Low-speed Diesel Engine Research Articles

Modelling and theoretical research on transient lubrication and wear coupling behavior of main bearing during start-up of low-speed diesel engine

This study endeavors to establish a transient mixed lubrication model for time-varying wear of main bearings under dynamic loading conditions during the startup process. This model accounts for variations in oil film thickness attributable to deformation, wear profiles, and journal misalignment throughout the operational cycle. The evolution of main bearing lubrication characteristics during the startup process was analyzed, as well as the evolution of wear profiles and their effects on lubricating properties under different numbers of startups. The results indicate that as the number of startup cycles increases, the wear rate gradually levels off. With increasing startup times, there is a decrease in contact pressure and a slight reduction in oil film pressure, resulting in enhanced lubricating performance. Furthermore, the influence of startup duration on wear is significantly greater than that of radial clearance.

Optimisation of Brayton cycle CO2-based binary mixtures: An application for waste heat recovery of marine low-speed diesel engines exhaust gas

The energy-saving capabilities and efficient operation of marine low-speed diesel engines (MLDE) is a key emphasis for the main power source for ocean transportation. The purpose of the supercritical carbon dioxide recompression Brayton cycle (SCRBC) is to capture and utilise waste heat emitted by the engine's exhaust gas. However, the SCRBC performance will be severely affected by the large temperature fluctuations of ocean-going vessels during operation and high ambient temperatures in the cabin. A SCRBC model was built using the exhaust gas test data as the boundary conditions and validated using the Sandia National Laboratory (SNL) test data. The physical characteristics of the working fluids were evaluated by adding other fluids to CO2 in specific proportions to modify the critical point and increase cycle efficiency. The results demonstrated that employing CO2-based binary working fluids with low alkane and hydrogen sulfide (H2S) enhanced the recovery power, with the most significant increase obtained by the addition of 16.48 % H2S, which increased the power by 9.72 kW and improved the Brayton cycle efficiency by 3.31 %. Compared to the MLDE at 100 % load, the total efficiency increased by 1.77 % and the BSFC decreased by 6.76 (g kW−1 h−1) using CO2-H2S as the working fluid. The analysis of the SCRBC system component exergy losses showed that the cooler had the highest exergy losses. Adding other fluids to CO2 reduced the exergy losses of each component with the SCRBC system exergy losses decreasing from 162.97 to 129.90 kW and the exergy loss efficiency decreasing from 24.24 % to 22.65 %. The use of CO2-based binary working fluids specifically designed for ambient temperature may be expanded to other engines to enhance the efficiency of waste heat recovery.

Design, optimisation and evaluation of the S-CO2 Brayton cycle for marine low-speed engine flue gas

Five configurations of the S-CO2 Brayton cycle (SCBC) were constructed for flue gas waste heat recovery (WHR) of the marine low-speed engine HHM-6EX340EF on the EBSILON platform using the bench test data to save energy, reduce emissions and the efficient operation of ocean-going vessels. Experimental data from Sandia National Laboratories (SNL) was utilised to validate the accuracy of the model and the effect of Brayton cycle parameters on flue gas WHR for different configurations was analysed. The parameters of the SCRBC (S-CO2 recompression Brayton cycle) were optimised by combining the Neural Network Fitting (NNF) model, Multi-objective optimisation algorithms, and multi-criteria decision-making methods. Finally, the thermodynamic analysis of the low-speed engine was comprehensively evaluated by comparing five configurations of SCSBC (S-CO2 simple Brayton cycle), SCCBC (S-CO2 recuperative Brayton cycle), SCHBC (S-CO2 reheating Brayton cycle), SCIBC (S-CO2 intercooling Brayton cycle), and SCRBC. The optimised SCIBC configuration had the highest net recuperated work of 195.76 kW and the highest Brayton cycle efficiency of 20.13 %. The engine efficiency was improved by 0.82 %, 1.45 %, 1.78 %, 1.86 % and 1.68 %, the BSFC was decreased by 3.20 g/kW·h, 5.56 g/kW·h, 6.82 g/kW·h, 7.04 g/kW·h, and 6.43 g/kW·h, and the output power increased by 86.90 kW, 153.33 kW, 195.76 kW, 189.36 kW, and 178.14 kW, respectively. The maximum exergy loss of the cooler was 162.67 kW with an exergy loss efficiency of 14.54 % in the SCSBC configuration. Optimising the flue gas heat exchanger and cooler will further enhance the efficiency and reduce emissions. Research on the parameter and configuration optimisation of the SCBC for marine low-speed diesel engines can be extended to other low-speed engines.

