Patent Publication Number: US-7588005-B2

Title: Dual intake valve assembly for internal combustion engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a Continuation-in-Part (CIP) of U.S. patent application Ser. No. 11/857,527 filed Sep. 19, 2007 now U.S. Pat. No. 7,523,733, which itself claims the benefit of U.S. Provisional Application No. 60/918,911 filed Mar. 20, 2007 by Ralph Moore, and hereby incorporates herein by reference the respective entireties thereof. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to internal combustion engines in general and, more particularly, to an intake valve assembly of an internal combustion engine. 
   2. Description of the Prior Art 
   In a conventional internal combustion engine, intake and exhaust poppet valves regulate the gas exchange. A valve train (i.e. cams, drive gears and chains, rocker arms, push rods, lifters, etc.) regulate the poppet valves. Fixed valve timing of the poppet valves of the conventional internal combustion engine, and especially of the intake valve, represents a compromise between two conflicting design objectives: 1) maximum effective pressure within a cylinder, thus torque, at the most desirable points in a range of engine operating speeds, and 2) a highest possible power peak output. The higher the RPM at which maximum power occurs, and the wider the range of an engine operating speed, the less satisfactory will be the ultimate compromise. Large variations in the effective flow opening of the intake valve relative to the stroke (i.e., in design featuring more than two valves) will intensify this tendency. 
   In conventional four-stroke internal combustion engines, during the ending phase of the exhaust stroke, both intake and exhaust valves to the combustion chamber are kept open simultaneously for a certain period (known in the art as a valve overlap period, or simply a valve overlap) in order to increase exhaust efficiency of the engine. However, as a consequence of both valves being open simultaneously, part of the exhaust gas burnt in the combustion chamber is blown past the open intake valve and into an intake passage of the engine where the exhaust gas is mixed with the air-fuel mixture flowing through the intake passage. The exhaust gases impair ignition of the air-fuel mixture and therefore adversely affect the engine performance. The instability and accompanying inefficiency are particularly acute in the medium to low speed operational ranges of the engine and during idling of the engine. 
   Typically, a range of engine operating speeds includes a low engine speed range (low engine speeds) and a high engine speed range (high engine speeds). Generally, the low engine speed range is defined as a speed range from an idle speed to a midrange speed, and high engine speed is defined as a speed range from the midrange speed to a maximum engine speed. In other words, the low engine speed is the engine speed at or near the lower end of the operating speed range of the engine, while the high engine speed is the engine speed at or near the upper end of the operating speed range of the engine. 
   At the same time, growing demand for minimizing exhaust emissions and maximizing fuel economy means that a low idle speed and high low-end torque along with high specific output of an internal combustion engine are becoming increasingly important. These imperatives have led to the application of variable valve timing systems (especially for intake valves). However, this approach is complex and expensive, and takes away from durability of the internal combustion engine. Moreover, effectiveness of the variable valve timing systems that regulate the valve train is limited to a downstream efficiency of the poppet valve. The poppet valve is far from ideal. Even when the valve is open a disk-shaped head of the poppet valve is directly in front of an intake port opening, where it sits directly in the way of the air or air/gas mixture flow stream. However, currently, the poppet valve is the only kind of valve that can operate in the severe environment of the internal combustion engine. 
   Thus, the intake valve assembly of the prior art, including but not limited to those discussed above, are susceptible to improvements that may enhance their performance and cost. The need therefore exists for intake valve assembly that is simple in design, compact in construction and cost effective in manufacturing, and, at the same time, provides both an improved low-end torque along with a high power output of the internal combustion engine. 
   SUMMARY OF THE INVENTION 
   The present invention provides a novel intake valve assembly for an internal combustion engine that includes a combustion chamber and an intake passage fluidly communicating with the combustion chamber through an intake port. 
   The intake valve assembly of the present invention comprises a primary valve provided to seal against a primary valve seat formed in the intake port, and a hollow secondary valve mounted about the primary valve substantially coaxially therewith and provided to seal against a secondary valve seat formed in the intake port. The primary valve is movable into and out of engagement with the primary valve seat between respective closed and open positions, while the secondary valve is movable into and out of engagement with the secondary valve seat between respective closed and open positions. The intake valve assembly further comprises a primary valve spring for normally biasing the primary valve toward a closed position thereof, a secondary valve spring for normally biasing the secondary valve toward a closed position thereof, and a secondary valve lifter fixed to the primary valve so as to be axially spaced from the secondary valve when both the primary valve and the secondary valve are in the closed position. 
   The primary valve is operated only mechanically, while the secondary valve is operated both mechanically by the secondary valve lifter and fluidly in response to pressure differential between the intake passage and the combustion chamber. The secondary valve is engageable with the primary valve through the secondary valve lifter after opening of the primary valve so that further movement of the primary valve away from the primary valve seat pushes the secondary valve away from the secondary valve seat. 
   The primary valve spring is normally contracted for continuously biasing the primary valve toward the closed position thereof, and the secondary valve spring is also normally contracted for continuously biasing the secondary valve toward the closed position thereof. 
