EXERGY STUDY TAKES PRACTICAL APPROACH TO DUAL-FUEL ENGINE PERFORMANCE

Exergy, also known as available energy, is a concept arising from Second Law of Thermodynamics which quantifies the maximum amount of work obtainable from a given system.

PhD student Beichuan Hong and his supervisors Professor Anders Christiansen Erlandsson and Dr Andreas Cronhjort from the KTH Royal Institute of Technology in Sweden set out to take a more practical approach to understanding the theoretical concept of exergy on engine component design than has been achieved in the past.

“Many previous studies have discussed engine performance from the exergy perspective,” said Hong. “However, since exergy is an advanced concept from thermodynamics, it usually explains the engine procedures in theoretical terms as irreversibility or entropy generation. This approach is not always easy to grasp. Therefore, people usually apply it at the macroscopic level without linking it to specific engineering problems.

“Although marine engines are quite energy efficient, it is important to look at the theoretical efficiency limit, and, as a system, the parts which hinder us from reaching that limit in practice. By taking a comprehensive view of the engine, we attempted to give some answers,” said Hong.

Their study examined the exergy losses inside the engine components of a 10-cylinder, V-bank Wärtsilä 31DF engine with a two-stage, serial turbocharger. The high and low pressure turbochargers were sequentially arranged with charge cooling after each compression stage, and the system used an air bypass valve and exhaust waste gate valves to improve the charging efficiency and for component protection.

The engine was operated at 25%, 50%, 75% and 100% load at a working speed of 750rpm. The exergy losses were characterized as: combustion-related, heat dissipation or gas exchange losses. A 1D simulation model of the engine was developed and validated on the commercial platform, GT-Suite. Dynamic sub-models were separately calibrated, including, for example, gas flow property across the inlet valve, heat transfer in the cylinders and pipes and mechanical friction at the crankshaft.

“Instead of focusing on a particular engine component, we reviewed the engine performance at the systematic level and analysed the inefficiencies as well as the reasons why these losses occur,” said Hong. “In short, this project tried to demonstrate how far we are from the ‘ideal’ engine and where the potential for further improvement is.”

The researchers found that combustion exergy destruction dominated for the engine, contributing more than half of the total exergy destruction. The flow exergy destroyed in gas exchange, as the second highest contributor, was slightly higher than the rest of the exergy destruction types, and it increased at high load. Heat dissipation maintained the same level of loss by engine mechanical friction and was less sensitive to fuel mode and operating loads.

Fuel Exergy

The engine was evaluated in dual-fuel mode with compressed natural gas (CNG) (using a premixed charge ignited by pilot diesel) and in conventional diesel mode. “Such an engine configuration enabled us to compare the difference between the two combustion modes,” said Hong.

Based on a calibrated 1D simulation, the researchers found that combustion irreversibility was the largest source of engine exergy losses amounting to at least 25% of fuel exergy. The exergy destruction was characterised as resulting from chemical reactions, heat conduction and mass transfer of the non-equilibrium combustion gases, with the chemical reactions always making up the largest proportion of the exergy destruction.

“According to our analysis, dual-fuel mode had a higher efficiency than diesel mode. For example, at the full-load operating point with the same brake mean effective pressure (BMEP), the energy efficiency at dual-fuel mode was 48.6% while diesel mode it was 47%. One reason for this is that the CNG fuel has a higher energy density (i.e. lower heating value) than most common fossil fuels. Also, the combustion of CNG premixed charge is easier to concentrate at top dead centre. This makes the combustion process faster and more homogeneous in gas mode, leading to less entropy generation.”

The fuel exergy losses cause a reduction in indicated mean effective pressure (IMEP) and exhaust energy which further limits the amount of flow energy used to drive the turbines or other waste energy recovery systems. To have a loss of over 25% of fuel exergy indicates it is still very promising to optimize in-cylinder processes for faster combustion, lower heat loss and better work extraction even though the engine is already energy efficient, says Hong.

