Medium Speed Marine Research Articles

Evaluating Ammonia and Methanol as Lower-Emission Alternatives to liquefied natural gas for Medium-speed Marine Engines: A Thermodynamic Analysis

Evaluating Ammonia and Methanol as Lower-Emission Alternatives to liquefied natural gas for Medium-speed Marine Engines: A Thermodynamic Analysis

Strategies to improve ammonia combustion in a dual fuel marine engine by using CFD

Maritime transportation is currently responsible for delivering most of the world trade, and it still plays a major role in the global economy and pollutant emissions. Due to their heavy loads and long distances, full electrification is generally assumed as an unsuitable pathway, leaving combustion engines as the dominant technology in the marine sector. For a long time, these engines have benefited from the low costs of heavy oils produced from oil refining, but these fuels present a difficult combustion and are associated with significant pollutant emissions and greenhouse gases. Today, carbon-free fuels such as hydrogen and ammonia are gradually getting a foothold in the shipping market.In the present paper, an investigation on several strategies to improve the oxidation of ammonia is carried out for a dual fuel (DF) medium speed marine engine. A CFD analysis using ANSYS FORTE® code is performed on an improved and more realistic bowl geometry by varying the premixed fuel according to methods validated in previous studies and introducing adequate kinetic mechanisms from the literature. Results show that the sole use of ammonia to replace natural gas, as premixed fuel, leads to incomplete combustion; the increase of diesel fuel can only mitigate this condition, but it cannot be considered as viable solution. Therefore, several blends of ammonia/hydrogen are tested to examine combustion development and pollutant emissions. The addition of hydrogen above 10% to the premixed charge composition demonstrates to promote the oxidation of ammonia and to effectively reduce CO2 emissions of 84% without a dramatic increase in nitrogen oxides. On the contrary, hydrogen percentages lower than 10% feature a worsening of the combustion of the premixed charge. Finally, the use of a fuel containing nitrogen promote a slight formation of NOx emissions.

The Impact of Pilot Diesel Injection Strategies on the Combustion and Emission Characteristics of Diesel–Natural Gas Dual-Fuel Medium-Speed Marine Engines Based on Large-Eddy Simulation

The Impact of Pilot Diesel Injection Strategies on the Combustion and Emission Characteristics of Diesel–Natural Gas Dual-Fuel Medium-Speed Marine Engines Based on Large-Eddy Simulation

Research on Machining Quality Prediction Method Based on Machining Error Transfer Network and Grey Neural Network

Machining quality prediction is the critical link of quality control in parts machining. With the advent of the Industry 4.0 era, intelligent manufacturing and data-driven technologies bring new ideas for quality control in complex machining processes. Quality control is complicated for multi-process, multi-condition, small-batch, and high-precision parts processing requirements. To solve this problem, this paper proposes a machining quality prediction method based on the machining error transfer network and the grey neural network. Initially, by constructing a processing error transfer network, the error transfer law in part processing is described, and the PageRank algorithm and the influence degree of the nodes are used to determine the critical quality features. Additionally, the problem of low prediction accuracy due to small sample data and multiple coupling relationships is solved using the grey neural network algorithm, and a high accuracy prediction of critical quality features is achieved. Finally, the effectiveness and reliability of the method are verified by the case of medium-speed marine diesel engine fuselage processing. The results indicate that this method not only effectively identifies critical quality features in the machining process of complex parts, but it also maintains a high predictive accuracy for these features, even with small samples and limited data.

Research on Critical Quality Feature Recognition and Quality Prediction Method of Machining Based on Information Entropy and XGBoost Hyperparameter Optimization

To address the problem of predicting machining quality for critical features in the manufacturing process of mechanical products, a method that combines information entropy and XGBoost (version 2.1.1) hyperparameter optimization is proposed. Initially, machining data of mechanical products are analyzed based on information entropy theory to identify critical quality characteristics. Subsequently, a quality prediction model for these critical features is established using the XGBoost machine learning framework. The model’s hyperparameters are then optimized through Bayesian optimization. This method is applied as a case study to a medium-speed marine diesel engine piston. After the critical quality characteristics in the machining process are identified, the machining quality of these vital characteristics is predicted, and the results are compared with those obtained from a machine learning model without hyperparameter optimization. The findings demonstrate that the proposed method effectively predicts the machining quality of mechanical products.