   Therefore, the present invention provides a novel dual intake valve assembly of an internal combustion engine that provides in effect a variable valve timing and significantly improves both low and high speed performance of the engine. Moreover, the present invention reduces cost and complexity of the valve assembly and valve train compared to the existing (conventional) variable valve timing systems, and requires minimal low cost modification to adapt the intake valve assembly of the present invention to existing engines. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the present invention will become apparent from a study of the following specification when viewed in light of the accompanying drawings, wherein: 
       FIG. 1  is a fragmentary, sectional transverse view of an internal combustion engine comprising an intake valve assembly according to a first exemplary embodiment of the present invention; 
       FIG. 2  is a sectional view of the intake valve assembly according to the first exemplary embodiment of the present invention with both primary valve and secondary valve in a closed position; 
       FIG. 3  is a sectional view of the intake valve assembly according to the first exemplary embodiment of the present invention with both primary valve and secondary valve in an open position; 
       FIG. 4  is a cross-sectional view of a primary poppet valve of the intake valve assembly according to the first exemplary embodiment of the present invention; 
       FIG. 5  is a cross-sectional view of a secondary poppet valve of the intake valve assembly according to the first exemplary embodiment of the present invention; 
       FIG. 6  is a cross-sectional view of the intake valve assembly according to the first exemplary embodiment of the present invention showing the secondary poppet valve and a secondary valve lifter mounted to the primary poppet valve; 
       FIG. 7  is an exploded view of the secondary valve lifter according to the first exemplary embodiment of the present invention; 
       FIG. 8  is a cross-sectional view of the primary poppet valve with the secondary valve lifter formed unitarily with a valve stem of the primary poppet valve according to alternative embodiment of the present invention; 
       FIG. 9  is a fragmentary, sectional transverse view of the internal combustion engine according to the first exemplary embodiment of the present invention during valve overlap at low engine speed; 
       FIG. 10  is a fragmentary, sectional transverse view of the internal combustion engine according to the first exemplary embodiment of the present invention during a crossover phase from an intake stroke to a compression stroke at low engine speed; 
       FIG. 11  is a fragmentary, sectional transverse view of the internal combustion engine according to the first exemplary embodiment of the present invention during valve overlap at high engine speed; 
       FIG. 12  is a fragmentary, sectional transverse view of the internal combustion engine according to the first exemplary embodiment of the present invention during the crossover phase from the intake stroke to the compression stroke at high engine speed; 
       FIG. 13  shows comparison diagrams of engine torque and power for a conventional stock engine and the engine equipped with the intake valve assembly of the first exemplary embodiment of the present invention; 
       FIG. 14  shows dynamometer test results for the conventional stock engine; and 
       FIG. 15  shows dynamometer test results for the engine equipped with the intake valve assembly of the first exemplary embodiment of the present invention; 
       FIG. 16  is a graph of cam and valve lift versus cam angle of an intake cam lobe and the primary poppet valve; 
       FIG. 17  is a fragmentary, sectional transverse view of an internal combustion engine comprising an intake valve assembly according to a second exemplary embodiment of the present invention; 
       FIG. 18  is a sectional view of the intake valve assembly according to the second exemplary embodiment of the present invention with both primary valve and secondary valve in a closed position; 
       FIG. 19  is a sectional view of the intake valve assembly according to the second exemplary embodiment of the present invention with both primary valve and secondary valve in an open position; 
       FIG. 20  is a cross-sectional view of the intake valve assembly according to the second exemplary embodiment of the present invention showing a secondary poppet valve and a secondary valve lifter mounted to the primary poppet valve; 
       FIG. 21  is a cross-sectional view of the primary poppet valve of the intake valve assembly according to the second exemplary embodiment of the present invention; 
       FIG. 22  is a cross-sectional view of the secondary poppet valve of the intake valve assembly according to the second exemplary embodiment of the present invention; 
       FIG. 23  is a fragmentary, sectional transverse view of an internal combustion engine comprising an intake valve assembly according to a third exemplary embodiment of the present invention; 
       FIG. 24  is a cross-sectional view of the intake valve assembly according to the third exemplary embodiment of the present invention showing a secondary poppet valve and a secondary valve lifter mounted to the primary poppet valve; 
       FIG. 25  is a cross-sectional view of the primary poppet valve of the intake valve assembly according to the third exemplary embodiment of the present invention; and 
       FIG. 26  is a cross-sectional view of the secondary poppet valve of the intake valve assembly according to the third exemplary embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The preferred embodiments of the present invention will now be described with the reference to accompanying drawing. 
   For purposes of the following description, certain terminology is used in the following description for convenience only and is not limiting. The words such as “upper” and “lower”, “left” and “right”, “inwardly” and “outwardly” designate directions in the drawings to which reference is made. The words “smaller” and “larger” refer to relative size of elements of the apparatus of the present invention and designated portions thereof. The terminology includes the words specifically mentioned above, derivatives thereof and words of similar import. Additionally, the word “a”, as used in the claims, means “at least one”. 
   Referring to  FIG. 1  of the drawings, a first exemplary embodiment of an internal combustion engine of the present invention, generally denoted by reference numeral  10 , is illustrated. 
   The engine  10  comprises a cylinder block  11  defining at least one hollow cylinder  12 , a cylinder head  14  fastened to the cylinder block  11  to seal the upper end of the cylinder  12 , and a piston  16  reciprocatingly mounted in the cylinder  12  and, in turn, conventionally connected to a crankshaft through a connecting rod (not shown). The cylinder  12  of the cylinder block  11 , the cylinder head  14  and the piston  16  define a combustion chamber  15 . The cylinder head  14  is provided with an intake passage  18  fluidly communicating with the combustion chamber  15  through an intake port  20 , and an exhaust passage  22  fluidly communicating with the combustion chamber  15  through an exhaust port  23 . As further illustrated in detail in  FIGS. 2 and 3 , the intake port is defined by a substantially annular valve seat member  24  secured to the cylinder head  14 . The valve seat member  24  has a first (or primary) substantially annular valve seat  24   a  and a second (or secondary) substantially annular valve seat  24   b  (best shown in  FIG. 3 . As illustrated in  FIGS. 2 and 3 , the primary valve seat  24   a  is larger in cross-section than the secondary valve seat  24   b . Moreover, as used herein, the term “gas” or “fluid” will refer to an air or air/fuel mixture flowing through the intake passage  18  into the combustion chamber  15  through the intake port  20 . 
   The engine  10  further comprises an intake valve assembly  30 , an exhaust valve assembly  32 , and a valve train (or valve actuating mechanism)  34  provided for actuating the intake and exhaust valve assemblies  30  and  32 . The valve train  34 , illustrated in  FIG. 1 , includes a first (intake) rocker arm  36   a  actuating the intake valve assembly  30 , a second (exhaust) rocker arm  36   b  actuating the exhaust valve assembly  32 , and a valve actuating cam  38 . In turn, the cam  38  has a first (intake) lobe  38   a  actuating the first rocker arm  36   a  and a second (exhaust) lobe  38   b  actuating the second rocker arm  36   b . The intake cam lobe  38   a  has a fixed cam profile including a leading (opening) flank  38 ′ and a trailing (closing) flank  38 ″. Rotation of the crankshaft (not shown) causes the piston  16  to reciprocate in the cylinder  11  and the valve actuating mechanism  34  to operate in conventional manner to perform the known four-stroke engine operating cycle comprising intake, compression, expansion and exhaust strokes. 
   As illustrated in detail in  FIGS. 2-4 ,  6  and  7 , the intake valve assembly  30  according to the first exemplary embodiment the present invention comprises a primary poppet valve  40  and a secondary poppet valve  42  mounted about the primary poppet valve  40  substantially coaxially therewith. The primary poppet valve  40  includes an elongated valve stem  44  and a disk-shaped primary valve head  46  provided at a lower end of the valve stem  44  for sealingly engaging the valve seat member  24 . The intake valve assembly  30  further includes a valve guide  48  supporting the valve stem  44  of the primary poppet valve  40  for reciprocatingly sliding in the cylinder head  14 . The valve guide  48  is fixed in the cylinder head  14  in any appropriate manner known in the art, such by press-fit connection. 
