Variable valve timing

Intake valve timing information is provided for control and diagnostics in a variable valve timing internal combustion engine application in which check valves are disposed in intake passages connecting engine cylinders to an intake plenum for impeding backflow out of the intake passages and into the intake plenum, by timed sampling of intake passage pressure and a determination of pressure change over an intake valve closing event indicating an amount of backflow out of the cylinder and into the intake passage. The amount of backflow indicates intake valve timing as the crank angle at the time of intake valve closing.

FIELD OF THE INVENTION 
This invention relates to internal combustion engine variable valve timing 
and, more particularly, to measuring valve event timing in an engine 
having variable valve timing control. 
BACKGROUND OF THE INVENTION 
The benefits of variable valve timing to control internal combustion engine 
cylinder intake charge are well-established. It is further known that 
backflow may be prevented in variable valve timing applications through 
placement of check valves, such as reed valves, in the engine intake 
manifold. Still further, active intake charge throttling for each cylinder 
has been proposed using a butterfly or rotary cylinder inlet valve. The 
control under such mechanizations, such as that described in U.S. Pat. No. 
5,372,108, requires instantaneous valve timing information. Crankshaft and 
camshaft phasing diagnostics may further be provided through the benefit 
of precise valve event timing. Absolute internal combustion engine 
camshaft position information, which may only be available at one camshaft 
angular position for each engine cycle, may be used to derive a rough 
approximation of valve timing in variable valve timing systems. Camshaft 
position sensor signal processing delays and engine speed transients can 
significantly reduce the accuracy of such a derivation. Indeed, the 
inaccuracy of the derivation may disqualify it for use in many engine 
control and diagnostic applications. 
An approach to determining valve timing in variable valve timing engine 
applications with sufficient precision for application in engine control 
and diagnostics is therefore desirable. It is further desired that such an 
approach add minimal cost and complexity to current variable valve timing 
control and diagnostic applications. 
SUMMARY OF THE INVENTION 
The present invention is a desirable approach to determining valve timing 
in internal combustion engine variable valve timing applications as it 
requires minimal additional control or diagnostic hardware over that of 
conventional control and diagnostic systems, and as it is sufficiently 
accurate to support even sophisticated control and diagnostic 
applications. 
More specifically, the approach of the present invention relies on 
synchronous sampling of a trapped volume pressure before and after an 
active engine cylinder intake valve closing event. A simple, often already 
available pressure transducer may be used to sample the pressure. A first 
pressure sample is taken when the cylinder is at a known position with a 
known cylinder volume. This first sample provides a baseline pressure 
upstream of the active cylinder between a check valve and the active 
cylinder intake valve. The check valve is positioned in the engine inlet 
air passage between an intake plenum and the cylinder intake valve. A 
second pressure sample is taken when the intake valve for the active 
cylinder is closed, such as during the active cylinder compression stroke. 
In typical engine applications, the active cylinder intake valve closes 
just after the piston in the active cylinder reaches its bottom dead 
center position, resulting in backflow of a volume of cylinder charge out 
of the cylinder and into the upstream cylinder intake passage. The reed 
valve in the passage will close shortly after the beginning of the active 
cylinder compression stroke and before the active cylinder intake valve 
closes, trapping the backflowing cylinder charge between the closed reed 
valve and the closing intake valve. The amount of delay between active 
cylinder bottom dead center position and the time of intake valve closing 
will dictate, for given engine operating conditions, the amount of charge 
pushed out of the cylinder past the intake valve. The backflowing charge 
will cause a measurable disruption in the air pressure in the region 
between the intake valve and the check valve. The magnitude of this 
disruption may be used to derive the time of active cylinder intake valve 
closing. The disruption is provided for by taking the first pressure 
sample before any backflow of the cylinder charge is likely to have 
occurred. The second sample is then taken after the active cylinder intake 
valve closes, trapping the backflowing charge between the closed check and 
intake valves. The change in cylinder charge volume which indicates the 
amount of backflowing charge during the time period between the two 
pressure samples, is determined directly from the pressure disruption and, 
after accounting for any flow losses that may have occurred, the change in 
cylinder charge volume is used in a direct derivation of the time of 
active cylinder intake valve closing, which may be expressed for 
convenience as the engine crankshaft angular position at the time of 
active cylinder intake valve closing. Precise variable valve timing 
diagnostics and control may be carried out using the precise engine 
position information, including a closed-loop valve timing control, in 
which a desired phase offset may be compared to the actual measured 
offset, and the offset error driven controllably toward zero through a 
classical or modern control technique. Still, further, a check valve 
diagnostic may be provided by comparing the pressure disruption between 
successive cylinder events or between events of a single cylinder to 
determine if the variation in the pressure disruption is acceptably small 
and stable. 
