Control system and control method of internal combustion engine Number:7,367,318 from the United States Patent and Trademark Office (PTO) owispatent An internal combustion engine (1) provided with a valve mechanism (VM) able to change a valve opening characteristic of at least one of an intake valve (Vi) and exhaust valve (Ve), a cylinder pressure sensor (15) for detecting a cylinder pressure in a combustion chamber (3), and an ECU (20), wherein the ECU (20) calculates the amount of air sucked into the combustion chamber (3) based on an intake air pressure during valve overlap between the intake valve (Vi) and the exhaust valve (Ve), the exhaust gas pressure during valve overlap, a cylinder pressure during the compression stroke detected by a cylinder sensor (15), and a gas passage effective area during valve overlap.<H combustion, internal, Control, system, , 7367318.html, Patent, pto, 7367318

Title: Control system and control method of internal combustion engine

Abstract: An internal combustion engine (1) provided with a valve mechanism (VM) able to change a valve opening characteristic of at least one of an intake valve (Vi) and exhaust valve (Ve), a cylinder pressure sensor (15) for detecting a cylinder pressure in a combustion chamber (3), and an ECU (20), wherein the ECU (20) calculates the amount of air sucked into the combustion chamber (3) based on an intake air pressure during valve overlap between the intake valve (Vi) and the exhaust valve (Ve), the exhaust gas pressure during valve overlap, a cylinder pressure during the compression stroke detected by a cylinder sensor (15), and a gas passage effective area during valve overlap.

Patent Number: 7,367,318 Issued on 05/06/2008 to Moriya, et al.

Inventors: Moriya; Hidenori (Susono, JP), Ogino; Ryusuke (Suntou-gun, JP)
Assignee: Toyota Jidosha Kabushiki Kaisha (Toyota, JP)

Appl. No.: 10/582,226

Filed: October 7, 2005

PCT Filed: October 07, 2005

PCT No.: PCT/JP2005/018907

371(c)(1),(2),(4) Date: June 08, 2006

PCT Pub. No.: WO2006/038737

PCT Pub. Date: April 13, 2006

Foreign Application Priority Data


Oct 07, 2004 [JP]			2004-295090
Aug 19, 2005 [JP]			2005-238999

Current U.S. Class: 123/435 ; 123/568.14; 123/90.15; 701/103

Current International Class: F02M 7/00 (20060101); F01L 1/34 (20060101); G06F 17/00 (20060101); F02M 25/07 (20060101)

Field of Search: 701/103,104,108,110,111,114,115 123/90.15,568.11,568.14,435

References Cited [Referenced By]

U.S. Patent Documents


4967711	November 1990	Morikawa et al.
6840235	January 2005	Koseki et al.
6912995	July 2005	Miura
7003390	February 2006	Kaga
7107140	September 2006	Yoshino et al.
7151994	December 2006	Fuwa
7181336	February 2007	Muto et al.
2004/0117104	June 2004	Muto et al.
2004/0139949	July 2004	Koseki et al.
2004/0220718	November 2004	Uchida et al.
2005/0039723	February 2005	Miura
2005/0065707	March 2005	Kaga
2005/0216179	September 2005	Yasui et al.
2006/0047406	March 2006	Chatfield et al.
2006/0161333	July 2006	Muto et al.

Foreign Patent Documents


A-02-040054	Feb., 1990	JP
A-2003-184613	Jul., 2003	JP
A-2004-108262	Apr., 2004	JP
A-2004-278359	Oct., 2004	JP
A-2005-307847	Nov., 2005	JP

Primary Examiner: Miller; Carl S.
Assistant Examiner: Castro; Arnold
Attorney, Agent or Firm: Oliff & Berridge, PLC

Claims

The invention claimed is:

1. A control system of an internal combustion engine having a valve mechanism able to change a valve opening characteristic of at least one of an intake valve and exhaust valve and burning an air-fuel mixture comprised of fuel and air inside a combustion chamber to generate power, characterized by being provided with a cylinder pressure detecting means for detecting a cylinder pressure at said combustion chamber and an intake air calculating means for calculating an amount of air sucked into said combustion chamber based on an intake air pressure during valve overlap between said intake valve and said exhaust valve, an exhaust gas pressure during valve overlap, a cylinder pressure during the compression stroke detected by said cylinder pressure detecting means, and a gas passage effective area during said valve overlap.

2. A control system of an internal combustion engine as set forth in claim 1, wherein the exhaust gas pressure during valve overlap is estimated based on the cylinder pressure detected by the cylinder pressure detecting means before or at the start of valve overlap.

3. A control system of an internal combustion engine as set forth in claim 2, wherein the exhaust gas pressure during valve overlap is also estimated based on a load of the internal combustion engine.

4. A control system of an internal combustion engine as set forth in claim 3, wherein when the load of said internal combustion engine is higher than a predetermined load, the exhaust gas pressure during valve overlap is estimated higher than the cylinder pressure detected by the cylinder pressure detecting means before or at the start of valve overlap.

5. A control system of an internal combustion engine as set forth in claim 1, wherein said internal combustion engine has a plurality of said combustion chambers and is provided with said cylinder pressure detecting means for each combustion chamber and the intake air pressure during the valve overlap in any combustion chamber is estimated based on the cylinder pressure at intake bottom dead center of another combustion chamber where the intake stroke was executed before that combustion chamber.

6. A control system of an internal combustion engine as set forth in claim 1, wherein the system is further provided with a second intake air calculating means for calculating the amount of air sucked into said combustion chamber by a different technique from said intake air calculating means forming the first intake air calculating means and calculates the amount of air sucked into said combustion chamber used for control of said internal combustion engine based on the amount of air sucked into said combustion chamber calculated by said first intake air calculating means and the amount of air sucked into said combustion chamber calculated by said second intake air calculating means.

7. A control system of an internal combustion engine as set forth in claim 6, wherein the amount of air sucked into said combustion chamber in the current cycle calculated by said second intake air calculating means is corrected based on the amount of air sucked into said combustion chamber in the previous cycle calculated by said first intake air calculating means so as to calculate the amount of air sucked into said combustion chamber in the current cycle.

8. A control system of an internal combustion engine as set forth in claim 7, wherein the amount of air sucked into said combustion chamber in the current cycle calculated by said second intake air calculating means is corrected based on the difference between the amount of air sucked into said combustion chamber in the previous cycle calculated by said first intake air calculating means and the amount of air sucked into said combustion chamber in the previous cycle calculated by said second intake air calculating means so as to calculate the amount of air sucked into said combustion chamber in the current cycle.

