Abstract:
A method for controlling intake valves in an internal combustion with intake valves operated by more than one type of actuation device includes steps to accomplish smooth transitions among intake valve operating modes.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This is a division of Application No. 09/650,434, filed Aug. 29, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a control method for engine valve actuation. 
     BACKGROUND 
     In U.S. Pat. No. 6,009,841, an engine with a hybrid valvetrain is disclosed in which one intake valve is actuated by a source other than a camshaft. This first intake valve is randomly operable meaning that the valve opening and closing events are independent of engine crankshaft position, thus, a fully variable valve. A second intake valve is actuated by a camshaft and includes a deactivator. Operation of the second valve may be discontinued or restored within one engine cycle, termed selectable intake valve herein. The exhaust valve(s) of the hybrid valvetrain is conventionally camshaft actuated. As disclosed in U.S. Pat. No. 6,009,841, the advantage of such a system over fully camless engine operation is improved fuel economy. 
     In U.S. Pat. No. 6,009,841, the method is described in which air is admitted using a randomly operable intake valve when the engine is operating in a lowest range in torque, using a selectable intake valve when the engine is operating in a medium range in torque, and using both the randomly operable intake valve and the selectable intake valve when the engine is operating in a highest range in torque. 
     In U.S. Pat. No. 5,647,312, a method is described in which air is admitted using a randomly operable intake valve when the engine is operating in a lower range in torque and speed and air is admitted using a selectable intake valve when the engine is operating at higher speed or higher torque. 
     Both U.S. Pat. No. 6,009,841 and U.S. Pat. No. 5,647,312 teach that making transitions among operating modes involves activating and deactivating valves. The inventors herein have recognized that by simply turning valves on and off leads to large excursions in engine torque which would be noticeable and annoying to the operator. The inventors herein have also recognized that the potential for hybrid valvetrain fuel economy benefits, as disclosed in U.S. Pat. No. 6,009,841, depends on suitable methods to accomplish transitions between operating modes; otherwise, the fuel efficiency potential is unrealized. 
     SUMMARY OF THE INVENTION 
     A method for controlling an internal combustion engine, the engine containing at least one cylinder, a randomly operable intake valve, a selectable intake valve with a predetermined closing time, a throttle valve in the engine&#39;s intake system, and an engine control unit, is provided. The method includes the steps of providing a randomly operable intake valve closing time which is after the predetermined valve closing and initiating operation of the selectable intake valve. In a further step, the throttle valve is closed and the randomly operable intake valve closing time is advanced so that substantially constant engine torque is provided. 
     A method for controlling an internal combustion engine, the engine containing at least one cylinder, a randomly operable intake valve, a selectable intake valve with a predetermined valve closing time, a throttle valve in the engine&#39;s intake system, and an engine control unit, has a first step of activating the randomly operable intake valve with a closing time after the predetermined valve closing time and a second step of opening the throttle valve and retarding the randomly operable intake valve closing time. 
     A method for controlling an internal combustion engine, the engine containing at least one cylinder, a randomly operable intake valve, a selectable intake valve with a predetermined valve closing time, a throttle valve in the engine&#39;s intake system, and an engine control unit, has a first step of advancing a randomly operable intake valve closing time and closing the throttle valve and a second step of ceasing operation of the randomly operable intake valve when additional advancement of the randomly operable intake valve closing time has no appreciable effect on inducted air remaining in the cylinder after closing of the randomly operable intake valve. 
     A method for controlling an internal combustion engine, the engine containing at least one cylinder, a randomly operable intake valve, a throttle valve in the engine&#39;s intake system, and an engine control unit, has the steps of closing the throttle valve and advancing a randomly operable intake valve closing time, in the event that the randomly operable intake valve closing time is retarded from a predetermined valve closing time and closing the throttle valve and retarding a randomly operable intake valve closing time, in the event that the randomly operable intake valve closing time is advanced from the predetermined valve closing time. The predetermined valve closing time is that which maximizes air inducted which is retained in the cylinder after closing of the randomly operable intake valve. 
     A method for controlling an internal combustion engine, the engine containing at least one cylinder, a randomly operable intake valve, a throttle valve in the engine&#39;s intake system, and an engine control unit, has the steps of opening the throttle valve and advancing a randomly operable intake valve closing time, in the event that the randomly operable intake valve closing time is advanced from a predetermined valve closing time and opening the throttle valve and retarding a randomly operable intake valve closing time, in the event that the randomly operable intake valve closing time is retarded from the predetermined valve closing time. 
     The advantages of the above methods are numerous. The transitions among operating modes provide a smooth torque trajectory. Thus, the transitions are not detectable by the operator. The time over which a transition occurs is brief, two to twenty engine cycles. The methods disclosed herein permit realization of the fuel efficiency potential of an engine with a hybrid electric valvetrain. 
     Other advantages, as well as objects and features of the present invention, will become apparent to the reader of this specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic of a hybrid valvetrain engine showing cross-sections of the cylinder head and the fuel vapor recovery and purge system to which aspects of the present invention apply; 
     FIG. 1A is a cross-section schematic of the cylinder head with the cross-section taken through the selectable intake valve; 
