Energy efficiency analysis of marine high-powered medium-speed diesel engine base on energy balance and exergy

Highlights

• Evaluation of energy efficiency of high-powered medium-speed marine diesel engine by both energy balance and exergy analysis.

• Model of diesel engine thermodynamic cycle is developed.

• Effects of combustion parameters on energy loss are investigated to improve the energy efficiency.

Abstract

High-powered, medium-speed diesel engines are widely used in merchant ships. Improving energy efficiency by using energy rationally is critical to reducing both environmental pollution and transport charges. In this paper, the energy efficiency of marine high-powered, medium-speed diesel engine is investigated. A new thermal cycle model of the marine engine is developed using AVL-Boost software. The thermodynamic data of the engine thermal cycle are obtained from the AVL-Boost software. The energy efficiency of the marine diesel engine is evaluated using both energy balance and exergy analysis. From the energy balance analysis, about 25% of the total energy is lost through exhaust heat. This forms the largest energy loss. However, using exergy analysis the largest energy loss originates from the irreversible exergy loss produced during the combustion process. This represents about 36% of the total energy loss. To explore ways to reduce energy loss and improve the energy efficiency, the effects of critical combustion parameters in the thermal cycle (combustion quality index, combustion starting angle and combustion duration angle) on energy distributions are discussed. The decrease in the combustion quality index, combustion starting angle and combustion duration angle (within a reasonable range) all contribute to reduce the total energy loss, increasing the indicated work and improving energy efficiency.

Introduction

Diesel engines are widely used in transportation as the primary items of equipment for generating power. They are reliable and have a wide power and speed range. Over the coming decades, they will continue to occupy a dominant position in mobile power generation [1,2]. Improving energy efficiency is an ongoing goal of diesel engine engineering, primarily due to the attention directed towards exhaust emissions [3,4]. Energy efficiency analysis can clarify the law of energy conversion, distribution and use, identify the unreasonable and wasteful use of energy, and provide a reference for the rational use of energy [5,6].

The shipping industry is the major mode of transport for the world trade [1]. High-powered, medium-speed diesel engines have been widely used as main engines and diesel generators [1,4]. More than 90% of merchant ships use diesel engines as their main engines and almost all generators are diesel driven [7]. Although powerful, the marine diesel engine has large fuel consumption e.g. a marine engine with a power output of 10,000 kW consumes approximately 5000 g/kWh of fuel. Fuel costs account for 30–55% of the total vessel operational cost [8,9]. Furthermore, the power output generated from the engine accounts for only 30–45% of the total fuel energy with the remainder being discharged as waste heat. Recently, depressed shipping markets caused by global economic risks have forced shipping companies to focus on cost savings, resulting in an increased need to reduce energy consumption [10].

Considerable research into the energy efficiency of diesel engines already exists [11,12]. Taymaz et al. [13,14] employed a thermal balance method to analyze the energy efficiency of diesel engine. They evaluated the heat losses at different engine loads and speeds in diesel engines with the effect of ceramic coating and reported a reduction in heat loss to the coolant between 5 and 25%. Models of engine thermal balance and engine body heat transfer were introduced by Yu et al. [15] to evaluate the engine heat dissipation and to determine the engine coolant requirements. They used a lumped parameter model and their stable-state analysis results showed that the combustion heat distribution in subsystems can be the basis of improved engine design. Using energy balance analyses, Yuksel et al. investigated the effects on the heat distribution and performance of a four-stroke engine by adding constant quantity hydrogen to the gasoline-air mixture [16]. Their results showed a significant reduction in heat loss to the cooling water and unaccounted losses by about 36% and 30% (respectively) of the average values could be achieved by using hydrogen supplementation. Simultaneously, heat loss through the exhaust gases was nearly the same as that of gasoline.