In yet a further aspect of this invention, more than one intake valve 
closing event timing measurement may be made for each engine cycle simply 
by carrying out the pressure disruption measurement for a plurality of the 
cylinders of the engine. Precise phasing information may thereby be 
provided for use in the described control and diagnostics functions. Still 
further, the approach of the present invention may be self-diagnosing 
through a comparison of successive valve timing measurements and a 
determination of measurement correlation reasonableness. Unreasonably poor 
correlation of the timing information may indicate a fault condition 
arising, for example, from a pressure or synchronization inaccuracy, or 
from a poor seal in the system. 
The precise measurement for control and diagnostics of the present 
invention, even when applied to determine a plurality of intake valve 
timing events, adds little cost to engine control and diagnostics, as the 
required sensors are already available on many current engine 
applications. The derivation of timing from the sensed pressure and the 
resulting control and diagnostics may be carried out through control and 
actuation hardware already available on many current engine applications.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a spark ignited four stroke cycle internal combustion 
engine 10 includes a cylinder block 11 having multiple cylinders 12 only 
one being shown. Each cylinder has a piston 14 reciprocable therein and 
connected by a connecting rod 15 to a crankshaft 16. A cylinder head 18 
closes the end of the cylinder 12 above the piston and includes at least 
one intake valve 19 and one exhaust valve 20 respectively controlling an 
intake port 22 and an exhaust port 23 connecting with the cylinder. 
Variable timing valve actuation means 24 are provided of any conventional 
type for varying the piston crank angle corresponding to an angular 
position of the crankshaft 16 relative to intake and exhaust valve timing 
corresponding to an angular position of the camshaft 36 (FIG. 2). The 
intake port 22 connects with a manifold runner 26 to define an intake 
passage 27 into which fuel is sprayed by a fuel injector 28. Upstream of 
the fuel injector 28 is an intake plenum 30 connecting with each of the 
passages 27 for each respective engine cylinder and connecting with an air 
intake tube 31 having a main throttle 32 of the butterfly or rotary type. 
An auxiliary one way or check valve 34, such as a reed valve of 
conventional design, is located in each of the intake passages 27 between 
the plenum 30 and the intake valve 19. The presence of the reed valve 
prevents or limits backflow of gases from the engine cylinders such as 
cylinder 12 into the intake manifold plenum 30 and thus permits use of 
camshaft phase control of the engine valves to provide for generally 
understood variable valve timing advantages. Conventional pressure 
transducer 46 is located in the intake passage 27 to sense the air 
pressure therein and output signal Ptv indicating the intake passage 
pressure. Likewise, a conventional pressure transducer 48 is located in 
intake plenum 30 to provide an output signal Pi indicating such pressure. 
The pressure signals Ptv and Pi may be continuous analog signals which are 
periodically sampled and interpreted by a conventional engine controller 
44. Engine controller 44 may be a conventional single chip 
micro-controller having such generally known elements as a central 
processing unit, a read only memory unit, a random access memory unit, and 
input/output control units for communicating controller input and output 
information. Included with such input information in this embodiment is a 
signal RPM in the form of a periodic analog signal the frequency of which 
is proportional to the rate of rotation of the crankshaft 16, and a 
synchronization signal CAM generated when an engine camshaft 36 (FIG. 2) 
rotates past a reference angle. Signal RPM may be used to indicate engine 
crankshaft position occurrences corresponding, for example, to occurrence 
of top or bottom dead center position of the engine cylinders, and signal 
CAM may be used to indicate when the camshaft 36 is at a predetermined 
rotational angle. Engine controller 44, with benefit of the pressure 
signals Ptv and Pi, the signals RPM and CAM and other conventional sensor 
signals, provides for engine control and diagnostic operations generally 
understood in the art. Such operations include valve timing control 
operations dictated by a command .phi.c output by the engine controller to 
the variable valve timing actuation means 24. 
