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  • Octane 101

    Posted on June 1st, 2008 ninarussin

    Is premium fuel worth the premium price?
    By Nina Russin

    Photo courtesy of Ford Motor Company

    Photo courtesy of Ford Motor Company

    In our supersized society, we tend to assume that more is better. If a big gulp is better than a small gulp, then premium fuel must be better than regular unleaded. Many drivers believe that octane is a performance enhancer: that’s a misconception. Octane’s purpose is to prevent engine damage that can occur due to pre-ignition or detonation, or what most people call “engine knock.”

    Internal combustion engines create power through a series of controlled explosions called combustion. Here’s how it works: a fuel injector sprays gasoline into the engine cylinder, and mixes with air entering through the intake valve. The intake and exhaust valves close, sealing off the cylinder. The piston moves down in the cylinder, compressing the vaporized gasoline to make it more volatile.

    A spark plug fires, igniting the mixture. The explosion pushes the piston back up in the cylinder. As each piston moves up in the engine cylinder, it creates energy to spin the crankshaft: the power that eventually finds its way to the wheels. The spent fuel exits the engine cylinders through the exhaust valves and makes its way to the tailpipe.

    In order for the engine to work properly, the fuel must only ignite when the spark plug fires. But because gasoline is so flammable, explosions can occur in the engine cylinders at other times as well.

    Internal combustion engines are inherently inefficient: some gasoline passes through the cylinders without burning. If this gas vapor auto-ignites after the spark plug fires, the result is detonation. Detonation can burn pistons, shatter spark plugs, or even crack the engine’s cylinder heads.

    Pre-ignition is caused by a hot spot in the combustion chamber that ignites the fuel mixture before the spark plug fires. It sound like pinging or knocking, and over time, can cause similar damage to detonation.

    Octane reduces gasoline’s tendency to auto-ignite, protecting the engine against detonation or pre-ignition. Manufacturers will recommend the minimum amount of octane necessary to protect the engine against damage. High compression engines tend to need more octane, because they are more susceptible to detonation. Supercharging, which uses a blower to increase the volume of air moving through the engine, can also raise the car’s octane requirements.

    Octane requirements increase when vehicle weight goes up significantly: for example, when a truck is towing a large trailer. Extreme heat can make an engine more likely to detonate than moist, cooler air, raising the engine’s need for octane.

    The bottom line is this: don’t use gasoline with a lower octane rating than the manufacturer recommends because the engine may not run as efficiently, and the likelihood of damage from detonation or pre-ignition increases. On the other hand, using premium fuel when the manufacturer recommends regular is a waste of money.

    Why diesel fuel 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.

    E85

    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 gas versus cool gas

    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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