Fuel Management

[Crimson KING]

I'm no instructor by any means, but I have been trained by some of the best so I'm going to do my best to explain some common misconceptions about which fuel we use in our engines these days.

First off I have to explain what fuel is, and what it does.
So we all know when the low fuel light comes on we go to the pump and fill up. But what is fuel? Well fuel is energy. And with all types of energy, it was created by none other than our sun. But the sun didn't create all things equal. No we have many different types of fuels to choose from. We have 100% gas. We have different blends of 100% gas. We have different octane ratings. We have Compressed natural gas and diesel and more.
But what is 87,89,91,93 and so on? Yeah it's the octane rating of the gas, but what does it mean? Well let me give you this...
Fuel - Liquid, US Gallons GGE GGE % BTU/Gal kWh/Gal
Gasoline (base)[2] 1.0000 100.00% 114,000 33.41
Gasoline (conventional, summer)[2] 0.9960 100.40% 114,500 33.56
Gasoline (conventional, winter)[2] 1.0130 98.72% 112,500 32.97
Gasoline (reformulated gasoline, ethanol)[2] 1.0190 98.14% 111,836 32.78
Gasoline (reformulated gasoline, ETBE)[2] 1.0190 98.14% 111,811 32.77
Gasoline (reformulated gasoline, MTBE)[2] 1.0200 98.04% 111,745 32.75
Gasoline (10% MBTE)[3] 1.0200 98.04% 112,000 32.83
Gasoline (regular unleaded)[4] 1.0000 100.00% 114,100 33.44
Diesel #2[4] 0.8800 113.64% 129,500 37.95
Biodiesel (B100)[4] 0.9600 104.17% 118,300 34.80
Bio Diesel (B20)[4] 0.9000 111.11% 127,250 37.12
Liquid natural gas (LNG)[4] 1.5362 65.10% 75,000 21.75
Liquefied petroleum gas (propane) (LPG)[4] 1.3500 74.04% 84,300 24.75
Methanol fuel (M100)[4] 2.0100 49.75% 56,800 16.62
Ethanol fuel (E100)[4] 1.5000 66.67% 76,100 22.27
Ethanol (E85)[4] 1.3900 71.94% 81,800 24.04
Jet fuel (naphtha)[5] 0.9700 103.09% 118,700 34.44
Jet fuel (kerosene)[5] 0.9000 111.11% 128,100 37.12

What are we looking for in fuel? The British thermal unit per gallon, or BTU.
As the chart shows each type of fuel has different BTU ratings, in other words, how long it takes to burn.
Without getting too in depth to the science of it all(there's not enough time in the day), we will explain it like this;
in the US we use basically 86-87 Octane... 89-90 octane.. and 90-94 octane.
For argument purposes lets say these gases burn in....
87 Octane burns in .87 of a second
89 Octane burns in .89 of a second
93 Octane burns in .93 of a second
So obviously 87 octane burns the quickest, and 93 burns the slowest.
Now what does this mean for you?
Well cars nowadays are all engineered and designed to use a certain type of gas. A car that recommends 87 octane runs best on 87 octane. A car that recommends 93 octane runs best on .... 93 octane!
But why? Why not just use 93 in everything? If it burns slower than 87 that must mean I get better economy right? Wrong. This is why.
We all know our motors are timed to do everything in a certain order. We have chains and belts to keep all that good stuff working together and in time as designed.
Yeah fuel trims change as our O2 sensors read the exhaust and they increase or decrease our injectors pulse width, or time that they spray fuel.
But in each of our cars there's a little box with lots of gadgets and processors and capacitors and such. That little box keeps everything in motion. It's the boss. It tells everything what to do. But what we're worried about is timing.
Say our imaginary motor here is set to detonate its fuel at 10 degrees after top dead center. Why not 0 degrees before top dead center or right at top dead center? Well if the fuel was being detonated before the piston reached the top, you probably wouldn't get very far, since the piston would be coming up, or at its peak, rather than going down. But anyway, so our motor is set to detonate at 10 ATDC. Now in this motor the engineers have told us that we should use 87 octane gasoline. Why? Because the motor is timed that at a certain RPM, this fuel is going to burn most efficiently, and most complete at 10 Degrees ATDC.
So what happens when you use 93 octane gas in the same motor? It's simple, this motor is timed to burn it's fuel in .87 seconds at 10 degrees ATDC. But now that we have 93 octane, our fuel is burning at .93 of a second.... but where does that leave your timing? You can't burn both 87 octane and 93 octane at the same amount of time... 93 octane burns slower right? So now we're throwing our timing off, because we are taking longer to burn our fuel. Instead of 10 degrees like we had with 87 octane... we're now at 12-14 degrees ATDC.

