APPARATUS AND METHOD FOR CONTROLLING KNOCK IN AN INTERNAL

COMBUSTION ENGINE

BACKGROUND OF THE INVENTION

1. Field of the Invention

[0001] The present invention relates to an apparatus and method for controlling

knock in an internal combustion engine.

2. Description of the Related Art

[0002] When the ignition timing of the internal combustion engine is too advanced,

knocking occurs due to abnormal combustion, which can damage the cylinders and the

pistons. Some internal combustion engines are thus provided with a knock control

apparatus to prevent knocking. The knock control apparatus determines whether knocking

has occurred, and regulates operations of the internal combustion engine to prevent

knocking.

[0003] The determination by the knock control apparatus depends on a signal output

from a knock sensor. A typical knock sensor is a vibration pickup that is mounted onto the

cylinder block. The knock sensor senses vibrations of the cylinder block in the frequency

range specific to knocking and converts the vibrations into an electrical output signal. The

knock control apparatus determines that knocking occurs when a knock intensity value

LVPK, which is converted from the output signal of the knock sensor, is equal to or greater

than a knock criterion level VKD. In contrast, the knock control apparatus determines that

knocking does not occur when the knock intensity value LVPK is lower than the knock

criterion level VKD. In accordance with the determination result, the knock control

apparatus controls the internal combustion engine to prevent knocking, in other words, the

knock control apparatus executes a knock prevention control. The knock prevention

control is generally achieved by a responsive ignition timing control that results in more

effective knock prevention. More specifically, the ignition timing is retarded when

knocking is detected or advanced when knocking is not detected. This allows the knock

control apparatus to control the internal combustion engine such that knocking is minimized

within the allowable range or the engine conditions are maintained at the level immediately

before knocking occurs.

[0004] The cylinder block vibrates not only due to knocking, but also due to driving

the injectors, valves, and other components of the internal combustion engine. Thus, the

output signals of the knock sensor include a true knock signal generated due to knocking as

well as a false knock signal generated due to mechanical operations of the engine

components. Therefore, if the false knock signal is improperly determined as the true

knock signal, the ignition timing is unnecessarily retarded, which can lower the engine

power output and the emission efficiency.

[0005] In view of this problem, the following knock control apparatuses for an

internal combustion engine have been proposed. Japanese Patent Application Publication

No. JP-A-Hei 6-101556 describes a knock control apparatus that calculates the angle of

ignition timing retardation, performed under the retardation control, based on the output

signal of the knock sensor, and determines whether the output signal of the knock sensor is

a false knock signal based on the calculated retardation angle. To be more specific, the

knock control apparatus presets a certain maximum threshold and minimum threshold. If

the retardation angle falls within the range between the preset maximum threshold and the

preset minimum threshold, the output signal is determined to be a false knock signal. In

contrast, if the retardation angle falls out of the range, the output signal is determined to be

a true knock signal.

[0006] Another knock control apparatus for an internal combustion engine is

described in JP-A-2003-278592. The knock control apparatus described in

JP-A-2003-278592determines whether a signal, output from the knock sensor when

knocking occurs is a true knock signal based on the knock intensity LVPK. The knock

intensity LVPK is a value converted from the output signal of the knock sensor. More

specifically, if the current knock intensity value LVPK is equal to or lower than its previous

value, the output signal is determined to be a true knock signal. In contrast, if the current

knock intensity value LVPK exceeds its previous value, the output signal is determined to

be a false knock signal. In addition, if true knocking is detected, retarding the ignition

timing prevents the knocking. This results in lower knock intensity LVPK. In contrast, if

false knocking is detected mainly due to mechanical noise from the pistons and other

components, retarding the ignition timing helps the false knocking grow louder. This

results in higher knock intensity LVPK. Thus, the knock control apparatus determines if

true or false knocking is detected based on the knock intensity LVPK, obtained under the

ignition timing retardation control, or on the behavior of the output signal of the knock

sensor.

[0007] However, a number of mechanical devices are mounted in the internal

combustion engine so that mechanical noises also cause the cylinder block to vibrate.

