Variable compression ratio type engine with fuel containing alcohol

An internal combustion engine which is provided with a variable compression ratio mechanism able to change a mechanical compression ratio and a variable valve timing mechanism able to control a closing timing of an intake valve. The expansion ratio is made higher at the time of engine low load operation compared with at the time of engine high load operation. A fuel containing alcohol is used as the fuel, and the expansion ratio at the time of engine low load operation is made to fall when an alcohol concentration in the fuel is high compared with when the alcohol concentration in the fuel is low.

TECHNICAL FIELD

The present invention relates to a spark ignition type internal combustion engine.

BACKGROUND ART

When using a fuel which contains alcohol as a fuel, the higher the alcohol concentration in the fuel, the higher the octane value and the harder it becomes for knocking to occur. Therefore, the higher the alcohol concentration in the fuel, the higher the compression ratio can be made. Therefore, there is known an internal combustion engine which is provided with a variable compression ratio mechanism which can change a mechanical compression ratio and a variable valve timing mechanism which can control a closing timing of the intake valve, which uses a fuel which contains alcohol as a fuel, and which raises the actual compression ratio the higher the alcohol concentration in the fuel (see Patent Literature 1).

CITATIONS LIST

Patent Literature

SUMMARY OF INVENTION

Technical Problem

In this regard, if making a fuel like alcohol which contains oxygen burn, a large amount of water with a large specific heat is produced compared with when making usual gasoline burn and as a result the combustion temperature falls. If the combustion temperature falls, the combustion pressure falls and the expansion end pressure falls. Therefore, when using usual gasoline, the expansion end pressure becomes the atmospheric pressure or more, but when using a fuel which contains alcohol, even if raising the actual compression ratio, sometimes the expansion end pressure will end up falling to below the atmospheric pressure, that is, over expansion will end up occurring. However, if such over expansion occurs, the heat efficiency will greatly fall.

An object of the present invention is to provide a spark ignition type internal combustion engine which can prevent over expansion when using a fuel containing alcohol and thereby can secure a high heat efficiency.

Solution to Problem

According to the present invention, there is provided a spark ignition type internal combustion engine comprising a variable compression ratio mechanism able to change a mechanical compression ratio and a variable valve timing mechanism able to control a closing timing of an intake valve, and an expansion ratio is made higher at the time of engine low load operation compared with at the time of engine high load operation, wherein a fuel containing alcohol is used as a fuel, and the expansion ratio at the time of engine low load operation is made to fall when an alcohol concentration in the fuel is high compared with when the alcohol concentration in the fuel is low.

Advantageous Effects of Invention

At the time of engine low load operation, the expansion ratio is made higher compared with the time of engine high load operation, so when a fuel containing alcohol is used as fuel, there is a possibility of over expansion. In this case, the higher the alcohol concentration in the fuel, the more readily over expansion occurs. However, in the present invention, when the alcohol concentration in the fuel is high, the expansion ratio at the time of engine low load operation is made to fall compared with when the alcohol concentration in the fuel is low, so even when the alcohol concentration in the fuel is high, over expansion can be prevented from occurring.

DESCRIPTION OF EMBODIMENTS

FIG. 1shows a side cross-sectional view of a spark ignition type internal combustion engine.

Referring toFIG. 1,1indicates a crank case,2a cylinder block,3a cylinder head,4a piston,5a combustion chamber,6a spark plug arranged at the top center of the combustion chamber5,7an intake valve,8an intake port,9an exhaust valve, and10an exhaust port. Each intake port8is connected through an intake branch pipe11to a surge tank12, while each intake branch pipe11is provided with a fuel injector13for injecting fuel toward a corresponding intake port8. Note that each fuel injector13may be arranged at each combustion chamber5instead of being attached to each intake branch pipe11.

The surge tank12is connected through an intake duct14to an air cleaner15, while the intake duct14is provided inside it with a throttle valve17which is driven by an actuator16and an intake air amount detector18which uses for example a hot wire. On the other hand, the exhaust port10is connected through an exhaust manifold19to a catalytic converter20which houses for example a three-way catalyst, while the exhaust manifold19is provided inside it with an air-fuel ratio sensor21.

In the embodiment which is shown inFIG. 1, a fuel containing alcohol is used as fuel. The alcohol-containing fuel which is stored in a fuel tank22is fed to each fuel injector13. In this embodiment according to the present invention, the alcohol concentration in the fuel used extends over a broad range from 0% to 85% or so, therefore the alcohol concentration in the fuel which is injected from the fuel injector13also changes over a broad range. Inside the fuel tank22, an alcohol concentration sensor23is attached for detecting the alcohol concentration in the fuel which is injected from the fuel injector13.

On the other hand, in the embodiment shown inFIG. 1, the connecting part of the crank case1and the cylinder block2is provided with a variable compression ratio mechanism A which is able to change the relative positions of the crank case1and cylinder block2in the cylinder axial direction so as to change the volume of the combustion chamber5when the piston4is positioned at compression top dead center and is further provided with an actual compression action start timing changing mechanism B which is able to change a start timing of an actual compression action. Note that in the embodiment shown inFIG. 1, this actual compression action start timing changing mechanism B is comprised of a variable valve timing mechanism which is able to control the closing timing of the intake valve7.

