Crankcase scavenged four-stroke engine

A crankcase scavenged and lubricated four-strok engine (1) comprising:

• a cylinder (2)
• a piston (3) movably mounted in the cylinder in order to rotatingly drive a crankshaft (4) via a connection rod (5);
• the cylinder and the piston conjointly delimiting a combustion chamber (6);
• the cylinder having an intake opening (7) and an intake valve (8) for opening and closing the intake opening;
• the cylinder having an exhaust opening (9) and an exhaust valve (10) for opening and closing the exhaust opening, followed by an exhaust duct (28);
• a mixture-preparation device (15) for supplying an air/fuel-lubricant mixture or for supplying in two steps an air/lubricant mixture and an air/fuel mixture;
• a crankcase (11) connected to the cylinder and delimiting together with the underside of the piston (3) a crankcase volume (12), to which at least air (13) and lubricant (14) is supplied by the mixture preparation device;
• an intake channel (16) connecting to the crankcase or cylinder to supply at least air (13) to the crankcase volume (12);
• an overflow channel (17) connecting to the crankcase or cylinder and to the intake opening (7) with intake valve (8) to supply the intake at least air and lubricant from the crankcase volume;
• a valve drive assembly (18) driven by said crankshaft (4) for actuating the intake valve (8) and the exhaust valve (10), characterized in that it further comprises:
• a valve drive assembly housing (19, 20, 21, 21′, 22) comprising one or more sections (19, 20, 21, 21′, 22) which is/are separate from the crankcase volume but in communication exclusively with the crankcase volume through at least one small size passage (23, 24, 25, 26, 27), and each section (19, 20, 21, 21′, 22) of the valve drive assembly housing has a lowest part situated higher in the axial direction of the cylinder bore, than the lowest part of the crankcase.