Vibrations of the cylinder block vary with the engine operating conditions. Due to this,

the knock control apparatus, described in JP-A-Hei 6-101556, cannot avoid the problem of

improper determination, such that the false knocking is improperly determined as true

knocking, if the angle of ignition timing retardation, which is calculated based on the output

signal of the knock sensor, falls out of the range between the preset maximum threshold and

the preset minimum threshold. In turn, the knock control apparatus, described in

JP-A-2003-278592, also has the similar problem, such that the output signal that indicates

false knocking is improperly determined as true knocking, if the current knock intensity

LVPK is equal to or lower than its previous value. As described above, any type of knock

control apparatuses may not ensure sufficient accuracy of knock determination when the

knock determination is based on the output signal of the knock sensor.

SUMMARY OF THE INVENTION

[0008] The present invention provides a knock control apparatus for an internal

combustion engine, which achieves highly accurate knock determination relative to

conventional knock control apparatuses that use an output signal of a knock sensor. The

present invention also provides a knock control method for an internal combustion engine.

[0009] A first aspect of the invention is directed to a knock control apparatus for an

internal combustion engine, the apparatus including: a first determination device that

determines whether the internal combustion engine is knocking based on an output signal of

a knocking detector provided in the internal combustion engine; a controller that retards an

ignition timing of the internal combustion engine when the first determination device has

determined that the internal combustion engine is knocking; and a second determination

device that confirms whether the internal combustion engine is knocking based on the

output torque of the internal combustion engine, which is obtained when the controller

retards the ignition timing.

[0010] When an air-fuel mixture is burned in a combustion chamber of the internal

combustion engine, spontaneous ignition or abnormal combustion may occur, which causes

knocking. Retarding the ignition timing when knocking occurs in the internal combustion

engine prevents spontaneous ignition or abnormal combustion in the combustion chamber.

Nonetheless, this does not greatly lower combustion efficiency of the internal combustion

engine. Thus, retarding the ignition timing when the internal combustion engine is

knocking results in a small variation in output torque of the internal combustion engine. In

contrast, retarding the ignition timing when false engine knocking occurs lowers the

combustion efficiency of the internal combustion engine. Thus, retarding the ignition

timing when false engine knocking occurs results in a large variation in output torque of the

internal combustion engine. Therefore, whether the internal combustion engine is

knocking is determined based on the output torque of the internal combustion engine that is

obtained when the ignition timing is retarded.

[0011] The thus-configured knock control apparatus includes: the first

determination device that determines whether the internal combustion engine is knocking

based on the output signal of the knocking detector provided in the internal combustion

engine; and the controller that retards the ignition timing of the internal combustion engine

if the first determination device determines that the internal combustion engine is knocking.

Thus, as in the case with the conventional apparatuses using a knock sensor, if it is

determined that the internal combustion engine is knocking based on the output signal of

the knock sensor, the engine ignition timing is retarded.

[0012] The knock control apparatus of the invention further includes the second

determination device that confirms whether the internal combustion engine is knocking .

based on the output torque of the internal combustion engine obtained when the controller

retards the ignition timing. Thus, when the output signal indicates false knocking, and the

first determination device improperly determines the false knocking as true knocking, the

second determination device confirms whether the internal combustion engine is knocking

based on the output torque of the internal combustion engine. Therefore, even if various

mechanical noises are produced depending on engine operating conditions, the use of the

output torque of the internal combustion engine, which is independent of these mechanical

noises, allows a proper determination whether the internal combustion engine is knocking.

Consequently, the knock control apparatus of the invention achieves highly accurate knock

determination relative to the conventional apparatuses using the output signal of the knock

sensor.

[0013] More specifically, according to the first aspect of the invention, the second

determination device may confirm that the internal combustion engine is not knocking, if a

variation rate of the output torque of the engine, obtained when the controller retards the

ignition timing, exceeds a variation rate in the output torque of the internal combustion

engine that indicates true knocking. Also, according to the first aspect of the invention,

the second determination device may confirm that the internal combustion engine is

knocking, if the variation rate of the output torque of the internal combustion engine,

obtained when the controller retards the ignition timing, falls within a range of variation

rates in the output torque of the internal combustion engine that indicate true knocking.

