Page 23 - Introduction to Marine Engineering

Diesel engines 11

Fuel injector
Cylinder

Piston
Exhaust
port _
Scavenge
port

Connecting rodi

Crank
Exhaust
(b)

Rotation

Compression
(d)
Figure 2.3 Two-stroke cycle

are forced apart—one up, one down—by the expanding gases (Figure
2.5{a)). The upper piston opens the exhaust ports as it reaches the end
of its travel (Figure 2.5(b)). The lower piston, a moment or two later,
opens the scavenge ports to charge the cylinder with fresh air and
remove the final traces of exhaust gas (Figure 2.5(c)). Once the pistons
reach their extreme points they both begin to move inward. This closes
off the scavenge and exhaust ports for the compression stroke to take
place prior to fuel injection and combustion (Figure 2.5(d)). This cycle is
used in the Doxford engine, which is no longer manufactured although
many are still in operation.