What Does the “Octane Rating” of Fuel Actually Mean?

Ah, the joys of vehicle ownership! Traffic jams! Construction! Costly insurance! Speed traps! Searching endlessly for parking! Having the check engine light come on just as you were about to buy that new game system and having the mechanic charge you thousands of dollars for parts and labour… and, of course, the greatest joy of all: that weekly soul-crushing, wallet-emptying visit to the petrol station. As you try to avert your gaze from the rapidly climbing numbers on the pump gauge, you have probably found yourself idly pondering the various fuel grades on offer: 87, 89, and 93 octane. Unless you happen to own a high-performance sports car, you have likely never even considered anything but sticking to the lowest – and cheapest – octane rating. But what is the “octane rating” anyway? How do these different grades of gasoline differ, and how do they actually affect the performance of your car, if at all? Well, put the pedal to the metal and feel the need for speed as we dive into the surprisingly fascinating science of octane rating and combustion chemistry.

In simple terms, a fuel’s octane rating measures its ability to resist engine knock, a phenomenon caused by uneven combustion in the engine cylinder. In a regular four-stroke petrol engine, a mixture of fuel and air is injected into the cylinder, then compressed by the rising piston. At the top of the piston’s stroke, a spark plug ignites the compressed fuel-air mixture, creating hot, high-pressure gas that forces the cylinder downwards. While the spark plug is supposed to be the sole source of ignition, under certain conditions the fuel can spontaneously ignite at other spots in the cylinder, leading to uneven combustion and generating a loud “knocking” or “pinging” sound. This is similar but not identical to the phenomenon of pre-detonation, in which the fuel spontaneously ignites before the spark plug fires due to compression – an effect known as dieseling. The type of combustion which produces knocking, known as detonation, is faster and more violent than regular combustion or deflagration, causing the pressure inside the cylinder to rise rapidly. If not corrected, over time engine knock can inflict serious damage to the engine and eventually cause it to fail.

Knocking can result from a variety of factors, including cylinder geometry and hot spots in the engine. However, the most common cause is fuel which is too volatile, making it susceptible to spontaneous ignition. The fuel which we Brits call “petrol” and almost everyone else calls “gasoline” is not one homogenous compound but rather a mixture of dozens of hydrocarbons and additives such as rust inhibitors, stabilizers, and lubricants. Most of these hydrocarbons are octanes, molecules composed of 8 carbon and 18 hydrogen atoms. While all octanes have the same chemical formula, their constituent atoms can be rearranged structurally and geometrically to create different isomers. In total, there are 24 isomers of octane – from regular n-octane to 2,2,3,3-Tetramethylbutane – all of which differ in their volatility and susceptibility to spontaneous ignition. The least susceptible is 2,2,4-Trimethylpentane or iso-octane while the most susceptible is normal or n-heptane, and it is these isomers which are used to determine the octane rating of commercial fuel.

Octane rating is measured using specially-designed machines called octane testing engines. These engines are calibrated using Primary Reference Fuels or PRFs, composed of iso-octane and n-heptane mixed in various proportions. Due to their respective volatility, iso-octane is given an octane rating of 100 and n-heptane 0. Thus, a mixture of 90% iso-octane and 10% n-heptane has an octane rating of 90. Various PRF mixtures are fed into the octane testing engine and the cylinder height – and thus compression – of the engine adjusted until knocking intensity reaches a certain level. These results are then used to create a set of standard tables against which commercial fuels can be compared. Once calibrated, the same octane testing engines are used to test samples of commercial fuel. The intensity of the knocking produced by a certain blend is compared to the standard tables to find the PRF mixture that produces the same effects to yield its equivalent octane rating. Thus, if a certain fuel blend produces the same amount of knocking under the same conditions as a PRF mixture of 93% iso-octane and 7% n-heptane, that fuel will have an octane rating of 93. These tests are typically conducted under both idling and full-power conditions, yielding a pair of values known as the Research Octane Number or RON and Motor Octane Number or MON, respectively. These two values are then combined to yield the average octane rating listed on the petrol pump.

