Factors that Determine Octane Requirement

[WardL]

All of these are general statements that apply to "normal" engines. The octane requirement is the minimum octane of the gasoline that will not cause engine knock. Most of this information is taken from the American Petroleum Institute.

Octane requirements are lowest in a new engine by as much as 3-6 octane. As the engine gets broken in, sometimes as much as 15,000 miles later, the octane requirement will stabilize to the manufacturer's octane recommendations. As you drive your brand new car off the lot, it may run fine on 81 to 84 octane gasoline. As the engine ages, the octane requirement will increase until it levels out at 87 (assuming 87 is the manufacture's fuel recommendation).

Hotter the ambient air and coolant, the greater the octane requirement.

Higher the altitude above sea level, the lower octane requirement.

Lower the humidity the greater the octane requirement. Low desert not looking good.

Octane requirement increases with advancing spark timing and increased engine compression ratio.

Heavier engine load (WOT) may require higher octane requirements.

Malfunctions of emission control systems can affect octane requirements.

Computer-controlled engines can adjust spark timing and air-fuel ratio based on knock sensors, ambient air temperature, barometric pressure and load requirements.

Many new engines recommend premium fuel for optimum performance in terms of gas mileage and emissions. In some cases, the engine performance is essentially the same with regular or premium fuel use. If the engine is turbocharged or has high-performance design characteristics, use of regular fuel will cause significant performance loss and possibly damage.

I'm sure there are more octane factors not mentioned here.

[Tom Kirkham]

Fuel mixture, lean being more knock sensitive
Engine speed, lower being more knock sensitive
Higher compression is more sensitive to knock
A bigger bore is more knock sensitive
multiple spark plugs per cylinder decrease knock sensitivity
central spark plug is less knock sensitive than side mounted spark plug
aluminum cylinder heads are less knock sensitive than iron heads
colder heat range spark plugs are less knock sensitive
oil contamination in the combustion chamber increases sensitivity
swirl, and turbulence decrease knock sensitivity
quench and squish decrease sensitivity
wedge heads decrease sensitivity
supercharging increases sensitivity
inter-cooling decreases sensitivity
water and alcohol injection decreases sensitivity
insulating the underside of the manifold decreases sensitivity
blocking off the heat riser passage in an intake decreases sensitivity
Air gap intake manifolds are less sensitive
Dome pistons are more sensitive to knock than flat tops, or dished pistons

[*13*]

Great thread! Thanks for sharing the knowledge...

[Rick Parker]

A flexable right ankle can adjust knock sensativity.

[FWB]

I opted to superglue a mattress to my front door for a huge decrease....

[Tom Kirkham]

Knock is what limits the amount of power an engine can produce.

Knock or pinging is when the combustion flame front turns from a conflagration (smooth controlled burning) to detonation which has an erratic and multiple flame fronts (i.e. explosion). Pre-ignition is when the cylinder is lit off by something other than the spark of the spark plug. This can be the electrode of the spark plug being too hot, or an exposed spark plug thread, or even carbon glowing in the combustion chamber. Detonation is not pre-ignition, however pre-ignition can and usually does cause detonation.
In a multi-cylinder engine prior to the advent of computer controls, most engines were tuned to the cylinder with the most sensitivity to knock.

If you look at the lists presented above you will see that the factors listed can be put into several broader categories:

Unburned fuel air charge quality.
This is basically how homogenous the fuel air charge is at the time of combustion. Items in this category after the intake valve are: how much the charge swirls around the cylinder (think water going down a toilet), tumble is very similar in that how much the charge tumbles (think end over end). Side note: tumble and swirl hurt air flow, but not necessarily power.
Squish is when the piston comes up very close to the head and squishes out the charge. This is a very violent action that mixes up the fuel air charge. Wedge heads are the poster boy for this. However, almost all modern heads take advantage of this to some degree. That is one of the reasons that 4 valve heads combustion chamber have a clover leaf appearance.
Items before the intake valve are manifold design, charge velocity through the manifold. The higher the velocity the charge air has through the intake runners the more homogenous the charge will be when it gets to the chamber. If the charge does not have enough velocity the fuel will drop out and puddle on the intake manifold walls. Port injection, direct injection, and individual intake runner manifolds are methods to control the variability of how much fuel is getting to each cylinder. When fuel is introduced prior to a supercharger, the supercharger is an excellent fuel air charge mixer (homogenizers.)
Another part of charge quality is the Air to Fuel Ratio. The closer a fuel air charge is to stoichiometric (where all of the air and all of the fuel are used in the combustion process), the easier the charge is to ignite. Fuel rich mixtures and fuel mixtures leaner than stoichiometric are more difficult to ignite and thus are more resistant to knock. Side note: A stratified combustion chambers takes advantage of this phenomenon, to have an area in the combustion chamber that is close to stoichiometric to begin the combustion process and another area that is much leaner to complete the combustion. This is the basic idea behind a Honda CVCC engine .

