Auto-ignition quality is one of the most important properties in gasoline. Auto-ignition quality is today described by the Motor Octane Number (MON) and the Research Octane Number (RON). For modern, light duty, gasoline engines it has been quite well established that RON is the most accurate number. However, no study has been performed on hand held forest and garden products, such as chainsaws. Is the auto-ignition quality best described in the same way for these engines as for light duty engines?
In this paper, a matrix of six different fuels with different combinations of MON and RON values were tested on a Husqvarna 550 XP Mark II, a modern air cooled, sequential stratified scavenging 2-stroke chainsaw engine. Ignition timing sweeps were performed and knock limited spark advance (KLSA) were calculated. Then the data has been analyzed with a multi-variate analysis of KLSA against both MON and RON to try to determine how MON and RON should be combined to best describe the anti-knock quality on this specific engine.
The results show that neither MON nor RON describes the anti-knock quality well on this product. The AKI (the average of RON and MON) described the anti-knock quality best of the conventionally recognized anti-knock quality measurements. From the multivariate analysis the K-value was determined to be 0,59. Using a fuel with a more ideal combination of MON and RON on this type of products without knock control can limit the maximum cylinder pressure and cylinder top temperature. This is likely to extend the life time of the forest and garden equipment significantly.
Small engine gasoline fuel, also known as alkylate gasoline, has a good combination of RON and MON that makes it outperform other market fuels from an Anti-knock quality perspective on the tested engine. At the same time small engine gasoline has other benefits, such as less health hazardous exhaust emissions, that can improve the work environment for loggers. This tailor made forest and garden fuel remain mysteriously anonymous outside of Europe.