compression ratio on fuel

[Dirka]

Can anyone help explain this concept to me?
After which point is it better to run a higher octane fuel?

Thanks!

[Jason S]

I'll take a stab at this:

Higher octane rated fuel can take more compression. High octane doesn't mean the fuel has more thermal power.  As an example, LPG (propane) is rated at 115 octane, but only has around 85% of the BTU's of an equivalent amount of  gasoline.

A higher compression engine, let's say 9.1 or greater, can squeeze a low octane fuel/air mixture to the point of detonation. This is where the fuel/air mixture ignites without spark from the spark plug. It can be pinging under load or engine run-on (i.e., dieseling) after shut down. Pinging and engine run-on will also occur on a hot low compression engine using low octane fuel.

After which point is it better to run a higher octane fuel?

That depends upon the engine and how it's built. Different aspects of the combustion chamber, such as the design and material of the heads, piston design, quench area, ignition timing, air temperature, etc. can all have an influence on the need for higher octane.

A 9.5:1 compression engines using aluminum heads may run happy with 87 octane with an occasional addition of 89 octane; while the same engine with iron heads may require 89 or 91 octane all the time.

Computer controlled engines can allow high compression engines to run on lower octane rated gasoline by modifying the ignition timing in real time, the engine won't produce the same amount of power but will operate without knocking or pinging.

There are plenty of writeups online, in magazines and books on built and tested 350's engines making 400 horsepower on 87 octane gasoline. The underlying factor that I read on a lot of these engines is relatively cool air temperature, optimized combustion chamber design and material and timing.

So I'm sure I've skipped over a lot of things; However, in short if you have a regular production stock engine with a properly tuned ignition system- then 87 octane (or thereabouts) should be just fine for regular driving.  If you have added compression, I'll say 9.0 or greater, then you're engine will probably need a higher octane rated fuel.

[bd]

To help with some fundamental concepts concerning what Jason has posted, compression ratio is a mathematical relationship.  Compression ratio is the combined volume of the cylinder plus the combustion chamber with the piston at its lowest point of travel, divided by the volume of the cylinder plus combustion chamber with the piston at its highest point of travel.  Effectively, a high compression engine squeezes a given volume of air-fuel mixture into a smaller space than a low compression engine.

When the air-fuel mixture in a cylinder is compressed by the piston as it moves upward, the temperature of that mixture increases (it becomes hotter).  The more the mixture is compressed, the hotter its temperature becomes.  So, a higher compression engine increases the temperature of the air-fuel mixture more than a lower compression engine.  Significantly, heat directly affects the rate at which an air-fuel mixture burns.  A hot air-fuel mixture burns faster than a cool air-fuel mixture.

Fuel octane relates to how fast fuel burns in the presence of oxygen.  High octane fuel burns more slowly than low octane fuel.  Since high octane fuel burns more slowly, the resulting pressure in the combustion chamber develops more slowly.  So, higher octane fuel provides greater control over the rate of burn, the speed of flame travel, and the development and timing of combustion pressure.

Now, for optimum engine power production, the "compressed air-fuel mixture" must be ignited by the spark plug when the piston is located very near its highest point of travel.  If the mixture ignites too early, the resulting explosion will try to push the piston back down the cylinder before it has reached the top of its travel and the engine will "ping" and lose power.  'Ping' is the result of improperly timed, or uncontrolled burn and detonation.  Unresolved, chronic detonation destroys engines.

Since higher compression increases the air-fuel mixture temperature more than lower compression, higher compression engines require higher octane fuels to control the rate of air-fuel combustion and minimize detonation (ping).

[Dirka]

Thanks those are both great helps!
A compression ratio of 9.8 is higher or lower than 9.1?

[zieg85]

The higher the number the higher the compression

[VileZambonie]

9.8 and 9.1 are not compressions ratio's. These would be CR's:

9.8:1 or 9.1:1 etc