High Compression Low Octane

[capnbass91]

I am quite impressed with the SkyActive and EcoBoost that are out there, direct injection, turbo/supercharging, and even higher CRs.

I just filled up with 87 octane and it cost $4.199/gal, about average for OC, and there is no ethanol free stations anywhere.

After this tank I'll try a tank of 91 and see if there's a noticeable difference.

Thanks for all the info!

[gone-ot]

...here's an OSU college 'thesis' paper on the subject that's worth reading:

https://kb.osu.edu/dspace/bitstream/...ate_Thesis.pdf

[MPGranger]

about the atkinson cycle engine. I thought the big deal with it was that the connecting rods also cycled so a cylinder had a combustion event every 360 degrees of crank rotation. (Versus the 720 degrees with the otto cycle) But atkinson cycles are low on power/high on efficiency since the full compression isn't fully used.

and a miller cycle engine leaves the intake valves open until the combustion event so a supercharger keeps packing the cylinder. So the static compression ratio is useless here as well, and so is the dynamic (since the valve is open almost to top dead center)

[mechman600]

Originally Posted by MPGranger

about the atkinson cycle engine. I thought the big deal with it was that the connecting rods also cycled so a cylinder had a combustion event every 360 degrees of crank rotation. (Versus the 720 degrees with the otto cycle) But atkinson cycles are low on power/high on efficiency since the full compression isn't fully used.

Atkinson is still a four cycle engine with power stroke every 720 degrees.

and a miller cycle engine leaves the intake valves open until the combustion event so a supercharger keeps packing the cylinder. So the static compression ratio is useless here as well, and so is the dynamic (since the valve is open almost to top dead center)

Increased static compression ratio = increased expansion ratio (regardless of intake valve timing), so static still means a lot when it comes to efficiency.

[MPGranger]

HowStuffWorks "How an Atkinson Cycle Engine Works"
Animated Engines - Atkinson

I should have mentioned that the atkinson achieved 4 strokes with only 360 degrees of the crank due to some tricky connecting rods and linkages

But Mazda's skyactivD (2.2L diesel) lowered compression to a measelly 14:1, but improved FE. So wouldn't that suggest that there are diminishing returns for just bumping up compression ratio?

[MPGranger]

Low CR engines use lower octane fuel because the octane rating is about Knock AKA predetonation. Modern high compression engines have variable timing and ignition and so forth just in case you use lower than optimal octane rating so you don't do damage to your engine. This protection, however, will cost you some FE.

In summary, an engine that doesn't need higher octane won't become more efficient with higher octane.

HowStuffWorks "What does octane mean?"

Also some gas brands put additional cleaning additives in just their high octane fuel to help sell their more expensive dino-juice. Their are also top-tier gasoline brands that put additional cleaners in all their gas choices. But relax, in the US, their are additives that must be in the gas already.

[ERTW]

Yamaha 5 valve tech will not knock - 93 octane with 13.5:1 cr! The corvette LT5 was said to run fine on 87 octane with 11:1 cr.

[metromizer]

Originally Posted by ERTW

Yamaha 5 valve tech will not knock - 93 octane with 13.5:1 cr! The corvette LT5 was said to run fine on 87 octane with 11:1 cr.

A lot of recent engineering has gone into homogenous charge, better mixing of the fuel and air within the combustion chamber. That, coupled with better 'active' fuel/spark controls I think is what allow today's engines to use higher compression ratios with relatively low octane fuels.

[Allch Chcar]

That said, I have heard rumors that Honda's LEA series will adjust for higher octane and they are 'only' 10.8:1. The number I heard was 100 RON which should be about 94-95 AKI octane.