9. A control system of an internal combustion engine as set forth in claim 8 wherein, when the difference between the amount of air sucked into said combustion chamber in the previous cycle calculated by said first intake air calculating means and the amount of air sucked into said combustion chamber in the previous cycle calculated by said second intake air calculating means is a predetermined value or more, correcting the amount of air sucked into said combustion chamber in the current cycle calculated by said second intake air calculating means based on said difference so as to calculate the amount of air sucked into said combustion chamber in the current cycle, is prohibited.

10. A control system of an internal combustion engine as set forth in claim 1, wherein said gas passage effective area is calculated based on lift amounts of the intake valve and exhaust valve during said valve overlap and the engine speed during said valve overlap.

11. A control method of an internal combustion engine having a valve mechanism able to change a valve opening characteristic of at least one of an intake valve and exhaust valve and burning an air-fuel mixture comprised of fuel and air inside a combustion chamber to generate power, characterized by calculating the amount of air sucked into the combustion chamber based on an intake air pressure during valve overlap between said intake valve and said exhaust valve, an exhaust gas pressure during valve overlap, a cylinder pressure at said combustion chamber during a compression stroke of said internal combustion engine, and a gas passage effective area during said valve overlap.

Description

TECHNICAL FIELD

The present invention relates to a control system and control method of an internal combustion engine burning an air-fuel mixture comprised of fuel and air inside a combustion chamber to generate power, more particularly relates to a control system and control method of an internal combustion engine having a valve mechanism able to change a valve opening characteristic of at least one of an intake valve and exhaust valve.

BACKGROUND ART

In the past, a control system of an internal combustion engine provided with a means for calculating the amount of change of cylinder pressure between immediately after closing of the intake valve and immediately before ignition based on a cylinder pressure, crank angle, and throttle opening degree and a means for calculating the amount of intake air from the amount of change of the cylinder pressure and an engine speed has been known (for example, see Japanese Patent Publication (A) No. 2-40054). Further, in the past, an internal combustion engine able to set valve overlap for making an intake valve and exhaust valve simultaneously open for improving the output or efficiency and reducing emissions has been known in the past. Further, as this type of internal combustion engine, an engine provided with a control system able to calculate the amount of gas remaining in a combustion chamber (amount of internal EGR) due to the valve overlap between the intake valve and exhaust valve has been known (for example, see Japanese Patent Publication (A) No. 2004-108262).

The control system described in Japanese Patent Publication (A) No. 2004-108262 calculates the cylinder temperature and cylinder pressure based on signals from an exhaust temperature sensor, intake pressure sensor, and exhaust pressure sensor at the time of closing of the exhaust valve, calculates the gas constant of the exhaust gas in accordance with the burned air-fuel ratio, and calculates the amount of cylinder gas at the time of closing of the exhaust valve based on the cylinder temperature, cylinder pressure, and gas constant. Further, this control system calculates the amount of blowback gas during overlap between the intake valve and exhaust valve based on signals from a crank angle sensor, water temperature sensor, cam angle sensor, and accelerator opening degree sensor and calculates the amount of gas remaining in a combustion chamber (amount of internal EGR) due to the valve overlap based on the amount of cylinder gas and the amount of blowback gas.

DISCLOSURE OF THE INVENTION

As explained above, according to a conventional control system, it is possible to calculate the amount of intake air and the amount of residual gas (amount of internal EGR) when the valve overlap of the internal combustion engine is set. However, in the conventional example, calculation of the amount of intake air or amount of residual gas requires a large number of parameters. For this reason, in a conventional internal combustion engine, to obtain these parameters, a large number of sensors are required and an increase in cost is forced.

Therefore, the present invention provides a practical control system and control method of an internal combustion engine able to precisely calculate the amount of air sucked into the combustion chamber at a low cost even when valve overlap between the intake valve and exhaust valve is set.

A control system of an internal combustion engine according to the present invention is a control system of an internal combustion engine having a valve mechanism able to change a valve opening characteristic of at least one of an intake valve and exhaust valve and burning an air-fuel mixture comprised of fuel and air inside a combustion chamber to generate power, characterized by being provided with a cylinder pressure detecting means for detecting a cylinder pressure at the combustion chamber and an intake air calculating means for calculating the amount of air sucked into the combustion chamber based on an intake air pressure during valve overlap between the intake valve and the exhaust valve, an exhaust gas pressure during valve overlap, a cylinder pressure during a compression stroke detected by the cylinder pressure detecting means, and a gas passage effective area during the valve overlap.

Note that, preferably, the exhaust gas pressure during valve overlap is estimated based on the cylinder pressure detected by the cylinder pressure detecting means before or at the start of valve overlap.

Further, preferably, the exhaust gas pressure during valve overlap is estimated also based on the load of the internal combustion engine.

Further, preferably, when a load of the internal combustion engine is higher than a predetermined load, the exhaust gas pressure during valve overlap is estimated to be higher than the cylinder pressure detected by the cylinder pressure detecting means before or at the start of valve overlap.

Further, preferably, the internal combustion engine has a plurality of combustion chambers and is provided with the cylinder pressure detecting means for each combustion chamber and the intake air pressure during the valve overlap in any combustion chamber is estimated based on the cylinder pressure at intake bottom dead center of another combustion chamber where the intake stroke was executed before that combustion chamber.

Further, preferably, the system is further provided with a second intake air calculating means for calculating the amount of air sucked into the combustion chamber by a different technique from the intake air calculating means forming the first intake air calculating means and calculates the amount of air sucked into the combustion chamber used for control of the internal combustion engine based on the amount of air sucked into the combustion chamber calculated by the first intake air calculating means and the amount of air sucked into the combustion chamber calculated by the second intake air calculating means.

Further, preferably, the amount of air sucked into the combustion chamber in the current cycle calculated by the second intake air calculating means is corrected based on the amount of air sucked into the combustion chamber in the previous cycle calculated by the first intake air calculating means so as to calculate the amount of air sucked into the combustion chamber in the current cycle.

Further, the amount of air sucked into the combustion chamber in the current cycle calculated by the second intake air calculating means is corrected based on the difference between the amount of air sucked into the combustion chamber in the previous cycle calculated by the first intake air calculating means and the amount of air sucked into the combustion chamber in the previous cycle calculated by the second intake air calculating means so as to calculate the amount of air sucked into the combustion chamber in the current cycle.

