JP2002242714A - 4-cycle engine for automobile - Google Patents

Abstract

PROBLEM TO BE SOLVED: To achieve the appropriate effect of internal EGR and further enhance the effect of reducing pumping losses by closing an exhaust valve prior to an intake top dead center in the low-load range of an engine. SOLUTION: A valve timing changer is provided for an intake valve and an exhaust valve. In the low-load range of the engine, the closing timing ExC of the exhaust valve which is defined by the point of transition of the characteristic of an exhaust-valve cam lift from an acceleration section to a constant- speed section is set before the intake top dead center by a predetermined period, and the opening timing InO of the intake valve which is defined by the point of transition of the characteristic of an intake-valve cam lift from the constant- speed section to the acceleration section is set behind the intake top dead center so that the period θIn from the intake top dead center to the opening timing of the intake valve is longer than the period θEx from the closing timing of the exhaust valve to the intake top dead center.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a four-stroke engine for automobiles, which actively utilizes so-called internal EGR by leaving burned gas in a combustion chamber.

[0002]

2. Description of the Related Art Conventionally, an external EGR passage is connected between an exhaust system and an intake system of an engine, and exhaust gas recirculation (EGR) is performed through the external EGR passage.
It is generally known to reduce x. However, according to such an external EGR, a passage or an EGR
In addition to the need for a valve, there is also a problem that a deposit is likely to occur in the intake system due to EGR.

[0003] Therefore, instead of a method in which exhaust gas is recirculated through an external EGR passage, a method using a so-called internal EGR by actively burning burned gas in a combustion chamber has been considered. Generally, the overlap between the opening periods of the intake valve and the exhaust valve is increased. That is, a variable valve timing device that can change the opening / closing timing of the intake valve and the exhaust valve is used, and the internal EG
In the region where R is required, the amount of the remaining burned gas is increased by making the above-mentioned overlap large, for example, by returning the exhaust gas during the overlap period.

However, when the overlap between the opening periods of the intake and exhaust valves is increased as described above, the intake valve and the exhaust valve are relatively widely opened at the intake top dead center, so that interference with them is avoided. Therefore, it is necessary to provide a deep recess on the top surface of the piston, and this has a problem of adversely affecting combustion.

[0005] Another conventional technique for leaving burned gas in a combustion chamber is disclosed in JP-A-10-266.
As disclosed in Japanese Patent Application Laid-Open No. 878, an exhaust valve is considered to be closed before the intake top dead center.

In the invention disclosed in this publication, the exhaust valve is closed before top dead center within a load range from low to medium load of the engine, and the exhaust valve closing timing is advanced as the required load decreases, and Within the load range, the intake valve is opened after the top dead center, and the opening timing of the intake valve is advanced as the required load decreases. According to the description of this publication, within the above-mentioned load range, the residual burned gas in the combustion chamber is increased by accelerating the closing timing of the exhaust valve, and the residual burned gas is used to multi-point in the combustion chamber. Self-ignition is caused, thereby increasing the combustibility and suppressing NOx by avoiding a local increase in combustion temperature.
In addition, with the throttle valve kept almost fully open, the intake air amount is controlled by adjusting the residual burned gas amount by changing the exhaust valve closing timing, and the pumping loss is reduced while controlling the output according to the required load. We try to reduce it.

[0007]

SUMMARY OF THE INVENTION The above-mentioned Japanese Patent Laid-Open No. 10-2 is disclosed.
In the invention disclosed in Japanese Patent Application Publication No. 66878, the amount of residual burned gas is adjusted by changing the closing timing of the exhaust valve while the throttle valve is kept substantially fully open, thereby controlling the amount of intake air according to the required load. However, if the amount of residual burned gas increases too much, combustion stability will be impaired.
In reality, it is necessary to control the amount of residual burned gas to such an extent that combustion stability is not impaired, and to adjust the intake air amount with a throttle valve at low load, which inevitably causes pumping loss. . In the invention described in the above publication, the intake valve is opened after the top dead center when the exhaust valve is closed before the top dead center.However, the period from the top dead center to the intake valve opening timing is from the exhaust valve closing timing. It is about the same as the period until top dead center.

[0008] In the above-mentioned publication, there is room for improvement in terms of pumping loss reduction and the like.