   The primary valve head  46  is movable into and out of engagement with the valve seat member  24  between respective closed and open positions of the primary poppet valve  40 . In the closed position, the primary valve head  46  of the primary poppet valve  40  engages the primary valve seat  24   a  of the valve seat member  24  (as shown in  FIGS. 1 and 2 ), while in the open position thereof the primary valve head  46  is axially spaced from the primary valve seat  24   a  (a shown in  FIGS. 3 ,  8 ,  9 ,  10  and  11 ). The primary poppet valve  40  is biased toward the closed position thereof by a primary valve spring  50  which engages an upper end of the valve stem  44  using a conventional valve spring holder  51   a  and a keeper  51   b . Preferably, the primary valve spring  50  is in the form of a coils spring mounted concentric to the valve stem  44  of the primary poppet valve  40 . Moreover, the primary valve head  46  of the primary poppet valve  40  is complementary to the primary valve seat  24   a . Accordingly, when the primary valve head  46  of the primary poppet valve  40  engages the primary valve seat  24   a  of the valve seat member  24  in the closed position thereof (shown in  FIGS. 1 and 2 ), the intake port  20  is blocked and the combustion chamber  15  is hermetically sealed from the intake passage  18 . 
   The secondary poppet valve  42 , illustrated in detail in  FIGS. 2 ,  3  and  5 - 7 , includes a hollow stem portion  54  and a secondary valve head  56  provided at a lower end of the stem portion  54  for sealingly engaging the valve seat member  24 . The secondary valve head  56  is conical or dome-shaped with a front surface  57  thereof configured to complement and nest over a back surface  47  of the valve head  46  of the primary poppet valve  40 , as illustrated in detail in  FIG. 5 . The hollow stem portion  54  defines a substantially cylindrical bore  58  extending through both the stem portion  54  and the secondary valve head  56  of the secondary poppet valve  42 . Consequently, the hollow stem portion  54  of the secondary poppet valve  42  is reciprocatingly and coaxially mounted to and about the valve stem  44  of the primary poppet valve  40  to allow the secondary valve head  56  to slide back and forth between the valve seat member  24  of the intake port  20  and the primary valve head  46  of the primary poppet valve  40 . 
   The secondary valve head  46  is movable into and out of engagement with the valve seat member  24  between respective closed and open positions of the secondary poppet valve  42 . In the closed position, the secondary valve head  56  of the secondary poppet valve  42  engages the secondary valve seat  24   b  of the valve seat member  24  (as shown in  FIGS. 1 ,  2 ,  8  and  9 ), while in the open position thereof the secondary valve head  56  is axially spaced from the secondary valve seat  24   b  (a shown in  FIGS. 3 ,  10  and  11 ). The secondary poppet valve  42  is biased toward the closed position thereof by a secondary valve spring  60  which is non-movably coupled (fixed) to the valve guide  48  at an upper end thereof and to the stem portion  54  of the secondary poppet valve  42  at a lower end of the secondary valve spring  60 . Preferably, the secondary valve spring  60  is in the form of a coils spring mounted about the valve stem  44  of the primary poppet valve  40  substantially concentrically thereto. Further preferably, the secondary valve spring  60  is fixed to the valve guide  48  by engaging a helical groove  49  formed thereon and to the secondary valve  42  by engaging a helical groove  59  formed on the stem portion  54 . Moreover, the secondary valve head  56  of the secondary poppet valve  42  is complementary to the secondary valve seat  24   b . Accordingly, when the secondary valve head  56  of the secondary poppet valve  42  engages the secondary valve seat  24   b  of the valve seat member  24  in the closed position thereof (shown in  FIGS. 1 ,  2 ,  8  and  9 ), the intake port  20  is blocked and the combustion chamber  15  is hermetically sealed from the intake passage  18 . 
   Therefore, both the primary poppet valve  40  and the secondary poppet valve  42  are continuously (or normally) biased in the closed positions thereof by the primary and secondary valve springs  50  and  60 , respectively. Moreover, the primary valve spring  50 , being normally contracted, biases the primary poppet valve  40  in the closed position by its expansion force. Conversely, the secondary valve spring  60 , being normally extended, biases the secondary poppet valve  42  in the closed position by its contraction force. However, as illustrated in  FIG. 2 , both the primary and secondary poppet valves  40  and  42  are biased toward their closed positions in the same direction, specifically, in the vertically upward direction. As further illustrated in  FIGS. 1 ,  2 ,  8  and  9 , the intake port  20  is blocked and the combustion chamber  15  is hermetically sealed from the intake passage  18  only when the secondary poppet valve  42  is in the closed position, i.e. when the secondary valve head  56  of the secondary poppet valve  42  engages the secondary valve seat  24   b  of the valve seat member  24 . On the other hand, if the primary intake valve  40  is closed, the secondary intake valve  42  is also in its closed position. 
   The intake valve assembly  30  further comprises a mechanical secondary valve lifter  52  immovably fixed to the elongated valve stem  44  of the primary poppet valve  40  between the distal ends thereof so as to extend radially outwardly from the valve stem  44 , as illustrated in detail in  FIGS. 2 ,  3 ,  6  and  7 . Preferably, the secondary valve lifter  52  is in the form of a substantially cylindrical collar immovably retained in a groove  45  formed in the valve stem  44  by machining. Further preferably, the secondary valve lifter  52  comprises an actuator member  66  and an internally threaded nut member  68  (shown in detail in  FIG. 6 ). The actuator member  66  mates with the groove  45  in the valve stem  44 . In turn, the actuator member  66  includes two separate complementary pieces  66   a  and  66   b  each having complementary semi-cylindrical threaded surface  67 , as illustrated in  FIG. 7 . Prior to assembly, the complementary pieces  66   a  and  66   b  of the actuator member  66  are placed in the groove  45  on either side of the valve stem  44 , then the nut member  68  is threaded over the threaded surfaces  67  thereof to lock the actuator member  66  in place into the groove  45  in the primary poppet valve  40 . Alternatively, the secondary valve lifter  52  can be formed unitarily with the valve stem  44  of the primary poppet valve  40  as a single-piece member, as illustrated in  FIG. 8 . As further illustrated in  FIG. 6 , the actuator member  66  of secondary valve lifter  52  has an actuator surface  69  (preferably annular in configuration) provided on axially bottom end thereof so as to extend radially outwardly from the valve stem  44 . In turn, the stem portion  54  of the secondary poppet valve  42  has a contact (back) surface  55  (preferably annular in configuration) provided on axially top end thereof and substantially complementary to the actuator surface  69  of the secondary valve lifter  52 . 
   The intake valve assembly  30  is mechanically controlled by the single intake lobe  38   a . In other words, both the primary and secondary valves  40  and  42 , respectively, are actuated by the same (single) cam lobe  38   a . However, the geometry of the cam lobe is novel to this valve assembly. The primary and secondary valves  40  and  42  are arranged coaxially and linearly (i.e. stacked one on top of the other). Both valves have a clearance area: a valve lash (or valve clearance) of the primary intake valve  40  defined as a distance between a distal end of the valve stem  44  of the primary intake valve  40  and the rocker arm  36   a , and a valve lash (or valve clearance) of the secondary intake valve  42  defined as a distance between the engagement surface  53  of the secondary valve lifter  52  and the contact surface  55  of the secondary poppet valve  42  in axial direction along the valve stem  44  of the primary poppet valve  40  when both the primary and secondary poppet valves  40  and  42  are in their closed positions. In other words, the valve lash provides a free movement or a distance the valve train has to travel before mechanical contact is achieved. 
