What you need to know about main engine continuous low load operation

I believe that many of you, have heard in the last couple of years about running the main engine at low load or slow steaming due increasing fuel price and lately due more stringent environmental regulations.

The 2-stroke engines are designed and optimized for operation in the load range above 60 % CMCR, but is possible to use them at continuous low loads down to 10% CMCR if we pay special attention as they are some recommendations on what needs to be observed when operating the engine at loads lower than 60 % CMCR.

It is very important to be aware that at lower engine load between approximately 60% and the auxiliary blower switch-on/off point, the turbocharger efficiency is relatively low and within this power range the engine operates with a lower air/fuel ratio resulting in higher exhaust gas temperatures.

The electronic controlled engines are more suitable for continuous low load operation than the conventional engines, due to their electronically  controlled common rail injection system.

Example of common rail injection system

These engines allow for higher injection pressure and selective fuel injector cut-off at very low loads, thus reducing excessive carbon deposits, exhaust gas economiser and turbocharger fouling.

The engine makers have issued a set of recommendations that should be observed, in order to limit the adverse affects of continuous low load operation as much as possible. The following needs to be in order:

    • The fuel injection valves should be in good working order.
    • When operating on HFO, the fuel viscosity required at the fuel pump inlet for conventional engines must be in the range of 13 to 17 cSt; for electronically controlled engines must be in the range 10 to 20 cSt. However, it is recommended to maintain the viscosity at the lower end of the range 13 to 17 cSt as specified in the engine operating manual, without exceeding 150°C at engine inlet. Sufficient trace heating of the fuel system on the engine must be ensured.
    • Keep the LT cooling water close to upper limit at 36°C in order to maintain the optimum scavenge air temperature and to minimize effects of possible cold corrosion.
    • For DF (dual fuel) engines operating in gas mode or (Low Sulphur) liquid fuels keep the LT cooling water set point at 25 °C to maintain a low (optimized) scavenge air temperature.
    • Clean the turbocharger as per manufacturer’s instruction manual.

Apart from above the following should be observed, monitored and adjusted accordingly:

    • The cylinder oil feed rate is load and sulphur dependent and is recommended to be properly adjusted as per the fuel that it is in use (about cylinder lubrication you can read in here). Frequent piston underside inspections must be carried out to monitor piston running conditions and signs of over-lubrication, as over-lubrication can lead to scuffing due to hard alkaline deposits on the piston crown.
    • The exhaust gas temperature after the cylinders should be kept above 250°C in order to reduce and avoid cold corrosion, fouling of exhaust gas receiver and turbocharger nozzle ring. If the exhaust gas temperature drops below this value, the engine load should be increased.
    • If the exhaust gas temperature gets too high (>450°C after cylinders), the auxiliary blower may be switched to “continuous running”. However, it has to be taken into account that not all auxiliary blowers and circuit breakers may be suitable for continuous running at electrical loads above nominal current.
    • Repeatedly switching on/off of the auxiliary blower must be avoided. If necessary, the auxiliary blower controls have to be switched to “manual operation”, or operation in this load area has to be avoided.
    • Inspect and lubricate the bearings more frequently if considered necessary due to increased operation of the blower. This also includes the inspections of the non-return valves for the scavenging air.
    • A concern during continuous low load operation is the accumulation of unburned fuel and lubricating oil in the exhaust manifold, as such deposits can ignite after the engine load is increased again. This may result in severe damage to the turbocharger due to sudden over-speeding. Therefore, it should be considered to periodically (twice a week) increase the engine load as high as possible, however at least 70% for at least 1 hour, in order to burn off accumulated carbon deposits. The load-up has to be done very carefully (i.e. during 2 hours) in order to avoid adverse piston running conditions due to carbon that has built up on the crown land of the piston head and to avoid possible exhaust manifold fire.
    • Exhaust manifold and other related components (scavenging air receiver, exhaust gas valves, turbocharger grid, etc.) need more frequent inspections and possible cleaning. Depending on result of inspections, the regular engine load-up intervals might be adapted if no excessive deposit accumulation is detected.
    • An economiser with closely-spaced fins may also require more frequent soot blowing.
    • On Dual-Fuel (DF) engines operating in gas mode, the described regular loading up to high loads is not required. The deposit formation is minimal compared to diesel mode operation.

In order to improve the piston running performance and reduce the risk of cold corrosion in cylinder liners, when the engine is continuously running at low loads, the temperature range of the cylinder cooling water outlet is increased. For example, Wärtsilä recommends keeping the cylinder cooling water outlet temperature as close as possible to the alarm limit. As a consequence of the increase of the cylinder cooling outlet water temperature, the respective alarm and slowdown settings need to be adjusted in some engines as well.

Example of alarm settings on Wartsila engines

In order to further optimize the engine operation at low load, Wärtsilä has developed A Slow Steaming Upgrade Kit that involves cutting out of a turbocharger.

Example of Wartsila Slow Steaming Kit

This increases the scavenge air delivery at low load for better combustion and more optimum temperatures of engine components. With this kit the following is achieved:

    • With the increased scavenge air pressure the auxiliary blower on/off threshold is at lower loads compared to engines with all turbochargers operative.
    • A considerable reduction in SFOC with cut out turbocharger and increased scavenge air pressure in the low-load range.
    • Due to better combustion at lower loads the risk of turbocharger and economizer fouling is decreased and the formation of deposits due to unburnt fuel is reduced.

The time interval between engine load-up to burn off carbon deposits can be increased based on inspection results. In order to burn off the deposits, a high enough exhaust gas temperature at turbine inlet is needed. The engine needs to be loaded up until the exhaust gas temperature at turbine inlet corresponds to 380°C. If this temperature is not possible to reach, the engine needs to be loaded up to the maximum load that can be reached with one turbocharger cut-out.

In combination with the above described slow steaming kit, Wärtsilä also recommends the installation of electronically controlled cylinder lubrication, called Retrofit Pulse Lubrication System which provides optimal lubrication due to the precisely timed feeding of oil into the piston ring pack and savings in lubricating oil.

If you have any questions regarding above, please feel free to use our existing forum Seafarer’s World and will try to answer to all your queries.

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Source and Bibliography:

  • Wartsila 2 Stroke –  Service letter RT174 – 27/11/2014

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