     FIG. 1B is a cross-section schematic of the cylinder head with the cross-section taken through the randomly operable intake valve; 
     FIG. 2 is a typical engine operating map onto which the regions employing the different modes of hybrid valvetrain operation are illustrated; 
     FIG. 3 a  is a graph of randomly operable intake valve lift profiles for advanced and retarded closing times according to an aspect of the present invention; 
     FIG. 3 b  is a graph of intake valve closing time -showing the quantity of fresh charge trapped in the cylinder; 
     FIG. 3 c  is a graph of intake valve closing time showing the trapped fresh charge reduction factor according to an aspect of the present invention; 
     FIG. 3 d  is a graph of intake valve closing time showing the temperature of the trapped fresh charge; 
     FIG. 4 a  is a graph of valve lift profiles for both randomly operable and selectable valves; 
     FIG. 4 b  is a graph showing the effect on trapped fresh charge of varying randomly operable intake valve closing when the selectable intake valve is operated concurrently; 
     FIG. 5 a  illustrates a time history of throttle position for a transition according to one aspect of the present invention; 
     FIG. 5 b  illustrates a time history of randomly operable intake valve closing for a transition according to an aspect of the present invention; 
     FIG  5   c  illustrates a time history of manifold absolute pressure for a transition according to an aspect of the present invention; 
     FIG  5   d  illustrates a time history of mechanical valve status for a transition according to an aspect of the present invention; 
     FIG. 6 is a flowchart of steps involved in making a transition from a region of lower engine speed and lower engine torque to other operating conditions according to an aspect of the present invention; 
     FIG. 7 is a flowchart of steps involved in making a transition from a region of higher engine speed and lower engine torque to other operating conditions according to an aspect of the present invention; 
     FIG. 8 a  is a flowchart of steps involved in making a transition from a region of higher engine torque to other operating conditions according to an aspect of the present invention; 
     FIG. 8 b  is a flowchart of steps involved in making a transition from a region of higher engine torque to other operating conditions according to an aspect of the present invention; 
     FIG. 9 is a flowchart of steps involved in making a transition between medium load in engine torque and lowest load in engine torque according to an aspect of the present invention; and 
     FIG. 10 is a flowchart of a method by which inputs of demanded engine torque and rpm. can be used to compute intake valve closing and throttle valve position as functions of time to command to actuators according to an aspect of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to FIG. 1, engine  10  contains at least one cylinder  4 . The cylinder head of engine  10  contains selectable intake valve  18 , randomly operable intake valve  16 , and exhaust valves  20 . Throttle valve  14  is disposed in the intake  12  to engine  10 . Combusted gases are rejected through exhaust line  24 . Engine control unit  26  is used to: activate and deactivate the selectable intake valve  18 , actuate the randomly operable intake valve  16 , and control the position of the electronically controlled throttle valve  14 . Various engine sensors  28 , such as an exhaust gas oxygen sensor, a mass air flow sensor, and an engine speed sensor, provide signals to the engine control unit  26 . 
     Referring now to the cross-section shown in FIG. 1A, the selectable intake valve  18  and the exhaust valve  20  are camshaft actuated by camshafts  2  and  3 , respectively. As such, the timing of the events is based on engine rotational position. In the cross-section shown in FIG. 1B, the randomly operable intake valve  16  is driven by an electromechanical actuator. An electrohydraulic actuator may also be used. The valve events of the randomly operable intake valve  16  are fully flexible and controlled by the engine control unit  26 . The exhaust valve  20  is actuated by camshaft  3 . Also shown in FIGS. 1A and 1B are a piston  5  which reciprocates within the cylinder  4 , an intake port  6 , and an exhaust port  8 . Intake ports  6  and exhaust ports  8  are coupled to respective intake and exhaust manifolds (not shown) to form respective intake line  12  and exhaust line  24 . 
     A conventional fuel vapor recovery and purge system for an automotive type engine also is shown in FIG.  1 . Engine  10  communicates with an intake  12  and an exhaust  24 . Fuel is metered into the intake by a fuel injector  42 . The throttle valve  14  is situated in the intake line  12 . The fuel tank.  48  contains an in-tank fuel pump  50  which supplies fuel through a fuel supply line  44  to the fuel injector  42 . The fuel tank  48  is replenished through the fuel filler tube  46 ; fuel cap  68  is removed to allow filling. The liquid components of the fuel fall through tube  62 . Gaseous components may proceed through vapor recovery line  66 . During fuel tank  48  filling, the volume not containing liquid fuel is occupied by gaseous components which are pushed into the vapor recovery lines  64  and  66  by the entering liquid fuel. The vapor recovery lines  64  and  66  lead to the carbon canister  52  which contains activated carbon to absorb fuel vapors. The carbon canister  52  is purged regularly. Purging is accomplished by opening valves  54  and  58  which allows fresh purge air to flow through the fresh purge air intake line  56 . The gases exiting carbon canister  52  contains both fresh air and fuel vapors which proceed through valve  58  and line  60 . Line  60  is introduced into the intake  12  downstream of the throttle valve  14 . Flow from the carbon canister circuit, through elements  56 ,  54 ,  52 ,  58 , and  60 , into the intake  12  and ultimately to the engine  10  for combustion occurs under the conditions of a vacuum in the intake  12  downstream of the throttle valve  14 . 
