Journal of Primeasia

 Introduction

Marine diesel engines have remained largely unchanged in their fundamental operating principles for more than a century, despite increasing regulatory pressure to reduce emissions and improve fuel efficiency. Two-stroke diesel engines continue to dominate large commercial vessels because of their high torque output and mechanical simplicity. However, these advantages are accompanied by significant inefficiencies and environmental challenges, particularly in relation to exhaust energy losses and lubrication-derived pollution. This systematic review examines existing exhaust energy recovery concepts, lubrication-related emissions, and multi-expansion engine architectures, identifying unresolved limitations and synthesizing pathways that motivate emerging hybrid propulsion concepts. A substantial proportion of the chemical energy in marine fuel is lost as waste heat through exhaust gases. Conventional exhaust gas economizers partially recover this energy by generating steam for auxiliary systems, but their contribution to overall propulsion efficiency remains modest (Lee et al., 2019; MAN Energy Solutions, 2022). Turbocharging represents another widely adopted recovery strategy, converting exhaust kinetic energy into increased intake air pressure. However, turbochargers are fundamentally constrained by exhaust pressure availability and cannot extract the full thermodynamic potential of high-temperature exhaust streams. As a result, a large fraction of recoverable energy remains unused.

Advanced waste heat recovery systems, including organic Rankine cycles and multi-stage turbocharging, have been proposed, yet these systems introduce additional complexity, cost, and maintenance demands. Importantly, most existing technologies treat exhaust energy recovery as an auxiliary process rather than an integral extension of the propulsion cycle itself.

Two-stroke marine engines rely on total-loss cylinder lubrication, where oil is injected directly onto cylinder liners and subsequently burned during operation. While necessary for preventing liner wear and scuffing, this system produces sludge as a byproduct, especially when sulfur-rich fuels are used. Sulfur combustion leads to sulfuric acid formation, which is neutralized by alkaline additives in cylinder oil, generating environmentally hazardous waste (MARPOL Annex VI, 1997).

Sludge disposal remains a persistent environmental concern. Although onboard incineration is permitted under MARPOL regulations (Annex I, 1973), incinerators emit additional pollutants and require clean fuel for operation. In practice, enforcement gaps have contributed to illegal sludge discharge at sea, underscoring the environmental consequences of engine architectures that inherently generate hazardous waste.

To address exhaust energy losses, several multi-expansion engine concepts have been explored (Figure 1). The five-stroke engine, notably developed by Ilmor Engineering, demonstrated the theoretical benefits of reusing exhaust gases for additional expansion work (Gerhardy & Denger, 2009). However, systematic analyses reveal a critical limitation: the reduction in exhaust energy after low-pressure expansion impairs turbocharger performance, restricting scalability for heavy-duty marine applications.

Figure 1. Schematic representation of a multi-expansion marine engine architecture.

Four-stroke engines offer advantages in lubrication efficiency and emissions control due to oil recirculation but face unfavorable power-to-weight ratios for large vessels. Consequently, no existing architecture fully resolves the combined challenges of efficiency, emissions, and mechanical suitability for marine propulsion.

The reviewed literature reveals a clear gap between exhaust energy recovery technologies and core propulsion design. Existing systems improve efficiency incrementally but do not fundamentally restructure the propulsion cycle to reuse exhaust energy as active mechanical work. This gap has motivated recent hypothesis-driven proposals that integrate exhaust expansion, synchronized combustion, and advanced thermal management within a unified four-stroke framework.

Rather than presenting experimental validation, these emerging concepts serve as theoretical syntheses of established thermodynamic principles. As such, they represent a valuable direction for future computational modeling, simulation-based optimization, and experimental prototyping. This review highlights the need for systematic evaluation of integrated exhaust reuse architectures as a promising pathway toward cleaner and more efficient marine propulsion systems.

2. Materials and methods

2.1 Methodological Scope and Research Approach

The present study follows a theoretical, hypothesis-driven, and computational modeling methodology to investigate the feasibility of a novel marine propulsion architecture termed the Marine Exhaust Boost Recycle Engine (MEBRE). The development of a physical prototype is not feasible due to limitations in laboratory infrastructure, industrial access, capital investment, and large-scale marine engine testing facilities. Consequently, this research does not claim experimental validation; rather, it aims to establish a rigorous conceptual and analytical framework grounded in established internal combustion engine theory, marine propulsion practice, and comparative engine architecture analysis (Heywood, 1988, 2018; Lee et al., 2019).