Waste Heat

There were two categories of waste heat considered: the flow energy contained by the exhaust after passing through the turbines and after-treatment and the energy carried by the coolers. For these, the total waste heat accounted for more than 30% of the fuel energy. However, the exergy analysis indicated that the energy quality of these waste heat sources was not high, said Hong.

“Theoretically, for the tested marine engine system, converting waste heat to mechanical work may recover a maximum of 9% of the fuel energy (e.g. 492kW in diesel mode for 5500kW engine output power). An alternative solution for this situation is that it could be directly used as a heating system. In the case of a co-generation system the efficiency of energy recovery could be much higher.”

Gas Exchange Losses

Exergy destruction in the gas exchange system was caused by flow losses resulting from piping restrictions caused by valve throttling, by fluid friction in the pipes and by the flow energy recovery of the turbochargers. Most of the exergy destruction occurred in the turbocharging system where the high-pressure turbocharger contributed around 40% of the total flow exergy destruction. Unlike the flow losses in the piping system, the flow exergy in the turbocharger was not entirely wasted, because it contributed to the work extracted by the turbine and the air boosting by the compressor.

“While intake and exhaust valves accounted for almost 20% of the flow exergy destruction, 70%-80% of the flow exergy destruction in the gas paths occurred at the turbocharging system. For each charging stage, the compressor always caused more exergy destruction than the turbine side. However, the flow losses by the valves also rose with load due to the increasing mass flow,” said Hong.

Unlike the flow losses in the engine piping system that are simply wasted, the work extraction by the turbine and the air boosting by the compressor also caused exergy destruction. “Our exergy analysis shows that the current two-stage turbocharging system in the tested marine engine has a pretty high efficiency for extracting the flow exergy. However, the turbines also destroy 15% of flow exergy whilst recovering the rest and sending to the compressor. Perhaps, exergy-related R&D goals like reducing entropy generation could be a consideration in further research on the turbomachinery side,” said Hong.

“Of particular interest to us is the quantification of available energy inside the exhaust flow. The flow exergy is equivalent to 65% of the crank shaft output power. Hence the management of exhaust energy recovery indeed plays an important role in improving marine engine efficiency. When we compared the marine engine results to a truck engine pre-study, we found that 20% of flow exergy destruction occurs at intake and exhaust valves, while for truck engines, it was around 7%. This was rather unexpected and implies the sensitivity of the valve motions. Also, further analysis is needed to investigate the effects of Miller timing strategy on flow exergy utilization.”

Therefore, the next step of Hong’s research plan focuses on the flow exergy in the exhaust system. “The aim of this project is to understand how to reduce flow losses and optimize the usage of flow exergy. One part of our project is to integrate Computational Fluid Dynamics simulations with measurement data for modifying the potential design flaws, especially for the configuration of the exhaust system such as the pipe geometry and valve profiles. This study will also enhance our understanding of engine exhaust exergy, especially how it relates to the performance of the turbine or other waste heat recovery systems.”

Fast flow sensors

Apart from its engineering application, one of Hong and his supervisors’ ongoing studies involves characterising exhaust energy pulses using fast measurement techniques. “One big challenge for fast flow exergy measurement comes from the development of fast flow sensors. The flow sensors are expected to survive in the marine engine exhaust and keep their high sampling accuracy. For fast pressure sensors, there are many commercial ones that can be used. However, for fast temperature and flow velocity measurement, we need to design and fabricate one ourselves. Luckily, our colleagues from KTH Fluid Physics Laboratory are collaborating with us to develop high-accuracy engine pulse-orientated sensors,” said Hong.

Engine simulation approach

Wärtsilä supported the study with both data and supervision, and the company states: “The study presents its results and findings based on simulation model provided by Wärtsilä and not on real engine measurements. This means that the accuracy of result is dependent on calibration input of the simulation models operating conditions. In real testing and measurements, the result is varying contingent on operating and ambient conditions. As such, the final result is dependent on accuracy level of the model where absolute values are not the most relevant findings but rather the behaviour and principle of energy and exergy losses that occur in medium-speed engines. The study is part of the continuous development process within Wärtsilä and provides valuable input to further improve the efficiency of our engines and continue to have the highest efficiency among medium speed.”