     In FIG. 2, an operating map of a typical spark-ignited engine is displayed. The upper curve  40  represents the maximum torque that the engine delivers as a function of speed. Operating regions are shown at which the randomly operable valve operates along, the selectable valve operates alone, and both valves operate. At higher torque across all speeds, region  30 , both intake valves are employed. Using both intake valves admits the maximum air possible thereby allowing the engine to develop its maximum torque. If intake air is admitted through one valve solely, the velocity through that intake port and valve is roughly twice as high as admitting the fresh charge through two valves, if the valves are of comparable size. This leads to higher turbulence in the cylinder at the time of combustion. Although high turbulence is a desirable condition at marginal combustion conditions, it leads to excessively rapid combustion or combustion harshness at robust operating conditions exemplified by region  30 . In region  36  of FIG. 2, the selectable intake valve  18  only is employed. Region  36  does not require maximum airflow as maximum torque is not demanded. Region  30  is to be selected when region  36  cannot admit sufficient airflow or when the combustion is too harsh using the selectable intake valve  18  only. 
     Criteria are provided for determining when a transition is desired. The decision when to make a transition from region  36  to region  32  or region  34  is based on whether the selectable intake valve  18  or the randomly operable intake valve  16  provides more efficient operation. Efficiency is based on energy consumed in rotating the intake camshaft, energy consumed in actuating the randomly operable intake valve  16 , and pumping losses, that is, the energy consumed in replenishing spent combustion gases with a fresh charge. 
     Region  32  is a region in which the intake valve closing of the randomly operable intake valve  16  can be adjusted to match the demand for engine torque. Lift profiles for the randomly operable intake valve  16  are shown in FIG. 3 a.  The intake valve closing time can be advanced or retarded from the timing which provides the maximum trapped fresh charge, as shown in FIG. 3 b.  By tailoring the intake valve closing time, the desired fresh charge is trapped. As shown in FIG. 3 b,  both retarding and advancing intake valve closing lessen the quantity of trapped fresh charge. In the case of retarded closing time, some of the inducted fresh air charge is pushed out of the combustion chamber prior to intake valve closing. The amount of fresh air retained in the cylinder is plotted in FIG. 3 b.    
     At retarded intake valve closing times the fresh charge temperature is increased; this is in contrast to advanced intake valve closing times which have little effect on fresh charge temperature, as shown in FIG. 3 c.  There may be reasons to prefer advancement or retardation of closing time which will become apparent in the development of the method. Nevertheless, controlling engine torque by adjusting intake valve closing time is preferred over throttling because it lessens pumping losses and hence leads to higher overall efficiency. As demanded torque is reduced, engine torque control by adjusting the closing time of the randomly operable intake valve  16  may lead to unstable combustion. Depending on the combustion stability desired for a particular application, a torque level may be determined, below which engine torque control is accomplished by throttling. Thus, the distinction between regions  32  and  34  is that throttling is employed within region  34 . 
     Combustion stability is related to the standard deviation of the power produced within a cylinder on a cyclic basis. A low standard deviation, that is, constant power produced from cycle to cycle, indicates stable combustion and vice versa. Herein, degradation in combustion stability refers to an increase in the standard deviation and an improvement in combustion stability refers to a decrease in the standard deviation. 
     Within region  34  of FIG. 2, it is desirable to mitigate the level of throttling necessary to control engine torque. Thus, the intake valve closing is as far advanced or retarded, depending on control method being employed, as possible while maintaining satisfactory combustion stability. It may be found that as engine torque is reduced (within region  34 ), i.e., the throttle valve is closed, the intake valve closing time must be altered to provide continued robust combustion. 
     As the demand for engine torque or engine speed changes during the course of operation, it will be found desirable to move between regions in FIG.  2 . The transition among the various regions of operation should be imperceptible to the operator of the vehicle. 
     A transition from region  32  to  34  is provided when the combustion stability in region  32  becomes poorer than desired. A smooth transition from region  32  to region  34  is accomplished by closing the throttle valve to attain the desired torque level. 
     The transition from region  32  to region  30  is provided when the valve closing time of the randomly operable intake valve  16  is at that which provides the maximum trapped fresh charge. It may be found that it is desirable to cause a transition from region  32  to  30  based on a limit imposed by tolerable combustion harshness rather than running out of authority in intake valve closing time of the randomly operable valve. Further increase in engine torque is achieved by activating the selectable intake valve  18 . Simultaneously, the randomly operable intake valve time is retarded such that the air flow before and after the transition cycle is substantially constant. 
     Engine torque control in region  30  is achieved by controlling the valve closing time of the randomly operable intake valve  16 . As illustrated in FIG. 2 b,  torque can be managed by late intake valve closing of the randomly operable intake valve  16 , whereas, early intake valve closing has minimal effect on the trapped charge. 
     A transition from region  36  to region  30  is desired when the selectable intake valve  18  alone does not supply sufficient fresh air charge. When a transition is demanded, the randomly operable intake valve  16  is activated with a retarded closing time in which trapped fresh charge is unaffected. The closing time of the randomly operable intake valve  16  is advanced to increase trapped fresh charge mass as needed. The reverse transition (from region  30  to region  36 ) is accomplished when the trapped fresh charge could be provided by the selectable intake valve  18  alone. It may be found that instead of basing the transition on capacity constraint the combustion harshness is the determining factor. That is, there may be operating conditions that the selectable intake valve  18  alone can provide sufficient fresh charge, but the resulting combustion harshness is beyond a desired level. In this case, a transition from region  36  to region  30  is accomplished based on combustion harshness. 
     Region  32  or  34  is preferred over region  36  when the following equation holds true: 
     