   Conventionally, valve lash is used to ensure a positive seal between the valve and its seat. Accordingly, the valve lash of the primary intake valve  40  is conventional. The mechanical valve timing of the secondary intake valve  42  is just before top dead center and just after bottom dead center. This requires an abnormal amount of distance (or clearance) between the secondary valve lifter  52  fixed to the primary valves stem  44  and the secondary valve  42 . 
   There are mechanical limits to which valve trains can operate valves. An opening ramp on the leading flank of the intake cam lobe starts the intake rocker arm upward rather slowly in the initial stages to take up any residual slack and reduce the shock-loading transferred to the valve train. However, once the valve is moving, it is best to accelerate it at a maximum rate. This same principle holds true in the last stages of closing of the valve. The valve train has to slow the valve down before it returns it down to its seat. In other words, the conventional cam lobe includes the leading flank and the trailing flank having a substantially constant gradient between minimum and maximum lifts. 
   Because the secondary valve lifter  52 , which operates the secondary valve  42 , is fixed to the primary valve  40 , and the amount of distance required between the secondary valve lifter  52  and the secondary valve  42 , a conventional cam profile (with constant gradient) would have a velocity of the secondary valve lifter  52  too high at the time it made contact with the secondary valve  42 . Because of this fact, a cam profile of the intake cam lobe  38   a  according to the present invention is designed to accommodate the dual valve assembly. Specifically, the cam profile of the leading flank  38 ′ of the intake cam lobe  38   a  is such that it contacts the primary valve  40  conventionally and starts moving it at a rate that will allow it to slow down and safely contact the secondary valve  42 . The same principal is applied to the trailing flank  38 ″ of the intake cam lobe  38   a . The cam profile of the intake cam lobe  38   a  has to slow down the primary valve  40  to a safe rate to first return the secondary valve  42  to its seat  24   b  then return the primary valve  40  to its seat  24   a . In other words, the leading flank  38 ′ and the trailing flank  38 ″ of the intake cam lobe  38   a  of the present invention have a variable gradient between minimum and maximum lifts. 
   More specifically, as illustrated in  FIG. 16 , the leading flank  38 ′ of the intake cam lobe  38   a  conventionally starts upward rather slowly in the initial stages to take up any residual slack and reduce the shock-loading transferred to the valve train (segment I of the cam lift, or the opening ramp of the cam lobe profile). Once the primary valve  40  is moving, the gradient of the leading flank  38 ′ increases (segment II of the cam lift of the cam lobe profile) so as to accelerate opening of the primary valve  40 . Then, the gradient of the leading flank  38 ′ significantly decreases (segment III of the cam lift) so as to slow down and safely contact the secondary valve  42 . Subsequently, the gradient of the leading flank  38 ′ considerably increases again (segment IV of the cam lift) so as to accelerate both the primary valve  40  and the secondary valve  42  at a maximum rate toward their respective open position. When the primary and secondary valves  40  and  42  are reaching their fully open positions, the gradient of the leading flank  38 ′ again decreases (segment V of the cam lift). 
   Similarly, the gradient of the trailing flank  38 ″ of the intake cam lobe  38   a  first gradually increases (segment VI of the cam lift). Subsequently, the gradient of the trailing flank  38 ″ considerably increases (segment VII of the cam lift) so as to accelerate both the primary valve  40  and the secondary valve  42  at a maximum rate toward their respective closed position. Then, the gradient of the trailing flank  38 ″ significantly decreases (segment VIII of the cam lift) so as to slow down before the secondary valve  42  engages the secondary valve seat  24   b . Once the secondary valve  42  is safely seated, the gradient of the trailing flank  38 ″ increases again (segment IX of the cam lift) so as to accelerate closing of the primary valve  40 . Finally, the gradient of the trailing flank  38 ″ significantly decreases (segment X of the cam lift) so as to slow the primary valve  40  down before it returns it down to its seat  24   a.    
   In other words, the leading flank  38 ′ and the trailing flank  38 ″ of the intake cam lobe  38   a  according to the present invention have a variable gradient between minimum and maximum lifts of the primary valve  40 . 
   The primary poppet valve  40  has a fixed duration and lift defined by a geometry (or profile) of the intake lobe  38   a  of the valve actuating cam  38  suitable for high speed performance, while the secondary poppet valve  42  has a variable duration and lift when actuated fluidly (pneumatically) and fixed duration and lift when actuated mechanically suitable for both low and high engine speed performance defined by the geometry of the intake lobe  38   a  of the valve actuating cam  38 , by a distance between the engagement surface  53  of the secondary valve lifter  52  and the contact surface  55  of the secondary poppet valve  42  in axial direction along the valve stem  44  of the primary poppet valve  40  when both the primary and secondary poppet valves  40  and  42  are in their closed positions (commonly known in the art as a valve lash or valve clearance), and by a spring rate (coefficient of elasticity) of the secondary valve spring  60 . More specifically, the secondary valve  42  is operated mechanically by the secondary valve lifter  52  and fluidly (or pneumatically) in response to pressure differential between the intake passage  18  and the combustion chamber  15 . The secondary valve  42  is engageable with the primary valve  40  through the secondary valve lifter  52  after opening of the primary valve  40  so that further movement of the primary valve  40  away from the primary valve seat  24   a  pushes the secondary valve  42  away from the secondary valve seat  24   b . Free movement of the secondary valve  42  (the amount controlled pneumatically) is always restricted between the secondary valve lifter  52  and the back surface  47  of the valve head  46  of the primary poppet valve  40 . Such an arrangement of the intake valve assembly  30  provides the fluidly actuate the secondary intake valve  42  with the ability to operate at high engine speeds. In other words, when the primary valve  40  is fully open—the secondary valve  42  is also opened by the secondary valve lifter  52  (as illustrated in  FIG. 3 ), and when the primary valve  40  is closed—the secondary valve  42  is also closed (as illustrated in  FIGS. 1 and 2 ). 
