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 (< 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.

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