The methodological objective is to evaluate whether recycling exhaust gas for secondary mechanical work, combined with water-assisted thermal energy conversion and hybrid turbocharging, can theoretically improve efficiency, reduce emissions, and mitigate known limitations of existing marine diesel engines. The study builds upon prior work on turbocharged four-stroke engines and multi-expansion concepts such as the five-stroke engine developed by Ilmor Engineering, while explicitly addressing their documented shortcomings.

2.2 Baseline Engine Configuration and Reference Models

The proposed engine is conceptually based on a six-cylinder, four-stroke marine diesel engine, representative of propulsion systems used in heavy commercial vessels. Baseline assumptions regarding compression ratio, bore–stroke ratio, air–fuel ratio, and exhaust gas temperature are derived from standard marine engine literature and manufacturer documentation (Heywood, 2018; MAN Energy Solutions, 2022).

Two benchmark engine configurations are used for comparative evaluation:

• A conventional four-stroke turbocharged marine diesel engine, and
• The five-stroke engine architecture developed by Ilmor Engineering, which incorporates secondary exhaust expansion but suffers from limited turbocharger scalability (Gerhardy & Denger, 2009).

These reference models provide the comparative baseline for evaluating improvements in exhaust utilization, turbocharger performance, and mechanical integration.

2.3 Dual Piston Group Architecture and Mechanical Coupling

The defining methodological innovation of the MEBRE architecture is the division of cylinders into two mechanically synchronized piston groups:

• Primary piston group (high-pressure cylinders):

These operate as conventional four-stroke diesel cylinders, producing the main combustion power.

• Secondary piston group (low-pressure cylinders):

These cylinders are dedicated to managing and expanding exhaust gases after the primary exhaust stroke.

The two piston groups are mechanically connected through a shared crankshaft and linking rods, allowing energy from the primary combustion stroke to assist the secondary exhaust expansion stroke. The fundamental hypothesis is that using exhaust gas twice—first to drive a turbocharger and then to perform direct mechanical work—can significantly increase overall energy utilization.

The total indicated work per cycle is expressed as:

Wtotal=pHP dVHP+pLPdVLP

where subscripts HP and LP denote high-pressure and low-pressure cylinders, respectively.

2.4 Exhaust Energy Reuse and Turbocharger–BLDC Hybridization

The engine employs a modified turbocharging system in which exhaust gas energy is supplemented by a brushless DC (BLDC) motor mounted on the turbocharger shaft. The turbocharger shaft therefore receives power from two sources:

Pshaft=Pexhaust+PBLDC

A free-wheel clutch mechanism is incorporated between the driven shaft and the BLDC motor to ensure that electrical power is not wasted when exhaust energy alone is sufficient. This configuration allows the turbocharger to function simultaneously as a turbocharger and an electrically assisted supercharger, particularly during low-speed, transient, or emergency operating conditions.

The inclusion of the BLDC motor is justified not by insufficiency of exhaust energy under nominal conditions, but by the need to:

• Eliminate turbo lag caused by fouled or inefficient turbine blades,
• Enable stable operation on low-quality heavy fuel oil, and
• Provide rapid power augmentation during emergency maneuvers (e.g., collision avoidance, piracy threats).

2.5 Water Injection Strategy and Thermodynamic Modeling

A central methodological component is the direct injection of cold distilled water into both the high-pressure combustion cylinders and the low-pressure exhaust expansion cylinders. Water is selected due to its exceptionally high specific heat capacity and latent heat of vaporization.

The absorbed thermal energy is calculated as:

Qabs=m[wcp,w(Tsat-Tinj)+hfg]

where mw is the injected water mass flow rate.

The vaporization of water produces rapid steam expansion, which contributes additional mechanical force while simultaneously reducing peak combustion temperature. This mechanism mitigates the formation of nitrogen oxides (NO?), whose production rate is strongly dependent on peak temperature:

NOx f(Tpeak)

By redirecting thermal energy into steam expansion rather than excessive temperature rise, the methodology allows higher air-to-fuel ratios without exceeding NO? thresholds. The same mechanism also suppresses sulfur dioxide formation by limiting extreme combustion temperatures.