   On the other hand, the medium that regulates the variable valve timing of the secondary valve  42  between the two fixed mechanical actuation positions is the pressure and flow of the gas acting directly on the secondary intake valve  42 . For the secondary intake valve  42  to work properly in the gas flow, a return spring force of the secondary valve spring  60 , i.e. the spring rate) has to be low enough to produce minimum resistance to gas flow. For that reason, and the fact that atmospherically controlled valves cannot be opened early (before top dead center) or closed late (after bottom dead center) the speed range of operation of the secondary valve  42  is very limited without the use of mechanical control. When gas flow and pressure in the intake passage  18  fall below the minimum to open the intake port  20  (usually at the low engine speed), the mechanical valve lifter  52  will open to secondary valve  42  at the fixed point. A similar control is in effect at the intake valve closing. The secondary valve  42  will be returned to the secondary valve seat  24   b  by the cam profile, either against the mechanical valve lifter  52  from its return spring tension or against the back surface  47  of the primary valve  40  from gas flow and pressure in the intake passage  18 . 
   The exhaust valve assembly  32  is substantially conventional and includes an exhaust poppet valve  62  normally biased toward a closed position thereof by an exhaust valve spring  64 , as shown in  FIG. 1 . Preferably, the exhaust valve spring  64  is in the form of a compression coils spring. The exhaust poppet valve  62  has a fixed duration and lift defined by the geometry of the exhaust lobe  38   b  of the valve actuating cam  38 . 
   The operation of the secondary valve  42  is hybrid in nature. In other words, the secondary valve  42  is operated both mechanically by the same intake lobe  38   a  of the valve actuating cam  38  as the primary poppet valve  40  using the secondary valve lifter  52  fixed to the valve stem  44  of the primary poppet valve  40  as its mechanical lifter, and fluidly (or pneumatically) by pressure differential between the intake passage  18  and the combustion chamber  15 . Specifically, the secondary poppet valve  42  can be displaced toward its open position either mechanically, when the secondary valve lifter  52  engages the valve stem  44  of the secondary poppet valve  42  due to the movement of the primary poppet valve  40  in an opening direction, or fluidly (pneumatically), when the pressure differential between the intake passage  18  and the combustion chamber  15  reaches a predetermined value capable to overcome the biasing force of the secondary valve spring  60 . More specifically, when gas pressure differential between the intake passage  18  and the combustion chamber  15  is higher than the predetermined value to open the secondary poppet valve  42  defined by the spring rate of the secondary valve spring  60  (i.e. the gas pressure in the intake passage  18  is higher than the gas pressure in the combustion chamber  15  and the biasing force of the secondary valve spring  60 ), the secondary poppet valve  42  would be opened without intervention of the mechanical secondary valve lifter  52  (if the primary poppet valve  40  is open). Also, when gas pressure differential between the intake passage  18  and the combustion chamber  15  falls below the predetermined value to open the secondary poppet valve  42  (i.e. the gas pressure in the intake passage  18  is lower than the gas pressure in the combustion chamber  15  and the biasing force of the secondary valve spring  60 ), the mechanical secondary valve lifter  52  will open the secondary poppet valve  42  at the fixed point. Similarly, when gas pressure differential between the intake passage  18  and the combustion chamber  15  falls below the predetermined value, the secondary poppet valve  42  will be returned to its seat  24   b  fluidly due to the gas pressure differential or mechanically by the back surface  47  of the valve head  46  of the primary poppet valve  40  due to the spring tension of the primary valve spring  50  as the primary poppet valve  40  moves toward its closed position. Accordingly, the present invention provides in effect a variable valve timing. Also, only minimal low cost modification is required to adapt the intake valve assembly  30  of the present invention to existing engines. 
   The mechanical opening and closing points of the secondary poppet valve  42  are determined by the distance (or valve clearance) between the secondary valve lifter  52  and the stem portion  54  of the secondary poppet valve  42  when both the primary and secondary poppet valves  40  and  42  are in their closed positions. The fluid operated opening and closing duration and a lift rate of the secondary poppet valve  42  are determined by the spring rate of the secondary valve spring  60 , opposing the pressure and flow differential of gases between the intake passage  18  and the combustion chamber  15 . 
   The operation of the intake valve assembly  30  of the present invention at low speeds of the engine  10 , illustrated in  FIGS. 9 and 10 , is as follows. 
     FIG. 9  illustrates the valve overlap (i.e. the overlap of the ending phase of the exhaust stroke and the beginning phase of the intake stroke) at low engine speed when the piston  16  is moving up and is near its top dead center (TDC) position. During this time, the combustion chamber  15  is filled with exhaust gas, and the exhaust poppet valve  62  is still open to enable the exhaust gas to escape from the combustion chamber  15 . As the piston  16  is reaching its top dead center (TDC) position to begin the intake stroke, the valve actuating mechanism  34  for the associated intake valve assembly  30  is operated so that the valve stem  44  of the primary poppet valve  40  is pushed downwardly in an opening direction by the cam lobe  38   a  and the first rocker arm  36   a  forcing the primary poppet valve  40  away from the primary valve seat  24   a  through the closed secondary poppet valve  42 , thus producing a reduced valve overlap period wherein both the primary intake poppet valve  40  and the exhaust poppet valve  62  are simultaneously open (as compared to conventional engines). However, initially, as the primary poppet valve  40  moves downwardly, the secondary poppet valve  42  remains seated on the secondary valve seat  24   b  due to the biasing force of the secondary valve spring  60 . At the same time, as the pressure of the exhaust gas in the combustion chamber  15  is higher than the pressure of the air-fuel mixture in the intake passage  18  at the low engine speeds, the secondary intake poppet valve  42  is pressed against the secondary valve seat  24   b  by the pressure differential between the combustion chamber  15  and the intake passage  18 . It will be appreciated that during this phase of the intake stroke, although the primary poppet valve  40  is open, the intake port  20  is blocked by the secondary poppet valve  42  so as to prevent fluid communication between the combustion chamber  15  and the intake passage  18 , thus preventing back-flow of exhaust gas through the intake port  20  into the intake passage  18  and, consequently, dilution of the air-fuel mixture in the intake passage  18 . This, in turn, increasing fuel economy and reduces exhaust emission. 
   Therefore, during the reduced valve overlap period at low engine speeds, the secondary poppet valve  42  is closed until the secondary valve lifter  52  engages the valve stem  44  of the secondary poppet valve  42  due to the movement of the primary poppet valve  40  in an opening direction. Further downward movement of the primary poppet valve  40  (in the opening direction) opens the secondary poppet valve  42 , which opens the intake port  20  and provides fluid communication between the combustion chamber  15  and the intake passage  18 . 
