Homogeneous charge compression ignition engine and air intake and exhaust system thereof

Abstract

An EGR passage partially refluxes exhaust gas to a combustion chamber as an EGR gas. A heat exchanger cools the EGR gas. A heating intake passage branches off from a branching portion formed in a portion of an intake passage on an upstream side of a downstream end of the EGR passage, and its downstream end communicates with a portion of the EGR passage on an upstream side of the heat exchanger. A switch valve adjusts respective amounts of intake air passing through the intake passage and through the heating intake passage. When an EGR valve is closed, an ECU switches the switch valve such that the intake air passes through one of the intake passage and the heating intake passage; when the EGR valve is open, the ECU switches the switch valve such that the intake air passes solely through the intake passage.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a homogeneous charge compression ignition engine and an air intake and exhaust system thereof for refluxing exhaust gas to a combustion chamber as an EGR gas.

2. Description of the Related Art

In recent years, in the field of internal combustion engines, attention is being given to a homogeneous charge compression ignition engine capable of attaining a satisfactory fuel efficiency and thermal efficiency, and various studies are being conducted in this regard. In most homogeneous charge compression ignition engines, fuel and air are mixed with each other in an intake passage, and the resultant air fuel mixture is supplied to a combustion chamber. Further, the air fuel mixture trapped in the combustion chamber undergoes self-ignition during compression stroke with an increase in temperature and pressure due to rising of a piston. As is known in the art, in such a homogeneous charge compression ignition engine, the operation range allowing stable control of the homogeneous charge compression ignition (HCCI) is still rather small, which is a problem to be solved in putting this engine into practical use. In view of this, with a view toward solving this problem, an attempt is being made to put into practical use a homogeneous charge compression ignition engine as applied to a stationary engine whose normal operation range is relatively small, such as a gas heat pump (GHP) gas engine. Further, there has also been proposed an engine in which operational switching is effected as appropriate such that homogeneous charge compression ignition is effected in a range near a low/medium rotation and low/medium load range, which is frequently adopted in actual operation, and that spark ignition (SI) is effected in a high rotation range and an ultra-low load and high load range.

In a homogeneous charge compression ignition engine, the operation range allowing stable control of the homogeneous charge compression ignition is small. In the following, this problem will be discussed in detail. For example, in the low load operation range, the amount of air fuel mixture supplied to the combustion chamber is small, and the in-cylinder temperature does not easily increase, so the ignition property deteriorates, and a misfire is liable to occur. As is known in the art, in order to suppress occurrence of a misfire, there is adopted a method using a so-called internal EGR, according to which a negative overlapping period is provided in the valve timing of the intake valve and the exhaust valve so that a part of the gas already burnt may be allowed to remain in the combustion chamber for the next cycle of combustion. By thus utilizing the internal EGR, the internal EGR gas at high temperature and the air fuel mixture newly supplied to the combustion chamber are mixed with each other to increase the in-cylinder temperature, so the ignition property at the time of homogeneous charge compression ignition is improved, thus suppressing occurrence of a misfire. However, under a still worse condition as in the case of a low outdoor air temperature, the temperature in the combustion chamber is low, and a high-temperature internal EGR is hard to obtain, so even when an internal EGR is used, homogeneous charge compression ignition is rather hard to effect, and there is a fear of occurrence of a misfire.

Apart from the internal EGR, there is known, as a means for increasing the in-cylinder temperature of the combustion chamber of, for example, a diesel engine, a means which prevents occurrence of a misfire by causing heated intake air (air fuel mixture) previously heated by a heating mechanism such as a heat exchanger to flow into the combustion chamber, that is, by performing intake air heating.

On the other hand, in a homogeneous charge compression ignition engine, there occurs in the high load operation range an abnormal combustion such as knocking or premature ignition. As is known in the art, in order to suppress occurrence of such an abnormal combustion, there is utilized an external exhaust gas recirculation (EGR). Since the external EGR gas immediately after its extraction from the exhaust passage is at high temperature, it is cooled by an EGR cooler provided at some midpoint in the EGR passage so that the volumetric efficiency of the intake air may not be deteriorated. Further, by refluxing the EGR gas cooled by the EGR cooler into the combustion chamber, the combustion in the combustion chamber is slowed down due to an increase in inert gas.

As an example of the heat exchanger as the heating device or the heat exchanger as the EGR cooler, JP 2005-517857 A cited herein as Patent Document 1 discloses a technique according to which the heating of the intake air and the cooling of the external EGR gas are effected by using a heat exchanger 12 (see FIG. 2 of Patent Document 1 described above). In this technique, the heat exchange mechanism is made relatively simple by effecting the cooling of the EGR gas and the intake air heating with the same coolant.

However, while having a single casing, the heat exchanger of Patent Document 1 described above contains a first heat exchange portion and a second heat exchange portion that are separate from each other, and the cooling of the EGR gas and the intake air heating are effected in each of those separate systems, i.e., within each of the first heat exchange portion and the second heat exchange portion. Thus, while it can achieve space saving as compared with the related-art technique using two heat exchangers, the technique as disclosed in Patent Document 1 described above involves an increase in size of the heat exchanger as compared with an ordinary heat exchange system consisting of a single heat exchanger. Further, in an internal combustion engine using the heat exchanger of Patent Document 1 described above, it is necessary to connect to the single heat exchanger a large number of pipes, including pipes constituting the intake passage and the EGR passage and a pipe for coolant, resulting in a rather strict limitation in terms of design in arranging them around the internal combustion engine.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide a homogeneous charge compression ignition engine and an air intake and exhaust system thereof which help to achieve further space saving and which allow EGR gas cooling and intake air heating.