2.6 Onboard Water Management and Environmental Control

The methodology explicitly incorporates existing shipboard systems. Modern commercial vessels are equipped with both a Fresh Water Generator (FWG) and an Oily Water Separator (OWS). In the proposed system:

• The FWG supplies distilled water for injection,
• The OWS treats oil-contaminated water resulting from condensation and blow-by,
• Treated water is either safely discharged or recycled through the FWG.

Thus, while water injection introduces additional water handling, it does not fundamentally increase pollution risk beyond existing MARPOL-regulated systems.

2.7 Conceptual Origin of Water Injection Mechanism

The conceptual basis for water-assisted expansion is informed by a real-world thermofluid phenomenon observed during an accidental grease fire. When water was introduced into a high-temperature oil environment, instantaneous vaporization caused violent volumetric expansion, dispersing burning oil. While hazardous in open environments, this phenomenon illustrates the extreme expansion potential of water-to-steam phase change, which is deliberately and safely harnessed within a controlled engine cylinder in the proposed design.

This observation motivated the hypothesis that controlled water injection could achieve three objectives:

• Provide supplemental mechanical expansion work,
• Reduce NO? formation by limiting peak temperature, and
• Enable higher air-to-fuel ratios without excessive emissions.

2.8 Rotary Valve System and Sealing Strategy

To improve gas flow and reduce mechanical losses, traditional poppet valves are replaced with rotary valves. Rotary valves eliminate valve lift restrictions and valve float at high speed, improving volumetric efficiency.

Gas flow is modeled using:

m=CdAeff2 p

Recognizing the sealing challenges of rotary valves, the methodology proposes a water–oil hybrid sealing film, combining cooling and lubrication. Although theoretical, this approach addresses thermal expansion and leakage concerns and is identified as a priority area for future experimental validation.

2.9 Firing Order Synchronization and System Stability

A rhythmic synchronized firing order is proposed to ensure that high-pressure combustion strokes and low-pressure exhaust expansion strokes do not interfere destructively. Crankshaft torque balance is analyzed to minimize torsional stress and vibration:

T(theta)=Ia

This synchronization is critical to maintaining mechanical reliability in a multi-expansion architecture.

2.10 Comparative Analysis and Methodological Limitations

Performance is theoretically compared against conventional four-stroke and five-stroke engines in terms of exhaust utilization, turbocharger response, airflow efficiency, and mechanical complexity. All results are predictive and hypothesis-based. The methodology establishes a structured foundation for future CFD simulations, 1-D engine modeling, and experimental prototyping.

3. Results and Discussion

The Marine Exhaust Boost Engine (MEBRE) concept emerges from a long-standing challenge in internal combustion engine (ICE) engineering: how to extract additional useful work from fuel energy that is otherwise lost as exhaust heat, while simultaneously mitigating emissions and maintaining operational reliability in heavy-duty applications. When examined through a systematic review lens, the proposed architecture does not stand in isolation; rather, it synthesizes multiple established but previously disconnected ideas—exhaust energy recovery, staged expansion, water-assisted combustion, rotary valve breathing, and electrically assisted turbocharging—into a single theoretical framework. This discussion situates the MEBRE concept within the broader body of engine research, critically evaluates its theoretical advantages and limitations, and clarifies its potential relevance to future marine propulsion systems.

3.1 Reframing Efficiency Limits in Marine Internal Combustion Engines

Conventional marine diesel engines remain constrained by fundamental thermodynamic inefficiencies. Even state-of-the-art large-bore diesel engines typically convert only 30–48% of fuel chemical energy into mechanical work, with incremental improvements reaching approximately 50% under optimized conditions (Heywood, 1988; Heywood, 2018). Exhaust gas enthalpy, which can account for more than one-third of total energy losses, represents a persistent inefficiency that turbocharging alone cannot fully recover. As reviewed in prior thermodynamic analyses, single-stage turbochargers are inherently limited by pressure ratios, turbine efficiency, and turbo lag, particularly under transient load conditions common in marine operations (Lee et al., 2019).

The MEBRE design addresses this limitation by conceptually extending the expansion process beyond the conventional exhaust stroke. By introducing a secondary low-pressure piston group mechanically linked to the primary high-pressure pistons (Figure 1 and Figure 2), the engine attempts to reclaim exhaust energy through a secondary mechanical expansion phase rather than dissipating it entirely through the turbine. This approach conceptually aligns with earlier multi-stage expansion engines, including five-stroke architectures, but adapts them to a marine-scale diesel context where low rotational speed and high torque dominate operational requirements.

