Lesson 2/Learning Event 2
Learning Event 2:
DESCRIBE PRINCIPLES OF OPERATION AND CONSTRUCTION OF TWOSTROKE-CYCLE
The same basic rules hold true for the two-stroke-cycle engine, whether it is a compression-ignition or a spark-
ignition type of engine. Because two-stroke-cycle compression-ignition engines have ONE power stroke for
each turn, they produce twice as many power strokes as the four-stroke-cycle engine, and the flywheel can still
be lighter. Let's find out how the two-stroke-cycle diesel engine is different from the four-stroke-cycle diesel
engine that was just discussed.
The same events of intake, compression, power, and exhaust take place in all compression-ignition engines. On
the four-stroke-cycle engine, all four events occur during four strokes of a piston and two turns of the
crankshaft. In the two-stroke-cycle engine, all four events occur during two strokes of a piston and in one turn
of the crankshaft. To make this possible, the engine must be built a little different from the four-stroke-cycle
type of engine.
The present military two-stroke-cycle diesel engine, such as that used in the M911 tractor truck, does not have
any intake valves. Instead, it has inlet ports about halfway down the cylinder sleeve. There is no intake stroke
in the two-stroke-cycle diesel engine to draw air into the cylinder. A Roots-type air blower is used to pump air
into the cylinder. The up and down movement of the piston opens and closes the inlet ports causing them to
function as inlet valves. The engine has exhaust valves located in the cylinder head. The exhaust valves are
operated by a camshaft, tappets, pushrods, and rocker arms like most other engines. But, the operating
principles of this engine are different from most other engines. Let's find out how it operates.
The Roots-type blower is made with two rotors with lobes that mesh and work together as a pump to pack air
into the engine.
The blower housing mounts the two rotors and driving gears. Air is drawn in through openings on one side of
the housing and is exhausted through holes on the opposite side. The blower is driven from the engine
crankshaft by gears and shafts. On in-line engines, the blower is bolted to the side of the cylinder block. It is
mounted on the top between the cylinder banks of the V-type engine.