But why don't our knock sensors adjust our timing to correct this? Well it's not pinging. There's no engine knock. The Computer still sees everything is working, and technically evrything is working. But you're not completely burning that fuel. You have nothing to gain by upping your octane rating. You're just burning dollars. You've shot unburnt fuel right into the exhaust. No economy gains, no power gains.... nothing.
Long term effects of using the wrong octane? More hydro carbons. What do HCs do? Break stuff. Had a gummed up EGR? How about a p0420 Catalyst code? Maybe you have maybe you haven't. If you're lucky enough to say you haven't, count your luck. Cause I've seen it happen many times. Years go by and people still use the wrong type of gas.

Do yourself a favor and use what your manufacturer recommends. There's a reason why they put that number in your owners manual.

Please comment and ask questions if you're unclear about anything, as I said earlier, I'm no professor, just a tech tryin to spread the wealth.

 

[blackx-runner]

Everything I have ever read list those as minimum octane readings. Not recommended or required octane readings. Its my understanding that the ECU will advance the timing, to an extent, until it gets feedback from the knock sensors. And it does this continuously during driving. Does it have enough range to take full advantage of higher octane fuel? I don't know. But there is some adjustment range, so I wouldn't say its a complete waste running higher octane fuel.
I also don't see how you can say by changing octane you will be dumping unburnt fuel into the exhaust. Where is the correlations with spark timing, octane rating, and stoichimetric mix that causes the excess fuel?

 

[Gadget]

There is a lot of stuff in there that is right, but right for the wrong reasons.

The way the octane level is determined is by running the gasoline in a special laboratory engine that has variable compression ratio. The compression ratio is raised until knock is induced and that is how the octane rating is determined.

Generally there is more chemical energy in lower octane fuel and in general it will combust faster than higher octane fuel under the same condition.

The higher octane is the slower it will burn. If you use a higher octane fuel than required you may have less power and less fuel economy and there will be more combustion byproducts left behind such as carbon that will dirty up the engine.

I did run dyno tests on my XRunner and found that the power on the dyno was exactly the same between 87 and 93. I also found that the MPG was a hire better on 87. That was all before boost of course.

G

 

[Crimson KING]

It's not excess fuel, it's just unburnt fuel. Our injectors aren't spraying more fuel, they're spraying the exact same amount of fuel. But remember, it's a fuel that doesn't burn at the same time as designed. An ECM doesn't know the difference between 87 and 93. It can tell ethanol from gas but not 87 from 93.
Yes, any octane will work, but what is "recommended" will work best. And yes, you're right, the ECM can and does negotiate with everything else to maintain a 14.7:1 air/fuel ratio. MAF's, MAPs, O2s, Knock Sensors, and everything else all work to keep our motors running right. You're o2 will see a rich/lean fuel mixture and they will make your injectors increase/decrease pulse width to account for that. However it's not a "problem" to change octane ratings. Our o2's are already doing enough work managing your injectors without having the wrong octane rating, why make them work double hard? Why make your engine work harder to account for the wrong type of fuel? In the end, it's the same components that usually go. Your o2 sensors, catalytic converters, or EGR valves.
Technology is always evolving, and maybe one day it will make a difference, but as of now, tests prove that 93 octane gas only benefits motors designed to run on 93 octane.
As far as a naturally aspirated engine is concerned, with a factory untuned fuel managment system, use the recommended octane rating.

There is a lot of stuff in there that is right, but right for the wrong reasons.