Figure 2. Simplified schematic of the proposed multi-expansion cylinder arrangement showing a high-pressure (HP) combustion cylinder coupled to a low-pressure (LP) expansion cylinder via a one-way exhaust transfer valve. The HP cylinder performs the primary combustion and initial expansion; partially expanded exhaust is routed through the transfer valve into the LP cylinder for a second expansion, improving recovery of exhaust enthalpy. The figure also indicates the exhaust outlet directed toward the turbocharger turbine to preserve boost while enabling staged expansion.

3.2 Exhaust Energy Recycling as a System-Level Strategy

From a system integration perspective, the dual piston-group architecture represents a shift from component-level optimization toward whole-cycle energy management. As illustrated in Table 1, the synchronized firing order allows combustion, exhaust, secondary expansion, and compression assistance to occur concurrently across different cylinders. This rhythmic overlap is not presented as an experimentally verified mechanism, but rather as a theoretically coherent timing strategy designed to smooth torque delivery and reduce crankshaft imbalance.

Table 1. Rhythmic Synchronized Firing and Expansion Cycle of the Marine Exhaust Boost Engine (MEBRE).

Each high-pressure combustion event is deliberately paired with a low-pressure exhaust expansion event, ensuring that exhaust gas energy is utilized twice before final release to the turbocharger.

The linking-rod and shared crankshaft mechanism enable mechanical energy transfer from HP cylinders to LP cylinders without introducing abrupt torque spikes.

The rhythmic sequencing distributes thermal and mechanical loads evenly across the crankshaft, reducing vibration and fatigue.

After the secondary expansion stroke, exhaust gas exits the LP cylinder with sufficient momentum to drive a large turbocharger, even after partial energy extraction.

One LP cylinder is always in a clearing or preparation phase, ensuring uninterrupted cyclic operation (see Figure 2.0).

This rhythmic firing strategy is central to the conceptual validity of the MEBRE architecture. It demonstrates how exhaust energy recovery, crankshaft balance, turbocharger efficiency, and emission mitigation are treated as interdependent system elements, rather than isolated optimizations. The table thus provides a reproducible conceptual framework suitable for future computational modeling and simulation-based validation.

The conceptual advantage lies in the coupling of high-pressure combustion work with low-pressure exhaust recovery. Unlike traditional compound engines or bottoming cycles—which often require separate turbines or Rankine loops—the MEBRE approach 

integrates exhaust recovery directly into the reciprocating mechanism. In theory, this could reduce parasitic losses associated with auxiliary recovery systems while maintaining mechanical simplicity at the system boundary, even if internal linkages become more complex.

3.3 Rotary Valve Adoption and Engine Breathing Reconsidered

A central pillar of the MEBRE concept is the replacement of conventional poppet valves with a rotary valve system (Figure 3). Decades of engine research have shown that while poppet valves offer excellent sealing, they remain a fundamental bottleneck to airflow, particularly at higher volumetric demands (Watson, 1991). Historical attempts to eliminate this bottleneck—ranging from desmodromic systems to camless actuators—have largely failed to achieve widespread adoption due to complexity, cost, or durability concerns.

Figure 3. Flow between chambers X and Y controlled by a poppet valve.

Rotary valves, by contrast, offer an intuitively elegant solution: unobstructed flow paths, elimination of valve springs, and reduced valvetrain mass. The discussion of Ralph Watson’s successful long-term rotary valve operation and contemporary developments by Vaztec provides important contextual grounding, demonstrating that sealing challenges, while significant, are not insurmountable. The proposed use of a water–oil mixture as a dynamic sealant 

reflects an attempt to leverage onboard marine infrastructure (e.g., oily water separators) rather than introducing entirely new subsystems.

Nevertheless, the discussion must acknowledge that rotary valve sealing remains one of the most critical uncertainties. Thermal expansion, wear, and long-term sealing stability under continuous high-load marine operation remain unresolved without simulation or experimental validation. Thus, within a review framework, the rotary valve is best interpreted as a high-potential but high-risk enabling technology rather than a guaranteed improvement.

