Case Study -1
The vessel main engine (6L67GFC) had suffered from high exhaust
temperature on unit No.4 for several weeks. Following actions had been taken on
unit No.4 but without improvement:
o Replaced exhaust valve
with a ready spare.
o Replace fuel injector
o Replace fuel pump
o Port inspection
through scavenging receiver reviewed that the cylinder was in reasonably good
o Renewed the piston
rings on the exhaust actuator and found the condition of the oil cylinder was
o The engine speed was
limited to 95 rpm due to the high exhaust temperature (about 460 degrees at 103
o The indicator cards
taken by the chief engineer shows that exhaust valve No.2 close and open too
early (high exhaust temperature, high compression pressure and longer
"tail" on the PV diagram).
o Based on the above
information, we decided to check the fuel and exhaust timing on unit No.2 and
fuel and exhaust timing checked:
Cylinder No. 2 & 4
Exhaust valve open at (A) 7.5 mm
lift 85 108.4
Exhaust valve close at (B) 6.5 mm
lift 226 249.5
Exhaust cam lead (measured) 24.5
Exhaust cam lead (shop trial record)
Fuel cam lead (measured) 18.2 mm
Fuel cam lead (shop trial record) 18.5
mm 17.6 mm
The fuel cam lead has no change, but
the exhaust cam lead of the No.2 unit was in the position of about 24 degrees
Sea trial has carried out after the
correction of exhaust cam timing on No.2. The engine speed has increased to 110
rpm within a short period.
It confirmed that the engine was
running in normal condition again with a maximum exhaust temperature of 378
degrees at 110rpm.
real reason behind the high exhaust temperature in unit No.4 was approved to be
the wrong exhaust timing on unit No.2. This situation seems very strange.
However, it can explain as follows.
firing order of the engine is 1-5-3-4-2-6.
the engine runs in an ahead direction unit No.2 fire right after unit No.4, the
No.4 exhaust valve opens at 288.4 degrees and the scavenging port start opening
at 319 degrees.
exhaust gas flows out by itself between 288.4 and 319 degrees and start to be
forced out by scavenging air between crank angle 319 and 41 degrees.
to the wrong exhaust timing, the No.2 exhaust valve opens early at 324.8
degrees instead of 349 degrees. During this time, the hot exhaust gas gash out
from unit No.2 with higher pressure results in pressure pulsation of the
exhausted receiver. During a narrow range, the scavenging angle of unit No.4,
the exhaust pressure was even higher than the scavenging pressure. The
consequence is that the breath of unit No.4 was incomplete, and poor combustion
took place during the next cycle due to lack of air.
the pressure fluctuation, the temperature (where exhaust thermometer exposes
to) also fluctuated. Based on the same theory, the No. 4 exhaust thermometer
exposed longer timing in the hot gas than other units.
to the early closing of the No.2 exhaust valve (Just 5 degrees after the
scavenging port closed), the compression pressure of unit No.2 was very high
(56 bar instead of 45-47 bar in other units).
the exhaust or fuel cam shifted, it is usually retarded as the engine normally
runs ahead. However, the cam moved possibly in an advanced position if the
engine started in an astern order while the incident took place.