[0014] Further, according to the first aspect of the invention, the variation rate of the

output torque of the internal combustion engine may be a variation in output torque of the

internal combustion engine per degree of retardation angle.

[0015] As described above, the variation rate of the output torque of the internal

combustion engine is the variation in output torque of the internal combustion engine per

degree of retardation angle. Therefore, although the retardation angle is variable with the

output signal of the knock detector in a knock prevention control, a relationship between the

retardation angle and the output torque is evaluated with respect to the same criterion for

determining true or false knocking. This allows a more proper determination of whether

the internal combustion engine is knocking.

[0016] According to the first aspect of the invention, if the second determination

device confirms that the internal combustion engine is not knocking, the controller may

reset the retarded ignition timing, which has been retarded when the first determination

device has tentatively determined that the internal combustion engine is knocking, to an

ignition timing before the retardation.

[0017] The controller thus-configured resets the retarded ignition timing to an

ignition timing before the retardation, if the second determination device confirms that the

internal combustion engine is not knocking. This prevents the ignition timing from being

retarded when the first determination device improperly determines that the internal

combustion engine is knocking. Accordingly, the output torque of the internal combustion

engine is prevented from decreasing due to the retarded ignition timing.

[0018] According to the first aspect of the invention, if the second determination

device confirms that the internal combustion engine is knocking, the controller may

maintain the retarded ignition timing.

[0019] The controller thus-configured maintains the ignition timing, which has been

retarded when the first determination device has determined that the internal combustion

engine is knocking, if the second determination device confirms that the internal

combustion engine is knocking. This allows the controller to maintain the retarded

ignition timing because it has been accurately determined that the internal combustion

engine is knocking by the first determination device and the second determination device.

[0020] A second aspect of the invention is directed to a method of controlling

engine knock, the method including: determining whether the internal combustion engine is

knocking based on an output signal of a knocking detector provided in the internal

combustion engine; retarding an ignition timing of the internal combustion engine if it is

determined that the internal combustion engine is knocking; and confirming whether the

internal combustion engine is knocking based on an output torque of the internal

combustion engine that is obtained when the ignition timing is retarded.

BRIEF DESCRIPTION OFTHE DRAWINGS

[0021] The foregoing and further objects, features and advantages of the invention

will become apparent from the following description of example embodiments with

reference ,to the accompanying drawings, wherein like numerals are used to represent like

elements and wherein:

FIG. 1 is a schematic diagram of a knock control apparatus for an internal combustion

engine according to one embodiment of the invention.

FIG. 2 is a flowchart showing the steps of a knock control process by the knock control

apparatus of FIG. 1.

FIG. 3 is a graph showing a relationship between ignition timing retardation angle and

engine output torque for the knock control apparatus of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] One embodiment of the invention, in which a knock control apparatus for an

internal combustion engine is applied to a vehicle engine, will be described below with

reference to FIGs. 1 to 3. As shown in FIG. 1, the engine includes: a cylinder block 1 with

four cylinders Ia (only one of them is illustrated); and a cylinder head 2 disposed on the top

of the cylinder block 1. A piston 3 is provided within each cylinder Ia for reciprocating

motion. The piston 3 connects to a crankshaft 5 via a connection rod 4. The piston 3, the

cylinder Ia, and the cylinder head 2 define a combustion chamber 13 corresponding to each

cylinder.

[0023] The cylinder head 2 includes an intake port 2a and an exhaust port 2b, which

are provided for each combustion chamber 13. The intake port 2a connects an intake

branch passage 6a, formed in an intake manifold 6, to the combustion chamber 13. The

intake port 2a and the intake branch passage 6a form a part of an intake passage. The

exhaust port 2b connects an exhaust branch passage 7a, formed in an exhaust manifold 7, to

the combustion chamber 13. The exhaust port 2b and the exhaust branch passage 7a form

a part of an exhaust passage.

[0024] In the cylinder head 2, an intake valve 9 and an exhaust valve 10 are

provided for opening and closing the intake port 2a and the exhaust port 2b, respectively.