The present invention has been made in view of the above circumstances, and in a low engine load range, an exhaust valve is closed before an intake top dead center so that an appropriate internal EGR effect can be obtained. An object of the present invention is to provide a four-stroke engine for an automobile which can be further enhanced.

[0010]

SUMMARY OF THE INVENTION The present invention relates to an intake valve closing timing defined by a transition point from an acceleration section to a constant speed section in an exhaust valve cam lift characteristic in a low load range in a warm state of an engine. The intake valve opening timing, which is set a predetermined time before the top dead center and defined at the transition from the constant speed section to the acceleration section in the intake valve cam lift characteristic, is set after the intake top dead center, and The period from the point to the opening timing of the intake valve is set to be longer than the period from the closing timing of the exhaust valve to the top dead center of the intake valve.

According to the present invention, in a low load range in a warm state of the engine, the exhaust valve is closed a predetermined period before the intake top dead center, so that the burned gas remains in the combustion chamber and the internal EGR effect is reduced. Is obtained, and NOx is reduced. Also, when the exhaust valve closing timing is before the intake top dead center and the intake valve opening timing is after the intake top dead center, the pressure in the combustion chamber rises once after the exhaust valve is closed and passes through the intake top dead center. And when the intake valve is opened, it becomes a level corresponding to the intake pressure,
In the process of such a combustion chamber pressure change, the intake top dead center TD
The difference between the pressure before reaching C and the pressure after passing the intake top dead center TDC is a pumping loss, but the period from the intake top dead center to the intake valve opening timing is from the exhaust valve closing timing to the intake By delaying the intake valve opening timing to an extent longer than the period until the dead center, the difference becomes smaller than in the case where the intake valve opening timing is earlier, so that the pumping loss is reduced.

In the present invention, there is provided a variable valve timing device which can change the valve opening / closing timing for at least one of the exhaust valve and the intake valve, and the valve timing is changed from the exhaust valve closing timing with respect to the intake top dead center. It is preferable to change the valve opening / closing timing according to the operation state so that the period until the intake valve opening timing is reduced at least as the load increases from a medium load.

In this manner, on the low load side, the period from the exhaust valve closing timing to the intake valve opening timing is made relatively long, so that the effects of suppressing NOx and reducing pumping loss can be sufficiently obtained, and at the same time, the high load can be obtained. On the load side, output is ensured by reducing the period.

In the present invention, the period from the exhaust valve closing timing to the intake top dead center is 5 ° or more in crank angle and the intake top dead center is sandwiched between the exhaust valve closing timing and the intake top dead center in a low load range in a warm state of the engine. It is preferable that the period from the closing timing of the exhaust valve to the opening timing of the intake valve is set to 20 ° or more in crank angle.

By doing so, NO in a low load region
The effects of suppressing x and reducing pumping loss are sufficiently obtained.

Further, the present invention is preferably applied to a direct injection type engine having an injector for directly injecting fuel into a combustion chamber. According to this engine, it is possible to increase the combustion stability while increasing the air-fuel ratio by stratified combustion in a low-load region and increase the allowable amount of internal EGR (residual burned gas). Therefore, the internal EGR effect by setting the exhaust valve closing timing before the intake top dead center can be sufficiently exhibited.

[0017]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 schematically shows the entire structure of a four-cycle engine for an automobile to which the present invention is applied.
In this figure, reference numeral 1 denotes an engine body, which has a plurality of cylinders, each of which has a combustion chamber 5 formed above a piston 4 inserted into a cylinder bore. An intake port 7 and an exhaust port 8 are opened in the combustion chamber 5, and these ports 7, 8 are opened and closed by an intake valve 9 and an exhaust valve 10.

The intake valve 9 and the exhaust valve 10 are opened and closed by a valve mechanism including camshafts 11 and 12 and the like. The valve mechanism for the intake valve 9 and the valve mechanism for the exhaust valve 10 are respectively provided with variable valve timing devices 13 and 14 that can change the valve opening / closing timing. The variable valve timing devices 13 and 14 are provided between a cam pulley and a camshaft interlocking with the crankshaft, and change the phase of the camshaft with respect to the crankshaft, thereby keeping the valve opening period constant and the opening timing and time. The closing time can be changed. Since various types of such variable valve timing devices 13 and 14 have been conventionally known, illustration and description of a specific structure will be omitted.

