A premixed compression self-ignition engine comprising: a crankcase; a crankshaft and a fulcrum shaft supported on the crankcase such that axes of the crankshaft and the fulcrum shaft are parallel to each other; a trigonal link having an intermediate portion rotatably supported on a crankpin of the crankshaft; a connecting rod having a small end portion connected to a piston configured to be raised and lowered in a cylinder bore and a large end portion relatively rotatably connected to one end portion of the trigonal link; and a swing link having a base end portion (21 a) rotatably supported on the fulcrum shaft and a swinging end portion relatively rotatably connected to another end portion of the trigonal link, wherein the engine further comprises: an intake port provided in the crankcase and open to a crank chamber in the crankcase; a reed valve mounted in the intake port and configured to be opened by pressure reduction in the crank chamber accompanying an upward stroke of the piston; throttle means (25, 26) for controlling a flow rate of fresh air supplied to the crank chamber through the intake port when the reed valve is opened; a scavenging port configured to be opened by the piston in a downward stroke of the piston and guide the fresh air from the crank chamber into the cylinder bore; an exhaust port provided in a cylinder head and open to a top portion of the cylinder bore; and an exhaust valve configured to be opened and discharge exhaust gas in the cylinder bore to the exhaust port before the piston opens the scavenging port in the downward stroke of the piston. a crankcase; a crankshaft and a fulcrum shaft supported on the crankcase such that axes of the crankshaft and the fulcrum shaft are parallel to each other; a trigonal link having an intermediate portion rotatably supported on a crankpin of the crankshaft; a connecting rod having a small end portion connected to a piston configured to be raised and lowered in a cylinder bore and a large end portion relatively rotatably connected to one end portion of the trigonal link; and a swing link having a base end portion (21 a) rotatably supported on the fulcrum shaft and a swinging end portion relatively rotatably connected to another end portion of the trigonal link, wherein the engine further comprises: an intake port provided in the crankcase and open to a crank chamber in the crankcase; a reed valve mounted in the intake port and configured to be opened by pressure reduction in the crank chamber accompanying an upward stroke of the piston; throttle means (25, 26) for controlling a flow rate of fresh air supplied to the crank chamber through the intake port when the reed valve is opened; a scavenging port configured to be opened by the piston in a downward stroke of the piston and guide the fresh air from the crank chamber into the cylinder bore; an exhaust port provided in a cylinder head and open to a top portion of the cylinder bore; and an exhaust valve configured to be opened and discharge exhaust gas in the cylinder bore to the exhaust port before the piston opens the scavenging port in the downward stroke of the piston. an intake port provided in the crankcase and open to a crank chamber in the crankcase; a reed valve mounted in the intake port and configured to be opened by pressure reduction in the crank chamber accompanying an upward stroke of the piston; throttle means (25, 26) for controlling a flow rate of fresh air supplied to the crank chamber through the intake port when the reed valve is opened; a scavenging port configured to be opened by the piston in a downward stroke of the piston and guide the fresh air from the crank chamber into the cylinder bore; an exhaust port provided in a cylinder head and open to a top portion of the cylinder bore; and an exhaust valve configured to be opened and discharge exhaust gas in the cylinder bore to the exhaust port before the piston opens the scavenging port in the downward stroke of the piston.