Further, preferably, when the difference between the amount of air sucked into the combustion chamber in the previous cycle calculated by the first intake air calculating means and the amount of air sucked into the combustion chamber in the previous cycle calculated by the second intake air calculating means is a predetermined value or more, correcting the amount of air sucked into the combustion chamber in the current cycle, calculated by the second intake air calculating means based on the difference so as to calculate the amount of air sucked into the combustion chamber in the current cycle, is prohibited.

Further, preferably, the gas passage effective area is calculated based on the lift amounts of the intake valve and exhaust valve during the valve overlap and the engine speed during the valve overlap.

The control method of an internal combustion engine according to the present invention is a control method of an internal combustion engine having a valve mechanism able to change a valve opening characteristic of at least one of an intake valve and exhaust valve and burning an air-fuel mixture comprised of fuel and air inside a combustion chamber to generate power, characterized by calculating the amount of air sucked into the combustion chamber based on an intake air pressure during valve overlap between the intake valve and the exhaust valve, an exhaust gas pressure during valve overlap, a cylinder pressure at the combustion chamber during a compression stroke of the internal combustion engine, and a gas passage effective area during the valve overlap.

According to the present invention, it becomes possible to realize a practical control system and control method of an internal combustion engine able to precisely calculate the amount of air sucked into a combustion chamber at a low cost.

Below, the present invention will be able to be understood more fully from the attached drawings and preferred embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of the configuration of an internal combustion engine to which the control system according to the present invention is applied.

FIG. 2 is a flow chart for explaining an intake air amount calculation routine in the internal combustion engine of FIG. 1.

FIG. 3 is a graph illustrating the relationship between the ratio between intake air pressure at a predetermined timing during valve overlap and the cylinder pressure at a predetermined timing immediately before or at the start of valve overlap and the ratio between the intake air pressure and exhaust gas pressure, at a predetermined timing during valve overlap.

FIG. 4 is a graph illustrating the relationship between the ratio between the intake air pressure at a predetermined timing during valve overlap and the cylinder pressure at a predetermined timing immediately before or at the start of valve overlap and the ratio between an intake air pressure and exhaust gas pressure at a predetermined timing during valve overlap.

FIG. 5 is a flow chart for explaining the routine for estimating the intake air pressure at a predetermined timing during valve overlap based on the cylinder pressure.

FIG. 6 is a view of an intake air model.

FIG. 7 is a view of the relationship between the throttle valve opening degree and the flow rate coefficient.

FIG. 8 is a view of the function .PHI.(Ps/Pa).

FIG. 9 is a view of the basic concept of a throttle model.

FIG. 10 is a view of the basic concept of an intake pipe model.

FIG. 11 is a view of the basic concept of an intake valve model.

FIG. 12 is a view relating to the definitions of the amount of intake air Mc and combustion chamber intake air flow rate mc.

FIG. 13 is a flow chart for explaining an intake air amount calculation routine in another embodiment of the present invention.

BEST MODE FOR WORKING THE INVENTION

A control system of an internal combustion engine according to the present invention calculates an amount of change of cylinder pressure due to valve overlap when valve overlap between an intake valve and exhaust valve is set and calculates an amount of air sucked into a combustion chamber based on the amount of change of this cylinder pressure and the cylinder pressure detected by a cylinder pressure detecting means at a predetermined timing.

Here, an amount of residual gas Me remaining in the combustion chamber due to the valve overlap when valve overlap between the intake valve and exhaust valve is set is expressed by the following equation (1) when an intake air pressure at a predetermined timing during valve overlap (timing when crank angle becomes .theta..sub.1) is Pm(.theta..sub.1), an exhaust gas pressure at the predetermined timing is Pe(.theta..sub.1), a temperature of the exhaust gas at that time is Te, and a gas constant is R(J/(kgK)), Me=S.phi.(Pm(.theta..sub.1)/Pe(.theta..sub.1))Pe(.theta..sub.1)/ {square root over (RTe)} (1)

In the equation (1), S is an effective area allowing passage of gas during valve overlap, that is, a gas passage effective area. The gas passage effective area S is expressed by the following equation (2). In equation (2), Ne(.theta..sub.1) is an engine speed at the timing when the crank angle becomes .theta..sub.1. Further, Ri is a valve diameter of an intake valve Vi, Re is a valve diameter of an exhaust valve Ve, Li(.theta.) is an amount of lift of the intake valve Vi, Le(.theta.) is an amount of lift of the exhaust valve Ve, IVO is a crank angle at the timing of opening of the intake valve Vi, and EVC is a crank angle at the timing of closing of the exhaust valve Ve. Further, in the equation (2), the value obtained by integrating the {square root over ((Li(.theta.)Le(.theta.)))}{square root over ((Li(.theta.)Le(.theta.)))} from IVO to EVC (i.e. .intg. {square root over ((Li(.theta.)Le(.theta.)))}{square root over ((Li(.theta.)Le(.theta.)))}d.theta.) is the value determined in accordance with the amount of advance (VVT advance) of the variable valve timing mechanism.

Further, in the equation (1), .phi.(Pm(.theta..sub.1)/Pe(.theta..sub.1)) is a term related to the ratio between the intake air pressure and the exhaust gas pressure and is basically expressed by the following equation (3). When the value of Pm(.theta..sub.1)/Pe(.theta..sub.1) is small, it is expressed by the following equation (4). However, in the equation (3) and equation (4), .kappa. is the specific heat ratio.

.pi..times..times..function..times..degree..intg..times..function..theta..- function..theta..times.d.theta..function..theta..PHI..function..function..- theta..function..theta..times..kappa..kappa..function..theta..function..th- eta..kappa..function..theta..function..theta..kappa..kappa..times..times..- times..times..function..theta..function..theta..gtoreq..kappa..kappa..kapp- a..PHI..function..function..theta..function..theta..kappa..times..kappa..k- appa..kappa..kappa..times..times..times..times..function..theta..function.- .theta.<.kappa..kappa..kappa. ##EQU00001##

On the other hand, between the amount of residual gas Me remaining in the combustion chamber due to the valve overlap and the amount of change .DELTA.Pc of the cylinder pressure due to the valve overlap, in general the relationship of the following equation (5) stands. Due to this, from the equation (1) and equation (5), the amount of change .DELTA.Pc of the cylinder pressure is expressed as in the following equation (6) based on the amount of residual gas Me remaining in the combustion chamber due to the valve overlap. In equation (6), .alpha. is a constant determined based on experiments etc. Further, from the amount of change .DELTA.Pc of this cylinder pressure and the cylinder pressure Pc (.theta..sub.2) detected by the cylinder pressure detecting means at a predetermined timing during the compression stroke (timing when the crank angle becomes .theta..sub.2, timing after intake valve closing and before start of combustion (before spark ignition or before compression ignition)), the amount of air sucked into the combustion chamber M.sub.air can be expressed by the following equation (7). However, in equation (7), .beta. is a constant determined based on experiments etc. .DELTA.Pc .varies. Me {square root over (Te)} (5) .DELTA.Pc=.alpha.S.phi.(Pm(.theta..sub.1)/Pe(.theta..sub.1))Pe(.theta..su- b.1) (6) M.sub.air=.beta.(Pc(.theta..sub.2)-.DELTA.Pc) (7)

