You Really Can Fool Some of the People All of the Time
I have been hearing a lot of rubbish about petrol on television recently. Shell V-Power, Caltex Vortex, BP Ultimate etcetera etcetera etcetera all flog their stuff with agressive marketing and people are tanking up with the "good" stuff because the commercials imply that it's better for their engine. When the oil companies use superlatives like "Super", "Extra" and "High"... well it must be better, right? And of course they wouldn't be charging 10 to 20 cents a litre more unless they were putting some really good stuff in there, right? Sorry... NOT!
"High Octane" is not synonymous with "good" or "better", and does not mean that it is better for your engine. Chances are pretty good you don’t need high octane fuel in your motor car.
High-octane fuels only become necessary when your engine has a high compression ratio. It’s a very long and complicated story... which I intend to tell. Are you sitting comfortably? Then we'll begin.
First important fact that you must accept:
All petrol, regardless of its octane rating, has pretty much the same amount of energy per litre.
What?! "Sacrilege" you say? Well, actually in truth, some higher-octane fuels actually have a few LESS percent energy per litre, so as not to argue over this small point, for the sake of this discussion we will all agree that the petrol that you buy at the pump, regardless of octane rating, has the same amount of potential energy per litre.
Second important fact that you must accept:
Octane is NOT a measure of power but of the fuels’ resistance to ignition from heat. A higher-octane fuel, under identical combustion chamber conditions, will burn slower.
How can this be? If all of the above is true, how do we get more power out of high-octane petrol? We do, don’t we?
Well... under certain conditions... then yes, we do. Here’s how:
But first you must understand "heat of compression". There is a 2,000 year old fire starting device that still amazes me no matter how many times I see this.
A length of bamboo is hollowed out leaving one end capped. A stick, about the same length as the bamboo, is whittled down until it fits snugly into the bamboo cylinder. A bit of dried grass or wood shavings are placed in the bottom of the bamboo cylinder and the snugly fitting stick was violently rammed down the bamboo tube. The heat generated from rapidly compressing the air in the tube is sufficient to ignite the tinder.
The same thing can happen in the cylinder of an engine (and in fact, diesel engines rely on this principle). The piston, quickly squeezing the fuel/air mixture into a small space, can generate enough heat of compression to ignite the fuel well before the spark plug fires, with unpleasant results. If the fuel prematurely ignites while the piston is on its way up, the burning of the fuel, in conjunction with the rising piston, creates even more pressure, resulting in a violent explosion. This explosion is equivalent to hitting the top of the piston with a very large hammer. If you want to be able to see through the top of your piston, ignore those sounds that are usually called: "pre-ignition", "ping" or "engine knock".
Trust me on this one; in my somewhat reckless youth and with less than brilliant motor cars; using this method, I turned a few pistons into paper weights.
What we really want is a very rapid burn of the fuel, not an explosion. And we want the burning of the fuel to take place while the piston is in a better position to convert this pressure into productive work, like on its way down. Think of this burning as a very fast "push" on the top of the piston. Despite the violent noises you hear from some exhaust systems, it really is a rapid push on the top of the piston making the crankshaft go around, not explosions.
So that we can ignite the fuel at exactly the right time with the spark plug, instead of from the heat of compression, they actually put flame retardants into petrol to keep it from igniting prematurely. The more resistant the fuel is to ignition from the heat of compression, the higher its octane rating.
Are you with me so far?
Higher compression ratios = higher combustion chamber pressures = higher heat… and it is with these higher combustion chamber temperatures that the magic happens.
At higher temperatures the fuel is burned more efficiently. So, while it’s true that the higher-octane fuel does not posses any more energy than low octane fuel, the increased octane allows the extraction of more of the potential energy that has always been there. Conversely, lower compression ration engines utilize a little less of the fuel energy potential (2-4% reduction) but there is also less heat generated in the combustion process.
So how do you know if you need high-octane fuel? I suggest you look in your Owners’ Manual! (Duh, I wondered what that thing was for. I didn't actually know that it contained useful information).
Manufacturers really do want you to get the maximum efficiency out of your engine (it makes them look better and also make you want to make a return purchase). They do their best to give a good balance between horsepower and engine life. It’s in their best interests to do so.
There is ABSOLUTELY NO BENEFIT to using a higher octane than your engine needs. The only benefit is increased profits to the oil companies that have cleverly convinced some of the public that their new "Super-Duper, Premium-High-Test, Clean-Burning, Used-By-Famous-Racing-Types-All-Around-The-World, Extra-Detergent-Laden-Keep-Your-Pipes-Clean, Extra-High-Octane" fuel is your engines’ best friend.
I tire of people insisting that they got better fuel economy, better acceleration, and less dental plaque by switching to a high-octane fuel. I'd like to remind these people that in every pharmacy is a special miracle pill that is often prescribed by doctors, it works wonders because people believe that it works wonders; it’s called a "placebo". The Placebo Effect does wonders, if you make people believe that the car will be more efficient or more powerful, then they'll change their habits with the loud pedal.
Never confuse faith with physics!
If you are getting pinging or knocking with what should be the correct octane for your engine, start by checking the ignition timing, also check that the spark plug is the correct heat range. For older cars, check for excessive carbon build-up on the top of the piston, the carbon takes up space and increases the compression ratio. If all is well and correct, and you still are getting knocking, then try the next higher octane. You won’t go faster, you won’t go farther, but you will prevent an unsightly hole in your pistons.
In reality this subject is a whole lot more complicated than I want to bother with. If you are curious to know more, then google the following and enjoy the education (and then please don't whinge to me when I'm proved right).
Antiknock Index, Octane Rating, Octane, Stoichiometric Combustion, Thermal Efficiency, Flame Front, Highest Useful Compression Ratio, Compression Ratio, Placebo, Stockholm Syndrome.
How does he do it? The man is a genius.