Pull up to the pumps at a gas station and you’re usually faced with three options: regular, mid-grade and premium. The grades are often rationalized as "All right," "Why not do something special for my car?," and "Wow." Yet there is more to the choice than just good-better-best.
The staggered prices reflect the measure of octane in the fuel. Octane is a molecule of composed of hydrogen and carbon, a hydrocarbon. Octane raises a fuel's resistance to autoignition, so the more octane in a fuel, the more pressure the fuel can take before it spontaneously combusts. If the fuel doesn't autoignite, or explode before its meant to, then the spark plug can do its job, you get the full performance of the engine and everyone's happy. If the fuel does autoignite, bad things can happen to your engine -- but we’re getting a bit ahead of ourselves here.
First, a little primer on compression ratios, octane spark timing and engine performance. Engines that place an emphasis on performance, such as the 6.2-liter V8 in the Corvette ZR1 or the 3.8-liter flat-six in the Porsche 911, are high compression ratio engines. That is, when the fuel and air are compressed by the pistons in the combustion chambers, that mixture is compressed to a much higher pressure than that in the combustion chamber of the 1.4-liter four cylinder found in the Chevy Cruze.
"In general," said Rick Davis, combustion technical specialist at General Motors, "a higher compression ratio is the big enabler for a lot of engine performance attributes. It's a fundamental contributor to engine efficiency, whether it be fuel economy or whether it be the ability to make more torque or more power."
Or for the science-minded among you, as summed up by John Juriga, Hyundai's director of powertrain, "Increased compression ratio improves thermal efficiency, and increased thermal efficiency improves power and fuel efficiency."
Increased compression ratios, however, require premium octane fuels. That's why performance cars with high compression ratios require 91 or higher octane -- they need the fuel to remain stable under high pressure so the engine can run as it was created.
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Engineers spend a lot of time working out the ideal spark timing, i.e., when the spark plug should ignite the compressed gas and air. Spark advance is the term for how far ahead the spark plug fires before the piston reaches top dead center (TDC), and a rough rule of thumb is the more advance you can tune in, the more performance you can get out. Yet the amount of advance needs to be balanced against the sensation known as "knock."
That is the dreaded sound caused by an extra combustion event in the cylinder. Ideally, there is only one explosion in the combustion chamber, the one that happens when the spark plug ignites the fuel. That spark creates what's called a combustion front -- like an explosion in a Michael Bay film -- that is meant to expand evenly throughout the chamber as the piston is rising. If, however, for some reason there's a pocket of fuel that ignites on its own, outside of the combustion front, then you have two explosions in the chamber. That second explosion greatly raises the pressure in the chamber, and because it tends to happen along the edges, it blasts components that are least able to withstand the increased load, like the edges of the piston and the piston rings.
If this is all sounding a bit too technical, understand that knock is something that can only make your mechanic happy, as it will eventually result in your needing your engine rebuilt.
So, to get rid of knock, the amount of spark advance is cut short. By allowing the air to be more compressed by the piston before the spark ignites, there is less opportunity for a random pocket of air to ignite on its own. The drawback is a reduction in performance, since the spark can't fire at the optimal time.
Enough of the science -- how does this affect you, your car and your gas budget?
An engine is calibrated to run on a specific octane. Most entry-level and moderately priced cars are built to run on 87 octane because manufacturers know that buyers don't want to take a beating at the premium pump every week. There’s no real advantage to putting higher-octane gas in these cars. At the other end of the spectrum are the high-performance machines that require premium. As we’ve explained, without premium gas, these engines are not able to produce the full measure of their power.
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But increasingly, companies like GM and Hyundai are developing engines that can both take advantage of the performance benefits of premium gas, while still run on regular. And in some cases, even produce nearly as much power. One example is the 430-horsepower LS3 engine in the Corvette. It can run on plain old 87 octane gas, but you'll get a little more out of it if you feed it premium.
"In a ‘Premium Recommended’ engine," said GM's Davis, "the design has been pushed to take advantage of that higher octane. You can see a slight performance loss and potentially a slight efficiency loss by running a regular octane fuel, depending on the conditions, because the calibration would have to pull a little spark advance out to compensate for the lower octane."
But if it's been calibrated to run on premium fuel, how can they keep it from knocking with regular gas? First and foremost, knock sensors.
