Enhancing Marine Engine Efficiency: A Solution for Low-Speed Operation

Author: Daniel G. Teleoaca – Marine Chief Engineer

Marine engines are the unsung heroes of the shipping industry, tirelessly powering vessels across vast oceans and seas.

However, these workhorses face a unique challenge when it comes to low-speed operation. Low speed operation can cause various problems for marine engines, such as increased fuel consumption, reduced power output, higher emissions, and more wear and tear. The inefficiency of marine engines at lower speeds can have significant economic and environmental implications.

Preceding the implementation of emission-limiting regulations, some of the ships, especially containers, were generally engineered to achieve maximum cruising velocities of 30 knots. Presently, operators are obligated to comply with regulatory frameworks such as the carbon intensity indicator (CII) and the energy efficiency existing ship index (EEXI).

As a consequence, cruising veers off at approximately 18 knots, which is roughly two-thirds the speed for which the engines were originally designed. As a result, engines operate extremely inefficiently at low loads, consuming significantly more fuel and emitting significantly more CO2 than is required.

Without intervention, Wartsila predicted in 2022 that by 2023, over one-third of container ships would be non-compliant, based on an analysis of the global fleet. Moreover, in the absence of intervention, 80% of container ships will be classified under the lowest CII category by 2030.

In this article, we’ll explore the reasons behind this inefficiency and the options available to improve marine engine performance when running at low speeds.

Understanding the Inefficiency

Marine engines are designed to operate at a certain range of speed and load, depending on the type and size of the engine, the ship’s hull form, the propeller characteristics, and the operating conditions. When the engine operates outside this range, it can suffer from inefficiency and performance loss. There are several key reasons for this inefficiency:

  • Reduced Combustion Efficiency: A cause of marine engine inefficiency at low speed is the incomplete combustion of fuel in the cylinders. The combustion process in a marine engine depends on many factors, such as the fuel quality, the air-fuel ratio, the injection timing, the compression pressure, the ignition temperature, and the combustion duration. When the engine operates at low speed and load, some of these factors can be adversely affected, resulting in incomplete combustion of fuel. Incomplete combustion can lead to lower power output, higher fuel consumption, higher emissions of carbon monoxide (CO), hydrocarbons (HC), particulate matter (PM), and smoke, and more carbon deposits in the cylinders and turbocharger.

  • Mechanical Losses: At low speeds, the engine’s mechanical components, such as pistons, bearings, and crankshafts, experience higher frictional losses. This additional resistance leads to decreased engine efficiency. Moreover, the turbocharger is a device that uses the exhaust gas from the engine to drive a compressor that increases the air pressure and density in the intake manifold. The turbocharger improves the engine performance by allowing more air and fuel to be burned in each cylinder. The turbocharger efficiency depends on the pressure ratio between the exhaust gas and the intake air, which is called the boost pressure. The boost pressure is highest at high engine speed and load, when there is more exhaust gas available to drive the turbocharger. When the engine operates at low speed and load, there is less exhaust gas available, and the boost pressure drops. This means that less air is supplied to the cylinders, resulting in lower power output, higher fuel consumption, higher emissions of nitrogen oxides (NOx), and more turbo lag.

  • Propeller Inefficiency: One of the main causes of marine engine inefficiency at low speed is the mismatch between the engine and the propeller. The propeller is a device that converts the rotational energy of the engine into thrust force for propulsion. The propeller efficiency depends on the ratio of the propeller speed to the ship speed, which is called the advance ratio. The propeller efficiency is highest at a certain advance ratio, which corresponds to a certain engine speed and load. When the ship operates at low speed, the advance ratio increases, and the propeller efficiency decreases. This means that more engine power is wasted as friction and turbulence in the water, rather than converted into useful thrust.