The way the octane level is determined is by running the gasoline in a special laboratory engine that has variable compression ratio. The compression ratio is raised until knock is induced and that is how the octane rating is determined.

Generally there is more chemical energy in lower octane fuel and in general it will combust faster than higher octane fuel under the same condition.

The higher octane is the slower it will burn. If you use a higher octane fuel than required you may have less power and less fuel economy and there will be more combustion byproducts left behind such as carbon that will dirty up the engine.

I did run dyno tests on my XRunner and found that the power on the dyno was exactly the same between 87 and 93. I also found that the MPG was a hire better on 87. That was all before boost of course.

G

Aha! Now you are layin down what I'm trying to say. You got what I want to say out much quicker and more effective. Kudos to you sir. :top:

 

[blackx-runner]

I always run 93 with 10% ethanol because its damn near impossible to find anything around here without ethanol except reg 87.
When I travel and can get premium without ethanol I don't see any mileage difference over what I regularly run.

 

[Crimson KING]

Usually takes a few trips to get all of the ethanol out unless you're filling up below empty. I like to disconnect my battery and short cables together for 10 minutes just to make sure my Keep Alive Memory and all learned adaptive values are reset. That way your ECM is learning all new values on the 100% gas.

Try this though, just for arguments sake. Go through a few tanks of 100% 87 octane. Reset your ECM after the first full tank then fill up on your second. Just give it a whirl, I think you'll notice a change.

 

[cwatt]

It seems the argument here is running higher than recommended fuel won't create additional power (a lot of fuel is unburnt). Sorry to be the one who says it, but arguing higher octane use here is pointless unless you have access to 93+. I think the real story is why can't we just use lower octane? G has addressed running lower than recommended doesn't loose power either. According to the logic expressed above, it would advance detonation, yet there is no measurable power loss. How can this be?

Argument: why shouldn't I just go out and start running 87?

 

[Torspd]

Due to the faster burning rate of lower octane fuels, they have the ability to "spontaneously combust". For that reason alone,higher cylinder pressures, caused either by a "conventional" high compression N/A engine or a boosted engine, is why those types of engines need to run a high octane rated fuel. Strictly to prevent spontaneous combustion, which in turn leads to knock/detonation.

That does not account for advances in technology such as Mazda's new skyactive engines which run 13:1 compression and 87 octane, and still get 30+ MPG.

As the op states though, which is a valid point, when a vehicle was designed to run best with an octane rating which burns at stated speed, changing that speed, could have a negative effect on economy, if the ECU is not adaptable.

 

[Murderface]

Before I boosted my X I had to run 91 octane or else I could hear the engine knock. Same with my wife's old 03 Altima V6, if I didn't run 91 it would knock. In neither of these engines is 91 octane listed as recommended octane rating

 

[*IrunnIt*]

The same happens when I put regular in mine, and I get horrible gas milege only in jersey, new york gas I different its waay better

 

[blackx-runner]

Usually takes a few trips to get all of the ethanol out unless you're filling up below empty. I like to disconnect my battery and short cables together for 10 minutes just to make sure my Keep Alive Memory and all learned adaptive values are reset. That way your ECM is learning all new values on the 100% gas.

This was more of the point I was getting at. The ECUs do, and can adapt to changes in the fuel. If they couldn't then clearing the ECU memory so they can relearn the adaptive values would be entirely pointless.
Is it worth the money to run a higher octane fuel than necessary? Probably not. Are you getting any performance gains from doing so? Again probably not. Is there any harm in running a higher octane fuel? Probably only the couple bucks extra out of your wallet each tank.

 

[cwatt]

Before I boosted my X I had to run 91 octane or else I could hear the engine knock. Same with my wife's old 03 Altima V6, if I didn't run 91 it would knock. In neither of these engines is 91 octane listed as recommended octane rating

Do the X-Runners not call for 91?

 

[blackx-runner]

Nope, 87. For some reason the FJ is about the only thing in the toyota line that recommends 91? And its the same engine?

 

[Gadget]

All the 1GRs are 87 min octane.

Early on the FJs owner's manual required 91, but there was an amendment made and new pages for the owner's manual were mailed to owners.

87 is fine in all 1GR engines.

G