Patent Description:
In a crosshead type engine described in Patent Literature <NUM>, a hydraulic mechanism is provided between a piston rod and a crosshead pin. In Patent Literature <NUM>, the hydraulic mechanism is operated to cause the piston rod to move up and down so that a compression ratio of the crosshead type engine may be varied. Patent Literature <NUM> discloses a variable compression ratio control device according to the preamble of claim <NUM>. Patent Literature <NUM>, <NUM>, <NUM> and <NUM> disclose further prior art.

In Patent Literature <NUM>, fuel efficiency is improved by changing the compression ratio, for example, when a supplied fuel is changed from diesel oil to gas. However, development of a technology capable of further improving the fuel efficiency of an engine is longed for.

The present disclosure has an object to provide a compression ratio control device capable of improving fuel efficiency of an engine, and to provide an engine.

The above-mentioned problem is solved by a compression ratio control device according to claim <NUM>. Advantageous embodiments are disclosed in the dependent claims. The problem is also solved by an engine according to claim <NUM>, which comprises the compression ratio control device.

According to the compression ratio control device and the engine of the present disclosure, it is possible to improve the fuel efficiency of the engine.

Now, with reference to the attached drawings, an embodiment of the present disclosure is described in detail. The dimensions, materials, and other specific numerical values represented in the embodiment are merely examples used for facilitating the understanding of the disclosure, and do not limit the present disclosure otherwise particularly noted. Elements having substantially the same functions and configurations herein and in the drawings are denoted by the same reference symbols to omit redundant description thereof. Further, illustration of elements with no direct relationship to the present disclosure is omitted.

<FIG> is an explanatory view for illustrating an overall configuration of an engine <NUM>. As illustrated in <FIG>, the engine <NUM> includes a cylinder <NUM>, a piston <NUM>, a piston rod <NUM>, a crosshead <NUM>, a connecting rod <NUM>, a crankshaft <NUM>, a flywheel <NUM>, a cylinder cover <NUM>, an exhaust valve cage <NUM>, a combustion chamber <NUM>, an exhaust valve <NUM>, an exhaust valve drive device <NUM>, an exhaust pipe <NUM>, a scavenge reservoir <NUM>, a cooler <NUM>, and a cylinder jacket <NUM>.

The piston <NUM> is provided in the cylinder <NUM>. The piston <NUM> is configured to reciprocate inside the cylinder <NUM>. One end of the piston rod <NUM> is mounted to the piston <NUM>. A crosshead pin <NUM> of the crosshead <NUM> is coupled to another end of the piston rod <NUM>. The crosshead <NUM> is configured to reciprocate together with the piston <NUM>. A movement of the crosshead <NUM> in a right-and-left direction (a direction perpendicular to a stroke direction of the piston <NUM>) of <FIG> is restricted by a guide shoe 116a.

The crosshead pin <NUM> is axially supported by a crosshead bearing 118a provided at one end of the connecting rod <NUM>. The crosshead pin <NUM> is configured to support one end of the connecting rod <NUM>. Another end of the piston rod <NUM> and the one end of the connecting rod <NUM> are connected to each other through intermediation of the crosshead <NUM>.

Another end of the connecting rod <NUM> is coupled to the crankshaft <NUM>. The crankshaft <NUM> is rotatable with respect to the connecting rod <NUM>. When the crosshead <NUM> reciprocates as the piston <NUM> reciprocates, the crankshaft <NUM> rotates. A rotation speed detection sensor <NUM> is provided in the engine <NUM>. The rotation speed detection sensor <NUM> is provided in a vicinity of the crankshaft <NUM>. The rotation speed detection sensor <NUM> is configured to detect an angle of the crankshaft <NUM>, to thereby detect the engine rotation speed.

The flywheel <NUM> is mounted to the crankshaft <NUM>. Rotations of the crankshaft <NUM> and the like are stabilized by an inertia of the flywheel <NUM>. The cylinder cover <NUM> is provided at a top end of the cylinder <NUM>. The exhaust valve cage <NUM> is inserted through the cylinder cover <NUM>.

One end of the exhaust valve cage <NUM> faces the piston <NUM>. An exhaust port 126a is opened at the one end of the exhaust valve cage <NUM>. The exhaust port 126a is opened to the combustion chamber <NUM>. The exhaust chamber <NUM> is formed inside the cylinder <NUM> so as to be surrounded by the cylinder cover <NUM>, the cylinder <NUM>, and the piston <NUM>.

A valve body of the exhaust valve <NUM> is located in the combustion chamber <NUM>. The exhaust valve drive device <NUM> is mounted to a rod portion of the exhaust valve <NUM>. The exhaust valve drive device <NUM> is arranged in the exhaust valve cage <NUM>. The exhaust valve drive device <NUM> moves the exhaust valve <NUM> in a stroke direction of the piston <NUM>.

When the exhaust valve <NUM> moves toward the piston <NUM> side, the exhaust port 126a is opened. When the exhaust port 126a is opened, an exhaust gas generated in the cylinder <NUM> after the combustion is discharged from the exhaust port 126a. After the exhaust gas is discharged, when the exhaust valve <NUM> moves toward the exhaust valve cage <NUM> side, the exhaust port 126a is closed.

The exhaust pipe <NUM> is mounted to the exhaust valve cage <NUM> and a turbocharger C. An inside of the exhaust pipe <NUM> communicates with the exhaust port 126a and a turbine of the turbocharger C. The exhaust gas discharged from the exhaust port 126a is supplied to the turbine of the turbocharger C through the exhaust pipe <NUM>, and is then discharged to the outside.

An active gas is pressurized by a compressor of the turbocharger C. In this state, the active gas is, for example, air. The pressurized active gas is cooled by the cooler <NUM> in the scavenge reservoir <NUM>. A bottom end of the cylinder <NUM> is surrounded by the cylinder jacket <NUM>. A scavenge chamber 140a is formed inside the cylinder jacket <NUM>. The active gas after the cooling is forcibly fed into the scavenge chamber 140a.

Scavenging ports 110a are formed on a bottom end side of the cylinder <NUM>. The scavenging port 110a is a hole passing from an inner peripheral surface to an outer peripheral surface of the cylinder <NUM>. A plurality of scavenging ports 110a are formed at intervals in a circumferential direction of the cylinder <NUM>.