       
         
           PWroiv+Wroiv+FW′siv+CLroiv&gt;PWsiv+FWsiv+CLsiv 
         
       
     
     in which PWroiv is the pumping work of the engine with the randomly operable intake valve  16 , Wroiv is the work extracted from the engine to drive the randomly operable intake valve  16 , PWsiv is the pumping work of the engine using only the selectable intake valve  18 , FWsiv is the friction work lost in driving the selectable intake valve  18 , FW′siv is the friction work lost in driving the camshaft of the actuated intake valve when the selectable intake valve  18  is deactivated. FW′siv is considerably less than FWsiv, but not negligible, due to rotational friction in the camshaft even when the selectable intake valve  18  is deactivated. CLroiv and CLsiv are cycle losses associated with operating the randomly operable and selectable intake valves, respectively. Cycle losses are difference between the ideal cycle work that could obtained for an Otto cycle and the actual amount generated. The actual work generated is less than ideal cycle work due to heat transfer, combustion time losses (i.e., finite combustion duration), combustion phasing, and others. The choice between using the selectable intake valve  18 , region  36 , and using the randomly perable intake valve  16 , regions  32  and  34 , is based on minimizing the losses due to valve actuation and pumping work. When the above equation is violated, the control system selects region  36 ; i.e., the selectable intake valve  18  is actuated only. 
     The quantities described above may be computed or estimated in the following ways. Pumping work (PW) is a function of manifold pressure, engine rpm, and engine displacement primarily and could be contained in a lookup table or in equation form in the engine&#39;s control unit. The energy loss associated with driving the intake camshaft is primarily a function of engine rpm. This is a quantity which could be measured in a representative engine and the data applied to all engines of the same type. This could be lookup table or an equation in the engine&#39;s control unit. There would be two distinct tables or equations, one for the case with the selectable intake valve  18  activated and for the case when it is deactivated. The power that is absorbed from the engine to actuate the randomly operable intake valve  16  is a quantity that would be determined in the course of the development of the randomly variable intake valve. The design variables that would affect the power requirement is the size of the valve, the lift profile that is selected, and the drivers used to actuate the valve. For example, a faster valve lift expends more energy. The factors external to the valve design that would determine the power consumption is the efficiency of the engine alternator in generating electrical power, system losses in storing and retrieving energy, losses in transforming voltage, and pressure in the cylinder at the time of valve actuation. All of these quantities, except for cylinder pressure, depend on the system design. Thus for a given design, the power consumed in actuating the randomly operable intake valve  16  is a function of cylinder pressure primarily. Other dependencies may be determined in- the course of development. The randomly operable intake valve  16  alternatively may be actuated electro-hydraulically or by other means. In the electro-hydraulic case, power is consumed in driving a pump used to develop hydraulic fluid pressure, hydraulic losses in system lines (highly dependent on hydraulic fluid temperature), and electrical losses in actuating controlling solenoid valves as well as the effect of the lift profile, valve size, and cylinder pressure effects mentioned above. 
     Accomplishing the transition between regions  32  or  34  to region  36  is shown in the timeline shown in FIG.  5 . If at the time of the transition, the randomly operable intake: valve  16  is operating with an advanced closing time, in the next engine cycle, a retarded closing time is selected which traps the same mass of fresh charge. In FIG. 3 b,  trapped fresh charge decreases on both sides of the peak. Thus, there is a retarded timing which can be selected in which trapped fresh charge and, thus, developed engine torque- matches that of the advanced timing. In the next engine cycle, the selectable intake valve  18  can be enabled. The trapped fresh charge is not appreciably affected by activating the selectable intake valve  18  when the randomly operable intake valve  16  is operating at retarded valve closing time. Over the next engine cycles, the randomly operable intake valve closing time is advanced simultaneously as the throttle valve is closed. These operations are controlled in concert such that the trapped fresh charge is substantially constant herein meaning providing substantially constant engine torque or changing smoothly along the desired torque trajectory. A change in intake valve closing time of the randomly operable intake valve  16  can be accomplished in a single engine cycle. In contrast, even though a change in throttle valve position can be accomplished rapidly, intake manifold filling considerations cause the intake manifold pressure to react over several engine cycles. Thus, the transition illustrated in FIG. 4 occurs in a matter of a few to a couple dozen engine cycles. As the timing of the randomly operable intake valve  16  continues to be advanced, eventually it no longer has an affect on the amount of trapped fresh charge. At which point, it can be turned off. 
     The reverse transition (region  36  to region  32  or region  34  of FIG. 2) occurs analogously: the randomly operable intake valve  16  is activated at an advanced timing in such a manner to not impact trapped fresh charge. The closing time of the randomly operable intake valve  16  is retarded concurrently with opening the throttle such that the trapped fresh charge is substantially constant. When the randomly operable valve closing time is sufficiently retarded, the selectable intake valve  18  is no longer having any affect on the trapped fresh charge and may be turned off. 
     The distinction between making a region  32  to region  36  transition and a region  34  to region  36  transition is that the initial throttle position is fully open and the initial manifold pressure is atmospheric in the former case and partially open, i.e., less than atmospheric, in the latter case. 
     It is desirable to limit the number of transitions which the engine&#39;s control unit must manage. Thus, as the demanded engine torque and speed approaches a new region (within FIG.  2 ), the transition may be delayed until the demanded engine torque and speed traverse the border by a predetermined amount. The boundaries of FIG. 2 can be considered to be bands. As a boundary is approached, the transition is not made until the demanded operating condition exceeds the farther edge of the boundary. That is, a transition from region  36  to region  30  would occur at the higher engine torque edge of the boundary between the two regions. Conversely, a transition from region  30  to region  36  would occur at the lower engine torque edge of the boundary between the two regions. 
     Transitions among regions of FIG. 2 which involve closing or opening the throttle valve, may require a minimum of one and as many as  20  engine revolutions due to manifold filling lags. The throttle valve can be actuated on the order of 100 ms. However, causing air to fill the manifold takes multiple engine revolutions to overcome the inertia of the gases. 
     Herein, substantially constant engine torque means either a constant torque or a trajectory in torque along the desired path; i.e., engine torque deviation from the desired trajectory is small or unnoticeable to the operator of the vehicle. 
     Retarding or advancing spark timing is a powerful tool which can be used to smooth transitions. The advantage of spark advance is that it can be changed in one engine cycle. Furthermore, spark timing has a wide range of authority in controlling engine torque. Spark timing, however, negatively impacts fuel economy, typically. Thus, it is a secondary tool to refine transitions. 
