What is the effect of altitude?
The effect of increasing altitude may be nonlinear, with one study reporting a decrease of the octane requirement of 1.4 RON/300m from sea level to 1800m and 2.5 RON/300m from 1800m to 3600m [17]. Other studies report the octane number requirement decreased by 1.0 – 1.9 RON/300m without specifying altitude [24]. Modern engine management systems can accommodate this adjustment, and in some recent studies, the octane number requirement was 0.2 – 0.5 Antiknock Index/300m. The reduction on older engines was due to:- – reduced air density provides lower combustion temperature and pressure. – fuel is metered according to air volume, consequently as density decreases the stoichiometry moves to rich, with a lower octane number requirement. – manifold vacuum controlled spark advance, and reduced manifold vacuum results in less spark advance.