When the piston <NUM> moves toward a bottom dead center position side with respect to the scavenging ports 110a, the active gas is sucked from the scavenging ports 110a into the cylinder <NUM> by a pressure difference between the scavenge chamber 140a and the inside of the cylinder <NUM>. A scavenging pressure detection sensor <NUM> is provided in the scavenge chamber 140a. The scavenging pressure detection sensor <NUM> is configured to detect a scavenging pressure, which is a pressure of the active gas supplied into the cylinder <NUM> (combustion chamber <NUM>).

A gas fuel injection valve (not shown) is provided in a vicinity of the scavenging ports 110a, or a portion of the cylinder <NUM> from the scavenging ports 110a to the cylinder cover <NUM>. The fuel gas is injected from the gas fuel injection valve, and then flows into the cylinder <NUM>.

A pilot injection valve (not shown) is provided in the cylinder cover <NUM>. An appropriate amount of fuel oil is injected from the pilot injection valve into the combustion chamber <NUM>. The fuel oil is vaporized, ignited, and combusted through heat of the combustion chamber <NUM>, thereby increasing the temperature in the combustion chamber <NUM>. Mixture of the fuel gas and the active gas compressed by the piston <NUM> is ignited by the heat of the combustion chamber <NUM>, and is combusted. The piston <NUM> is configured to reciprocate through an expansion pressure generated by the combustion of the fuel gas (mixture). An injection amount detection sensor <NUM> is provided in the cylinder cover <NUM>. The injection amount detection sensor <NUM> is configured to detect an injection amount of the fuel supplied from the gas fuel injection valve (not shown) into the combustion chamber <NUM>. Moreover, a pressure detection sensor <NUM> is provided in the cylinder cover <NUM>. The pressure detection sensor <NUM> is configured to detect a pressure in the cylinder <NUM> (combustion chamber <NUM>).

The rotation speed detection sensor <NUM>, the scavenging pressure detection sensor <NUM>, the fuel injection amount detection sensor <NUM>, and the pressure detection sensor <NUM> are connected to a compression ratio controller <NUM> described later, and are configured to output detection values (detection signals) to the compression ratio controller <NUM>. In <FIG>, flows of the signals are indicated by broken line arrows.

In this case, the fuel gas is produced by, for example, gasifying a liquefied natural gas (LNG). However, the fuel gas is not limited to those produced by gasifying the LNG, and there may also be used fuel gas produced by gasifying, for example, a liquefied petroleum gas (LPG), a light oil, or a heavy oil.

A compression ratio varying mechanism V is provided for the engine <NUM>. A compression ratio control device <NUM> configured to control a compression ratio of the combustion chamber <NUM> is provided for the engine <NUM>. The compression ratio control device <NUM> includes detectors such as the rotation speed detection sensor <NUM>, the scavenging pressure detection sensor <NUM>, the injection amount detection sensor <NUM>, and the pressure detection sensor <NUM>, and the compression ratio controller <NUM>. The compression ratio controller <NUM> is configured to control the compression ratio varying mechanism V based on the signals obtained from the detectors such as the rotation speed detection sensor <NUM>, the scavenging pressure detection sensor <NUM>, the injection amount detection sensor <NUM>, and the pressure detection sensor <NUM>. A detailed description is now given of the compression ratio varying mechanism V and the compression ratio control device <NUM>.

<FIG> and <FIG> are a schematic configuration view and a schematic configuration diagram for illustrating the compression ratio varying mechanism V and the compression ratio control device <NUM>, respectively. <FIG> is an extracted view for illustrating a coupling portion between the piston rod <NUM> and the crosshead pin <NUM>. <FIG> is a functional block diagram for illustrating the compression ratio control device <NUM>. As illustrated in <FIG>, a flat surface portion <NUM> is formed on an outer peripheral surface of the crosshead pin <NUM> on the piston <NUM> side. The flat surface portion <NUM> extends in a direction substantially perpendicular to the stroke direction of the piston <NUM>.

A pin hole <NUM> is formed in the crosshead pin <NUM>. The pin hole <NUM> is opened in the flat surface portion <NUM>. The pin hole <NUM> extends from the flat surface portion <NUM> toward the crankshaft <NUM> side (bottom side of <FIG>) along the stroke direction.

A cover member <NUM> is provided on the flat surface portion <NUM> of the crosshead pin <NUM>. The cover member <NUM> is mounted to the flat surface portion <NUM> of the crosshead pin <NUM> by a fastening member <NUM>. The cover member <NUM> covers the pin hole <NUM>. A cover hole 160a passing in the stroke direction is provided in the cover member <NUM>.

The piston rod <NUM> includes a large-diameter portion 114a and a small-diameter portion 114b. An outer diameter of the large-diameter portion 114a is larger than an outer diameter of the small-diameter portion 114b. The large-diameter portion 114a is formed at the another end of the piston rod <NUM>. The large-diameter portion 114a is inserted into the pin hole <NUM> of the crosshead pin <NUM>. The small-diameter portion 114b is formed on the one end side of the piston rod <NUM> with respect to the large-diameter portion 114a. The small-diameter portion 114b is inserted into the cover hole 160a of the cover member <NUM>.

A hydraulic chamber 154a is formed inside the pin hole <NUM>. The pin hole <NUM> is partitioned by the large-diameter portion 114a in the stroke direction. The hydraulic chamber 154a is a space defined on a bottom surface 154b side of the pin hole <NUM> partitioned by the large-diameter portion 114a.

The compression ratio varying mechanism V includes a hydraulic pressure adjustment mechanism O. The hydraulic pressure adjustment mechanism O includes a hydraulic pipe <NUM>, a hydraulic pump <NUM>, a check valve <NUM>, a branch pipe <NUM>, and a selector valve <NUM>.

One end of an oil passage <NUM> is opened in the bottom surface 154b. Another end of the oil passage <NUM> is opened to an outside of the crosshead pin <NUM>. The hydraulic pipe <NUM> is connected to the another end of the oil passage <NUM>. The hydraulic pump <NUM> communicates with the hydraulic pipe <NUM>. The hydraulic pump <NUM> supplies working oil supplied from an oil tank (not shown) to the hydraulic pipe <NUM> based on an instruction from the compression ratio controller <NUM>. The check valve <NUM> is provided between the hydraulic pump <NUM> and the oil passage <NUM>. A flow of working oil flowing from the oil passage <NUM> side toward the hydraulic pump <NUM> is suppressed by the check valve <NUM>. The working oil is forcibly fed into the hydraulic chamber 154a from the hydraulic pump <NUM> through the oil passage <NUM>.

