gas octane question for PB F150

[Samson16]

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soph·ist·ry
noun

1. the use of fallacious arguments, especially with the intention of deceiving.
   "trying to argue that I had benefited in any way from the disaster was pure sophistry"
   • a fallacious argument.
     plural noun: sophistries

*Just in case for those who are reading along...

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[nomarhits400]

I say again, you're misleading people by using sophistry. At the end of the day "compression ratio" is fact. Using the term "effective" in front of it says it all. It is not compression ratio, it is a way of accounting for a reduced mass of air in the cylinder. It also ignores the fact of the otto cycle greater efficiency if it could be run up to the same compression ratio as diesel cycles. The reality is all of the bells and whistles of things like variable cam timing that are there to improve efficiency do not override basic engine design. Compressio ratio is 10.5:1. Gasoline direct injection (curiously absent from much of this discussion) gives much ability to prevent detonation since fuel is not in the cylinder during the compression stroke (detonation only occurs during compression of a fuel air mixture to the point of self ignition prior to the spark). It has become clear that Ford designed this engine for 91 octane at full power but then derated it for 87 octane as a sales ploy since they knew that most operation of the engine would be at partial power and they could control detonation with add-ons. Octane is not a measure of energy content.

 

[UGADawg96]

 

[dolsen]

Good lord some people just don’t know how to admit they’re wrong

 

[amschind]

I say again, you're misleading people by using sophistry. At the end of the day "compression ratio" is fact. Using the term "effective" in front of it says it all. It is not compression ratio, it is a way of accounting for a reduced mass of air in the cylinder. It also ignores the fact of the otto cycle greater efficiency if it could be run up to the same compression ratio as diesel cycles. The reality is all of the bells and whistles of things like variable cam timing that are there to improve efficiency do not override basic engine design. Compressio ratio is 10.5:1. Gasoline direct injection (curiously absent from much of this discussion) gives much ability to prevent detonation since fuel is not in the cylinder during the compression stroke (detonation only occurs during compression of a fuel air mixture to the point of self ignition prior to the spark). It has become clear that Ford designed this engine for 91 octane at full power but then derated it for 87 octane as a sales ploy since they knew that most operation of the engine would be at partial power and they could control detonation with add-ons. Octane is not a measure of energy content.

Let's address this in order;
1) You are correct that mechanical compression ratio is fact. Nobody is disputing that.
2) Effective compression ratio is the only metric that matters. In a naturally aspirated engine, this is equivalent to mechanical compression ratio, which is I think the root of the confusion in this discussion. HOWEVER, once any kind of supercharging, from crankcase supercharging to a roots type charger to a modern turbosupercharger, mechanical compression ratio is NECESSARY BUT INSUFFICIENT to calculate effective compression ratio. It would be a bit like comparing two engines and declaring that one has a larger displacement because the bore size is larger: you cannot calculate displacement without bore AND stroke. Likewise, you cannot calculate effective compression ratio without mechanical compression ratio and manifold pressure (which is STILL an oversimplification, but is much closer than just displacement.

Think about this: the 3.5L with the wastegate wide open and the intake valve open well into the compression stroke has a far lower effective compression ratio than the same engine with the wastegate shut, manifold pressure at 18 PSI and the intake valve closing as the compression stroke begins. The numbers that we fundamentally need are moles of N2/O2/CO2/H2O (cursed EGR) in the cylinder, percent O2 (which is gonna be a bit less than 21% because of EGR), and charge air starting temp, which should allow us to avoid knock when that mass is compressed by the compression stroke and thus heated.

As a different example, look at super and turbosupercharged aviation engines in the 1920-1945 era: the same engine with the same mechanical compression ratio had INFINITELY MANY effective compression ratios based upon ambient pressure/altitude and air temperature. Even a naturally aspirated engine can have infinitely many effective compression ratios given extreme circumstances (typically changes in altitude): take a drive in the mountains on a hot day and a drive by the ocean on a cold day to see for yourself. If effective compression were always the same, we would have no need for mass airflow sensors, just an RPM sensor and a fuel demand table.