3.4 Integration with Advanced Combustion and Pre-Chamber Ignition

To further enhance efficiency and combustion stability, the proposed engine architecture integrates a pre-chamber combustion system inspired by Maserati’s high-performance Nettuno V6 engine. In this design, ignition occurs first within a small pre-chamber, generating high-energy flame jets that propagate rapidly into the main combustion chamber through calibrated orifices. This process ensures faster, more complete combustion, improved lean-burn capability, and reduced emissions. The configuration and operation of the pre-chamber and injector arrangement are illustrated in Figures 4 and 5.

Figure 4. Pre-chamber fuel injection system integrated with high-pressure combustion chamber.

Figure 5. Top view of pre-chamber fuel injection system showing flame jet pathways.

In the present concept, multiple computer-controlled fuel injectors are employed within and around the pre-chamber to precisely meter fuel delivery, particularly under lean operating conditions (Figures 4 and 5). For enhanced reliability in extreme or cold environments, a high-wattage laser ignition system is incorporated as an auxiliary ignition source. Furthermore, the pre-chamber geometry is refined using a shaped-charge–inspired profile to directionally focus combustion energy toward the piston crown, thereby increasing thermal efficiency and power output while minimizing emissions (Figure 4).

3.5 Comparative Assessment and System-Level Advantages

Although rotary valve sealing efficiency may be marginally lower than that of conventional poppet valves, the associated losses are minor when compared to the cumulative inefficiencies introduced by traditional valve trains and their auxiliary systems. Empirical and conceptual comparisons indicate that rotary valve engines can achieve superior overall efficiency, reduced mass, and simplified construction. The reduction in required components, as summarized in Table 2, underscores the economic and operational advantages of the rotary valve system. Collectively, these characteristics support the feasibility of rotary valve technology as a practical, scalable, and high-efficiency alternative for future internal combustion engine applications.

Table 2. Comparison of Components in Poppet Valve Engine vs. Rotary Valve Engine. Comparative components list between the average poppet valve engine and "Vaztec" rotary valve engine.

3.6 Water Injection, Combustion Temperature Control, and NOx Mitigation

One of the more innovative—and controversial—elements of the MEBRE concept is direct distilled water injection into both high- and low-pressure combustion environments (Figure 5). Water injection is not a novel idea; it has been historically applied in aviation engines and high-performance systems to suppress knock and control combustion temperature. However, the MEBRE framework reframes water injection as a primary thermodynamic lever rather than an auxiliary safeguard.

The theoretical basis for this approach is well supported by combustion chemistry. Thermal NOx formation, governed primarily by the Zeldovich mechanism, increases exponentially at combustion temperatures above approximately 1,600°C (Zeldovich, 1946). By absorbing latent heat during phase change, injected water reduces peak flame temperatures while simultaneously generating additional expansion work through steam formation. The discussion appropriately links this mechanism to the possibility of operating at higher air-to-fuel ratios—beyond traditional diesel stoichiometric limits—without exceeding NOx thresholds.

This conceptual trade-off is particularly relevant in marine contexts, where regulatory pressure on NOx and sulfur oxide emissions continues to intensify. However, it must be emphasized that water injection introduces its own challenges, including corrosion risk, injector durability, and control complexity. The reliance on onboard fresh water generation systems partially 

mitigates logistical concerns, but long-term impacts on engine materials remain speculative within the current framework.

3.7 Electrically Assisted Turbocharging and Operational Resilience

The integration of a BLDC motor and magnetic freewheel into the turbocharger assembly (Figure 6 and Figure 7) represents a strategic response to turbo lag, fuel quality variability, and emergency operational demands. Electrically assisted turbocharging has been explored in automotive research, but its application in low-speed, high-mass marine engines introduces a distinct operational logic.

Figure 6. Sequential stages of pre-chamber combustion with distilled water spray integration. High energy flame jet moving direction and direct water spray in the live combustion process

Figure 7. Schematic of exhaust-driven turbo-compressor with BLDC motor assist. Highly complex turbocharger to act as turbocharger and occasionally super charger both

Rather than prioritizing transient acceleration alone, the MEBRE concept frames electric assistance as a reliability and adaptability feature. The ability to operate effectively on low-grade heavy fuel oil, maintain boost during fouling conditions, and deliver rapid power increases during emergencies (e.g., collision avoidance) reflects real-world marine priorities. The magnetic freewheel mechanism, while novel, is conceptually aligned with existing electromechanical coupling strategies used in high-torque industrial systems.