Referring to FIG. 2, mechanical portions of the variable timing valve 
actuation means 24 are illustrated comprising a cam phaser 35 capable of 
varying the phase angle of the camshaft 36, which actuates both the intake 
and exhaust valves 19 and 20 through conventional valve gear (not shown), 
relative to the phase angle of the crankshaft 16 by which the camshaft is 
driven. The phaser 35 is driven by the crankshaft 16 through a chain 38 
and is in turn connected through a geartrain 39 to drive a balance shaft 
40 at crankshaft speed with a fixed phase angle. An internal planetary 
gear mechanism in the phaser 35 is adjustable through a control shaft 42 
to vary the camshaft phase angle. An example of a phaser of this type is 
found in U. S. Pat. No. 5,327,859, assigned to the assignee of this 
application. 
The routines illustrated in FIGS. 3 and 4 describe a sequence of engine 
controller operations for carrying out the intake valve timing operations 
of the preferred embodiment of this invention. These operations may be 
embodied in the form of a sequence of controller software instructions 
stored in a controller read only memory (not shown) and executed in a step 
by step manner following predetermined engine synchronization events. 
Generally, these routines provide for timed sampling of trapped volume 
pressure to determine the degree of charge backflowing out of an active 
engine cylinder 12 of FIG. 1 following cylinder bottom dead center 
position until the intake valve 19 of the active cylinder 12 reaches a 
closing position isolating the backflowing volume in the intake passage 
27. The check valve 34 of FIG. 1 closes shortly after the cylinder bottom 
dead center position, blocking the path of backflow and trapping the 
backflowing charge. The amount of charge thus trapped indicates the 
precise amount of time that the cylinder compression stroke was pushing 
the cylinder charge up into the intake passage 27. This amount of time 
indicates a difference in time between occurrence of active cylinder 
bottom dead center position (directly corresponding to engine crankshaft 
angular position) and occurrence of active cylinder intake valve closing 
(directly corresponding to engine camshaft angular position), providing a 
precise measure of the actual phasing between the crankshaft and camshaft 
which may also be described as the intake valve timing relative to the 
crank angle of the piston. Such information is used in precision phasing 
control, fuel, air and ignition control, and diagnostics. 
More specifically, the operations of FIGS. 3 and 4 provide for pressure 
sensing using conventional pressure transducers to determine the volume of 
the charge trapped, as a function of engine speed and then correct the 
determined volume with an engine speed dependent flow loss coefficient. 
The corrected volume is then applied to a predetermined function or to a 
conventional lookup table to determine a time of occurrence of the active 
cylinder intake valve closing event, expressed as the crank angle at the 
time of the intake valve closing event. Engine control and diagnostic 
operations are then carried out with benefit of the determined crank 
angle. 
To provide for such operations, two synchronization events are required in 
this embodiment. First, a cylinder bottom dead center BDC event is used to 
trigger a baseline pressure measurement through the operations of FIG. 3, 
and next a cylinder crank event is used to signal a time for measuring a 
trapped volume pressure in accord with initial operations of the routine 
of FIG. 4. The pressure measurements are made under assumed substantially 
constant engine operating conditions, such as engine EGR and net mean 
effective pressure NMEP conditions, for example by minimizing the time 
between such measurements. In this embodiment, such time is minimized by 
taking the measurement samples during a single engine cycle, such as 
shortly before and shortly after a single intake valve closing event. In 
the embodiment of this invention in which a plurality of intake valve 
closing timing events are measured, such measurements may occur shortly 
before and shortly after the corresponding intake valve closing events for 
the current active engine cylinder. 