Therefore, as executed by a control system of an internal combustion engine according to the present invention, if obtaining the intake air pressure Pm(.theta..sub.1), exhaust gas pressure Pe(.theta..sub.1), and engine speed Ne(.theta..sub.1) at a predetermined timing during valve overlap and the cylinder pressure Pc(.theta..sub.2) detected at a predetermined timing, it is possible to calculate the amount of air sucked into the combustion chamber at a low cost and good precision without using a large number of sensors.

Further, as explained above, when calculating the amount of change .DELTA.Pc of the cylinder pressure due to the valve overlap based on the intake air pressure Pm(.theta..sub.1) and exhaust gas pressure Pe(.theta..sub.1) during valve overlap, the exhaust gas pressure Pe(.theta..sub.1) is preferably estimated based on the cylinder pressure detected by the cylinder pressure detecting means Pc(.theta..sub.0) immediately before or at the start of valve overlap (at the timing when the crank angle becomes .theta..sub.0).

That is, the exhaust gas pressure before opening the intake valve for valve overlap or at the time of opening of the intake valve generally matches the cylinder pressure. If the load of the internal combustion engine is not that large, the change in exhaust gas pressure before and after opening of the intake valve for the valve overlap, is small. Therefore, the exhaust gas pressure Pe(.theta..sub.1) during valve overlap can be estimated based on the cylinder pressure Pc(.theta..sub.0) detected by the cylinder pressure detecting means before or at the start of valve overlap. If at the time of low load of the internal combustion engine, for example, Pe(.theta..sub.1) may be assumed to be Pc(.theta..sub.0). Due to this, a sensor for detecting the pressure of the exhaust gas becomes unnecessary, so it becomes possible to reduce the cost required for calculation of the amount of air sucked into the combustion chamber.

On the other hand, if the load of the internal combustion engine rises to a certain extent, due to the effects of exhaust pulsation etc., the change in the exhaust gas pressure during valve overlap becomes larger and replacement of the exhaust gas pressure Pe(.theta..sub.1) during valve overlap by the cylinder pressure Pc(.theta..sub.0) detected by the cylinder pressure detecting means before or at the start of valve overlap becomes difficult.

That is, until the load of the internal combustion engine rises to a certain extent, the ratio between the intake air pressure Pm(.theta..sub.1) and the exhaust gas pressure Pe(.theta..sub.1) during valve overlap entered for the function .phi. of the equation (3) generally matches with the ratio of the intake air pressure Pm(.theta..sub.1) during valve overlap and the cylinder pressure Pc(.theta..sub.0) detected before or at the start of valve overlap. The values of the two increase along with a rise in the load. As opposed to this, if the ratio between the intake air pressure Pm(.theta..sub.1) and the cylinder pressure Pc(.theta..sub.0) rises above a predetermined value .epsilon. determined by experiments or experience, the correlation Pm(.theta..sub.1)/Pe(.theta..sub.1)=Pm(.theta..sub.1)/Pc(.theta..sub.0) no longer stands.

For this reason, when the ratio between the intake air pressure Pm(.theta..sub.1) and cylinder pressure Pc(.theta..sub.0) exceeds a predetermined value .epsilon., the ratio between the intake air pressure Pm(.theta..sub.1) and exhaust gas pressure Pe(.theta..sub.1) during valve overlap is assumed to be fixed at a predetermined value .epsilon., the exhaust gas pressure Pe(.theta..sub.1) during valve overlap is preferably determined based on the intake air pressure Pm(.theta..sub.1) during valve overlap and the predetermined value .epsilon. as Pe(.theta..sub.1)=Pm(.theta..sub.1)/.epsilon.. Due to this, when the exhaust gas pressure during valve overlap is not actually measured, even if the load of the internal combustion engine rises, the amount of air sucked into the combustion chamber can be calculated precisely without effect due to the change in the exhaust gas pressure accompanying valve overlap.

Further, in an internal combustion engine having a plurality of combustion chambers, a cylinder pressure detecting means may be provided for each combustion chamber. In this case, preferably the amount of change .DELTA.Pc of the cylinder pressure is calculated for each combustion chamber and the amount of air sucked into each combustion chamber is calculated based on the amount of change .DELTA.Pc of the cylinder pressure in each combustion chamber and the cylinder pressure Pc(.theta..sub.2) in each combustion chamber detected by each cylinder pressure detecting means at a predetermined timing. Due to this, it is possible to obtain a precise grasp of the variations in the amount of intake air between combustion chambers, so it is possible to improve the precision of control of the air-fuel ratio etc. in each combustion chamber.

Further, the intake air pressure during valve overlap in any combustion chamber may be estimated based on the cylinder pressure at intake bottom dead center of another combustion chamber where the intake stroke was executed before the combustion chamber.

In general, the intake air pressure and cylinder pressure becomes generally the same at intake bottom dead center. Further, the timing when valve overlap is executed at a certain combustion chamber generally matches with the timing when the intake bottom dead center arrives in another combustion chamber where the intake stroke is executed 1/N cycle before the combustion chamber (where the four strokes of intake, compression, expansion, and exhaust are designated as 1 cycle and N indicates the number of cylinders). Therefore, by estimating the intake air pressure based on the cylinder pressure, the sensor for detecting the intake air pressure becomes unnecessary and the cost of calculating the amount of air sucked into each combustion chamber can be reduced much more.

Below, the drawings will be referred to so as to explain the best mode for working the present invention in detail.

FIG. 1 is a schematic view of the configuration showing an internal combustion engine to which the control system according to the present invention is applied. The internal combustion engine 1 shown in the figure is one which burns an air-fuel mixture comprised of fuel and air inside a combustion chamber 3 formed in a cylinder block 2 to make a piston 4 move back and forth in the combustion chamber 3 and thereby generate power. The internal combustion engine 1 is preferably configured as a multicylinder engine. The internal combustion engine 1 of the present embodiment is configured as for example a four-cylinder engine.