The electronic control unit30is comprised of a digital computer which is provided with a ROM (read only memory)32, RAM (random access memory)33, CPU (microprocessor)34, input port35, and output port36, which are connected with each other through a bidirectional bus31. The output signal of the intake air amount detector18and the output signals of the air-fuel ratio sensor21and alcohol sensor23are input through corresponding AD converters37to the input port35. Further, an accelerator pedal40is connected to a load sensor41which generates an output voltage which is proportional to the amount of depression L of the accelerator pedal40. The output voltage of the load sensor41is input through a corresponding AD converter37to the input port35. Further, the input port35is connected to a crank angle sensor42which generates an output pulse every time the crankshaft rotates by for example 30°. On the other hand, the output port36is connected through the drive circuit38to each spark plug6, each fuel injector13, throttle valve drive actuator16, variable compression ratio mechanism A, and variable valve timing mechanism B.

FIG. 2is a disassembled perspective view of the variable compression ratio mechanism A which is shown inFIG. 1, whileFIG. 3is a side cross-sectional view of the illustrated internal combustion engine. Referring toFIG. 2, at the bottom of the two side walls of the cylinder block2, a plurality of projecting parts50which are separated from each other by a certain distance are formed. Each projecting part50is formed with a circular cross-section cam insertion hole51. On the other hand, the top surface of the crank case1is formed with a plurality of projecting parts52which are separated from each other by a certain distance and which fit between the corresponding projecting parts50. These projecting parts52are also formed with circular cross-section cam insertion holes53.

As shown inFIG. 2, a pair of cam shafts54and55is provided. Each of the cam shafts54and55has circular cams56fixed on it which is able to be rotatably inserted in the cam insertion holes51at every other position. These circular cams56are coaxial with the axes of rotation of the cam shafts54and55. On the other hand, between the circular cams56, as shown by the hatching inFIG. 3, extend eccentric shafts57arranged eccentrically with respect to the axes of rotation of the cam shafts54and55. Each eccentric shaft57has other circular cams58rotatably attached to it eccentrically. As shown inFIG. 2, these circular cams58are arranged between the circular cams56. These circular cams58are rotatably inserted in the corresponding cam insertion holes53.

When the circular cams56which are fastened to the cam shafts54and55are rotated in opposite directions as shown by the solid line arrows inFIG. 3(A)from the state shown inFIG. 3(A), the eccentric shafts57move toward the bottom center, so the circular cams58rotate in the opposite directions from the circular cams56in the cam insertion holes53as shown by the broken line arrows inFIG. 3(A). As shown inFIG. 3(B), when the eccentric shafts57move toward the bottom center, the centers of the circular cams58move to below the eccentric shafts57.

As will be understood from a comparison ofFIG. 3(A)andFIG. 3(B), the relative positions of the crank case1and cylinder block2are determined by the distance between the centers of the circular cams56and the centers of the circular cams58. The larger the distance between the centers of the circular cams56and the centers of the circular cams58, the further the cylinder block2from the crank case1. If the cylinder block2separates from the crank case1, the volume of the combustion chamber5when the piston4is positioned at compression top dead center increases, therefore by making the cam shafts54and55rotate, the volume of the combustion chamber5when the piston4is positioned at compression top dead center can be changed.

As shown inFIG. 2, to make the cam shafts54and55rotate in opposite directions, the shaft of a drive motor59is provided with a pair of worm gears61and62with opposite thread directions. Gears63and64engaging with these worm gears61and62are fastened to ends of the cam shafts54and55. In this embodiment, by driving the drive motor59, it is possible to change the volume of the combustion chamber5when the piston4is positioned at compression top dead center over a broad range. Note that the variable compression ratio mechanism A shown fromFIG. 1toFIG. 3shows an example. Any type of variable compression ratio mechanism may be used.

On the other hand,FIG. 4shows the variable valve timing mechanism B which is attached to the end of the cam shaft70for driving the intake valve7inFIG. 1. Referring toFIG. 4, this variable valve timing mechanism B is provided with a timing pulley71which is rotated by an engine crankshaft through a timing belt in the arrow direction, a cylindrical housing72which rotates together with the timing pulley71, a shaft73which is able to rotate together with an intake valve drive cam shaft70and rotate relative to the cylindrical housing72, a plurality of partitions74which extend from an inside circumference of the cylindrical housing72to an outside circumference of the shaft73, and vanes75which extend between the partitions74from the outside circumference of the shaft73to the inside circumference of the cylindrical housing72, the two sides of the vanes75being formed with advancing use hydraulic chambers76and retarding use hydraulic chambers77.

The feed of working oil to the hydraulic chambers76and77is controlled by a working oil feed control valve78. This working oil feed control valve78is provided with hydraulic ports79and80which are connected to the hydraulic chambers76and77, a feed port82for working oil which is discharged from a hydraulic pump81, a pair of drain ports83and84, and a spool valve85for controlling connection and disconnection of the ports79,81,82,83, and84.