Features of the operation of the lubrication system for lubricating ships' low-speed engines

The article examines the optimization of high-alkaline cylinder oil consumption in lubrication systems of marine low-speed engines (MODs), as well as the peculiarities of operation of the cylinder lubrication system in view of modern trends in the development of low-speed marine engines. An overview of the lubricator lubrication systems of the cylinders of marine main engines was carried out and possible prospects for improving the lubricator lubrication systems of the cylinders of marine diesel engines were considered. Attention is paid to the causes of occurrence and methods of combating a whole range of problems, which represent the possibility of increased wear of cylinder sleeves, pistons, piston rings, this is related to the type and grade of fuel and its sulfur content. Sulfur in the fuel is neutralized with the help of cylinder oils with high alkalinity, problems arise when changing the grade of fuel, this requires changing the brand of cylinder oil (CLO BN). The possibility of using a cylinder oil mixing system (ACOM) to neutralize sulfur in the fuel and improve protection against low-temperature corrosion of MOD cylinder liners is proposed and considered. This system mixes and doses cylinder oil depending on the load of the ship's engine and the type of fuel. ACOM allows the dosing of oil on marine engines operating in dual fuel consumption (SDF) modes, where the ratio between pilot (ignition) and gas fuel is defined, the system mixes two different types of cylinder oil into one with the required BN base number, it can be determined as the oil's ability to neutralize acids produced during use. The results of optimal consumption of cylinder oil, which ensures minimal wear of cylinder liners of a diesel engine, are given. Features of lubrication regimes and control of the technical condition of the cylinder group of marine low-speed diesel engines during their operation at reduced rotation frequency are described

Optimization and 4E analysis of integration of waste heat recovery and exhaust gas recirculation for a marine diesel engine to reduce NOx emissions and improve efficiency

This paper proposes an integration of waste heat recovery system and exhaust gas recirculation system to break the inherent trade-offs between NOx and carbon emissions of marine low-speed diesel engine. It is significant important to break these trade-offs, because Energy Efficiency Existing Ship Index may not be met, though Energy Efficiency Design Index and Tier III can be already achieved for old ships. The waste heat recovery system consists of supercritical carbon dioxide Brayton cycle and Kalina cycle, where the integration of supercritical carbon dioxide Brayton cycle and Kalina cycle is achieved in the parallel style. This paper uses the classical thermodynamics in terms of engineering approach to carry out the parameter analysis of combined system first. The optimization is conducted in terms of efficiency, energy density and techno-economy with the introduction of global optimal policy. The results showed that the highest efficiency of 52.32% at Tier III mode can be achieved, at which the net power, exergy efficiency, area per unit power output and levelized energy cost can be achieved by 1568.57 kW, 45.75%, 0.589 m2/kW and 0.114$/kWh at main engine load of 85%. Further, the net power output of waste heat recovery system can reach 1085.76 kW and 2043 kW at Tier II mode and Tier III mode, while the recovering rate of waste heat recovery system can reach over 5% at Tier III mode. Finally, the Energy Efficiency Existing Ship Index of a container ship equipping with such combined system can be reduced from 9.50 to 9.05.

Structural optimization of S-CO2 Brayton cycle compressor impeller based on evolutionary algorithm

As the critical components for marine low-speed diesel engine flue gas waste heat recovery (WHR) supercritical carbon dioxide (S-CO2) Brayton cycle system, the structure of the compressor impeller is optimized by the evolutionary algorithm (EA) based on the co-simulation of the CAESES, ANSYS CFX and Opti Slang. The law of impeller pressure ratio, efficiency and power consumption in S-CO2 Brayton cycle (SCBC) as a function of rotational speed, inlet temperature, pressure and impeller structural parameters are revealed, and the method of improving SCBC efficiency for marine low-speed diesel engine flue gas waste heat recovery is studied. The optimized impeller structure is greatly enhanced in aerodynamic performance and safety, and the isentropic efficiency is increased by 2.54%, the pressure ratio is increased by 35.64%, and the temperature rise is increased by only 4.6%. A 100kW S-CO2 compression cycle test bench was set up to verify the simulation-optimized impeller results. The final results show that the optimized impeller structure, aerodynamic performance and safety are greatly improved. It provides theoretical support for selecting and optimising compressor impellers for marine low-speed diesel engine flue gas waste heat recovery S-CO2 Brayton cycle.