The branch pipe <NUM> is connected to the hydraulic pipe <NUM> between the oil passage <NUM> and the check valve <NUM>. The selector valve <NUM> is provided to the branch pipe <NUM>. The selector valve <NUM> is, for example, an electromagnetic valve. The selector valve <NUM> is controlled to an open state or a closed state based on an instruction from the compression ratio controller <NUM>. The selector valve <NUM> is closed during operation of the hydraulic pump <NUM>. When the selector valve <NUM> is opened while the hydraulic pump <NUM> is stopped, the working oil is discharged from the hydraulic chamber 154a toward the branch pipe <NUM> side. The selector valve <NUM> communicates with the oil tank (not shown) on a side of the selector valve <NUM> opposite to the oil passage <NUM>. The discharged working oil is retained in the oil tank. The oil tank is configured to supply the working oil to the hydraulic pump <NUM>.

The large-diameter portion 114a is configured to slide on an inner peripheral surface of the pin hole <NUM> in the stroke direction in accordance with an oil amount of the working oil in the hydraulic chamber 154a. As a result, the piston rod <NUM> moves in the stroke direction. The piston <NUM> moves together with the piston rod <NUM>. A top dead center position of the piston <NUM> becomes variable through the movement of the piston rod <NUM> in the stroke direction.

The compression ratio varying mechanism V includes the hydraulic chamber 154a and the large-diameter portion 114a of the piston rod <NUM>. The compression ratio varying mechanism V moves the top dead center position of the piston <NUM> so that the compression ratio is variable. The compression ratio varying mechanism V can vary the top dead center position and the bottom dead center position of the piston <NUM> in the cylinder <NUM> of the engine <NUM> through adjustment of the oil amount of the working oil to be supplied to the hydraulic chamber 154a.

Description has been given of the case in which the one hydraulic chamber 154a is provided. However, a space 154c on the cover member <NUM> side of the pin hole <NUM> partitioned by the large-diameter portion 114a may also be a hydraulic chamber. This hydraulic chamber may be used together with the hydraulic chamber 154a or may be used individually.

In <FIG>, a configuration relating to control for the compression ratio varying mechanism V is mainly illustrated. As illustrated in <FIG>, the compression ratio control device <NUM> includes the compression ratio controller <NUM>. The compression ratio control device <NUM> is formed of, for example, an engine control unit (ECU). The compression ratio control device <NUM> is formed of a central processing unit (CPU), a ROM storing programs and the like, a RAM serving as a work area, and the like, and is configured to control the entire engine <NUM>.

The compression ratio controller <NUM> is configured to control the hydraulic pump <NUM> and the selector valve <NUM> to move the top dead center position of the piston <NUM>. In such a manner, the compression ratio controller <NUM> controls a geometrical compression ratio of the engine <NUM>.

<FIG> and <FIG> are respectively a schematic configuration view and a schematic configuration diagram for illustrating a compression ratio varying mechanism Va and a compression ratio control device 180a in a modification example. <FIG> is an extracted view for illustrating the coupling portion between the piston rod <NUM> and the crosshead pin <NUM> in the modification example. <FIG> is a functional block diagram for illustrating the compression ratio control device 180a in the modification example.

The compression ratio varying mechanism Va includes the hydraulic chamber 154a and the large-diameter portion 114a of the piston rod <NUM>. The compression ratio varying mechanism Va includes a hydraulic pressure adjustment mechanism Oa. The hydraulic pressure adjustment mechanism Oa includes the hydraulic pump <NUM>, a swiveling pipe <NUM>, a plunger pump <NUM>, a relief valve <NUM>, a plunger driver <NUM>, and a relief valve driver <NUM>.

The hydraulic pump <NUM> supplies the working oil supplied from the oil tank (not shown) to the swiveling pipe <NUM> based on an instruction from the compression ratio controller <NUM>. The swiveling pipe <NUM> is a pipe configured to connect the hydraulic pump <NUM> and the plunger pump <NUM> to each other. The swiveling pipe <NUM> is configured to be able to swivel between the plunger pump <NUM> moving together with the crosshead pin <NUM> and the hydraulic pump <NUM>.

The plunger pump <NUM> is mounted to the crosshead pin <NUM>. The plunger pump <NUM> includes a plunger 304a having a rod shape and a cylinder 304b having a tubular shape configured to slidably receive the plunger 304a.

The plunger pump <NUM> moves as the crosshead pin <NUM> moves so that the plunger 304a comes into contact with the plunger driver <NUM>. The plunger pump <NUM> is slid in the cylinder 304b through the contact of the plunger 304a with the plunger driver <NUM>, thereby increasing the pressure of the working oil in the cylinder 304b to supply the working oil increased in pressure to the hydraulic chamber 154a. A first check valve 304c is provided in an opening provided at an end of the cylinder 304b on a discharge side for the working oil. A second check valve 304d is provided in an opening formed in a side peripheral surface of the cylinder 304b on a suction side.

The plunger driver <NUM> is driven to a contact position, which is brought into contact with the plunger 304a and a non-contact position, which is not brought into contact with the plunger 304a based on instructions from the compression ratio controller <NUM>. The plunger driver <NUM> comes into contact with the plunger 304a, to thereby press the plunger 304a toward the cylinder 304b.

The first check valve 304c is closed when a valve body is biased toward an inside of the cylinder 304b. When the first check valve 304c is closed, after the working oil has been supplied to the hydraulic chamber 154a, flowing back of the working oil into the cylinder 304b is suppressed. When a pressure of the working oil in the cylinder 304b becomes equal to or more than a biasing force (opening pressure) of a biasing member of the first check valve 304c, the valve body of the first check valve 304c is pushed by the working oil, thereby being opened.

The second check valve 304d is closed when a valve body is biased toward an outside of the cylinder 304b. When the second check valve 304d is closed, after the working oil has been supplied to the cylinder 304b, the flowing back of the working oil into the hydraulic pump <NUM> is suppressed. Moreover, when the pressure of the working oil supplied from the hydraulic pump <NUM> becomes equal to or more than a biasing force (opening pressure) of a biasing member of the second check valve 304d, the valve body of the second check valve 304d is pushed by the working oil, thereby being opened. The opening pressure of the first check valve 304c is set to be higher than the opening pressure of the second check valve 304d.

The relief valve <NUM> is mounted to the crosshead pin <NUM>. The relief valve <NUM> is connected to the hydraulic chamber 154a and the oil tank (not shown). The relief valve <NUM> includes a rod 306a having a rod shape, a main body 306b having a tubular shape, and a valve body 306c. The main body 306b is configured to slidably receive the rod 306a. An internal flow passage is formed inside the main body 306b. The working oil discharged from the hydraulic chamber 154a flows through the internal flow passage. The valve body 306c is arranged in the internal flow passage of the main body 306b.

