Is Premium Fuel Worth the Premium Price?

Why Diesel Doesn't Have Octane

Diesel is a heavier grade of fuel than gasoline. Diesel engines don't have spark plugs because they can’t  ignite the fuel. Instead, the engine uses extreme compression to create combustion.

Pistons in the diesel engine compress air in the cylinders to make it hotter. When an injector sprays diesel fuel an engine cylinder, the air inside is hot enough to make the fuel burn and expand.  The burning fuel creates enough power to move the piston back and spin the crankshaft.

The ticking sound we hear when a diesel car is running is detonation: it's a common occurrence in diesel engines. So engineers use components robust enough to withstand the stress of random explosions inside the cylinders.

Detergent Additives

Many gasolines have detergent additives to prevent deposits from forming inside the engine. Fuel injector tips can become clogged over time with paraffin and dirt. Clogged fuel injectors can't deliver enough fuel to the engine cylinders and fuel economy suffers.

While detergent additives can do a good job of preventing dirt from building up on the injector tips, they are less successful at removing deposits that have already accumulated. Fuel injector cleaners that go into the gas tank are equally ineffective: they simply aren't strong enough to do the job.

Any fluid strong enough to clean the injector tips is also strong enough to do damage to the fuel lines that run to and from the fuel rails. That's why technicians inject these materials directly into the fuel rails, after they block the fuel lines off.

Some manufacturers recommend against using these caustic injector cleaners, because they can cause damage to the injector tips. The best way to clean these injectors is to remove them and clean them ultrasonically: a job best done by a qualified technician.


Some new cars, also called flex-fuel vehicles, are made to be E85 compatible. E85 is a gasoline mixture that contains up to eighty-five percent ethanol by volume. Ethanol, which is produced from corn, is a renewable fuel. Using renewable fuel reduces our dependency on oil, which as we all know is a limited resource.

The bad news is that using E85 reduces a car's fuel economy. E85 is also more expensive than conventional gasoline. So drivers who need to stretch their gasoline dollars should think twice before filling up with E85.

Hot vs. Cool

Like any other liquid, gasoline expands when it gets hot. Gasoline density effects power: the denser the liquid, the more power it can create. Therefore, gasoline that is warm due to higher ambient temperatures will contain less effective power than cool gasoline. The debate of warm versus cool gas has been the subject of recent media attention, with arguments on both sides. While some experts claim that ambient temperatures have a significant effect on underground tanks, others claim that the tanks are relatively temperature proof.

Does it pay to fill up the tank during cooler times of the day: in the early morning or at night? The answer is ‘yes,’ but not necessarily because of the gasoline temperature. The hotter the ambient temperature, the more likely gasoline is to escape around the neck of the fuel tank in vapor form. That's because gasoline vaporizes at very low temperatures: much lower than ambient air.

Modern fueling stations have a collar around the filler nozzle to minimize evaporative emissions. In addition, cars have specially designed as caps that seal off the neck of the fuel tank, to minimize pollution from gasoline fumes. Filling up in the cooler weather minimizes evaporative emissions, causing less pollution and probably saving some money as well.

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