3) You're correct that the diesel cycle gains most of its efficiency over the Otto cycle by virtue of higher compression, but compression ignition is also a source of its greater efficiency. This is why HCCI* (homogenous charge compression ignition)remains such a huge topic of research. Spark ignition burns as a premixed flame but expands out as a subsonic flame front from a point source (the spark or sparks); compression ignition begins at multiple points and completes far faster, which leads to more complete combustion and provides additional benefits for a compression ignition engine beyond the compression advantage. HCCI engines typically use a combo of direct and port injection, often with the port injector making a very lean but well mixed intake charge and the in cylinder injector creating a small rich area which ignites first and whose pressure spike ignites the rest of the intake charge. Mazda's SkyactivX also has a backup spark plug.

That detour is actually useful, because your mention of direct injection as a means of preventing detonation is actually inaccurate. In a diesel -cycle diesel fuel burning engine, the injection event determines first time at which there is ANY fuel in the cylinder, and thus the first time at which combustion is possible. Thanks to the high effective compression (either through a >20:1 mechanical compression ratio or an equivalent 15-16:1 mechanical compression ratio+turbosupercharger pressurizing the intake charge), the air is hot enough that the diesel fuel droplets burn instantly. The key difference is that diesel OIL burns in a fundamentally different process from gasoline (see cetane vs octane ratings).

Gasoline vaporizes and mixes with the charge air at a molecular level; this is called a premixed flame, like in a bunsen burner. Diesel burns like a log, albeit many gazillions of very small logs due to the extreme pressures of modern diesel fuel injectors; this is why diesel injectors use very expensive piezo electric injection while even direct injected gasoline engines can get away with far lower injection pressures. Diesel droplets, even though they are VERY finely atomized, burn at the surface of each tiny droplet, where the heat from the charge air and then from the combustion event generates syngas at the droplet surface which THEN diffuses away and combusts. ONLY gases burn, whether gasoline vapor in air, natural gas in air, or syngas generated by combustion on the surface of a log or a miniscule diesel droplet.

All of that is to say that the direct injection in a spark ignition gasoline engine MUST occur before the start of combustion because the fuel must vaporize before it burns. That time delay can be small: very fine droplets in hot air vaporize pretty fast. However, it is NOT the same as a diesel, in that injection and initiation of combustion via the spark are completely distinct events. This is NOT simple, and the ongoing quest for controllable HCCI is limited by the fact that all compression ignition engines thus far (save for the SkyactivX) have worked because the diffusion flame process of diesel oil combustion limits the reaction rate and thus prevents detonation even when the droplets are absolutely tiny. In an HCCI gasoline engine where the fuel and oxidizer are mixed at a molecular level at the start of combustion, you don't have a chemical process providing a brake on the reaction rate, and thus that event has historically been avoided because it's essentially a fuel air munition detonating inside your combustion chamber. The process by which HCCI is INTENDED to work straddles the thin dividing line between "detonation" and "regular old spark ignition"

I do NOT understand the physics of detonation and shockwave propagation, but that's where the state of the art is. If we have any engineers who can, I'd be delighted to hear them explain it. The point that I can make is that direct injection does NOT control combustion in a compression ignition engine in the same way that it does in a spark gasoline engine, and KIND OF MAYBE does in an HCCI gasoline engine.

4) Finally, I fundamentally agree that the engine is derating itself to work with 87 octane, but the statement that "it's designed to work with 91 octane" is unnecessarily limited. If it were a naturally aspirated engine, that would be true. If it were a naturally aspirated engine with variable cam timing to mimic a "mini-Atkinson cycle", that would be mostly true. However, the Ecoboost is a turbosupercharged engine: it's mechanical and effective compression ratios are not equivalent, and aren't even defined. Consider this: why can E85 generate more power in an Ecoboost tuned for it? E85 has a highly variable Octane rating, but let's say 112. In a naturally aspirated engine tuned for 91, the higher octane of the E85 would get you nothing, and you would actually lose power due to the ~30% lower volumetric energy density of ethanol. The Ecoboost is NOT naturally aspirated. In our case, by increasing boost, you can increase the number of moles of gas and O2 in the cylinder while the higher octane rating of the E85 prevents that from detonating during compression. Just as you said above, the higher compression ratio (the EFFECTIVE compression is higher, despite the same bore and stroke, because the manifold pressure is higher due to more effort by the turbosupercharger) is able to generate increased efficiency and power. That's why a lot of the really crazy Ecoboost tunes turn to E85: it's an easy way to get high octane without resorting to race gas, and as long as you can deliver enough fuel to overcome the lower volumetric energy density vs gasoline, you generate more power.

