High Compression Low Octane

[capnbass91]

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 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.

[mechman600]

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.

[gone-ot]

...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.

[user removed]

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

[mechman600]

Originally Posted by Old Tele man

...they (Ford) are playing with the valve-timing so as to vary the effective-CR

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

[serialk11r]

Originally Posted by mechman600

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.

[baldlobo]

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

[user removed]

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

[mechman600]

Good read.

The main concept for reducing pumping loss is the late intake valve closing (LIVC) Miller cycle with high compression ratio. This enables a high expansion ratio even in LIVC state. In the HR12DDR, intake valve closing (IVC) is retarded until 100 ° after bottom dead center (ABDC)—corresponding to an effective CR of about 7:1. To reduce the rest of the pumping loss on un-boosted partial load, Nissan combined internal EGR with exhaust continuously variable valve timing (CVTC) and external EGR.

And a magnetic clutch driven supercharger. Like Mad Max?

[mwebb]

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