Antioxidants and Fuel

[Hugh Janus]

Kevin Cameron has been writing about motorcycles for nearly 50 years, first for <em>Cycle magazine</em> and, since 1992, for <em>Cycle World</em>. (Robert Martin/)

The problem of edible fats and oils deteriorating over time has been with us forever—my mother spoke of bacon kept too long in the fridge as “going rancid” (developing an unpleasant taste). Petroleum molecules have vulnerable structures which can be similarly attacked if they are “unsaturated,” that is, if they contain double carbon bonds. When oxygen attaches itself at such points, separate chains can join together, causing thickening.

At the AMA’s Atlanta roadrace in 1973, one of the big air-cooled four-stroke entries ran so hot that its engine oil transformed into an amber-colored rubbery gel. The team owner—simultaneously outraged and fascinated—made the rounds of the paddock, showing us all this curiosity.

How did it happen? Modern petroleum oils of good quality are reformed in a process that changes olefins and aromatics into saturated structures less vulnerable to oxygen attack. Synthetic oils share this reduced vulnerability.

But 50 years ago there were evidently less desirable molecular varieties in whichever oil that gentleman was using, and when it became very hot, oxygen cross-linked it into a gel in a process vaguely similar to vulcanizing rubber.

Problems With Fuel Oxidation

Another concern—maybe more worrisome today—is that fuel in long storage (in an inactive vehicle, for example) may “go bad,” forming gum—especially in the presence of catalytic metals such as copper. So-called “cracked” gasolines contain olefins that are especially vulnerable. Normally the high turnover rate of fuel stocks at filling stations prevents trouble, but if it’s sitting in your bike’s tank over the winter…

The word “catalysis” sounds frightening, but merely refers to substances that can change the rate at which a chemical reaction takes place without themselves being consumed. I like to think of the catalyst (in the above case copper, iron, or lead) as the mugger’s large assistant, who pins the victim while his wallet is lifted. In the molecular case, the electric field around the nucleus of a heavy-metal atom can deform the shape of the target molecules in a manner that makes the desired chemical reaction more likely to occur.

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In the instance of fuel-system corrosion, metals act as catalysts to accelerate oxidation. Fuel blenders therefore add “metal inactivators,” molecules that attach themselves to metal surfaces to form an unreactive protective layer.

Stale Fuel and Fuel Storage

Motorcyclists of a certain age have had the disagreeable experience of being unable to start a bike that has been long stored “wet.” A common result of such storage is that the most volatile components of the gasoline have evaporated out through the tank breather, leaving the remaining fluid as a kind of “lamp oil.” Its refusal to evaporate and form a spark-ignitable mixture may prevent the engine from starting.

Even if you replace the lamp oil with fresh fuel, the engine still may not run properly; while it may start, it stalls rather than returning to idle. In this case the gum that has formed in the fuel blocks the carburetor’s tiny idle jets. This occurs when the fuel in carburetor float bowls evaporates over weeks or months. You’ll need to remove and disassemble and clean the carburetors, with special attention to those idle jets. Here’s an excuse to buy that ultrasonic cleaner you’ve been wanting.

Modern bikes with both digital fuel injection and an evaporative emissions canister system (to prevent evaporating fuel from entering the atmosphere) are much less likely to suffer from the above.

Fuel Stabilizers

Fuel stabilizers are mainly antioxidants that block the gum-forming reaction. Similar chemistry provides protection for lube oils and edible fats.

Antioxidants were originally developed in the 1920s for application to edible oils, but later proved their value in lube oils, where they slowed the process experienced by our friend at the Atlanta race mentioned above. Sludge formation was also an oxidative process well known in an earlier time. Before the development of oil additives such as antioxidants and dispersants, high-hour engines generated sludge that could eventually block oil circulation, leading to engine failure.

Related: Get Your Bike Ready For Summer In The Winter

These reactions occur during early steps in a reaction chain leading toward actual combustion, which ultimately combines atmospheric oxygen with the carbon and hydrogen of the fuel, oil, or fat to form the final combustion products carbon dioxide and water. At first, thermal agitation knocks those reactive fragments (called radicals) loose; further reactions form a chain leading to gel, gum, or sludge.

By reacting with certain of these radicals, antioxidants divert them from their normal reaction path. Think of the antioxidant as an agent sent to calm down a crowd by passing out hundred-dollar bills. Harmony is restored!

Source

[Buckster]

Gay.