Study on the forming process of the cylinder wall fuel film and the piston top land ablation mechanism of a medium speed marine diesel engine under cold start condition

The study presents an explanation of the piston top land ablation mechanism in a specific type of marine medium-speed diesel engine, utilizing multi-dimensional Computational Fluid Dynamics (CFD) software. At present, there is a lack of research on the process analysis of fuel droplets attached to the cylinder wall at diesel engine cold start condition and the mechanism of piston top land ablation. The study results indicate that during low-speed cold start condition, a multitude of diesel droplets impact the wall, causing the combustion area concentrated near the wall of the combustion chamber. The last stage fuel spray droplets, driven by the vortex in the cylinder, adhere to the cylinder wall before evaporation during the post-combustion stage. After multi-cycle operation of the diesel engine under cold start condition, the mass of fuel film adhering to the cylinder wall and the cumulative fuel mass on the top of the gas ring increase. As the engine getting warm, higher cylinder wall temperature leads to rapid evaporation of the fuel film on the cylinder wall and the gas ring. Deflagration happens when the fuel vapor burns under high temperature and high-pressure condition, and subsequent pressure oscillation destroys the thermal boundary layer of the piston top land. Piston material ablation happens under high temperature and high-pressure shock.

Research on identification of key processes in machining process based on PageRank algorithm

Aiming at the difficulty of quantitatively evaluating the critical processes in the manufacturing process of complex mechanical products, a critical process identification method based on the PageRank algorithm is proposed with the goal of identifying key processes in the machining process. Based on the complex network theory, the error transfer network model of the machining process is established in this paper. Adopting the actual machining process as the data set of the complex network, the weights of the machining feature nodes are calculated by the PageRank ranking algorithm, and the nodes are ranked according to the weight values to assess the influence and importance of the nodes in the network model. Finally, taking the connecting rod machining process of a medium-speed marine diesel engine as an example, the results show that the method can quickly and effectively identify the key processes in the machining process.

Research on combustion process optimization of marine diesel engines based on dual-injector system

Marine diesel engines are widely used with their strong power and mature technology. However, in recent years, the emphasis on environmental protection has made combustion pollutants the focus of attention, and the new regulations of the International Maritime Organization (IMO) and various countries have put forward new requirements and challenges for diesel engine combustion and emission performance. Due to the large size and large amount of circulating fuel injection, the incomplete mixing of fuel in the cylinder and the local violent combustion of high-power medium-speed marine diesel engines are important reasons for low thermal efficiency and high pollutant emissions. To achieve efficient and clean combustion of fuel in the cylinder, a dual-injector combustion system is designed on a dual-fuel engine basis, aiming to add a micro-injection before the central injection and provide positive in-cylinder flow and hot atmosphere for the atomized combustion of the central-injection fuel, so that the combustion in the cylinder is reasonably arranged, the thermal efficiency is improved and NOx emissions are reduced. The 3D simulation model was established by CFD software CONVERGE, and the physical model was calibrated according to the data of the pure diesel mode of the dual-fuel engine, and the influence of the small pilot-injection fuel volume on the development of spray and combustion in the cylinder at high and low load was analyzed. It was found that the small pilot-injection fuel volume at a high load had little effect on the atomization evaporation of the main-injection fuel and the hot atmosphere in the cylinder, so the improvement effect of combustion quality was not obvious, while the pilot-injection volume with the same pilot-injection capacity as high load relatively accounted for a large proportion at the low load with a small cycle injection volume. Without changing the injector specification settings of the original dual-fuel engine, the thermal efficiency was increased by 6.13% and the NOx generation was reduced by 22.42% by optimizing the injection timing, injection duration, and injection amount.