The intake valve 9 and the exhaust valve 10 are driven by an intake camshaft 11 and an

exhaust camshaft 12, respectively. The intake camshaft 11 and the exhaust camshaft 12

operate in conjunction with rotations of the crankshaft 5.

[0025] In the cylinder head 2, an ignition plug 8 and a fuel injection valve 14 are

also provided for each combustion chamber 13. A distal end of the ignition plug 8, that is

an electrode 8a, is located at the center within the combustion chamber 13 between the

opening of the intake port 2a and the opening of the exhaust port 2b. A distal end 14a of

the fuel injection valve 14 is located within the combustion chamber 13 on the right of the

opening of the intake port 2a in FIG. 1. The fuel injection valve 14 injects fuel from its

hole, formed at the distal end 14a, directly into the combustion chamber 13.

[0026] The engine has several sensors for detecting respective engine operating

conditions. For example, a knock sensor 20 is provided on the cylinder block 1. The

knock sensor 20 is designed to output a knock signal KCS in response to the intensity of

vibrations of the cylinder block 1. The knock sensor 20 may be considered as a knock

detector in the knock control apparatus of the invention. Adjacent to the crankshaft 5, a

rotational speed sensor 21 is provided for outputting the rotational speed signal NES in

response to the rotational speed of the crankshaft 5 (engine speed). An air flow meter 22

is provided in the engine intake passage. The air flow meter 22 outputs an intake air

volume signal GAS in response to the volume of intake air passing through the intake

passage. A torque sensor 23 is connected to the crankshaft 5. The torque sensor 23 is

designed to output a torque signal TOS in response to the torque transmitted to the

crankshaft 5, that is, engine output torque.

[0027] The signals output from the respective sensors 20 to 23 are received by an

engine electronic control unit 30 (hereinafter referred to as "ECU"). The ECU 30 includes

a CPU, a memory, an input-output circuit, and a drive circuit. The ECU 30 reads and

calculates parameters of the engine operating conditions based on the output signals. In

addition, the ECU 30 regulates the operations of the ignition plug 8, the fuel injection valve

14, and other devices to control the combustion mode, ignition timing, fuel injection, and

other various engine operating conditions.

[0028] The steps of the knock control process for the above-described engine will

be described below with reference to FIGs. 2 and 3. A series of the steps shown in FIG. 2

are repeated by the ECU 30 at predetermined intervals.

[0029] As shown in FIG. 2, it is first tentatively determined whether the knocking is

occurring in the engine in step S 100. To be more specific, a peak hold value of the knock

signal KCS for a preset knock determination period is acquired for each cylinder. Also, a

logarithmic transformation value of the peak hold value is calculated as knock intensity

LVPK. The preset knock determination period includes the time at which knocking starts

in the target cylinder. The knock intensity LVPK is compared to a preset knock criterion

level VKD to determine the presence or absence of knock. In other words, if the knock

intensity LVPK exceeds the knock criterion level VKD, the ECU 30 tentatively determines

that knocking is occurring in the engine. If the knock intensity LVPK is equal to or lower

than the knock criterion level VKD, the ECU 30 tentatively determines that knocking is not

occurring in the engine. Step SlOO may be considered as a step to be performed by a first

determination device of the knock control apparatus of the invention. The knock criterion

level VKD is empirically determined, and stored in the memory of the ECU 30.

[0030] If the ECU 30 tentatively determines that knocking is occurring in the engine

(YES in step SlOO), an engine output torque TA (torque pre-control value), obtained prior to

the ignition timing retardation control, is stored in the memory of the ECU 30 (step SlOl).

The ignition timing retardation control will be discussed later. A torque transmitted to the

crankshaft 5, that is, engine output torque TO, is calculated based on the torque signal TOS,

and stored in the memory of the ECU 30 as the torque pre-control value TA.

