Compression of our motor and octane

I usually use the Costco near me and since they only have 87 and 93, it can be like $.50-$.60 difference. That being said, depending on how much fuel is in my tank, I switch between the grades so it probably costs me $10-$15 a month to run 93 as much as possible.

Other stations gas cause the same stumble on 87 as originally I thought it was a gas issue for those curious.

 

Yes it is.

But it actually has no value other than measuring it with engine not running. Cylinder pressure is what counts which can change depending on several factors. So use of what fuel you use could vary.

 

The compression ratio still has value. It is the maximum value the cylinder obtains depending on conditions. It can not go higher, only lower.

It would be interesting to see on a graph how often the intake valve alter cylinder pressure.

 

I'll be damned, they must have lied to me when taking thermo and internal combustion engines

 

Here's an interesting article...
It's sort of a triple-edged word for me as it makes a case for increased ethanol blending and I'm a 91 E0 'believer'
It might explain some of the difference seen between 87 ethanol blend vs 91 ethanol blend.
http://www.energyresourcefulness.or...ctane_levels.html#holding_octane_to_old_level

My take away is, octane sensitivity(RON minus MON)is a thing.
Blendstock is 88 RON. Ethanol percentages gives us regular or premium. 91 AKI will have a higher percentage of ethanol; increasing mostly RON and thus increasing octane sensitivity.

 

direct injection has basically made the old notion that Higher compression engine require higher octane fuel all but moot.. having direct injection the engine can adjust AF ratios on the fly. the only case where higher octane fuel is required is compressions higher than 14:1, as well as turbo applications. my maxda cx-5 with a 13:1 compression ratio runs perfectly on 87octane.. DI is the exact reason ford is producing smaller turbo DI engines that can run on 87 octane...toyota is toying with the pseudo atikinson cycle.. but is only active on low RPM (500-2200rpms roughly) on our engines.

 

I run 85 octane.

 

I don't know if this has been posted yet or not. Here is a link for the specs for the 2GR-FKS engine that is in our 3rd Gen Tacomas. http://www.toyotareference.com/guts/tacoma16_engine.pdf

Note the fuel octane rating. This engine was designed to run 91+. Toyota has tuned it to be able to run 87 nicely, for a variety of reasons.

I personally started running a higher octane. I prefer it. My 2 cents.

 

I had a 9-3 with the HOT. great fun! But crappie oiling system for the turbo. Oil lines were too small for super cold weather. Blew a turbo when the overnight temps got to -4 F. Started it and it imploded during warm up just idling.

 

There is debate as to whether that is RON or AKI. 91 RON=87 AKI

 

Ouch. That is interest coming from the Swedes. Hopefully you were still under warranty at the time.

 

News flash there is more heat energy in 87 than there is in 93. I know your not going to like this but ethanol has some pretty good anti knock properties. Race cars any one?

 

Nope $1,400 later... We have a 9-5 also and it had no issues with the cold.

 

This thread tho lol. I started off with 87 and my gas mileage was around 18. I switched to 92 and since then I get 19.5 regular commute which is an even mix of street and highway. On the weekends we drive to visit family and highway I average about 20.5. The kicker is my truck was new and I didn't get past the break in period so this all could be fake news lol. Will probably switch back to 87 just to see.

 

That is a bummer.

When the kids were little, we tried to find an MT 9-5 wagon. No luck at the time. We ended up with a used V70 MT. Let's just say that the wrong Swede went out of business. We both wish that we grabbed the new Subaru 2.5GT wagon that we test drove. We'd probably still have it.

We're staying Japanese here on out.

Regardless, a 1.5mpg bump just about covers the cost, so it works out well for you.

 

THREAD REVIVE!

Just skimming through this one.

I can personally attest to running ethanol vs E-free. Out here we have 88 E-free gas. I've been running it for 6 months now and ever since switching from 87 eth, have seen about +1.5 mpg difference.

Don't run ETH. It is literally filler food under the guise of "cleaner" fuel to appease the greentards.

 

Other than using the seat-O- meter how are you measuring this difference in performance?

 

You do realize ethanol has a much higher octane rating? How come serious high performance stuff burns ethanol? Maybe the "greentards" like the performance?

 

Nice to see a real world result. I agree with your analysis of why; that pretty much sums it up.

 

Right, Alcohol has a higher octane which means it is more resistant to detonation, however, alcohol produces less power per volume than gasoline. It produces less heat during the power stroke (heat is the energy) and exerts less force on the piston (heat expansion is where the power comes from that drives the piston during combustion) with all other things (compression or boost, timing, fueling) being equal. For that reason, an ICE requires more alcohol with each power stroke. IIRC stoichiometry for alcohol is 8:1 ( air/alcohol) which is far richer than the 14:1 for gas. Stoichiometry changes with engine demands, and that ratio is a guide but it also explains the general difference in fuel requirements.

Where alcohol is useful is with running a lot of compression or boost and fuel efficiency is not an important factor. You can simply compress it more and make more power on a pure alcohol engine. On a flex fuel engine there are some compromises because of the air fuel ratio required for either creates some practical limitations. All things being equal as far as cylinder pressure, timing, cylinder head material, cam design and so on, an alcohol fueled vehicle would get 2/3 to 3/4 the mileage of the same setup with gasoline. 20 MPG would become 15 MPG or so. Aside from that, there is the hydroscopic issue where alcohol attracts water which is not great to have in your fuel, and alcohol more readily contaminates oil. Like lots of things, there are trade-offs. Burning alcohol also creates other pollutants than gasoline which are not really any better for us. You can make more power with alcohol with an engine designed to work with alcohol's stoichiometry, and tolerance for pressure. What you can't do is get better mileage. alcohol engines simply require more fuel per power cycle.

For most of us, we would be better off with straight gasoline. For a race application alcohol might be a better choice. If it were endurance racing, though it would change your pit strategy because you would need to fuel more if you were limited to the same tank volume as a competitor running gas. Its a trade-off which is the truth about almost all things in the automotive world.