Gasoline 101: Origin, Additives, and Octane | MC GARAGE

On any given day, Americans burn through some 368 million gallons of gasoline. We have a serious appetite for the stuff, but besides fussing over its price, how much thought do you give to gasoline? When so much pride and concern is fixed to your bike's performance, how much do you know about the flammable liquid that makes it all happen?

Strap in, kiddos, because in this issue and the next you're going to get a crash course on gasoline.

Despite terms like “dinosaur juice” and “fossil fuel,” petroleum products aren’t formed from the remains of prehistoric reptiles. According to people who study this stuff, we have ancient marine plants like plankton to thank for all the black gold we burn.

Before it gets to your bike’s tank, gasoline must be refined. Raw petroleum is pumped out of the ground as crude oil that’s composed of a stew of hydrocarbon chains of various lengths. These chains vaporize at different temperatures, with the shorter, lighter chains going gaseous first. At the refinery the crude is distilled and the various chain lengths are boiled off and collected. They run the gamut from gases like propane to solids like paraffin, with the volatile liquids that make up gasoline residing near the middle of the range. On average, a barrel of crude oil (42 gallons) yields about 21 gallons of gasoline.

There are currently 139 refineries operating in the US, and those facilities are responsible for providing the base fuel that’s sold to various retailers and distributed to the nation’s 160,000 or so service stations. “The base gas is shared among marketers,” says Jim Macias, fuel technology manager at Shell. “It has to meet very stringent industry standards, so the quality is good.”

It’s what’s added to this fungible base stock that differentiates the gas that’s sold at a name-brand station versus what you pump at the no-name quickie mart on the corner. In addition to government-mandated detergents (as well as ethanol, but we’ll get to that next month) intended to reduce tailpipe emissions by keeping engine internals clean, many companies blend their own additive package into the tanker trucks before they depart for the service station.

These chemical cleaners cost money to develop and produce (and market), and not every retailer pours in the same amount. The EPA (Environmental Protection Agency) has a minimum concentration, and “about 50 percent of the market only puts in the minimum,” Macias says. “But we’ve found that amount to be ineffective in keeping the engine clean.” Many auto manufacturers agree, which is how the so-called Top Tier gas standard came to be.

Top Tier fuels contain a higher concentration of detergent additives (2.5 times more at minimum for all grades of gas) and are free of metallic additives that can damage catalytic convertors. Some 30 retailers in the US have jumped on the Top Tier bandwagon (see toptiergas.com for a list) since the standard was introduced in 2004.

So there’s a difference in additive concentrations among retailers, but even after you decide on a brand you still have to make a decision at the pump: regular, mid-grade, or premium?

Octane is the main difference between the three grades of gas. Octane is a family of hydrocarbons, but it’s more commonly used as shorthand for octane rating, the index of a fuel’s ability to resist detonation. Detonation, or “engine knock,” is when the air/fuel mixture in the combustion chamber ignites spontaneously ahead of the normal, uniform flame propagating from the spark plug. (That is, after the spark plug has fired. A spontaneous burn before the plug fires is called preignition and occurs as the result of a hot spot within the combustion chamber, often a piece of carbon.) Detonation creates a massive pressure spike in the combustion chamber that manifests itself as a ringing sound as the engine literally resonates from the shock. Detonation is bad news. It hammers the piston and can pit the piston crown, crack ring-lands, and do all sorts of other nasty stuff.

The octane rating of a gas is determined in a laboratory using a single-cylinder “knock” engine with a variable compression ratio. Technicians run the engine on a test gas and increase the compression ratio until knock occurs and then compare the results to pure octane, which has an octane rating of 100. Two different tests are employed when settling on an octane rating, the Research Octane Number (RON) and Motor Octane Number (MON). The tests follow slightly different parameters, and in the end neither is used as the sole indicator of knock resistance. Instead, the two numbers are averaged to get the fuel’s Anti-Knock Index (AKI), which is the number you see at the pump. The equation (R+M)/2 is even visible below the octane number.

So which number should you go with? Whatever your bike’s manufacturer calls for. But a higher-octane gas is better, right? No, at least not in terms of power or throttle response. A higher octane rating, by itself, does not increase engine power unless it prevents detonation. If an engine does not experience detonation on 87-octane fuel, it won’t make more power or run any better on 91.

But as far as keeping your engine clean, premium gas has “the highest concentration of the good stuff,” Macias says of Shell’s premium gas. “It has five times the cleaning agents required by the EPA.” The idea here is that a vehicle engineered to run on high-octane gas is likely built to a higher state of tune, so internal cleanliness is essential for proper performance. So if you frequent the quickie mart for discount gas, the occasional tank of premium from a name-brand station may do your engine some good.

We’ve never heard of anybody intentionally running their bike on a lower-than-specified-octane gas, but what if you had no choice? “Keep the revs down and listen for detonation,” says Brad Puetz, Kawasaki’s media relations supervisor. And if you hear that telltale pinging, pull over and let the engine cool down.

Check back next month to learn about ethanol, stale gas, and fuel stabilizers.

When most folks think about combustion, they assume that when the spark plug fires the air-fuel mixture inside the combustion chamber explodes all at once like a firecracker. In reality, it’s a controlled burn that originates at the spark plug and progresses toward the cylinder walls. The image above was taken inside the combustion chamber of a test engine and shows the “flame kernel” expanding from the plug 13,707 microseconds (call it 0.014 of a second) after ignition.

Gas companies make a big deal about engine deposits, and plenty of aftermarket chemical companies sell potions intended to purge your engine of sediments. So what are these residues made of, and where do they come from?

Colloquially known as “gunk,” the black deposits found on intake valves and piston crowns are primarily carbon. This crud accrues as gas vapor breaks down as a result of coming into contact with hot engine parts. It can also form as the result of excessive oil in the combustion chamber—from leaky rings or worn valve seals or guides. Carbon deposits impede gas flow, just like cholesterol buildup slows the flow of blood through your heart.