows to the accumulator chamber  104 , and from there to the air springs  45 ,  49  (in series), and then to the pressure relief valve  106 . 
     The main difference between the system described above and the current system is in the way the air compressor  100 ′ is driven. In this embodiment, the compressor driving cam  138  is disposed on a tubular compressor driving shaft  154 . The compressor driving shaft  154  is coaxial with the intake camshaft  50 . One end of the intake camshaft  50  is disposed inside one end of the compressor driving shaft  154 . An overrunning clutch  156  disposed between the end of the intake camshaft  50  and the compressor driving shaft  154  selectively connects the end of the intake camshaft  50  to the compressor driving shaft  154  such that the compressor driving shaft  154 , and therefore the air compressor  100 ′, can be selectively driven by the intake camshaft  50 . It is contemplated that the air compressor driving shaft  154  could alternatively be selectively connected to another rotating shaft of the engine  10 , such as the exhaust camshaft  60  or the crankshaft  14 . 
     A secondary shaft  158 , which is coaxial with the compressor driving shaft  154 , has one end disposed inside the other end of the compressor driving shaft  154 . An overrunning clutch  160  disposed between the end of the secondary shaft  158  and the compressor driving shaft  154  selectively connects the end of the secondary shaft  158  to the compressor driving shaft  154  such that the compressor driving shaft  154 , and therefore the air compressor  100 ′, can be selectively driven by the secondary shaft  158 . The secondary shaft  158  is driven by an electric motor  162 . 
     The electric motor  162  is disposed inside a cavity (not shown) formed between the compressor cover  102 ′ and a second cover  164  ( FIGS. 9 and 10 ) that is fastened to the compressor cover  102 ′. The secondary shaft  158  passes through an aperture  166  ( FIG. 11 ) in the compressor cover  102 ′ and extends inside the cavity. The end of the secondary shaft  158  that is in the cavity has a gear  168  disposed thereon. The motor  162  has a motor shaft  170  that extends generally perpendicularly to the secondary shaft. The motor shaft  170  has a gear  172  disposed thereon which engages the gear  168  of the secondary shaft  158  so as to the drive the secondary shaft  158  with the motor  162 . 
     As would be understood, due to the overrunning clutches  156 ,  160 , the one of the intake camshaft  50  and the secondary shaft  158  which rotates the fastest during the operation of the engine  10  is the one that drives the compressor driving shaft  154 , and therefore the air compressor  100 ′. 
     With reference to  FIG. 12 , a method of operating the system shown in  FIGS. 7 to 11  will be described. The method begins at step  200  when a control unit (not shown) of the engine  10  receives an indication of a desire to start the engine  10 . This indication could, for example, come from a signal received when an ignition key is turned or when a start button is pressed. Then at step  202 , before starting the engine  10 , the motor  162  is used to drive the compressor driving shaft  154 , and therefore the air compressor  100 ′. Then at step  204 , the control unit determines if a predetermined condition has been reached. It is contemplated that the predetermined condition could be a predetermined air pressure indicative of an air pressure inside the air springs  45 ,  49 . The air pressure could be sensed by a pressure sensor sensing the pressure directly inside one or more of the air springs  45 ,  49 , or inside the air supply line  90 . Alternatively, the predetermined condition could be a predetermined amount of time for which the air compressor  100 ′ is driven by the motor  162 . When the predetermined condition is reached, the air compressor  100 ′ has supplied enough air to the air springs  45 ,  49  such that the air springs  45 ,  49  bias the valves  44 ,  48  towards their closed positions. The motor  162  will continue to drive the air compressor  100 ′ and the engine  10  will not be started until the predetermined condition is reached. This ensures that the piston  22  of the engine  10  will not contact the valves  44 ,  48  when the engine  10  is started, which might have occurred if air leaked out of the air springs  45 ,  49  while the engine  10  was not in use, as previously explained. 