     A quantity which may be determined in development is rpm t  (identified in FIG. 2) which is the threshold rpm between regions  32  and  36 . This quantity is discussed below in regard to the control strategy employed in selecting among regions for operation. 
     FIG. 3 a  shows valve lift profile for early and late closing time of the randomly operable intake valve  16 . FIG. 3 b  indicates the resulting trapped fresh charge as a function of the closing time of the randomly operable intake valve  16 . A maximum trapped fresh charge occurs at a particular valve closing time. At valve closing times advanced or retarded from that maximum reduce the quantity of trapped fresh charge. The amount of trapped fresh charge is the primary factor determining the amount of torque that the engine produces. When only the randomly operable intake valve  16  is operating, either a retarded timing or an advanced timing may be selected to give a particular desired trapped fresh charge. The trapped fresh charge can be normalized by dividing the trapped fresh charge at any given intake valve closing time. (IVC) by maximum trapped fresh charge. The normalized quantity is termed trapped fresh charge reduction factor. As shown in  3   c,  trapped fresh charge reduction factor ranges between 0 and 1. 
     When operating within region  34  of FIG. 2, the primary method by which engine torque is controlled is by throttling and secondarily by intake valve close timing. As shown in FIG. 3 b,  either an advanced or retarded valve close timing could be used to provide a desired trapped fresh charge. In FIG. 3 d,  the resulting fresh charge temperature is shown. At advanced timings, the fresh charge temperature is substantially constant; whereas, fresh charge temperature increases as a function of retardation. This is due to the fact that at advanced timings, the intake valve is closed prematurely to limit the amount of trapped fresh charge. At retarded timings, the cylinder is filled with fresh charge and as the piston moves up, the fresh charge is pushed out of the cylinder. In this case, the fresh charge comes in contact with the hot cylinder surfaces and hot intake valve multiple times and is heated more than the advanced intake valve closing time case. Higher fresh charge temperature may be found to be beneficial in improving combustion stability. Thus, within region  36 , in which combustion stability is a concern, retarded closing time of the randomly operable intake valve  16  may be preferred due to improved combustion stability. 
     FIG. 4 a  shows the valve lift profiles for operation with both the selectable intake valve  18  and the randomly operable intake valve  16 . FIG. 4 b  indicates the resulting trapped fresh charge as a function of the closing time of the randomly operable intake valve  16 . A maximum trapped fresh charge occurs at a particular valve closing time of the randomly operable intake valve  16 . At times advanced of the maximum, a very slight reduction in trapped fresh charge may occur. That is, the randomly operable intake valve  16  does not have substantive authority over trapped fresh charge by advancing the closing time. In FIG.4 b,  the randomly operable intake valve  16  closing time does have a range of authority over trapped fresh charge when the closing time is retarded beyond the closing time of the selectable valve, i.e., retarded timing. 
     In FIG. 2, the regions among which the engine control unit must select operation are illustrated. In addition to providing control within each region, smooth transitions between regions must be managed. A time line of a transition between region  32  to region  34  is outlined in FIGS. 5 a-d.  If the randomly operable intake valve  16  is operating at an advanced timing  50 , it must be switched to a retarded timing which provides identical trapped fresh charge at the start of the transition, as shown in FIG. 5 b.  The ability to find a retarded timing which provides the same trapped fresh charge as an advanced timing is supported by FIG. 3 a,  as discussed above. If the randomly operable intake valve  16  is operating at a retarded timing  52 , no action is necessary. The selectable valve can be opened in the same engine cycle or shortly thereafter, FIG. 5 c . The throttle valve (designated TP for throttle position in FIG. 5 a ) is closed. Simultaneously, the randomly operable intake valve closing time is retarded, FIG. 5 d , such that the desired trajectory in engine torque is achieved. As the randomly operable intake valve timing is retarded beyond a certain point, it is no longer impacting the quantity of trapped fresh charge. At this point, the randomly operable intake valve  16  can be turned off, FIG. 4 b.    
     For the purposes of discussing the control strategy, region  30  of FIG. 1 is called region three, region  36  of FIG. 1 is called region two, and the combined region containing both regions of  32  and  34  of FIG. 2 is called region one. For purposes of discussing control strategy in FIG. 9, region  32  and  34  of FIG. 2 are termed regions four and five, respectively. 
     FIG. 6 indicates the steps that would be taken to assess whether a transition from region one were called for and then to make the transition. The system is operating in region one in block  100 . Blocks  102 ,  104 , and  106  are assessment steps as to whether a transition is warranted. The order in which assessment blocks  102 ,  104 , and  106  occur is arbitrary. In block  102 , the losses operating in region  1  compared to losses operating in region two are evaluated. These losses are described above as consisting of all losses associated with operating the selectable intake valve  18  compared to the randomly actuated intake valve. The object of block  102  is to select the more efficient operating region. If region two is more efficient, a transition from region one to region two is accomplished beginning in block  108 . The intake valve timing of the randomly operable intake valve  16  is compared to that which would give maximum trapped fresh charge in block  110 . If the timing is advanced from the maximum trapped fresh charge condition, the intake valve timing is changed to a retarded intake valve timing which gives substantially similar trapped fresh charge in block  112 . The selectable intake valve  18  is activated in block  114 . The purpose for the change from an advanced timing to a retarded timing in block  112  is illustrated in FIGS. 3 and 4. With only the randomly operable intake valve  16  activated as in FIG. 3, trapped fresh charge drops on both sides of the maximum. But, with both valves open as in FIG. 4, trapped fresh charge remains substantially constant on the advanced side and drops on the retarded side. To allow valve timing a range in authority in controlling trapped fresh charge, the randomly operable intake valve  16  should be operating at a retarded timing. In block  116 , the throttle valve is closed concurrently with advancing the valve closing time of the randomly operable intake valve  16 . This is accomplished such that engine torque is substantially constant. Block  118  is a check to determine whether the closing time of the randomly operable intake valve  16  is sufficiently advanced such that it no longer has authority over fresh trapped charge. If not, block  116  is repeated. When the test in block  118  fails, the randomly actuated intake valve can be turned off in block  120  because it no longer affects engine torque. Block  122  indicates that the engine is operating within region two. 
     In block  102  of FIG. 6, if losses in region one are less than region two, checks  104  and  106  are made. In block  104 , a check is made to determine if the randomly operable intake valve  16  admits sufficient trapped fresh charge so that demanded engine torque can be satisfied. If not, a transition from region one to region three is requested in block  124 . If the test of block  104  is passed, an additional test is made in block  106  to determine if the combustion harshness is acceptable. Because the randomly operable intake valve  16  induces more turbulence in the combustion gases than when intake gases are inducted through both valves, the resulting combustion can become too rapid or harsh. In block  106 , existence of harsh combustion causes a request for a transition from region one to region three. The selectable intake valve  18  is turned on after the randomly operable intake valve close timing is retarded in block  126 . The closing time for the randomly operable intake valve  16  is selected so that substantially constant engine torque is achieved during the transition of block  130 . Now the engine is operating in region three in block  128 . 
