September 8, 2012 § 3 Comments

The words high octane conjure up the vision of powerful, almost inexorable.  Scribes in the press refer to a high octane economy.  As it turns out, in the transportation world, high octane is not necessarily better.  But it could be. 

The octane number is a characteristic that defines the ability of a fuel to tolerate high compression engines.  In such an engine the fuel is compressed in the cylinder to a greater extent than in regular engines.  Consequently, when ignited, the energy released is greater than in the conventional cylinder.  This provides a high torque to the wheels.  More importantly to the fuel economics and the environment, more energy is produced for the same amount of fuel.  In simple terms, the efficiency of the engine is increased.

The octane number of the fuel determines whether this can be accomplished.  If the octane number is too low, the fuel will ignite prematurely.  This is known as “knocking” in the parlance because of the sound produced, a bit like a rattle. Each car engine manual defines the octane rating permitted.  Regular engines use 87 octane fuel and most others use 91 octane.  Some sporty cars require 93 octane.  This is why gas stations carry three grades.  Inexplicably, though, the mid grade is 89 octane, not 91.  No car is rated for that value.  Ultra-high compression engines such as Indy race cars require octane numbers over 100. That is why they use pure ethanol or methanol, both with octane numbers over 110.

But for a given car, higher octane is not necessarily better.  The 93 octane grade often carries the descriptor “super” or “hi-test”.  This can be deceiving.  A car designed to use 87 octane gasoline will derive no benefit from the higher grade.  This is a case of more not being better.

But more can be better if we change the engine.  While this may seem an impractical suggestion, consider first an important fact.  Each of the three most viable substitutes for oil derived transportation fuel has an extremely high octane number.  Ethanol, methanol and methane (the principal constituent of natural gas) clock in at 113, 117 and 125, respectively. Comparing just the liquids in that list, regular gasoline scores an anemic 87. 

But ethanol has 33% less energy content and methanol has about 45% less.  This is why E85, which contains 85% ethanol, has never been popular with consumers.  Flex fuel vehicles (FFV’s) tolerate any mixture ranging from pure gasoline to E85.  But without subsidies E85 costs as much or more than gasoline, especially in drought years such as this.  And it delivers 28% fewer miles to the gallon.  Not a formula for consumer acceptance.

Methanol is much cheaper to produce than ethanol, especially with cheap shale gas.  So even taking into consideration the lower energy content an M85 blend would be good value.  But one would have to fill up about twice as often than with gasoline.

The principal disadvantage of methane (natural gas) as a fuel is the low volumetric density.  Compressed natural gas (CNG) occupies four times the volume as gasoline.  Ongoing research at RTI and elsewhere targets halving that disadvantage.

An elegant solution would be a super-high compression engine, with compression ratio around 16.  Regular engines are just under 9.  At these high compressions, all three gasoline substitutes would deliver very high efficiencies. One could expect the energy density disadvantage to be completely eliminated.  Methanol in particular would be significantly cheaper than gasoline per mile driven.  At present low cost natural gas would be the raw material of choice.  But biomass sourced methanol will be in our future.  Policy measures to ensure this ought to include federally funded research to reduce the cost.

The President recently set a goal of 50% reduction in imported oil by 2020.  A significant component of that will be more shale oil from the Bakken and similar deposits.  We are already producing 10% of our requirements from there, up from essentially zero a scant five years ago.  But a big factor ought to be displacement of oil by alternatives.  The three discussed above can play big parts.  They would have starring roles if we standardized on FFV’s with high compression engines. 

In one fell swoop we would also take a major stride towards the goal of 54.5 miles per gallon for passenger cars by 2025.  This ambitious goal is likely not realizable without a major technical advance such as efficient engines running on high octane fuels.  Finally, such engines are relatively simple to design and produce.  Detroit ought not to balk.

Vikram Rao

See announcement of web cast by Vikram Rao and Ann Korin


§ 3 Responses to HIGH OCTANE

  • Mark Corigliano says:

    Great article Vik. I’m wondering which of the three alternatives you think is most viable. My gut says CNG, which eliminates the need to add capital intensive methanol production. The range issue can be addressed by developing safe and cost-effective at-home refueling pumps. Do you know of any progress in this area?

    • rtecrtp says:

      My favorites are methanol and natural gas, for different reasons.

      If current active research with Metal-organic Frameworks (MOF’s) is successful, the 4X volumetric penalty for CNG with respect to gasoline will drop to 2X. Operating at pressures close to 500 psi, as opposed to 3500 psi for conventional CNG, has two consequences. One is thinner walled, lighter, tanks. The other relates to your question on in-home refueling. The lower pressure should reduce the capital cost and time to refuel. And yes, re-fueling at home takes the sting out of the range penalty. But the reduction in tank volume using MOF’s will also be a key to consumer acceptance; the current CNG tank in the Honda Civic takes up a lot of trunk space.

      The Department of Energy is funding multiple efforts in the development of MOF based storage and also refueling systems. One such is an RTI collaboration with Texas A&M University on MOF’s.

      Vikram Rao

  • Mark J Creager says:

    In 1979, I bought a Subaru FE model. The car ran on regular gas, it had a carburetor and it averaged over 56 miles per gallon until the day it died , when I hit a deer. I traveled from Salt Lake City Utah to Las Vegas Nevada on 8.6 gallons of gas! The compression ratio of the engine was 9.7 to 1. Four horizontally-opposed cylinders, the basic Boxster engine. Your article says that 54.5 is not easily obtainable yet it’s already been done. I owned one and drove it for nearly 13 years. When I was in high school, two brothers appeared on .the news, they invented a diesel car that got roughly 84 miles to the gallon, but nothing came of that. In 1980 Subaru developed a car that got 70 miles to the gallon, but the United States would not allow it to be sold? Fuel injection is actually highly inefficient it leaves a lot of the fuel unburnt. The highest efficiency experimental vehicles used fuel injection( a single injector) at the beginning of a long throat tunnel that mixes the air similar to a carburetor however the throat tunnel is approximately two and a half feet long with indentations to stir the air and a mix the air-fuel mixture so that there is no liquid fuel it’s all vapor. Oddly though the industry has chosen to go with an injector for each cylinder Which squirts little blobs of liquid fuel, a lot of which remains unburned, because only Vapor Burns, not liquid.

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