The relief valve <NUM> is configured to move as the crosshead pin <NUM> moves so that the rod 306a comes into contact with the relief valve driver <NUM>. The relief valve driver <NUM> is driven to a contact position, which is brought into contact with the rod 306a and a non-contact position, which is not brought into contact with the rod 306a based on instructions from the compression ratio controller <NUM>. The relief valve driver <NUM> comes into contact with the rod 306a, to thereby press the rod 306a toward the main body 306b. When the rod 306a is pressed toward the main body 306b, the rod 306a opens the valve body 306c. When the valve body 306c is opened, the working oil stored in the hydraulic chamber 154a is returned to the oil tank.

Each of the plunger driver <NUM> and the relief valve driver <NUM> includes a mechanism including a cam plate configured to perform operation control through, for example, a change in relative position to the plunger pump <NUM> or the relief valve <NUM>. Moreover, each of the plunger driver <NUM> and the relief valve driver <NUM> includes a mechanism configured to use an actuator to drive the relative position of the cam plate.

In <FIG>, a configuration relating to control for the compression ratio varying mechanism Va is mainly illustrated. As illustrated in <FIG>, the compression ratio control device 180a includes the compression ratio controller <NUM>. The compression ratio control device 180a is formed of, for example, an engine control unit (ECU). The compression ratio control device 180a is formed of a central processing unit (CPU), a ROM storing programs and the like, a RAM serving as a work area, and the like, and is configured to control the entire engine <NUM>.

The compression ratio controller <NUM> is configured to control the hydraulic pump <NUM>, the plunger driver <NUM>, and the relief valve driver <NUM> to move the top dead center position of the piston <NUM>. In such a manner, the compression ratio controller <NUM> controls a geometrical compression ratio of the engine <NUM>.

Incidentally, an upper limit value (hereinafter referred to as "cylinder-internal-pressure upper limit value") of the pressure in the cylinder <NUM> is defined for the engine <NUM> from the view point of durability of the cylinder <NUM>. <FIG> is a graph for showing an example of the pressure in the cylinder <NUM> measured by the pressure detection sensor <NUM>. In <FIG>, a vertical axis represents the pressure (cylinder internal pressure) in the cylinder <NUM>, and a horizontal axis represents a crank angle.

As shown in <FIG>, as the crank angle approaches the top dead center from the bottom dead center, the mixture (the air and the fuel) in the cylinder <NUM> is compressed by the piston <NUM>, and the temperature and the pressure in the cylinder <NUM> increase (compression stroke). When the crank angle reaches a point A before the crank angle reaches the top dead center from the bottom dead center, the mixture in the cylinder <NUM> is combusted, and the combustion gas is expanded by heat generated by the combustion (the combustion stroke and the expansion stroke). A force for pushing down the piston <NUM> is generated through an increase in pressure by the expansion of the combustion gas.

In this embodiment, of the pressures in the cylinder <NUM> measured by the pressure detection sensor <NUM>, a pressure in the compression stroke in which the crank angle is before the point A is referred to as "compression pressure Pcomp". Moreover, of the pressures in the cylinder <NUM> measured by the pressure detection sensor <NUM>, a pressure in the combustion stroke and the expansion stroke in which the crank angle is after the point A is referred to as "combustion pressure P". Moreover, the maximum pressure of the combustion pressure P is referred to as "maximum combustion pressure Pmax". The maximum combustion pressure Pmax is the maximum pressure in the cylinder <NUM> measured by the pressure detection sensor <NUM> in one combustion cycle. A broken line of <FIG> indicates a compression pressure after the point A estimated from the pressure measured in the compression stroke. A point B of <FIG> indicates a peak position (peak value) of the estimated compression pressure. Moreover, a point C of <FIG> indicates a peak position (peak value) of the combustion pressure P, that is, a position of the maximum combustion pressure Pmax.

As described above, the cylinder-internal-pressure upper limit value (combustion pressure upper limit value) is defined for the engine <NUM>. Therefore, the engine <NUM> needs to suppress the maximum combustion pressure Pmax so as to be equal to or less than the cylinder-internal-pressure upper limit value. The maximum combustion pressure Pmax changes in accordance with a scavenging pressure Ps, which is a pressure of the active gas supplied to the combustion chamber <NUM>. Specifically, as the scavenging pressure Ps becomes larger, the maximum combustion pressure Pmax becomes larger. As the scavenging pressure Ps becomes smaller, the maximum combustion pressure Pmax becomes smaller.

The scavenging pressure Ps changes in accordance with engine load. Specifically, as the engine load (for example, the engine rotation speed) becomes larger, the scavenging pressure Ps becomes larger. As the engine load becomes smaller, the scavenging pressure Ps becomes smaller. Consequently, the maximum combustion pressure Pmax reaches the highest value at an engine full load (<NUM>% load) at which the scavenging pressure Ps becomes larger to the highest value, that is, the engine load becomes larger to the highest value. Therefore, the compression ratio of the engine <NUM> is usually set so that the maximum combustion pressure Pmax at the engine full load is the cylinder-internal-pressure upper limit value when the compression ratio of the combustion chamber <NUM> is fixed.

<FIG> and <FIG> are graphs showing a relationship between the engine load and the maximum combustion pressure Pmax. In each of <FIG> and <FIG>, a vertical axis represents the maximum combustion pressure Pmax, and a horizontal axis represents the engine load. <FIG> is a graph for showing a relationship between the engine load and the maximum combustion pressure Pmax when the compression ratio of the combustion chamber <NUM> is fixed, not according to the invention. <FIG> is a graph for showing the relationship between the engine load and the maximum combustion pressure Pmax when the compression ratio of the combustion chamber <NUM> is fixed and when the compression ratio is variable. In <FIG> and <FIG>, a one-dot chain line indicates the cylinder-internal-pressure upper limit value Pmax Limit.

A solid line of <FIG> indicates the maximum combustion pressure Pmax changing in accordance with the engine load when the compression ratio of the combustion chamber <NUM> is fixed. As shown in <FIG>, when the compression ratio of the combustion chamber <NUM> is fixed, the maximum combustion pressure Pmax is the cylinder-internal-pressure upper limit value Pmax Limit in the engine full load state. As the maximum combustion pressure Pmax becomes larger, a fuel consumption rate can be reduced (that is, the fuel efficiency can be improved). Therefore, the fuel efficiency is improved in the engine full load state in which the maximum combustion pressure Pmax is the cylinder-internal-pressure upper limit value Pmax Limit.