To advance the phase of the cams of the intake valve drive cam shaft70, inFIG. 4, the spool valve85is made to move to the right, working oil which is fed from the feed port82is fed through the hydraulic port79to the advancing use hydraulic chambers76, and working oil in the retarding use hydraulic chambers77is drained from the drain port84. At this time, the shaft73is made to rotate relative to the cylindrical housing72in the arrow direction.

As opposed to this, to retard the phase of the cams of the intake valve drive cam shaft70, inFIG. 4, the spool valve85is made to move to the left, working oil which is fed from the feed port82is fed through the hydraulic port80to the retarding use hydraulic chambers77, and working oil in the advancing use hydraulic chambers76is drained from the drain port83. At this time, the shaft73is made to rotate relative to the cylindrical housing72in the direction opposite to the arrow.

When the shaft73is made to rotate relative to the cylindrical housing72, if the spool valve85is returned to the neutral position which is shown inFIG. 4, the operation for relative rotation of the shaft73is stopped. The shaft73is held at the relative rotational position at that time. Therefore, it is possible to use the variable valve timing mechanism B so as to advance or retard the phase of the cams of the intake valve drive cam shaft70by exactly the desired amount.

InFIG. 5, the solid line shows when the variable valve timing mechanism B is used to advance the phase of the cams of the intake valve drive cam shaft70the most, while the broken line shows when it is used to retard the phase of the cams of the intake valve drive cam shaft70the most. Therefore, the opening time of the intake valve7can be freely set between the range shown by the solid line inFIG. 5and the range shown by the broken line, therefore the closing timing of the intake valve7can be set to any crank angle in the range shown by the arrow C inFIG. 5.

The variable valve timing mechanism B which is shown inFIG. 1andFIG. 4is one example. For example, a variable valve timing mechanism or other various types of variable valve timing mechanisms which are able to change only the closing timing of the intake valve while maintaining the opening timing of the intake valve constant can be used.

Next, the meaning of the terms used in the present application will be explained with reference toFIG. 6. Note thatFIGS. 6(A), (B), and (C) show for explanatory purposes an engine with a volume of each combustion chamber of 50 ml and a stroke volume of each piston of 500 ml. In theseFIGS. 6(A), (B), and (C), the combustion chamber volume shows the volume of a combustion chamber when a piston is at compression top dead center.

FIG. 6(A)explains the mechanical compression ratio. The mechanical compression ratio is a value determined mechanically from a stroke volume of a piston and the combustion chamber volume at the time of a compression stroke. This mechanical compression ratio is expressed by (combustion chamber volume+stroke volume)/combustion chamber volume. In the example shown inFIG. 6(A), this mechanical compression ratio becomes (50 ml+500 ml)/50 ml=11.

FIG. 6(B)explains the actual combustion ratio. This actual combustion ratio is a value determined from the actual stroke volume of the piston and the combustion chamber volume from when the compression action is actually started to when the piston reaches top dead center. This actual combustion ratio is expressed by (combustion chamber volume+actual stroke volume)/combustion chamber volume. That is, as shown inFIG. 6(B), even if the piston starts to rise in the compression stroke, no compression action is performed while the intake valve is opened. The actual compression action is started after the intake valve closes. Therefore, the actual combustion ratio is expressed as above using the actual stroke volume. In the example shown inFIG. 6(B), the actual combustion ratio becomes (50 ml+450 ml)/50 ml=10.

FIG. 6(C)explains the expansion ratio. The expansion ratio is a value determined from the stroke volume of the piston and the combustion chamber volume at the time of an expansion stroke. This expansion ratio is expressed by the (combustion chamber volume+stroke volume)/combustion chamber volume. In the example shown inFIG. 6(C), this expansion ratio becomes (50 ml+500 ml)/50 ml=11.

Next, a superhigh expansion ratio cycle which is used in the present invention will be explained with reference toFIG. 7andFIG. 8. Note thatFIG. 7shows the relationship between the theoretical thermal efficiency and the expansion ratio in the case of using gasoline as the fuel, whileFIG. 8shows a comparison between the ordinary cycle and superhigh expansion ratio cycle used selectively in accordance with the load in the present invention.

FIG. 8(A)shows the ordinary cycle when the intake valve closes near the bottom dead center and the compression action by the piston is started from near substantially compression bottom dead center. In the example shown in thisFIG. 8(A)as well, in the same way as the examples shown inFIGS. 6(A), (B), and (C), the combustion chamber volume is made 50 ml, and the stroke volume of the piston is made 500 ml. As will be understood fromFIG. 8(A), in an ordinary cycle, the mechanical compression ratio is (50 ml+500 ml)/50 ml=11, the actual combustion ratio is also about 11, and the expansion ratio also becomes (50 ml+500 ml)/50 ml=11. That is, in an ordinary internal combustion engine, the mechanical compression ratio and actual combustion ratio and the expansion ratio become substantially equal.

The solid line inFIG. 7shows the change in the theoretical thermal efficiency in the case where the actual combustion ratio and expansion ratio are substantially equal, that is, in the ordinary cycle. In this case, it is learned that the larger the expansion ratio, that is, the higher the actual combustion ratio, the higher the theoretical thermal efficiency. Therefore, in an ordinary cycle, to raise the theoretical thermal efficiency, the actual combustion ratio should be made higher. However, due to the restrictions on the occurrence of knocking at the time of engine high load operation, the actual combustion ratio can only be raised even at the maximum to about 12, accordingly, in an ordinary cycle, the theoretical thermal efficiency cannot be made sufficiently high.

