Dual fuel diesel engine at variable operating conditions: a numerical and experimental study

The dual-fuel (diesel/natural gas, NG) concept represents a solution to reduce emissions from diesel engines by using natural gas as an alternative fuel. As well known, the dual-fuel technology has the potential to offer significant improvements in the emissions of carbon dioxide from light-duty compression ignition engines. A further important requirement of the DF operation in automotive engines is a satisfactory response in a wide range of load levels. In particular, the part-load levels could present more challenging conditions for an efficient combustion development, due to the poor fuel/air ratio. Basing on the above assumptions, the authors discuss in this article the results of a combined numerical and experimental study on the effect of different injection timings on performance and pollutant fractions of a common rail diesel engine supplied with natural gas and diesel oil. The study of dual-fuel engines that is carried out in this paper aims at the evaluation of the CFD capability to analyze the main phenomena that characterize this particular technology. Actually, in order to put into evidence the key processes that take place during the dual-fuel operation, say the typical flame propagation throughout the premixed methane-air medium that is activated by the early self-ignition of the diesel fuel, the fluiddynamic calculations are extremely useful. The different conditions have been induced by changes in the operating parameter setting; in particular, the tests have been performed by varying the injection timing for a fixed NG ratio, both at full and at part load. Actually, the start of the liquid fuel injection can considerably influence the combustion development and therefore THC and NOx fractions production. The calculations have been validated with the experimental data and a comparison between diesel and dual fuel diesel/CNG operation has been made, in terms of performance and pollutant levels. A detailed description of the phenomena that govern the engine response at several operating conditions and at different load levels is then provided.