However, as shown in <FIG>, when the compression ratio of the combustion chamber <NUM> is fixed, the maximum combustion pressure Pmax does not reach the cylinder-internal-pressure upper limit value Pmax Limit in a load state in which the engine load is lower than the engine load in the engine full load state. Consequently, in the example shown in <FIG>, there is a room for improving the fuel efficiency in a load state in which the engine load is lower than the engine load in the engine full load state.

Consequently, in the embodiment of <FIG>, at least in a state in which the engine load is equal to or less than a predetermined load, the compression ratio controller <NUM> controls the compression ratio of the combustion chamber <NUM> (compression ratio varying mechanism V) so that the maximum combustion pressure Pmax approaches the cylinder-internal-pressure upper limit value Pmax Limit set in advance. In this embodiment, the compression ratio controller <NUM> can acquire the detection value (the cylinder internal pressure including the maximum combustion pressure Pmax) output from the pressure detection sensor <NUM>. Consequently, the compression ratio controller <NUM> compares the maximum combustion pressure Pmax detected by the pressure detection sensor <NUM> and the cylinder-internal-pressure upper limit value Pmax Limit with each other, and then controls the compression ratio so that the maximum combustion pressure Pmax approaches the cylinder-internal-pressure upper limit value Pmax Limit.

The compression ratio controller <NUM> controls the compression ratio varying mechanism V so that the compression ratio of the combustion chamber <NUM> becomes variable between a compression ratio ε0 and a compression ratio εn. The compression ratio ε0 is a compression ratio at which the compression ratio of the combustion chamber <NUM> is the lowest. The compression ratio εn is a compression ratio at which the compression ratio of the combustion chamber <NUM> is the highest.

A solid line of <FIG> indicates the maximum combustion pressure Pmax, which changes in accordance with the engine load when the compression ratio of the combustion chamber <NUM> is variable in this embodiment. In this embodiment, the compression ratio controller <NUM> controls the compression ratio varying mechanism V so that the compression ratio of the combustion chamber <NUM> is a lowest compression ratio ε0 in the engine full load state. As shown in <FIG>, when the compression ratio of the combustion chamber <NUM> is the lowest compression ratio ε0 in the engine full load state, the maximum combustion pressure Pmax is the cylinder-internal-pressure upper limit value Pmax Limit. In this configuration, a broken line of <FIG> indicates the maximum combustion pressure Pmax, which changes in accordance with the engine load when the compression ratio of the combustion chamber <NUM> is fixed to the lowest compression ratio ε0.

The compression ratio controller <NUM> controls the compression ratio varying mechanism V so that the compression ratio of the combustion chamber <NUM> is a compression ratio larger than the lowest compression ratio ε0 in a load state in which a load is smaller than the load in the engine full load state. As described above, the maximum combustion pressure Pmax changes in accordance with the scavenging pressure Ps, but also changes in accordance with the compression ratio of the combustion chamber <NUM>. Specifically, as the compression ratio becomes larger, the maximum combustion pressure Pmax becomes larger. As the compression ratio becomes smaller, the maximum combustion pressure Pmax becomes smaller.

Consequently, even when the scavenging pressure Ps decreases, and the maximum combustion pressure Pmax thus becomes smaller, the maximum combustion pressure Pmax can be made larger through changing the compression ratio of the combustion chamber <NUM> to a compression ratio larger than the lowest compression ratio ε0. As a result, the maximum combustion pressure Pmax can be caused to approach the cylinder-internal-pressure upper limit value Pmax Limit also in the load state in which the load is smaller than the load in the engine full load state.

As described above, the compression ratio controller <NUM> varies the compression ratio of the combustion chamber <NUM> so that the maximum combustion pressure Pmax is maintained to the cylinder-internal-pressure upper limit value Pmax Limit even when the engine load becomes smaller. An engine load region R1 shown in <FIG> is a range in which the maximum combustion pressure Pmax can be maintained to the cylinder-internal-pressure upper limit value Pmax Limit through changing the compression ratio of the combustion chamber <NUM> in the range from the lowest compression ratio ε0 to the highest compression ratio εn.

In the engine load region R1, the compression ratio controller <NUM> can obtain a larger compression ratio when the compression ratio of the combustion chamber <NUM> is variable (the solid line of <FIG>) than the compression ratio when the compression ratio of the combustion chamber <NUM> is fixed (the broken line of <FIG>). As described above, as the compression ratio becomes larger, the maximum combustion pressure Pmax becomes larger.

Consequently, in the engine load region R1, the maximum combustion pressure Pmax when the compression ratio of the combustion chamber <NUM> is set to a compression ratio larger than the lowest compression ratio ε0 (the solid line of <FIG>) can be made larger than the maximum combustion pressure Pmax when the compression ratio is set to the lowest compression ratio ε0 (the broken line of <FIG>). As described above, the compression ratio controller <NUM> increases the compression ratio of the combustion chamber <NUM> as much as possible in the range in which the maximum combustion pressure Pmax does not exceed the cylinder-internal-pressure upper limit value Pmax Limit in the engine load region R1, thereby being able to improve the fuel efficiency.

An engine load region R2 shown in <FIG> is a range in which the maximum combustion pressure Pmax is less than the cylinder-internal-pressure upper limit value Pmax Limit even when the compression ratio of the combustion chamber <NUM> is set to the highest compression ratio εn. In this graph, the engine load region R1 is an engine load region including the engine full load. Moreover, the engine load region R2 is a load region in which the load is smaller than the load in the engine load region R1.

In the engine load region R2, the maximum combustion pressure Pmax is less than the cylinder-internal-pressure upper limit value Pmax Limit whether the compression ratio of the combustion chamber <NUM> is fixed (broken line) or variable (solid line). However, when the compression ratio of the combustion chamber <NUM> is variable (solid line) in the engine load region R2, the compression ratio controller <NUM> can achieve the larger compression ratio εn than the compression ratio when the compression ratio of the combustion chamber <NUM> is fixed (broken line).

Consequently, in the engine load region R2, the maximum combustion pressure Pmax when the compression ratio of the combustion chamber <NUM> is variable (solid line) can be made larger than the maximum combustion pressure Pmax when the compression ratio is fixed (broken line). In such a manner, the compression ratio controller <NUM> increases the compression ratio of the combustion chamber <NUM> as much as possible, to thereby improve the fuel economy also in the engine load region R2.