Temperature of the charge.
The lower the inlet temperature charge temperature the less sensitive the charge is to detonate. Methods to reduce inlet charge temperature are: inter-coolers, fresh air inlets (hood scoops), isolation boxes (turkey pans), water injection, air gap manifolds, insulating the bottom of intake manifolds, insulating exhaust systems to decrees under-hood temperatures. Running fuel through coolers, blocking off manifold heat (heat risers), lowering engine running (or water) temperature. Aluminum cylinder heads are other ways to lower the combustion charge temperature. Drag Racers and Bonneville racers will also pack their intake manifolds in ice (or even dry ice). Now most of these items are not designed primarily to decrease charge sensitivity to knock, but to increase the density of the charge, and thus make more power.

Compression.
Just the act of compressing air will cause the temperature of the air to go up. And it turns out this temperature rise is very significant. So for a given set of initial conditions, a fuel air charge will detonate at some compression ratio point. So the higher the compression, the higher the temperature and thus the more knock sensitive the engine is. There are test engines where the compression ratio of the engine can be change while the engine is running. This is how knock ratings of fuel are determined. If I recall correctly, Saab had a high boost turbocharged experimental engine that could change the compression ratio on the fly, so that a computer could optimize the compression ratio and boost as the car was driven. The head was basically on a hinge, which allowed the head to pivot up and down to control the compression ratio. Another thing to consider is the “dynamic compression ratio.” The more valve overlap (when both the intake and exhaust valve are open at the same time) a camshaft has the less dynamic compression ratio the engine has. This will reduce octane requirements, as well as increases Exhaust Gas Recirculation (EGR) which further reduces octane requirements. EGR dilutes the combustion charge with exhaust gas, causing the combustion temperatures to go down. Large valve overlap is what makes hot rod engines run rough at low RPMs.

Engine speed.
The faster the engine is running the less time there is available for the charge to detonate.

Flame front length.
The longer the flame front has to travel the more time the front has to change from conflagration to detonation. Items in this category are: centralized spark plugs, (less distance to travel from the center out, than from one side to the other by half). Multiple spark plugs also decrease the length the flame front needs to travel. Quench means to remove heat. The quench portion in the head stop the flame front and thus detonation. Wedge heads have typically large quench areas. Which are also good for squish. Dome pistons are made to increase compression ratio for a given head by protruding into the combustion chamber. The down side is the flame front now has a much longer path to follow. Obviously the bigger the cylinder bore the longer the flame front has to travel. This is one of the reasons that modern engines are going to under-square designs. By going to an under-square design an engine manufacture can for a given displacement, run more compression ratio, making the engine more efficient.

Ignition timing.
With ignition timing you want the engine the spark to happen at the right time. Well duh. What you want is the combustion to push down on the piston at the right time. In an Otto cycle engine, the engine is most efficient if all of the combustion takes place at Top Dead Center (TDC). If the spark and combustion took no time at all you would want the spark to fire at when the piston is at TDC. The problem is the spark and combustion process DO take time. So if the engine sparks at top dead center, the charge will be burning as the piston goes down the cylinder. If the engine speed is high enough the charge will still be burning as it leaves the combustion chamber through the exhaust valve. Obviously, not the hot ticket to engine efficiency. On the other hand if the spark happens prior to the piston reaching TDC then some amount of energy is going into stopping the piston from coming up the cylinder. So then there must be some optimal setting for the spark timing. Unfortunately it changes with load, inlet air temperature, coolant temperature, etc. And of course it must not be so early that it causes the engine to knock. If the spark timing is too early the pressure rise (and thus temperature rise) will be to fast causing the fuel air charge to detonate.

Now for something to consider: A little detonation at very small throttle openings will not hurt the engine. An analogy: Throw a firecracker into your living room. After the smoke clears there will be no damage. Now throw a stick of dynamite into your house… In other words at small throttle openings there is not enough charge in the cylinder to damage anything. However at large throttle openings you can destroy and engine just as quickly as if you had thrown a stick of dynamite.

Hope this helps

[Gaz64]

Originally Posted by WardL

As the engine ages, the octane requirement will increase until it levels out at 87 (assuming 87 is the manufacture's fuel recommendation).

Combination of rings bedding in, (which doesn't take long) and the carbon buildup that ALL engines burning a hydrocarbon get.

The compression ratio goes up from the carbon buildup.

Water injection keeps the carbon buildup to a minimum and also suppresses/minimises detonation.

[rattlesnakepete]

I believe there are two forms in which combusion can physically occur, 1) weak defligration and 2) strong detonation.

In defligration the preheat comes from radiation/conduction of the previously combusted material. Almost all forms of combustion common to us are weak defligrations, even including the discharge of a firearm cartridge.

In detonation the preheat comes from a propagating shock wave and not radiation or conduction. Detonations involve very large magnitude pressures, millions of psi. It is ubiquitous to use the term detonation in context with I/C engines but I believe it is technically incorrect.

In interesting subject and thread nontheless.