Characteristics of Particulate Matter Emissions for Low-Sulfur Heavy Oil Used in Low-Speed Two-Stroke Diesel Engines of Ocean-Going Ships

Characteristics of Particulate Matter Emissions for Low-Sulfur Heavy Oil Used in Low-Speed Two-Stroke Diesel Engines of Ocean-Going Ships

Cam Profile Design of Low-Speed Marine Diesel Engine Based on the Grey Theory

The camshaft profile is one of the important parameters of the engine, the reasonableness or otherwise of its design directly influences the engine’s power output, fuel consumption, emissions, noise and reliability. However, most existing cam profile optimization designs are based on the basic software operation, profile parameter selection and tuning lack a scientific theoretical basis. In order to address this issue, this study establishes a single valve kinematic model of the gas delivery mechanism of diesel engines by using the AVL-EXCITE Timing Drive, and performs a model-based multinomial kinematic acceleration cam profile design. Subsequently, an orthogonal design method is employed to obtain the parameter combinations that can satisfy the dynamic requirements. The research investigates the influence of cam profile design parameters (C 4, p, q, r, and s) on valve fullness, maximum jerk, cam contact stress, and valve seating force through a systematic study. Lastly, based on the grey correlation theory, a thorough analysis of the performance indices of cams with different profile parameter combinations is conducted. The optimization problem of the profile parameters with multiple performance indicators is transformed into a single-objective grey relational optimization problem, resulting in the optimal cam profile parameter combination of C4 =0.1, p=14, q=20, r=28 and s=36. The optimal method proposed in the paper is straightforward to implement and is applicable to the optimum design of cam profiles of the engine, which is of great importance for the optimal design of cam fabrication processes.

Crankcase Explosions in Marine Diesel Engines: A Computational Study of Unvented and Vented Explosions of Lubricating Oil Mist

Accidental crankcase explosions in marine diesel engines are presumably caused by the inflammation of lubricating oil in air followed by flame propagation and pressure buildup. This manuscript deals with the numerical simulation of internal unvented and vented crankcase explosions of lubricating oil mist using the 3D CFD approach for two-phase turbulent reactive flow with finite-rate turbulent/molecular mixing and chemistry. The lubricating oil mist was treated as either monodispersed with a droplet size of 60 μm or polydispersed with a trimodal droplet size distribution (10 μm (10 wt%), 250 μm (10 wt%), and 500 μm (80 wt%)). The mist was partly pre-evaporated with pre-evaporation degrees of 60%, 70%, and 80%. As an example, a typical low-speed two-stroke six-cylinder marine diesel engine was considered. Four possible accidental ignition sites were considered in different linked segments of the crankcase, namely the leakage of hot blow-by gases through the faulty stuffing box, a hot spot on the crankpin bearing, electrostatic discharge in the open space at the A-frame, and a hot spot on the main bearing. Calculations show that the most important parameter affecting the dynamics of crankcase explosion is the pre-evaporation degree of the oil mist, whereas the oil droplet size distribution plays a minor role. The most severe unvented explosion was caused by the hot spot ignition of the oil mist on the main bearing and flame breaking through the windows connecting the crankcase segments. The predicted maximum rate of pressure rise in the crankcase attained 0.6–0.7 bar/s, whereas the apparent turbulent burning velocity attained 7–8 m/s. The rate of heat release attained a value of 13 MW. Explosion venting caused the rate of pressure rise to decrease and become negative. However, vent opening does not lead to an immediate pressure drop in the crankcase: the pressure keeps growing for a certain time and attains a maximum value that can be a factor of 2 higher than the vent opening pressure.

Production of biofuel from AD digestate waste and their combustion characteristics in a low-speed diesel engine

Anaerobic digestion biogas plants generate large amounts of digestate that cannot always be valorised as fertilizer. This study proposes an alternative use through pyrolysis of the digestate for the production of liquid fuels for compression ignition engines. The digestate pyrolysis oil (DPO) and two types of biodiesel were produced and mixed with different alcohols. A total of five blends of DPO, biodiesel and alcohol were prepared and characterized, showing that their acidity and viscosity were higher than for pure diesel, and their heating value was lower. Blends containing 60 % biodiesel, 20 % DPO, and 20 % butanol were then tested in an engine, showing that the maximum in-cylinder pressure and heat release rate were 4.6 % and 3 % lower, respectively, compared to diesel, and the engine thermal efficiency at full load was 6–8% lower. The nitric oxide and smoke emissions were 7 % and 40 % lower, respectively, but the carbon dioxide emissions were 7–10 % higher than with diesel. The blends showed retarded start of combustion by 1.5° crank angle, which delays the ignition by about 6.4 %. This study concludes that blends can be used as a fuel for agriculture and marine diesel engines, although their viscosity should be reduced by improving the pyrolysis conditions.

