The octane number game
by Hank Hogan
American vehicle manufacturers have been pushing for higher octane gasoline to be available in the U.S. marketplace, as they are under pressure to improve vehicle fuel economy from today’s 34 miles per gallon (mpg) to 54.5 mpg by 2025.
“Higher octane fuels will enable engine design and calibration to provide improved performance for the customer, both in terms of fuel economy and in terms of torque/power/vehicle acceleration,” said Stephen Russ, senior technical leader of global gas engine engineering at the Ford Motor Company.
Studies have shown that higher efficiency can be achieved in burning fuel by increasing the compression ratio of the engine. Original equipment manufacturers (OEMs) could also install turbochargers, which would optimise the compression ratio while making the engine lighter weight for the same horsepower.
“The use of higher-octane fuels enables multiple and synergistic engine improvements, like higher compression ratios, turbocharging, down-sizing/down-speeding, etc., all of which enable greater engine efficiencies,” said Clarence Woo, executive director of the Singapore-based Asian Clean Fuels Association (ACFA). He referred to a study published in SAE technical paper No. 2014-01-2610 (CO2 Emission Reduction Synergies of Advanced Engine Design and Fuel Octane Number), wherein the engine compression ratio increased from 10.2:1 to 12.2:1 when fuel was switched from 95 to 102 RON. According to the study, fuel economy could be improved by as much as 15%, depending on the operating conditions,.
Europe uses higher-octane fuel than the United States. If the U.S. were to follow Europe, this could result in at least a 6% fuel efficiency gain and a corresponding decrease in greenhouse gas emissions, said Mary Beth Stanek, director of vehicle technologies and government relations at General Motors (GM).
Octane rating can be measured in several ways. In Europe, which uses a research octane number (RON), regular gasoline is 95 RON and premium is 98 RON. The U.S. averages the RON and the motor octane number (MON). There is a difference of between eight and 12 points in these two methods. Thus, regular gasoline in the U.S., which is 87, is effectively 91-92 RON.
Automakers, including GM, Ford and others, advocate for higher-octane gasoline in the U.S. The U.S. Environmental Protection Agency (EPA) has started to consider this, citing the need to regulate CO2 emissions as their regulatory justification.
However, according to published reports, it is unlikely that the EPA will establish any octane guidelines until after 2025, when the agency will issue regulations as part of a planned rollout of the new fuel economy standards.
It may take that long for all the stakeholders to hammer out some critical details. For instance, what would the new octane number be, and how would gasoline be boosted to achieve this? One proposal is to blend gasoline with 30% ethanol, said Lewis Gibbs, a fuel expert who is now a consultant after retiring from the Chevron Corporation. This would boost RON by eight points from the current E10. The end product would have a lower emission footprint than alternatives like aromatics if the ethanol came from renewable sources. However, the energy content would be lower than straight gasoline, which would have a negative impact on fuel economy.
“The critical question is whether or not the increase in compression ratio effect on fuel consumption exceeds the loss in energy content,” Gibbs said.
Other questions abound, such as how to phase in the change, and how to lessen the potential price shock to businesses and consumers.
Stanek favours a “breakpoint and phase-in” approach. That is, at some agreed date, a higher-octane fuel and vehicles that could take advantage of it would become available. After that, there would be a gradual switchover to the new fuel and vehicles.
However, lower octane fuel could potentially harm new powertrains, so misfueling at the pump could be a problem. The industry has looked at this issue, considering such approaches as colour coding pumps and/or installing different shaped nozzles.
Another issue is the price differential between lower and higher-octane fuel. According to the American Automobile Association (AAA), there is currently a 50-cent differential between regular and premium gasoline in the U.S., or roughly a 25% price difference. There have been suggestions that the price differential could disappear once the fuel volume mix changes, because higher octane gasoline today is a small amount of the total sold. The argument is that once higher octane gasoline becomes mainstream, and the volume produced increases, the price could go down, much like what happened when diesel fuel switched to ultra-low sulphur.