On the other hand, under this situation, the inventors strictly differentiated between the mechanical compression ratio and actual combustion ratio and studied the theoretical thermal efficiency and as a result discovered that in the theoretical thermal efficiency, the expansion ratio is dominant, and the theoretical thermal efficiency is not affected much at all by the actual combustion ratio. That is, if raising the actual combustion ratio, the explosive force rises, but compression requires a large energy, accordingly even if raising the actual combustion ratio, the theoretical thermal efficiency will not rise much at all.

As opposed to this, if increasing the expansion ratio, the longer the period during which a force acts pressing down the piston at the time of the expansion stroke, the longer the time that the piston gives a rotational force to the crankshaft. Therefore, the larger the expansion ratio is made, the higher the theoretical thermal efficiency becomes. The broken line ε=10 inFIG. 7shows the theoretical thermal efficiency in the case of fixing the actual combustion ratio at 10 and raising the expansion ratio in that state. In this way, it is learned that the amount of rise of the theoretical thermal efficiency when raising the expansion ratio in the state where the actual combustion ratio is maintained at a low value and the amount of rise of the theoretical thermal efficiency in the case where the actual combustion ratio is increased along with the expansion ratio as shown by the solid line ofFIG. 7will not differ that much.

If the actual combustion ratio is maintained at a low value in this way, knocking will not occur, therefore if raising the expansion ratio in the state where the actual combustion ratio is maintained at a low value, the occurrence of knocking can be prevented and the theoretical thermal efficiency can be greatly raised.FIG. 8(B)shows an example of the case when using the variable compression ratio mechanism A and variable valve timing mechanism B to maintain the actual combustion ratio at a low value while raising the expansion ratio.

Referring toFIG. 8(B), in this example, the variable compression ratio mechanism A is used to lower the combustion chamber volume from 50 ml to 20 ml. On the other hand, the variable valve timing mechanism B is used to retard the closing timing of the intake valve until the actual stroke volume of the piston changes from 500 ml to 200 ml. As a result, in this example, the actual combustion ratio becomes (20 ml+200 ml)/20 ml=11 and the expansion ratio becomes (20 ml+500 ml)/20 ml=26. In the ordinary cycle shown inFIG. 8(A), as explained above, the actual combustion ratio is about 11 and the expansion ratio is 11. Compared with this case, in the case shown inFIG. 8(B), it is learned that only the expansion ratio is raised to 26. This is the reason that it is called the superhigh expansion ratio cycle.

Generally speaking, in an internal combustion engine, the lower the engine load, the worse the thermal efficiency, therefore to improve the thermal efficiency at the time of vehicle operation, that is, to improve the fuel efficiency, it becomes necessary to improve the thermal efficiency at the time of engine low load operation. On the other hand, in the superhigh expansion ratio cycle shown inFIG. 8(B), the actual stroke volume of the piston at the time of the compression stroke is made smaller, so the amount of intake air which can be taken into the combustion chamber5becomes smaller, therefore this superhigh expansion ratio cycle can only be employed when the engine load is relatively low. Therefore, in the present invention, at the time of engine low load operation, the superhigh expansion ratio cycle shown inFIG. 8(B)is used, while at the time of engine high load operation, the ordinary cycle shown inFIG. 8(A)is used.

Next, the operational control as a whole will be explained with reference toFIG. 9.

FIG. 9shows the changes in the intake air amount, mechanical compression ratio, expansion ratio, expansion end pressure, actual compression ratio, closing timing of the intake valve7, and opening degree of the throttle valve17in accordance with the engine load at a certain engine speed. Note that inFIG. 9, the broken lines show the case of use of gasoline as the fuel, while the solid lines show the case of use of an alcohol-containing fuel with a certain alcohol concentration as the fuel. Further, in this embodiment according to the present invention, ordinarily the average air-fuel ratio in the combustion chamber5is feedback controlled to the stoichiometric air-fuel ratio based on the output signal of the air-fuel ratio sensor21so that the three-way catalyst in the catalytic converter20can simultaneously reduce the unburned HC, CO, and NOXin the exhaust gas.

First, if explaining the case as shown by the broken lines inFIG. 9, that is, the case of using gasoline as fuel, at the time of engine high load operation, as explained above, the ordinary cycle which is shown inFIG. 8(A)is executed. Therefore, at this time, as shown inFIG. 9, the mechanical compression ratio is lowered, so the expansion ratio is low. As shown inFIG. 9by the broken lines, the closing timing of the intake valve7is advanced as shown by the solid line inFIG. 5. Further, at this time, the amount of intake air is large. At this time, the opening degree of the throttle valve17is held full open or substantially full open.

