Method for controlling an operating condition of a vehicle engine

A residual ratio factor characterizing the amount of residual exhaust gas left in a selected cylinder at the end of a piston intake stroke is determined from tabular and surface models based on previously gathered dynamometer data from a test vehicle at various engine speeds. The residual ratio factor is then used to calculate the mole fractions of air and residual exhaust gas in the selected cylinder, which, in turn, are used to determine mass airflow at an engine intake port at the end of the intake stroke. The mass airflow can then be used to derive further models for determining an engine operating parameter, such as fuel/air ratio, required for achieving at preselected vehicle operating condition.

FIELD OF THE INVENTION

The present invention generally relates to vehicle engine control systems. More specifically, the invention pertains to fueling adjustments based on airflow models derived from test vehicles dynamometer data.

BACKGROUND OF THE INVENTION

Conventional airflow models for use in computer control of vehicular engines suffer from the fact that gas densities and volumetric efficiencies used in control algorithms are not constant, thereby requiring use of complex error correction factors. Such correction factors, in turn, are highly dependent on hard-to-achieve precise measurements of engine operating parameters, such as manifold absolute pressure. Additionally, prior approaches require complex combinations of software tabular and surface data to properly calibrate the controller to estimate normally unmeasured parameters, such as cylinder temperature.

The complexity of cylinder temperature calibration requires large amounts of time in specialty dynamometer cells generating huge data sets for calibration and verification. Advanced engine systems utilize devices which affect exhaust gas residual content in a selected cylinder at the completion of an intake stroke. These devices typically include variable valve timing devices or manifold tuning valves and all require complex modifiers to parameters such as volumetric efficiency to obtain acceptably useful calibration.

Hence, there is a need for an improved model approach to modeling volumetric efficiency and gas density for use in controlling operating conditions of a vehicle engine.

SUMMARY OF THE INVENTION

A method for controlling an operating condition of a vehicle engine includes determining a residual ratio factor from dynamometer data generated by a test vehicle engine at various engine speeds; calculating mole fractions of air and residual exhaust gas in a selected cylinder of the engine at completion of an intake stroke for the selected cylinder, the calculation being a function of engine speed and the residual ratio factor; using the mole fractions of air and residual exhaust gas to determine mass air flow of the engine; and using the determined mass air flow to estimate an operating parameter of the vehicle engine required to achieve a desired vehicle operating condition.

DETAILED DESCRIPTION

The method of the invention is based on model refinements to both volumetric efficiency and gas density. We begin by defining the volumetric efficiency as the ratio of the actual cylinder volume to the cylinder volume upon intake valve closure for that cylinder. This definition is consistent with the classical definition of a mole fraction and therefore the refined definition of volumetric efficiency is equal to the mole fraction of air in the cylinder. Neglecting fuel, we presume that the contents of a selected cylinder upon closure of the intake valve are limited to air and exhaust gas residual. Hence, the mole fraction of the residual exhaust gas is simply 1—the mole fraction of air. Conversely, the mole fraction of air is given by 1—the mole fraction of the residual exhaust gas. Hence, since the method uses a model of the residual exhaust, the mole fraction of air is calculated from the determined mole fraction of the residual exhaust.

Knowing the relative amounts of air and residual exhaust gas from the residual model and the temperatures of same, it then becomes possible with the method of the invention to calculate the actual temperature of the mixed air and residual exhaust gas in a selected cylinder upon closure of the intake valve, thereby eliminating a great deal of calibrating data harvesting required with conventional approaches.

The only remaining unknown then becomes the cylinder pressure at intake valve closure, which is calculated from manifold absolute pressure (MAP), engine speed and intake manifold gas temperature. This pressure is then calibrated to provide the measured airflow. The residual based model of the invention begins with collecting data from which a residual partial pressure ratio factor can be determined. With reference toFIG. 1, a graph is shown of collected data points for various engine speeds where mass airflow Ma is plotted versus a pressure ratio Rpof manifold absolute pressure to barometric pressure. The pressure ratio at zero mass airflow, or the X intercept of the various engine speed data graphs is shown at110. This intercept yields the residual partial pressure ratio, Rpr, for various engine speeds. While only a single point110is shown inFIG. 1, it is to be noted that in the real world situation, the X intercepts for each of the speed graphs (i.e., 1000 rpm, 2000 rpm, etc.) are separate crossover points. Hence, if the engine speed is known in the engine control algorithm, a table lookup procedure can be utilized from dynamometer data such as that shown inFIG. 1to derive the residual partial pressure ratio factor Rpr.