"The primary mechanism on basically all production engines is a barometer," said Davis, "a sensor on the engine that can detect if knock occurs. A knock event causes some vibration that's picked up by a sensor, and then the control system slightly retards the spark timing to compensate. That's where the performance loss comes in, as the spark is retarded from its optimum value to avoid the knock."
To get an idea how much performance is cut when running regular versus premium, we can look to Hyundai's 4.6-liter V8, currently doing service in the Genesis sedan. This engine is rated at 385 horsepower and 333 lb-ft of torque on premium. With regular unleaded, those numbers drop to 378 hp and 324 lb-ft.
"Our customers typically use regular fuel," said Hyundai's Juriga, "so we typically develop our engines to run on that. However, on some more performance-oriented applications you can design to a premium fuel. You calibrate the engine to have a greater spark advance so it gives you more performance, but the prudent thing to do is to be able to run on regular fuel as well."
Enter the anti-knock system. "With knock systems, or electronic spark control," said Juriga, "there are accelerometers in the engine, typically one or two depending on the engine. They're tuned to measure the vibration that you would get during a knock condition, so they can identify whether something was regular combustion or knocking combustion or some other vibration from an accessory drive or the AC compressor kicking on. If the customer puts in the premium fuel, it runs fine, no problem. If the customer puts in regular fuel the engine sensors register knock, so they retard the spark advance until the knock goes away. This happens very, very quickly, so if you have a well-calibrated system the customer probably won't even recognize any type of knock."
Anti-knock sensors aren't the only features that allow engines to perform well while serving up the benefits of needing less expensive gas.
"A couple of key technologies allow us to do that, and one is direct injection," said GM's Davis. "One of its big benefits is it provides a cooling of the of the intake charge in the engine. That cooling suppresses the autoignition reactions that lead to knock. In a sense, by adding direct injection you can lower the octane requirement of the engine. The other contributor to knock is the combustion process itself. By a careful design of the combustion system geometry, the shape of the combustion chamber, as well as the airflow characteristics and the flame propagation, you can actually make the combustion system more tolerant of the lower octane fuels."
The research into eliminating knock extends to other fuels as well, such as diesel and biofuel. In diesel applications it isn't octane, but cetane that denotes a better resistance to causing the spark knock. Even though the nature of diesel combustion lends itself to knock, hence the diesel clatter we come to expect from every big rig. Fuels with higher cetane and developments like common-rail injection systems and exhaust gas recirculation have greatly reduced the noise of diesels. The Volkswagen Golf TDI is so refined that at idle you'd have to be parked somewhere quiet and listening closely in order to know you were in a diesel.
When it comes to biofuel, Bentley's new Continental Supersports can run on premium gas or E85 or any combination of the two without suffering any loss of performance.
"The blend of fuel delivered to the engine is constantly monitored by the fuel quality sensor," said Brian Gush, Bentley's director of chassis, powertrain and motorsport, "and engine parameters are adjusted automatically to ensure vehicle performance remains consistent regardless of the fuel-ethanol content, up to a maximum of 85 percent. Ignition advance supplied by the engine control system is optimized for the full operating range of gasoline-ethanol blends and any variability in octane is compensated for by a knock control system to achieve the best performance and economy."
In traditional, less expensive systems, adjusting for knock comes after the fact. That is, the engine has no idea what kind of fuel is in the car, it merely responds by altering the cylinder's spark timing. In the Supersports, preventing knock takes place before and after the combustion event because the car keeps track of what kind of gas is about to go into the cylinders.
So you get the 621 horsepower from its 6.0-liter W12 at all times, because the control system automatically adjusts the fuel injection, fuel pressure, ignition angle, throttle position, turbo boost pressure, camshaft position, and on-board diagnostics. So when people want to know what you get for your $370,000 -- that can be part of your answer.
Nevertheless, there is still more to come. "The effort and design work that goes into lowering an engine's octane requirement is still a growing science and it's an area that sees a lot of research," said Davis, GM's combustion specialist. "Variable camshaft timing has been an enabler, and there will be others coming like better control system capability and higher fidelity control over fuel delivery."
The real point for you to take from their efforts is this: More power, less money spent at the pump. Or as GM spokesman Tom Read put it, "It's a big win ultimately for the consumer."
When first published, this story cited an incorrect horsepower number for the Chevy Corvette and an incorrect displacement for the Chevy Cruze engine. The article has since been corrected.