The intake port of each combustion chamber 3 is connected to an intake pipe (intake manifold) 5, while the exhaust port of each combustion chamber 3 is connected to an exhaust pipe 6 (exhaust manifold). Further, the cylinder head of the internal combustion engine 1 is provided with an intake valve Vi and exhaust valve Ve for each combustion chamber 3. Each intake valve Vi opens/closes a corresponding intake port, while each exhaust valve Ve opens/closes a corresponding exhaust port. Each intake valve Vi and each exhaust valve Ve is opened/closed by a valve mechanism VM including a variable valve timing mechanism. Further, the internal combustion engine 1 has a number of spark plugs 7 corresponding to the number of cylinders. Each spark plug 7 is provided at the cylinder head to approach the inside of the corresponding combustion chamber 3.

The intake pipe 5, as shown in FIG. 1, is connected to the surge tank 8. The surge tank 8 has an air feed line L1 connected to it. The air feed line L1 is connected through an air cleaner 9 to a not shown air intake port. Further, in the middle of the air feed line L1 (between the surge tank 8 and air cleaner 9), a throttle valve (in the present embodiment, an electronic control type throttle valve) 10 is installed. On the other hand, the exhaust pipe 6, as shown in FIG. 1, has a front-end catalyst device 11a including, for example, a three-way catalyst and a back-end catalyst device 11b including, for example, a NOx storing and reducing catalyst, connected to it.

Further, the internal combustion engine 1 has a plurality of injectors 12. Each injector 12, as shown in FIG. 1, is arranged at the cylinder head to approach the inside of the corresponding combustion chamber 3. Further, each piston 4 of the internal combustion engine 1 is configured by a recessed top face type which has a recess 4a on its top face. Further, in the internal combustion engine 1, in the state with air sucked into each combustion chamber 3, each injector 12 directly injects gasoline or other fuel toward the recess 4a of the piston 4 in each combustion chamber 3.

Due to this, in the internal combustion engine 1, a layer of the air-fuel mixture comprised of the fuel and air is formed near the spark plug 7 in a state separated from the surrounding air layer (stratification), so it is possible to use an extremely lean air-fuel mixture for execution of stable stratified combustion. Note that the internal combustion engine 1 of the present embodiment is explained as a so-called direct injection engine, but is not limited to this. The present invention may also be applied to an intake pipe (intake port) injection type internal combustion engine as a matter of course.

The above-mentioned spark plugs 7, throttle valves 10, injectors 12, valve mechanism VM, etc. are electrically connected to the ECU 20 functioning as the control system of the internal combustion engine 1. The ECU 20 includes, all not shown, a CPU, ROM, RAM, input/output ports, storage device, etc. The ECU 20, as shown in FIG. 1, has the crank angle sensor 14 of the internal combustion engine 1 and other various types of sensors electrically connected to it. The ECU 20 uses various types of maps etc. stored in the storage device and controls the spark plugs 7, throttle valves 10, injectors 12, valve mechanism VM, etc. so that the desired output is obtained based on the detection values of the various types of sensors etc.

Further, the internal combustion engine 1 has a number of cylinder pressure sensors including semiconductor devices, piezoelectric devices, piezomagnetic devices, or optical fiber detection devices, etc. (cylinder pressure detecting means) 15 corresponding to the number of cylinders. Each cylinder pressure sensor 15 is arranged at the cylinder head so that its pressure receiving face approaches the inside of the corresponding combustion chamber 3 and is electrically connected through a not shown A/D converter etc. to the ECU 20. Each cylinder pressure sensor 15 outputs a pressure applied to the pressure receiving face in the combustion chamber 3 (cylinder pressure) as a relative value with respect to the atmospheric pressure and gives a voltage signal corresponding to the pressure applied to the pressure receiving face (cylinder pressure) (signal showing the detection value) to the ECU 20.

Further, the internal combustion engine 1 has an intake pressure sensor 16 detecting the intake air pressure inside the surge tank 8 (intake pressure) as an absolute pressure. The intake pressure sensor 16 is also electrically connected, through a not shown A/D converter etc., to the ECU 20 and sends a signal showing the detected absolute pressure of the intake air in the surge tank 8 to the ECU 20. Note that detection values of the crank angle sensor 14 and the intake pressure sensor 16 are successively given every other incremental time interval to the ECU 20 and stored in a predetermined storage region (buffer) of the ECU 20 in predetermined amounts. Further, the detection value of each cylinder pressure sensor 15 (cylinder pressure) is corrected in absolute pressure based on the detection value of the intake pressure sensor 16 and stored in a predetermined storage region (buffer ) of the ECU 20 in predetermined amounts.

Next, referring to FIG. 2, the routine for calculating the amount of air sucked into each combustion chamber 3 in the above-mentioned internal combustion engine 1 will be explained. When the internal combustion engine 1 is started, the ECU 20 repeatedly executes the intake air calculation routine shown in FIG. 2 for each combustion chamber 3. The intake air calculation routine of FIG. 2 is basically for calculating the amount of air sucked into each combustion chamber 3 using equations (1) to (7). When the timing for execution of this routine arrives, the ECU 20 first judges if the valve opening timing of the intake valve Vi is advanced (S10).

When the ECU 20 judges at S10 that the valve opening timing of the intake valve Vi is advanced, the ECU 20 reads from the predetermined storage region the cylinder pressure Pc(.theta..sub.0) detected by the cylinder pressure sensor 15 at a predetermined timing immediately before or at the start of valve overlap between the intake valve Vi and exhaust valve Ve (timing when crank angle becomes .theta..sub.0) for the combustion chamber 3 concerned and reads the intake air pressure Pm(.theta..sub.1) detected by the intake pressure sensor 16 at a predetermined timing during valve overlap between the intake valve Vi and exhaust valve Ve (timing when crank angle becomes .theta..sub.1) (S12). Further, at S12, the ECU 20 finds the engine speed Ne(.theta..sub.1) at the timing when the crank angle becomes .theta..sub.1 based on the detection value of the crank angle sensor 14 at a predetermined timing during valve overlap (timing where crank angle becomes .theta..sub.1) and obtains the VVT advance at the timing when the crank angle becomes .theta..sub.1 from the valve mechanism VM.

