Figure 1-13.Two-stroke cycle diesel engine.
Two-Stroke Cycle Diesel Engine
A two-troke diesel engine (fig. 1-13) shares the
same operating principles as other internal combustion
engines. It has all of the advantages that other diesel
engines have over gasoline engines.
A two-stroke diesel engine does not produce as
much power as a four-stroke diesel engine; however, it
runs smoother than the four-stroke diesel. This is
because it generates a power stroke each time the piston
moves downward; that is, once for each crankshaft
revolution. The two-stroke diesel engine has a less
complicated valve train because it does not use intake
valves. Instead, it requires a supercharger to force air
into the cylinder and force exhaust gases out, because
the piston cannot do this naturally as in four-stroke
The two-stroke diesel takes in air and discharges
exhaust through a system called scavenging.
Scavenging begins with the piston at bottom dead
center. At this point, the intake ports are uncovered in
the cylinder wall and the exhaust valve is open. The
supercharger forces air into the cylinder, and, as the air
is forced in, the burned gases from the previous
operating cycle are forced out (fig. 1-14).
COMPRESSION STROKE. As the piston
moves towards top dead center, it covers the intake
ports. The exhaust valves close at this point and seals
the upper cylinder. As the piston continues upward, the
air in the cylinder is tightly compressed (fig. 1-14). As
in the four-stroke cycle diesel, a tremendous amount of
heat is generated by the compression.
POWER STROKE. As the piston reaches top
dead center, the compression stroke ends. Fuel is
injected at this point and the intense heat of the
compression causes the fuel to ignite. The burning fuel
pushes the piston down, giving power to the crankshaft.
The power stroke ends when the piston gets down to the
point where the intake ports are uncovered. At about this
point, the exhaust valve opens and scavenging begins
again, as shown in figure 1-14.
The operation of the valves in a timed sequence is
critical. If the exhaust valve opened in the middle of the
intake stroke, the piston would draw burnt gases into the
combustion chamber with a fresh mixture of fuel and
air. As the piston continued to the power stroke, there
would be nothing in the combustion chamber that would