With this configuration, the compression ratio controller <NUM> controls the compression ratio so that the compression ratio is the highest compression ratio in the range in which the maximum combustion pressure Pmax is less than the cylinder-internal-pressure upper limit value Pmax Limit. Specifically, the compression ratio controller <NUM> controls the compression ratio so as to be maintained to the highest compression ratio εn in the case in which the maximum combustion pressure Pmax is less than the cylinder-internal-pressure upper limit value Pmax Limit when the compression ratio is the highest compression ratio εn.

<FIG>, <FIG>, <FIG>, <FIG>, and <FIG> are graphs for showing performance of the engine <NUM> according to this embodiment. <FIG> is a graph for showing a relationship between a fuel consumption rate (fuel efficiency) and the engine load in the engine load region R1 shown in <FIG>. In <FIG>, a vertical axis represents the fuel consumption rate, and a horizontal axis represents the engine load. In <FIG>, engine loads becomes smaller in the order of Ea, Eb, Ec, Ed, and Ee. That is, a relationship among the engine loads Ea, Eb, Ec, Ed, and Ee is represented as Ea>Eb>Ec>Ed>Ed. The engine load Ea indicates an engine full load (<NUM>% load). The engine loads Ea, Eb, Ec, Ed, and Ee of <FIG> are also defined as the engine loads of <FIG>. Moreover, in <FIG>, a broken line indicates the lowest fuel consumption rate at which the fuel consumption rate is the lowest.

<FIG> is a graph for showing a relationship between the maximum combustion pressure Pmax and the engine load in the engine load region R1 shown in <FIG>. In <FIG>, a vertical axis represents the maximum combustion pressure Pmax, and a horizontal axis represents the engine load. Moreover, in <FIG>, a one-dot chain line indicates the cylinder-internal-pressure upper limit value Pmax Limit. The cylinder-internal-pressure upper limit value is a constant value independent of the engine load.

<FIG> is a graph for showing a relationship between the compression pressure Pcomp and the engine load in the engine load region R1 shown in <FIG>. In <FIG>, a vertical axis represents the compression pressure Pcomp, and a horizontal axis represents the engine load. In this graph, the compression pressure Pcomp is the estimated peak value of the compression pressure such as the point B of <FIG>. Moreover, in <FIG>, a one-dot chain line indicates a target value (hereinafter referred to as "target compression pressure") of the estimated peak value of the compression pressure. The maximum combustion pressure Pmax can be caused to approach the cylinder-internal-pressure upper limit value Pmax Limit by causing the peak value of the compression pressure Pcomp to approach the target compression pressure. When the peak value of the compression pressure Pcomp is the target compression pressure, the maximum combustion pressure Pmax is the cylinder-internal-pressure upper limit value Pmax Limit.

As shown in <FIG>, the target compression pressure changes in accordance with the engine load, and is thus not a constant value. Specifically, the target compression pressure is a value that becomes smaller as the engine load becomes smaller, and becomes larger as the engine load becomes larger. This is because a difference Δ between the peak value of the compression pressure Pcomp indicated by the point B of <FIG> and the peak value (maximum combustion pressure Pmax) of the combustion pressure P indicated by the point C of <FIG> becomes larger as the engine load becomes larger. Even when the difference Δ becomes larger as the engine load becomes larger, the maximum combustion pressure Pmax can be a constant value independent of the engine load through increasing the target compression pressure as the engine load becomes larger.

<FIG> is a graph for showing a relationship between the scavenging pressure Ps and the engine load in the engine load region R1 shown in <FIG>. In <FIG>, a vertical axis represents the scavenging pressure Ps, and the horizontal axis represents the engine load. As shown in <FIG>, the scavenging pressure Ps becomes larger as the engine load becomes larger, and becomes smaller as the engine load becomes smaller.

<FIG> is a graph for showing a relationship between an effective compression ratio εef and the engine load in the engine load region R1 shown in <FIG>. In <FIG>, a vertical axis represents the effective compression ratio εef, and the horizontal axis represents the engine load. As shown in <FIG>, the effective compression ratio εef becomes smaller as the engine load becomes larger, and becomes larger as the engine load becomes smaller. The effective compression ratio εef is an actual compression ratio of the combustion chamber <NUM>, and is indicated by a ratio between a volume in the cylinder <NUM> at a moment when the scavenging ports 110a are closed and a volume of the combustion chamber <NUM> when the piston <NUM> reaches the top dead center.

As shown in <FIG>, when the engine load becomes smaller from the engine full load state in the order of the engine loads of Ea, Eb, Ec, Ed, and Ed, the compression ratio controller <NUM> changes the compression ratio of the combustion chamber <NUM> in the order of compression ratios of ε0, ε1, ε2, εn-<NUM>, and εn. The compression ratio is a value which becomes larger in the order of ε0, ε1, ε2, εn-<NUM>, and εn. That is, a relationship among the compression ratios ε0, ε1, ε2, εn-<NUM>, and εn is represented as ε0<ε1<ε2<εn-<NUM><εn.

Specifically, the compression ratio controller <NUM> sets the compression ratio of the combustion chamber <NUM> to the compression ratio ε0 at the engine load Ea (engine full load). The maximum combustion pressure Pmax can be brought to the cylinder-internal-pressure upper limit value Pmax Limit by setting the compression ratio to the compression ratio ε0 at the engine load Ea. Moreover, the compression ratio controller <NUM> sets the compression ratio of the combustion chamber <NUM> to the compression ratio ε1 at the engine load Eb. The maximum combustion pressure Pmax can be brought to the cylinder-internal-pressure upper limit value Pmax Limit by setting the compression ratio to the compression ratio ε1 at the engine load Eb.

Moreover, the compression ratio controller <NUM> sets the compression ratio of the combustion chamber <NUM> to the compression ratio ε2 at the engine load Ec. The maximum combustion pressure Pmax can be brought to the cylinder-internal-pressure upper limit value Pmax Limit by setting the compression ratio to the compression ratio ε2 at the engine load Ec. Moreover, the compression ratio controller <NUM> sets the compression ratio of the combustion chamber <NUM> to the compression ratio εn-<NUM> at the engine load Ed. The maximum combustion pressure Pmax can be brought to the cylinder-internal-pressure upper limit value Pmax Limit by setting the compression ratio to the compression ratio εn-<NUM> at the engine load Ed. Moreover, the compression ratio controller <NUM> sets the compression ratio of the combustion chamber <NUM> to the compression ratio εn at the engine load Ee. The maximum combustion pressure Pmax can be brought to the cylinder-internal-pressure upper limit value Pmax Limit by setting the compression ratio to the compression ratio εn at the engine load Ee.