In summary, then, the octane rating of a fuel doesn’t indicate the actual proportion of any particular octane in the fuel itself, but rather how the fuel performs compared to a given Reference Fuel. Nor does the octane rating indicate the energy density of the fuel, despite “high octane” being used in everyday speech to mean “high powered”. Putting 93 octane fuel in a car designed for 89 will not make it go faster; in fact, it will do very little but cost you more at the pump. Instead, different fuel blends are optimized for specific engines, with higher-octane fuels being preferred for engines which use higher compression ratios, supercharging, or turbocharging to squeeze more energy out of the fuel. If lower-octane fuel is used in such engines, the higher compression may cause it to knock or pre-detonate, potentially damaging the engine. Conversely, if higher-octane fuel is burned in a lower-compression engine, it may slightly improve fuel efficiency and lower CO2 emissions under extreme conditions like driving at high speeds or towing a heavy trailer. Otherwise, however, it will confer very little advantage, so unless your vehicle’s owner’s manual specifically recommends higher-octane fuel, you are likely better off sticking to the cheaper, regular blend.

In North America, gasoline is typically offered in three different octane ratings: 87, 89-90, and 91-94 – though in certain regions, 85 octane fuel can also be found. This blend is mainly sold in high-elevation areas, where engines require a more volatile fuel to run due to the thinner air. But as this advantage applies only to older, carbureted engines and not more modern, fuel-injected engines, 85 octane fuel is being slowly phased out in most markets.

On the opposite end of the spectrum, aviation gasoline or avgas for aircraft engines -which typically have very high compression ratios – have octane ratings between 96-100, somewhat controversially on the higher end general using lead to reach that 100 mark, though very recently a drop in replacement has been created to get the lead out, but still certified to work in all general aviation engines that otherwise require 100 low lead fuel. That said for reasons from practical to bureaucratically ridiculous, rivaling how much the FAA drug its feet on signing off on this fuel in the first place even long after it was proven to have met all their requirements, it may yet take many, many years for this lead free 100 octane fuel to be widely adopted, if at all despite seemingly everyone from pilots to politicians in agreement the leaded fuel must go. More on this later. In any event, during the Second World War, when piston engine performance reached its zenith, military aviation fuels were produced with octane ratings as high as 150.

But how is the octane rating of fuel actually controlled? Today, this is mainly accomplished by blending gasoline with ethanol AKA grain alcohol, which lowers the volatility of the fuel and makes it less susceptible to knocking and predetonation. Compared to regular gasoline, pure ethanol has an octane rating of 109, though most fuel blends contain between 10-15% ethanol. The anti-knock properties of ethanol were discovered in the early 1920s by American inventor Charles Kettering, who was working at General Motors’ research laboratories in Dayton, Ohio. However, as the use of ethanol as a fuel additive could not be patented, Kettering and his colleague chemist Thomas Midgely Jr. set out to find another substance that could. In December 1921, they hit upon an organometallic compound called tetraethyl lead or TEL, which could eliminate engine knocking at very low concentrations. Better yet, its manufacture could be patented. In 1923 General Motors and Standard Oil formed a subsidiary called the Ethyl Corporation to manufacture and market the additive, which was renamed “Ethyl” to avoid the toxic connotations of lead. However, it was not long before workers at ethyl factories began succumbing to the effects of this highly neurotoxic substance, suffering hallucinations, psychosis, and even death due to absorbing TEL through their skin. At one Ohio plant alone in 1924, five workers were killed and 35 driven insane. To counter accusations that tetraethyl lead was unsafe, on October 30, 1924, Thomas Midgely Jr. held an infamous press conference in which he poured TEL over his hands and inhaled its vapours for 60 seconds. In reality, however, Midgely was acutely aware of the dangers of TEL, having recently recovered from a severe bout of lead poisoning.

Yet despite these dangers, tetraethyl lead continued to be sold for another six decades, almost completely overtaking ethanol and benzene as the most widely-used antiknock fuel additive worldwide. It was not until the 1970s that the chronic health effects of lead accumulation in the environment – especially on the physical and mental development of children – was widely recognized, and countries around the world began to ban the substance. Today, nearly every country has outlawed TEL and switched to ethanol as their primary octane booster, with only Iraq and Yemen continuing to consume large quantities of leaded gasoline.

