Compression Ratio And Octane

[jnmeade]

Is there a relationship between compression ratio and minimum fuel octane? In answering, please give examples to support your comments.
The reason for my question is I think I've seen varying relationships. It seems that Lycs and Cons have to be down around 6-7:1 to run 87 octane, a little higher to run 91 AKI and if they're up around 9-10 they demand 100LL.
On the other hand, the Rotax 80hp at 9:1 runs regular car gas and the 100hp at 10.5:1 wants 91 AKI or 100LL.
If there is some other determinant for minimum octane, such as cam dwell or stroke or bore, feel free to educate me.
Now, before you all go off on some tangent, please try to initially answer the simple question of physics on the relationship between compression ratio and octane. Then, feel free to take the thread wherever your own interests take you, such as what engines will really do versus what the manufacturer publishes and so forth.

 

[Checkout_my_Six]

Yes there is a direct correlation. Octane is a measure the fuels ability flash or combust. The higher the octane the harder it is to get the combustion flame front started. Higher octane fuels are used in high compression engines to control the combustion event and to prevent pre-ignition and detonation.

 

[genna]

There is a correlation, but it's not the only thing that controls what octane you can run. Ignition timing is more of a factor(or at least more of a factor that can be easily adjusted).

 

[Henning]

8.5-9.5:1 (most cars) runs perfectly well on 87 octane, I have even run 10.25:1 on 87 octane. It's not just CR that matters, but ignition timing as well, and how clean your combustion chamber is and even the shape of it, plus there is load factor. The lower the octane rating, the cleaner it burns.

 

[Checkout_my_Six]

There is a correlation, but it's not the only thing that controls what octane you can run. Ignition timing is more of a factor(or at least more of a factor that can be easily adjusted).

Yes, but adjusting the timing can affect (lower) horsepower and Hot CHTs. Combustion engineers attempt to extract energy from the fuel burn efficiently. Timing that is too retarded will produce less takeoff power. If adjusted in the other direction one sees higher HP but with excessive CHTs. So this is a balancing act to gain max HP with the cylinder operating within material limits.

 

[genna]

Yes, but adjusting the timing can affect (lower) horsepower and Hot CHTs

Correct. So is adjusting compression .. Regardless, OP seem to have asked why different engines react differently to CR changes. Timing and, as Henning pointed out, other design factors have big effect on what octane can be used in the engine. And you can relatively easy retard timing to use lower octane(nearly all new cars do it automatically now), but messing with CR required quite a bit of work

More compression=~ more power
Faster timing=~ more power.

Both result in higher temps

Edit: To finish my though here before someone jumps on me . Both eventually result in pre-ignition and will require higher octane to combat it.

 

[ronnieh]

As check my six said higher compression needs higher octane. But, there are other factors to consider. Bore diameter is a big factor. The smaller the head (diameter across the combustion chamber) the higher the compression can be per octane number. In a big bore engine more time is required for the flame front to go from one side to the other allowing more time for the combustion process to start in another area of the head due to compression heat which is one cause of the "knocking" that can be heard. Thus the octane requirement might be higher per compression ratio. The compression ratios you are quoting are theoretical ratios and not effective ratios so yes cam profile will figure in. On a two stroke, port timing.

A high quality racing fuel (perhaps 110+) designed for big bore application for example the mountain motors used in the pro mod drag racing cars will need a relative fast rate of combustion to cover the distance across the combustion chamber and still resist a secondary flame front due to the heat of combustion. Shape of the head also can figure in as in a hemi verses a wedge.

In short in addition to mechanical compression ratios, there is a host of other factors of which I have touched on a few.

 

[Omalley1537]

Compression is a factor but the true compression that's important is dynamic (running) compression, not Static (the relationship of cylinder volume at TDC and BDC). Dynamic takes into account cam timing. Two engines can have the same stroke, bore, and combustion chamber/piston volume but have very different octane requirements (even with everything else equal) due to cam differences. A cam with a late intake closing event bleeds off compression because the piston can't start making compression until the valve is closed.

 

[docmirror]

Yes, generally speaking higher compression requires higher octane. Now - Gosh, where to start. Combustion chamber geometry, fuel stratification/atomization, valve timing, ign timing, metalurgy, spark plug location, rod length/stroke ratio, exhaust efficiency, are just a few things that come into play.

Let's just focus on one thing, and that's fuel stratification/atomization. In recent years, there have been leaps and bounds improvement in getting all the fuel to mix with all the air coming into the cylinder so the entire volume of mixed gas truly is mixed(charge homogeneity) and the effective F/A ratio is consistent just before combustion event. This is soooooo critical because if there is a too rich or lean mixture in a localized area of the combustion chamber, and that area happens to be near the hot exhaust valve, or maybe the ground lug of the spark plug, you are going to have either pre-ignition, or detonation(depending on what happens at the compression event). This will require higher octane to inhibit. So, on more modern engines like the Rotax it appears they've done a better job of charge efficiency, and maybe better atomization to take advantage of the higher compression without increasing octane. And - that's just one aspect of a very complex chemical, mechanical action. There's a ton more to look at when trying to suppress detonation and/or pre-ignition while maximizing thermodynamic efficiency.