At the center of the combustion chamber 5, a spark plug 16
And the plug tip faces the combustion chamber. Further, the tip of the injector 18 faces the combustion chamber 5 from the side, and fuel is directly injected from the injector 18 into the combustion chamber 5.

An intake passage 20 and an exhaust passage 30 are connected to the engine body 1. In the intake passage 20, an air cleaner 21, an air flow sensor 22, a throttle valve 23 and a surge tank 24 are arranged in this order from the upstream side.
Is provided. The throttle valve 23 is mechanically connected to an accelerator pedal (not shown), and is opened to an opening corresponding to the accelerator pedal depression amount. A throttle opening sensor 25 for detecting the opening of the throttle valve 23 is provided.

In the exhaust passage 30, an O ₂ sensor 31 for detecting the air-fuel ratio by detecting the oxygen concentration in the exhaust gas is provided, and a catalyst 32 for purifying the exhaust gas is provided downstream thereof. . The catalyst 32 may be constituted by a three-way catalyst. However, in order to enhance the purification performance when the air-fuel ratio is lean and the stratification operation is performed, even when the air-fuel ratio is under a condition leaner than the stoichiometric air-fuel ratio. It is desirable to use a catalyst that can effectively purify NOx. In the present embodiment, NOx in the exhaust gas is absorbed in an oxygen-excess atmosphere, and when the air-fuel ratio changes from lean to rich and the oxygen concentration decreases, the absorbed NOx is released and the atmosphere is released. Lean NOx that is designed to reduce NOx by existing reducing materials such as CO
A catalyst is used.

Reference numeral 40 denotes a control unit (ECU) for controlling the engine. The ECU 40 includes the air flow sensor 22, the throttle opening sensor 25, and O _2.
A signal from the sensor 31 is input, a crank angle signal for detecting an engine rotation speed and the like is input from a crank angle sensor 35, and a signal from a water temperature sensor 36 and the like for detecting the temperature of engine cooling water is also input. ing.

Further, the ECU 40 outputs a signal for controlling the fuel injection to the injector 18 and outputs a signal for controlling the same to the variable valve timing devices 13 and 14.

The above-mentioned ECU 40 is provided with an operating state determining means 4.
1, a valve timing control means 42 and a fuel injection control means 43 are included. The operating state discriminating means 41 compares the engine speed detected by measuring the cycle of the crank angle signal from the crank angle sensor 35 and the engine load checked by the signals from the air flow sensor 22, the throttle opening sensor 25 and the like. The operating state of the engine is determined on the basis of this.

The valve timing control means 42 controls the variable valve timing devices 13 and 14 in accordance with the operation state determined by the operation state determination means 41, thereby determining the opening / closing timing of the intake valve 11 and the exhaust valve 12 in detail. It is set and changed as described.

The fuel injection control means 43 controls the fuel injection amount and the injection timing from the injector 18 according to the operation state determined by the operation state determination means 41.
For example, in a predetermined region on the low load side of the engine (region B in FIG. 4 to be described later or a low / medium speed region from low load to medium load including this region B), the air-fuel ratio is made leaner than the stoichiometric air-fuel ratio. The fuel injection amount and the injection timing are controlled such that the fuel is injected in the latter half of the compression stroke, thereby causing the air-fuel mixture to be unevenly distributed around the ignition plug 16 to perform stratified combustion. On the other hand, in an area other than the predetermined area, the air-fuel ratio is set to the stoichiometric air-fuel ratio or a value close to the stoichiometric air-fuel ratio, and the fuel is injected in the intake stroke to diffuse the air-fuel mixture to perform uniform combustion. Control the quantity and injection timing.

FIG. 2 shows a cam lift curve for indicating the opening / closing timing of the intake / exhaust valve.
ExV means an exhaust valve. InO and InC are the opening and closing times of the intake valve, and ExO and ExC are the opening and closing times of the exhaust valve. Here, the opening timings InO and ExO of the intake valve and the exhaust valve are defined by a transition point from a constant speed section to an acceleration section in the cam lift characteristic,
The closing timings InC and ExC of the intake valve and the exhaust valve are defined at the transition point from the acceleration section to the constant speed section in the cam lift characteristic (see FIG. 3).

