SFOC optimisation for IMO Tier II engines
One of the goals in the marine industry today is to reduce the impact of carbon dioxide emissions and thereby to reduce fuel consumption across the load range.
The need to cut fuel cost may often result in operation of the ship at reduced ship speed and, consequently, at reduced engine load. According to MAN Diesel & Turbo, this has placed more emphasis on operational flexibility in terms of SFOC (specific fuel oil consumption) at part/low-load operation of the main engine. However, MAN points out that on two-stroke engines, reduction of the SFOC can adversely affect NOx emissions, taking the engine out of compliance with IMO Tier II demands.
Depending on the intended operation range of the main engine, the engine may be SFOC-optimised in one of several percentage SMCR (specified maximum continuous rating) ranges.
The high-load range corresponds to a modern, normal, standard-tuned engine. For part-load and low-load optimisation of engines with high-efficiency turbochargers designed to IMO Tier II requirements, various engine tuning methods are available.
An SFOC reduction of 5 g/kWh makes it possible to reduce fuel cost by a maximum of about 3% of the specific consumption. The daily consumption will be reduced further due to the low load.
In general, NOx emissions will increase if SFOC is reduced and vice versa. MAN says that in the standard configuration, its engines are optimised close to the IMO NOx limit, so a significant fuel saving will take the engine outside Tier II NOx limits.
The IMO NOx limit is given as a weighted average of the NOx emission cycle values at 25, 50, 75 and 100% load:
5% x NOx (25) + 11% x NOx (50) + 55% x NOx (75) + 29% x NOx (100).
This relationship can be used to shape or tailor the SFOC profile over the load range, i.e. the SFOC can be reduced at low load at the expense of higher SFOC in the high-load range without exceeding the IMO NOx limit.
The variable exhaust valve timing arrangement on the ME/ME-C electronic engines allows greater potential for reducing SFOC compared with the mechanically-actuated valves on the MC/MC-C/ME-B engine types.
The engine tuning methods available are as follows:
Exhaust gas bypass
Exhaust gas bypass (EGB) technology, individually tailored at around 6% EGB, is available for both the ME/ME-C and MC/MC-C/ME-B engine families. SFOC potential is better on the ME type engine, where EGB can be combined with variable exhaust valve timing.
The turbochargers on the ME/ME-C engines for part load and low load are matched at 100% load with fully open EGB. These percentages vary slightly for other engine types. EGB allows SFOC to be decreased at low load at the expense of higher SFOC at high load.
The variable turbine area or turbine geometry (VT) method requires special parts allowing the turbocharger(s) on the engine to vary the area of the nozzle ring. It is available for both the ME/ME-C and MC/MC-C/ ME-B type engines, and as with EGB, VT offers greater SFOC potential on the ME/ME-C type engines, where it can be combined with variable exhaust valve timing.
The nozzle ring area has a maximum at the higher engine load range. When the engine load for is reduced, the area gradually starts to decrease.
Again, SFOC is reduced at low load at the expense of higher SFOC at full load.
Engine control tuning
Engine control tuning (ECT) can be implemented without change of engine components, and can be implemented as an engine running mode. Only pmax and engine control parameters are changed. Because it used variable valve timing and injection profiling, ECT is only available for ME/ME-C engine types. Two different service optimisation possibilities are available: part-load optimisation reduces SFOC at all loads below about 85% compared with a standard-tuned engine; and low-load optimisation reduces SFOC at all loads below about 70%, at the expense of higher SFOC at higher load.
Random shifting between the part-load and low-load modes is not allowed by the authorities. A mode shift in case of a change in trade pattern is permitted if reported and approved by the flag state representative, usually a classification society. Hence, on a longer term basis, the owner can select one or the other modes for the engine, provided the authorities are informed.
Fuel saving potential
Reduced CO2 emissions, and thereby lower fuel consumption, is an increasing demand that will be even stronger in the future. This may result in lower service ship speeds and, the lower the ship speed, the lower the required propulsion power and, thereby, the lower the fuel consumption.
However, most shipowners still want to retain the ability to operating ships at the earlier higher ship speed, at least occasionally. This means that the SMCR power of the main engines may still be maintained, while the changed trading pattern of the ship may result in operation with a relatively lower load on the main engine, with only few days of operation on high engine loads.
Under such conditions, the application of one of the previously described engine tuning methods, such as Variable Turbine area, optimised for low-load operation, will help reduce fuel consumption.
For a typical trading pattern, the potential specific fuel saving calculated for a 6S80ME-C8.2 engine is about 2.6%, or for a 6S80MC-C8.2 engine the corresponding calculation shows a 1.5% saving. In all cases, the daily fuel consumption will be lowered mostly because of the lower power requirement due to the lower ship speed.
It should be noted that use of these engine tuning methods will result in a lower exhaust gas temperature at low-load operation, which has to be considered when exhaust boilers are employed.
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