Modern spark ignition engines can take advantage of better fuel octane quality
either towards improving acceleration performance or fuel economy via an active
ignition management system. Higher fuel octane allows for spark timing advance
and consequently higher torque output and higher engine efficiency.
Additionally, engines can be designed with higher compression ratios if a higher
anti-knock quality fuel is used. Due to historical reasons, Research Octane
(RON) and Motor Octane Number (MON) are the metrics used to characterize the
anti-knock quality of a fuel. The test conditions used to compute RON and MON
correlated well with those in older engines designed about 20 years ago. But the
correlation has drifted considerably in the recent past due to advances in
engine infrastructures mainly governed by stringent fuel economy and emission
standards. In prior research, the octane response of modern engines seemed to
correlate better with RON than MON; however, the impact of octane sensitivity
(RON-MON) has not been evaluated in detail. In this study, the aforementioned
relationship between engine octane appetite and octane sensitivity was studied
in a single cylinder direct injection spark ignition (DISI) engine using six
fuels with two levels of RON, each of which had three octane sensitivity levels
ranging from 5 to 15. Experiments were conducted under three compression ratios
ranging from 9.5:1 to 11.5:1. The results show that both higher RON and octane
sensitivity have positive impacts on the engine thermal efficiency, with RON
being more influential than octane sensitivity. It is also found that the effect
of octane sensitivity was more pronounced at lower RON fuels.
