TWO-STROKE TRACTOR ENGINES

Two-stroke tractor engines are invariably of the compression-ignition type. Some small cultivators have two-stroke spark-ignition engines also.

In the two-stroke cycle each downward stroke of the piston is first a power stroke and then an exhaust stroke, and each upward stroke of the piston provides for replenishment of the air charge as well as its subsequent compression. The air for combustion assists removal of the exhaust gases; it is therefore known as 'scavenge' air, and its admission to the engine as 'scavenging’. For efficient scavenging a supply of air is required at a pressure above atmospheric; this can be secured by provision of a 'blower' or by adapting the engine crankcase as an air pump ('crankcase compression scavenging'). A two-stroke engine with a rotary blower of lobed type is shown in Figure 4; in this design exhaust valves are used in the cylinder head, so that the advantages of 'uniflow' scavenging are obtained - i.e. the incoming air sweeps the spent charge out of the cylinder in a generally uniform direction, with relatively little intermixing between the two.

Crankcase-compression scavenging is employed in the exceptionally simple single-cylinder 'valveless' engines of which considerable numbers have been built. An example is shown in Figure 5, and the sequence of events in an engine of this kind is shown in the lower diagram in Figure 6. On the compression stroke of the piston, air is inducted into the crankcase through a non-return valve. On the power stroke the air in the crankcase is compressed to a pressure slightly (perhaps 2 lb./sq.in.) over atmospheric. Near the end of this stroke the piston uncovers an exhaust port in one side of the cylinder wall, and then a 'transfer port' in the other, communicating with the crankcase; the top of the piston (the 'piston crown') is so shaped that the air thus entering the cylinder takes a path promoting scavenging and sweeps residual burnt gases out through the exhaust port. As the piston begins the next stroke it covers both ports and a new cycle follows.