Is the “K Value” of an Engine Truly Fuel Independent? 2020-01-0615
The octane appetite of an engine is frequently characterised by the so-called K value. It is usually assumed that K is dependent only on the thermodynamic conditions in the engine when knock occurs, and not dependent on the fuel. In this work we test this hypothesis: further analysis was conducted on experimental results from SAE 2019-01-0035 in which a matrix of fuels was tested in a single cylinder engine. The fuels consisted of a relatively small number of components and the RON and MON were decorrelated. This simplifies the analysis of the chemical kinetic proprieties. It was shown previously that K increases with engine speed because a higher temperature and pressure is reached along the compression isentrope before knock onset.
Through dividing the original fuels matrix into subsets, it was possible to explore the variation of K value with fuel properties. It was found that K value tends to increase slightly with RON. The explanation for this finding is that higher RON leads to advanced ignition timing (i.e. closer to MBT conditions) and advanced ignition timing results in faster combustion because of the higher pressures and temperatures reached in the thermodynamic trajectory. The Livengood-Wu integral can be employed to show that for higher octane fuels, knock onset occurs at a higher temperature and pressure.
This explanation is consistent with the explanation as to why K value increases with engine speed. Thus, it is shown that the fuel octane quality can impact K value, but the mechanism appears to be through its influence on the thermodynamic trajectory in the engine.
Roger Cracknell, Masaharu Kassai, Taisuke Shiraishi, Andrea Festa, Sandro Gail, Allen Aradi, Masahiko Shibuya
Shell Global Solutions (UK), Nissan Motor Co., Ltd., University of Bath, Shell Global Solutions (Deutschland) GmbH, Shell Global Solutions (US), Shell Lubricants Japan K. K.