Research and Optimization on the Exhaust Flow Characteristics Based on Energy-Splitting Method of the Low-Speed Marine Diesel Engine

Abstract In this study, an optimization scheme for exhaust flow characteristics based on the energy-splitting method is proposed. The low-speed marine diesel engine adopts the exhaust energy-splitting method during the exhaust process. Based on this, a one-dimensional calculation model, a three-dimensional calculation model, and a three-dimensional model of the gas collection box are established. After splitting, the vortex structure in the same phase of the gas collection box has a larger scale, higher flowrate, more significant entropy increase, and more severe turbulence dissipation. The length and diameter of the high-grade gas collection box are optimized. The results show that after optimization, the flow energy dissipation brought by the vortex structure is reduced, and the outlet pressure and flow velocity of the gas collection box are increased.

An Experimental Study of the Effects of Cylinder Lubricating Oils on the Vibration Characteristics of a Two-Stroke Low-Speed Marine Diesel Engine

Abstract Two-stroke, low-speed diesel engines are widely used in large ships due to their good performance and fuel economy. However, there have been few studies of the effects of lubricating oils on the vibration of two-stroke, low-speed diesel engines. In this work, the effects of three different lubricating oils on the vibration characteristics of a low-speed engine are investigated, using the frequency domain, time-frequency domain, fast Fourier transform (FFT) and short-time Fourier transform (STFT) methods. The results show that non-invasive condition monitoring of the wear to a cylinder liner in a low-speed marine engine can be successfully achieved based on vibration signals. Both the FFT and STFT methods are capable of capturing information about combustion in the cylinder online in real time, and the STFT method also provides the ability to visualise the results with more comprehensive information. From the online condition monitoring of vibration signals, cylinder lubricants with medium viscosity and medium alkali content are found to have the best wear protection properties. This result is consistent with those of an elemental analysis of cylinder lubrication properties and an analysis of the data measured from a piston lifted from the cylinder after 300 h of engine operation.

Design and research of a heavy load fatigue test system based on hydraulic control for full-size marine low-speed diesel engine bearings

As the power density design indicators of marine low-speed diesel engines gradually increased, resulting the loads of marine low-speed diesel engine bearings become increasingly complex. It's easy to cause crankshaft fatigue damage, leading to a very significant economic loss. Due to the maximum thrust loads of full-size-bearing is larger than 2000kN, the bearings performance tests are very expensive on a complete diesel engine. So, newly developed bearings must be verified on a specialized test rig. The research goal is to design a novel fatigue life test system (Bearing diameter: 300mm–520mm, Speed: 5r/min-200r/min, Thrust: 0kN–2800kN) that can simulate the working conditions of full-size low-speed marine diesel engine bearings. Adopting the advanced hydraulic-based control passive pressurization technology. Multi-platform joint simulation technology is used to make sure that the maximum thrust of the test rig is proportional to the eccentricity of the eccentric shaft. The final eccentricity of the shaft is determined by 3mm for test rig. The results based on 475mm×150mm bearing is published for the first time. The test load can be well enveloped in the load of the target diesel engine, with maximum load fluctuation less than 0.2%. The test rig can meet the performance testing requirements of full-size bearings.

Research on the characteristics of the exhaust energy splitting method for the two-stroke low speed marine diesel engine

Efficient waste heat recovery (WHR) is the key to improve the thermal efficiency of the marine diesel engine and reduce the cost. According to the characteristics of long stroke of low-speed marine diesel engine, the exhaust energy splitting (EES) method based on exhaust energy distribution is proposed in this paper. EES is a new method of improving waste heat grade by separating high temperature exhaust gas and low temperature scavenging air. Compared with other studies on increasing exhaust temperature, EES significantly increases exhaust temperature and improves waste heat recovery efficiency without negatively affecting engine performance. Moreover, due to the adjustable EES valve, the optimal performance of engine and WHR system can be achieved under various operating conditions by adjusting the exhaust splitting phase (ESP). The One-Dimension and Three-Dimension simulation models for a two-stroke low speed marine diesel engine with waste heat recovery system are established. The exhaust energy distribution and the splitting phase to be optimized are also explored synchronously. The results show that by using the exhaust energy splitting method, temperature after turbine can be increased by more than 100 K compared with the baseline of the original engine system. Using Organic Rankine cycle (ORC) as WHR system, the waste heat recovery efficiency can reach over 17 %. Compared with the original engine, the total output power of the optimized EES low-speed engine and ORC system Increase by 100.90 kW under 75 % load and 154.72 kW under 100 % load, and the NOx emissions can be reduced by 5 %.