Therefore, the effects of marine engine inefficiency at low speed can be summarized as follows:

  • Lower power output: The engine produces less power than it is capable of, resulting in lower ship speed or lower reserve power for maneuvering or emergency situations.
  • Higher fuel consumption: The engine consumes more fuel than it needs to produce a given amount of power, resulting in higher operating costs and lower profitability.
  • Higher emissions: The engine emits more pollutants than it should, resulting in environmental damage and potential non-compliance with emission regulations.
  • More wear and tear: The engine suffers from more stress and damage due to friction, corrosion, erosion, vibration, overheating, fouling, etc., resulting in higher maintenance costs and lower reliability.

Options to improve marine engine efficiency and performance at low speed

The inefficiency of marine engines at low speeds is a persistent challenge, but there are several innovative solutions available to mitigate this issue. Some of these options are:

  • Variable Geometry Turbochargers (VGTs): VGTs are turbochargers that can adjust their geometry to optimize airflow at different engine speeds. They help maintain higher combustion efficiency, even at low speeds, reducing fuel consumption and emissions.

  • Slow Steaming Strategies: Slow steaming involves deliberately operating a vessel at reduced speeds to conserve fuel. It has become a popular strategy in the shipping industry, allowing ships to run more efficiently at lower RPMs, thus saving fuel.
  • Dual-Fuel Engines: Dual-fuel engines are designed to run on a combination of natural gas and diesel fuel. These engines offer improved combustion efficiency and emissions control, making them an attractive option for low-speed operation.

  • Waste Heat Recovery Systems: Waste heat recovery systems capture and reuse the heat generated by the engine’s exhaust. They can be used to produce additional power or drive other ship systems, enhancing overall energy efficiency.

  • Upgraded Propellers: Shipowners can consider investing in more efficient propeller designs, specifically tailored to their vessels’ operating profiles. Modern propeller designs are more adaptable to a wide range of ship speeds.

  • Improved Hull Design: The vessel’s hull design can also impact its performance at lower speeds. Optimized hull shapes can reduce hydrodynamic resistance and improve overall efficiency.

  • Hybrid Power Systems: Some vessels employ hybrid power systems that combine traditional diesel engines with electric propulsion. This setup allows for efficient power delivery at various speeds, including low-speed operation.

  • Engine derating: Engine derating is a method of reducing the maximum power output of an engine by adjusting its settings or components. Engine derating can improve the engine efficiency at low speed by reducing the mismatch between the engine and the propeller, and by optimizing the combustion process and the turbocharger operation. Engine derating can also reduce the emissions of NOx, CO, HC, and PM. However, engine derating can also reduce the reserve power of the engine, and may require the approval of the engine manufacturer and the classification society.
  • Turbocharger cut-out: Turbocharger cut-out is a method of disconnecting one or more turbochargers from an engine by closing a valve or opening a bypass. Turbocharger cut-out can improve the engine efficiency at low speed by increasing the boost pressure and the air supply to the cylinders. Turbocharger cut-out can also reduce the emissions of CO, HC, and smoke. However, turbocharger cut-out can also increase the emissions of NOx, and may cause the turbocharger to overheat or surge.

In conclusion, addressing the inefficiency of marine engines at low speeds is critical for both economic and environmental reasons. The shipping industry has made significant strides in developing technologies and strategies to improve engine efficiency during slow steaming and low-speed operation. These solutions not only reduce fuel consumption but also contribute to lower emissions and a more sustainable maritime industry. As technology continues to advance, marine engines are likely to become more versatile, making them more efficient across a broader range of operating speeds, ultimately benefiting the entire global shipping industry.

If you have any questions regarding above, please feel free to use our existing forum Seafarer’s World, Telegram Chief Engineer’s Log Chat or Instagram and will try to answer to all your queries. You can use the feedback button as well!

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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.

If you like my posts, please don’t forget to press Like and Share. You can also Subscribe to this blog and you will be informed every time when a new article is published. Also you can buy me a coffee by donating to this website, so I will have the fuel I need to keep producing great content! Thank you!

Source and Bibliography:

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