Two-stroke I.C. engines

Claim 1

A two-stroke internal combustion throttle controlled engine with crankcase compression and scavenging, comprising: a cylinder having a wall, a piston slidable inside the cylinder wall upwardly by a compression stroke to a top dead center position and downwardly by a working stroke to bottom dead center position, a crankcase below said cylinder and having an inlet port for receiving a throttle controlled combustible air-fuel gaseous mixture which is subject to compression within the crankcase by said piston during its downward working stroke to effect a relatively low mixture pressure at low load operation with partial throttle and a relatively high mixture pressure at full load operation with full throttle, a plurality of transfer ports formed in said cylinder wall with their upper limits all at the same predetermined distance below the top dead-center piston position and being controlled by movement of said piston so that all the transfer ports will start to be uncovered simultaneously by the descending piston, exhaust port means in said cylinder wall and means to ensure operation thereof before said transfer ports are uncovered by said piston, a plurality of transfer port passages extending through the cylinder wall and communicating at their lower ends with the interior of said crankcase and at their upper ends with said transfer ports, and hence with the upper working space in the cylinder above the piston when the piston reaches a predetermined level on its downward working stroke and uncovers the transfer ports, at least one but not all of said transfer port passages having an unrestricted gas flow path from end to end for scavenging the said upper working space in the cylinder by the said mixture compressed during each downward working stroke, and means for improving the scavenging effectiveness of said mixture and combustion conditions during said low load operation by increasing then the velocity of said mixture via at least said one passage and respective transfer port into said upper working spaces, said velocity increasing means being characterized by gas pressure responsive check valve means disposed in at least one other of said transfer passages and responsive solely to the pressure of said mixture in said crankcase and being normally biased towards its closed position for restricting passage of said low pressure mixture through said other transfer passage during said low load operation and thereby increasing the said velocity at said one port to cause said improved scavenging and combustion conditions, said valve means being automatically opened up by the relatively high pressure of said crankcase mixture at said full load operation and by intermediate crankcase pressures which exceed the pressure in said upper working space in said cylinder.