Wednesday, May 14, 2008

Octane Rating

Just a little FYI. I was stumped today on what exactly an octane rating on gasoline meant.
The octane rating is based on results using a test engine. It is the resistance to autoignition that is being measured. Autoingition is undesirable in a gasoline engine as this produces knocking. It is called an octane rating as iso-octane is defined as 100 and n-heptane is defined as 0. So a mixture of 87% iso-octane and 13% n-heptane would have an octane rating of 87%. However that doesn't mean that a fuel with a 97 octane rating has 97% octane, it just has characteristics as if it was 97% octane and 3% n-heptane.


8 comments:

Anonymous Coward said...

I read in Scientific American that high octane fuel makes no difference in modern engines...

BK said...

It makes no difference in older engines.

Modern engines can sense (they have a sensor, it's not magic) the - something or other - and 'advance the timing', taking advantage if the higher compressibility of the higher octane fuel.

If you don't have the sensor then there's no advantage.

bk said...

bah. 'taking advantage of the...'

Anonymous Coward said...

I found the article:

"Most modern cars, however, are designed to employ a specific compression ratio, a measure of how much room is available to the fuel when the piston is at the bottom and the top of the cylinder. This compression ratio—somewhere in the neighborhood of eight to one—tolerates lower octane fuels (such as regular gasoline, good old 87 octane) without knocking. "The compression ratio is fixed by the designer of the engine," Green says. "The regular fuel will burn properly and the premium fuel will burn properly and therefore there is no reason you should pay the extra money." High-performance engines, such as those in some sports cars or older, heavier automobiles, often boast much higher compression ratios. These cars—for example, Shepherd's Subaru WRX—require premium gasoline and will definitely knock without it. "I have to put the 92 octane in," he says. "It has a turbocharger.""

BK said...

Uh, yeah. That's not quite what you claimed initially.

High performance engines have higher compression ratios. My own vehicle has a compression ratio of 10.4:1 (it's Australia: my ride, a 3.5l V6, is not unusual). Higher octane fuel is recommended.

The critical thing is when you spark. I don't get knocking with regular octane because the engine management system makes the plugs spark earlier, so you don't get pinking. The ride is not as smooth though, and the power development not as impressive because you're not igniting at the peak compression (and I've noticed a 10% improvement in fuel economy with premium vs regular on long trips).

I agree, completely: if you have a low compression ratio engine then getting premium petrol does you no good (although Shell for example put detergents in that seem to help). It does all depend on where in the cycle you spark.

Anonymous Coward said...

Actually the article says later on that you can get better fuel economy on premium... Makes sense, although I doubt it makes up for the price...


"Such high compression ratios—and the premium fuels that go with them—could be turned to efficiency, rather than speed, Green notes, especially if put into the engines of lighter cars like his Honda Civic. Other automotive fuels, such as ethanol, can also offer high octane ratings, allowing oil companies to use more volatile gasoline in such blends. But for standard cars on the road today, purchasing premium gasoline is simply paying a premium for a fuel that delivers no added benefits. "If you think you need it," Green says, "you're being very eccentric.""

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