How Gasoline Works

What’s the right gas for my car? Is one brand better than another? What do they mean by “winter gas”? If you’re a technician, chances are motorists will ask your advice about the most important fluid to every car: gasoline. How much do you know about motor fuels? Here are some answers to the most commonly asked questions. If you know, they’ll know.

What is gasoline?

Gasoline is a mixture of chemicals made up of different molecules containing carbon and hydrogen called, logically, hydrocarbons. Heptane, Octane, Nonane and Decane are common constituents, having from seven to 10 carbons per molecule. Additives for lubricity, corrosion protection, emission control, volatility, detergency and anti-knock properties are added to the blend.

Where do the emissions come from?

Liberating the heat trapped in gasoline requires combustion to break down the molecules in gasoline, and recombine the parts with oxygen and unfortunately, nitrogen, both from the air. The carbon in the hydrocarbons forms CO and CO2 with the oxygen in the air, while the heat of combustion also causes oxygen to combine with nitrogen for the air to form NOx. Water vapour is also produced.

What does the octane rating mean?

“Octane” is one of the components of gasoline that is very knock-resistant in gasoline engines. Octane ratings are based on a hypothetical mixture of a form of octane called iso-octane and heptane, another gasoline component that has poor anti-knock properties. “87 octane” fuel, for example, has the same anti-knock properties as a blend of 87 percent isooctane and 13 percent heptane in a special single cylinder test engine. There are in fact two octane numbers, the Research Octane Number (low speed 600-RPM test) and a Motor Octane Number (higher speed 900-rpm, both in a special variable compression test engine). The octane rating given by oil companies is an average of the two numbers.

Is higher-octane fuel better for my engine?

No, the best fuel for a gasoline engine is the lowest octane that the engine will accept without knocking. Some retailers add value to their higher octane fuels by adding more additives, but any octane rating above that amount that prevents knocking is wasted money.

My engine pings occasionally. Should I switch to a higher octane fuel?

Not necessarily. Light pinging or knocking is tolerable, but heavy knock, or knock that’s continuous needs looking into. Always advise the customer to bring the vehicle in for a check before experimenting with fuels as a cure for persistent engine knock. Most consumers don’t know about knock sensors.

Can I blend low and high octane fuels in my tank?

In theory, yes, but there can be density differences between the two gasolines, so it’s best to not empty the tank and then mix low and high octane in a 50-50 proportion unless you’re sure you can mix them well. It’s possible to get a “slug” of low octane fuel in the lines, enough to damage and engine from severe knock, especially in modified or older, carburetted designs.

Should I use fuel system additives?

Additives that clean injectors or mop up water in fuel systems can play a part in vehicle maintenance, but not in place of regular maintenance. Periodic injector cleaning by a professional is still the best way, as is engine flushing. De-icer can be a good idea in some winter regions, although some winter gasoline blends have enough ethanol to take care of moisture without additives. Remember that excessive use of methanol-containing additives will dry fuel systems, but is hard on plastic parts in fuel systems like pumps and O-rings. Ether is a bad idea with modern fuel injected engines, so if you see a can of “quick start” used for cold starting, suggest a tune-up or diagnostic to avoid potential damage to emission control systems. Similarly, old style “top oil” additives are not needed with modern valve and valve seat materials. If the customer wants extra upper cylinder lubrication, recommend a modern product that’s sensor safe.

How do I prevent getting water in my tank?

Additives are one method, but there are things a driver can do to minimize moisture. Keeping the tank at least half full reduces the chance of condensation on the tank inner walls from watering the gasoline. Similarly, fueling early in the morning, while the storage tanks are cool, can help as can avoiding stations that have just refilled their tanks. Competent retailers wait several hours after a refill to allow settling would add to HC emissions, but many do not. It helps to buy fuel consistently from the same source.

What is “winter gas”?

Gasolines are formulated differently in the summer and winter. Colder temperatures require a more volatile gasoline for easy starting, while low volatility fuel in the hot days of summer adds to HC emissions and can vapour lock carburetor vehicles. Refiners tweak their blends seasonally.

Can mothballs increase gasoline octane?

This one has roots that go back to the 1930’s. Napthalene, the ingredient in some mothballs, has a blending motor octane number of 90, so when added to the 60-80 octane fuel back in the day, it could make a difference. With today’s fuels, it’s as likely to reduce octane rather than help it, as well as adding the potential to precipitate out and clog fuel system parts like injectors. Many mothballs today don’t even contain napthalene; it’s often para-dichlorobenzene, which isn’t an octane enhancer anyway.