Therefore in its broader aspects, the method begins by determining a residual ratio factor, such as the residual partial pressure ratio110ofFIG. 1, from dynamometer data generated by a test vehicle engine at various engine speeds. The method calculates a mole fraction of air and residual exhaust gas in a selected cylinder of the engine at completion of an intake stroke for the selected cylinder, the calculation being a function of engine speed and the residual ratio factor. The mole fractions of air and residual exhaust gas are used to determine mass airflow of the engine and the determined mass airflow is then used to estimate an operating parameter of the vehicle engine required to achieve a desired vehicle operating condition, such as fuel to air ratio, spark timing, or engine output torque.

In a more detailed example of the method of the invention, an operating condition of a vehicle engine is controlled by first calculating mole fractions of residual exhaust and air in a selected cylinder of the engine at the end of that cylinder's intake stroke. Gas pressure in the selected cylinder is calculated upon closure of the intake valve. The temperature of the mixed intake air and residual exhaust gas resident in the selected cylinder upon the closure of the intake valve is then calculated, and then mass airflow at an intake port of the engine is calculated using the calculated gas pressure and calculated gas temperature and the mole fraction of air for a selected cylinder. Using the mass airflow, an estimate is made of an operating parameter of the vehicle engine to achieve a preselected vehicle operating condition. The details of each of these steps are illustrated below with reference toFIGS. 2–7.

With reference toFIG. 2, a block diagram200sets forth the determination of residual exhaust and air mole fractions in a selected cylinder of the engine using tabular and/or surface models, measured engine parameters and calculations.

The basic inputs to the determination of mole fractions inFIG. 2are intake cam position at block202, exhaust cam position at block204, engine speed at block206, manifold absolute pressure at block208and barometric pressure at block210.

Using the intake and exhaust cam positions, a valve overlap modifier is calculated at block212according to
mvo=f(ICP,ECP)
The above function is derived from lookup tables representing a three-dimensional surface.

At block214a residual partial pressure ratio is derived from a table lookup and is a function of engine speed
Rpr=f(Ne).

At block216a pressure ratio is calculated according to
Rp=MAP/BARO
where MAP is manifold absolute pressure and BARO is barometric pressure.

The valve overlap modifier, residual partial pressure ratio and the pressure ratio are then used at block218to calculate the mole fraction of residual exhaust gas in the selected cylinder in accordance with
Xr=(Rpr/Rp)*mvo.

Finally, at block220the mole fraction of air is derived from the mole fraction of residual exhaust gas assuming that air and exhaust are the only two gases resident in the cylinder at the end of the intake stroke
Xa=1−Xr.

FIG. 3sets forth a block diagram300showing the determination of gas pressure in the selected cylinder at intake valve closure using tabular and/or surface models, measured engine parameters and calculations.

The basic inputs for the determination of gas pressure in the cylinder at intake valve closing are manifold absolute pressure at block302, gas temperature at the engine intake port at block304which is derived from a variety of surface and tabular lookups, engine speed at block312, the position of a variable charge motion device at block314, the exhaust cam position at block316and the intake cam position at block318. A variable charge motion device is an element in advanced engine systems located in the intake manifold or intake port close to the valve which blocks part of the port with the intent of promoting or increasing gas motion. Additional inputs are a manifold tuning valve flag at block306and a short runner valve flag at block308. These flags serve to indicate the state of these valves which are also present in some advanced engine systems for providing intake manifold tuning features.

At block310gas density in the intake port is calculated according to
ρi=MAP/RTi
where R is the universal gas constant and Tiis gas temperature in the intake port.

At block320dynamic pressure in the cylinder is derived from a model comprising a surface representation and is a function of the states of any manifold tuning valve MTV or short runner valve SRV present in the system, engine speed Neand the calculated gas density in the intake port, or
Pd=f(MTV,SRV,Ne,ρi).

At block322a variable charge motion device position modifier mvcmis derived from a surface lookup model and is a function of engine speed and the position pvcmof the variable charge motion device, or
mvcm=f(Ne,pvcm).

At block324a cam position modifier mvvtis derived from a surface model and is a function of the exhaust cam ECP and intake cam ICP positions or
mvvt=f(ECP,ICP).

At block326gas pressure at the cylinder of interest at intake valve closing is calculated in accordance with
Pcyl=(mvcm*mvvt*Pd)+MAP.

With reference toFIG. 4, block diagram400sets forth the determination of the mixed intake and residual gas temperature in a selected cylinder at intake valve closing using tabular and/or surface models, measured engine parameters and calculations.

Inputs to the gas temperature determination model ofFIG. 4are derived gas temperature in the exhaust port Teat block402, engine speed Neat block404, a derived exhaust back pressure dPe at block406(which is determined in accordance with eitherFIG. 6orFIG. 7as will be discussed below), and barometric pressure BARO at block408.

At block410residual exhaust gas temperature in the selected cylinder at the opening of the intake valve is determined from a lookup table model as a function of the exhaust gas temperature at block402.

At block412a polytropic exponent k is derived via table lookup and is a function of engine speed.