Specifically, an interrupt is generated upon occurrence of a crank event 
corresponding to a piston bottom dead center event. In this embodiment, a 
piston bottom dead center event occurs in a four cycle engine between a 
cylinder intake stroke and a cylinder compression stroke. Such event may 
also be described as occurring when the piston in an active cylinder is at 
its BDC position, which is the crank angle of the piston when it is fully 
retracted in the cylinder, providing an inlet air region in the cylinder 
of maximum volume. Such maximum volume is substantially constant for a 
given engine, and is known as a substantially constant value used for 
design and analysis of the engine. Upon occurrence of the bottom dead 
center interrupt, any ongoing controller operations are temporarily 
suspended and the controller is directed to carry out an interrupt service 
routine illustrated by the series of operations of FIG. 3, which are 
executed starting at a first step 60 and proceeding to a next step 62 at 
which Ptv is sampled. Ptv is the air pressure in intake passage 27 as 
output by sensor 46. It has been determined experimentally that Ptv at 
cylinder bottom dead center position is substantially the same as Pi,bdc 
which is the intake plenum pressure transduced by sensor 48 of FIG. 1, at 
cylinder bottom dead center position. As such, and in accord with this 
invention, the pressure sampled at the step 62 may be the signal Pi output 
from sensor 48 of FIG. 1. The signal Pi then, within the scope of this 
invention, would be substituted for each occurrence of the signal Ptv in 
the routine of FIG. 3, such as at steps 62-66, and would be used to 
determine the pressure ratio at step 78 of FIG. 4, to be described. After 
reading the pressure signal at the step 62, whether the signal Ptv from 
the sensor 46 in the intake passage 27 (FIG. 1) or the signal Pi from the 
sensor 48 in the intake plenum 30 (FIG. 1), a next step 64 is executed to 
filter the pressure signal through a conventional filter process at a step 
64. The filter process may include a first order lag filter process for 
reducing the impact of sensor or signal noise on the accuracy of the 
pressure indicated by signal Ptv (or equivalently Pi, within this 
invention). The pressure signal characteristic, such as shown in FIG. 5, 
is not significantly changed through such lag filter process. The filtered 
trapped volume pressure value Ptv (or Pi) is next stored in controller 
random access memory at a step 66 with the label Ptv,bdc, indicating it 
represents the trapped volume pressure (or the plenum pressure) at 
cylinder bottom dead center position BDC. The routine next returns to any 
other operations that may be required, under conventional BDC interrupt 
servicing practices, to properly service the BDC interrupt. Upon 
completing such additional operations, the controller may exit the service 
routine and resume execution of any operations that may have been 
temporarily suspended at the time of the BDC interrupt to allow such 
interrupt to be serviced. 
The sequence of operations illustrated by FIG. 4 make up an interrupt 
service routine that is executed for each occurrence of a crankshaft event 
that occurs only when the intake valve for the cylinder under measurement 
is closed. Such valve must be closed to allow a measurement of the amount 
of change in trapped volume in the corresponding intake passage 27. For 
example, the interrupt that initiates execution of the operations of FIG. 
4 may be the next crank event following the BDC event that initiated the 
described routine of FIG. 3. This ensures the intake valve 19 (FIG. 1) 
will be closed and that a minimal change in engine operating conditions 
has occurred since the cylinder BDC event occurred that triggered the 
sampling of Ptv of FIG. 3. 
FIG. 5 illustrates a relationship between engine crankshaft angle and 
trapped volume pressure over a range of engine operating conditions. 
Curves 120-124 illustrate the significant drop in pressure in the trapped 
volume upon opening of the intake valve such as intake valve 19 of FIG. 1. 
The curve 120 represents the relationship calibrated at engine operating 
conditions of about a fifty degree camshaft phase retard from a reference 
phase angle, a 70 kPa manifold absolute pressure in intake plenum 30 of 
FIG. 1, no recirculation EGR of engine exhaust for mixing with engine 
intake air, and a net mean effective cylinder pressure NMEP of about 330 
kPa. At about 410 crank angle degrees, curve 120 illustrates the 
significant pressure drop in intake passage 27 occurring upon opening of 
the intake valve 19 (FIG. 1). The pressure remains at the same level as 
the manifold absolute pressure at cylinder bottom dead center position, 
which is the 540 degree position in FIG. 5, at which position the pressure 
in intake passage is sampled through execution of the operations of FIG. 