     Once the predetermined condition is reached, then at step  206  the engine  10  is started, and as a result, at step  208 , the engine  10  drives the air compressor  100 ′ via the intake camshaft  50 . The motor  162  is then stopped at step  210 . It is contemplated that the motor  162  could alternatively be stopped as soon as the predetermined condition is reached (i.e. between steps  204  and  206 ). The air compressor  100 ′ continues to be driven by the intake camshaft  50  until the engine  10  is stopped, at which point the method ends at step  212 . 
     Turning now to  FIG. 13 , another air spring system will be described. For simplicity, the elements shown in  FIG. 13  which are similar to those of  FIGS. 1 to 6  have been labelled with the same reference numerals and will not be described again in detail. 
     The air spring system shown in  FIG. 13  is the same as the one shown in  FIG. 3 , but with the addition of a second air compressor  250 . The air compressor  250  is an electrical air compressor powered by a battery  252 . The battery  252  is preferably the same battery that is used for the engine  10 . A switch  254  is used to turn the electrical air compressor  250  on or off. The electrical air compressor  250  is preferably disposed inside the cylinder head  28 . As can be seen, the electrical air compressor  250  fluidly communicates with the accumulator chamber  104 , the air supply line  90 , and the air springs  45 ,  49  so as to supply air to the air springs  45 ,  49 . It is contemplated that in the case that the air compressor  250  could bypass the accumulator chamber  104  and connect directly to the air supply line  90 . This could be done should the air compressor  250  be of a type that provides pressurized air with relatively small pressure fluctuations. 
     With reference to  FIG. 14 , a method of operating the system shown in  FIG. 13  will be described. The method begins at step  300  when a control unit (not shown) of the engine  10  receives an indication of a desire to start the engine  10 . This indication could, for example, come from a signal received when an ignition key is turned or when a start button is pressed. Then at step  302 , before starting the engine  10 , the switch  254  is closed and the electrical air compressor  250  is turned on to supply air to the air springs  45 ,  49 . Then at step  304 , the control unit determines if a predetermined condition has been reached. When the predetermined condition is reached, the air compressor  250  has supplied enough air to the air springs  45 ,  49  such that the air springs  45 ,  49  bias the valves  44 ,  48  towards their closed positions. The air compressor  250  will continue supply air to the air springs  45 ,  49  and the engine  10  will not be started until the predetermined condition is reached. It is contemplated that the predetermined condition could be a predetermined air pressure indicative of an air pressure inside the air springs  45 ,  49 . The air pressure could be sensed by a pressure sensor sensing the pressure directly inside one or more of the air springs  45 ,  49 , or inside the air supply line  90 , such as pressure sensor  256 . Alternatively, the predetermined condition could be a predetermined amount of time for which the air compressor  250  is driven. 
     Once the predetermined condition is reached, then at step  306  the engine  10  is started, and as a result, at step  308 , the engine  10  drives the air compressor  100  via the intake camshaft  50 . The switch  254  is then opened and the electrical air compressor  25  stopped at step  310 . It is contemplated that the electrical air compressor  250  could alternatively be stopped as soon as the predetermined condition is reached (i.e. between steps  304  and  306 ). The air compressor  100  continues to be driven by the intake camshaft  50  until the engine  10  is stopped, at which point the method ends at step  312 . 
     Modifications and improvements to the above-described embodiments of the present invention may become apparent to those skilled in the art. The foregoing description is intended to be exemplary rather than limiting. The scope of the present invention is therefore intended to be limited solely by the scope of the appended claims.
A method of supplying air to an air spring biasing one of an intake valve and an exhaust valve of an internal combustion engine to a closed position is disclosed. The method includes: driving an air compressor with a motor prior to starting of the internal combustion engine, the air compressor fluidly communicating with the air spring to supply air to the air spring; determining that a predetermined condition has been reached; starting the engine once the predetermined condition has been reached; and driving the air compressor with a rotating shaft of the engine once the engine has started.