     FIG. 7 shows steps involved in checking for and making transitions from operating region two, block  200 . The checks to determine if a transition is warranted are blocks  202 ,  204 , and  206 , which can performed in any order. Block  202  checks whether region one or two is more efficient. If region one is more efficient, a transition from region two to region one is requested, block  208 . The randomly operable intake valve  16  is activated with a closing time advanced of maximum trapped fresh charge in block  210 . By advancing the closing time, the randomly operable intake valve  16  has a minimal effect on trapped fresh charge. The closing time is retarded in block  212  until in block  214  further retardation would affect trapped fresh charge. When block  214  is satisfied, the randomly operable intake valve closing time is retarded concurrently with opening the throttle valve, block  216 . This is accomplished such that engine torque remains substantially constant until the desired throttle opening is achieved. The selectable intake valve  18  may be turned off as shown in block  218 . Blocks  216  and block  218  may be accomplished in arbitrary order. Now the engine is operating in region one, block  220 . 
     In block  202  of FIG. 7, if the losses of region two re less than those of region one, checks are made in blocks  204  and  206  to determine whether sufficient engine torque can be developed using the selectable intake valve  18  alone and whether combustion harshness is acceptable, respectively. If either of these checks fails, a transition from region two to region three is demanded, block  222 . The randomly operable intake valve  16  is turned on with an advanced closing time. The valve closing time is retarded in block  226  with a check to see if further retardation will affect trapped fresh charge, block  228 . When the test of block  228  passes, the intake valve timing is retarded further while the throttle valve is opened in such a manner to maintain substantially constant engine torque in block  230 . The throttle is opened fully or to a lesser opening depending, if desired to support other functions. The engine is now operating within region three, block  232 . 
     FIG. 8 a  shows steps involved in checking for and making transitions from operating region three, block  300 . Blocks  302 ,  304 ,  306 ,  316  and  318  are checks to determine if a transition is warranted. In block  302 , the current rpm is compared to rpm t , which is a predetermined value of rpm which indicates the boundary between regions one and two (rpm t  is shown in FIG. 2.) The purpose of step  302  is to determine if a transition is made whether it is to region one or region two. If rpm is less than rpm t , then checks in block  304  and  306  determine whether enough engine torque can be produced and whether combustion harshness would be acceptable with the randomly operable intake valve  16 . If either check  304  or  306  fail, control is returned to block  300 . If both checks  304  and  306  pass, a transition to region one is made in block  308 . In block  312 , the selectable intake valve  18  is closed, the randomly operable intake valve  16  is advanced to maintain engine torque. If rpm is greater than rpm t , checks  316  and  318  are made to determine whether enough engine torque can be produced and whether combustion harshness would be acceptable with the selectable intake valve  18 . Failure in either check  316  or  318  returns control to block  300 , operating region three. If both check  316  and  318  pass, a transition to region two is requested in block  320 . The throttle valve is closed and the closing time of the randomly operable intake valve  16  is advanced in block  322 . A check in  324  determines whether further advancement of the closing time of the randomly operable intake valve  16  affects trapped charge. If so, return to block  322 . If not, deactivate the randomly operable intake valve  16  in block  326  and the system is now operating within region two in block  328 . 
     FIG. 8 b  indicates an alternative to FIG. 8 a.  Blocks  302 ,  304 ,  306 ,  316 , and  318  of FIG. 8 a  are replaced with blocks  350 ,  352 , and  354  of FIG. 8 b.  In block  350 , four questions with binary responses are asked. Question one is “can enough engine torque be produced with the selectable intake valve  18 .” Question two is “can enough engine torque be produced with the randomly operable intake valve  16 .” Question three is “whether combustion harshness would be acceptable with the selectable intake valve  18 .” Question four is “whether combustion harshness would be acceptable with the randomly operable intake valve  16 .” Block  352  shows the direction of control based on the responses of the four questions. Control proceeds along path A if the four answers are all positive or yes. Path A leads to block  354  in which a check is made to determine if the losses in region one are less than losses in region two. If positive result from block  354 , a transition from region three to region one is demanded, block  308 . If negative, a transition from region three to region two is demanded, block  320 . Control proceeds along path B if the answers to questions one and three are positive and either one or both of the answers to questions two and four are negative. Path B calls for a transition from region three to region two, block  320 . Similarly, a positive answer to both questions two and four (with either one or both answers to questions one and three negative) leads to control along path C which leads to block  312 , a transition from region three to region one. Any other result than those discussed above, leads to result D which is a return to block  300 , operating region three. The remaining control steps of FIG. 8 b  are discussed above in regard to FIG. 8 a.    
     In FIG. 9, if operating within region four, block  400 , a transition would be requested when combustion stability is unacceptable, block  402 . To make the transition block  404 , the throttle valve is closed while the randomly operable intake valve closing time is advanced maintaining substantially constant engine torque, block  406 . The engine is operating within region five, block  408 . 
     If operating within region five, block  420  of FIG. 9, a check is made to determine if combustion stability would be acceptable without throttling, block  422 . If so, a transition from region five to region four is requested, block  424 . The throttle is opened while intake valve closing time is retarded such that substantially constant engine torque is developed during the transition in block  426 . The engine is now operating within region four, block  428 . 
     In FIG. 8, checks are made in blocks  316  and block  304  to determine whether enough engine torque can be produced with the selectable intake valve  18  and the randomly operable intake valve  16 , respectively. Checks are made concerning combustion harshness in blocks  306  and  318 . It may be found in the course of development that combustion harshness, torque production, or other measure is the sole criteria by which a transition should be demanded. The strategies discussed in relation to FIGS. 6-9 may be simplified in accordance. 
     An algorithm to calculate throttle position and intake valve closing for transitions involving a throttle change to be disposed in the electronic control unit is outlined below. Specifically, transitions involving a throttle valve change are any transitions involving region  36  and transitions between regions  32  and  34  of FIG.  2 . Desired trapped fresh charge (des_trp_chg) depends on demanded or desired engine torque (des_tq) and engine speed (rpm), i.e., 
     