In order to achieve the above-mentioned object, a homogeneous charge compression ignition engine according to the present invention includes: a combustion chamber; an intake passage serving as a passage for intake air to the combustion chamber; an exhaust passage serving as a passage for exhaust gas from the combustion chamber; an EGR passage, communicating with the exhaust passage and the intake passage, for refluxing a part of the exhaust gas from the combustion chamber to the combustion chamber as an EGR gas; a heat exchanger, provided at some midpoint in the EGR passage, for cooling the EGR gas; an EGR valve, provided at some midpoint in the EGR passage, for adjusting an opening and closing of the EGR passage; a heating intake passage which branches off from a branching portion formed in a portion of the intake passage on an upstream side of a downstream end of the EGR passage and whose downstream end communicates with a portion of the EGR passage on an upstream side of the heat exchanger; a switch valve for adjusting respective amounts of intake air passing through the portion of the intake passage on a downstream side of the branching portion and through the heating intake passage; and a control means which, when the EGR valve is closed, switches the switch valve such that, on the downstream side of the branching portion, the intake air flowing into the combustion chamber passes through at least one of the intake passage and the heating intake passage, and which, when the EGR valve is open, switches the switch valve such that, on the downstream side of the branching portion, the intake air flowing into the combustion chamber passes solely through the intake passage.

Further, in order to achieve the above-mentioned object, an air intake and exhaust system for use in a homogeneous charge compression ignition engine according to the present invention is used for a homogeneous charge compression ignition engine having a combustion chamber and an exhaust passage serving as a passage for exhaust gas from the combustion chamber. The air intake and exhaust system includes: an intake passage leading to the combustion chamber; an EGR passage, communicating with the exhaust passage and the intake passage, for refluxing the exhaust gas from the combustion chamber to the combustion chamber as an EGR gas; a heat exchanger, provided at some midpoint in the EGR passage, for cooling the EGR gas; an EGR valve, provided at some midpoint in the EGR passage, for opening and closing the EGR passage; a heating intake passage which branches off from a branching portion formed in a portion of the intake passage on an upstream side of a downstream end of the EGR passage and whose downstream end communicates with a portion of the EGR passage on an upstream side of the heat exchanger; a switch valve for adjusting respective amounts of intake air passing through the intake passage and through the heating intake passage on a downstream side of the branching portion; and a control means which, when the EGR valve is closed, switches the switch valve such that, on the downstream side of the branching portion, the intake air flowing into the combustion chamber passes through at least one of the intake passage and the heating intake passage, and which, when the EGR valve is open, switches the switch valve such that, on the downstream side of the branching portion, the intake air flowing into the combustion chamber passes solely through the intake passage.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings;

FIG. 1 is an overall schematic view of a homogeneous charge compression ignition engine and an air intake and exhaust system thereof according to an embodiment of the present invention;

FIG. 2 is a chart illustrating how an EGR valve and a switch valve of the homogeneous charge compression ignition engine of FIG. 1 are controlled;

FIG. 3 is a schematic view of the operation ranges when the homogeneous charge compression ignition engine of FIG. 1 is used;

FIG. 4 is an overall schematic view of a first modification of the homogeneous charge compression ignition engine of FIG. 1;

FIG. 5 is an overall schematic view of a second modification of the homogeneous charge compression ignition engine of FIG. 1; and

FIG. 6 is an enlarged schematic view illustrating an operation range allowing homogeneous charge compression ignition without using any supercharger in the homogeneous charge compression ignition engine of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following, a preferred embodiment of the present invention will be described with reference to the drawings.

(Overall Construction)

With reference to FIG. 1, the overall construction of a homogeneous charge compression ignition engine and an air intake and exhaust system thereof according to an embodiment of the present invention will be described. In the following description, the term “intake air” means a gas supplied into a combustion chamber (e.g., mixture of intake air and gas fuel or mixture of intake air, gas fuel, and external EGR), and the term “air fuel mixture” means a mixture of intake air and gas fuel.

As shown in FIG. 1, a homogeneous charge compression ignition engine 1 has a combustion chamber 10, an intake passage 11p leading to the combustion chamber 10, and an exhaust passage 12p constituting a passage for the exhaust gas from the combustion chamber 10, and effects switching as appropriate between spark ignition and homogeneous charge compression ignition according to the operating conditions (load and engine RPM). In this way, by switching between homogeneous charge compression ignition and spark ignition according to the operating conditions, it is possible to attain both the high fuel efficiency due to homogeneous charge compression ignition and the satisfactory starting property and high output due to spark ignition.

Further, the homogeneous charge compression ignition engine 1 has a supercharger 11t, a throttle 3, a fuel valve 2v (fuel supply amount adjusting device), a heating intake passage 20p, a switch valve 11v, an EGR passage 30p, an EGR valve 30v, a heat exchanger 40, an intake valve 51v, an exhaust valve 52v, and an ignition plug 53c. (Those components will be described in detail below). Further, the homogeneous charge compression ignition engine 1 has an electronic control unit (ECU, which corresponds to a control device) 5. Electrically connected to the ECU 5 are control cables 5a through 5i corresponding to the exhaust valve 52v, the ignition plug 53c, the intake valve 51v, the EGR valve 30v, the switch valve 11v, the throttle 3, the fuel valve 2v, a bypass control valve 40v, and the supercharger 11t, respectively. Further, the ECU 5 controls the operation of the exhaust valve 52v, the ignition plug 53c, the intake valve 51v, the EGR valve 30v, the switch valve 11v, the throttle 3, the fuel valve 2v, the bypass control valve 40v, and the supercharger 11t.

