TECHNICAL FIELD

The present invention relates to a variable compression ratio internal combustion engine which can change a mechanical compression ratio.

BACKGROUND ART

Known in the past has been an internal combustion engine comprising a variable compression ratio mechanism which can change a mechanical compression ratio of the internal combustion engine. As such a variable compression ratio mechanism, various mechanisms have been proposed. As one among these, one which can change the effective length of a connecting rod used in the internal combustion engine may be mentioned (for example, PTL 1). In this regard, the “effective length of a connecting rod” means the distance between a center of a crank receiving opening which receives a crank pin and a center of a piston pin receiving opening which receives a piston pin. Therefore, if the effective length of a connecting rod becomes longer, a combustion chamber volume when the piston is at top dead center of the compression stroke becomes smaller, and therefore the mechanical compression ratio increases. On the other hand, if the effective length of a connecting rod becomes shorter, the combustion chamber volume when the piston is at top dead center of the compression stroke becomes larger, and therefore the mechanical compression ratio falls.

As a variable length connecting rod which can be changed in effective length, known is one which is provided with a connecting rod body with a small end on which an eccentric member (eccentric arm or eccentric sleeve), which can turn with respect to the connecting rod body, is provided (for example, PTL 1). The eccentric member has a piston pin receiving opening which receives the piston pin. This piston pin receiving opening is provided so as to offset with respect to a turning axis of the eccentric member. In such a variable length connecting rod, if changing the turning position of the eccentric member, the effective length of the connecting rod can be changed accordingly.

Specifically, the eccentric member pivots in one direction due to the upward inertial force acting on the piston pin due to the reciprocating motion of the piston to thereby lengthen the effective length of the connecting rod. As a result, the piston rises with respect to the connecting rod body and the mechanical compression ratio is switched from a low compression ratio to a high compression ratio. On the other hand, the eccentric member pivots in the other direction due to the downward inertial force acting on the piston pin due to the reciprocating motion of the piston and the downward explosive force acting on the piston pin due to combustion of the air-fuel mixture to thereby shorten the effective length of the connecting rod. As a result, the piston descends with respect to the connecting rod body and the mechanical compression ratio is switched from a high compression ratio to a low compression ratio. Therefore, in a variable compression ratio internal combustion engine provided with a variable length connecting rod, the mechanical compression ratio is switched by inertial force from a low compression ratio to a high compression ratio and is switched by inertial force and explosive force from a high compression ratio to a low compression ratio.

CITATION LIST

Patent Literature

PLT 1: Japanese Patent Publication No. 2016-142137A

SUMMARY OF INVENTION

Technical Problem

In this regard, the inertial force is far smaller than the explosive force. For this reason, it is difficult to obtain a sufficient response when switching the mechanical compression ratio from a low compression ratio to a high compression ratio. Further, the inertial force is proportional to the square of the engine speed of the internal combustion engine, so in the low speed region of the internal combustion engine, a sufficient inertial force cannot be obtained and the response further deteriorates.

Therefore, in view of the above problem, an object of the present invention is to improve the response when raising the mechanical compression ratio in a variable compression ratio internal combustion engine provided with a variable compression ratio mechanism.

Solution to Problem

The summary of the present disclosure is as follows.

(1) A variable compression ratio internal combustion engine comprising: a variable compression ratio mechanism able to change a mechanical compression ratio; an exhaust promotion mechanism able to reduce cylinder residual gas after an exhaust stroke of cylinders; and a control device configured to control the mechanical compression ratio by the variable compression ratio mechanism and control an operation of the exhaust promotion mechanism, wherein the control device is configured to operate the exhaust promotion mechanism in at least a partial time period of a time period from when it is demanded that the mechanical compression ratio be raised to when the mechanical compression ratio finishes being changed.

(2) The variable compression ratio internal combustion engine described in above (1), wherein the variable compression ratio mechanism is a variable length connecting rod comprising a connecting rod body provided with a crank receiving opening receiving a crank pin and an effective length changing member provided with a piston pin receiving opening receiving a piston pin and attached to the connecting rod body to be able to move so as to change a length between a center of the piston pin receiving opening and a center of the crank receiving opening.

(3) The variable compression ratio internal combustion engine described in above (2), wherein the control device is configured not to operate the exhaust promotion mechanism when an engine speed is a predetermined reference speed or more.

(4) The variable compression ratio internal combustion engine described in any one of above (1) to (3), wherein the control device is configured not to operate the exhaust promotion mechanism when an engine load is a predetermined threshold value or less.

(5) The variable compression ratio internal combustion engine described in any one of above (1) to (4), further comprising a detector detecting the mechanical compression ratio finishing being changed, wherein the control device is configured to operate the exhaust promotion mechanism from when it is demanded that the mechanical compression ratio be raised to when the detector detects the mechanical compression ratio finishing being changed.

Advantageous Effects of Invention

According to the present invention, it is possible to improve the response when raising the mechanical compression ratio in a variable compression ratio internal combustion engine provided with a variable compression ratio mechanism.