       
           des   —   trp   —   chg=fnc ( des   —   tq,  rpm). 
       
     
     In an engine with fixed valve events, des_trp_chg, and desired manifold pressure (des_MAP) are related by 
     
       
         
           des 
           — 
           MAP=a*des 
           — 
           trp 
           — 
           chg+b 
         
       
     
     where a and b are functions of rpm. 
     In an engine with flexible valve events, the effect of valve timing can be included as 
     
       
           des   —   MAP=c*des   —   trp   —   chg/trp   —   chg   —   rf+d   (1) 
       
     
     where c and d are functions of rpm and trp_chg_rf is a trapped fresh charge reduction factor, defined as. 
     
       
           trp   —   chg   —   rf=trp   —   chg ( IVC )/ trp   —   chg ( IVC   m ) 
       
     
     where trp_chg(IVC) is the trapped fresh charge at the given IVC and trp_chg(IVC m ) is the trapped fresh charge at IVC m , which is the IVC which gives the maximum trapped fresh charge. IVC is intake valve closing time. It is apparent from FIG. 3 c,  that trp_chg_rf ranges between 0 and 1 and that 
     
       
           trp   —   chg   —   rf=fnc ( IVC ) 
       
     
     at a given MAP and rpm. 
     Or, in the general case, trp_chg_rf=fnc (IVC, MAP, rpm), the detailed form of the equation will be determined in development. 
     Solving for trp_chg_rf in equation 1 above, 
     
       
           trp   —   chg   —   rf= ( c*des   —   trp   —   chg )/( des   —   MAP−d )= fnc ( IVC, MAP,  rpm). 
       
     
     As mentioned above, the relationship between IVC and trp_chg_rf is not known, a priori. However, given such a relationship, the equation can be solved for IVC. IVC depends on 
     
       
           IVC=fnc (,  MAP, des   —   trp   —   chg,  rpm). 
       
     
     The desired throttle position (TP) is related to MAP through sonic and subsonic relationships. These relationships are known to one skilled in the art and are the subject of U.S. Pat. No. 5,526,787, which is assigned to the assignee of the present invention and which is incorporated by reference herein. It is covered, also, within “Internal Combustion Engine Fundamentals” by J. B. Heywood (McGraw Hill, 1988), which is hereby incorporated by reference herein. 
     