[0031] After step SlOl, the ECU 30 executes the ignition timing retardation control

for retarding the ignition timing of the ignition plug 8 (step S 102). More specifically, the

ECU 30 calculates the engine speed NE based on the rotational speed signal NES as well as

the intake air volume GA based on the intake air volume signal GAS. The engine speed

NE, the intake air volume GA, and the knock intensity LVPK are used to variably set a

retardation angle ITLA with respect to a reference ignition timing IT of the ignition plug 8.

The retardation angle ITLA is stored in the memory of the ECU 30 as retardation angle

ITLAK. The reference ignition timing IT of the ignition plug 8 is preset under the ignition

timing control performed by the ECU 30. The ignition timing control allows the reference

ignition timing IT of the ignition plug 8 to be variably set depending on the engine speed

NE, the intake air volume GA, arid other parameters.

[0032] Under the ignition timing retardation control, the ignition timing ITO of the

ignition plug 8 is determined in accordance with the following expression (1).

ITO=IT+ITLA (1)

Step S 102 may be considered as a step to be performed by a controller for retarding the

engine ignition timing ITO in the knock control apparatus of the invention.

[0033] After step S 102, an engine output torque TB (torque post-control value),

obtained after the retardation control, is stored in the memory of the ECU 30 (step S103).

More specifically, the engine output torque TO is calculated based on the torque signal TOS

output from the torque sensor 23, and stored in the memory of the ECU 30 as the torque

post-control value TB. For example, the torque post-control value TB is obtained at the

point in time after each ignition timing ITO for all the cylinders has been subjected to the

retardation control.

[0034] After step S 103, the ECU 30 calculates an engine torque variation rate TDR

(step S 104). Specifically, the torque variation rate TDR is calculated in accordance with

the following expression (2).

TDR=|TB-TA|/ITLAK (2)

As shown by the expression (2), the torque variation rate TDR is the variation in engine

output torque TO per unit retardation angle.

[0035] After step S 104, the ECU 30 confirms whether the engine is not knocking

(step S 105). More specifically, if the torque variation rate TDR exceeds a specified torque

variation rate TDRK, the ECU 30 confirms that the engine is not knocking. If the torque

variation rate TDR is equal to or lower than the specified torque variation rate TDRK, the

ECU 30 confirms that knocking is occurring in the engine. The specified torque variation

rate TDRK is the maximum variation rate of the engine output torque TO that indicates true

engine knocking. In other words, if the variation rate of the engine output torque, obtained

when the ignition timing ITO is retarded, falls within the range of the variation rates that

indicate true engine knocking, the ECU 30 confirms that knocking occurs in the engine.

Step S 105 may be considered as a step to be performed by a second determination device in

the knock control apparatus of the invention.

[0036] Now, a method for calculating the specified torque variation rate TDRK will

be described with reference to FIG. 3. In FIG. 3, the retardation angle is set to "0" when

the engine ignition timing ITO is set at MBT (the ignition timing for optimum output and

fuel consumption).

[0037] As shown in FIG. 3, in both the cases of true knocking (shown by the

dot-and-dashed curve) and false knocking (shown by the solid curve), as the ignition timing

ITO of the ignition plug 8 is retarded relative to the MBT, the engine output torque TO

decreases. If the ignition timing is retarded by a given retardation angle ITLAα relative to

the MBT. The torque decreases by an amount TDN during true knocking, while the torque

decreases by an amount TDG during false knocking. A comparison between the amount

TDN and the amount TDG establishes the relationship represented by the following

expression (3).

TDN<TDG (3)

[0038] More specifically, a comparison between the torque variation rate TDRN

during true knocking and the torque variation rate TDRG during false knocking establishes

the relationship represented by the following expression (4).

TDRN(=TDN/ITLAα)<TDRG(=TDG/ITLAα) (4)

[0039] The torque variation rate TDRN that indicates true knocking may be

considered as a variation rate in an output torque of the internal combustion engine during

true engine knocking, according to the invention.

[0040] As the engine operation control parameters, such as engine speed NE and

intake air volume GA, vary, the torque variation rate TDRN during true knocking and the

torque variation rate TDRG during false knocking fluctuate. Therefore, in step S 105,

while satisfying the relationship shown by the following expression (5), the specified torque

variation rate TDRK is set at the maximum variation rate of the engine output torque TO

during true engine knocking.