On the other hand, as shown inFIG. 9by the broken lines, if the engine load becomes lower, the closing timing of the intake valve7is retarded along with this to reduce the amount of intake air. Further, at this time, as shown inFIG. 9, the mechanical compression ratio is increased as the engine load becomes lower so that the actual compression ratio is held substantially constant, therefore the expansion ratio also is increased as the engine load becomes lower. Note that, at this time as well, the throttle valve17is held in the full open or substantially full open state, therefore the amount of intake air which is fed into the combustion chamber5is controlled without relying on the throttle valve17by changing the closing timing of the intake valve7.

When the engine load becomes lower from the engine high load operating state in this way, the mechanical compression ratio is made to increase along with the decrease in the amount of intake air under a substantially constant actual compression ratio. That is, the volume of the combustion chamber5when the piston4reaches compression top dead center is made to decrease in proportion to the decrease of the amount of intake air. Therefore, the volume of the combustion chamber5when the piston4reaches compression top dead center changes in proportion to the amount of intake air. Note that at this time, the air-fuel ratio inside the combustion chamber5becomes the stoichiometric air-fuel ratio, so the volume of the combustion chamber5when the piston4reaches compression top dead center changes in proportion to the amount of fuel.

If the engine load becomes further lower, the mechanical compression ratio is made to further increase. If the engine load falls to a certain load L of the medium load region, the mechanical compression ratio reaches the limit mechanical compression ratio forming the structural limit of the combustion chamber5. If the mechanical compression ratio reaches the limit mechanical compression ratio, in the region where the load is lower than the engine load L when the mechanical compression ratio reaches the mechanical compression ratio, the mechanical compression ratio is held at the limit mechanical compression ratio. Therefore, at the time of low load side engine medium load operation and at the time of engine low load operation, that is, at the engine low load operation side, the mechanical compression ratio becomes maximum and the expansion ratio also becomes maximum. In other words, at the engine low load operation side, the mechanical compression ratio is made the maximum so that the maximum expansion ratio is obtained.

On the other hand, in the example which is shown inFIG. 9, if the engine load falls to the load L, the closing timing of the intake valve7becomes the limit closing timing at which the amount of intake air which is fed into the combustion chamber5can be controlled. If the closing timing of the intake valve7reaches the limit closing timing, in the region of a load lower than the engine load L when the closing timing of the intake valve7reaches the limit closing timing, the closing timing of the intake valve7is held at the limit closing timing. If the engine load becomes the load L or less, the mechanical compression ratio and the closing timing of the intake valve7are held constant in this way, so the actual compression ratio is held constant.

If the closing timing of the intake valve7is held at the limit closing timing, a change of the closing timing of the intake valve7will not longer be able to be used to control the amount of intake air. In the embodiment which is shown inFIG. 9, at this time, in the region where the load is lower than the engine load L when the closing timing of the intake valve7reaches the limit closing timing, the throttle valve17is used to control the amount of intake air which is fed into the combustion chamber5. The lower the engine load becomes, the smaller the opening degree of the throttle valve17is made.

Further, if the engine load falls, the combustion pressure falls, so the expansion end pressure also falls. Therefore, as shown inFIG. 9by the broken line, along with a fall in the engine load, the expansion end pressure also falls. In this case, the expansion end pressure falls the most when the engine load falls the most, but as will be understood fromFIG. 9, even when the expansion end pressure falls the most, the expansion end pressure will not become less than the atmospheric pressure.

On the other hand, as shown inFIG. 9by the one-dot and dash line, by advancing the closing timing of the intake valve7along with a fall in the engine load, it is also possible to control the amount of intake air without relying on the throttle valve17. Therefore, if expressed to be able to encompass both the case which is shown by the broken line and the case which is shown by the one-dot and dash line inFIG. 9, in the example which is shown inFIG. 9, the closing timing of the intake valve7is made to move, as the engine load becomes lower, in a direction away from intake bottom dead center BDC until the limit closing timing L at which the amount of intake air which is fed into a combustion chamber can be controlled. In this way, the amount of intake air can also be controlled by making the closing timing of the intake valve7change as shown inFIG. 9by the broken line and can be controlled by making it change as shown by the one-dot and dash line, but below, the case of making the closing timing of the intake valve7change as shown inFIG. 9by the broken line will be explained as an example.

In this regard, as explained before, in the superhigh expansion ratio cycle which is shown in FIG.8(B), the expansion ratio is made 26. The higher this expansion ratio, the more preferable, but as will be understood fromFIG. 7, even for the practical by usable lower limit actual compression ratio ε=5, if 20 or more, a considerably high theoretical thermal efficiency can be obtained. Therefore, in the present invention, the variable compression ratio mechanism A is formed so that the expansion ratio becomes 20 or more.

FIG. 10is a PV graph which shows logarithmically both the volume V of the combustion chamber5and a pressure P of the combustion chamber5. InFIG. 10, the solid line shows the relationship between the volume V and the pressure P at the time of engine low load operation in the case of use of gasoline as the fuel. As shown inFIG. 10by the solid line, it is learned that when gasoline is used as the fuel, even at the time of engine low load operation, the expansion end pressure will be the atmospheric pressure or more. In this regard, when using a fuel which contains alcohol as the fuel like in the present invention, sometimes the expansion end pressure ends up becoming the atmospheric pressure or less.