(Regarding the Air Intake and Exhaust System)

An air intake and exhaust system 60 of the homogeneous charge compression ignition engine 1 of this embodiment is used to supply intake air to the combustion chamber 10, to scavenge exhaust gas from the combustion chamber 10, etc., and includes the intake passage 11p, the exhaust passage 12p, the heating intake passage 20p, the switch valve 11v, the EGR passage 30p, the EGR valve 30v, the heat exchanger 40, and the ECU 5. Further, apart from the above-mentioned intake air supply, etc., the air intake exhaust system 60 performs mixing of external EGR with air fuel mixture, intake air heating, etc.

(Mixer)

The homogeneous charge compression ignition engine 1 has a mixer 4 at some midpoint in the intake passage 11p; a gas fuel is supplied to the mixer 4 through a fuel supply path 2p communicating with the mixer 4. (That is, fuel supply path 2p serves as the passage for the gas fuel, and communicates with intake passage 11p). Further, air and the fuel are mixed with each other at the mixer 4.

(Fuel Valve)

The fuel valve 2v is provided at some midpoint in the fuel supply path 2p. Further, under control of the fuel valve 2v by the ECU 5, the degree of opening of the fuel valve 2v is adjusted, whereby the amount of gas fuel supplied to the intake passage 11b is adjusted.

(Throttle)

As shown in FIG. 1, the throttle 3 has a step motor (not shown) for driving a shaft 3c and a valve portion 3v, and the valve portion 3c is rotatable around the shaft 3c. Further, the ECU 5 controls the step motor of the throttle 3, whereby the degree of opening of the passage is adjusted by the valve portion 3v, thereby adjusting the amount of intake air supplied to the combustion chamber 10 through the intake passage 11p.

(Supercharger)

The supercharger 11t is of an electric turbo type whose supercharging pressure is variable, and is formed as a centrifugal compressor driven by an electric motor M. The supercharger 11t is arranged at some midpoint in the intake passage 11p. Further, a bypass route 40a and a main route 40b are respectively connected to the upstream side of the supercharger 11t and to the outlet portion of the supercharger 11t, and communication through those routes is provided for the intake passage 11p. At some midpoint of the bypass route 40a, there is provided a bypass control valve 40v for opening and closing the bypass route 40a.

When the supercharger 11t is not used, the electric motor M is controlled to be kept at rest, and the bypass control valve 40v is controlled to be kept open. As a result, the bypass route 40a functions as a normal intake passage. On the other hand, when performing supercharging by using the supercharger 11t, the electric motor M is controlled to be driven to be rotated, and the bypass control valve 40v is controlled to be closed. As a result, the main route 40b functions as a supercharging passage.

(Combustion Chamber)

The combustion chamber 10 is an internal space defined in the internal combustion engine by a cylinder and a piston; a piston 50 reciprocates vertically as seen in the drawing. The air fuel mixture is supplied to the combustion chamber 10 through the intake passage 11p. After the combustion, the exhaust gas is discharged through the exhaust passage 12p. The intake valve 51v and the exhaust valve 52v are arranged at the opening of the intake passage 11p to the combustion chamber 10 and at the opening of the exhaust passage 12p to the combustion chamber 10, respectively. Those valves are opened and closed as appropriate upon rising and lowering of the piston 50 during the strokes of intake, compression, combustion/expansion, and exhaust, thereby effecting intake and exhaust. More specifically, the intake valve 51v and the exhaust valve 52v are respectively driven to be opened and closed by cams 51c and 52c formed on the outer peripheries of camshafts (not shown), and their opening/closing timing (phase with respect to the crank angle) is varied by a well-known variable valve timing mechanisms provided on the camshafts. The ignition plug 53c is used for ignition at the time of spark ignition, and its operation is controlled by the ECU 5.

(EGR Passage)

The exhaust gas from the combustion chamber 10 passes through the exhaust passage 12p, and is discharged to the exterior through an exhaust port (not shown); a part of the exhaust gas discharged into the discharge passage 12p is supplied to the combustion chamber 10 again. The EGR passage 30p serves to reflux a part of the exhaust gas from the combustion chamber 10 to the combustion chamber 10 as EGR gas; it branches off from the exhaust passage 12p at a branching position 12b, and a downstream end 30b thereof communicates with the intake passage 11p. As a result, the inner space of the exhaust passage 12p and that of the intake passage 11p communicate with the inner space of the EGR passage 30p at the upstream end (branching position 12b) and the downstream end 30b of the EGR passage 30p.

Halfway through the EGR passage 30p, there are provided the EGR valve 30v for adjusting the opening/closing condition of the EGR passage 30p and the heat exchanger 40 for cooling the EGR gas in that order from the upstream side (with respect to the flowing direction of the EGR gas). Further, as described below, the EGR passage 30p has a communicating portion 20b between the EGR valve 30v and the heat exchanger 40, establishing communication with the downstream end of the heating intake passage 20p.

(Heat Exchanger)

The heat exchanger 40 is provided at some midpoint in the EGR passage 30p, and functions as a cooling device for the EGR gas. Further, in the case where the switch valve 11v has been switched so as to allow passage of intake air (air fuel mixture) through the heating intake passage 20p, the heating exchanger 40 also functions as an intake air heating device.

The heat exchange medium of the heat exchanger 40 is the circulation coolant of the engine; through passing of this circulation coolant through its interior, the heat exchanger 40 functions as a heat exchange device (see arrows A and A′ in FIG. 1). The circulation coolant that has attained high temperature as the result of cooling the cylinder block during operation of the engine is circulated through a radiator outside the engine, and is cooled at the radiator, whereby the temperature of the coolant is maintained at approximately 70 to 80 degrees.