At block412the exhaust absolute pressure Peis calculated in accordance with
Pe=BARO+dPe.

At block422the unmixed residual gas temperature in the engine intake port Treis calculated in accordance with

Finally, at block428the mixed intake and residual gas temperature in the cylinder of interest at intake valve closing is calculated in accordance with

Tcyl=[(Xr*Cpr*Tre)+(Xa*Cpa*Ti)][(Xr*Cpe)+(Xa*Cpa)]
where Tiis the gas temperature at the engine intake port, Cpris the specific heat of the residual exhaust gas and Cpais the specific heat of air.

With reference toFIG. 5, block diagram500sets forth the determination of mass air in the selected cylinder at intake valve closure and mass air flow at the engine intake port using tabular and/or surface models, measured engine parameters and calculations.

The basic inputs to this model are gas pressure in the cylinder at intake valve closing as derived from the model ofFIG. 3at block502, gas temperature in the cylinder at intake valve closing at block504as determined by the model ofFIG. 4, intake cam position ICP at block506, mole fraction of air Xaat block510, the number of cylinders Ncin the engine at block516and engine speed Neat block518.

At block508, the gas density in the cylinder at intake valve closing is calculated in accordance with
ρcyl=Pcyl/(R*Tcyl)
where ρcylis the gas density, Pcylis the cylinder gas pressure at intake valve closing, R is the universal gas constant and Tcylis the mixed intake air and residual gas temperature in the cylinder at intake valve closing.

At block512, the cylinder volume at intake valve closing is derived via a table lookup and is a function of the intake cam position.

At block514mass air in the cylinder at intake valve closure is calculated in accordance with
Macyl=Xa*Vcyl*ρcyl
where Macylis the mass air, and Vcylis the cylinder volume at intake valve closure derived at block512.

Finally, at block520engine intake port mass airflow Mapis calculated in accordance with
Map=Macyl*Nc*Ne.

Exhaust system back pressure dPeis determined via the model ofFIG. 6for those vehicle engines not employing a turbocharger. Exhaust gas temperature at block602and exhaust gas absolute pressure at block604are used to calculate exhaust gas density at block606in accordance with
ρe=Pe/RTe.

The exhaust gas density and the exhaust gas mass at block608are then used to calculate exhaust gas volume flow in accordance with
Ve=Me/le.

Finally, via a table lookup, the exhaust system pressure drop is derived at block612and is a function of exhaust gas volume flow.

Engines employing a turbocharger with a fan or turbine acting as an air pump for intake air enhancement use the exhaust back pressure model ofFIG. 7. Model700is similar to model600but takes into account the effects of the turbocharger turbine on the gas pressure and temperatures used in deriving total exhaust back pressure.

At block712the exhaust gas density after the turbine is calculated at block712using exhaust absolute pressure after the turbine at block702and exhaust gas temperature after the turbine at block704in accordance with
ρeat=Peat/(R*Teat)
where ρeatis the exhaust gas density after the turbine, Peatis the exhaust gas pressure after the turbine and Teatis the exhaust gas temperature after the turbine, each derived from tabular or surface-type lookup models.

At block714the exhaust gas density before the turbine is calculated in accordance with
ρebt=Pebt/(R*Tebt)
using exhaust gas temperature before the turbine, Tebt, and exhaust absolute pressure before the turbine at block708, Pebt, both derived from surface lookup models.

At block718the exhaust volume flow after the turbine is calculated in accordance with
Veat=Meat/ρeat
where Veat, is the exhaust volume flow after the turbine, Meatis the exhaust mass flow after the turbine and ρeatis exhaust gas density after the turbine.

At block722, exhaust volume flow before the turbine is calculated using the exhaust gas density before the turbine at block714and the exhaust mass flow before the turbine at block716, or
Vebt=Mebt/ρebt.

At block724, the exhaust system pressure drop dPe is derived from a table lookup as a function of the exhaust volume flow after the turbine at block718.

At block726, the turbine pressure drop is derived from a surface model at block726as a function the exhaust volume flow before the turbine at block722and the position of a waste gate at block720, pw. The waste gate is essentially a controllable relief valve to ensure that the turbine of the turbocharger does not run too fast, by opening a bleed-off passage to the main exhaust system.

Finally, at block728, total exhaust back pressure is calculated in accordance with
dPts=dPt+dPe
where dPtis the pressure drop of the turbine and dPe is the pressure drop of the exhaust back pressure. This value dPtsis then used at block406of the model ofFIG. 4for those vehicles employing a turbocharger.

Using the method of the invention has been shown to significantly lower the number of tables and surfaces and the required collection of calibration data required with conventional control schemes. With the use of detailed mass, pressure and temperature information, model based engine operating parameter control becomes feasible, including spark timing control, air/fuel ratio control and engine output torque control.