What is Compression Ratio...? and how is it related to High Octane Gas...?

The internet has a lot of information regarding what Compresssion Ratio means for engines.
But there are very few articles that mean much to motorcycle engines and about how the CR influences the characteristic of a motorcycle.

In theoretical terms, the compression ratio is the ratio of volume between the top-dead-center and bottom-dead-center..








Here are the CR for some common Indian Motorcycles:

Model	Wet Weight in KG	Compresion Ratio
KTM D390	163	12.8:1
R15	136	10.4:1
CBR 250	164	10.7:1
RE Classic 350	187	8.5:1
Avenger Cruise	155	9.5:1

What this number actually signifies is how much the air-fuel mixture gets compressed to before being ignited. That is, how much pressure would the combustion produce inside the engine.

To Simplify:
    More the CR, more the internal combustion pressure
    More the CR, more the heat produced during combustion
    More the CR, more energy is squeezed out of the same volume of petrol- hence higher mileage.

So it sounds like having a higher CR is good! Then why do some engines have lower CR?

Because of Point 1 and 2.
More internal combustion pressure means more wear and tear on the engine components.
Then how does Duke or R15 manage higher CR?

Duke has Nikasil coated engines which are also forged engines.
R15 does also boast a forged piston hence the increased cost than every other 150cc bikes
Ref: https://en.wikipedia.org/wiki/Nikasil

Because of this, the engines can take a lot more abuse and last a lot longer! hence they can sustain higher CR.

Point 2, all engines produce heat, but as CR increases the heat produced increases exponentially; ref: Thermodynamics.
Therefore, more efficient heat dissipation methods (oil cooling, radiators, liquid immersion) must be used; increasing the production cost.

Therefore, it becomes impractical to have a basic commuter motorcycle to boast all these features while keeping the cost down.
Enter: FZ, Pulsar 150, Hero Impulse, etc

Now the problem with compression occurs when the CR increases a certain threshold.
This is the only reason why we don't have infinitely high CR even if we could create engines out of diamond-like materials.

Uncontrolled combustion of air-fuel mixture during compression.
Whenever gas is compressed, it releases heat. This heat at times is enough to combust the air-fuel mixture.
And in low octane petrol - have molecules of hydrocarbon that combust at lower temperatures than pure forms of petrol.

Consider high-octane petrol to be ideal and purest form of petrol. They withstand high compression.

This is the reason why engines with high compression ratio are better off with high-octane petrol if the engine knocks when using regular petrol. 

Not because the higher octane fuel gives higher mileage or is cleaner.
The higher octane fuel returning more mileage is just a petrol-pump myth! ref: More Myths


Now how does this influence the characteristics of an engine?
    Will write about it in another article real soon ;) Keep riding and stay subscribed if you dont want to miss it!!

NOTE: Figures mentioned above are approximations acquired from respective brand sites; and images are outsourced from wikipedia with free usage licenses.