(Heating Intake Passage)

Heating intake passage 20p constitutes a part of the intake passage when performing intake air heating, and branches off from a branching portion 11b formed on the upstream side of the downstream end 30b of the EGR passage 30p (with respect to the flowing direction of the intake air) in the intake passage 11p. The downstream end of the heating intake passage 20p is formed so as to communicate with the upstream side of the heat exchanger 40 in the EGR passage 30p (upstream side with respect to the flowing direction of the EGR gas) at the communicating portion 20b.

(Switch Valve)

The switch valve 11v serves to adjust the amount of intake air (air fuel mixture) passing through the part of the intake passage 11p on the downstream side of the branching portion 11b (with respect to the flowing direction of the intake air) and through the heating intake passage 20p. The air fuel mixture sent from the upstream side to the branching portion 11b passes the branching portion 11b. After that, the proportion in which it circulates through the two circulation passages of the intake passage 11p and the heating intake passage 20p, on the downstream side of the branching portion 11b, is determined by adjustment of the switch valve 11v. More specifically, switching is effected in one of the following patterns: (a) a route in which, on the downstream side of the branching portion 11b, all the air fuel mixture passes through the intake passage 11p as it is and flows into the combustion chamber 10 (intake passage: 100%), (b) a route in which, on the downstream side of the branching portion 11b, all the air fuel mixture passes through the heating intake passage 20p to pass through a part of the EGR passage 30p before passing through the intake passage 11p again to flow into the combustion chamber 10 (heating intake passage: 100%), and (c) a route which is somewhere between the above two routes (i.e., route in which, on the downstream side of branching portion 11b, a part of the air fuel mixture passes through intake passage 11p, with a part of the air fuel mixture passing through heating intake passage 20p).

More specifically, the switch valve 11v contains a valve (not shown) for the intake passage 11p and a valve (not shown) for the heating intake passage 20p, each of which is controlled to be opened or closed, whereby the respective amounts of intake air passing through the portion of the intake passage 11p on the downstream side of the branching portion 11b and through the heating intake passage 20p are adjusted. While in this embodiment the switch valve is constructed as described above, this should not be construed restrictively.

(ECU)

The ECU 5 controls the switch valve 11v as follows: First, when the EGR valve 30v is closed, the switch valve 11v is switched such that the air fuel mixture flowing into the combustion chamber 10 passes, on the downstream side of the branching portion 11b (with respect to the intake air flowing direction), through at least one of the intake passage 11p and the heating intake passage 20p. On the other hand, when the EGR valve 30v is open, the switch valve 11v is switched such that the air fuel mixture flowing into the combustion chamber 10 passes solely through the intake passage 11p on the downstream side of the branching portion 11b (i.e., air fuel mixture does not pass through heating intake passage 20p, with all the air fuel mixture passing through intake passage 11p).

(Regarding Exhaust Gas and External EGR)

Next, the exhaust gas and the external EGR will be described. During homogeneous charge compression ignition, an abnormal combustion is liable to occur in the operation range on the high load side. In view of this, in the homogeneous charge compression ignition engine 1, by opening the EGR valve 30v in the operation range on the high load side, the EGR gas is cooled by the heat exchanger 40 provided at some midpoint in the EGR passage 30p, and is supplied to the combustion chamber 10 together with the air fuel mixture, whereby the combustion in the combustion chamber 10 is slowed down.

(Regarding Internal EGR)

Next, the internal EGR will be described. The homogeneous charge compression ignition engine 1 has a negative overlapping period for the valve timing during homogeneous charge compression ignition operation. Here, the negative overlapping period is a period in which both the exhaust valve 52v and the intake valve 51v are closed near the exhaust top dead center, with the exhaust valve 52v being closed before the exhaust top dead center is reached. As a result, it is possible to allow a part of the already burned gas (internal EGR gas) to remain in the combustion chamber 10 for the next cycle of combustion. By providing the negative overlapping period and by utilizing the internal EGR, the internal EGR gas that is at high temperature is mixed with the air fuel mixture newly supplied into the combustion chamber 10 to increase the in-cylinder temperature, so the ignition property at the time of homogeneous charge compression ignition is improved. Further, by controlling the length of the negative overlapping period, it is possible to control the ignition time to some degree.

(Supercharging and External EGR)

Next, supercharging and the external EGR will be described. During high load operation, the supply amount of the air fuel mixture including gas fuel increases, so the ignition property is improved to an excessive degree, and knocking attributable to too intense a combustion is liable to occur. In order to suppress the occurrence of knocking, it is necessary to reduce the internal EGR amount in the combustion chamber 10 for the purpose of reducing the ignition property. Thus, control is effected to reduce the negative overlapping period. However, a reduction in the internal EGR simultaneously causes a lag in ignition timing and a reduction in combustion rate, so it is impossible to control the ignition timing and the combustion rate as they are in a properly balanced manner. In view of this, the ignition timing is first controlled by raising the intake air temperature through supercharging, maintaining a proper timing. Further, when the engine operation range is on the high load side, external EGR is performed to suppress abnormal combustion, slowing down the combustion.

(Operation)

Next, an operation of the homogeneous charge compression ignition engine 1, constructed as described above, will be illustrated. First, in the operation range where spark ignition is effected, the ECU 5 controls the ignition plug 53c, etc., thereby effecting spark ignition.

In the operation range where homogeneous charge compression ignition is effected, the ECU 5 performs the following control. First, the air fuel mixture produced at the mixer 4 passes the intake passage 11p while being adjusted in intake amount by the throttle 3, and reaches the position of the branching portion 11b where the switch valve 11v is arranged.