Investigation of oil emission mechanisms in a marine medium-speed dual-fuel engine

Pilot-ignition Otto marine engines are known for greatly reduced emissions of air pollutants (sulphur oxides, nitrogen oxide, particulates) compared to marine diesel engines. However, lubricating oil emissions still are about one order of magnitude higher than in land-based systems. To identify reduction potentials, a better understanding of oil emission mechanisms has to be gained. For this purpose, mass spectrometric oil emission measurements and fluorescence lubricating film thickness measurements were performed on a medium-speed marine engine. With the fluorescence measuring system, the varying lubricating oil film on the cylinder wall can be visualised and analysed in sub-crank-angle resolution. By applying the developed calibration method to the measurement data, the oil film thickness can be determined in µm. It is shown that the oil film left by the piston rings on the liner as it moves down is almost halved after ignition compared to during intake stroke. The authors have further been able to detect and time operating point dependent ring rotation and investigations show a connection between ring rotation and cylinder liner temperature distribution. Aligning ring gaps allow blow-by to happen. This and other high intensity events such as engine knock, load shedding or the transition from diesel-mode to gas-mode, heavily disturb the oil layer and cause peaking oil emissions.

Experimental investigation on spray and flame characteristics of a marine diesel engine with single and double injection

Multiple injection is one of the advanced technologies employed in modern diesel engines to improve combustion efficiency, reduce pollutant emissions, and minimize combustion noise. The application of multiple injection in marine diesel engines differs from its use in vehicles or heavy-duty engines, as it is not commonly combined with Exhaust Gas Recirculation (EGR). However, there is a scarcity of studies specifically examining the combustion characteristics of medium-speed marine diesel engines utilizing multiple injection. Given the large space scale of marine diesel engine, a constant volume chamber with a visible diameter of 240 mm was used, and an injector with a nozzle diameter of 0.465 mm was employed in the experiment. The spray development and combustion process were recorded by Mie-scattering and flame natural luminosity imaging respectively. Both conventional and double injection combustion processes were analyzed in detail. The results show that although the liquid phase spray does not fully penetrate to the cylinder wall in marine diesel engines, the flame penetrates to the cylinder wall rapidly, the flame burns near the wall almost throughout the entire combustion duration. The combustion characteristics of the double injection are significantly different, the flame propagation speed of the pilot and main injection fuel is one-third to one-half of that of a single injection with a long duration. For single injection spray with long injection duration, the flame penetration velocity is determined by the sequential ignition velocity of the fuel. While the flame penetration velocity for sprays with short injection duration mainly depends on the jet penetration velocity. The investigation on the impact of dwell time, fuel distribution, and injection pressure on the combustion process of double injection also provides valuable insights for optimizing multiple injection strategies.

Numerical Analysis of Dual Fuel Combustion in a Medium Speed Marine Engine Supplied with Methane/Hydrogen Blends

Compression ignition engines will still be predominant in the naval sector: their high efficiency, high torque, and heavy weight perfectly suit the demands and architecture of ships. Nevertheless, recent emission legislations impose limitations to the pollutant emissions levels in this sector as well. In addition to post-treatment systems, it is necessary to reduce some pollutant species, and, therefore, the study of combustion strategies and new fuels can represent valid paths for limiting environmental harmful emissions such as CO2. The use of methane in dual fuel mode has already been implemented on existent vessels, but the progressive decarbonization will lead to the utilization of carbon-neutral or carbon-free fuels such as, in the last case, hydrogen. Thanks to its high reactivity nature, it can be helpful in the reduction of exhaust CH4. On the contrary, together with the high temperatures achieved by its oxidation, hydrogen could cause uncontrolled ignition of the premixed charge and high emissions of NOx. As a matter of fact, a source of ignition is still necessary to have better control on the whole combustion development. To this end, an optimal and specific injection strategy can help to overcome all the before-mentioned issues. In this study, three-dimensional numerical simulations have been performed with the ANSYS Forte® software (version 19.2) in an 8.8 L dual fuel engine cylinder supplied with methane, hydrogen, or hydrogen–methane blends with reference to experimental tests from the literature. A new kinetic mechanism has been used for the description of diesel fuel surrogate oxidation with a set of reactions specifically addressed for the low temperatures together with the GRIMECH 3.0 for CH4 and H2. This kinetics scheme allowed for the adequate reproduction of the ignition timing for the various mixtures used. Preliminary calculations with a one-dimensional commercial code were performed to retrieve the initial conditions of CFD calculations in the cylinder. The used approach demonstrated to be quite a reliable tool to predict the performance of a marine engine working under dual fuel mode with hydrogen-based blends at medium load. As a result, the system modelling shows that using hydrogen as fuel in the engine can achieve the same performance as diesel/natural gas, but when hydrogen totally replaces methane, CO2 is decreased up to 54% at the expense of the increase of about 76% of NOx emissions.

