An engine comprising: a cylinder block having a cylinder; a reciprocating piston movable through a stroke in the cylinder; a variable compression ratio mechanism configured to be linked to the piston for variably adjusting a geometrical compression ratio (ε) of the engine by varying at least a top dead center position of the piston, so that the top dead center position obtained at a low geometrical compression ratio is set to be lower than that at a high geometrical compression ratio, responsively to a controlled variable; an engine-and-vehicle sensor provided for detecting an engine operating condition including at least an engine load; and a controller configured to be electronically connected to at least the variable compression ratio mechanism and the sensor for controlling the variable compression ratio mechanism by calculating a target compression ratio (εm) based on the engine load detected by the sensor, setting the target compression ratio (εm) to a high value at low engine load operation and to a relatively low value at high engine load operation as compared to the target compression ratio (εm) set at the low engine load operation, and changing the controlled variable to bring the geometrical compression ratio (ε) closer to the target compression ratio (εm), characterized in that a substantially entire area of a crown of the piston, defining a part of a wall surface of a combustion chamber, which chamber is defined between the piston and the cylinder, is formed of a non-metallic material (55, 56; 55, 71a) having a higher heat-insulating and heat-reserving property as compared to a base structural material of each of the combustion chamber and the piston. a cylinder block having a cylinder; a reciprocating piston movable through a stroke in the cylinder; a variable compression ratio mechanism configured to be linked to the piston for variably adjusting a geometrical compression ratio (ε) of the engine by varying at least a top dead center position of the piston, so that the top dead center position obtained at a low geometrical compression ratio is set to be lower than that at a high geometrical compression ratio, responsively to a controlled variable; an engine-and-vehicle sensor provided for detecting an engine operating condition including at least an engine load; and a controller configured to be electronically connected to at least the variable compression ratio mechanism and the sensor for controlling the variable compression ratio mechanism by calculating a target compression ratio (εm) based on the engine load detected by the sensor, setting the target compression ratio (εm) to a high value at low engine load operation and to a relatively low value at high engine load operation as compared to the target compression ratio (εm) set at the low engine load operation, and changing the controlled variable to bring the geometrical compression ratio (ε) closer to the target compression ratio (εm), characterized in that a substantially entire area of a crown of the piston, defining a part of a wall surface of a combustion chamber, which chamber is defined between the piston and the cylinder, is formed of a non-metallic material (55, 56; 55, 71a) having a higher heat-insulating and heat-reserving property as compared to a base structural material of each of the combustion chamber and the piston.