Anyway, that's far too long, but I hope that it helps and maybe even sparks some discussion about HCCI and its bizarre relationship with detonation.

*That got longer than I wanted, but as a brief key:
SCCI=Stratified charge, compression ignition, like a modern diesel
SCSI= stratified charge, spark ignition, like a gen 1 Ecoboost
HCSI= homogenous charge, spark ignition, like any port injected or carburated gasoline engine
(Gen 2 Ecoboost is kind in between SCSI and HCSI because of the dual injector setup)
HCCI=homogenous charge compression ignition, which uses an injector setup like a Gen 2 ecoboost but ignites the fuel with compression; most production engines have spark ignition backup

 

[Snakebitten]

A really nice disortation.
(I'll read it slower the second time)

But do you really think it'll sway even a nanometer?

 

[dolsen]

A really nice disortation.
(I'll read it slower the second time)

But do you really think it'll sway even a nanometer?

Some people refuse to learn because they’re so confident they know better than everyone else, even in the face of irrefutable data and information

 

[Snakebitten]

Some people refuse to learn because they’re so confident they know better than everyone else, even in the face of irrefutable data and information

I know YOU won't mind if I disagree.

I don't think it really has to do with what is believed to be right or wrong.

 

[dolsen]

I know YOU won't mind if I disagree.

I don't think it really has to do with what is believed to be right or wrong.

How dare you disagree. I should block you for that

I enjoy civil disagreement, otherwise how would we learn new perspectives and ideas. Being stuck in an echo chamber is a recipe for a boring life without any true growth

 

[nrmedic]

The local Sheetz had a sale on E15 88 this week. Instant loss of about 3mpg. But for the $1.99/gal vs $3.89/gal for E10 93, I cannot complain.

 

[EricR]

The local Sheetz had a sale on E15 88 this week. Instant loss of about 3mpg. But for the $1.99/gal vs $3.89/gal for E10 93, I cannot complain.

It's nice these engines are designed to give us options: 87, 89, 91 or 93 (where available).

 

[HammaMan]

Lol looks like ol what’s his face is still at it. He clearly doesn’t understand what he’s saying, he did me a favor by blocking me so I cannot see his posts

I was first
I'm not missing anything. I view the forum in other ways and just ignore him.

I run 'toptier' fuels and have had no noticeable difference between 87 and 93 tanks of fuel outside of paying more for them.

Hopefully with the coming food crisis we'll see less food turned into mileage consuming fuels that increase their CO2 burden by another 25%. Corn into gas is but a farmer subsidy.

 

[Suns_PSD]

A bit of an aside but I did call Livernois to ask them about running my 91-93 street tune on 91 octane ethanol free fuel as opposed to the 93 octane E-10 I run now.

They said that 91 octane is within the adjustment range of the modified ECM. However, I will definitely lose power and the engine will in fact retard ignition timing. He said it's worth about 8-10 rwhp going from 93 to 91.

I asked him about reported mpg gains or losses and he said: 'lots of customers have tested this, with no change in economy, just less power and more money spent'.

It's possible that the increased energy in the ethanol free fuel, is offset by the concurrent loss in timing, or close enough to certainly not cover the 20% increase in fuel cost.

That's good enough for me, I'll stick with the inexpensive 93 E-10 from Costco.

 

[stonefly]

I use 91 octane typically "up to 10% ethanol" towing or not. I think it makes a difference in towing over 87 for sure.
I wish I could get 93. 87, 89, 91 available in San Diego.

 

[dolsen]

I use 91 octane typically "up to 10% ethanol" towing or not. I think it makes a difference in towing over 87 for sure.
I wish I could get 93. 87, 89, 91 available in San Diego.

I don’t think 93 is available in CA

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