Further, when the EGR valve 30v is closed, the switch valve 11v is switched such that the air fuel mixture flowing into the combustion chamber 10 passes at least one of the intake passage 11p and the heating intake passage 20p on the downstream side of the branching portion 11b. Here, when the air fuel mixture passes the heating intake passage 20p, the air fuel mixture passes the heat exchanger 40, so the heating of the air fuel mixture is effected at the heat exchanger 40. Thus, the air fuel mixture, which is at around the outdoor air temperature, passes the heating intake passage 20p, and is supplied to the combustion chamber 10 in a state in which its temperature has been previously raised by several to several tens of degrees. When, for example, the outdoor air temperature is low, the temperature in the combustion chamber is low, and it is difficult to obtain a high temperature internal EGR, so homogeneous charge compression ignition does not easily occur; thus, when homogeneous charge compression ignition is effected, there is a fear of occurrence of a misfire. However, by thus causing a previously heated intake air (air fuel mixture) to flow into the combustion chamber 10, it is possible to suppress occurrence of a misfire and to enlarge the operation range where homogeneous charge compression ignition is possible.

The case in which the EGR valve 30v is closed corresponds to the case in which the engine is in the operation range where there is no need to use external EGR. As shown in FIG. 3, an operation range is determined by engine load and engine RPM; the homogeneous charge compression ignition range (operation range suitable for homogeneous charge compression ignition) corresponds to the central portion of the drawing. Of this homogeneous charge compression ignition range (including ranges indicated by symbols a, b, c, and d), the ranges a, b, and c are operation ranges where no external EGR is used.

When the EGR valve 30v is open, the switch valve 11v is switched such that the air fuel mixture flowing into the combustion chamber 10 passes solely through the intake passage 11p on the downstream side of the branching portion 11b. That is, when the EGR valve 30v is open, the switch valve 11v is controlled such that no air fuel mixture passes through the heating intake passage 20p.

Further, since the EGR valve 30v is open, a part of the exhaust gas from the combustion chamber circulates through the EGR passage 30p, and is cooled by the heat exchanger 40 before flowing into the intake passage 11p from the downstream end 30b communicating with the intake passage 11p and refluxing to the combustion chamber 10. (That is, both the air fuel mixture and the external EGR gas are sent to the combustion chamber 10). In this way, when the EGR valve 30v is in the open state, the heat exchanger 40 functions as a cooling device for the external EGR.

In the high load operation range, abnormal combustion such as knocking or premature ignition occurs; however, by thus utilizing the external EGR, it is possible to suppress occurrence of abnormal combustion. More specifically, the external EGR gas, which is at high temperature (e.g., approximately 300° C. before reaching the heat exchanger), is cooled by the heat exchanger (EGR cooler) 40 provided at some midpoint in the EGR passage 30p. Further, the cooled EGR gas refluxes into the combustion chamber 10, whereby the combustion in the combustion chamber 10 is slowed down due to an increase in inert gas, and occurrence of abnormal combustion in the combustion chamber 10 is suppressed.

The case in which the EGR valve 30 is open corresponds to the case in which the engine is in the operation range where it is necessary to use the external EGR. FIG. 3 shows operation ranges that are determined by engine load and engine RPM; of the homogeneous charge compression ignition ranges (ranges a through d), the range d is the operation range where the external EGR is used. More specifically, in the high load operation range, the external EGR is utilized as supercharging of the air fuel mixture is effected. As a result, it is possible to both secure the requisite ignition property and slow down the combustion due to an increase in intake air temperature caused by supercharging and due to a local reduction in temperature attributable to the external EGR, with the result that it is possible, in the range on the high load side, to enlarge the range where homogeneous charge compression ignition is possible.

(Control Example for the Homogeneous Charge Compression Ignition Engine and the Air Intake and Exhaust System)

Next, a control example for the homogeneous charge compression ignition engine 1 and the air intake and exhaust system 60 will be described with reference to FIG. 2. FIG. 2 is a chart illustrating the opening/closing control and switching control of the EGR valve 30v and the switch valve 11v of the homogeneous charge compression ignition engine 1. The horizontal axis of FIG. 2 indicates the magnitude of the engine load. The upper chart in FIG. 2 illustrates the opening degree of the EGR valve. In this chart, the lowermost portion (see portion (i) of FIG. 2) corresponds to the state in which the EGR valve 30v is closed, and the opening degree of the EGR valve 30v increases as the line extends upwards therefrom (see portion (ii) of FIG. 2). The lower chart in FIG. 2 illustrates how the switch valve 11v is switched. In this chart, the uppermost position (see portion (1) of FIG. 2) corresponds to the state in which all the air fuel mixture is passing through the heating intake passage 20p on the downstream side of the branching portion 11b, and the lowermost position (see portion (3) of FIG. 2) corresponds to the state in which all the air fuel mixture is passing through the intake passage 11p on the downstream side of the branching portion 11b. The intermediate position (see portion (2) of FIG. 2) corresponds to the intermediate state, that is, the state in which, on the downstream side of the branching portion 11b, a part of the air fuel mixture passes through the intake passage 11p, with a part of the air fuel mixture passing through the heating intake passage 20p.

Further, as shown in FIG. 2, in the homogeneous charge compression ignition engine 1, the operation range is divided according to the engine load into an NA (natural aspiration) range, a supercharging range, a supercharging/external-EGR range, etc., and operation control is effected according to each load range. The horizontal axis in FIG. 2 indicates a part of the entire range, which means there exist a range of lower load and a range of higher load than the range shown in the drawing.

As shown in FIG. 2, in the NA range and the supercharging range, the EGR valve 30v is in the closed state, and the heating intake passage 20p and the intake passage 11p are used as the intake path. That is, the external EGR is not used, but the heat exchanger 40 is used solely for intake air heating.

