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.

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Navigating Green Waters: Propulsion and Engine Optimization for EEXI Compliance

The maritime industry is at the helm of significant change as it sails toward a more sustainable future. With the Energy Efficiency Existing Ship Index (EEXI) regulation coming into effect in 2024, vessel owners and operators are tasked with optimizing propulsion systems and engines to reduce greenhouse gas emissions. If you want to read more about EEXI please follow THIS LINK.

In this article, we’ll dive into the challenges, available technology, and what marine engineers need to do to navigate this sea of change successfully.

Challenges on the Horizon

Complying with the EEXI regulation presents several challenges, primarily centered around improving energy efficiency while minimizing emissions. Some of the key challenges include:

  1. Evaluating Existing Systems: Vessel owners must assess their current propulsion and engine systems to determine their energy efficiency and EEXI compliance. This often requires complex calculations and data analysis.

  2. Investment Costs: Upgrading propulsion systems and engines can be a significant investment. Owners need to balance these costs with the long-term benefits of improved efficiency and compliance.

  3. Technology Integration: Implementing new technologies and optimizing engines can be a complex process. Ensuring these systems work seamlessly with existing onboard systems is crucial.

  4. Regulatory Compliance: Meeting EEXI requirements necessitates compliance with stringent emissions standards. Staying up to date with evolving regulations is an ongoing challenge.

Technology on the Market

To address these challenges, a range of innovative technologies and solutions are emerging in the maritime sector:

  1. Fuel-Efficient Engines: Modern, fuel-efficient engines with advanced combustion technologies and improved design are becoming more widely available.

  2. Exhaust Gas Cleaning Systems: Technologies like scrubbers and selective catalytic reduction (SCR) systems help reduce emissions from engines, aligning with EEXI standards. More about this if you follow THIS LINK.

  3. Alternative Fuels: The adoption of alternative fuels such as LNG, hydrogen, and ammonia can significantly reduce greenhouse gas emissions.

  4. Energy Recovery Systems: Systems that recover and reuse waste energy from the engine, such as waste heat recovery systems, contribute to greater efficiency.

  5. Propulsion Efficiency Solutions: Upgrading propulsion systems with modern propellers and thrusters designed for efficiency can reduce fuel consumption.

    Source and Credit: MOL

    One of the most common methods to improve the attained EEXI is to limit the engine power or shaft power of the ship. This can be done by re-setting the fuel index by limiting the fuel rack using either mechanical stop or setting the control system in combination with an approved override functionality as defined in the IMO guidelines. This method is called Engine Power Limitation (EPL) or Shaft Power Limitation (ShaPoLi). To read more about this, please follow THIS LINK.

    However, this method also poses some challenges and risks for the ship operation, such as reduced maneuverability, increased fuel consumption, increased maintenance costs, and potential safety issues.

    Therefore, ship operators need to consider other measures to optimize the propulsion and engine performance of their ships, such as installing energy saving devices, using alternative fuels, or upgrading the propulsion system. Some of the available technologies on the market that can help achieve this are:

    • FuelOpt: This is a propulsion optimization system developed by Yara Marine Technologies that provides an integrated ShaPoLi feature that complies with the EEXI framework. The system enhances vessel efficiency while minimizing the impact of engine or shaft power limitations on daily operations. FuelOpt can also reduce fuel consumption and emissions by controlling the propeller pitch and engine load in real time.
    • Rotating sails: These are vertical cylinders that rotate around their axis and use the Magnus effect to create a forward thrust. They can be installed on existing ships as an auxiliary propulsion system that can reduce fuel consumption and emissions by up to 20%. Some examples of rotating sails are Flettner rotors and Norsepower rotor sails .
    • Bulbous bow: This is a protruding bulb at the bow of a ship that modifies the water flow around the hull and reduces the drag. It can improve the hydrodynamic efficiency of a ship and reduce fuel consumption and emissions by up to 15%. However, it requires careful design and optimization for different ship types and speeds.
    • Propeller fins: These are appendages attached to the propeller blades that increase the thrust and efficiency of the propeller. They can reduce fuel consumption and emissions by up to 5%. Some examples of propeller fins are Becker Mewis Ducts and Propeller Boss Cap Fins .
    • Alternative fuels: These are fuels that have lower carbon intensity than conventional marine fuels, such as liquefied natural gas (LNG), biofuels, hydrogen, ammonia, or methanol. They can reduce greenhouse gas emissions from ships by up to 100%, depending on their production and use. However, they also require new infrastructure, storage, handling, and safety measures.
    • Propulsion systems: These are systems that convert energy into propulsive force, such as diesel engines, electric motors, gas turbines, or fuel cells. They can be upgraded or replaced with more efficient or low-carbon technologies that can reduce fuel consumption and emissions. Some examples of propulsion systems are hybrid propulsion, diesel-electric propulsion, or hydrogen fuel cell propulsion .

What Marine Engineers Need to Do

Marine engineers play a pivotal role in ensuring vessels comply with EEXI regulations and optimizing propulsion and engine systems. Here’s what they should consider:

  1. Data Analysis: Conduct detailed data analysis to determine the current energy efficiency of propulsion and engine systems. This forms the foundation for improvement strategies.

  2. Collaboration: Collaborate with naval architects, designers, and technology providers to select the most suitable propulsion and engine optimization solutions.

  3. Regular Maintenance: Implement a rigorous maintenance schedule to keep engines and propulsion systems in optimal working condition, reducing energy wastage.

  4. Training: Stay up to date with the latest technologies and best practices through continuous education and training programs.

  5. Monitoring and Reporting: Implement systems for real-time monitoring of engine and propulsion system performance. Regularly report on energy efficiency improvements and emissions reductions.

  6. Documentation: Maintain comprehensive records of all upgrades, modifications, and maintenance activities related to propulsion and engines for compliance verification.

The EEXI regulation is expected to have a significant impact on the shipping industry in 2024 and beyond. As the maritime industry charts a course towards greater sustainability, marine engineers are the navigators guiding vessels through these uncharted waters. By leveraging the available technology and adhering to best practices, marine engineers can help vessel owners and operators meet the challenges of EEXI compliance while contributing to a cleaner, greener future for the maritime world.