TDRN<TDRK<TDRG (5)

[0041] In the knocking confirmation process of the invention, the relationships

between the specified torque variation rate TDRK and the respective engine operation

control parameters are established empirically. Function data (function map) that defines

these relationships is stored in the memory of the ECU 30. The ECU 30 calculates the

engine operation control parameters based on the signals output from the respective sensors

in the engine. Then, the ECU 30 calculates the specified torque variation rate TDRK with

reference to the function map.

[0042] If it is confirmed that the engine is not knocking (NO in step S 105), the ECU

30 executes the retardation control to reset the ignition timing ITO of the ignition plug 8 to

the ignition timing before the retardation (step S 106). To be more specific, the ignition

timing ITO of the ignition plug 8 is reset in accordance with the following expression (6).

ITO=IT (6)

[0043] After step S 106, the series of steps end temporarily. In turn, the series of

steps also end temporarily, if the ECU 30 tentatively determines that knocking is not

occurring in the engine (NO in step SlOO), or if the ECU 30 confirms that knocking is

occurring in the engine(YES in step S 105).

[0044] In accordance with the knock determination process, if the ECU 30

tentatively determines in step SlOO that knocking is occurring in the engine, and then

confirms in step S 105 that the engine is not knocking, the ECU 30 ultimately determines

that the false engine knock has occurred. If the ECU 30 determines that the false engine

knock has occurred, the retarded ignition timing ITO (IT + ITLA) is reset to the ignition

timing before the retardation ITO (IT). In contrast, if the ECU 30 tentatively determines in

step SlOO that knocking is occurring in the engine, and then confirms in step S 105 that

knocking is occurring in the engine, the ECU 30 ultimately determines that the true engine

knock has occurred. In such a case where the ECU 30 determines that the true engine

knock has occurred, the retarded ignition timing ITO (IT + ITLA) is maintained.

[0045] According to the embodiment of the invention, the following functions and

effects are achieved as has been discussed in detail. (1) When an air-fuel mixture is

burned in the engine combustion chamber, spontaneous ignition or abnormal combustion

may occur, which results in knocking. Retarding the ignition timing ITO when the engine

is knocking prevents the spontaneous ignition or abnormal combustion in the combustion

chamber. Nonetheless, this does not greatly lowers the engine combustion efficiency.

Thus, retarding the ignition timing ITO when the engine is knocking results in a minimal

decrease in engine output torque TO. In contrast, retarding the ignition timing ITO when

the false engine knocking is occurring lowers the engine combustion efficiency. Thus,

retarding the ignition timing ITO when the false engine knocking is occurring results in a

high decrease in engine output torque TO. Therefore, whether knocking is occurring in

the engine is determined based on the engine output torque TO obtained when the ignition

timing ITO is retarded.

[0046] The knock control apparatus for an engine according to the embodiment of

the invention is designed to execute the knock determination in step SlOO and the

retardation control. The knock determination is intended to tentatively determine if the

engine is knocking based on the knocking signal KCS of the knock sensor 20 provided in

the engine. The retardation control is intended to retard the engine ignition timing ITO, if

the engine is tentatively determined to be knocking in step SlOO. Thus, when the engine is

tentatively determined to be knocking, the engine ignition timing ITO is retarded, as in the

case with the conventional apparatuses.

[0047] Further, the knock control apparatus of the invention is designed to perform

the knock determination in step S 105. The knock determination is intended to determine

whether the engine is not knocking based on the engine output torque, which is obtained

when the ignition timing ITO is retarded under the retardation control. Thus, if the output

signal indicates false knocking, and the ECU 30 improperly determines the false knocking

as true knocking in step SlOO, the ECU 30 re-determines in step S 105 whether the engine is

not knocking based on the engine output torque. Therefore, even if various mechanical

noises are produced depending on the engine operating conditions, the use of the engine

output torque TO, which is independent of these mechanical noises, allows the ECU 30 to

properly determine whether knocking occurs in the engine. Consequently, the knock

control apparatus of the invention achieves highly accurate knock determination relative to

the conventional apparatuses using the knocking signal KCS of the knock sensor 20.