Experimental Study of the Influences of Operating Parameters on the Performance, Energy and Exergy Characteristics of a Turbocharged Marine Low-Speed Engine

With appropriate strategy of EVC timing and fuel injection, the engine NOx emission could be reduced with acceptable deterioration of fuel consumption. However, for the mechanism of these favorable results, few studies focus on improving the fuel economy from the perspective of energy and exergy analysis, which could be helpful for providing a deeper comprehensive to researchers. Therefore, an experimental study for the combination of EVC timing and fuel injection strategy is conducted based on a marine low-speed engine with 340 mm bore, then the energy and the exergy analysis are separately carried out according to the obtained experimental results. The experimental results show that delaying the exhaust valve closing (EVC), complemented with an appropriate injection strategy, could improve the marine low-speed diesel engine brake specific fuel consumption (BSFC) with the NOx emission almost kept constant. The following energy balance analysis demonstrates that the losses from exhaust gas and heat transfer account for about 50% of total energy. However, from the perspective of exergy analysis, an opposite conclusion could be obtained; the proportion of exhaust gas and heat transfer exergy could reach about 15%, and the ratio of heat transfer could be higher relatively. The losses caused by irreversibility is the biggest source of all, and the irreversibility from combustion could takes about 70% of the total irreversibility. Finally, the reduction of total irreversibility could reach about 4% by optimizing the parameters of marine low-speed engines.

Significant impacts of injection parameters on marine diesel engines

Nowadays, over 90% of the shipping industry mainly relies on marine transportation. The ability of 2-stroke low-speed diesel engines to consume heavy fuel and even crude oil makes them occupy an important position in the main propulsion power market. Also, it is quite convenient for them to operate reversely, which is needed for the astern situation. Compared with other propulsion power sources such as gas turbines or electro-propulsion, low-speed diesel engines have relatively better part-load performance and long-time stability, making them stand out. Most importantly, their efficiency can reach up to 60% when combined with exhaust boilers and turbine-driven alternators. The majority of those advantages are gained from the delicately designed fuel injection system and the governor system. This article discusses 2-stroke low-speed marine diesel engines' fuel injection systems and how some important parameters influence engine performance. Future opportunities and key challenges in this exciting field are also highlighted.

Numerical Research on Effects of Variable Port Timing on Performance of Marine Low-Speed Two-Stroke Engine

Enhancing the effective expansion ratio to further improve the fuel consumption, this study implemented a kind of Variable Port Timing (VPT) by designing a vertically moving sleeve on the outside of the scavenging port of a low-speed two-stroke diesel engine with a 340 mm bore. A 3D Computational Fluid Dynamics (CFD) model was constructed and calibrated to investigate the influence of the VPT strategy on the engine performance and the internal gas exchange process. The results indicated that the VPT can reduce the negative work from the compression stroke and increase the expansion work from the expansion stroke, which effectively enhances the fuel economy. However, the reduction in the mass flow rate would lead to the severe deterioration of the turbocharging system’s performance. The related matching analysis between the sleeve and the scavenging ports revealed that the sleeve velocity had a minimal influence on the scavenging flow rate, while increasing the height of the scavenging port can restore a certain mass flow rate, but will decrease the in-cylinder swirl intensity, deteriorating the combustion in the cylinders. The optimal approach is to raise the position of the scavenging port, achieving a Scavenging Port Closing (SPC) at a 235°CA, which will restore the scavenging flow rate of the original level to 90.7% and improve the indicated fuel consumption by 2.9 g/kWh.