In this embodiment, at least when the engine load is equal to or less than the predetermined load (engine full load), the compression ratio controller <NUM> controls the compression ratio of the combustion chamber <NUM> so that the maximum combustion pressure Pmax approaches the cylinder-internal-pressure upper limit value Pmax Limit set in advance. The compression ratio controller <NUM> increases the compression ratio as the engine load becomes smaller from the engine full load state. As a result, even when the scavenging pressure Ps becomes smaller as shown in <FIG>, the maximum combustion pressure Pmax can be caused to approach the cylinder-internal-pressure upper limit value Pmax Limit as shown in <FIG>. As a result, as shown in <FIG>, the fuel consumption rate can be minimized (that is, the fuel efficiency can be improved) at each of the engine loads Ea to Ee.

<FIG> is a flowchart for illustrating control processing for the compression ratio by the compression ratio controller <NUM>.

First, the compression ratio controller <NUM> derives the current cylinder internal pressure based on the signal output from the pressure detection sensor <NUM> (Step S102). Then, the compression ratio controller <NUM> determines whether or not the maximum combustion pressure Pmax is smaller than the cylinder-internal-pressure upper limit value Pmax Limit (Step S104). When the maximum combustion pressure Pmax is smaller than the cylinder-internal-pressure upper limit value Pmax Limit (YES in Step S104), the compression ratio controller <NUM> proceeds to Step S106. Meanwhile, when the maximum combustion pressure Pmax is equal to or more than the cylinder-internal-pressure upper limit value Pmax Limit (NO in Step S104), the compression ratio controller <NUM> proceeds to Step S110.

When the determination of YES is made in Step S104, the compression ratio controller <NUM> controls the compression ratio varying mechanism V so as to increase the compression ratio of the combustion chamber <NUM> (Step S106). After the compression ratio controller <NUM> increases the compression ratio of the combustion chamber <NUM>, the compression ratio controller <NUM> determines whether or not the compression ratio of the combustion chamber <NUM> is the maximum compression ratio εn (Step S108). When the compression ratio of the combustion chamber <NUM> is the maximum compression ratio εn (YES in Step S108), the compression ratio controller <NUM> proceeds to Step S116. When the compression ratio of the combustion chamber <NUM> is not the maximum compression ratio εn (NO in Step S108), the compression ratio controller <NUM> returns to Step S102, and again executes the processing in Step S102 to Step S104.

When a determination of NO is made in Step S104, the compression ratio controller <NUM> determines whether or not the maximum combustion pressure Pmax is larger than the cylinder-internal-pressure upper limit value Pmax Limit (Step S110). When the maximum combustion pressure Pmax is larger than the cylinder-internal-pressure upper limit value Pmax Limit (YES in Step S110), the compression ratio controller <NUM> proceeds to Step S112. Meanwhile, when the maximum combustion pressure Pmax is equal to or less than the cylinder-internal-pressure upper limit value Pmax Limit, that is, when the maximum combustion pressure Pmax is the cylinder-internal-pressure upper limit value Pmax Limit (NO in Step S110), the compression ratio controller <NUM> proceeds to Step S116.

When the determination of YES is made in Step S110, the compression ratio controller <NUM> controls the compression ratio varying mechanism V so as to decrease the compression ratio of the combustion chamber <NUM> (Step S112). After the compression ratio controller <NUM> decreases the compression ratio of the combustion chamber <NUM>, the compression ratio controller <NUM> determines whether or not the compression ratio of the combustion chamber <NUM> is the minimum compression ratio ε0 (Step S114). When the compression ratio of the combustion chamber <NUM> is the minimum compression ratio ε0 (YES in Step S114), the compression ratio controller <NUM> proceeds to Step S116. When the compression ratio of the combustion chamber <NUM> is not the minimum compression ratio ε0 (NO in Step S114), the compression ratio controller <NUM> returns to Step S102, and again executes the processing in Step S102, Step S104, and Step S110.

When the determination of YES is made in Step S108 or Step S114, and the determination of NO is made in Step S110, the compression ratio controller <NUM> controls the compression ratio varying mechanism V so that the compression ratio in the combustion chamber <NUM> is maintained (Step S116), and finishes the control processing for the compression ratio.

In the above-mentioned embodiment, description is given of the example in which the compression ratio controller <NUM> changes the compression ratio in accordance with the maximum combustion pressure Pmax measured by the pressure detection sensor <NUM>. However, the maximum combustion pressure Pmax is not required to be measured by the pressure detection sensor <NUM>. For example, the compression ratio controller <NUM> may estimate the maximum combustion pressure Pmax based on the scavenging pressure Ps measured by the scavenging pressure detection sensor <NUM> in place of the pressure detection sensor <NUM>.

Specifically, the compression ratio controller <NUM> may estimate the maximum combustion pressure Pmax based on the scavenging pressure Ps, the compression ratio, and a specific heat ratio. The compression ratio controller <NUM> may compare the estimated maximum combustion pressure Pmax and the cylinder-internal-pressure upper limit value Pmax Limit with each other, and may then control the compression ratio so that the maximum combustion pressure Pmax approaches the cylinder-internal-pressure upper limit value Pmax Limit.

Moreover, the compression ratio controller <NUM> may vary the compression ratio in accordance with the compression pressure Pcomp. For example, the compression ratio controller <NUM> estimates the peak value of the compression pressure Pcomp from the cylinder internal pressure measured by the pressure detection sensor <NUM>. In this case, the compression ratio controller <NUM> includes a ROM storing, in advance, a map indicating a target compression pressure corresponding to the engine load or the engine rotation speed. The compression ratio controller <NUM> refers to the map stored in the ROM, thereby being capable of varying the compression ratio to a compression ratio at which the estimated peak value of the compression pressure is the target compression pressure. As described above, the compression ratio controller <NUM> varies the compression ratio to the compression ratio at which the peak value of the compression pressure Pcomp is the target compression pressure so that the maximum combustion pressure Pmax can be caused to approach the cylinder-internal-pressure upper limit value Pmax Limit at each engine load.