That is, if making a fuel like alcohol which contains oxygen burn, a large amount of water with a large specific heat will be produced compared with when making usual gasoline burn. As a result, the combustion temperature will fall and the combustion pressure will fall. If the combustion pressure falls, the expansion end pressure falls and as a result, as shown inFIG. 10by the broken line, sometimes the expansion end pressure ends up becoming less than the atmospheric pressure, that is, sometimes over expansion ends up occurring. However, if over expansion occurs in this way, the heat efficiency will greatly fall, so it is necessary to prevent such over expansion from occurring.

In this regard, when using a fuel which contains alcohol as the fuel, the higher the alcohol concentration in the fuel, the more the combustion pressure falls and the more the expansion end pressure falls. On the other hand, the expansion end pressure rises as the expansion ratio is made to fall. Therefore, to prevent over expansion, it is sufficient to make the expansion ratio fall the higher the alcohol concentration in the fuel. Therefore, in the present invention, when the alcohol concentration in the fuel is high, the expansion ratio at the time of engine low load operation is made to fall compared to when the alcohol concentration in the fuel is low.

Note that, in this embodiment according to the present invention, as shown inFIG. 11, the expansion ratio is made higher as the alcohol concentration in the fuel becomes higher. Further, the higher the alcohol concentration in the fuel becomes, the harder it is for knocking to occur, therefore, it is possible to raise the actual compression ratio the higher the alcohol concentration in the fuel. Therefore, in the present invention, when the alcohol concentration in the fuel is high, the actual compression ratio is made higher compared with when the alcohol concentration in the fuel is low. In this case, in the embodiment according to the present invention, as shown inFIG. 12, the higher the alcohol concentration in the fuel, the greater the actual compression ratio.

Now, to make the expansion ratio fall, there are two methods: the method of making the mechanical compression ratio fall and the method of advancing the opening timing of the exhaust valve9. The solid lines ofFIG. 9show the changes in the mechanical compression ratio etc. in the case of making the mechanical compression ratio fall to thereby make the expansion ratio fall at the time of engine low load operation. Note that the solid line ofFIG. 9shows the case where fuel which contains a certain concentration of alcohol is used as the fuel and where the actual compression ratio is raised across the board without regard as to the engine load.

Referring toFIG. 9, as shown by the solid line, at the time of engine high load operation, the mechanical compression ratio is made higher by exactly the amount by which the actual compression ratio is made higher.

Therefore, at this time, the expansion ratio also becomes higher than the case which is shown by the broken lines, that is, the case of using gasoline. On the other hand, at this time, the expansion end pressure becomes lower compared with the case of using gasoline. Further, at this time, the throttle valve17is held in the full open or substantially full open state.

If the engine load becomes lower, as shown inFIG. 9by the solid line, the closing timing of the intake valve7is retarded to decrease the amount of intake air. Further, at this time, the mechanical compression ratio is made to increase as the engine load becomes lower so that the actual compression ratio is held substantially constant, therefore the expansion ratio is also increased as the engine load becomes lower. Note that at this time as well, the throttle valve17is held full open or substantially full open in state, therefore the amount of intake air which is fed into the combustion chamber5is controlled, without relying on the throttle valve17, by changing the closing timing of the intake valve7. Further, at this time, the expansion end pressure gradually falls.

Next, if the engine load becomes further lower, the mechanical compression ratio is further made to increase. If the engine load falls to the load L1(>L), the mechanical compression ratio reaches the maximum mechanical compression ratio. On the other hand, in the example which is shown inFIG. 9, if the engine load falls to L1, the closing timing of the intake valve7becomes the limit closing timing at which the amount of intake air which is fed into a combustion chamber5can be controlled. If the closing timing of the intake valve7reaches the limit closing timing, the change of the closing timing of the intake valve7can no longer be used to control the amount of intake air, therefore at this time the throttle valve17is used to control the amount of intake air which flows into the combustion chamber5. If the engine load becomes lower than L1, the lower the engine load becomes, the smaller the opening degree of the throttle valve17is made.

On the other hand, in the example which is shown inFIG. 9, as shown by the solid line, if the engine load falls to L2(<L), the expansion end pressure falls down to atmospheric pressure. Therefore, at the time of low load operation where the engine load is lower than the load L2where the expansion end pressure becomes the atmospheric pressure, the expansion ratio is made to fall by decreasing the mechanical compression ratio. As will be understood fromFIG. 9, at the time of engine low load operation, if the expansion ratio is held constant, the expansion end pressure will rapidly fall down to less than atmospheric pressure along with the fall in the engine load. To prevent the expansion end pressure from becoming less than the atmospheric pressure at this time, it is necessary to make the expansion ratio fall when the engine load falls.

Therefore, in the present invention, at the time of engine low load operation, the amount of fall of the expansion ratio is made larger at the engine low load side compared with the engine high load side. Note that, in this case, in the example which is shown inFIG. 9, as the engine load becomes lower, the mechanical compression ratio is made to become lower and along with this the expansion ratio is made to become lower. On the other hand, in the example which is shown inFIG. 9, at the engine low load operation region where the engine load is lower than L2, to maintain the actual compression ratio constant, the closing timing of the intake valve7is advanced as the mechanical compression ratio is made to fall. At this time, the opening degree of the throttle valve17is made to close more compared with when using gasoline so that the amount of intake air becomes the required amount of intake air corresponding to the load.

