The aim of this paper is to identify and investigate the potential and limitations of diesel–gas combustion concepts for high speed large engines operated in gas mode with very small amounts of pilot fuel (<5% diesel fraction). Experimental tests were carried out on a flexible single cylinder research engine (displacement 6.24 dm3) equipped with a common rail system. Various engine configurations and operating parameters were varied and the effects on the combustion process were analyzed. The results presented in this paper include a comparison of the performance of the investigated dual fuel concept to those of a state-of-the-art monofuel gas engine and a state-of-the-art monofuel diesel engine. Evaluation reveals that certain limiting factors exist that prevent the dual fuel engine from performing as well as the superior gas engine. At the same NOx level of 1.3 g/kWh, the efficiency of the dual fuel engine is ≈3.5% pts. lower than that of the gas engine. This is caused by the weaker ignition performance of the injected pilot fuel compared to that of the gas scavenged prechamber of the gas engine. On the other hand, the dual fuel concept has the potential to compete with the diesel engine. The dual fuel engine can be operated at the efficiency level of the diesel engine yet with significantly lower NOx emissions (3.5 g/kWh and 6.3 g/kWh, respectively). Since the injection of pilot fuel is of major importance for flame initialization, and thus for the main combustion event of the dual fuel engine, optical investigations in a spray box, measurements of injection rates, and three-dimensional (3D) computational fluid dynamics (CFD) simulation were conducted to obtain even more detailed insight into these processes. A study on the influence of the diesel fraction shows that diminishing the diesel fraction from 3% to lower values has a significant impact on engine performance because of the effects of such a reduction on injection, ignition delay, and initial flame formation. The presented results illustrate which operating strategy is beneficial for engine performance in terms of low NOx emissions and high efficiency. Moreover, potential measures can be derived which allow for further optimization of the diesel–gas combustion process.

Interest is growing in using fully flexible diesel-gas dual fuel engines for power generation and propulsion on land and sea. Benefits such as the flexibility to adapt the type of fuel to the market situation, fail-safe operation and lower NOx emissions than diesel engines are convincing arguments for engine operators. However, diesel-gas engine concepts still suffer from lower efficiency than state-of-the-art monovalent diesel engines and spark ignited gas engines when operated in the corresponding fuel mode. To meet stringent NOx emission legislation, high diesel substitution rates are necessary, which in turn often lead to poor combustion stability. Especially with these small diesel fractions, the challenge remains to ensure stable ignition, fast combustion of the air-fuel mixture and low hydrocarbon emissions. The aim of this paper is to identify and investigate the potential and limitations of diesel-gas combustion concepts for high speed large engines operated in gas mode with very small amounts of pilot fuel (&lt; 5 % diesel fraction1). Experimental tests were carried out on a flexible single cylinder research engine (swept volume approximately 6 1) equipped with a common rail system. Various engine configurations and operating parameters were varied and the effects on the combustion process were analyzed. The results presented in this paper include a comparison of the performance of the investigated dual fuel concept to those of a state-of-the-art monovalent gas engine and a state-of-the-art monovalent diesel engine. Evaluation reveals that certain limiting factors exist that prevent the dual fuel engine from performing as well as the superior gas engine. On the other hand, the potential is already present for the dual fuel concept to compete with the diesel engine. Since the injection of pilot fuel is of major importance for flame initialization and thus for the main combustion event of the dual fuel engine, optical investigations in a spray box, measurements of injection rates and 3D-CFD simulation were conducted to obtain even more detailed insight into these processes. A study on the influence of the diesel fraction shows that diminishing the diesel fraction from 3 % to lower values has a significant impact on engine performance because of the effects of such a reduction on injection, ignition delay and initial flame formation. An investigation of the influence of the injection timing reveals that with diesel fractions of ≤ 1.5 %, the well-known relationship between the injection timing and combustion phasing of conventional engine concepts is no longer valid. The presented results illustrate which operating strategy is beneficial for engine performance in terms of low NOx emissions and high efficiency. Moreover, potential measures can be derived which allow for further optimization of the diesel-gas combustion process.

The balancing of the electric grid has become more challenging due to the expansion of fluctuating renewable energy sources for electric power generation. The importance of power plants driven by internal combustion engines will increase since they can react flexibly and quickly to changes in the energy demand. With regard to the emission of pollutants and CO2, gas fueled engines are favored for gensets. However, it is more challenging to meet the dynamic load requirements with a gas engine than with a conventional diesel engine because the load acceptance of the gas engine is limited by the occurrence of knocking combustion. Dual fuel engines are a good compromise between these two engine concepts; they can use gaseous fuel during steady state engine operation and increase the diesel share during transient modes to improve the dynamic behavior. The high number of degrees of freedom of dual fuel combustion concepts requires advanced operating strategies. The aim of this paper is to investigate and evaluate strategies to improve the transient behavior of a 20-cylinder large bore diesel-gas engine (displacement 6.24 dm3 per cylinder) for a genset application. In the investigations, the latest turbocharging technology is applied in combination with a turbine waste gate. A wide range diesel injector that covers the whole diesel injection range of approximately 1 % to 100 % diesel fraction1 of the rated power fuel mass provides the basis for the most flexible diesel injection. A 1D simulation tool was used to model and optimize the genset in transient operation. The combustion process was simulated with Vibe heat release rate models. The optimized transient engine operating strategies were validated on a highly dynamic single cylinder research engine test bed. The paper provides a comparison of different strategies that use these technologies to improve the dynamic behavior of the genset in island mode operation during a 50 % load step. Key to meeting the challenging requirements is an optimized diesel injection strategy or even a switch from gas operation mode to diesel operation mode during the load step. Based on the results of simulation and engine testing, potential ways to minimize engine speed drop and recovery time after the load demand increase are evaluated.

Maritime environmental regulations stipulate lower emissions from the shipping industry. To cope with these rules, improving the combustion processes, make use of cleaner alternative fuels and implement exhaust gas cleaning systems is necessary. Alternative fuels, like fish oil, have a potential to reduce soot formation during the combustion process and will be deeply investigated in this paper. For this purpose, two different types of fish oil and their blends with marine gas oil (MGO) have been tested in a constant volume pre-combustion cell (CVPC). The CVPC laboratory was built in collaboration between MARINTEK and NTNU. To generate similar injection condition in the combustion cell as in an internal combustion engine, the CVPC is heated using a chemical heating process. The CVPC is used as a fundamental investigation tool for studying the fuel injection system for large engine applications. Parameters that were studied include the combustion, spray development, fuel evaporation process and ignition delay. The general experimental setup of the test facility is described and the optical methods applied are explained for the investigation of fish oil. The aim is to study soot intensity and spray development and to compare the results to pure low-sulphur MGO as a reference fuel. The results conclude that the combustion and ignition properties of fish oil are very similar to marine gas oil and this makes it applicable as an alternative fuel for power generation in the maritime industry. The tests also showed a significant decrease in soot formation for the two fish oils.

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