3, yielding the value Ptv,bdc. The pressure stabilizes at about 660 crank 
angle degrees when the intake valve closes. The pressure remains at a 
stable value until the next intake valve opening, during which time the 
routine of FIG. 4 may be executed to sample pressure and determine the 
precise time of intake valve closing in accord with this invention. 
Curve 122 represents the relationship between crank angle and trapped 
volume pressure for the engine operating conditions of about 30 degrees of 
camshaft phase retard from the reference position, an absolute pressure in 
intake plenum 30 of about 60 kPa, no EGR, and an NMEP of about 379 kPa. 
Under these conditions, the pressure drop corresponding to intake valve 
opening occurs at about 390 crank angle degrees. The pressure occurring at 
cylinder BDC is sampled and stored as Ptv,bdc. The pressure in the intake 
passage 27 is again read after the intake valve closes, such as after a 
crank angle of about 640 degrees through the operations of FIG. 4, to be 
described. 
Finally, curve 124 represents the pressure-angle relationship for engine 
operating conditions of about ten degrees camshaft phase retard from the 
reference position, about forty kPa of intake plenum absolute pressure, 
about eleven percent EGR and NMEP of about 193 kPa. Under these 
conditions, curve 124 illustrates an intake valve opening angle of about 
370 crank angle degrees. At about 540 crank angle degrees, the cylinder 
BDC event occurs and the pressure Ptv,bdc is read and stored. Following 
intake valve closing at about 620 crank angle degrees, the pressure may 
again be read and the time of intake valve closing determined in accord 
with this invention. 
Upon occurrence of an interrupt indicating the intake valve 19 (FIG. 1) is 
closed, such as the crank event following the event indicating active 
cylinder bottom dead center position, any current controller operations of 
sufficiently low priority are temporarily suspended and the operations of 
FIG. 4 executed starting at a step 70 and proceeding to a step 72 to read 
the trapped volume pressure signal Ptv provided by pressure transducer 46 
of FIG. 1. The Ptv value may next be filtered at a step 74 through a 
conventional first order lag filter process, for example to minimize the 
effect of sensor and signal noise on signal accuracy. 
The filtered value may then be used to update a stored trapped volume 
pressure value for the active cylinder at a step 76 by storing the sensed 
pressure as Ptv, ivc in controller random access memory. The trapped 
volume pressure is next used at a step 78 to determine a pressure ratio as 
the ratio of Ptv, ivc to Ptv,bdc. 
Referring to FIG. 6, a relationship between CAM retard or relative CAM 
position and the pressure ratio determined at the step 78 at the FIG. 4 is 
illustrated at first and second representative engine speed values. The 
relationship of FIG. 6 illustrates a number of determined pressure ratios 
for CAM retard positions indicating a correlation between the pressure 
ratio and the CAM retard over a number of measurement values. Curve 130 
illustrates the relationship for an engine speed of about 1500 RPM, and 
curve 132 represents the relationship for an engine speed of about 2200 
RPM. The measurements made for a given engine at varying CAM retard values 
indicate a stable pressure ratio that is a function substantially of only 
camshaft position and is substantially insensitive to intake plenum 
pressure or an amount of EGR. Accordingly, as FIG. 6 illustrates, pressure 
ratio is a robust indicator of camshaft angular position relative to 
crankshaft angular position and thus is a robust measure of the engine 
events, such as valve events corresponding to actual cam position in 
accord with this invention. 
Furthermore, the pressure ratio for a given camshaft angular position 
relative to the crankshaft angular position should not vary for a great 
number of engine operating conditions and if such a variation is detected 
such as by comparing successive pressure ratio values, a fault condition 
may be present. As will be described, such fault condition may be assumed 
to be a leak in the check valve 34 of FIG. 1. 
