link between compression ratio and octane rating (alcohol engine)?

Question

I would like to know what is the relationship between octane rating and compression ratio.

What i know is that octane rating is a measure of anti detonation of a fuel. Diesel fuel have an octane rating of ~20 and its compression ratio is ~20:1. Gasoline however have an octane rating of ~80 and compression ratio of ~8:1.

So i would like to assume that lower octane rating will give high compression ratio.

ive got 2 question :

1) I dont understand when using an alcohol+gasoline(say e85,its octane rating is 105) fuel, the engines compression ratio should be ~14:1.
2) What happens in an e85 car when im not using the correct fuel percentage?say maybe 15% alcohol

Answer 1

With a gasoline engine, the higher the octane rating, the higher the compression ratio can be before you get pre-ignition, or ignition of the fuel before the spark plug fires.
Most high compression, high performance engines use the highest octane gasoline available.

The higher the compression ratio, the higher the octane rating should be, at least for gasoline engines.

For a regular gasoline engine, an octane rating of 80%-85% is considered satisfactory, however if you want to pay more for your gas without getting any compensation for it, you can buy and use high octane gasolines in a regular engine. In other words, you can use a high octane fuel in a regular engine, but there is no reason to use it, and it is just a waste of cash.

E85 ethanol fuel has a higher octane rating than a standard fuel pump gasoline, however it contains 30% less energy than regular gasoline, and will decrease your mileage by approximately 30% when compared to using regular 100% gasoline.

An E85 rated engine just signifies that the engine is satisfactory for the use of an 85% ethanol fuel.
The engine is actually designed for 100% gasoline, but is modified so that it can be used for any combination of an ethanol (10%, 15%, 50% etc. up to 85% max)/gasoline mix.

It takes 4 gallons of 100% ethanol to produce the same mileage and energy as 3 gallons of regular gasoline.
Think of that every time you buy ethanol gas and compare the price to the price of regular gas.

Answer 2

There is no direct relationship between the compression ratio and the octane rating. The compression ratio is purely a function of the physical layout of the engine, and the octane rating is purely a function of the chemical properties of the fuel.

Also, in order to maximize the efficiency of combustion, you GENERALLY want to maximize the compression ratio of your engine. But as you compress any gas, it heats up. If you compress it too far, it will heat up so much that the fuel will autoignite before the spark plug fires. This is called preignition, knock, or detnoation. It is very bad for engines. In the worst cases it can melt pistons, crack pistons, blow out head gaskets, etc.

So generally, you need a certain MINIMUM octane rating in order to run a given compression ratio. That means that you can run a higher octane rating with no ill effects (except the impact to your wallet). But if you run too low of an octane rating it COULD damage your engine. (Most modern cars that require high octane fuel have knock sensors that will adjust the timing of the spark plugs to prevent harmful detonation if you happen to get a bad tank of fuel.)