In the supercharging/external-EGR range, the EGR valve 30v is in the open state, and the heating intake passage 20p is not used at all. That is, solely the external EGR is used, and the heat exchanger 40 is used solely as the EGR cooler for cooling the external EGR gas.

The control illustrated in FIG. 2 is only shown by way of example; the control of the EGR valve 30v and the switch valve 11b is not restricted to the one shown in the drawing.

(Effects)

In the following, effects of the present invention as compared with those of a diesel engine will be described. In a diesel engine, intake air heating is effected for the purpose, for example, of suppressing generation of unburned fuel component and white smoke during low load operation as in the case of engine start. Further, by using external EGR, exhaust gas, which is an inert gas, is supplied to the combustion chamber, and the maximum combustion temperature is lowered to reduce the amount of nitrogen oxides generated. To elaborate on this, in the diesel engine, fuel is directly injected into the combustion chamber, so generation of differing concentrations of fuel in the combustion chamber is inevitable, whereby a high temperature portion is locally generated, resulting in generation of a large amount of nitrogen oxides. That is, in the diesel engine, external EGR is used for the purpose of suppressing generation of nitrogen oxides; in particular, in recent years, when countermeasures against exhaust gas are being emphasized, the operation range in which it is used along with intake air heating is being enlarged in the low load operation range.

On the other hand, in homogeneous charge compression ignition, during low load operation as in the case of low outdoor air temperature and during medium load operation, an improvement in ignition property is achieved by heating the intake air (air fuel mixture), and occurrence of a misfire is suppressed. Further, in homogeneous charge compression ignition, the use of external EGR during high load operation is effective in view of suppression of knocking. However, during low load operation as in the case of low outdoor air temperature and during medium load operation, the use of external EGR is not desirable in view of deterioration in ignition property. That is, in homogeneous charge compression ignition, intake air heating helps to achieve an improvement in ignition property and external EGR suppresses ignition property, so they are used in different operation ranges, which means external EGR and intake air heating are not used simultaneously.

To elaborate on this, in homogeneous charge compression ignition, an air fuel mixture obtained by substantially uniformly mixing fuel and air with each other is caused to undergo self ignition in the combustion chamber, so, as compared with a diesel engine in which fuel exists in an uneven fashion, and as compared with spark ignition (as in the case of a gasoline engine, for example) in which a local high temperature portion is generated in the flame, a local high temperature portion is not generated easily, and the maximum combustion temperature is low. Thus, the amount of nitrogen oxides generated is small, and there is no need to use external EGR for the purpose of suppressing generation of nitrogen oxides.

As described above, in the homogeneous charge compression ignition engine 1, the use of external EGR and intake air heating are not effected simultaneously, so the heat exchanger 40 only effects one of the cooling of EGR gas and intake air heating according to the operation range (which is determined according to the engine load and the engine RPM). Thus, due to the above-mentioned construction of the homogeneous charge compression ignition engine 1, the heat exchanger 40, which is used as the EGR cooler, can also be used for intake air heating without involving a change in construction, so, even when compared with a generally adopted construction which uses a single heat exchanger having only one heat exchange portion, there is involved no increase in the size of the heat exchanger. Thus, with the simple construction, it is possible to effect cooling of the EGR gas and intake air heating. Further, by using an existing EGR passage having an EGR cooler, it is possible to realize the above-mentioned construction through a change in and addition of a simple piping structure. Further, there is no need to connect to a single heat exchanger a large number of pipes, including the pipes constituting the intake passage and the EGR passage and the pipe for coolant, and the restrictions in terms of design in arranging the pipes around the internal combustion engine are mitigated, so it is possible to achieve further space saving and conduct cooling of the EGR gas and intake gas heating.

Further, in the EGR passage 30p, the EGR valve 30v is provided on the upstream side of the downstream end of the heating intake passage 20p (communicating portion 20b), so, in the EGR passage 30p, the heated intake air (air fuel mixture) is not allowed to flow into the upstream side of the communicating portion 20b, and it is possible to reflux the heated intake air to the combustion chamber 10 via the portion of the EGR passage 30p on the downstream side of the communicating portion 20b and the intake passage 11p. Further, when the EGR valve is closed, it is possible to prevent EGR gas from flowing into the heating intake passage 20p.

Further, the switch valve 11v is provided at the branching portion 11b, so it is possible to send the intake air (air fuel mixture) heated by the heat exchanger 40 positively into the heating intake passage 20p. On the other hand, the intake air (air fuel mixture) not passing through the heat exchanger 40 does not enter the heating intake passage 20p, all of it passing through the intake passage 11p. Thus, the circulation of the intake air is effected efficiently.

Further, the heat exchanger 40, through which the circulation coolant of the engine passes, can utilize the engine heat, making it possible to effect the cooling of the EGR gas and the intake air heating with a still simpler construction.

Further, with its simple construction, the air intake and exhaust system 60 of the homogeneous charge compression ignition device of the present invention described above can effect the cooling of EGR gas and intake gas heating. Further, by utilizing an existing EGR passage having an EGR cooler, it is possible to realize the above construction through a change in and addition of a simple piping structure.

(Regarding Expansion of the Range Allowing Operation)

Further, it is possible to expand the range allowing operation toward the low load side by using the homogeneous charge compression ignition engine 1 and the air intake and exhaust system 60. This will be described with reference to FIG. 3. FIG. 3 is a schematic view illustrating the operation range in the case in which the homogeneous charge compression ignition engine 1 is used; the horizontal axis indicates engine RPM, and the vertical axis indicates engine load.

