How Gasoline Has Changed Through The Years

In its early days, “gasoline” was simply that fraction of crude oil that boiled off in a temperature range of 86–437 degrees Fahrenheit. That simple definition existed because such a fuel evaporated easily enough to allow spark-ignition engines to start and run, and because that was how the fuel was manufactured: by boiling it out of crude oil in a tall fractional distillation tower. Fractions of lower volatility, such as diesel, light lube oil, heavier lube oils, and so on, boiled out at progressively higher temperatures.

In an engine, such a simple fuel had the tendency to detonate (knock, an abnormal and often damaging form of combustion), varying on where the crude oil was found (California, Borneo, Romania). To be safe, early automakers gave their engines extremely low compression ratios, around 3:1. Because this greatly reduced torque and power, there was lively interest in understanding detonation and finding means of eliminating it. Fuels intended for power-dependent activities such as aviation were chosen by reputation; there was as yet no system for measuring or standardizing a fuel’s knock resistance.

In England, engine research pioneer Harry Ricardo formalized the use of alcohol as a high-quality fuel (it had seen some use in auto racing since 1903!), so from 1922 on and for a number of years its use in the Isle of Man TT races was legal. Over time, other pathways to improved fuels were discovered. The so-called “BTX liquid,” or benzole, a byproduct of manufacturing coke from coal, soon became a fuel additive. When English racers spoke of “50/50” they were referring to a 50/50 mixture of the best petrol with BTX, a cocktail of benzene, toluene, and xylene. All of these, we now know, have high anti-knock ratings.

Related: Confusing Detonation And Preignition

Ricardo created his “toluene scale,” attempting to relate the anti-knock value of a given fuel to that of toluene, but it was not widely adopted. In the US, the work of Graham Edgar resulted in an isooctane/normal-heptane anti-knock index which remains in use to this day. By using a special test engine whose compression ratio could be changed as it ran, the knock threshold of a fuel under test could be compared with that of various mixtures of two other fuels, one of high anti-knock value and the other so bad that “you could almost hear it knocking in the can.” If the behavior of the test fuel was closest to that of a mixture of 87 percent isooctane and 13 percent n-heptane, the test fuel was said to be of “87 octane.” (Octane Number is often abbreviated to ON.) CFR test engines continue to be built and operated in fuels labs worldwide to this day; CFR stands for the US Cooperative Fuel Research committee which brought it into being.

The octane scale was a tremendous step forward because it allowed engines to be reliably matched to appropriate fuels. No more pushing the throttle to the firewall only to hear the rattle of detonation and know that your engine will be smoking junk within seconds.

In the 1920s came Thomas Midgley’s discovery of the strong anti-knock action of the compound tetraethyl lead (TEL), often simply referred to as “lead.” Some people today want to blame Midgley for discovering TEL as well as the long-serving but ozone-layer-destroying refrigerants called Freon (TEL is highly poisonous, as in a single drop on your skin can kill you), but as an employee of Delco, he was just one among the hundreds of thousands of industrial chemists who have served our society. Millions upon millions of repetitive tests have been necessary to create present-day chemical technology, which is why we know the names of so few of those who performed them.

Both the octane scale and TEL were cornerstones of the performance Allied warplanes enjoyed in World War II. One gram of TEL, added to a gallon of isooctane, could raise its octane number from 100 to 108. Adding a second gram gave 112, and adding a total of 6 grams (common during WWII) produced a 120 rating. German aero engines, operating on lower-octane fuels, had to be given roughly 20 percent more displacement to equal the power of the Allied engines.

Higher ON by itself cannot increase power; this is a common misconception. But its higher knock resistance allows an engine’s compression ratio or supercharger boost to be safely raised, both of which can result in increased power.

TEL wasn’t the whole story of higher-octane fuels. Ways were found to economically produce large volumes of high-ON alkylates which, when blended into simple gasolines, greatly improved their knock resistance.

When the war ended in 1945, the US was well-equipped to produce high-octane fuels for which the only use was in large aircraft piston engines. One of the last big B-29 raids flown in August of 1945 required almost 6 million gallons of aviation 115/145.

Soon the rising performance of big V-8 engines started boosting post-war auto sales, leading directly to the supercar era of the 1960s. In certain auto engines, compression ratios climbed as high as 12:1—possible thanks to “super-premium” pump gasolines based upon wartime avgas technology. In 1965 a friend and I drove down to Providence, Rhode Island, to look at a Mercury 427 Thunderbolt. Like Honda’s 1983 Interceptor, it was a homologation special, built so it could be raced legally. It reputedly gave a stock 600 hp.

