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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Propelling Ahead: Major Trends in Marine Propulsion Define Maritime Landscape in 2023

Date: September 01, 2023

By: Daniel G. Teleoaca – Marine Chief Engineer

The maritime propulsion landscape is undergoing a remarkable transformation in 2023, fueled by innovation and a growing commitment to sustainability. As the world seeks cleaner and more efficient modes of transportation, the maritime industry is embracing a range of major trends in propulsion that promise to reshape the way ships navigate the seas while reducing their environmental impact.

1. Electrification Takes Center Stage

Electrification has emerged as a defining trend in marine propulsion, revolutionizing the way ships are powered. From ferries and cruise liners to cargo vessels, electrification is reshaping maritime transportation.

Source and Credit: Kawasaki Heavy Industries

Electric propulsion systems, powered by high-capacity batteries, offer unparalleled efficiency, lower emissions, and reduced noise pollution. This trend aligns perfectly with the industry’s push toward environmental responsibility and sustainable practices.

2. Hydrogen Fuel Cells Gain Momentum

The maritime sector’s quest for cleaner energy sources has ushered in the rise of hydrogen fuel cell propulsion.

Source and Credit: Green Car Congress

These cells, which generate electricity by combining hydrogen with oxygen, emit only water vapor as a byproduct. Major shipping companies are investing in research and development, making strides towards hydrogen-powered vessels. This trend not only reduces emissions but also showcases the industry’s commitment to embracing innovative technologies for a greener future.

3. Sustainable Biofuels and LNG

Biofuels derived from renewable sources and liquefied natural gas (LNG) are gaining traction as alternative propulsion solutions.

Source and Credit: Wartsila

These fuels offer reduced carbon emissions compared to traditional fossil fuels, contributing to a more sustainable maritime industry. LNG, in particular, has seen increased adoption due to its potential to significantly lower greenhouse gas emissions, aligning with international emission reduction goals.

4. Advanced Hybrid Systems

Hybrid propulsion systems that combine traditional engines with electric or alternative power sources are becoming a staple in modern marine engineering.

Source and Credit: Wartsila

These systems optimize efficiency by seamlessly switching between power sources based on operational requirements, resulting in reduced fuel consumption and emissions. Hybrid solutions offer flexibility and versatility, making them well-suited for a variety of vessel types and applications.

5. Digital Twin Technology for Performance Optimization

Digital twin technology, a virtual replica of a vessel’s systems and operations, is playing a pivotal role in optimizing propulsion performance. By leveraging real-time data and simulations, ship operators can fine-tune propulsion systems for maximum efficiency and reliability. This trend enhances operational decision-making, reduces downtime, and prolongs the lifespan of propulsion equipment.

Charting a Sustainable Course Forward

As the maritime industry adapts to evolving environmental regulations and societal expectations, these major propulsion trends highlight a collective effort to transition towards a more sustainable and efficient maritime future. From electrification and hydrogen fuel cells to advanced hybrid systems and digital innovations, the maritime sector is harnessing the power of innovation to navigate towards cleaner, greener waters. The trends of 2023 are not just shaping the way ships move; they are steering the industry towards a more responsible and eco-conscious future.

What you need to know about vessel thrusters (bow and stern)

The purpose of the thruster units is to turn the vessel when is operating at slow speeds or when is not under way, to keep the ship in position in a cross wind and to move the ship towards or away from a mooring position as required. The thrust is produced by rotation of a propeller unit which is housed in a transverse cylindrical ducting, where the propeller unit is rotated by means of a vertical electric motor via bevel gears.

Example of a thruster unit

The propeller blade pitch is controllable in order to obtain the desired magnitude and direction of thrust.

The thruster comprises of a number of separate sections:

    • The electric motor unit with drive shaft and bevel gearing driving the propeller unit hub;
  • Example of thruster electrical motor

    • The propeller unit with blades mounted in the hub;
  • Example of thruster propeller

    • The hydraulic unit which changes the pitch of the propeller blades;
  • Example of hydraulic power pack

    • The control system which regulates the blade pitch in accordance with demand from the bridge.

Example of thruster controller

Power is transmitted from the electric motor through the flexible coupling, input shaft and bevel gears to the propeller shaft, rotating the propeller in a single direction.

The propeller part, usually consists of four propeller blades and a propeller hub. The propeller hub and gear case house a hydraulic servomotor and sliding block mechanism. The propeller blades are connected to blade carriers by blade bolts, and this ensures easy exchange of blades in the thruster tunnel. The gear case, which carries the propeller parts, is connected to the thruster tube by bolts and this ensures easy overhauling of all parts inside the thruster tube.