In FIG. 3, the central portion indicated by symbol HCCI corresponds to the range where homogeneous charge compression ignition is effected, and the remaining, peripheral range thereof corresponds to the spark ignition (SI) range. In this way, operation is conducted while effecting switching as appropriate between homogeneous charge compression ignition and spark ignition according to the engine load and the engine RPM.

Further, in the case of an engine performing no intake air heating, it is only in the range b in FIG. 3 that operation is conducted by natural aspiration. That is, when no intake air heating is conducted, operation is conducted through spark ignition in the range a of the drawing. However, in the case in which intake air heating is effected as in the homogeneous charge compression ignition engine 1, homogeneous charge compression ignition operation is possible not only in the range b but also in the range a.

In the following, this will be described more specifically. First, in the homogeneous charge compression ignition in the range a, which is a low load range, a misfire is liable to occur since the amount of fuel supplied is small. However, by performing intake air heating in the range a, that is, by closing the EGR valve 30v and effecting switching at the switch valve 11v such that the intake air (air fuel mixture) passes through the heating intake passage 20p, an improvement in ignition property is achieved, and occurrence of a misfire can be suppressed, so homogeneous charge compression ignition is possible. As a result, it is possible to expand the range allowing operation toward the low load side.

By performing the above-mentioned control in the homogeneous charge compression ignition engine 1, it is possible to expand the range allowing operation toward the low load side (range a). Further, by adjusting the switch valve 11v as appropriate, the amounts of intake air distributed to the intake passage 11p and the heating intake passage 20p are controlled according to the outdoor air temperature, whereby it is possible to adjust the temperature of the intake air flowing into the combustion chamber 10. Thus, irrespective of the outdoor air temperature, it is possible to expand the operation range toward the low load side by adjustment of intake air distribution. In the range c of the drawing, operation is effected by supercharging, and, in the range d of the drawing, operation is possible by supercharging and utilization of external EGR.

(Modifications)

Next, modifications of the homogeneous charge compression ignition engine of the above embodiment will be described with reference to FIGS. 4 and 5; the description will center on the differences between the above embodiment and the modifications. FIG. 4 is an overall schematic view of a homogeneous charge compression ignition engine according to a first modification, and FIG. 5 is an overall schematic view of a homogeneous charge compression ignition engine according to a second modification. The portions that are the same as those of the above embodiment are indicated by the same reference numerals, and a description thereof will be omitted.

(First Modification)

First, a first modification will be described. As shown in FIG. 4, in a homogeneous charge compression ignition engine 100 and an air intake and exhaust system 160 according to this modification, a switch valve 111v and an EGR valve 130v are provided at the communicating portion 20b between the heating intake passage 20p and an EGR passage 130p. More specifically, the switch valve 111v, which is an opening/closing valve, is provided at the connecting portion between the heating intake passage 20p and the EGR passage 130p, and the EGR valve 130v, which is an opening/closing valve, is provided in the portion of the EGR passage on the upstream side of the connecting portion 20b. Here, a control cable 105d (105e) connecting the switch valve 111v and the EGR valve 130v to an ECU 105 is one obtained by integrating the control cables 5d and 5e of the above embodiment. In this modification, the switch valve and the EGR valve are arranged at one position in the form of a valve 100v, so it is possible to form the homogeneous charge compression ignition engine and the air intake and exhaust system in a simple construction.

(Second Modification)

First, a second modification will be described. As shown in FIG. 5, in a homogeneous charge compression ignition engine 200 and an air intake and exhaust system 260 of this modification, an EGR valve 230v is provided near the communicating portion 20b (downstream end of the heating intake passage) between the heating intake passage 20p and an EGR passage 230p, and is provided on the upstream side with respect to the flowing direction of the EGR gas in the EGR passage 230p. With this construction also, it is possible to obtain the same effect as that of the above-mentioned embodiment.

The present invention is not restricted to the above-mentioned embodiment but can be carried out in various modifications without departing from the scope as defined in the claims.

For example, while in the above embodiment the fuel supply path 2p is arranged so as to communicate with the portion of the intake passage 11p on the upstream side of the branching portion 11b, this should not be construed restrictively. For example, it may also communicate with the portion of the intake passage 11p on the downstream side of the connecting portion (downstream end 30b) to the EGR passage 30p (see the position indicated by arrow C of FIG. 1). By arranging the fuel supply path 2p as in the above embodiment or at the position indicated by the arrow C, it is possible to supply fuel halfway through the route for intake air no matter which of the intake passage 11p and the heating intake passage 20p may be selected by the switch valve 11v. On the other hand, when the fuel supply path is arranged at a position in the portion of the intake passage 11p between the branching portion 11b and the connecting portion (downstream end 30b) (see the position indicated by arrow B of FIG. 1), the communicating position of the fuel supply path is not halfway through the route for the intake air when the heating intake passage 20p is selected, so the fuel supply becomes rather incomplete, which is not desirable.

Further, while in the above embodiment the throttle 3 is arranged in the portion of the intake passage 11p on the upstream side of the branching portion 11b, this should not be construed restrictively; it may also be provided on the downstream side of the connecting portion to the EGR passage 30p (downstream end 30b) (see the position indicated by arrow C of the drawing). By arranging the throttle 3 at the position of the above embodiment or at the position indicated by the arrow C, it is possible to send intake air forwards or suck intake air in from the rear side no matter which of the intake passage 11p and the heating intake passage 20p may be selected by the switch valve 11v. On the other hand, when the throttle is arranged at a position in the portion of the intake passage 11p between the branching portion 11b and the connecting portion (downstream end 30b) (see the position indicated by arrow B of FIG. 1), the adjustment of the amount of intake air becomes rather difficult when the heating intake passage 20p is selected, which means this arrangement is undesirable.