     FIG. 10  illustrates a crossover phase from the intake stroke to the compression stroke at low engine speed when the engine  10  has reached the end of the intake stroke and the piston  16  is just started moving up to compress the gas in the combustion chamber  15  and is near its bottom dead center (BDC) position. During this time, the combustion chamber  15  is filled with the air-fuel mixture, the exhaust valve  62  is closed, while the primary poppet valve  40  is closing but still off the primary valve seat  24   a . As the piston  16  is rising and compressing the air-fuel mixture, the gas pressure in the cylinder  12  increases well above the gas pressure inside the intake passage  18 . It should be appreciated that at the low engine speeds the speed of the gas flow, thus the pressure, in the intake passage  18  is relatively low. Therefore, the gas pressure in the intake passage  18  is not enough to overcome the gas pressure in the combustion chamber  15  and the closing biasing force of the secondary valve spring  60 . The gas pressure differential between the intake passage  18  and the combustion chamber  15  and the biasing force of the secondary valve spring  60  presses the secondary intake poppet valve  42  against the secondary valve seat  24   b . It will be appreciated that during this phase of the intake stroke, although the primary poppet valve  40  is still open, the intake port  20  is blocked by the secondary poppet valve  42  so as to prevent fluid communication between the combustion chamber  15  and the intake passage  18 , thus preventing reverse pulsing of the air-fuel mixture through the intake-port  20  back into the intake passage  18  and, consequently, improving engine torque and power. 
   Therefore, the intake valve assembly  30  of the present invention in effect reduces the valve open duration at low engine speeds as compared to conventional engines. 
   The operation of the intake valve assembly  30  of the present invention at high speeds of the engine  10 , illustrated in  FIGS. 11 and 12 , is as follows. 
     FIG. 11  illustrates the valve overlap (i.e. the overlap of the ending phase of the exhaust stroke and the beginning phase of the intake stroke) at high engine speed when the piston  16  is moving up and is near its TDC position. During this time, the exhaust poppet valve  62  is still open to enable the exhaust gas to escape from the combustion chamber  15 , but is quickly closing. As the piston  16  is moving up toward its TDC position to conduct the intake stroke, the valve actuating mechanism  34  for the associated intake valve assembly  30  is operated so that the valve stem  44  of the primary poppet valve  40  is pushed downwardly in an opening direction by the cam lobe  38   a  and the first rocker arm  36   a  forcing the primary poppet valve  40  away from the primary valve seat  24   a  through the secondary poppet valve  42 . As the primary intake poppet valve  40  moves downwardly, the secondary intake poppet valve  42  is rapidly opening, thus increasing valve overlap period (as compared to the engine operation at low engine speeds), because at the high engine speeds the fluid pressure in the intake passage  18  is well above the pressure in the combustion chamber  15 .  FIG. 11  illustrates the beginning phase of the intake stroke during the high speed engine operation, when the primary intake valve  40  is opening, while the secondary intake valve  42  is fluidly opening earlier than during the same valving phase at low engine speeds. In other words, when the primary intake valve  40  is opening at high engine speeds, the secondary intake valve  42  is opening simultaneously as the high pressure differential between the intake passage  18  and the combustion chamber  15  (due to the high speed of the exhaust flow) as the piston  16  reaches TDC and is reversed at a high rate of acceleration of the intake flow velocity keeps the secondary intake valve  42  open against the back surface  47  of the valve head  46  of the primary poppet valve  40 . This improves volumetric efficiency and a high end power of the engine  10 . 
     FIG. 12  illustrates a crossover phase from the intake stroke to the compression stroke at high engine speed. The piston  16  has just completed its downward travel at very high velocity, and has just reached its BDC position. For that reason, the gas pressure in the combustion chamber  15  is well below the gas pressure in the intake passage  18 . During this time, the exhaust poppet valve  62  is closed, and the piston  16  is moving up toward its TDC position to perform the compression stroke. In the initial phase of the compression stroke the air-fuel mixture continues to fill the cylinder  12  against the rising piston  16 . The still high pressure of the air-fuel mixture flowing through the intake passage  18  keeps the secondary intake valve  42  open against the primary intake valve  40 . The primary intake valve  40  and, correspondingly, the secondary intake valve  42 , are timed to close before the air-fuel mixture flow reverses. 
   Therefore, the intake valve assembly  30  of the present invention reduces the opening angle and timing of the secondary intake valve  42  at the low engine speeds so as to improve low speed performance and fuel economy of the internal combustion engine, and increases the opening angle and timing of the intake port of the secondary intake valve  42  at high engine speeds to improve a peak power output. Accordingly, the intake valve assembly  30  of the present invention provides in effect a variable valve timing. 
   Comparison diagrams of engine torque and power for the conventional stock engine and the improved engine equipped with the intake valve assembly of the present invention are shown in  FIG. 13 . Detailed dynamometer test results are shown in  FIGS. 14  (for stock engine) and  15  (for test engine equipped with the intake valve assembly of the present invention). The tested stock engine is a single cylinder, four-stroke engine having an engine displacement 19.02 in 3 . The test engine is the same single cylinder engine having the intake valve assembly of the present invention. 
     FIGS. 17-19  illustrate a second exemplary embodiment of an internal combustion engine of the present invention, generally depicted by the reference character  110 . Components, which are unchanged from the previous exemplary embodiments of the present invention, are labeled with the same reference characters. Components, which function in the same way as in the first exemplary embodiment of the present invention depicted in  FIGS. 1-12  are generally designated by the same reference numerals to which  100  has been added, sometimes without being described in detail since similarities between the corresponding parts in the two embodiments will be readily perceived by the reader. 
   The internal combustion engine  110  of the second exemplary embodiment of the present invention (shown in  FIGS. 17-19 ) corresponds substantially to the internal combustion engine  10  of the first exemplary embodiment (shown in  FIGS. 1-12 ), and only an intake valve assembly generally depicted by the reference character  430 , which differs, will therefore be explained in detail below. 
   The intake valve assembly  130  according to the second exemplary embodiment of the present invention, as illustrated in detail in  FIGS. 18-22 , comprises a primary poppet valve  140  and a secondary poppet valve  142  mounted about the primary poppet valve  140  substantially coaxially therewith. The primary poppet valve  140 , illustrated in detail in  FIGS. 20 and 21 , includes an elongated valve stem  144  and a disk-shaped primary valve head  146  provided at a lower end of the valve stem  144  for sealingly engaging the valve seat member  24 . The intake valve assembly  130  further includes a valve guide  48  (shown in  FIGS. 17-19 ) supporting the valve stem  144  of the primary poppet valve  140  for reciprocatingly sliding in the cylinder head  14 . The valve guide  48  is fixed in the cylinder head  14  in any appropriate manner known in the art, such by press-fit connection. 