Going back to aviation, as alluded to, today the largest consumer of leaded gasoline is the recreational aviation industry, as many older piston aircraft engines, which are the majority flying in general aviation, cannot accept any other type of fuel. The most common type of avgas used today, 100 octane low-lead blue or 100LL, contains 2.12 grams of lead per US gallon – half the amount in the previous 100/130 green blend but still twice the amount allowed in regular automotive gasoline prior to 1988. While this reduction was a step in the right direction, the aviation sector still releases around 100 tons of lead into the atmosphere each year.

While you might think this should have been a simple problem to solve, given the huge variety of engines and an insane amount of red tape and expense to solve the problem in a way that can be shown that no in-flight issues will occur via using a different blend of fuel that reaches that octane level, combined with the relatively low volume produced annually, let’s just say not a lot of companies were willing to devote the many millions of dollars and years of effort to do it, especially also given uncertain adoption on the other side, let alone getting the FAA to sign off on it. That said, after almost a decade and a half of work by a company called GAMI, as mentioned, this has recently been done and a true drop-in replacement created and certified for all engines rated to run 100LL. The fuel is GAMI’s G100UL. However, rather than widely adopt it or push this, not only did the FAA inexplicably drag their feet on signing off on it for many months after all tests passed, since it has been approved, various entities connected to the FAA allegedly are doing their best to make sure this particular fuel never gets widely adopted, or it takes as long as humanly possible, despite the chief entity allegedly behind this literally called the Eliminate Aviation Gasoline Lead Emissions or EAGLE.

Explaining the nuances and all of the political wranglings on this one is beyond the scope of this video and could fill out its own extremely lengthy piece. But as our personal favorite aviation author and host for avweb, the absolute treasure that is Paul Bertorelli, summed up in an August of 2023 article he wrote, titled GAMI Should Be Allowed To Sink Or Swim On Its Own, “Small companies like GAMI are at a distinct disadvantage because… the ASTM process is horse trading. Big oil companies, the very ones who would vote on giving G100UL an approved spec, have interests to protect and horses to trade. Small companies don’t. … I have little doubt that if GAMI did achieve the consensus standard, EAGLE would create some other barrier or objection to G100UL’s deployment. We shouldn’t forget that EAGLE’s ‘stakeholders’ include current avgas producers and the oil industry broadly. Avgas is a small market, but it’s profitable, and companies selling it will do all they can to protect that market… And thus politics intrudes constantly in ways favorable to vested interests, but counter to sound public policy.“

In any event, going back to the original introduction of leaded gasoline, as a side note, it is worth noting that Thomas Midgely Jr. was also responsible for the creation of chlorofluorocarbons or CFCs, a family of organic compounds which were widely used throughout the 20th century as refrigerants and propellants for spray cans. In the 1970s, however, it was discovered that CFCs were responsible for severely depleting the earth’s ozone layer, which protects us from ultraviolet radiation from the sun. Thus, Thomas Midgely Jr. is arguably a serious contender for the single most environmentally destructive individual in human history. And for those who believe cosmic justice, know that Midgely also died a particularly bizarre and perhaps even karmic death. In 1940, Midgely contracted a severe case of polio, leaving him partially paralyzed. Resourceful inventor that he was, he constructed an elaborate system of cables and pulleys to help pull himself out of bed in the morning. However, on November 2, 1944, he was found dead, having become tangled in the cables and strangled by his own creation. Whoopsie doodle.

Today, in addition to ethanol, another commonly used anti-knocking agent is BTEX, a mixture of benzene, toluene, xylene, and ethyl-benzene also known as gasoline aromatics. Engine knock is also compensated for by modern electronic engine management systems. When knocking is detected, these systems automatically adjust the ignition timing so that the spark plugs fire when engine compression is not at its peak, ensuring smooth combustion. This is the reason you have likely never heard of engine knock; thanks to modern technology, it has mostly become a thing of the past. But while automatic ignition timing adjustment can eliminate knock and increase the life of the engine, this typically comes at the cost of combustion efficiency, leading to lower fuel economy and greater pollutant emissions. After all, in engineering as in the rest of life, there is no such thing as a free lunch.

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