In FIG. 2, when the exhaust valve is most advanced within the opening / closing timing variable range, the closing timing Ex is indicated by a solid line.
When C is before the intake top dead center TDC and is most retarded, the closing timing ExC is after the intake top dead center TDC as shown by the broken line, and when the intake valve is advanced most in the opening / closing timing variable range, it is shown by the broken line. As described above, the opening timing InO is before the intake top dead center TDC, and when it is most retarded, the opening timing InO is after the intake top dead center TDC as shown by the solid line. Therefore, when the exhaust valve as shown by the broken line is retarded and the intake valve is advanced, there is an overlap between the valve opening periods, but the exhaust valve as shown by the solid line is advanced and the intake valve is retarded. In this state, there is no overlap between the valve opening periods. The period from the exhaust valve closing timing ExC to the intake valve opening timing InO without such overlap is referred to as minus overlap (minus O / L) for convenience in the description of the embodiment.

Next, how to set and change the valve timing according to the operation state will be described with reference to FIGS. In the following description, numerical values representing the timing and period of the opening / closing timing of the intake valve and the exhaust valve are based on the crank angle, BTDC means before top dead center, and ATDC means top dead center. Means later.

In the warm state of the engine, in a low load region B surrounded by a dashed line in FIG. 4, as shown in FIG.
The exhaust valve closing timing ExC is set a predetermined period before the intake top dead center TDC, and the intake valve opening timing InO is after the intake top dead center TDC, and from the intake top dead center TDC to the intake valve opening timing InO. Is set longer than the period θEx from the exhaust valve closing timing ExC to the intake top dead center TDC.

The time when the exhaust valve closing timing ExC is a predetermined period before the intake top dead center TDC is a timing at which an appropriate amount of internal EGR is obtained while ensuring combustion stability at a low load. 5 ° or more before top dead center TDC, preferably BT
DC is set to about 5 to 15 °. The intake valve opening timing InO is preferably set to a timing that satisfies [θIn-θEx] ≧ 5 °, and is set, for example, to about 10 to 20 ° ATDC. In this region B, the period from the exhaust valve closing timing ExC to the intake valve opening timing InO is preferably 20 ° or more.

Further, in the middle / high speed region of the engine on the medium load to high load side (the region other than the fully open region and the region near the middle load region and the high load region), the exhaust valve closing timing ExC indicates the intake valve closing timing ExC. By setting the intake valve opening timing InO after the top dead center TDC for a predetermined period before the top dead center TDC and setting the intake valve opening timing InO after the intake top dead center TDC, minus O / L is set. Then, the minus O / L is made larger in the medium speed region of the medium load to high load region than in the high speed region.

That is, in FIG. 4, minus O / L is maximized in a middle speed range (region A) in a region extending from the engine middle load to a slightly higher load side.
Specifically, at medium speed and medium load (within the area A), FIG.
As shown in (b), the exhaust valve closing timing ExC is at least 20 ° before the intake top dead center TDC, preferably from BTDC 30 to TDC.
40 °, and the intake valve opening timing InO is later than the intake top dead center TDC, preferably ATDC 35-45.
° set. In the middle speed and middle load range, the closing timing InC of the intake valve is about 80 ° after the bottom dead center of the intake, and the opening timing of the exhaust valve is about 80 ° before the bottom dead center of the exhaust. According to the variable valve timing device used in the present embodiment, while the opening period of the intake valve and the exhaust valve is kept constant, the closing timing of the intake valve also changes with the change of the opening timing of the intake valve, Also, the exhaust valve opening timing changes in accordance with the change in the exhaust valve closing timing.

As shown in FIG. 6C, in the high-speed medium-load region, which is a region on the higher-speed side than the region A, the period has a negative O / L, but the period is smaller than that in the medium-speed medium-load region. The closing timing ExC is set to BTDC 20 to 30 °, and the intake valve opening timing InO is set to ATDC 25 to 35.
° set.

Further, as the vehicle approaches the fully open region from the region A, the exhaust valve is gradually retarded and the intake valve is gradually advanced, so that the minus O / L is gradually reduced, or Further, a state where a positive overlap occurs is reached. Then, in the middle-speed fully-open range, as shown in FIG. 6E, the exhaust valve closing timing ExC is set after the intake top dead center TDC.
For example, ATDC is set to about 10 °, and the intake valve opening timing InO is before the intake top dead center TDC, for example, BT
DC is set to about 10 to 15 °. Further, in the high-speed and high-load region, as shown in FIG.
Is set to, for example, about 10 ° ATDC after the intake top dead center TDC, and the intake valve opening timing InO is set to, for example, 10 to 15 ° ATDC after the intake top dead center TDC.

