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| Fig. 1: Octane gasoline label. (Source: Wikimedia Commons) |
Modern-day gasoline engines are engineered to have high efficiency as well as high performance. The quality of fuel used in these systems is commonly marked by a singular number, seen at gas stations. (See Fig. 1.) These numbers are known as its octane rating. Octane ratings play an indirect, but important role in achieving optimal energy use in engines. These ratings are expressed as either RON or MON, standing for Research Octane Number and Motor Octane Number, respectively. Both measure the fuels resistance to auto-ignition, which is essential to prevent knocking, a sharp, metallic pinging sound that occurs when fuel burns unevenly in an internal combustion engine. Knocking can reduce engine efficiency, but more importantly, can render significant damage to the engine entirely. [1,2]
The Research Octane Number (RON) is measured under conditions that simulate mild to moderate driving, with the engine running at a speed of 600 rpm and an intake air temperature of 52°C. The fuel's tendency to knock is compared to a reference mixture of iso-octane (assigned a value of 100) and n-heptane (assigned a value of 0), and the RON value is based on this comparison. RON of a sample fuel is measured experimentally through the RON of a known reference fuel, specifically a mix of said iso-octane and n-heptane. This reference RON can be calculated using the equation RON = (% iso-octane) × 100 + (% n-heptane) × 0, with percentages representing volume fractions. For example, a pre-made mix of 90% iso-octane and 10% n-heptane is known to have a RON of 90. The experiment then mixes iso-octane and n-heptane until knocking characteristics match the sample, where the RON is then determined. In contrast, the Motor Octane Number (MON) simulates more severe driving conditions, with a higher engine speed of 900 rpm and a governed charge temperature of 149°C. The method of finding MON is equivalent to finding RON, just done through an experiment conducted under more intense engine conditions, which includes preheated fuel mixture, higher engine speed, and variable ignition timing to further stress the fuel's knock resistance. The MON value is typically about 10 numbers lower than the RON (as fuel tends to perform worse under stress). [1,3]
Modern engine designs, particularly turbocharged direct injection (DIG) engines, exhibit a strong correlation between RON and performance, while the relationship with MON is less significant. In Australia, New Zealand, and most European countries, the octane rating displayed on gas pumps is the RON. In North America, the headline number is the Anti-Knock Index (AKI), which is the average of the RON and MON values. (See Fig. 1.) AKI is a simpler form of the extended Octane Index Value, explained below. [1] For instance, the Octane Index (OI), given by the formula
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| Fig. 2: Fuel Consumption across Octane Levels for 93-RON suited engine. [2] (Image Source: J. Finch) |
where S = RON - MON represents the sensitivity of the fuel, and K is the factor used in Octane Index describing the relative importance of RON and MON. The formula for Anti-Knock Index is derived when K is set to 0.5 (setting RON and MON at equal importance), written as
Differences between RON and MON show how a fuel behaves under varying driving conditions. Higher sensitivity indicates a greater drop in performance under stress. As engine technologies evolve, understanding these metrics is crucial for matching fuel properties to engine requirements and maximizing efficiency. [3,4]
Higher octane gasoline enables engines to operate at higher compression ratios, improving thermodynamic efficiency and extracting more work from the fuel. Direct injection and turbocharging technologies leverage high octane fuel to optimize combustion, allowing for more aggressive ignition timing and a better air-fuel mixture, which can enhance energy efficiency. Nevertheless, the total energy released from burning gasoline, approximately 4.2 × 107 joules per kilogram, remains constant regardless of the octane rating. [1,2]
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| Fig. 3: Fuel Consumption across Octane Levels for 97-RON suited engine. [2] (Image Source: J. Finch) |
Using lower octane fuel in an engine designed for high octane causes the engine control unit (ECU) to adjust parameters to prevent knocking. These adjustments, such as retarding ignition timing or enriching the fuel mixture, reduce efficiency and increase fuel consumption. In high-load conditions, such as rapid acceleration, these inefficiencies become more pronounced. [3,4] Tests on Euro 4 vehicles have shown a clear increase in energy consumption with decreasing RON, especially at wide-open throttle (WOT) settings, where the engine is under significant stress. (See Fig. 3.) [1]
High-end vehicles often have engines designed for maximum performance and efficiency, featuring higher compression ratios that make them more prone to knocking. To prevent this, these engines require high octane fuels, which resist knocking better and allow the engines to operate at their optimal settings. In contrast, cheaper cars typically have lower-compression engines that are less susceptible to knocking, allowing them to run efficiently on regular octane fuel. The advanced engine management systems in high-end cars can compensate for lower octane fuel, but doing so reduces energy efficiency, leading to increased fuel consumption and lower performance. [3]
Although gasoline with higher octane ratings is more energy efficient in high-performance engines and expensive cars, they do not inherently have more energy per unit that is constant for all gasoline, regardless of its octane rating. The octane level merely indicates the ability of the fuel to resist knocking under pressure, not its energy potential. However, when utilized in the correct system (i.e. high-performance engines), higher octane fuel can allow the engine itself to be more energy efficient. Thus, higher-octane fuel can indirectly increase energy efficiency. However, one must consider the overall impact and energy consumption of producing higher octane fuels, as they are often energy intensive. Fig. 2 and 3 show the difference in efficiency of engines suited for 93 and 97-RON respectively (low- and high-end engine). In it, it is clear that the more premier engine performs better (is more efficient) using gasoline with a higher octane rating. [4]
© Jackson Finch. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. Stradling et al., "Effect of Octane on Performance, Energy Consumption and Emissions of Two Euro 4 Passenger Cars," Transp. Res. Procedia 14, 3150 (2016).
[2] S.-J. Shuai et al., "Impact of Octane Number on Fuel Efficiency of Modern Vehicles." SAE Int. J. Fuels Lubr. 6, 702 (2013).
[3] E. Stauffer, J. A. Dolan and R. Newman, Fire Debris Analysis (Academic Press, 2007), p. 199.
[4] Z. Wang, H. Liu, and J.-X Wang, "Relationship Between Super-Knock and Pre-Ignition," Int. J. Engine Res. 16, 166 .2 (2015): 166-180.