In the present embodiment, the predetermined timing immediately before or at the time of start of valve overlap between the intake valve Vi and the exhaust valve Ve is made the time of start of valve overlap, that is, the time of opening of the intake valve Vi, and is made the timing where the crank angle becomes, for example, .theta..sub.0=BTDC20.degree.. Further, the predetermined timing during valve overlap is made the timing when the crank angle becomes, for example, .theta..sub.1=BTDC10.degree. (exhaust BTDC10.degree.). At S12, when it obtains the cylinder pressure Pc(.theta..sub.0) at the timing when the crank angle becomes .theta..sub.0 and the intake air pressure Pm(.theta..sub.1) at the timing when the crank angle becomes .theta..sub.1, the ECU 20 finds the value of the ratio Pm(.theta..sub.1)/Pc(.theta..sub.0) between the intake air pressure Pm(.theta..sub.1) and cylinder pressure Pc(.theta..sub.0) for the combustion chamber 3 concerned and judges if the value of Pm(.theta..sub.1)/Pc(.theta..sub.0) is a predetermined threshold value .epsilon. (in the present embodiment, .epsilon.=0.95) or less (S14).

Here, between the ratio Pm(.theta..sub.1)/Pc(.theta..sub.0) between the intake air pressure Pm(.theta..sub.1) and cylinder pressure Pc(.theta..sub.0) and the ratio Pm(.theta..sub.1)/Pe(.theta..sub.1) between the intake air pressure Pm(.theta..sub.1) and the exhaust gas pressure Pe(.theta..sub.1), which is the parameter used in the equation (3), the relationship illustrated in FIG. 3 stands. That is, in the range where the load of the internal combustion engine 1 is not that large, the value of Pm(.theta..sub.1)/Pe(.theta..sub.1) and the value of Pm(.theta..sub.1)/Pc(.theta..sub.0) increase along with the rise in the load and the relationship Pm(.theta..sub.1)/Pe(.theta..sub.1)=Pm(.theta..sub.1)/Pc(.theta..sub.0) holds.

That is, at the timing immediately before opening the intake valve Vi for valve overlap or at the time of opening, the exhaust gas pressure generally matches with the cylinder pressure. When the load of the internal combustion engine 1 is not that large, the change in exhaust gas pressure before and after opening the intake valve Vi for valve overlap is small. Therefore, in the range where the load of the internal combustion engine 1 is not that large, it is possible to estimate the exhaust gas pressure Pe(.theta..sub.1) during valve overlap, that is, at the timing when the crank angle becomes .theta..sub.1, based on the cylinder pressure Pc(.theta..sub.0) detected by the cylinder pressure sensor 15 immediately before or at the start of valve overlap, that is, at the timing when the crank angle becomes .theta..sub.0. It is possible to deem that Pe(.theta..sub.1)=Pc(.theta..sub.0), Pm(.theta..sub.1)/Pe(.theta..sub.1)=Pm(.theta..sub.1)/Pc(.theta..sub.0).

As opposed to this, if the load of the internal combustion engine 1 becomes higher by a certain extent, due to the effects of exhaust pulsation etc., the change in the exhaust gas pressure before and after opening the intake valve Vi for valve overlap becomes larger. That is, if the load of the internal combustion engine 1 becomes higher by a certain extent and the ratio Pm(.theta..sub.1)/Pc(.theta..sub.0) between the intake air pressure Pm(.theta..sub.1) and the cylinder pressure Pc(.theta..sub.0) becomes the predetermined value .epsilon. or more, the correlation of Pm(.theta..sub.1)/Pe(.theta..sub.1)=Pm(.theta..sub.1)/Pc(.theta..sub.0) no longer stands and replacement of the exhaust gas pressure Pe(.theta..sub.1) during valve overlap by the cylinder pressure Pc(.theta..sub.0) detected by the cylinder pressure detecting means immediately before or at the start of valve overlap becomes difficult.

Considering these points, in the internal combustion engine 1, when it is judged at S14 for each combustion chamber 3 that the value of Pm(.theta..sub.1)/Pc(.theta..sub.0) is the threshold value .epsilon. or less, the exhaust gas pressure Pe(.theta..sub.1) during valve overlap is replaced by the cylinder pressure Pc(.theta..sub.0) detected by the cylinder pressure sensor 15 immediately before or at the start of valve overlap and Pe(.theta..sub.1)=Pc(.theta..sub.0) is set by the ECU 20 (S16). Further, when it is judged at S14 for each combustion chamber 3 that the value of Pm(.theta..sub.1)/Pc(.theta..sub.0) exceeds the threshold value .epsilon., the ECU 20 sets the exhaust gas pressure Pe(.theta..sub.1) during valve overlap using the predetermined value .epsilon. as Pe(.theta..sub.1)=Pm(.theta..sub.1)/.epsilon. (S18). That is, in the processing of S18, the ratio between the intake air pressure Pm(.theta..sub.1) and the exhaust gas pressure Pe(.theta..sub.1) during valve overlap is assumed to be fixed to the guard value constituted by the threshold value .epsilon. (in the present embodiment, 0.95) and the exhaust gas pressure Pe(.theta..sub.1) during valve overlap is set based on the intake air pressure Pm(.theta..sub.1) during valve overlap and the threshold value .epsilon..

If executing the processing of S16 or S18, the ECU 20 sets the value of .intg. {square root over ((Li(.theta.)Le(.theta.)))}{square root over ((Li(.theta.)Le(.theta.)))}d.theta. corresponding to the VVT advance obtained at S12 using a predetermined function equation or map and uses this value and the engine speed Ne(.theta..sub.1) obtained at S12 to calculate the gas passage effective area S from the equation (2) (S20). If finding the gas passage effective area S, the ECU 20 judges whether the value of the intake air pressure Pm(.theta..sub.1) obtained at S12 divided by the exhaust gas pressure Pe(.theta..sub.1) during valve overlap set at S16 or S18 is the threshold value (2/(.kappa.+1)).sup..kappa./(.kappa.-1) or more (S22). In the present embodiment, as the threshold value (2/(.kappa.+1)).sup..kappa./(.kappa.-1), for example a constant obtained as .kappa.=1.32 is used.

As explained above, the equation expressing the .phi.(Pm(.theta..sub.1)/Pe(.theta..sub.1)) required when calculating the amount of change .DELTA.Pc of the cylinder pressure due to valve overlap changes in accordance with the value of Pm(.theta..sub.1)/Pe(.theta..sub.1). For this reason, when the ECU 20 judges at S22 that the value of Pm(.theta..sub.1)/Pe(.theta..sub.1) is the threshold value or more, it uses the equation (3) to calculate the value of .phi.(Pm(.theta..sub.1)/Pe(.theta..sub.1)) (S24). Further, when the ECU 20 judges at S22 that the value of Pm(.theta..sub.1)/Pe(.theta..sub.1) is lower than the threshold value, it uses the equation (4) to calculate the value of .phi.(Pm(.theta..sub.1)/Pe(.theta..sub.1)) (S26).