       
           TP=fnc ( MAP,  rpm). 
       
     
     The relationships above hold for a single operating condition. However, transitions which involve opening or closing the throttle valve (between region  36  and another other region of FIG. 2) occur over an interval and require concurrent ramping of IVC and throttle position. The invention herein uses a ramp in MAP to define ramps in IVC and TP. MAP is ramped based on the desired final MAP (des_MAP) and the current or initial MAP (MAP i ). A change in throttle position, with a typical electronically controlled throttle valve, can occur much faster than the manifold pressure can react due to the inertia of the gases. Depending on the magnitude of the change desired and engine speed, it may take from about one to twenty engine cycles for the manifold pressure to reach its equilibrium level. The desire for smooth transitions among regions of FIG. 2 suggests that the ramp in MAP be sufficiently slow such that manifold filling lag is minimal. A linear ramp in MAP may be preferred with the end points defined by MAP i  and des_MAP and the slope determined by manifold filling considerations. The ramp in MAP is MAP(t). The ramps in both IVC and TP are based on this ramp in MAP with the additional constraint of delivering des_trp_chg. Thus, IVC(t) and TP(t) can be computed based on manifold pressure ramp and desired trapped fresh charge: 
     
       
           IVC ( t )= fnc ( MAP ( t ),  des   —   trp   —   chg,  rpm) 
       
     
     and 
     
       
           TP ( t )= fnc ( MAP ( t ),  des   —   trp   —   chg,  rpm) 
       
     
     with rpm as a given. 
     In FIG. 10, the steps in computing IVC(t) and TP(t) are outlined. Inputs to block  500  are the desired or demanded torque, des_tq, and engine speed, rpm. Within block  500 , the desired trapped fresh charge (des_trp_chg) is computed. In block  502 , the desired manifold pressure (des_MAP), that is, the final MAP at the completion of the transition, is computed with des_trp_chg, rpm, and the operational status of the intake valves at the end of the transition. The operational status of the intake valves for a transition from region  36  to region  30  of FIG. 2 is for both the randomly operable intake valve  16  and the selectable intake valve  18  to be activated. Within block  504 , the ramp in MAP, des_MAP(t), is computed with inputs of initial MAP, MAP i , and des_MAP. As discussed above, a MAP trajectory may be linear, block  512 , and takes into account intake manifold filling considerations. The desired MAP trajectory characteristics may be reduced to an algorithm and disposed within the engine&#39;s control unit. The output of block  504 , des_MAP(t), along with des_trp_chg are inputs to both blocks  506  and  508  in which IVC(t) and TP(t) are computed, respectively. Typical automotive engine control systems incorporate a measure of delivered fresh charge, shown as block  510 . Measured fresh charge is input to block  510  in which trapped fresh charge can be computed. The actual trapped fresh charge is input to block  508  allowing error checking and updating of the throttle position equations or lookup tables. 
     Referring again to FIG. 1, modern automotive vehicles are equipped with vapor recovery and purge systems to manage fuel vapors evolving from the liquid fuel in the fuel tank  48  due to temperature cycling and due to fuel vapors that are displaced in the process of refilling the fuel tank. The system contains a carbon canister  52  which absorbs the fuel vapors. When a purge is called for by the engine&#39;s electronic control unit  26 , fresh air is drawn through the canister  52 . The fresh air and desorbed vapors are inducted into the engine entering the engine on the downstream side of the throttle valve  14 . Purge vapors are drawn through the engine by virtue of the vacuum or depressed pressure in the intake  12 . 
     As conventional automotive spark-ignition, internal combustion engines  10  are throttled under most operating conditions, scheduling carbon canister purge is not usually an impediment. Additional measures must be taken to purge an internal combustion engine  10  with a hybrid valvetrain. Within region  32  (FIG.  2 ), in an internal combustion engine  10  with a hybrid valvetrain the throttle  14  is open. Thus, there is no vacuum in the intake  14  (FIG.  1 ). If the engine control unit calls for a purge, the throttle  14  may be closed to accommodate the need to purge the carbon canister  52 . To overcome the torque reduction caused by closing the throttle  14 , the closing time of the randomly actuated intake valve  16  is changed to allow induction of sufficient trapped fresh charge to meet the demand for engine torque. When the engine control unit determines that the carbon canister  52  has been purged, the throttle valve  14  may be reopened while concurrently altering the closing time of the randomly actuated intake valve  16  to meet demanded engine torque. A transition to purging while operating within region  32  of FIG. 2 can be accomplished in the same manner as a transition from region  32  to region  34  as discussed above. The distinction is that the transition is requested based on a need to purge the vapor recovery system rather than combustion stability. Analogously, a transition from a purge is accomplished the same as a transition from region  34  to region  32  described above. 
     In high torque operating region  30  in FIG. 2, throttling is not employed. It may be possible to schedule purging of the fuel vapor purge system such that sufficient purge time would be scheduled outside region  30 . In normal engine operation, region  30  is rarely accessed. However, if purging were desired, the approach described above for purging region  32  would apply to region  30  with the distinction that region  30  cannot tolerate as much throttling due to the need to develop high torque. 
     While the best mode for carrying out the invention has been described in detail, those familiar with the art to which this invention relates will recognize alternative designs and embodiments for practicing the invention. Thus, the above-described preferred embodiment is intended to be illustrative of the invention, which may be modified within the scope of the following claims.