[0048] (2) The torque variation rate TDR of the engine output torque TO is the

variation in engine output torque TO per unit retardation angle. Therefore, although the

retardation angle ITLA varies with the knocking signal KCS of the knock sensor 20 in the

retardation control, a relationship between the retardation angle ITLA and the output torque

TO is evaluated with respect to the same criterion for determining true or false knocking.

This enables the ECU 30 to more properly determine whether knocking is occurring in the

engine.

[0049] (3) According to the embodiment of the invention, when the ECU 30

determines in step S 105 that the engine is not knocking, the ignition timing ITO (IT +

ITLA), which has been retarded as a result of the tentative determination in step SlOO that

the engine is knocking, is reset to the reference ignition timing IT before the retardation.

This prevents the ignition timing from being retarded when the ECU 30 improperly

determines in step SlOO that the engine is knocking. Accordingly, a decrease in the engine

output torque due to the retarded ignition timing ITO is avoided.

[0050] (4) According to the embodiment of the invention, when the ECU 30

determines in step S 105 that knocking occurs in the engine, the ignition timing ITO (IT +

ITLA) is maintained, which has been retarded as a result of the tentative determination in

step SlOO that the engine is knocking. This allows the ECU 30 to maintain the retarded

ignition timing because it has been accurately determined that the engine is knocking

through step S 105 as well as step SlOO.

[0051] The embodiment may be modified as appropriate in the manner described

below. The knock control apparatus for an internal combustion engine of the invention is

applied in a four-cylinder engine in the aforementioned embodiment. However, the knock

control apparatus may be applied to other types of engines with fewer than four cylinders or

with five or more cylinders.

[0052] The engine torque variation rate TDR is calculated based on the expression

(2) in the aforementioned embodiment. Alternatively, an engine torque reduction rate

TDD may be calculated based on the following expression (7).

TDD=(TB-TA)/ITLAK (7)

[0053] More specifically, because generally the retarded engine ignition timing ITO

results in lower torque, the difference between the torque pre-control value TA and the

torque post-control value TB may be used.

[0054] In the above embodiment, whether the occurrence of knocking in the engine

is determined tentatively based on the signal output from the knock sensor 20 in response to

the intensity of vibrations of the cylinder block 1. Alternatively, the tentative

determination may be based on the pressure in the combustion chamber 13. Accordingly,

the retardation angle ITLA may also be set based on the pressure in the combustion

chamber 13 under the retardation control.

[0055] In the aforementioned embodiment, the retardation angle ITLA relative to

the reference ignition timing IT of the ignition plug 8 may be variably set under the

retardation control. Alternatively, the retardation angle ITLA may be set at a constant

value. That is, a denominator in the expression (2) is the constant retardation angle

ITLAK. Thus, the difference between the torque pre-control value TA and the torque

post-control value TB may be solely used to obtain the torque variation rate TDR.

[0056] In the above-described embodiment, if the ECU 30 determines in step S 105

that the engine is not knocking, the retardation control is executed to reset the ignition

timing ITO of the ignition plug 8 to the ignition timing before the retardation.

Alternatively, the ignition timing ITO may be reset to an intermediate timing between the

ignition timing before the retardation and the retarded ignition timing.

[0057] In the above-described embodiment, if the ECU 30 determines in step S 105

that knocking occurs in the engine, the retarded ignition timing ITO (IT + ITLA) is

maintained. Alternatively, the ECU 30 may execute an additional retardation control to

further retard the ignition timing ITO.

[0058] If the ECU 30 determines in step S 105 that knocking is occurring in the

engine, knocking may continue to occur in the engine even after the ignition timing ITO has

been retarded under the retardation control in step S 102. Thus, when the ECU 30

determines in step S 105 that knocking is occurring in the engine, a second retardation

control is executed to further retard the ignition timing ITO, in addition to the first

retardation control in step S 102. This more reliably prevents the engine from knocking.

Furthermore, the retardation angle may vary depending on the torque variation rate TDR

calculated in step S 104 in the second retardation control.