From a review standpoint, this subsystem exemplifies how hybridization in marine engines may evolve differently from road vehicles—prioritizing robustness and fuel flexibility over peak efficiency alone.

3.8 AI-Assisted Control and System Complexity

The proposed AI-based engine management system (Figure 8, Figure 9) reflects a growing trend toward data-driven control in complex mechanical systems. By integrating combustion temperature, exhaust parameters, fuel quality, and environmental conditions into adaptive injection and timing strategies, the MEBRE framework acknowledges that such a complex engine architecture cannot be governed by static control maps alone.

Figure 8. Schematic of ratchet mechanism with integrated permanent and electromagnets.  Highly complex turbocharger to act as turbocharger and occasionally super charger both

Figure 9. Valve timing and injection phases for main and secondary cylinders with water spray integration. MEBRE engine’s complex synchronized timing diagram

However, this also introduces a critical discussion point: system complexity versus operational reliability. Marine engines are traditionally valued for their predictability and serviceability under harsh conditions. While AI-driven diagnostics and control offer potential gains in efficiency and maintenance forecasting, they also require redundancy, cybersecurity considerations, and crew training—issues that fall beyond thermodynamic performance but are central to real-world adoption.

4. Comparative Perspective and Conceptual Validity

When evaluated against conventional four-stroke diesel engines, established five-stroke concepts, and alternative valve actuation systems, the Marine Exhaust Boost Engine (MEBRE) is best understood as a hybrid integrative architecture rather than a disruptive replacement of existing engine paradigms. The proposed system does not challenge fundamental thermodynamic limits governing internal combustion engines; instead, it seeks incremental but cumulative efficiency gains through improved utilization of energy streams that are traditionally underexploited, particularly exhaust gas enthalpy.

This positioning is consistent with the intent of a review-based manuscript derived from a hypothesis-driven proposal. Rather than claiming verified improvements in brake thermal efficiency, emission reduction, or durability, the present work maps a coherent conceptual pathway that links prior research on multi-stroke expansion, exhaust energy recovery, water-assisted combustion, rotary valve breathing, and electrically assisted turbocharging into a unified system-level framework. In this sense, the MEBRE should be interpreted not as a finalized engineering solution, but as a structured research agenda that highlights how existing technologies may be recombined to address long-standing inefficiencies in marine internal combustion engines.

5. Limitations and Future Research Directions

The most significant limitation of the present study is the absence of numerical simulation, prototype development, and experimental validation. While the proposed firing sequences, thermodynamic reasoning, and mechanical configurations exhibit internal logical consistency, they cannot substitute for quantitative assessment. Critical performance indicators—such as brake specific fuel consumption, exhaust temperature profiles, sealing losses, transient turbocharger response, and long-term component durability—remain unverified within the current scope.

Future investigations should therefore prioritize multi-physics computational modeling, integrating combustion kinetics, heat transfer, structural stress, lubrication behavior, and dynamic balancing. Particular attention should be given to rotary valve sealing under thermal expansion, synchronization stability between primary and secondary piston groups, and the long-term effects of direct water injection on combustion chamber materials. Experimental validation through scaled test rigs or subsystem-level prototyping would be essential before any claims of commercial viability can be made.

Despite these limitations, the present work retains value within a systematic review context. By synthesizing concepts from exhaust energy recovery, combustion chemistry, mechanical design, and intelligent control systems, it identifies underexplored intersections in marine engine research that merit focused investigation. The framework thus serves as a conceptual bridge between established knowledge and future experimental efforts.

6. Advantages and Innovations

6.1 Thermal Efficiency Enhancement

A primary innovation of the MEBRE concept lies in its recycling of exhaust energy through a synchronized dual-piston architecture. The rhythmic firing order is theoretically designed to convert a greater proportion of thermal energy into mechanical work without imposing excessive stress on the crankshaft. Additionally, the proposed modified pre-chamber system builds upon existing pre-chamber ignition concepts by incorporating a shape-charge-inspired geometry, adapted from high-velocity jet formation principles used in aerospace and defense applications. By accelerating and directing the pre-ignition flame through narrow passages, the system aims to promote faster flame propagation, more uniform combustion, and higher torque output than conventional pre-chamber designs.