Moreover, the compression ratio controller <NUM> may estimate the maximum combustion pressure Pmax from the estimated peak value of the compression pressure and the difference Δ between the above-mentioned point B and point C of <FIG>. In this case, the compression ratio controller <NUM> includes a ROM storing, in advance, a map indicating a difference Δ corresponding to the engine load or the engine rotation speed. The compression ratio controller <NUM> refers to the map stored in the ROM, thereby being capable of estimating the maximum combustion pressure Pmax from the estimated peak value of the compression pressure and the difference Δ. The compression ratio controller <NUM> may compare the estimated maximum combustion pressure Pmax and the cylinder-internal-pressure upper limit value Pmax Limit with each other, and may then control the compression ratio so that the maximum combustion pressure Pmax approaches the cylinder-internal-pressure upper limit value Pmax Limit.

As described above, the engine <NUM> includes the detectors (for example, the rotation speed detection sensor <NUM> and the pressure detection sensor <NUM>) configured to detect the signals correlating with at least one of the engine load or the maximum combustion pressure in the combustion chamber <NUM>. The compression ratio controller <NUM> can control the compression ratio so that the maximum combustion pressure Pmax approaches the cylinder-internal-pressure upper limit value Pmax Limit set in advance based on the signals acquired from the detectors.

Moreover, depending on the type of a driven member (for example, a propeller for a ship) driven by the engine <NUM>, the engine load may vary even when the engine rotation speed is the same. For example, a fixed-pitch propeller and a variable-pitch propeller are given as the driven member driven by the engine <NUM>. While the fixed-pitch propeller has a fixed angle of blades, the variable-pitch propeller can change the angle of the blades. Therefore, even when the variable-pitch propeller has the same rotation speed as the rotation speed of the fixed-pitch propeller, the variable-pitch propeller may apply a different engine load in accordance with the angle of the blades.

When the engine <NUM> drives the fixed-pitch propeller to rotate, the compression ratio controller <NUM> can control the compression ratio so that the maximum combustion pressure Pmax approaches the cylinder-internal-pressure upper limit value Pmax Limit through use of the above-mentioned method. However, when the engine <NUM> drives the variable-pitch propeller to rotate, in some cases, the compression ratio controller <NUM> is not be able to control the compression ratio so that the maximum combustion pressure Pmax approaches the cylinder-internal-pressure upper limit value Pmax Limit through use of the above-mentioned method.

Therefore, in a case in which the compression ratio controller <NUM> drives the variable-pitch propeller to rotate, when the compression ratio controller <NUM> cannot use the above-mentioned method to control the compression ratio, the compression ratio controller <NUM> may derive, for example, the maximum combustion pressure Pmax based on the angle of the blades of the variable-pitch propeller and the engine rotation speed. Then, the compression ratio controller <NUM> may compare the derived maximum combustion pressure Pmax and the cylinder-internal-pressure upper limit value Pmax Limit with each other, and may then control the compression ratio so that the maximum combustion pressure Pmax approaches the cylinder-internal-pressure upper limit value Pmax Limit.

Specifically, the compression ratio controller <NUM> can acquire information on the angle of the blades of the variable-pitch propeller VP from an angle detection sensor <NUM> (detector, see <FIG> and <FIG>) configured to be able to detect the angle of the blades of the variable-pitch propeller VP. In this case, the compression ratio controller <NUM> includes a ROM storing, in advance, a map indicating the maximum combustion pressure Pmax corresponding to the angle of the blades of the variable-pitch propeller VP and the engine rotation speed. The compression ratio controller <NUM> refers to the map stored in the ROM, thereby being capable of deriving the maximum combustion pressure Pmax from the current angle of the blades of the variable-pitch propeller VP and the engine rotation speed.

The map stored in the ROM may be a map indicating a compression ratio corresponding to the angle of the blades of the variable-pitch propeller VP and the engine rotation speed. In this case, the compression ratio controller <NUM> refers to the map stored in the ROM, thereby being capable of deriving the compression ratio from the current angle of the blades of the variable-pitch propeller VP and the engine rotation speed. Moreover, the compression ratio controller <NUM> can derive the engine load based on the angle of the blades of the variable-pitch propeller VP, the engine rotation speed, and the fuel injection amount. Consequently, the map stored in the ROM may be the above-mentioned map (for example, the map indicating the compression ratio corresponding to the engine load).

The embodiment has been described above with reference to the attached drawings, but, needless to say, the present disclosure is not limited to the above-mentioned embodiment. It is apparent that those skilled in the art may arrive at various alternations and modifications within the scope of claims, and those examples are construed as naturally falling within the technical scope of the present disclosure.

For example, in the above-mentioned embodiment, description is given of the two-cycle type, uniflow scavenging type, and crosshead type engine <NUM> as examples. However, the type of the engine is not limited to the two-cycle type, the uniflow scavenging type, and the crosshead type. It is only required that the present disclosure be applied to an engine. Moreover, in the above-mentioned embodiment, description is given of the example in which the gas fuel (fuel gas) is supplied to the inside of the cylinder <NUM> (combustion chamber <NUM>). However, the configuration is not limited to this example, and a liquid fuel may be supplied to the inside of the cylinder <NUM> (combustion chamber <NUM>). Moreover, the engine <NUM> may be, for example, of a dual fuel type, which chooses a gas fuel or a liquid fuel to be used. Moreover, the engine <NUM> is not limited to an engine for a boat, and may be an engine for, for example, an automobile.

The present disclosure can be applied to the compression ratio control device and the engine.

Claim 1:
A compression ratio control device (<NUM>, 180a), comprising:
a detector (<NUM>, <NUM>, <NUM>, <NUM>, <NUM>) configured to detect a signal correlating with the maximum combustion pressure (Pmax) in a combustion chamber (<NUM>) in a cylinder (<NUM>); and
a controller (<NUM>) configured to control a compression ratio of the combustion chamber (<NUM>),
wherein the controller (<NUM>) controls the compression ratio to be closer to a lowest compression ratio (ε0) at an engine full load, characterized in that
the controller (<NUM>) changes the compression ratio, in a first engine load region (R1) including the engine full load, in a range between the lowest compression ratio (ε0) and a highest compression ratio (εn) so that the maximum combustion pressure (Pmax) is maintained to a cylinder internal-pressure upper limit value (Pmax Limit) defined from the view point of durability of the cylinder (<NUM>), and
the controller (<NUM>) maintains the compression ratio to be closer to the highest compression ratio (εn), in a second engine load region (R2) in which the engine load is smaller than that in the first engine load region (R1) and the maximum combustion pressure (Pmax) is less than the cylinder-internal-pressure upper limit value (Pmax Limit).