New GKI - Gasoline Knock Index for Rating of Fuel’s Knock Resistance on an Upgraded CFR Test Engine 2018-01-1743
In terms of international efforts for conservation of resources and reducing CO2-emission, the thermal efficiency of SI engines needs to be increased. One key enabler to achieve this goal is the availability of highly knock-resistant fuels: it allows to break up the trade-off between elevated compression ratio demanded for high part-load efficiency and a reduced knock tendency at high engine loads by a minimized requirement for adverse spark retard. In view of the world’s fuel map, which is dominated nowadays by qualities between 91 and 98 RON, there is a beginning transition towards increased knock resistance (above 100 RON) being observed in several countries. The corresponding standards for engine-based fuel quality rating provide a RON scale covering the range from 40 to 120.3, which basically seems to be enough. At a second glance the change in reference material from isooctane/n-heptane mixtures towards isooctane with TEL for RON > 100 changes the rating behavior of the method. Comprehensive research and development has been established considering the test engine operation mode as well as the data processing, conclusively defining an improved test method as a suggestion for future knock resistance rating. In order to increase transferability of the fuel’s quality number to modern engines, especially in view to RDE legislation, the operation mode is basically characterized by a stoichiometric mixture composition in combination with a closed-loop control of the combustion phasing. The CR achievable at predefined knock intensity, statistically assessed by a 95% cumulative frequency of knock pressure amplitudes, defines knock resistance without relevant limitations in scale expressed by the new fuel characteristic GKI - Gasoline Knock Index.