Petrol is the principal fuel for spark ignition engines. It is a hydrocarbon fuel blended to balance the requirements of engines – acceptable performance with fuel economy and low emissions. The fuel must provide easy starting capabilities within a wide range of temperatures, allowing the engine to warm up quickly, accelerate smoothly, while burning cleanly without leaving abrasive residues of fouling gums.
Therefore, petrol must be volatile to enable a combustible mixture with air. The stoichiometric air/fuel ratio is 14.7:1, higher than many other fuels because its percentage by weight of oxygen is zero. For comparison, the stoichiometric ratio for methanol is 6.4:1 while its oxygen content is 49.9 percent.
Erratic combustion causes engine damage and increased emissions. This circumstance can be avoided by a fuel with an octane rating sufficiently high so as to permit it to burn smoothly without knocking. To achieve maximum power, an engine must run on the edge of the lean condition – just short of detonation. Knock testing determines the octane rating of a fuel.
To measure octane numbers, the fuel industry developed in the early '30s a single cylinder, variable compression ratio knock test engine, often referred to as a CFR engine. This type of engine was manufactured by the Waukesha Engine Company, and is still in use today. Originally it was intended that a set of operation conditions applied to the Knock Test engine would simulate vehicle on the road. However, developments in engine and fuel technology quickly made the correlation between laboratory results and vehicle octane requirements ineffective. The early knock test procedure was put aside at the time, but now forms the basis of the Research Method used to determine the RON figures (Research Octane Number) most commonly quoted.
The solution at the time, was to use two sets of operating conditions – the second, more severe, giving rise to the Motor Method, which produces the lower MON (Motor Octane Number) octane ratings. Both methods employ the CFR engine, and relate the knocking characteristics of a fuel under test to a primary reference fuel, which is a blend of two pure hydrocarbons: iso-octane (2,2,4 – trimethylpentane) and n-heptane. By definition, the octane number of iso-octane is 100 while the number for n-heptane is zero.
For octane numbers below 100, the octane of the test fuel is that percentage of iso-octane by volume in a blend with n-heptane that would produce knocking of the same intensity at the same compression ratio applying one of the ASTM test methods. The octane rating of the reference fuel is the same for both the Research and Motor Methods, but is rarely equal for commercially available fuels. Generally, the Motor Method produces results eight to ten octane numbers lower. Naturally, when evaluating fuels, be aware of the test method.
The Motor Method specifies a higher rpm and inlet temperature than the Research Method. The difference between the Research and Motor octane numbers can be used to determine the anti-knock quality of the fuel in multi-cylinder engines, and under varying operating conditions. RON figures are better indicators for engines operating at full throttle and low rpm, while MON figures give more accurate indication for the severe conditions caused by full throttle high rpm, and part throttle low and high rpm.
Octane points are significant in that they raise the threshold of detonation and allow the real power increases to come from higher compression ratios. However, octane ratings are only one measure of a fuel. Energy release must also be considered. Methanol, with an octane rating with an average of 115, in liquid form carries only half the energy of the same mass of petrol. For a stoichiometric charge, the energy density yields are about equal, however, with methanol you are using twice the volume. Methanol gives a three to four percent power advantage over petrol by virtue of permitting higher compression ratios.
High octane numbers alone, do not guarantee power. When seeking power increases from fuel, oxygenates and energy release are the key.