Waste heat recovery system for marine engines optimized through a preference learning rank function embedded into a Bayesian optimizer

Waste heat recovery is a proven process to improve efficiency on engines and meets current necessities of the maritime industry. Since January 1, 2023, already built vessels must meet the energy efficiency indicators known as EEXI and CII. Aiming to reduce fuel consumption and mitigate pollution emissions, a novel waste heat recovery system composed of steam Rankine cycle, organic Rankine cycle, thermoelectric harvesters and desalination is presented. High, medium and low-grade waste heat from exhaust gas, jacket water, lubricating oil and engine block radiation are targeted for recovery. Performance assessment of each subsystem when implemented on a real case study 6-cylinder medium speed marine engine is analyzed. The equivalent electricity production concept was used for the assessment of the desalination subsystem. The proposed system effectively recovers waste energy, offering economic benefits, reducing pollution and satisfying the daily demand of fresh water. Also, optimal states of the waste heat recovery are provided via Bayesian optimization, which requires an evaluation function for the system to be optimized. However, this function is not available and cannot be straightforwardly established, since the quality of waste heat recovery depends on some indicators with a trade-off among them. Hence, a preference learning procedure that exploits expert knowledge is proposed to induce a function of this kind from those indicators in order to be embedded into the Bayesian optimization procedure afterward. Applied to the case study engine, a fuel consumption reduction of 15.04% is achieved. Fuel savings lead to an improvement in energy efficiency indicators, achieving a reduction of 6.98% on the EEXI and a 13.85% on the CII.

Effects of split diesel injection strategies on combustion, knocking, cyclic variations and emissions of a natural gas-diesel dual fuel medium speed engine

Accelerated by the harms and depletion risk of fossil fuels, transition to low-carbon or renewable fuels have become an urgent need for combustion engines to decarbonize transport sectors. Natural gas-diesel dual-fuel combustion is a promising method to achieve these goals by allowing the usage of natural gas in diesel engines. However, this concept has drawbacks of low combustion efficiency, high unburned hydrocarbon (HC) emissions and high cyclic variations at low engine loads. These drawbacks hinder the benefits of dual-fuel such as low NOx and soot emissions. To find solutions to these drawbacks, split diesel injections with variable injection timings and mass split ratios (ratio of first injection diesel mass to total diesel mass) were investigated experimentally and numerically on a medium speed marine engine at low load using constant natural gas and diesel amounts.Experimental results of the split injection were compared to single injection dual-fuel and conventional diesel operation. Single injection case resulted in slightly lower HC emissions and cyclic variations and higher NOx emissions than split cases in a narrow injection timing interval. Apart from that, split injections increased combustion efficiency and decreased cyclic variations in a broader timing interval. For split cases, early first injection timing prevented high premixed heat release peak, suppressed NOx emissions and knocking. Especially, using 70% split ratio with retarded second injections increased combustion efficiency, decreased knocking level and provided simultaneous reduction of HC and NOx emissions compared to lower split ratio cases. In short, despite not giving the minimum values at all conditions, split injection showed potential to find a remedy to high HC emissions and cyclic variations issues by keeping robust operation in a wider injection timing interval.The numerical results support the experiments in which better combustion and decreased HC emissions are attained using sufficiently early diesel injections. Simulations show that, compared to late single injection, early single injection dual-fuel case enables more homogeneous temperature distribution and better oxidization of methane near the cylinder wall and central region above the piston crown. Besides, split injection shows improved methane oxidation near the cylinder wall and inside top-land crevice volume.

The Impact of Slow Steaming on Fuel Consumption and CO2 Emissions of a Container Ship