Also, as shown in FIG. 6 (a), the minus O / L is made smaller in the low speed medium load region than in the medium speed medium load region. For example, the exhaust valve closing timing ExC is BTDC 20 to 30 °, and the intake valve opening timing InO Is set to ATDC 25-35 °.
As shown in FIG. 6D, the exhaust valve closing timing ExC is set to about 10 ° ATDC in the low-speed full-open range and the intake valve opening timing InO is set to about BTDC10 to 1 in the same manner as in the medium-speed full-open range.
It is set to about 5 °.

The valve opening / closing timings shown in FIGS. 5 and 6 are set when the cooling water temperature of the engine is higher than a predetermined temperature, while the cooling water temperature is lower than the predetermined temperature. In a cold state, the exhaust valve closing timing ExC and the intake valve opening timing InO may be set to approximately the intake top dead center so as to secure the combustion stability by reducing the internal EGR.

According to the engine of the present embodiment as described above, in the low load range B, the exhaust valve closing timing ExC is set before the intake top dead center TDC, so that the burned gas is not completely discharged. As the exhaust valve closes, the burned gas remains in the combustion chamber 5 and an internal EGR effect is obtained, whereby NOx is reduced. In this case, an injector 18 for directly injecting fuel into the combustion chamber 5 is provided as in the present embodiment,
In a predetermined operating region including the low load region B, in a direct injection type engine in which the air-fuel ratio is set to a lean air-fuel ratio larger than the stoichiometric air-fuel ratio and fuel is injected in the compression stroke to perform stratified combustion, the low-load region Also in B, the combustion stability at a lean air-fuel ratio is enhanced by stratified combustion, and a relatively large amount of internal EG
R is allowed, for example, from about 30% to about 40% in EGR rate.

Since the internal EGR amount increases as the exhaust valve closing timing ExC is advanced, if the exhaust valve closing timing ExC is advanced within a range in which the internal EGR amount is allowed (a range in which combustion stability is not impaired). Often, the exhaust valve closing timing ExC is BT
By setting DC to about 5 to 15 °, an appropriate internal EGR amount that does not impair the combustion stability in the low load region B can be obtained.

Further, as described above, the exhaust valve closing timing ExC is set a predetermined period before the intake top dead center TDC, and the intake valve opening timing InO is set after the intake top dead center TDC and from the intake top dead center. Since the period θIn from the exhaust valve closing timing to the intake top dead center is set to a period that is longer than the period θEx from the exhaust valve closing timing to the intake valve opening timing, a sufficient pumping loss reduction effect can be obtained.

The effect of reducing the pumping loss will be described with reference to FIG. When the exhaust valve closes before the intake top dead center TDC, the pressure in the combustion chamber rises once from the level corresponding to the exhaust pressure, falls after the intake top dead center TDC, and decreases when the intake valve is opened. In this process, the difference between the pressure until the intake top dead center TDC is reached and the pressure after passing the intake top dead center TDC is the pumping loss. Then, the intake top dead center TD
When the intake valve is opened relatively early after C (for example, when the period from the intake top dead center TDC to the intake valve opening timing is equal to or shorter than the period from the exhaust valve closing timing to the intake top dead center TDC) In (2), when the intake valve is opened, the pressure in the combustion chamber suddenly decreases to the intake pressure, and the difference increases, so that the pumping loss increases.

On the other hand, if the intake valve opening timing InO is delayed so that θIn> θEx, the timing at which the pressure in the combustion chamber decreases to the intake pressure is delayed. Pumping loss is reduced by the amount indicated by hatching. In particular, [θIn-θEx] ≧
By delaying the intake valve opening timing InO to about 5 °, the pumping loss reduction effect can be sufficiently enhanced. Most preferably, in order to reduce the pumping loss, the intake valve should be opened when the pressure in the combustion chamber becomes substantially equal to the intake pressure while the intake valve is closed.

