Just saving a read on octane and denotation, Was posted in CR-G
A lot of confusion, mystery, and myth surround the simple question of what grade of fuel to use, what octane ratings mean, what detonation is, and the value of racing fuels. Part of the reason for that is the marketing that oil companies have done for over 70 years, in which they have promoted high octane fuels as an easy avenue to increase performance, when in fact, this is a very much backwards perspective on the matter. It is engines originally built for higher performance that need it.
Let’s see if we can clear up some of the misinformation! Click on to page two where we'll start with "Detonation: what is it?"
Detonation: what is it?
Gasoline engines are designed to burn gasoline. They are not designed to “explode” or detonate it. There’s a huge difference. During normal operation, air and fuel is introduced into the combustion chamber, compressed so as to improve the force of expansion as it burns, then near top dead center, it is ignited by the ignition system. What is supposed to happen is that the fuel immediately adjacent to the spark plug is ignited, and as it burns it ignites the fuel surrounding it and so on outward from there very much like a wave of flame. It’s like making a circle of match heads on top of a table and dropping a lit match in the middle (or anywhere else in the circle). The flame will spread rapidly, but never instantly.
Detonation is different. Detonation is the simultaneous ignition of all of the unburned fuel in the cylinder at the time it occurs, all at once. Normal ignition is a heavy, sudden push that continues over 90 or more degrees of crank rotation; detonation is like hitting the piston with a hammer instead of pushing it, and often has the effect you would expect from doing that kind of thing.
Why and how does detonation happen?
Detonation occurs when the heat and pressure in the cylinder rise to the point where the gasoline is ignited by just that, and not by contact with a flame. Once that level of heat is reached, it affects all of the fuel that remains to be burned equally and at the same time, rather than progressively, as a flame traveling from droplet to droplet of the fuel mist, and all of it ignites in the same instant, creating insanely high spikes of pressure that can and do cause horrendous damage.
In practice, the causes are one or more of excessive heat in the combustion chamber that allows temperatures to rise too high during compression, timing that is too far advanced, allowing the flame to build too much pressure before it can effectively turn the crank, hot spots in carbon deposits that ignite the fuel early, or simply a fuel the does not have sufficient resistance to being ignited by heat and pressure alone. That is, a fuel with an insufficient octane number.
Most of the time, what happens is that ignition takes place normally, and the flame front begins to spread over the combustion chamber as intended. But, as the flame spreads, heat and pressures in the chamber rise until they reach the point where the fuel can no longer tolerate them, the remaining fuel from the previous intake cycle detonates, often resulting in an audible “ping”. It can produce a sound reminiscent of having filled the top end with marbles or something, and can be quite destructive if it happens early enough in the power stroke. The correction, assuming that the engine is timed and jetted appropriately is to increase the resistance of the gasoline to being ignited by sources other than open flame. That means, a fuel with a high enough octane number.
That’s the form detonation usually takes, and at that level, in which only a part of the whole fuel charge is detonated instead of burned, it isn’t nearly as harmful as it can get. In the extreme, where the gasoline is pre-ignited, that is, ignited earlier than intended by an overheated engine, glowing carbon, etc., a larger percentage, or even all of the fuel may be detonated at once, and the results can be catastrophic.
Up next: What are the factors that influence normal combustion?
What are the factors that influence normal combustion?
Compression, ignition advance, and the fuel’s tolerance of them. Compression is simple to understand. Compression creates heat, and more compression creates more heat. Possibly too much more.
Ignition timing is a little more complicated, but still pretty easy to understand. In a piston engine, there is a mechanical “sweet zone” in the rotation of the crankshaft that goes from just after top dead center (TDC) to around 90 degrees after top dead center (ATDC). In this zone pressure on top of the piston does the most efficient job of turning the crank. It’s also important here to note that throughout this zone of rotation, the combustion chamber volume is constantly expanding. Gasoline of any kind or blend burns at nearly the same rate under pressure regardless of the situation.
Now, picture the piston having just past TDC and starting down the bore. If you light the fuel now, it will take a certain amount of time for it to develop a significant amount of pressure to really do anything about pushing the piston down. While it’s trying to build pressure, the piston is moving away from it at the same time, and little is gained. So the timing is set to ignite the fuel in advance of TDC. This allows the burning fuel to build up a meaningful amount of “push” by the time the piston starts down, and ideally burning most of it within the “sweet zone”. Since the engine will speed up, but the fuel burn won’t, the faster the engine spins, the more advance it needs as it picks up speed.
But if the fuel is ignited too early, the pressure and heat may reach critical levels before the combustion chamber volume has begun to enlarge adequately, and the portion of the fuel that lies in front of the advancing flame may then detonate.
Octane is the fuel’s detonation resistance.
And that’s all it is. Gasoline is a blend of several hydrocarbon solvents, among them toluene, benzene, heptane, and octane, plus a number of less active ingredients designed to do things other than add to the fuel’s energy levels. The number “100 octane” is based on the detonation resistance of 100% iso-octane. When a fuel is labeled “95 octane”, it resists detonation under pressure as well as a blend of gas consisting simply of 5% n-heptane and 95% iso-octane.
Octane number indicates ONLY this resistance to detonation. High octane gas does not burn hotter, colder, easier, harder, cleaner, dirtier, or with any more or less power because of the octane number. Differences such as any of these other fuel characteristics that actually do occur are the result of the overall fuel blend used for that particular gasoline, and it is both possible and common to find major differences in these qualities in different gasolines that all have the same octane rating. Race gas is a perfect example of this, as we will see later.
Up next: Why can I use 91 octane when my manual says I need at least 95?