Example of thruster overhauling

The power transmission gear is located inside the gear case and consists of the vertical input shaft, the right angle reduction bevel gear and the horizontal propeller shaft, and serves to transfer the power from the prime mover to the propeller. The bevel gear and individual bearings are lubricated by the gravity oil filling the gear case.

The hydraulic power pack unit provides oil under pressure and this is used to change the pitch of the thruster unit blades.
The oil is drawn from the gravity tank, through the suction filter and into the oil service pump. The pressurized oil is pumped to the solenoid valve via the check valve and the flow of oil is controlled by the solenoid valve.
The hydraulically operated solenoid valve is a changeover valve for the distribution of the hydraulic oil to the respective servo cylinders depending on the command entered at the active control panel. When the command is entered on the control panel, the solenoid valve is actuated and pressurized hydraulic oil is supplied to one of the hydraulic circuits down the oil tube, through the feed ring and oil entry tube to the servomotor, causing displacement of the crosshead piston. The reciprocating movement of the piston is converted into a turning movement by the sliding block mechanism and this turns the propeller blades.
The vent side of the servomotor piston drains to the oil bath in the thruster body via a solenoid valve. From this pressurized oil bath, oil returns to the header tank. The main actuator power pack pump takes oil from the header tank and supplies it to the thruster unit via the solenoid control valves.

Example of a thruster hydraulic diagram

A shaft sealing mechanism is attached to the gear case in order to prevent leakage of oil out of the system.
When a pitch change command is entered, the propeller will tend to move excessively. The pilot check valve prevents any excessive movement of the propeller whilst changing pitch.

Operation of the bow thrusters requires starting a large induction motor and the power requirement of this electric motor is high, requiring that additional generators are started in order to avoid the risk of a vessel blackout.

It is important to note that, on modern vessels if there is insufficient power capacity available at the switchboard an additional generator is started by the power management system. The thruster drive motor cannot be started until sufficient power is available at the switchboard.

Usually, the main switchboard includes a bow thruster control panel, which includes a control position selection switch, lock-out relay trip reset, motor control and an ammeter. A series of status indicators are also included for monitoring the condition of the VCB, gravity tank, pump pressure etc. If any warning lamp is illuminated, the cause of the fault should be determined and remedied before operation of the bow thruster.
Feeder protection for the bow thrusters is achieved by means of the protection and monitoring panel located on the main switchboard bow thruster panel and offers both measurement and protection for the bow thruster drive motor.

The bevel gear and all the bearings inside the gear case are lubricated by the bath lubricating method. The lubrication oil in the gear case is slightly pressurized by the connection
with the gravity tank which is positioned above the waterline to prevent sea water from leaking into the oil system.

The thruster unit includes a feedback system for transmitting the angle of the propeller blades to the remote control panel located on the bridge. As the oil entry varies, the stroke of the oil entry tube also varies. The movement of the oil entry tube causes movement of the feedback lever. This movement is transmitted via the feedback chain to the blade angle transmitter located outside the thruster gear casing. This mechanical movement is then converted to an electrical signal by the blade angle transmitter and transmitted to the angle indicator on the bridge and local control panels.

To ensure safe, reliable operation of the bow thrusters, limits are imposed on the vessel’s speed and draught. If there is insufficient draught, the thruster will suffer a reduction in performance along with cavitation and the possibility of air drawing. The result of this will be increased vibration which may cause damage. Similarly, at speeds greater than 5 knots there is a risk of drawing air into the thruster, particularly when operating at shallow waters. This will degrade the performance and can cause cavitation damage and it can be detected by hunting of the main motor ammeter and should be avoided. If the vessel’s speed is below 5 knots and air drawing is occurring, reducing the propeller pitch will prevent further air drawing from taking place.

The main motor must only be started when the blades are in the neutral position (zero pitch), or in the allowable zone (blade pitch of ±3°). The system is interlocked to prevent the main motor from starting if the blade pitch is outside of the set limits. The interlock switches also prevent the main motor from starting when:

    • The cooling fan is stopped;
    • The power pack gravity tank level is low;
    • The control oil pressure is low

Under normal circumstances the main power supply is activated by the engineering department and after that the thruster operation and control is undertaken by the deck department from the bridge panels. The main switch at the local thruster control panel should be set at REMOTE in order to allow for this.
Control of the thruster on the bridge is either at the wheelhouse control stand or the control stands on the bridge wings.

It is important to note that, especially on bow thruster, when the hydraulic pump is started the fan is also started and the FAN RUN indicator in the panel will be illuminated. The main motor is interlocked with the fan and oil pump and will not start unless they are running.

Also is important to remember that there are EMERGENCY STOP pushbuttons in the wheelhouse panel, forward mooring station and in the bridge wing panels.

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!