Further, even in the middle load to high load region of the engine, the negative E / L is set and the internal EGR causes NOx.
Is reduced, especially in the middle to high load area.
Since the combustion stability is higher in the high-speed range than in the low-speed and low-load range, by increasing the minus O / L, a relatively large amount of internal EGR is performed and NOx is sufficiently reduced. Further, the exhaust valve closing timing ExC is set to the intake top dead center TDC.
Before the predetermined period, the intake valve opening timing InO is set to the intake top dead center TD.
By setting the temperature after C, the burned gas in the combustion chamber due to the internal EGR is sufficiently cooled, so that effects such as improvement of fuel efficiency due to improvement of thermal efficiency and suppression of increase in exhaust gas temperature can be obtained.

That is, the exhaust valve closing timing ExC is set to a predetermined period before the intake top dead center TDC and the intake valve opening timing IC
Assuming that nO is after the intake top dead center TDC, the pressure in the combustion chamber increases from the exhaust valve closing timing ExC to the intake top dead center TDC as shown in FIG. As a result, the combustion chamber pressure decreases. And the temperature rises with the pressure rise, and the temperature falls with the pressure drop,
During a period in which the temperature in the combustion chamber is increased by the pressure increase, the temperature difference between the combustion chamber and surrounding walls (a cylinder head or a cylinder wall having a relatively low temperature with a built-in water jacket) increases, thereby increasing the temperature. The amount of heat radiation to surrounding walls increases. Therefore, even if the temperature of the burned gas remaining in the combustion chamber at the time when the exhaust valve is closed is high, heat is sufficiently released during a period in which the pressure is high after the exhaust valve is closed, and then the pressure is reduced. As the temperature decreases. In this way, an effect of cooling the burned gas is obtained, and thus, as in the case of introducing the externally cooled EGR gas,
Combustion temperature and exhaust temperature decrease.

In this case, in order to increase the pressure in the combustion chamber from the exhaust valve closing timing ExC to the intake top dead center TDC, the intake valve opening timing InO needs to be at least after the intake top dead center TDC. Furthermore, if the intake valve opens relatively early even after the intake top dead center TDC, the pressure in the combustion chamber suddenly drops to the intake pressure at that point, impairing the heat dissipation effect.
If the intake valve opening timing is delayed, the heat radiation period can be gained,
For example, the combustion temperature and the exhaust gas are reduced by delaying the intake valve opening timing to such an extent that the period from the intake top dead center TDC to the intake valve opening timing InO is equal to or longer than the period from the exhaust valve closing timing ExC to the intake top dead center TDC. It contributes to lowering the temperature. Also, by delaying the opening timing of the intake valve in this way, the pumping loss reducing effect can be obtained as described above.

The lower combustion temperature improves the thermal efficiency. The fuel efficiency is improved by the effect of reducing the pumping loss, and the lower exhaust gas temperature suppresses the increase in the temperature of the catalyst. The durability is improved.
Further, the decrease in the combustion temperature also exerts the effect of suppressing knocking in the high load side region.

When the internal EGR is obtained by minus O / L, the intake valve and the exhaust valve are closed at the intake top dead center. Therefore, when the internal EGR is obtained by the conventional positive overlap, It is not necessary to provide a deep recess on the top surface of the piston as in the above.

Further, according to such minus O / L, the fact that the effective valve opening period of the intake valve and the exhaust valve decreases as the engine speed increases increases the minus O / L substantially. Since they operate in the same way, even if the minus O / L is made smaller in the high speed range than in the medium speed range, the internal E
Effects such as securing of GR and reduction of combustion temperature and exhaust temperature can be sufficiently obtained. Therefore, for example, at medium speed and medium load, FIG.
As shown in (b), the exhaust valve closing timing ExC is
~ 40 °, intake valve opening timing InO is ATDC35 ~ 4
While set at about 5 ° to increase minus O / L, at high speed and medium load, as shown in FIG. 6C, the exhaust valve closing timing ExC is about BTDC 20 to 30 °, and the intake valve opening timing InO is By setting the ATDC to about 25 to 35 ° and making the minus O / L smaller than the medium-speed / medium load, the internal EGR amount is prevented from becoming excessive, and the output is secured while the above effects are obtained. Is done.

Further, in the fully open range of the engine, the exhaust valve closing timing ExC is slightly lower than the intake top dead center TDC.
By setting the angle to about °, the internal EGR is reduced as much as possible, and the full opening torque is secured.