In an embodiment according to the present invention, the closing timing of the intake valve7, mechanical compression ratio, and opening degree of the throttle valve17become functions of the concentration of ammonia in the fuel in addition to the engine load and engine speed. In the embodiment according to the present invention, a plurality of maps of the closing timing IC of the intake valve7such as shown inFIG. 13(A)are stored for various alcohol concentrations as functions of the engine load L and engine speed N in advance in the ROM32, a plurality of maps of the mechanical compression ratio CA such as shown inFIG. 13(B)are stored for various alcohol concentrations as functions of the engine load L and engine speed N in advance in the ROM32, and a plurality of maps of the opening degree θ of the throttle valve17such as shown inFIG. 13(C)are stored for various alcohol concentrations as functions of the engine load L and engine speed N in advance in the ROM32.

FIG. 14shows an operational control routine. Referring toFIG. 14, first, at step100, the alcohol concentration sensor23is used to detect the alcohol concentration in the fuel which is fed into the combustion chamber5. Next, at step101, the closing timing IC of the intake valve7is calculated from the map which is shown inFIG. 13(A)in accordance with the detected alcohol concentration, next, at step102, the mechanical compression ratio CR is calculated from the map which is shown inFIG. 13(B)in accordance with the detected alcohol concentration, next, at step103, the opening degree of the throttle valve17is calculated from the map which is shown inFIG. 13(C)in accordance with the detected alcohol concentration. Next, at step104, the variable compression ratio mechanism A is controlled so that the mechanical compression ratio becomes the mechanical compression ratio CR, the variable valve timing mechanism B is controlled so that the closing timing of the intake valve7becomes the closing timing IC, and the throttle valve17is controlled so that the opening degree of the throttle valve17becomes the opening degree θ.

FIG. 15shows another embodiment. In this embodiment, to control the opening timing of the exhaust valve9, a variable valve timing mechanism B′ which has a structure similar to the variable valve timing mechanism B is provided for a cam shaft90which drives the exhaust valve9. In this embodiment, the expansion ratio at the time of engine low load operation is made to fall by advancing the opening timing of the exhaust valve9by the variable valve timing mechanism B′.

The broken lines ofFIG. 16, in the same way as inFIG. 9, show when gasoline is used as the fuel, while the solid lines ofFIG. 16show the case of using alcohol-containing fuel with a certain alcohol concentration as the fuel. As shown by the solid line inFIG. 16, as in this embodiment, in the engine low load operation region where the engine load is lower than the load L2at which the expansion end pressure becomes the atmospheric pressure, the opening timing of the exhaust valve9is advanced compared with when using gasoline, that is, the case which is shown by the broken lines. If the opening timing of the exhaust valve9is advanced, the expansion ratio falls.

In this case, in the embodiment according to the present invention, as shown inFIG. 17, the higher the alcohol concentration in the fuel, the more the amount of advance of the opening timing of the exhaust valve9is increased. Further, as will be understood from the solid lines ofFIG. 16, at the time of engine low load operation, the more the engine load falls, the more the amount of advance of the opening timing of the exhaust valve9is increased, therefore the more the engine load falls, the more the expansion ratio is lowered. Note that, in this embodiment, at the time of engine low load operation, the mechanical compression ratio is maintained at the maximum mechanical compression ratio, while the closing timing of the intake valve7is held at the limit closing timing.

In this embodiment as well, the closing timing of the intake valve7, the mechanical compression ratio, and the opening degree of the throttle valve17become functions of the concentration of ammonia in the fuel in addition to the engine load and engine speed. These closing timing of the intake valve7, mechanical compression ratio, and opening degree of the throttle valve17are stored in advance with respect to various alcohol concentrations in the form of the maps such as shown inFIGS. 13(A), (B), and (C).

Further, in this embodiment, the opening timing of the exhaust valve9also becomes a function of the ammonia concentration in the fuel in addition to the engine load and engine speed. Therefore, in this embodiment, a plurality of maps of the opening timing EO of the exhaust valve9such as shown inFIG. 18are stored for various alcohol concentrations as functions of the engine load L and engine speed N in advance in the ROM32.

FIG. 19shows an operational control routine. Referring toFIG. 19, first, at step200, the alcohol concentration sensor23is used to detect the alcohol concentration in the fuel which is fed into the combustion chamber5. Next, at step201, the closing timing IC of the intake valve7is calculated from the map such as shown inFIG. 13(A)in accordance with the detected alcohol concentration, next, at step202, the mechanical compression ratio CR is calculated from the map such as shown inFIG. 13(B)in accordance with the detected alcohol concentration, next, at step103, the opening degree of the throttle valve17is calculated from the map such as shown inFIG. 13(C)in accordance with the detected alcohol concentration.

