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    Is premium gas truly better than regular? (ReneS, Flickr)

    by: Jonathon Ramsey | AOL Autos
     

    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.

    Explosion Management

    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.

    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.

    The Trade-Off

    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.

    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."

    No Knocking

    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."

    Poll
    Which type of fuel do you fill up with?

    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."

    Diesels And Biofuels

    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.

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    Discuss
    1 - 20 of 128 Comments
    MsGloriaAKA Mar 12, 2011 7:50 PM
    What aboutJaguar?Manua says use Hightest.
    Report This
    Peggsclay Mar 08, 2011 11:17 PM
    87 octane is garbage like non fat milk..water lol
    Report This
    indicadls Mar 07, 2011 8:43 PM
    04 KIA Optima V6. I use 87 octane get 22 in city and 28 @ 65mph on highway. Done
    Report This
    Ebneila Feb 15, 2011 5:38 AM
    @dfowler138---- Theres no way your four cylinder SUV will do 0-65MPH in 4 seconds, even down hill in a favorable tail wind. Your time/speed measuring method is obviously faulty, one hint, 0-65 rather than 0-60
    Report This
    EBarBranch1 Jan 26, 2011 1:36 PM
    This is the olny contry in the world where diesel cost more than gasoline. It takes about 5 gallons of diesel or some other equivalent tobring one gallon of ethanol to market to add ito our gasoline. California special blends are the worst poorest mileage highest cost & worst performance. The rest of the world most cars are either diesel or natural gas. Flexfuel touted in GM & Ford vehicles has 2 stations in the LA area none till you get to the mid west. Send me the 4 cylinder Mercedes E class diesel 55mpg & keep your Chevy Volts & hybrids There are few of these in Europe
    Report This
    gatorlexus7 Nov 28, 2010 6:37 PM
    I have a 2009 Infiniti G37, same engine as the Nissan 370Z, manual says 91 octane, I use the mid grade 89, works well, no knock and no noticable perormance difference. I save .10 a gallon that way...
    Report This
    w4msp2 Nov 27, 2010 5:29 PM
    Why not blend fuel !/2 91 1/289 Sunoco sells?????? Mech for 40 yrs
    Report This
    odysseyike Nov 25, 2010 8:18 AM
    There's no way I would use anything than 87 octane, gass is too expensive as it is. But I'll stop here cause I'll go off on a rant again.
    Report This
    lillsirecho Sep 06, 2010 4:01 PM
    I have a 1995 thunderbird and consistantly get 27-28 mpg on the freeway using the cruise control set anywhere between 65-75 mph. The milage goes goes down to about 21-22 in the city. I have the 4.6 liter V8 and use only regular fuel at 87 octane. In the past, I have tried the other higher octanes and noticed no difference so I stick to the 87 octane.
    Report This
    ucrlesr Sep 05, 2010 10:57 AM
    I drive a 04 Buick Park AVenue Ultra which has no knowledge of what I put in it. It thinks any octane is just fine. I cannot tell the difference in performance or milage. I generaly get 25 city and mountain's but shoot up to 28 on interstate highways. I run the speed limits at all times. 55 to 80 mph still gets 28mpg. I love this car.
    Report This
    sschaum1 Sep 03, 2010 8:39 PM
    I have 2010 Lexas 350 6cyl. The maniel say's I should use high test gas. I also had a 2007 Lexas 350 6 cyl. I have been runing them both on regular gas. I see no diference runing them on regular gas
    Report This
    wmathess Sep 01, 2010 4:58 PM
    @dfowler138 - - - 0 to 65 in 4 seconds? Subaru Forester?? Okay, whatever you say!! lol If your Forester will do 0-65 in 4 seconds, I'll challenge you to a race in my 1969 Mustang GT350!!!
    Report This
    chadwest2 Aug 31, 2010 7:30 PM
    Here in Colorado it make no differance due to the elevation, So Toyota says..
    Report This
    dontimmins Aug 24, 2010 5:05 PM
    I have been driving diesel cars since 1972, and I wouldn't go back to gasoline for anything. The Volkswagen TDIs deliver everything that I need. And diesel fuel is acknowledged to be 35% more efficient than gasoline.
    Report This
    cmdseahorse Aug 24, 2010 4:48 PM
    Low Compression engine or High compression engine???? The author of this article is telling the truth- Octane requirement or "demand" has only to do with your engines Compression Ratio in the beginning of it's life. If you have a car with a gasoline engine that is 12 to 1 (12:1) compression ratio, it will NOT develop its full power on 87 Octane fuel as it is more or less an engine that is used for drag strip car racing (for example)- it will also knock and vibrate violently on this low octane fuel, prematurely igniting the fuel. You need high octane fuel for high compression engines only. However, if you have a low compression ratio engine (8 to 1 compression ratio for instance ) you Do Not need more octane than standard 87 octane pump gas because your engine operates just as well on a more easily ignited fuel 87 octane than it does on the slightly more difficult to ignite 89 or 92 octane fuels. The higher compression ratio engines in production performance cars Cobra Mustang or Dodge viper etc (High Output- High Compression Ratio engines for example ) require premium (92 octane) to keep the fuel from "beginning to ignite" before the engine is ready-- this is what causes "ping" , spark-knock" or "knock" caused by low octane fuel in a hi-performance car engine . (OCTANE-DEMAND INCREASE: This engine-knock can Also happen in a 87 octane engine that is older or has needed a tune-up for many thousands of miles typically causing a carbon-buildup inside the cylinders (usually on the cylinder head or the top of each piston) If this occurs you have to clean out the carbon --- Chevron/Techron fuel additive or BG 44K or some other fuel additive that actually cleans away the carbon as you drive the car can be the answer to cleaning up the engine Then the engine will again be able to run on the 87 octane again in one or two tankfuls down the road without the "knock" or "ping") .You put the additive in the gas tank yourself ,keeping the car out of the shop -- I sold the BG44K ( has concentratedChevron Techron in it) back in the late 80's to car dealers and auto mechanics direct with never a one complaint. Today I buy the Chevron "ProGuard" fuel injector cleaner additive at Walmart or Auto-parts stores parts and use it in all my cars and generator engines etc ( it will clean carbon out of any gasoline engine but is not recommended for air-cooled 2 cycle engines). Chevron puts it in their fuel at the pump also.
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    texashash Aug 16, 2010 10:57 PM
    Dscottsanjose, use regular gas & that should be just fine. If there is a slight knock U could retard the timing just a hair, that is if U are not a speed demon!! High milage cars need higher octain because of carbon build up that increases the compression.
    Report This
    dfgdfgdfsdfsdsdf Aug 16, 2010 7:45 PM
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    dscottsanjose Aug 16, 2010 12:01 PM
    what kind of gas is recommended for a earlier model toyt camry 91 to be exact
    Report This
    wmathess Aug 16, 2010 9:32 AM
    I buy BP Biodiesel!!! I guess everybody hates me now!!!
    Report This
    wmathess Aug 16, 2010 9:30 AM
    Woah! They forgot to put the Ford Super Duty 6.7L Powerstroke Turbo Diesel V8 on the most fuel-effecient cars list! I'm serious, I've topped 28mpg on the highway with this thing!!! But that's in Pennsylvania. The roads are more straight and ain't as curvy. NOW, West Virginia, where I live is a whole different deal!!!
    Report This
    1 - 20 of 128 Comments
     
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