   The primary valve head  146 , illustrated in detail in  FIGS. 18-22 , is movable into and out of engagement with the valve seat member  24  between respective closed and open positions of the primary poppet valve  140 . In the closed position, the primary valve head  146  of the primary poppet valve  140  engages the primary valve seat  24   a  of the valve seat member  24  (as shown in  FIGS. 17 and 18 ), while in the open position thereof the primary valve head  46  is axially spaced from the primary valve seat  24   a  (as shown in  FIG. 19 ). The primary poppet valve  140  is biased toward the closed position thereof by a primary valve spring  50  which engages an upper end of the valve stem  144  using a conventional valve spring holder  51   a  and a keeper  51   b . Preferably, the primary valve spring  50  is in the form of a coils spring mounted concentric to the valve stem  144  of the primary poppet valve  140 . Moreover, the primary valve head  146  of the primary poppet valve  140  is complementary to the primary valve seat  24   a . Accordingly, when the primary valve bead  146  of the primary poppet valve  140  engages the primary valve seat  24   a  of the valve seat member  24  in the closed position thereof (shown in  FIGS. 17 and 18 ), the intake port  20  is blocked and the combustion chamber  15  is hermetically sealed from the intake passage  18 . 
   The secondary poppet valve  142 , illustrated in detail in  FIGS. 20 and 22 , includes a hollow stem portion  154  and a secondary valve head  156  provided at a lower end of the stem portion  154  for sealingly engaging the secondary valve seat  24   b  of the valve seat member  24 . The secondary valve head  156  is conical or dome-shaped with a front surface  157  thereof configured to complement and nest over a back surface  147  of the valve head  146  of the primary poppet valve  140 , as illustrated in  FIGS. 17 ,  18  and  20 . The hollow stem portion  154  defines a stepped cylindrical bore  158  extending through both the stem portion  154  and the secondary valve head  156  of the secondary poppet valve  142 . Consequently, the hollow stem portion  154  of the secondary poppet valve  142  is reciprocatingly and coaxially mounted to and about the valve stem  144  of the primary poppet valve  140  to allow the secondary valve head  156  to slide back and forth between the valve seat member  24  of the intake port  20  and the primary valve head  146  of the primary poppet valve  140 . 
   The secondary valve head  156  is movable into and out of engagement with the valve seat member  24  between respective closed and open positions of the secondary poppet valve  142 . In the closed position, the secondary valve head  156  of the secondary poppet valve  142  engages the secondary valve seat  24   b  of the valve seat member  24  (as shown in  FIGS. 17 and 18 ), while in the open position thereof the secondary valve head  156  is axially spaced from the secondary valve seat  24   b  (a shown in  FIG. 19 ). Moreover, the secondary valve head  156  of the secondary poppet valve  142  is complementary to the secondary valve seat  124   b . Accordingly, when the secondary valve head  156  of the secondary poppet valve  142  engages the secondary valve seat  24   b  of the valve seat member  24  in the closed position thereof (shown in  FIGS. 17 and 18 ), the intake port  20  is blocked and the combustion chamber  15  is hermetically sealed from the intake passage  18 . 
   The secondary poppet valve  142  is biased toward the closed position thereof by a secondary valve spring  160  disposed between the primary poppet valve  140  and the secondary poppet valve  142 , as illustrated in  FIG. 20 . The secondary valve spring  160  includes at least one spring member in the form of a spring washer. Preferably, as illustrated in detail in  FIG. 21 , the secondary valve spring  160  includes two spring members  162  and  164  being in the form of spring washers (known also as disc springs, diaphragm springs or Belleville washers) separated by a spring spacer  165 . As shown in  FIGS. 20 and 21 , the spring washers  162  and  164  are disposed in the stepped cylindrical bore  158  within the hollow stem portion  154  of the secondary poppet valve  142  and mounted about the valve stem  144  of the primary poppet valve  140  substantially concentrically thereto. Specifically, a lower end (as shown in  FIG. 20 ) of the secondary valve spring  160  engages a first support surface  148  (preferably annular in configuration) formed on the primary poppet valve  140 , while an upper end of the secondary valve spring  160  engages a second support surface  170  (preferably annular in configuration) formed on the secondary poppet valve  142  In other words, the secondary valve spring  160  (preferably preloaded) extends between the first support surface  148  of the primary poppet valve  140  and the second support surface  170  of the secondary poppet valve  142  within the stepped cylindrical bore  158  of the secondary poppet valve  142 . 
   Therefore, both the primary poppet valve  140  and the secondary poppet valve  142  are continuously (or normally) biased in the closed positions thereof by the primary and secondary valve springs  50  and  160 , respectively. Moreover, both the primary valve spring  50  and the secondary valve spring  160 , being normally contracted, bias the primary poppet valve  140  and the secondary poppet valve  142  in the closed positions thereof by their expansion forces. Furthermore, as illustrated in  FIG. 18 , both the primary and secondary poppet valves  140  and  142  are biased toward their closed positions in the same direction, specifically, in the vertically upward direction. As further illustrated in  FIGS. 17 and 18 , the intake port  20  is blocked and the combustion chamber  15  is hermetically sealed from the intake passage  18  only when the secondary poppet valve  142  is in the closed position, i.e. when the secondary valve head  156  of the secondary poppet valve  142  engages the secondary valve seat  24   b  of the valve seat member  24 . On the other hand, if the primary intake valve  140  is closed, the secondary intake valve  142  is also in its closed position. 
   The intake valve assembly  130  further comprises a mechanical secondary valve lifter  152  immovably fixed to the elongated valve stem  144  of the primary poppet valve  140  between the distal ends thereof so as to extend radially outwardly from the valve stem  144 , as illustrated in  FIGS. 17-20 . Preferably, as illustrated in detail in  FIGS. 20 and 21 , the secondary valve lifter  152  is in the form of a collar immovably retained on the valve stem  144  by threaded connection. Further preferably, the secondary valve lifter  152  comprises an actuator member in the form of a threaded nut  166 , and a locknut  168  (shown in detail in  FIG. 6 ). Both the actuator member  166  and the locknut  168  threadedly engage a complementary cylindrical threaded surface  167  of the valve stem  144  of the primary poppet valve  140 . It should be understood that the locknut  168  is threaded over the threaded surfaces  167  of the valve stem  144  to lock the actuator member  166  in place on the primary poppet valve  140 . 
   As further illustrated in  FIGS. 20 and 21 , the actuator member  166  of secondary valve lifter  152  has an actuator surface  169  provided on axially bottom end thereof so as to extend radially outwardly from the valve stem  144 . In turn, the stem portion  154  of the secondary poppet valve  142  has a contact (back) surface  155  (preferably annular in configuration) provided on axially top end thereof and substantially complementary to the actuator surface  169  of the secondary valve lifter  152 . It will be appreciated that alternatively, the secondary valve lifter  152  can be substantially identical to the secondary valve lifter  52  according to the first exemplary embodiment of the present invention as illustrated in detail in  FIGS. 6 and 7 . 
   The operation of the intake valve assembly  130  according to the second exemplary embodiment of the present invention at both low and high speeds of the engine  110  is substantially similar to the low and high speed operation of the intake valve assembly  30  of the engine  10  according to the first exemplary embodiment of the present invention, as described in detail above and illustrated in  FIGS. 9-12 . 
