The 2013 SAE International High Octane Fuels Symposium (HOFS) this week in Washington, DC, explored the pros and the cons of high octane fuels, with a particular focus on using ethanol as the source of the octane improvement.
The “obvious driver” for the consideration of a high octane fuel (HOF), said MIT Prof. Emeritus John Heywood, one of the speakers, is that higher octane fuels would lead to higher compression ratios which would lead to more efficient engines and reduced fuel consumption. With the spread of turbocharged gasoline engines, he added, there is a double benefit: higher compression ratios and increased boost.
There is a growing consensus that our opportunities for moving vehicle fuel consumption in the positive direction and reducing the greenhouse gases for the next 20 years is really going to be focused on improving mainstream engines and fuels...Clearly as we get into the greenhouse gas emissions reductions business and try to be serious, we’ve really got a sense of urgency that to date has been largely lacking.—John Heywood
Octane background. “Octane” has two meanings. Octane itself is a C8 hydrocarbon with 18 structural isomers, and is a component of gasoline. Octane is also used as shorthand for the octane rating of fuel, which compares a fuel’s antiknock (resistance to auto-ignition) properties on a scale in which 100 is pure 2,2,4-trimethylpentane (iso-octane, an isomer of octane), which is highly knock resistant, and 0 is pure n-heptane.
The CRC Handbook of Mechanical Engineering notes that two standardized methods are used for determining the octane rating of a fuel: the research method and the motor method. (Thereby leading to the Research Octane Number and the Motor Octane Number).
Both methods test the fuel in a special variable compression ratio engine, in which the compression ratio is gradually increased to obtain a standard knock intensity reading from a knock meter. The octane rating is obtained from the volumetric percentage of iso-octane in a blend of iso-octane and n-heptane that produces the same knock intensity at the same compression ratio.
Differences between the methods are principally the higher operatin speed, higher mixture temperature, and greater spark advance employed in the motor method. MON ratings are lower than RON ratings because of the more stringent conditions—i.e., the higher thermal loading of the fuel.
Octane ratings exceeding 100 are extrapolated using the knock-limited indicated mean effective pressure (klimep), which is determined by increasing the engine intake pressure until knock occurs. The ratio of the klimep of the fuel under test to that for iso-octane is used to extrapolate the octane rating above 100.
The “octane rating” on most fuel pumps is the antiknock index (AKI), which is the average of the two methods (R+M)/2.
Straight-run gasoline in the refinery (naphtha) has a poor octane rating on the order of 40-50 RON. Refiners create higher octane fuels by blending with higher octane components—e.g., products produced via the reaction of light olefin gases with isobutane in the presence of a catalyst. (Iso-octane is formed by reacting iso-butane with butene.) Aromatics with double carbon bonds shared between more than one ring (e.g., naphthalene and anthracene) also increase the octane rating because the molecules are difficult to break.
(One of the general concerns raised at the Symposium from the refining side was that creating a higher octane fuel in the refinery increases the lifecycle greenhouse gas footprint of the fuel because of the additional energy required in its refining.)
Additives can also increase octane ratings—perhaps the most notorious of these were lead alkyls (e.g., tetraethyl lead)—i.e., leaded gasoline. Other octane improvers are MTBE and ethanol.
(The use of MTBE in motor gasoline grew rapidly in the early 1980s in response to octane demand resulting initially from the phaseout of lead from gasoline and later from rising demand for premium gasoline. MTBE was low-cost, had a high octane value, and was easily blended into gasoline stock—and was later found to have its own environmental problems.)
The US has three grades of gasoline: regular gasoline is 87 octane (in most of the country, 85 in some areas); mid-grade is 89; and premium is 91 – 93. In 2011, regular gasoline grade sales by the refiners accounted for 86% of all motor gasoline sales, according to the US Energy Information Administration (EIA). Midgrade sales accounted for 6%, and premium, 8%).
Octane and modern engine technology. In his presentation at the symposium, Thomas McCarthy, Chief Engineer, Research & Advanced Powertrain Engineering, Ford Motor, noted that before 1960, there was a gradual increase in octane over time, and that the engine technology of the time was able to take advantage of it. From the 1960 onwards, however, octane levels have been fairly flat.
Considering a future fuel should be done in the context of the most modern engine technology that we have, McCarthy suggested. On the gasoline side, that currently points to turbocharged direct-injection (GTDI) technology, which Ford calls EcoBoost, although every OEM has some form of GTDI engine application in the market.
We believe from a technology standpoint that there is room to grow, especially from a CO2 perspective, relative to this GTDI. That is effectively the subject of a lot of the technology development we are doing in Research and Advanced, is looking at things to increase the BMEP capability of the engine, further dilution capability, advanced boosting systems. We really believe that there are technology option to extend the CO2 capability of this GTDI. One of the challenges you have with a technology such as GTDI is the interaction with knock, because knock becomes a significant factor when you try to push the limits and extend the capability of engine technology.—Thomas McCarthy
Ethanol. While E85 is a highly knock-resistant fuel, McCarthy noted, it has had a fairly limited market impact due to limited availability and relatively poor customer value—i.e., the decreased energy content of fuel.
Ford is exploring several approaches to leveraging ethanol for improving engine performance. One is a dual fuel approach in partnership with Ethanol Boosting Systems (earlier post), that combines port fuel injected gasoline for light loads with direct injected E85 to suppress knock at higher loads. Another is splash blending to increase the octane level in the fuel. (Ford is presenting a paper at the upcoming SAE World Congress on mid-level ethanol splash blend potential.) To maximize the potential of mid-level blends, McCarthy noted, the engine must be optimized for the fuel.
A high-octane Tier III fuel. William Woebkenberg, Fuels Technical and Regulatory Affairs Engineer for Mercedes-Benz Research & Development North America Inc., went further and proposed the re-allocation of the current ethanol pool into a new Tier III fuel (a new gasoline specification for the assumed EPA Tier III rules to come). Under the Mercedes scenario, premium would be blended from Tier III and regular gasoline, and mid-grade would be eliminated. Tier III volumes would initially be equal to that of mid-grade volume, and would hit the markets in 2016 or 2017.
More specifically, the ethanol content for regular grade could decrease to 7-9%, Premium would stay at 10%, and Tier III could range from 15-30%, with corresponding RON of 98-101.
In his talk, Heywood noted that blending up to 15% ethanol to produce 98 RON fuel could give a 5-7% fuel consumption reduction relative to today’s engines. He also noted that the dual fuel approach (Heywood is a co-founder of EBS) would give about a 10% reduction in fuel consumption relative to a current GTDI engine.
[The upcoming Part 2 will address some of the concerns and challenges with creating a new fuel grade.]