When finding the gas passage effective area S at S20 and finding the value of .phi.(Pm(.theta..sub.1)/Pe(.theta..sub.1)) at S24 or S26, the ECU 20 uses the equation (6) to calculate the amount of change .DELTA.Pc of the cylinder pressure due to the valve overlap for the combustion chamber 3 concerned (S28). After the processing of S28, the ECU 20 reads from the predetermined storage region, for each combustion chamber 3 concerned, the cylinder pressure Pc(.theta..sub.2) detected by the cylinder pressure sensor 15 at the timing when the crank angle during the compression stroke becomes .theta..sub.2 (S30). Note that, in the present embodiment, the predetermined timing during the compression stroke is made the timing when the crank angle becomes for example .theta..sub.2=BTDC50.degree. (compression BTDC50.degree.).

Further, the ECU 20 uses the equation (7) to calculate the amount of intake air M.sub.air for the combustion chamber 3 concerned from the amount of change .DELTA.Pc of the cylinder pressure found at S28 and the cylinder pressure Pc(.theta..sub.2) obtained at S30 (S32). In this way, in the internal combustion engine 1, by obtaining the intake air pressure Pm(.theta..sub.1) and exhaust gas pressure Pe(.theta..sub.1) and engine speed Ne(.theta..sub.1) at a predetermined timing during valve overlap and the cylinder pressure Pc(.theta..sub.2) detected at a predetermined timing, it is possible to precisely calculate the amount of air sucked into each combustion chamber 3 at a low cost without using a large number of sensors.

Further, in the internal combustion engine 1, when the load is relatively low and it is judged at S14 that the value of Pm(.theta..sub.1)/Pc(.theta..sub.0) is the threshold value .epsilon. or less, the exhaust gas pressure Pe(.theta..sub.1) during valve overlap is replaced by the cylinder pressure Pc(.theta..sub.0) detected by the cylinder pressure sensor 15 immediately before or at the start of valve overlap. Due to this, the sensor for measuring the exhaust gas pressure becomes unnecessary, so the cost required for calculating the amount of air sucked into each combustion chamber 3 can be reduced.

Further, in an internal combustion engine 1 from which a sensor for measuring the exhaust gas pressure is omitted, when the load rises and it is judged at S14 that the value of Pm(.theta..sub.1)/Pc(.theta..sub.0) exceeds a threshold value .epsilon., if it is assumed that the ratio between the intake air pressure Pm(.theta..sub.1) and exhaust gas pressure Pe(.theta..sub.1) during valve overlap is fixed to a so-called guard value constituted by the threshold value .epsilon. (in the present embodiment, 0.95), the exhaust gas pressure Pe(.theta..sub.1) during the valve overlap is determined based on the threshold value .epsilon. (S18). In this way, when the load of the internal combustion engine 1 rises, the difference between the intake air pressure and the exhaust gas pressure is small and further the amount of residual gas itself becomes smaller, so even if the processing such as in S18 is executed, there is no effect due to changes in the exhaust gas pressure, the amount of air sucked into each combustion chamber 3 can be precisely calculated, and good, practical results can be obtained.

Further, in an internal combustion engine 1 provided with a plurality of combustion chambers 3 and a cylinder pressure sensor 15 provided for each combustion chamber 3, the amount of change .DELTA.Pc of the cylinder pressure is calculated for each combustion chamber 3 and the amount of air sucked into each combustion chamber 3 is calculated based on the amount of change .DELTA.Pc of the cylinder pressure in each combustion chamber 3 and the cylinder pressure Pc(.theta..sub.2) in each combustion chamber 3 detected by each cylinder pressure sensor 15. Due to this, it is possible to obtain a precise understanding of the variations in the amount of intake air between combustion chambers 3, so it is possible to improve the precision of control of the air-fuel ratio etc. in each combustion chamber 3.

On the other hand, when judging at S10 that the valve opening timing of the intake valve Vi is not advanced and no valve overlap between the intake valve Vi and the exhaust valve Ve is set, the ECU 20 sets the amount of change .DELTA.Pc of the cylinder pressure used at S32 to zero (S34). Due to this, when no valve overlap is set, at S32, the amount of air M.sub.air sucked into each combustion chamber 3 is calculated based on only the cylinder pressure Pc(.theta..sub.2) obtained at S30. Here, the cylinder pressure during the compression stroke is a relative high value and can be precisely detected without regard to the detection precision of the cylinder pressure sensor 15 and the resolution of the cylinder pressure data, etc. Therefore, by using the cylinder pressure in the combustion chamber 3 at a predetermined timing during the compression stroke, it is possible to precisely find the amount of air sucked in the combustion chamber 3.

Note that in the above-mentioned internal combustion engine 1, when a negative judgment is made at S14, it is assumed that the ratio between the intake air pressure Pm(.theta..sub.1) and exhaust gas pressure Pe(.theta..sub.1) during valve overlap are fixed to the guard value constituted by the threshold value .epsilon., but the invention is not limited to this. That is, as shown in FIG. 4, the relationship between the ratio Pm(.theta..sub.1)/Pc(.theta..sub.0) between the intake air pressure Pm(.theta..sub.1) and the cylinder pressure Pc(.theta..sub.0) and the ratio Pm(.theta..sub.1)/Pe(.theta..sub.1) between the intake air pressure Pm(.theta..sub.1) and exhaust gas pressure Pe(.theta..sub.1) may be approximated using a plurality of functions.

In the example of FIG. 4, the relationship between Pm(.theta..sub.1)/Pc(.theta..sub.0) and Pm(.theta..sub.1)/Pe(.theta..sub.1) is approximated using two lines. In the range where 0.ltoreq.Pm(.theta..sub.1)/Pc(.theta..sub.0).ltoreq..epsilon..sub.1 (where, .epsilon..sub.1 is a constant determined by experiments or experience), Pm(.theta..sub.1)/Pe(.theta..sub.1)=Pm(.theta..sub.1)/Pc(.theta..sub.0), while in the range where .epsilon..sub.1.ltoreq.Pm(.theta..sub.1)/Pc(.theta..sub.0).ltoreq.1.0, Pm(.theta..sub.1)/Pe(.theta..sub.1) is expressed by the following equation (8) (where, in equation (8), .epsilon..sub.2 is a constant determined by experiments or experience, where .epsilon..sub.2>.epsilon..sub.1). When this approximation technique is employed, when it is judged negatively at S14 of FIG. 2, at S18, the value of Pe(.theta..sub.1) is set in accordance with the following equation (9).