Modern marine propulsion systems must be reliable, energy efficient, environmentally friendly, and economical. Efforts to reduce fuel costs and carbon dioxide (CO2) emissions per nautical mile have a significant impact on the choice of propulsion system. Considering that there is no alternative for maritime transport, various technical and technological solutions are being considered that aim to improve efficiency and reduce the negative impact on the environment. One of the ways to achieve this goal is slow steaming, which reduces fuel consumption and CO2 emissions. The designed speed of the vessel has a significant impact on the efficiency of slow steaming. Slow steaming is particularly suitable for large container ships with a design speed of more than 20 knots. In this paper, the effects of slow steaming are analyzed using the example of a container ship with diesel-engine propulsion. Propulsion systems with low-speed and medium-speed marine diesel engines with mechanical power transmission are investigated. Data on the required engine power and propeller speed were used for the study, obtained from calculations during testing of the ship’s hull model. The effects of speed reduction on specific fuel consumption and emission reduction were analyzed using numerical models of two-stroke and four-stroke diesel engines. The models were calibrated and validated using data provided by the engine manufacturers. The paper analyses four different cases where one or two low-speed diesel engines, or three or four medium-speed diesel engines, are used for propulsion. The analysis concludes that slow steaming can effectively reduce fuel consumption and CO2 emissions, but the choice of the optimal propulsion system is highly dependent on maritime market conditions in maritime transportation. The choice of propulsion system affects the potential of slow steaming.

Combustion and Exhaust Gas Characteristics of Residual Oil Mixed with Air-Fine Mixture in Medium-Speed Marine Diesel Engine

In the marine engineering field, research has been conducted to reduce fuel consumption and emissions of harmful exhaust gases from medium- and low-speed diesel engines by improving supercharging performance, scavenging efficiency, and introducing electronic control technology, etc., and these technologies have been put to practical use. However, most of these technologies have been applied to newly constructed vessels, and oxygen-containing fuels such as water-emulsion fuel and DME fuel have not yet been put to practical use. In this study, the effects of heating high viscosity residual oil and mixing air into the fuel as fine bubbles on the combustion and exhaust characteristics of the fuel were investigated using a practical medium-speed marine diesel engine. As a result, the fuel consumption reduction effect was confirmed when fine air bubbles were mixed in heated residual oil, as is the case with them being blended into marine diesel oil. In particular, the maximum reduction was 7% at 75% load and 0.2L/min air volume, and the average reduction was 3.2% at 75% load. In terms of the exhaust gas characteristics of the fuel, no significant reduction in NOx was observed when microbubbles were mixed, but smoke values were reduced by up to 33%.

Utilization of renewable and sustainable diesel/methanol/n-butanol (DMB) blends for reducing the engine emissions in a diesel engine with different pre-injection strategies

In the face of the severe energy crisis and environmental crisis, actively finding and using alternative fuels rationally is one of the main ways to alleviate energy shortage and environmental pollution. Biomass fuels are alternative fuels with good development prospects. The addition of oxygenated biofuels can reduce the soot emission, but increase the nitrogen oxides (NOx) emission. Multiple injection strategies can be used to reduce the NOx emission. Therefore, this paper was mainly aimed at the parameters investigation of a medium-speed marine four-stroke diesel engine fueled with diesel/methanol/n-butanol (DMB) blended fuel. The effects of pre-injection (including pre-injection fuel timing (PT) and pre-injection fuel mass ratio (PMR)) on the engine combustion, performance, and emission parameters were obtained. In addition, an orthogonal test design was used to optimize the PT and PMR to determine the optimal combination. The simulation results showed that advancing PT and increasing PMR could increase cylinder pressure, which was increased by about 9.8%–14.57%. However, it decreased the cylinder temperature, and shorten ignition delay (ID) and combustion duration (CD). In addition, the use of the pre-injection strategies could reduce the emissions of the engine pollution, in which the emission of NOx was reduced by about 46.43%–87.18%, and the emission of hydrocarbons (HC) was reduced by about 24.05%–38.03%. However, the brake specific fuel consumption (BSFC) increased. The optimal combination parameters of PT and PMR obtained by orthogonal test design were −45 °CA and 0.3. At this point, the peak cylinder pressure was 8.83 MPa, the BSFC was 304.46 g/(kW·h), and the NOx emission was 168.74 ppm. Therefore, this study could provide some theoretical foundations for the optimization of fuel injection strategy coupled with the oxygenated fuels combustion process.