Returning to FIG. 4, the pressure ratio is next, at a step 80, stored in 
controller memory such as random access memory for use in engine control 
and diagnostic applications, to be described. After storing the pressure 
ratio, the present engine speed RPM is read at a step 82, such as from 
controller random access memory. Engine speed may be derived by comparing 
signal RPM (FIG. 1) to a threshold voltage level and generating a value 
proportional to the time rate that the signal RPM crosses the threshold, 
as is generally understood in the art. A flow loss coefficient is next 
referenced at a step 84 from controller non-volatile memory, such as read 
only memory, as a predetermined function of engine speed. The flow loss 
coefficient is determined through a. conventional calibration process to 
account for any effects of flow loss from the engine cylinder 12 into the 
passage 27 due to the closing intake valve 19, the irreversible combustion 
process in the cylinder 12, any deviation of the volume in the engine 
cylinder at the time of the reed valve closing to the volume in the 
cylinder at the time of cylinder bottom dead center position, and for 
trapped charge leakage. Flow loss from these causes generally depends on 
engine speed and through a conventional calibration process in which flow 
loss is measured or estimated over a range of engine speeds, a function or 
a conventional lookup table may be derived and stored in controller 
non-volatile memory, and flow loss values calculated or referenced 
therefrom as a function of present engine speed. 
After determining the flow loss coefficient, the cylinder volume at the 
time of intake valve closing is determined at a next step 86 as follows: 
EQU Vcyl,ivc=(Vcyl,bdc+Vtv)/(e.sup.1/.gamma.*ln(PR/c)-Vtv 
in which Vcyl,bdc is engine cylinder volume at bottom dead center position 
which is a calibratable constant for a given engine, Vtv is the trapped 
volume which is a calibratable constant for a given engine induction 
system, .gamma. is the specific heat ratio for the cylinder charge, set to 
about 1.35 for air, PR is the pressure ratio determined at the described 
step 78, and c is the flow loss coefficient determined at the described 
step 84. 
The determination of the cylinder volume at the time of intake valve 
closing provides for a measure of the amount of charge lost into the 
intake passage 27 due to the delay between the time of cylinder bottom 
dead center position and intake valve closing. The amount of this delay is 
directly related for a given engine to the amount of charge lost through 
backflow into the passage 27 and therefore to the amount of time between 
the known time of cylinder bottom dead center event to the unknown time of 
the intake valve closing. Accordingly, the cylinder volume at the time of 
intake valve closing may be used to measure the difference in time between 
the bottom dead center and intake valve closing and therefore the absolute 
time of the intake valve closing in accord with a critical feature of this 
invention. 
Returning to FIG. 4, after determining cylinder volume at intake valve 
closing at the step 86, the crank angle position .theta. at the time of 
intake valve closing may next be determined at a step 88 from simple 
algebraic manipulation of the following equation: 
EQU Vcyl,ivc=.pi.*B.sup.2/ 4 * {2*T+CH-(T*.theta..sup.2 
/2)*(1-T*(1-.theta..sup.2 /6).sup.2 /CRL)} 
in which Vcyl,ivc is the volume determined at the step 86, B is cylinder 
bore diameter (a measurable constant), T is crank throw which is half of 
engine stroke, CH is piston clearance height which is the distance between 
the top of a piston and the bottom of a cylinder head when the piston is 
at top dead center position for a pancake chamber design of common 
cylinder to cylinder compression ratio (a measurable constant), and CRL is 
connecting rod length (a measurable constant). 
Accordingly, the angle .theta. at intake valve closing is determined, which 
may be used for engine control and diagnostics. Specifically, the fuel 
pulse for the variable valve timing engine control application of the 
present embodiment may be determined as a predetermined function of 
.theta. (i.v.c.) for precise fuel delivery control at a step 90 in accord 
with generally understood variable valve timing control practices. It is 
well-established that engine performance and emissions depend 
significantly on the relationship between fuel injection timing for a port 
fuel injection application and the time of cylinder intake valve opening 
and closing. Furthermore, at a next step 92, ignition timing commands may 
be determined as a predetermined function of, among other generally known 
parameters, .theta. (i.v.c.) so that fuel injection timing, cylinder inlet 
air control and ignition of the cylinder charge may be properly 
coordinated in accord with generally understood engine control principles. 