.function..theta..function..theta..function..theta..function..theta..funct- ion..theta..function..theta..function..theta..function..theta. ##EQU00002##

Further, in the present embodiment, the surge tank 8 is provided with an intake pressure sensor 16 for detecting the intake air pressure, but the intake pressure sensor 16 may also be omitted. The intake air pressure Pm(.theta..sub.1) at a predetermined timing during valve overlap (timing when crank angle becomes .theta..sub.1) may also be estimated based on the cylinder pressure.

That is, the intake air pressure and the cylinder pressure become substantially equal at intake bottom dead center. Further, the timing when valve overlap is executed at a certain combustion chamber 3, in a four-cylinder engine, generally matches with the timing when intake bottom dead center arrives in another combustion chamber 3 where the intake stroke was executed 1/4 cycle (180.degree.) before the combustion chamber 3. Therefore, based on this, the intake air pressure during valve overlap in a certain combustion chamber 3 can be estimated based on the cylinder pressure at intake bottom dead center of the combustion chamber 3 where the intake stroke was executed 1/4 cycle before the combustion chamber 3. Due to this, the intake pressure sensor 16 for detecting the intake air pressure becomes unnecessary and the cost required for calculating the amount of air sucked into each combustion chamber 3 can be reduced even more.

FIG. 5 is a flow chart for explaining the routine for estimating the intake air pressure at a predetermined timing during valve overlap based on the cylinder pressure. The routine of FIG. 5 is executed by the ECU 20, for example, at a predetermined timing before S14 of FIG. 2. In this case, the ECU 20 reads from the predetermined storage region the detection value Pc (.theta..sub.BDC) of the cylinder pressure sensor 15 at the nearest intake bottom dead center in the combustion chamber 3 (prior combustion chamber) where the intake stroke was executed 1/4 cycle before the combustion chamber 3 concerned (S100). Further, the ECU 20 reads from a predetermined storage region the detection values Pc(.theta..sub.a) and Pc(.theta..sub.b) of the cylinder pressure sensor 15 at a predetermined two points in the compression stroke after intake bottom dead center of the combustion chamber 3 where the intake stroke was executed 1/4 cycle before the combustion chamber 3 concerned (S102). Note that so long as the crank angles .theta..sub.a and .theta..sub.b are selected so as to be included in the compression stroke, they may be any values.

Here, if the intake pressure sensor is omitted, the output of the cylinder pressure sensor 15 (relative pressure) cannot be corrected in absolute pressure based on the detection value of the intake pressure sensor 16, so the detection values Pc(.theta..sub.a) and Pc(.theta..sub.b) of the cylinder pressure sensor 15 are stored in the storage region as they are without being corrected in absolute pressure (in the state showing the relative pressure). Here, when the cylinder pressure (true value) after correction of the absolute pressure when the crank angle becomes .theta..sub.a is Pa, the cylinder pressure (true value) after correction of the absolute pressure when the crank angle becomes .theta..sub.b is Pb, and the absolute pressure corrected value of the cylinder pressure sensor 15 is Pr, Pa=Pc(.theta..sub.a)+Pr Pb=Pc(.theta..sub.b)+Pr Further, when the compression stroke of the internal combustion engine is deemed to be an adiabatic process and the specific heat ratio is made .kappa., the relationship PaV.sup..kappa.(.theta..sub.a)=PbV.sup..kappa.(.theta..sub.b) holds. This relationship can be expressed by the following equation (10). Further, by solving equation (10) for the absolute pressure corrected value Pr, the absolute pressure corrected value Pr is expressed as in the following equation (11).

.function..theta..kappa..function..theta..function..theta..kappa..function- ..theta..function..theta..kappa..function..theta..function..theta..kappa..- function..theta..kappa..function..theta..kappa..function..theta. ##EQU00003##

For this reason, the ECU 20, after the processing of S102, uses the detection values Pc (.theta..sub.a) and Pc(.theta..sub.b) of the cylinder pressure sensor 15 at a two predetermined points in the compression stroke of the prior combustion chamber 3 and the cylinder volumes V(.theta..sub.a) and V(.theta..sub.b) of the two predetermined points to calculate, by the equation (11), the absolute pressure corrected value Pr of the cylinder pressure sensor 15 provided at the prior combustion chamber 3 (S104). Note that the values of the cylinder volumes V(.theta..sub.a) and V(.theta..sub.b) used at S104 are calculated in advance and stored in the storage device. The ECU 20 reads the values of the cylinder volumes V(.theta..sub.a) and V(.theta..sub.b) from the storage device and uses them for the processing of S20.

If finding the absolute pressure corrected value Pr at S104, the ECU 20 uses the detection value Pc(.theta..sub.BDC) Of the cylinder pressure sensor 15 at intake bottom dead center obtained at S100 and the absolute pressure corrected value Pr found at S104 to calculate the intake air pressure Pm(.theta..sub.1) during valve overlap in the combustion chamber 3 concerned (S106). That is, the intake air pressure Pm(.theta..sub.1) during valve overlap at the particular combustion chamber 3 can be calculated as Pm(.theta..sub.1)=Pr+Pc.sub.-180(.theta..sub.BDC) when the cylinder pressure at intake bottom dead center of the combustion chamber 3 where the intake stroke was executed 1/4 cycle (in an N-cylinder engine, 1/N cycle) before the particular combustion chamber 3 is Pc.sub.-180(.theta..sub.BDC). In this way, by the routine of FIG. 5 being executed, without using an intake pressure sensor for detecting the intake air pressure, it is possible to precisely calculate the amount of air sucked into each combustion chamber 3 based on the cylinder pressure P(.theta.) and the cylinder volume V(.theta.) (based on the product P(.theta.)V.sup..kappa.(.theta.) between the cylinder pressure P(.theta.) and the value V.sup..kappa.(.theta.) of the cylinder volume V(.theta.) to the power of a specific heat ratio (predetermined exponent) .kappa.).

However, in the technique for calculating the amount of air M.sub.air sucked into a combustion chamber 3 explained above, finally the equation (7) is used to calculate the amount of intake air M.sub.air, so the cylinder pressure Pc (.theta..sub.2) is required. Here, the cylinder pressure Pc(.theta..sub.2), as explained above, is found based on a value detected at a predetermined timing during the compression stroke (timing after intake valve closes and before start of combustion (before spark ignition or before compression ignition)). Therefore, the technique for calculating the amount of intake air M.sub.air explained above can be used to ca