Further, while in the above embodiment the internal EGR is utilized so as to expand the operation range allowing homogeneous charge compression ignition mainly toward the low load side, and the supercharger 11t is utilized so as to expand the operation range toward the high load side, this arrangement is not indispensable. When no internal EGR or supercharger is used, the operation range allowing homogeneous charge compression ignition is reduced; it is possible, however, to apply the present invention in such cases and to utilize intake air heating and external EGR. Further, as the device for expanding the operation range allowing homogeneous charge compression ignition, there has been proposed increasing of the compression ratio in the combustion chamber instead of using internal EGR; the present invention is also applicable to such a homogeneous charge compression ignition engine.

Further, while the above embodiment has been described on the assumption that the present invention is to be applied to an internal combustion engine using gas fuel, there are no particular limitations in this regard; it is also applicable to other types of internal combustion engine such as a gasoline engine. For example, in the case of a gasoline engine, it is possible to use, instead of the mixer of the above embodiment, some other fuel supply device such as a carburetor or injector as appropriate as the fuel supply device.

Further, while in the above embodiment the circulation coolant of the engine is used as the heat exchange medium of the heat exchanger 40, this should not be construed restrictively. For example, when the gas engine of the above-mentioned embodiment is applied to the use of gas heat pump, it is also possible to use the hot water tubing for heating already existing in the apparatus. There are no particular limitations regarding the heat exchange medium to be used as long as it is at a temperature higher than the low outdoor air temperature and lower than the temperature of the EGR gas (exhaust gas).

Further, while in the above embodiment the supercharger 11t is used so as to expand the operation range allowing homogeneous charge compression ignition, this is not indispensable. When it is not so necessary to expand the operation range allowing homogeneous charge compression ignition toward the high load side, it is also possible to omit the control by the supercharger and the external EGR. The case in which it is not so necessary to expand the operation range toward the high load side corresponds, for example, to a case in which the operation range normally used is restricted to the low and medium load ranges as in the case of some stationary engines. Further, while it is not so effective as in the case in which a supercharger is used, it is also possible to expand the operation range allowing homogeneous charge compression ignition without using any supercharger as in the case of the range e of FIG. 6 if the external EGR is also used on the high load side under the control by the internal EGR, that is, in a state in which the internal EGR is reduced, with the amount of air fuel mixture sucked in increased. In FIG. 6, the ranges a, b, and e are ranges in which the amount of the internal EGR gas is controlled; the range a is a range in which intake air heating is effected at the time of low outdoor air temperature and low load to suppress occurrence of a misfire, and the range e is a range in which the external EGR is also used at the time of high load to suppress knocking and expand the operation range.

Claims

1. A homogeneous charge compression ignition engine, comprising:

a combustion chamber;

an intake passage serving as a passage for intake air to the combustion chamber;

an exhaust passage serving as a passage for exhaust gas from the combustion chamber;

an EGR passage, communicating with the exhaust passage and the intake passage, for refluxing a part of the exhaust gas from the combustion chamber to the combustion chamber as an EGR gas;

a heat exchanger, provided at some midpoint in the EGR passage, for cooling the EGR gas;

an EGR valve, provided at some midpoint in the EGR passage, for adjusting an opening and closing of the EGR passage;

a heating intake passage which branches off from a branching portion formed in a portion of the intake passage on an upstream side of a downstream end of the EGR passage and whose downstream end communicates with a portion of the EGR passage on an upstream side of the heat exchanger;

a switch valve for adjusting respective amounts of intake air passing through the portion of the intake passage on a downstream side of the branching portion and through the heating intake passage; and

a control means which, when the EGR valve is closed, switches the switch valve such that, on the downstream side of the branching portion, the intake air flowing into the combustion chamber passes through at least one of the intake passage and the heating intake passage, and which, when the EGR valve is open, switches the switch valve such that, on the downstream side of the branching portion, the intake air flowing into the combustion chamber passes solely through the intake passage.

2. A homogeneous charge compression ignition engine according to claim 1, wherein, in the EGR passage, the EGR valve is provided on the upstream side of the downstream end of the heating intake passage.

3. A homogeneous charge compression ignition engine according to claim 1, wherein the switch valve is provided at the branching portion.

4. A homogeneous charge compression ignition engine according to claim 1, wherein the switch valve and the EGR valve are provided at a communication portion between the heating intake passage and the EGR passage.

5. A homogeneous charge compression ignition engine according to claim 1, wherein the heat exchanger is one through which circulation coolant of an engine passes.

6. An air intake and exhaust system for use in a homogeneous charge compression ignition engine having a combustion chamber and an exhaust passage serving as a passage for exhaust gas from the combustion chamber, the air intake and exhaust system comprising:

an intake passage leading to the combustion chamber;

an EGR passage, communicating with the exhaust passage and the intake passage, for refluxing the exhaust gas from the combustion chamber to the combustion chamber as an EGR gas;

a heat exchanger, provided at some midpoint in the EGR passage, for cooling the EGR gas;

an EGR valve, provided at some midpoint in the EGR passage, for opening and closing the EGR passage;

a heating intake passage which branches off from a branching portion formed in a portion of the intake passage on an upstream side of a downstream end of the EGR passage and whose downstream end communicates with a portion of the EGR passage on an upstream side of the heat exchanger;

a switch valve for adjusting respective amounts of intake air passing through the intake passage and through the heating intake passage on a downstream side of the branching portion; and

a control means which, when the EGR valve is closed, switches the switch valve such that, on the downstream side of the branching portion, the intake air flowing into the combustion chamber passes through at least one of the intake passage and the heating intake passage, and which, when the EGR valve is open, switches the switch valve such that, on the downstream side of the branching portion, the intake air flowing into the combustion chamber passes solely through the intake passage.