In the low-speed full-open range, the opening and closing timing of the intake valve and the exhaust valve is set in substantially the same manner as in the medium-speed full-open range. Further, in the low-speed medium-load region, the exhaust valve closing timing ExC is brought close to the top dead center TDC so as to reduce the internal EGR as compared with the medium-speed medium load region in order to secure combustion stability.
L is reduced. However, even if the exhaust valve closing timing ExC and the minus O / L are not changed, the negative angle O / L becomes substantially smaller and the internal EGR decreases because the invalid angle becomes smaller as the engine speed becomes lower. It is also conceivable that the exhaust valve closing timing ExC and the minus O / L in the low-speed medium-load region are set to be substantially the same as those in the medium-speed medium-load region.

[0052]

As described above, according to the engine of the present invention, in a low load range, the exhaust valve closing timing is set to a predetermined period before the intake top dead center, and the intake valve opening timing is set to the intake top dead center. Point, and the period from the intake top dead center to the intake valve open timing is set to be longer than the period from the exhaust valve close timing to the intake top dead center. NOx is sufficiently reduced by the EGR effect,
The effect of reducing the pumping loss can be enhanced, and the fuel efficiency can be greatly improved.

[Brief description of the drawings]

FIG. 1 is a schematic diagram of a four-stroke engine according to one embodiment of the present invention.

FIG. 2 is a diagram showing a cam lift curve for indicating opening and closing timings of an intake valve and an exhaust valve.

FIG. 3 is a partially enlarged view of a cam lift curve.

FIG. 4 is an explanatory diagram showing, in a map form, how to set and change a valve timing according to an operating state.

FIG. 5 is a diagram showing a closing timing of an exhaust valve and an opening / closing timing of an intake valve in a low load range.

FIG. 6 is a diagram showing the closing timing of the exhaust valve and the opening / closing timing of the intake valve in each of the operating ranges of low-speed medium load, medium-speed medium load, high-speed medium load, low-speed full opening, medium-speed full opening, and high-speed full opening.

FIG. 7 is a diagram showing changes in the volume of the combustion chamber and the pressure in the combustion chamber from the latter half of the exhaust stroke to the earlier half of the intake stroke.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine main body 5 Combustion chamber 9 Intake valve 10 Exhaust valve 13, 14 Variable valve timing device 40 ECU 42 Valve timing control means 43 Fuel injection control means

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Tadashige Oba 3-1, Shinchi, Fuchu-cho, Aki-gun, Hiroshima Mazda F-term (reference) 3G092 AA01 AA06 AA09 AA11 BA05 BA06 BB01 BB06 DA01 DA02 DA09 DA12 EA01 EA06 EA07 FA21 FA25 GA05 HA01Z HA06Z HD05Z HE01Z HE03Z HE08Z

Claims (4)

[Claims] 1. An exhaust valve closing timing defined by a transition point from an acceleration section to a constant speed section in a cam lift characteristic for an exhaust valve in a low load region in a warm state of an engine before a predetermined period before a top dead center of intake. In addition to the setting, the intake valve opening timing defined by the transition point from the constant speed section to the acceleration section in the intake valve cam lift characteristic is set after the intake top dead center and from the intake top dead center to the intake valve opening timing. A four-cycle engine for an automobile, wherein the period is set to be longer than the period from the exhaust valve closing timing to the intake top dead center. 2. An intake valve according to claim 1, further comprising a variable valve timing device for changing at least one of an exhaust valve and an intake valve so that the valve opening / closing timing can be changed. 2. The four-stroke engine for an automobile according to claim 1, wherein the valve opening / closing timing is changed according to the operating state so that a period until the opening timing is reduced at least as the load increases from a medium load. 3. In a low load range in a warm state of the engine, a period from the exhaust valve closing timing to the intake top dead center is set to 5 ° or more in crank angle, and the crank angle is sandwiched between the intake top dead center. 3. The crankshaft according to claim 1, wherein a period from an exhaust valve closing timing to the intake valve opening timing is set to 20 ° or more in crank angle.
A four-stroke engine for an automobile according to the above. 4. The four-stroke engine for an automobile according to claim 1, further comprising an injector for directly injecting fuel into the combustion chamber.