Tuesday, January 25, 2011

Octane - don't knock it

In this post, I am only talking about internal combustion, 4-stroke, gasoline-fueled engines.

In the early days of gasoline engines, they discovered two things right away.

1. The first engines knocked like crazy.
2. If you compress the fuel mixture before igniting it, you got more power.

Engines "knock" or "ping" when the fuel ignites before it is supposed to.

One of the strokes of a 4-stroke engine compresses the fuel-air mixture (by design.)

The problem of knocking (pre-ignition) had to be solved. Experiments with various fuels showed that heptane couldn't be compressed very much before it exploded on its own, but octane could be compressed a lot (relatively speaking) without making it ignite on its own.

One doesn't want to use pure octane, for cost reasons, and further experiments showed it could be combined with other fuels and still be compressed sufficiently. Today's low compression (8:1, for example) engines can run on about 87% octane and 13% something else. Heptane. Ethanol. Cheeseburgers. Maybe not cheeseburgers.

Cars with higher performance engines (some Cadillacs, Corvettes, etc.) which put out more power (and are therefore usually higher compression) require a higher octane-rated fuel to not knock (pre-ignite the fuel.) Knocking not only makes your engine have less power, it can damage your engine.

Many "muscle cars" of the 1960s and early 1970s compressed the fuel mixture to over 12:1. Zowie. 95 octane, please.
Octane is tested in the laboratory to arrive at a theoretical performance level, a test known as RON (Research Octane Number.) It is also tested under real life conditions - which show how a fuel behaves under a real load - with a test called MON (Motor Octane Number.) Then an average of the two is taken. The octane rating of the fuel thus tested is arrived at, as the gas pump at your favorite filling station will attest, by the above method, abbreviated (R+M)/2. There are underground tanks which separately hold each level of octane, and the hoses on the gas pump are fed by the proper tank. If the gasoline delivery truck driver hasn't been drinking.
We are not going to go into fuel additives, except for talking a little bit about lead, below. Sorry.

Early on, scientists discovered a second way to get more "free" power out of a gasoline engine besides compressing the air-fuel mixture before igniting it with a spark plug. That second thing was adding a substance called tetraethyl lead to the fuel. And so this was added to all gasoline. Unfortunately, after decades of doing this, a thin layer of lead covered planet earth. Lead is not particularly healthful to flowers, humans, and other living things. By the 1970s, leaded fuel had been outlawed in the U.S. There are only a handful of asshole countries in the world that still put lead in their gasoline, though some deny it. Here is the current list of countries the CIA says still allow leaded gasoline to be used. I want to "out" them here, for all the good it will do.

Bosnia and Herzegovina
North Korea
(Source: Central Intelligence Agency, 2008)


Q. My car's user manual says it runs on unleaded regular gasoline. If I put premium unleaded instead, will it hurt my car's engine?

A. No. Why would it? You'll just waste money.

Q. Will I get more power if I use Premium fuel in my car that is supposed to use regular fuel?

A. No. Your little car isn't going to compress it any more than it did with regular gasoline. It can't.

Q. Is it ok to use regular gas in my car that is supposed to use premium gasoline?

A. No. You must use premium, if that's what your owner's manual says. Otherwise, your engine will knock. Do you remember why? Then why did you ask this question?

Q. Should I add lead to my gasoline?

A. Only if
1. You don't care about air pollution and killing things
2. Only if you have a desire to completely destroy your expensive catalytic converter in under five minutes.

There's that word catalytic again.


  1. Catalytic, great word. Now, see, I didn't know about octane, so I learned something.

    That's always cool.

  2. My old Land-Rover, built in an era when lead was abundant, would really prefer it to still exist.
    One of the advantages of lead as an additive, was that it coated and protected the valve-seats and valve contact areas, and reduced wear erosion of these parts.
    Modern unleaded fuels are unkind to older engines.
    Also, when older vehicles, classics, maybe, which are not in everyday use, are stored with fuel in the tanks, some of the more volatile fractions of the fuel, like benzenes, tend to evaporate out, the remaining fuel mixture may further separate, with ethanols in particular tending to layer at the bottom of the tank. Corrosion increases, and a brown sticky varnish-like substance may coat components, and block filter-gauzes and smaller fuelways (like carburettor jets, for instance). Tetraethyl lead was indeed a toxic substance. But benzene is carcinogenic by inhalation or skin absorbtion.

  3. Mayhap you were going to speak further on the subject of "knock", and explain that it's all down to the speed of the flame-front in the fuel-air mixture. If the spark occurred before before the piston reached top-dead-centre (tdc), then too rapid a flame-front meant that the expansion of combustion gases tried to hammer the piston back whence it came, maybe damaging the engine. The spark, ideally, is timed to occur before tdc, but the full combustion of the fuel-air mix to occur when the piston has started its downward return.
    From there you could get into how the distributor, with the help of a vacuum-diaphragm, and centrifugal weights, would alter the point in the piston's travel
    at which the spark occurred, in order to avoid knocking under load or harsh acceleration.
    Nowadays, a computer, using multiple sensors, controls the moment and amount of fuel injection.
    But in the old days, a driver had to do it all. There was a lever, often on the steering wheel, that allowed the driver to "advance" or "retard" the spark.
    Retarding the spark was especially important in starting with a starting-handle. Ignition before the piston reached top dead centre (tdc) might result in a broken arm, through kickback.

  4. I'm leaning a lot, thx all.

    great post.

  5. @Stephanie Barr - Thanks. I like learning new things too. Believe it or not. :)

    @Soubriquet - Mayhaps, indeed. But that post will be in another series which simply discusses how a gasoline engine works, rather than combined in this series. I need to back off this topic for awhile and try to regain some of the readers that always drift away when I do this. :)

    I have noted your use of "starting handle" and added it to my growing list of BritishSpeakisms. It's a "crank" - and the trick is to put your thumb on the same side as your fingers. (As well as retarding the spark. You left that part out.)

    @Jeff King - Well, thank you. I promise to change the subject now. I do appreciate your loyalty here.

  6. Well, you insert the starting handle into the dog (are you still with me?) on the front end of the crankshaft and use it to crank the engine until it starts.
    I have a starting handle for my land-rover. It used to be quite easy to start that way, until I gave it new piston-rings and reground the valves.
    Either that or the engine rebuild coincided with me getting weaker.

    Some older diesel engines, such as boat engines and those used in building-site dumper trucks, were started with the starter handle, but you flip a lever which opens a "decompressor" valve on the cylinders. Thus the engine is easy to turn over, and you build up speed on the flywheel, once you judge that to be enough, you flip the compression lever back, and the engine bursts into rattling black smokey life.



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