Next, at step204, the opening timing EO of the exhaust valve9is calculated from the map which is shown inFIG. 18in accordance with the detected alcohol concentration. Next, at step205, the variable compression ratio mechanism A is controlled so that the mechanical compression ratio becomes the mechanical compression ratio CR, the variable valve timing mechanism B is controlled so that the closing timing of the intake valve7becomes the closing timing IC, the throttle valve17is controlled so that the opening degree of the throttle valve17becomes the opening degree θ, and the variable valve timing mechanism B′ is controlled so that the opening timing of the exhaust valve9becomes EO.

FIG. 20shows still another embodiment. In this embodiment, usually the expansion ratio at the time of engine low load operation is lowered by advancing the opening timing of the exhaust valve9. When there is a request to lower the mechanical compression ratio, the expansion ratio at the time of engine low load operation is lowered by lowering the mechanical compression ratio.

That is, as will be understood if comparingFIG. 9andFIG. 16, the opening degree of the throttle valve17at the time of engine low load operation is made smaller in the case which is shown inFIG. 9compared with the case which is shown inFIG. 16, therefore the pumping loss becomes larger in the case which is shown inFIG. 9compared with the case which is shown inFIG. 16. Therefore, if considering the thermal efficiency, as shown inFIG. 16, it is preferable to make the expansion ratio fall by advancing the opening timing EO of the exhaust valve9. Therefore, in this example, usually the expansion ratio is made to fall by advancing the opening timing EO of the exhaust valve9.

However, sometimes a request is issued to lower the mechanical compression ratio. That is, when the mechanical compression ratio can be changed, the higher the mechanical compression ratio becomes, the flatter the combustion chamber5becomes. As a result, the higher the mechanical compression ratio, the harder it becomes for fuel in the peripheral parts of the combustion chamber5to burn and therefore the easier it becomes for unburned HC to be produced. Therefore, for example, at this time, when desiring to lower the amount of production of unburned HC, it is preferable to lower the mechanical compression ratio. In such a case, a request is issued to lower the mechanical compression ratio.

As one example of the case where a request is issued to lower the mechanical compression ratio in this way, the time of engine startup or the time of engine warmup operation may be mentioned. That is, at the time of engine startup and at the time of engine warmup operation, usually the catalyst20is not activated, therefore if unburned HC flows into the catalyst20at this time, the unburned HC slips through the catalyst20without being removed at the catalyst20. Therefore at the time of engine startup or at the time of engine warmup operation, it is preferable to make the amount of exhaust of unburned HC from the combustion chamber5fall. Therefore, in this example, at the time of engine startup or at the time of engine warmup operation, a request is issued to lower the mechanical compression ratio. In this embodiment, when a request is issued to lower the mechanical compression ratio in this way, the mechanical compression ratio is made to fall to thereby make the expansion ratio fall.

Referring to the operational control routine which is shown inFIG. 20, first, at step300, the alcohol concentration sensor23is used to detect the alcohol concentration in the fuel which is fed into the combustion chamber5. Next, at step301, it is judged if a request has been issued to lower the mechanical compression ratio. If no request has been issued to lower the mechanical compression ratio, the routine proceeds to step302where the mechanical compression ratio etc. are controlled as shown by the solid lines ofFIG. 16.

That is, at step302, the closing timing IC of the intake valve7is calculated from the map such as shown inFIG. 13(A)in accordance with the detected alcohol concentration, next, at step303, the mechanical compression ratio CR is calculated from the map such as shown inFIG. 13(B)in accordance with the detected alcohol concentration, next, at step304, the opening degree of the throttle valve17is calculated from the map such as shown inFIG. 13(C)in accordance with the detected alcohol concentration. Next, at step305, the opening timing EO of the exhaust valve9is calculated from the map which is shown inFIG. 18in accordance with the detected alcohol concentration.

Next, at step306, the variable compression ratio mechanism A is controlled so that the mechanical compression ratio becomes the mechanical compression ratio CR, the variable valve timing mechanism B is controlled so that the closing timing of the intake valve7becomes the closing timing IC, the throttle valve17is controlled so that the opening degree of the throttle valve17becomes the opening degree θ, and the variable valve timing mechanism B′ is controlled so that the opening timing of the exhaust valve9becomes EO.

On the other hand, when it is judged at step301that a request has been issued to lower the mechanical compression ratio, the routine proceeds to step307where, as shown by the solid line ofFIG. 9, the mechanical compression ratio etc. are controlled.

That is, at step307, the closing timing IC of the intake valve7is calculated from the map which is shown inFIG. 13(A)in accordance with the detected alcohol concentration, next, at step308, the mechanical compression ratio CR is calculated from the map which is shown inFIG. 13(B)in accordance with the detected alcohol concentration, next, at step309, the opening degree of the throttle valve17is calculated from the map which is shown inFIG. 13(C)in accordance with the detected alcohol concentration. Next, at step310, the opening timing EO of the exhaust valve9is fixed at the reference timing, then the routine proceeds to step306. At this time, at step306, the variable compression ratio mechanism A is controlled so that the mechanical compression ratio becomes the mechanical compression ratio CR, the variable valve timing mechanism B is controlled so that the closing timing of the intake valve7becomes the closing timing IC, and the throttle valve17is controlled so that the opening degree of the throttle valve17becomes the opening degree θ.

REFERENCE SIGNS LIST