Marine Diesel Engines - Decreasing Fuel Consumption and Exhaust Gas Emissions in Transportation - page 160

1.1

exhaust gas temperature
before turbine
fuel consumption
output (constant)
charging pressure

1.0

air throughput

0.9

0.8

0 10 20 30 40 50

pressure of exhaust gas before turbine [mbar]

Fig. 11.3 Impact of exhaust gas counter pressure on engine service data

11.3.3 Impact of Bad Weather on Engine Operation

Marine diesel engines need reserve power to compensate for increases in resis-
tance of the ship's hull and decreased efficiency of the engine caused by wear and
contamination. Minimum efficiency reserves must be 10-15%.
The engine might only keep 75-80% of its theoretical efficiency measured at
the dockyard under test drive conditions. The reserve efficiency of 20-25% can
cover the resistance increases and the efficiency losses without reducing the ship's
speed at sea or at inland water ways.
In bad weather, speed has to be decreased depending on the power of the waves
and wind. The captain has to alter course according to the direction of the waves
and the wind to save fuel and maintain the ship at slower speeds. Bad weather
increases the temperature and the counter pressure in the exhaust gas in all relevant
elements back to the outlet valves. Sailing in bad weather for a long time can
increase the risks of overheating the engine and of high-temperature corrosion in
the exhaust gas after treatment system [ 22 ].

11.3.4 Cooling Circuit

The heat produced by burning in the combustion chamber must be conducted
outside by a cooling medium, e.g., by air or water. Main ship engines usually use
water cooling. The cooling water may have a maximum temperature of 80-90C
(176-194F). Auxiliary engines often use air cooling. Tears in hot material sec-
tions which can arise from high temperature differences between heated and
cooled elements can be observed by micro temperature sensors. Dangerous situ-
ations have to be always avoided.
Ship engines have two independent cooling water units:
• Internal circulation unit which uses fresh water; and
• External circulation unit which uses sea water [ 23 ].