A command EST, indicating the time of ignition of at least an active 
engine cylinder is thus generated to be output to a conventional ignition 
driver (not shown) of an engine cylinder. Beyond the fuel and ignition 
control operations generally outlined at the steps 90 and 92, other engine 
control functions may generally understood in the art of variable valve 
timing engine control applications, for example, EGR valve control 
applications, may make use of the engine absolute position measurement 
provided at the step 88. For example, closed loop valve timing control may 
be provided by proceeding to a step 94 to generate a valve timing error 
Evt as a difference between the generated valve timing information 
provided through the sequence of operations of FIGS. 3 and 4 and a 
commanded valve position such as a command .phi.c issued by engine 
controller 44 to the variable valve timing mechanism 24 of FIG. 1. The 
valve timing error represents the difference between the desired and 
actual valve timing and may be applied to a conventional control function 
to controllably drive the error toward zero providing a more precise 
engine valve timing control. Specifically, the valve timing error value 
Evt may be applied at a next step 96 to a predetermined control function, 
such as, for example, a classical control function or a control function 
generated through modern control techniques to determine valve timing 
compensation which would be applied to the valve command .phi.c to drive 
the actual valve position toward the commanded position in a controlled 
manner. The compensation value determined at a step 96 would then be 
applied through the conventional valve control function of the present 
embodiment to properly drive the variable valve timing mechanism 24 of 
FIG. 1. 
Beyond the control functions to which the valve timing information yielded 
through the principals of this invention are provided, certain diagnostic 
operations may be carried out with benefit of the precision valve timing 
information. For example, a check valve diagnostic may be provided by 
generating a .DELTA.pressure ratio at a next step 98 as a difference 
between the pressure ratio determined and stored at the steps 78 and 80 
and the pressure ratio determined for a prior execution of the routine of 
FIG. 4, such as for a neighboring engine cylinder or for a common engine 
cylinder during a prior engine cycle. For example, for a given engine, the 
pressure ratio determined for any engine cylinder such as determined at 
the step 78 of FIG. 4 should remain substantially constant under varying 
engine conditions. If the pressure ratio varies from prior determined 
ratios for a given engine, it may be assumed that a leak has occurred in 
the trapped volume and it is assumed in this embodiment that the check 
valve 34 of FIG. 1 is the source of the leak. Accordingly, a diagnostic of 
the check valve 34 is provided through a simple comparison of pressure 
ratios between engine cylinders or for a given engine cylinder between 
engine cycles. Specifically, after carrying out any engine control 
operations benefiting from the precise engine absolute position detection 
and timing of the present embodiment, a .DELTA.pressure ratio value is 
generated at a next step 98 as a difference between the pressure ratio 
stored at the step 80 and a prior stored pressure ratio either from a 
different engine cylinder or from the same engine cylinder during a prior 
engine cycle. The .DELTA.pressure ratio is next compared to a threshold 
delta pressure ratio value THR at a step 100. THR may be determined 
through a conventional calibration process as, for a given engine, the 
maximum tolerable variation of pressure ratios over a wide range of engine 
operating conditions. Any .DELTA.pressure ratio exceeding the calibrated 
threshold at the step 100 would therefore indicate a leak likely from the 
check valve 34 of FIG. 1. When such a leak is detected, a next step 102 
would be executed to indicate such a fault condition either by storing a 
fault code in a controller memory or by illuminating an indicator in view 
of a vehicle operator. After indicating any such valve leak fault 
condition, or if no such leak was detected, a step 104 is executed to 
return to any operations that the engine controller 44 of FIG. 1 was 
carrying out at the time of the crank event that initiated the current 
execution of the routine of FIG. 4. 
The preferred embodiment for the purpose of explaining this invention 
should not be taken as limiting or restricting this invention since many 
modifications may be made through the exercise of ordinary skill in the 
art without departing from the scope of this invention.