6.2 Emission Reduction Potential

The combined effects of internal exhaust gas recirculation, reduced peak combustion temperatures through water injection, and improved mixture homogeneity suggest a strong theoretical capacity for NOx mitigation. By shifting a portion of thermal energy into steam expansion rather than peak flame temperature, the system seeks to decouple the traditional trade-off between high air-to-fuel ratios and nitrogen oxide formation.

6.3 Fuel Flexibility

The engine architecture is designed to accommodate a wide range of fuels, including diesel and low-quality heavy or crude oils. Electrically assisted turbocharging and enhanced combustion stability may allow reliable operation even under variable fuel quality conditions, a critical requirement in marine and industrial environments.

6.4 Application Versatility

Although optimized for marine propulsion, the MEBRE concept is theoretically adaptable to large-scale stationary power generation, industrial power plants, data centers, heavy transport vehicles, locomotives, and offshore platforms. Its suitability increases with scale, where height and system complexity are less restrictive.

6.5 Operational Cost Reduction

By extracting additional mechanical work from the same quantity of fuel, the system conceptually offers lower fuel consumption per unit power output, translating into reduced operating costs over long-duration, high-load duty cycles.

7. Challenges, Limitations, and Disadvantages

The primary drawback of the MEBRE design is system complexity, particularly in exhaust gas management and turbocharger integration. The addition of secondary piston groups, freewheel-assisted electric turbocharging, and water injection systems increases both mechanical and maintenance demands. Furthermore, direct water injection into the combustion process raises environmental concerns related to oily-water vapor formation and the potential for sulfur dioxide interaction in sulfur-containing fuels.

Cooling of exhaust gases may also reduce economizer effectiveness, impacting waste-heat recovery systems commonly used on ships. The reliance on advanced components—such as rotary valves, BLDC motors, and intelligent control systems—necessitates higher technical expertise and increases lifecycle maintenance costs.

From a structural standpoint, the vertically stacked dual-cylinder configuration results in a taller engine profile (Figure 2), shifting the center of gravity upward and limiting applicability in smaller platforms such as motorcycles or passenger vehicles. However, this constraint is less significant for large ships, heavy trucks, mining equipment, power plants, and military vehicles, where spatial and mass allowances are greater.

Finally, the lack of simulation capability and experimental validation remains a fundamental limitation. At present, performance expectations are based on engineering reasoning and analogical inference rather than quantitative proof.

8. Conclusion

This study presents a comprehensive theoretical framework for the Marine Exhaust Boost Engine, proposing a novel exhaust recycling strategy combined with alternative valve architecture and water-assisted combustion. By addressing long-standing challenges—such as exhaust energy loss, valve-float-limited RPM constraints, combustion inefficiency, and NOx formation—the concept offers a fresh perspective on how internal combustion engines may evolve rather than be abruptly replaced.

The adoption of rotary valves eliminates poppet-valve-related RPM limitations and reduces component count, potentially lowering manufacturing and maintenance costs. The modified pre-chamber design promotes faster and more complete combustion, improving power output and fuel economy. Most notably, direct water injection provides a conceptual solution to the historic trade-off between high air-to-fuel ratios and NOx emissions, allowing efficiency improvements without proportional environmental penalties.

While the Marine Exhaust Boost Engine is not presented as a validated technology, it represents a meaningful conceptual contribution to sustainable marine propulsion research. As the maritime sector navigates the long transition between fossil fuel dominance and full electrification, such integrative engine concepts may serve as important intermediate solutions and foundations for future hybrid propulsion systems.

Author contributions

A.S.A.I. conceptualized the study, developed the theoretical framework, and led the analysis of engine dynamics, combustion behavior, and fuel optimization strategies. He also contributed to the interpretation of propulsion efficiency metrics and their integration with hydrodynamic performance. E.R.A. contributed to the investigation of propeller–hull interactions, data synthesis, and comparative evaluation of traditional and advanced propulsion schemes. Both authors collaboratively designed the methodology, analyzed the results, and contributed to drafting, reviewing, and revising the manuscript. All authors have read and approved the final version of the manuscript.

Acknowledgment

The authors would like to acknowledge the support provided by the Canadian University of Bangladesh for facilitating the academic environment necessary to conduct this research. The authors are grateful to colleagues and peers from the Department of Engineering for their constructive discussions and technical insights related to marine propulsion systems, engine thermodynamics, and hydrodynamic modeling. The authors also acknowledge the use of publicly available technical literature and analytical tools that supported the comparative evaluations presented in this research.

References