Higher-octane gasoline might lead to improved fuel economy

Dr. Neil Canter, Contributing Editor | TLT Tech Beat January 2015

Modeling study shows that an increase in octane number reduces fuel consumption.

KEY CONCEPTS
• A modeling study determines how an increase in the octane number of gasoline affects fuel consumption and carbon dioxide emissions in 2040.
• According to the model, gasoline consumption declines between 2.1 and 4.5 percent in 2040.
• Carbon dioxide emissions will be reduced between 19 and 35 million metric tons in U.S. automobiles in 2040.

THE DRAMATIC INCREASE IN THE CORPORATE AVERAGE FUEL ECONOMY STANDARD to 54.5 miles per gallon by 2025 is accelerating efforts to improve the efficiency of automobiles. One approach has been to produce battery-powered vehicles and hybrid systems that combine an internal combustion engine with an electric motor.

In a previous TLT article, a technology was discussed that enables conventional gasoline-powered vehicles to convert to hybrid electric vehicle operation in a process known as hybrid conversion (1). The components needed for the conversion include a traction motor, motor drive, high voltage lithium-ion battery and a sophisticated controller to interface between the two power systems. Vehicles using this system have achieved on average a 20 percent savings in fuel costs and a 20 percent reduction in carbon dioxide emissions. 

One familiar term associated with gasoline is octane number. Dr. Raymond Speth, research scientist in the department of aeronautics and astronautics at the Massachusetts Institute of Technology in Cambridge, Mass., says, “Octane number represents a fuel’s resistance to engine knock, which makes it more difficult for fuel molecules to auto-ignite. Engine knock occurs when fuel auto-ignites prior to consumption by the spark-initiated flame.”

Speth indicates that there is a direct relationship between the efficiency of gasoline engines and the octane number of the fuel. He says, “The reason is that the engine compression ratio and the boost level of turbocharged engines are both related to engine efficiency and both limited by engine knock.”

Since the 1930s, the octane number for gasoline has been determined by the standard research octane number (RON) and the motor octane number (MON). Speth says, “This procedure worked well into the 1970s when gasoline was characterized by an average of the two parameters, which is known as the antiknock index (AKI). But changes in the fuel injection strategy and the engine design means that RON becomes a much better measure of actual engine behavior. In effect, the refining process used to meet the current gasoline specification does not affect actual engine performance.”

Speth believes that using RON as the measure of knock resistance is a better way to evaluate gasoline used in vehicles. He adds, “RON is used in most parts of the world as a measure of knock resistance.”

With the direct relationship existing between RON and engine efficiency, Speth and his co-workers decided to predict what would happen if they conducted a study on how much annual gasoline consumption and carbon dioxide emissions will change if the average RON of gasoline is increased. 

The motivation for doing this study was to assess the impact of using ethanol, which has a RON value of 109. Speth says, “We seek to determine if ethanol can have a larger role in improving the fuel efficiency of automobiles. The reason for this study is we believe there is room for improvement in optimizing the performance of automobiles at this time.”

MODELING STUDY
The researchers decided to change the octane ratings for gasoline from 87 AKI to 92 RON for regular and from 93 AKI to 98 RON for premium fuel and also to evaluate the change in fuel consumption and emissions in 2040. Speth says, “We chose to make the transition to higher octane number gasoline gradually to minimize the potential problems that may occur in having automotive manufacturers modify their fleets and to minimize the difficulties that refineries may face in making the necessary adjustments.”

This approach means that the number of new vehicles operating with higher octane gasoline will increase gradually and not force the refineries to have to blend more than two gasoline octane grades. In making these assumptions, the researchers then used three models to do the analysis. They include models for vehicle fuel consumption, the composition of the automobile fleet and a refinery model that contains a lifecycle carbon dioxide emissions analysis. 

The results from the vehicle fuel composition modeling shows that a six-point increase in the octane number leads to an increase in the estimated engine efficiency from between 2.4 and 3.5 percent. This leads to an estimated reduction in fuel consumption between 3.0 and 4.5 percent.

Speth says, “We included turbocharged engines in the analysis because a significant amount of the U.S. fleet of automobiles will include this technology as we head toward 2040. The baseline compression ratio for turbocharged engines will be lower, leading to an estimated 4.9 to 7.1 percent reduction in fuel consumption when using higher octane number gasoline.”


‘We included turbocharged engines in the analysis because a significant amount of the U.S. fleet of automobiles will include this technology as we head toward 2040.’
www.canstockphoto.com

In the automobile fleet modeling, the researchers determined that the majority of the vehicles will run on higher RON fuel by 2034 and that the U.S. fleet will contain 75 percent of these automobiles by 2040. The analysis indicates that overall gasoline consumption declines by between 2.1 and 4.5 percent, leading to an average fuel economy for all vehicles between 39.0 and 39.6 miles per gallon in 2040. At the same time, premium gas consumption will grow to just below 80 percent by 2040. Currently, less than 10 percent of all gasoline consumed in the U.S. is the premium grade.

For the refinery model, the researchers found that no significant capital investments are needed to produce higher octane gasoline. Speth indicates that the production of higher octane fuel will actually increase carbon dioxide emissions in refineries. He says, “The biggest reduction in carbon dioxide emissions is due to the reduction in the use of gasoline.”

The researchers estimate an overall carbon dioxide reduction between 19 and 35 million metric tons per year in 2040. 

Ethanol is an important part of the modeling strategy. Speth says, “In the main scenario we considered, the octane rating is increased without changing the ethanol blending level (kept at today’s effective level of 10 percent). We also considered scenarios in which E15 or E20 is widely used. In those cases, the benefits of increasing the octane rating are larger because the high octane provided by ethanol means that refineries do not have to do as much to meet the higher octane requirement.”

Future work will lead the researchers to examine two areas. Speth says, “We would like to develop a better understanding of how much increase in compression ratio or boost level can be achieved for a given increase in octane rating and what will be the air quality benefits from reduced fuel consumption and engine emissions.”

Additional information can be found in a recent article (2) or by contacting Speth at speth@mit.edu. 

REFERENCES
1. Canter, N. (2014), “Conversion of conventional vehicles to hybrids,” TLT, 70 (6), pp. 16-17.
2. Speth, R., Chow, E., Malina, R., Barrett, S., Heywood, J. and Green, W. (2014), “Economic and Environmental Benefits of Higher-Octane Gasoline,” Environmental Science & Technology, 48 (12), pp. 6561-6568.


Neil Canter heads his own consulting company, Chemical Solutions, in Willow Grove, Pa. Ideas for Tech Beat items can be sent to him at neilcanter@comcast.net.