Pre-combustion chamber-type engine

A pre-combustion chamber type engine comprising:

• a cylinder block having cylinder bores;
• a cylinder head mounted to the cylinder block;
• main combustion chambers in the cylinder bores;
• pistons that reciprocate in the cylinder bores;
• cavities formed in any one of the cylinder head and a group of the pistons;
• pre-combustion chamber structures installed in the cavities with heat insulation layers interposed therebetween;
• pre-combustion chamber formed in the pre-combustion chamber structures;
• communication holes formed in the pre-combustion chamber structures to communicate the pre-combustion chambers and the main combustion chambers;
• and
• fuel injection nozzles to inject fuel into the pre-combustion chambers;
• wherein the pre-combustion chamber structures are installed in the cavities in such a way as to be surrounded and heat-insulated by the heat insulation layers and the ratio of heat capacity of the pre-combustion chamber structures to the inner surface area of the pre-combustion chambers is set to 0.55 cal/.degree.C..multidot.cm.sup.2 or lower, and
• wherein when the ratio of the heat capacity of the pre-combustion chamber structures to the inner surface area of the pre-combustion chambers is set to 0.55 cal/.degree.C..multidot.cm.sup.2 or lower, the compression ratio as related to the inner surface area of the pre-combustion chambers is demarcated at the upper limit by an approximation equation:
• y=0.03579.sup.z -2.2
• where Z=1/(-0.01029s.sup.1/3 -1), y is the compression ratio and s is a non-dimensional number of the inner surface area.