The exergy analysis method has also been used in assessing the energy efficiency analysis of the internal combustion engine [[17], [18], [19]]. Caliskan et al. [20] reviewed exergetic efficiency analysis and assessed various types of engines. Their results show that exergetic efficiency is best achieved using four-stroke, four-cylinder, turbocharged diesel engines at about 30% excepting to stationary diesel engine. Exergetic efficiency can be higher at lower speeds: between 1140 rpm/min and 2200 rpm/min. Azoumah et al. [21,22] calculated the exergy efficiency and engine performance of a direct injection compression ignition engine using a variety of biofuels. Their results proposed a tradeoff zone of engine load (based on the second law of thermodynamics) that could accommodate environmental concerns and engine efficiency. This tradeoff zone lay between 60% and 70% of the maximum engine load (6 kW in this case). Muammer et al. [23] conducted the exergy analyses of a diesel engine, analyzing the effect of pre-injection timing on engine performance. Yamegueu et al. [21,24] used exergy analysis combined with gas emissions analysis to optimize the performance of a compression ignition (CI) engine using biofuels. They showed that this combination of exergy and gas emissions analyses is a very effective tool for evaluating the optimal loads that can be supplied by CI engines.

In summary, thermal energy is supplied to the internal combustion engine by the combustion reaction of the fuel and transformed into work, heat in exhaust and heat in cooling water. There are two main methods to analyze and evaluate the energy efficiency of the diesel engine; thermal balance (energy balance) and exergy analysis [25,26]. The energy balance analyses of internal combustion engines can determine the heat distribution and provide theoretical guidance and reference data for quantifying the reduction in heat loss. Energy balance analysis deals with energy conservation whereas the exergy analysis focuses on the availability of energy, which determines the ability of a system to do work in a specific environment [27,28]. Exergy can be destroyed by irreversible processes during combustion, heat transfer, friction and mixing in the thermodynamic cycles of internal combustion engines. This is in contrast to energy, which is neither created nor destroyed. Identifying sources of exergy destruction and reducing the exergy loss in an internal combustion engine is also crucial to enhance the engine efficiency [25,28]. Therefore, it is necessary to analyze the energy efficiency of internal combustion engines by comprehensively using both the energy balance and the exergy analysis.

However, in the past, researchers have evaluated the energy efficiency of internal combustion engines only using either energy balance analysis or exergy analysis. There is limited research that combines both energy balance and exergy analysis to assess the energy efficiency of the diesel engine [29,30]. In addition, compared with the internal combustion engines used in other fields (such as cars, trucks, engineering vehicles and trains), the marine diesel engines have high manufacturing cost, large size (some are more than ten meters high), high power requirements, a range of auxiliary equipment and complex operations. Concomitantly, the experiment conditions required by the marine engines are much higher than in other fields. Energy consumption and other costs in the experimental process are also quite high [8,10]. A number of factors combine to make the experimental study of high-powered medium-speed marine diesel engine difficult; they require high manpower, large material resources and considerable technical support. Therefore, to date there are few studies and references on the energy efficiency of marine high-powered medium-speed diesel engine. The establishment of thermodynamic cycle model, energy balance analysis and exergy analysis are necessary to improve the energy efficiency of marine diesel engine.

In this paper, the energy efficiency of high-powered, medium-speed marine diesel engine is investigated using a new approach combining both energy balance and exergy analyses. The software AVL-Boost is employed to analyze the thermal cycle of the diesel engine and the simulation results are validated by comparing output with experimental results. Combined with results from the thermal cycle model of the diesel engine, the energy efficiency of the marine diesel engine is analyzed using both energy balance and exergy. The amount of heat energy and exergy in the processes of conversion, transmission, utilization and losses are identified and quantified, and the largest losses of heat energy and exergy are found. To explore ways to reduce energy loss and improve the energy efficiency, the effects of critical combustion parameters in the thermal cycle (i.e. combustion quality index, combustion starting angle and combustion duration angle) on energy distributions are discussed.

The results provide theoretical guidance and reference data for improving the energy efficiency and decreasing energy consumption of diesel engine from the point of both “energy quantity” view and “energy quality” view. Moreover, this study also provides a theoretical foundation for studying the energy efficiency of diesel engine by both energy balance and exergy analysis method.