The first successful design of a three-port two-stroke engine was patented in 1889 by Joseph Day & Son of Bath. This employed the underside of the piston in conjunction with a sealed crank-case to form a scavenge pump (‘scavenging’ being the pushing-out of exhaust gas by the induction of fresh charge).
This engine completes the cycle of events – induction, compression, power, and exhaust – in one revolution of the crankshaft or two complete piston strokes.
Crankcase-to-cylinder mixture transfer. The piston moves down the cylinder and initially uncovers the exhaust port, releasing the burnt exhaust gases to the atmosphere. Simultaneously the downward movement of the underside of the piston compresses the previously filled mixture of air and atomised petrol in the crankcase.
Further outward movement of the piston will uncover the transfer port, and the compressed mixture in the crankcase will then be transferred to the combustion-chamber side of the cylinder. The situation in the cylinderwill then be such that the fresh charge entering the cylinder will push out any remaining burnt products of combustion – this process is generally referred to as cross-flow scavenging.
Cylinder compression and crankcase induction
The crankshaft rotates, moving the piston in the direction of the cylinder head. Initially the piston seals off the transfer port, and then a short time later the exhaust port will be completely closed. Further inward movement of the piston will compress the mixture of air and atomised petrol to about one-seventh to one-eighth of its original volume At the same time as the fresh charge is being compressed between the combustion chamber and the piston head, the inward movement of the piston increases the total volume in the crank-case so that a depression is created in this space.
About half-way up the cylinder stroke, the lower part of the piston skirt will uncover the inlet port, and a fresh mixture of air and petrol prepared by the carburettor will be induced into the crankcase chamber. Cylinder combustion and crankcase compression.
Just before the piston reaches the top of its stroke, a spark-plug situated in the center of the cylinder head will be timed to spark and ignite the dense mixture. The burning rate of the charge will rapidly raise the gas pressure to a maximum of about 50 bar under full load.
The burning mixture then expands, forcing the piston back along its stroke with a corresponding reduction in cylinder pressure. Considering the condition underneath the piston in the crankcase, with the piston initially at the top of its stroke, fresh mixture will have entered the crankcase through the inlet port.
As the piston moves down its stroke, the piston skirt will cover the inlet port, and any further downward movement will compress the mixture in the crankcase in preparation for the next charge transfer into the cylinder and combustion-chamber space.
The combined cycle of events adapted to a three cylinder engine and show the complete cycle in terms of opening and closing events and cylinder volume and pressure changes respectively.