Soon it was admitted that auto exhaust emissions were factors in urban air pollution, which had become a severe problem in Southern California and elsewhere. Oxides of nitrogen, produced only above a certain threshold combustion temperature, were leading to photochemical smog. Because combustion temperature rises with compression ratio, compression ratio had to drop, putting an end to those supercars.

Related: Gasoline, Explained

Carburetors are a passive fueling technology; they naturally run rich in summer and leaner in winter, so at the same time a variety of schemes were developed to lean out fuel mixtures, bringing us the era of stutter-and-stall.

Exhaust catalysts were developed that effectively burned up excess fuel that passed unburned through engines. Because the lead in TEL deactivated such cats, plans were made to phase lead out of pump gasoline. In the 1977 season I could feel one result—the piston domes of the TZ750 Yamaha roadracer I was responsible for felt “funny.” They had settled—the result of light but chronic detonation on this new, lower-ON high-test.

Compression ratios in the 1970s and 1980s dropped to around 8:1, causing fuel consumption to rise and power to drop. The fuel companies promptly compensated by adding higher percentages of octane-boosting aromatics (the same basic stuff in the benzole of the 1930s).

Back in the day, when you turned carburetor mixture screws this way and that to achieve best idle, you were seeking a best-power mixture, which is slightly rich. Regulators couldn’t allow that, so a variety of schemes were developed to prevent owners from adjusting idle mixture. It was a time of frustration.

Motorcycle manufacturers jetted their products down until some barely ran, creating a nice revenue stream for Dynojet’s tuning kits, which consisted of either tiny washers to raise (and enrich) tapered fuel metering needles, or new needles with altered diameters.

To reduce emissions of unburned hydrocarbons, fresh air was admitted to exhaust ports where fuel-rich exhaust gas was hot enough to resume burning. Cars carried “smog pumps” for this, and bike engines featured a variety of unpumped schemes for the same purpose.

During the time when many carburetor-equipped cars remained in the national fleet, regulators conceived a scheme to compensate for their worn fuel metering needles and excessive richness, adding so-called oxygenates such as alcohols or ethers to pump gasoline. Because these fuels are in effect already partly burned (that is, combined with oxygen), they contain less energy per gallon than normal gasoline. That would effectively lean out older, rich-running carbureted vehicles, thereby reducing unburned hydrocarbon emissions. Because the ON of the chosen additives was high, their use would allow compression ratios to rise somewhat, improving fuel mileage.

Meanwhile, the MTBE ether that the EPA had chosen for its oxygenate program turned out to be less than benign. MTBE turned out to be more soluble in ground water and less digestible by soil bacteria than previously believed, and consequently MTBE from leaking gas-station tanks started showing up in a series of well-publicized ground-water-pollution incidents. The EPA suffered pink faces as ethanol (the kind of alcohol found in intoxicating beverages) had to be substituted for MTBE. Within a few years, as much as 40 percent of the US corn crop would be used to produce the ethanol we now see proclaimed on gas pumps as a percentage of the fuel. Changes of this kind tend to be self-perpetuating; farmers quickly grew accustomed to receiving premium prices for corn destined to be burned rather than eaten, and their lobbying became familiar to Washington senators and other members of congress.

Now users of small carbureted engines began to have lean-mixture troubles, up to and sometimes including seizure. The additional ethanol, containing only two-thirds as much energy per gallon as gasoline, has the effect of leaning out carburetion. The usual 10 percent ethanol results in a mixture 3–4 percent weak, but the higher percentages of ethanol later offered produced even weaker mixtures in such engines. This is the basis of the current back-and-forthing between the AMA and EPA over the practice of adding higher percentages of ethanol to pump gasoline. Fuel systems controlled by oxygen sensors easily compensate for 10 percent ethanol, but larger percentages such as E85 require the special equipment carried by vehicles designated “flex fuel.” And carburetor-equipped engines just receive weaker mixtures that concern owners because of possibility of engine damage. It’s never wrong to read the information presented on the gas pump.

Better days were on the way in the form of computer-controlled digital fuel injection, where mixture is actively controlled by reference to electronic exhaust gas oxygen sensors. Active mixture control and equal fuel delivery to all cylinders produced an immediate drop in emissions of unburned hydrocarbons (UHC) and carbon monoxide (CO).

Present-day 91-octane pump gasoline is a fuel quite inferior in knock resistance to the fuel adopted by the US Army Air Corps in 1935, but is considered “socially superior” because it contains no TEL and consists in part of renewable ethanol.

So how can bikes now routinely get away with 12:1 or even 13:1 compression ratios without damaging knock? There are several reasons:

Happy motoring!