EVEN at the cheapest petrol station in your correspondent's neighborhood, filling up the family kidmobile with premium (91 octane) fuel now costs over $70. As the meter clocks up dizzying dollar amounts, he looks longingly at the regular (87 octane) pump. Switching from his vehicle’s recommended premium-grade fuel to the cheaper variety would lower his fuel bill by at least 20 cents a gallon (more than five cents a litre). The question is, would it be worth it?

On the surface, the decision appears easy. Because the name “premium” implies a souped-up fuel that packs an extra punch, many motorists actually believe it delivers more oomph or miles per gallon—and may therefore represent good value. The truth, however, is that premium contains no more energy than regular petrol—around 114,000 British Thermal Units per gallon, depending on the season, the region, the local pollution requirements, and the amount of bio-ethanol that has to be added to petrol in America by law to keep the country’s corn-growers in clover (see “Competition at the pump”, August 20th 2012). The difference between premium and regular petrol lies in the blend of hydrocarbons used to make the fuel, and the package of additives mixed into it.

Nowadays, petrol is made up of hydrocarbons (mainly paraffins, naphthenes and olefins) produced in a catalytic cracker or reformer. The refinery process breaks the crude oil’s large hydrocarbon molecules into smaller ones by vaporising them in the presence of a powdered catalyst (an absorbent mineral such as zeolite). The blend varies depending on where the crude came from, the refinery equipment used, and the grade of petrol being produced.

Additives are included to reduce carbon build-up inside the engine, improve combustion, inhibit corrosion and allow easier starting in cold climates. Fuels that meet the requirements for “Top Tier Detergent Gasolines” (a voluntary standard endorsed by BMW, General Motors, Honda, Toyota and Volkswagen) contain more detergent in their additive packages than the minimum required by the authorities.

Another key additive that blenders stir into their brew is ethanol. That is done these days primarily to boost the fuel’s octane rating. A higher octane rating allows an engine to use a compression ratio of, say, 12-to-one instead of a more usual ten-to-one. The greater the compression, the higher the temperature within the combustion chamber. And the higher the temperature, the greater the thermal efficiency and power produced. In a nutshell, high-compression engines designed for performance need high-octane petrol.

Though ethanol has less energy per gallon than petrol, it has a considerably higher research octane number (RON)—around 108 to premium’s 97. It should be noted that this is not the octane rating seen on the pump in America. The RON figure results from a laboratory test done using a special engine with a variable compression ratio.

In the fuel test, the compression is raised until the engine begins to “knock”—ie, the fuel in the cylinder ceases to burn smoothly and instead detonates before it can be ignited by the spark plug. The cylinder pressure at which this occurs is then compared with that achieved while the engine is running on a reference fuel (a mixture of iso-octane and n-heptane). The ratio of the two pressures provides the RON of the fuel in question.

A better way of measuring a fuel’s ability to resist knocking under load is the so-called motor octane number (MON) test. This uses a similar test engine, but with a preheated fuel mixture, a higher engine speed and variable ignition timing. Because it uses more real-world conditions, the MON rating is typically eight to ten points lower than the equivalent RON figure.

In Europe, the octane rating on the pump is simply the RON figure. America, by contrast, uses the average of the RON and the MON figures, called the AKI (anti-knock index). Thus, 97 octane “super unleaded” in Britain is roughly equivalent to 91 octane premium in the United States.

Whatever the test, the point is that knocking needs to be avoided at all cost. If allowed to continue, it can quickly cause an engine to disintegrate. That is because when the air-fuel mixture in the cylinder detonates spontaneously before reaching the top of its compression stroke, the rising piston confronts a wall of rapidly expanding gases from the explosion, which attempt to force the piston back down the cylinder. The stresses caused by suddenly trying to reverse the rotation of the engine can become high enough to shatter the pistons, connecting rods and parts of the crankshaft.

To prevent that happening, a high-compression engine uses a blend of hydrocarbons that is somewhat less combustible than normal. Ethanol has an auto-ignition temperature of 362ºC, while petrol bursts into flames without a spark between 246ºC and 280ºC, depending on the blend. Therefore, adding a little ethanol to petrol can raise the auto-ignition temperature enough to prevent the blend from igniting purely from the heat generated during compression.

On the face of it, then, a motorist would seem ill-advised to use regular petrol in a car with a high-compression engine. That was certainly the case in the past. But cars today have sensors that listen carefully for the knocking sound, and instantly retard the ignition system when they detect that detonation is about to happen.

The delay in delivering the retarded spark allows the piston to start moving downward on its expansion stroke before the ignition actually occurs. That provides additional room in the cylinder head for the gases to expand and thereby reduce their damaging peak pressure—and so burn in a more controlled manner.

To sum up, if the car’s handbook says that premium petrol is “recommended” (rather than insisting it is “required”), then the engine will automatically adjust itself to run smoothly on a lower octane fuel. Because of the retarded ignition, the engine will, of course, produce less power, and have slightly higher fuel consumption. But the poorer fuel economy is likely to be outweighed by the savings at the pump.

Even so, your correspondent remains reluctant to make the switch. One reason is that no one has been able to tell him what damage is done, if any, by running the engine permanently in a retarded state, and forcing the anti-knock system to remain active all the time.

Another reason is because all the vehicle’s emissions testing was done using the recommended grade of fuel. Despite the fact that modern fuel-injection systems adjust the air-fuel mixture for changing conditions, your correspondent still has no idea how much more pollution the car might dump into the atmosphere if he switched to regular. Premium certainly has a better additive package, which helps keep the tailpipe clean as well as the inside of the engine.

But his biggest reason for sticking with premium, though, is that he was well aware that the car needed 91 octane to work properly when he bought it. And having paid upfront for the higher performance, he is reluctant now to throw that benefit away.

As for those who earnestly believe (and quite a few do) that filling the family Toyota with premium will somehow make it go faster or deliver more miles to the gallon, all one can say is don’t bother. As one wit noted, the only thing it will make run faster is money from your pocket.