Experimental Study of Identifying Emission Sources of Acoustic Signals on the Cylinder Body of a Two-Stroke Marine Diesel Engine

In this paper, an experimental method was utilized to investigate acoustic emission (AE) characteristics and to identify emission sources of the nonlinear AE signal on the cylinder body of a large low-speed two-stroke marine diesel engine in real-working conditions on the sea in misfiring and normal firing modes. Measurements focused on the AE signal acquired in a transverse direction in low-frequency (20–80 kHz), medium-frequency (100–400 kHz) and high-frequency (400–900 kHz) ranges. The collected signals were analyzed on the crank angle and crank angle-frequency domains. The results showed that all potential sources of the nonlinear AE signal could be mapped in the low-frequency range. However, only the AE signal caused by the combustion process at around the top dead center could be well-observed in the medium-to-high-frequency range. The findings also revealed that in normal firing conditions, the AE energy radiated by friction in the down-stroke period was smaller than in the up-stroke process due to gas-sealing forces. Moreover, the AE energy in the misfiring condition was higher than in the normal firing state. These outcomes considerably contributed understandings to characteristics of friction and wear around the mid-stroke area of the cylinder on a two-stroke marine diesel engine.

Energy and exergy analysis of VCR engine fueled with rubber-seed oil methyl ester using response surface methodology

This study presents a comprehensive analysis of the thermodynamic performance of a variable compression ratio engine fueled with rubber-seed oil methyl ester blended with diesel. The objective is to evaluate the energy and exergy characteristics according to the first and second laws of thermodynamics for various compression ratios and supercharging pressures. A 3.5 kW engine with compression ratios values ranging from 18:1 to 22:1 and SC pressures of 0, 0.25, and 0.5 bar (g) at 80% load was considered. The inlet manifold pressure was controlled using a centrifugal-type blower (1.5 m3/min, 350 W, 240 V AC) with an adjustable valve. Key fuel energy parameters including shaft energy, cylinder pressure, brake specific energy consumption, thermal efficiency, heat release rate, absorbed energy by cooling water, exhaust gas temperature, exergy efficiency, exergy destruction, exhaust energy, and emissions were analyzed. Results showed significant improvements in shaft energy (35.51%), thermal efficiency (35.37%), and heat release rate (25.17 J/°CA) for compression ratios 20 and supercharging pressure of 0.25 bar, with a reduction in brake specific energy consumption of 2.912 kJ/s kW. The second law efficiency and irreversibility were observed to be 60.31% and 0.0118 kW/K, respectively. Experimental findings demonstrated a reduction in carbon monoxide (25.12%) and hydrocarbon (40%) under the aforementioned conditions. Response surface methodology was employed to predict optimal operating conditions, and the equations were validated using analysis of variance and p-test. The D-optimality test facilitated the determination of optimal operating parameters for different responses through multi-objective optimization techniques. Individual desirability values were combined to obtain a composite desirability of 0.9054. The study identified compression ratios 18 with a supercharging pressure of 1.78 bar as the optimum operating condition for 20% fuel blends in a low-speed agricultural diesel engine, resulting in a remarkable improvement of 41.24% in energy efficiency and 68.45% in exergy efficiency, with minimal emissions.

Improving the Overall Efficiency of Marine Power Systems through Co-Optimization of Top-Bottom Combined Cycle by Means of Exhaust-Gas Bypass: A Semi Empirical Function Analysis Method

The mandatory implementation of the standards laid out in the Energy Efficiency Existing Ship Index (EEXI) and the Carbon Intensity Indicator (CII) requires ships to improve their efficiency and thereby reduce their carbon emissions. To date, the steam Rankine cycle (RC) has been widely used to recover wasted heat from marine main engines to improve the energy-conversion efficiency of ships. However, current marine low-speed diesel engines are usually highly efficient, leading to the low exhaust gas temperature. Additionally, the temperature of waste heat from exhaust gas is too low to be recovered economically by RC. Consequently, a solution has been proposed to improve the overall efficiency by means of waste heat recovery. The exhaust gas is bypassed before the turbocharger, which can decrease the air excess ratio of main engine to increase the exhaust gas temperature, and to achieve high overall efficiency of combined cycle. For quantitative assessments, a semi-empirical formula related to the bypass ratio, the excess air ratio, and the turbocharging efficiency was developed. Furthermore, the semi-empirical formula was verified by testing and engine model. The results showed that the semi-empirical formula accurately represented the relationships of these parameters. Assessment results showed that at the turbocharging efficiency of 68.8%, the exhaust temperature could increase by at least 75 °C, with a bypass ratio of 15%. Moreover, at the optimal bypass ratio of 11.1%, the maximum overall efficiency rose to 54.84% from 50.34%. Finally, EEXI (CII) decreased from 6.1 (4.56) to 5.64 (4.12), with the NOx emissions up to Tier II standard.