Schlieren and mie-scattering visualization of an evaporating spray under marine diesel engine conditions

Due to the critical influence on combustion, extensive research has been carried out to deepen the understanding of diesel spray evolution. However, for the greater share of land-based diesel engines, most of the studies have focused on the injectors with small nozzle diameter and rare studies can be found on marine diesel engines, which has a larger mass flow rate and longer injection duration with large nozzle diameter. In this study, the influence of injection pressure, ambient temperature, ambient density and nozzle diameter on spray morphology for medium speed marine diesel engine injectors (with nozzle diameter up to 0.53 mm) were investigated under various non-reacting but evaporative conditions. The experiments were performed via a highly enhanced constant volume chamber (highest temperature of 900 K and highest pressure of 8 MPa) with an optical window diameter of 205 mm. Liquid and vapor penetration of sprays were recorded by Mie-scattering and schlieren optical configurations respectively. Results show that the vapor penetration velocity is mainly affected by injection pressure. Ambient temperature has the most significant effect on liquid penetration length, but less effect can be found on the vapor penetration velocity. Given the obvious deviation from heavy duty engine sprays, the empirical correlations for liquid penetration length and vapor spray evolution are updated to fit the spray characteristics of medium speed marine diesel engine injectors, which is helpful to understand the spray behavior for large nozzle diameter injector under marine diesel engine conditions.

Effects of hydrogen-rich products from methanol steam reforming on the performance enhancement of a medium-speed marine engine

In order to improve the heat utilization of exhaust gas from the small and medium-sized ships without exhaust boilers, a methanol steam reforming (MSR) is proposed in the study. The combustion model of marine engine blended hydrogen-rich reforming gas was developed, and verified in terms of cylinder pressure, BSFC, and NO emissions. The compositions of reforming gas were obtained by the MSR model and employed to investigate the effect of reforming gas at different blending ratios on the combustion and emission characteristics of the marine diesel engine in the AVL-Fire environment. The results obtained show that the performance, emission, and economic characteristics are improved due to the existence of reforming gas. Specifically, the indicated power is decreased by 0.5%–4.26% and the NO mass fraction is reduced by 2.95%–18.25% at different loads, but the CO mass fractions increased by 20.21%–54.73%. Similarly, the NOx weighted emission is reduced by 34.10%, which is close to the NOx emission limit of Tier II, but the CO weighted emission is increased by 30.89%. In addition, the equivalent fuel consumption rate is reduced by 0.45%–3.59% at the maximum blending ratio, and the converted fuel cost is reduced by 6%–10% at medium and high loads. Finally, based on the comprehensive performance analysis, the optimized blending ratio at each load is obtained as follows: Rg7.5 or Rg12.5 for 100% load, Rg10 for 75% load, Rg7.5 for 50% load, and Rg2.5 for 25% load. Thus, the MSR reaction is feasible to improve the energy efficiency and reduce NOx emission of the marine engine.

Optimisation of the Topping-Up Process of Lubricating Oil in Medium-Speed Marine Engines

Abstract In this paper, we examine the problem of optimising the process of topping up lubricating oil in medium-speed marine engines. This process is one of the methods that can be applied to improve the properties of lubricating oil. The amount of fresh oil added to lubricating oil system always balances its consumption, but the method used to top up depends on the marine engineer. Small amounts of fresh oil can be added at short intervals, or large ones at long intervals, and the element of randomness often plays a significant role here. It would therefore be valuable to find a method that can help the mechanical engineer to choose the right strategy. We apply a multi-criteria optimisation method for this purpose, and assume that the criterion functions depend on the concentration of solid impurities and the alkalinity, which are among the most important aspects of the quality and properties of lubricating oil. These criterion functions form the basis for multi-objective optimisation carried out with the use of the MATLAB computer program.

Theoretical investigation of the combustion performance of ammonia/hydrogen mixtures on a marine diesel engine

Ammonia is a good hydrogen carrier and can be well combined with hydrogen for combustion. The combustion performance of the mixtures of ammonia and hydrogen in a medium-speed marine diesel engine was investigated theoretically. The HCCI combustion mode was selected for reducing thermal-NOx production. The start fire characteristic of the NH3–H2 mixtures was studied under different equivalence ratio, hydrogen-doped ratio, and intake air temperature and pressure. Then, the combustion performance of the NH3–H2 mixtures (doping 30% hydrogen) was analyzed at a typical operation condition of engine. The addition hydrogen improved the laminar flame velocity of ammonia, and affected the NOx emission. For the medium-speed marine engine fueled with NH3–H2, reducing combustion temperature, introducing EGR and combining with post-treatment technology would be a feasible scheme to reduce NOx emission.