Large bore marine engines are a major source of fossil fuel consumption in the transport sector. The development of more efficient and cleaner marine engine systems are always required. However, as one of the most efficient and optimized thermal machines, the identification of further improvement of modern marine engines presents a challenge to both industry and academia.

Exergy analysis is a second-law based approach to indicate the maximum amount of work obtainable from a given system. Compared to the first law, exergy applied on marine engine can show the irreversibilities of engine processes which can cause the degradation of energetic quality, and hence impede the potential for high efficiency energy conversion.

In this study, an exergy analysis is used to identify losses and improvement potential of a large bore Wärtsilä 31DF four-stroke marine engine system with two-stage turbocharging. An exergy-based framework is implemented on a calibrated 1D engine model to view the evolution of exergy flow over each engine sub-system while operating on different load points fuelled with natural gas and diesel separately.

The overall distributions of engine energy and exergy are initially compared at a systematic level regarding the impact of fuel mode and operating load. The comparison shows that only 25% of the heat loss and exhaust energy post turbocharger is recoverable. According to the exergy analysis, the total engine irreversibility of diesel mode is generally higher than natural gas mode, and the fraction of irreversibility reduces as engine load increases for both fuel modes.

Furthermore, the engine irreversibilities are characterized as three types: combustion, heat dissipation, and gas exchange losses. For each irreversibility, the corresponding engine components are identified and their exergy destruction levels quantified. The first type, combustion irreversibility, is the largest source of engine exergy losses amounting to at least 24% of fuel exergy. The combustion irreversibility reduces with engine load for both fuels.  A crank resolved analysis showed that premixed gas combustion produces lower exergy losses compared to diesel diffusion combustion.  The second type, thermal exergy transferred and destroyed by heat losses, are summarized for the entire engine system. Cylinder heat transfer and charge air cooling processes account for the greatest fraction of exergy loss due to the heat dissipation. From the exergy view, the charge coolers present an opportunity to recover about 9% of the brake power at full load. The last type, gas exchange losses, are categorized by accounting the flow losses caused by the valve throttling, fluid friction in pipes and the irreversibility of the two-stage turbocharging system.  The flow exergy evolution over each gas exchange component and the exergy recovered by the two-stage turbocharging are also presented. Most of exergy destruction in gas paths occurs at turbocharging system, where the high pressure turbocharger contributes to around 40% of the total flow exergy destruction. The fraction of low pressure turbocharger irreversibility grows significantly with increasing load.

This study helped identify the major sources of irreversibility in a highly efficient marine engine system. Such analysis can further be used to quantify improvement potential for combustion process, heat recovery and gas exchange systems. 


Quantification of Losses and Irreversibilities in a Marine Engine for Gas and Diesel Fuelled Operation Using an Exergy Analysis Approach

Division / Sponsors: 
Internal Combustion Engine Division (ICED)

Paper Proceedings: https://asmedigitalcollection.asme.org/ICEF/ICEF2020/volume/84034

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