     FIGS. 23-26  illustrate a third exemplary embodiment of an internal combustion engine of the present invention, generally depicted by the reference character  210 . Components, which are unchanged from the previous exemplary embodiments of the present invention, are labeled with the same reference characters. Components, which function in the same way as in the second exemplary embodiment of the present invention depicted in  FIGS. 17-22  are generally designated by the same reference numerals to which  100  has been added, sometimes without being described in detail since similarities between the corresponding parts in the two embodiments will be readily perceived by the reader. 
   The internal combustion engine  210  of the third exemplary embodiment of the present invention (shown in  FIG. 23 ) corresponds substantially to the internal combustion engine  110  of the second exemplary embodiment (shown in  FIGS. 17-19 ), and only an intake valve assembly generally depicted by the reference character  230 , which differs, will therefore be explained in detail below. 
   The intake valve assembly  230  according to the third exemplary embodiment of the present invention, as illustrated in detail in  FIGS. 24-26 , comprises a primary poppet valve  240  and a secondary poppet valve  242  mounted about the primary poppet valve  240  substantially coaxially therewith. The primary poppet valve  240  includes an elongated valve stem  244  and a disk-shaped primary valve head  246  provided at a lower end of the valve stem  244  for sealingly engaging the valve seat member  24 . The intake valve assembly  230  further includes a valve guide  48  (shown in  FIG. 23 ) supporting the valve stem  244  of the primary poppet valve  240  for reciprocatingly sliding in the cylinder head  14 . The valve guide  48  is fixed in the cylinder head  14  in any appropriate manner known in the art, such by press-fit connection. 
   The secondary poppet valve  242 , illustrated in detail in  FIGS. 24 and 26 , includes a hollow stem portion  254  and a secondary valve head  256  provided at a lower end of the stem portion  254  for sealingly engaging the secondary valve seat  24   b  of the valve seat member  24 . The secondary valve head  256  is conical or dome-shaped with a front surface  257  thereof configured to complement and nest over a back surface  247  of the valve head  246  of the primary poppet valve  240 , similarly to the intake valve assembly  130  according to the second exemplary embodiment of the present invention illustrated in  FIGS. 17 and 18 . The hollow stem portion  254  defines a stepped cylindrical bore  258  extending through both the stem portion  254  and the secondary valve head  256  of the secondary poppet valve  242 . Consequently, the hollow stem portion  254  of the secondary poppet valve  242  is reciprocatingly and coaxially mounted to and about the valve stem  244  of the primary poppet valve  240  to allow the secondary valve head  256  to slide back and forth between the valve seat member  24  of the intake port  20  and the primary valve head  246  of the primary poppet valve  240 . 
   The secondary poppet valve  242  is biased toward the closed position thereof by a secondary valve spring  260  disposed between the primary poppet valve  240  and the secondary poppet valve  242 , as illustrated in  FIGS. 23 and 24 . Preferably, the secondary valve spring  260  is in the form of a coil spring. It will be appreciated that the coil spring  260  of any configuration, such as, for example, cylindrical or conical, is within the scope of the present invention. As shown in detail in  FIGS. 24 , the coil spring  260  is disposed in the stepped cylindrical bore  258  within the hollow stem portion  254  of the secondary poppet valve  242  and mounted about the valve stem  244  of the primary poppet valve  240  substantially concentrically thereto. Specifically, a lower end (as shown in  FIG. 24 ) of the coil spring  260  engages a first support surface  248  (preferably annular in configuration) formed on a back surface  247  of the valve head  246  of the primary poppet valve  240 , while an upper end of the coil spring  260  engages a second support surface  270  (preferably annular in configuration) formed on the secondary poppet valve  242 . In other words, the coil spring  260  extends between the first support surface  248  of the primary poppet valve  240  and the second support surface  270  of the secondary poppet valve  242  within the stepped cylindrical bore  258  of the secondary poppet valve  242 . Further preferably, one of the distal ends of the secondary coil spring  260  is supported on the first support surface  248  of the primary poppet valve  240  by engaging a circular groove  249  formed thereon, while the opposite distal end of the secondary coil spring  260  is supported on the second support surface  270  of the secondary poppet valve  242  by engaging a circular groove  272  formed thereon. 
   Therefore, both the primary poppet valve  240  and the secondary poppet valve  242  are continuously (or normally) biased in the closed positions thereof by the primary and secondary valve springs  50  and  260 , respectively. Moreover, both the primary valve spring  50  and the secondary valve spring  260 , being normally contracted, bias the primary poppet valve  240  and the secondary poppet valve  242  in the closed positions thereof by their expansion forces. Furthermore, as illustrated in  FIG. 23 , both the primary and secondary poppet valves  240  and  242  are biased toward their closed positions in the same direction, specifically, in the vertically upward direction. As further illustrated in  FIG. 23 , the intake port  20  is blocked and the combustion chamber  15  is hermetically sealed from the intake passage  18  only when the secondary poppet valve  242  is in the closed position, i.e. when the secondary valve head  256  of the secondary poppet valve  242  engages the secondary valve seat  24   b  of the valve seat member  24 . On the other hand, if the primary intake valve  240  is closed, the secondary intake valve  242  is also in its closed position. 
   The intake valve assembly  230  further comprises a mechanical secondary valve lifter  152  immovably fixed to the elongated valve stem  244  of the primary poppet valve  240  between the distal ends thereof so as to extend radially outwardly from the valve stem  244 , as illustrated in detail in  FIG. 24 . Preferably, the secondary valve lifter  152  of the intake valve assembly  230  is substantially identical to the secondary valve lifter of the intake valve assembly  130  according to the second exemplary embodiment of the present invention. 
   The operation of the intake valve assembly  230  according to the third exemplary embodiment of the present invention at both low and high speeds of the engine  210  is substantially similar to the low and high speed operation of the intake valve assemblies  30  and  130  of the engine according to the first and second exemplary embodiments of the present invention, as described in detail above and illustrated in  FIGS. 9-12 . 
   Therefore, the present invention provides a novel intake valve assembly of an internal combustion engine that provides in effect variable valve timing and significantly improves both low and high speed performance of the engine, reduces emissions and improves fuel economy. Moreover, the present invention requires minimal low cost modification to adapt this invention to existing engines. 
   The foregoing description of the preferred exemplary embodiments of the present invention has been presented for the purpose of illustration in accordance with the provisions of the Patent Statutes. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. The embodiments disclosed hereinabove were chosen in order to best illustrate the principles of the present invention and its practical application to thereby enable those of ordinary skill in the art to best utilize the invention in various embodiments and with various modifications as suited to the particular use contemplated, as long as the principles described herein are followed. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains. Thus, changes can be made in the above-described invention without departing from the intent and scope thereof. It is also intended that the scope of the present invention be defined by the claims appended thereto.