The Work of a Two-Stroke Engine

A two-stroke cycle is completed in two strokes: the compression and the power strokes, or one revolution of the crankshaft, while a four-stroke cycle has two revolutions. The operations to remove the burnt gases and to fill the cylinder with a fresh charge of air in a two-stroke engine are performed near the bottom dead centre by means of a separate air pump or a blower, which takes in air from the atmosphere and delivers it into an air chamber.

There are two exhaust valves in the cylinder head of a two-stroke engine instead of one intake and one exhaust valves as it is in a four-cycle engine. To deliver air into the cylinder a row of holes or ports is cut in the cylinder wall. These intake ports are closed by the piston when it is at the top of its stroke but they are opened when the piston is near the bottom of its stroke. The ports are connected to the blower outlet by the air chamber. When the piston is at the bottom of its stroke the intake ports and the exhaust valves are open. A slightly compressed air enters the cylinder through the intake ports and pushes out the burnt gases, left from the previous cycle through the exhaust valves.

When the piston is about a quarter of the way up, the exhaust valves and the intake ports are closed. The exhaust gases are blown out. Now the cylinder is full of fresh air. The rest of the upward stroke is a compression stroke which compresses the air into a small space at the top.

Before the piston reaches its top dead centre the fuel nozzle delivers a spray of fuel into the chamber full of hot compressed air. Ignition and expansion take place as in a four-cycle engine, and the piston goes down on its power stroke. When the piston is about three quarters of the way down, the exhaust valves open and the burnt gases begin to escape. As the piston continues to go down it opens the intake ports and fresh air is blown into the cylinder.

This air, which has a slightly higher pressure than the hot gases in the cylinder, pumps out the hot gases through the valves. This operation is called scavenging. The air which is admitted is called scavenge air and the ports through which the scavenge air is admitted are called scavenge ports.

After the operation of scavenging, the cylinder is full of fresh air, the piston is at the bottom dead center and the cycle is completed, all in one revolution. The compression and the power strokes of a two-cycle engine are not much different from the strokes of a four-cycle engine. The exhaust and intake actions take place nearly together and in a much shorter time.

In a two-cycle engine the piston cannot push the exhaust gases out, they are blown out by the air under pressure. Air is not admitted into the cylinder by the piston, it is forced in.

instead of, prep вместо

row, n ряд

hole, n отверстие

by means of при помощи, посредством

scavenge, v продувать

scavenging продувка

remove, v отводить, удалять

blower, n воздуходувка

blower outlet отверстие воздуходувки

slightly, adv слегка

blow out выталкивать

in order to для того, чтобы

force in, v нагнетать

fill with, v наполнять

chamber, n камера

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