High Compression Low Octane
 

I recently got a 2012 Ford Fiesta (2 year lease) and it has a 1.6L 4cyl engine with a quite high compression ratio, 11:1. A few months ago I got a repair manual for my old car that I'm selling, 97 Chrysler Sebring Convertible. It has a 2.5L V6 with a compression ratio of 9.4:1. There is a side note in the manual that CR a little above 9:1 usually requires premium gas to operate correctly, the Sebring gets away with 87 octane due to timing adjustment and other stuff.

In the owner's manual for my new Fiesta there are 2 sections about gas, 1 says 87 octane is the minimum, while the other says it's the recommmended gas. Since it has such a high CR, I was wondering if there would be any benefit to using higher octane gas. I know higher octane would have anywhere from absolutely none to no benefit in my Sebring, but considering the Fiesta has such a high CR, maybe the timing and other stuff would be adjusted by the car's computer to use the higher octane.

I calculated the benefit and with gas at about $4.34/gal the 91 octane would have be about 5% more efficient that 87, with a 20 cent difference in price and originally getting either 30 or 33mpg.

Unfortunately my Ultraguage was stolen :mad: so I can't use it to see the actual timing advance with the different octanes, will just have to use avg mpg.

I'm just curious that if one works should the reverse:
hi CR - hi octane needed - low octane used = less power/efficiency
hi CR - low octane needed - hi octane used = ?

The dealer gave me a full tank of gas when I got my car and got 30 mpg on that. I'm going to use 2 tanks of Shell 87 octane (almost done with the 1st) then try 1 on 91, see if there is a noticeable difference with similar driving styles.

Different cars react differently to high octane gas. It makes no difference what gas I put in my Matrix, so I go with the lowest octane. The best way to find out is to try it, so it looks like you are on the right track.

...they (Ford) are playing with the valve-timing so as to vary the effective-CR (which is functional combination of static-CR & dynamic-CR) as engine load and speed change.

Both camshafts can be controlled independently on the Fiesta with throttle by wire and electronic timing control. My last tank in my Fiesta was 47 MPG on cheap Wal Mart gas ($3.409 a gallon). My gauge read 51 when I refilled. I tried 91 octane 0 alcohol gas and got about 2-3 MPG better per tank average, but it cost $4.199 a gallon. When the regular was $3.909 a gallon 6 weeks ago the difference in mileage just about covered the difference in cost, but now that regular E10 has dropped to the point where the difference is 70 cents a gallon it is not even close.

The difference in technology between the Fiesta and your old Chrysler is like the difference in the first us jet, the P80 and a F15. Look at the new SKYACTIV tech from Mazda to see the next step in engine technology.

regards
Mech

Almost Atkinson-esque, no? Probably close, but not close to enough to have to say it out loud.

Well, all cams have their natural peak volumetric efficiency speed, and most engines with 1 cam profile have this peak rather high in the rev range, so basically all engines have some sort of Atkinson-esque stuff going on at low engine speed. With cam phasing I believe what happens is that at lower rpm they advance the intake cam so that it closes earlier if you need more power, although if you have too much overlap then you end up with a lot of fresh, hot exhaust trapped in the cylinder and you have a knock problem.

As an example, I think the new Subaru FA20 (on the BRZ/FRS, and some variant will appear in the new WRX) the intake cam seems to have a volumetric efficiency peak in the 6000s rpm, so it's very possible that the actual cam duration is longer than the cam duration on a Prius for example.

it may also have to do with most newer cylinder heads having a spherical-ish, non-protruding combustion chamber

reminds me of "l heads" of the flat head fords 6.0:1cr on 87(i think?; wasn't born) vs. lead fuel of the 60's 10:1 or more; vs 80' 8.0-9.0:1 cr

Green Car Congress: Details on Nissan

This gives you some ideas about high compression ratio with 13 to 1 and supercharging in a 3 cyl 1.2 liter engine.

regards
Mech

Good read.
And a magnetic clutch driven supercharger. Like Mad Max? :D

EGR reduces "knock"
 

although if you have too much overlap then you end up with a lot of fresh, hot exhaust trapped in the cylinder and you have a knock problem.

those "hot gases" are inert and do not support combustion , they reduce combustion pressure and by so doing reduce
Combustion Temperature
they absolutely do not cause "ping" or "knock"
they reduce it

they effectively reduce engine displacement by diluting the available combustion chamber size with inert gases , creating a kinda of a cushion of a compressible gas that does not combust inside the combustion chamber

they are EGR
having said that
cooler temperature EGR is better than hotter temperature EGR

for the reduction of
NOX

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!

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

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

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)

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

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

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?

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.

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.

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.