Atkinson motor and fuel octane

[Rkbrumbelow]

Yikes! I haven’t been so overwhelmed since high school algebra!

Don't fret, it is part humour, and all factual. One of my degrees is in Chemistry, and I used to teach chem Labs.

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[SilverWind]

="Octane rating, on its own, does not produce more power. It simply allows higher compression. So an engine that does not take advantage of this through high compression ratio or forced induction (turbo/supercharger) won't make more power. Ethanol, however, has less power density than gasoline. So regular octane fuel without ethanol will produce the highest efficiency. Availability of that will depend on where you are. Often only Premium fuel is available ethanol-free and if the cost difference is greater than the increase in efficiency it's not worth spending more.

Holy frigging heck finally somebody else that understands octane does NOT equal horsepower. It simply prevents knocking in engines with a higher compression ratio. If an engine is rated for 87 octane than 87 octane is perfectly fine for it. The only time you'd have to adjust is if you notice a knock or other issues then it's time to increase your octane.

Also word to the wise I'd say away from the mid-grade fuel (don't recall the Oct rating) because not many people use it and it can be some old gas sometimes.

 

[Probity]

Also word to the wise I'd say away from the mid-grade fuel (don't recall the Oct rating) because not many people use it and it can be some old gas sometimes.

I thought mid-grade gas was blended at the pump from the regular and premium storage tanks?
What Is Midgrade Gas? | The Daily Drive | Consumer Guide® The Daily Drive | Consumer Guide®

 

[Old Ranchero]

Yes it will. Fuel loses its flash point over time and leads to absorption of impurities I.e moisture.

I read an article on a motorcycle forum some time ago that today's gasoline blends start to breakdown after about 2 weeks. They advised using fresh gas after that window for best performance. Being retired and living in a small town I only average about 10 miles per day (some days 0) and to ensure "fresh" fuel tend to only completely fill our vehicles before a long trip or gas prices spiking upwards.

 

[vap0rtranz]

Don't fret, it is part humour, and all factual. One of my degrees is in Chemistry, and I used to teach chem Labs.

I've been schooled again!

Now that we got the chemistry out of the way, is the general consensus that for the Hybrid ...?

• normal driving - don't waste $ on high octane
• towing - don't waste $ on high octane
• high altitude driving - don't waste $ on high octane
• aggressive "sport" driving - don't waste $ on high octane

 

[Probity]

I've been schooled again!

Now that we got the chemistry out of the way, is the general consensus that for the Hybrid ...?

• normal driving - don't waste $ on high octane
• towing - don't waste $ on high octane
• high altitude driving - don't waste $ on high octane
• aggressive "sport" driving - don't waste $ on high octane

Generally agree, but I'm pretty thickheaded, explain to me again why towing with Atkinson cycle wouldn't benefit from higher octane?

 

[vap0rtranz]

I cannot

Generally agree, but I'm pretty thickheaded, explain to me again why towing with Atkinson cycle wouldn't benefit from higher octane?

I cannot explain it but I will tell you why I'm putting 91/93 in while towing: ethanol. I need all the energy I can get while towing and evidently ethanol has less energy than pure gas. For whatever reason 91/93/Premium is the only grade I see at pumps that's not dilluted with ethanol. If a gas station actually had 85/87 grade with 0% ethanol, I'd pump with that instead of premium. So my choice has nothing to do with octanes, compression, knock but because I want pure gas while towing.

 

[Probity]

I'm learning about NA Atkinson cycle engines like a lot of us, lots of news you can use here.

My paradigm for towing is not really getting more power from higher octane, but maybe helping you keep the power you've normally got if knock conditions occur (more eloquently stated by someone else I found on a google search):

If your engine experiences pre-ignition while towing, it will definitely have more power from higher octane fuel than if the ignition is pulling timing. While the higher octane fuel is not giving it more power directly, if the engine needs it, the higher octane fuel will provide the means for which the engine can maintain the power it should normally have. Pre-ignition can occur due to greater stress on the engine, towing is a prime example of greater stress, especially when doing so up an incline.

So for the Maverick 2.5l is that not true?

 

[JASmith]

TLDR: intended fuel is 87 and the motor would have been tuned using that. You shouldn't see a performance or efficiency increase using a higher octane since all maps are tuned to take away timing.

Not always, the maps are often quite complex now, and the engine really has no clue what octane fuel it is being fed, and the same octane at sea level on a dry day that's 120oF outside would have different knock resistance compared to high altitude icy humid air. Heck, even the same engine can benefit from higher octane just based on age, as carbon buildups can reduce knock resistance which higher octane fuel can compensate for.

So sometimes the maps can have a huge range of timing and a/f ratios and what not programmed in, and rely on the knock sensor that are extremely sensitive to "learn" what is appropriate for the particular conditions that day and the fuel. The only way to know for sure would be to throw in some 100 octane and run it on a dyno, but chances are its not worth the effort and just stick with whatever the user manual indicates is the "minimum recommended" fuel, which in the case of the 2.5H is 87. Higher octane can't hurt anything, but probably is a waste of money.

 

[JASmith]

regular gasoline on both the 2.0 and 2.5 hybrid. for the escape these are the EPA stats with the type if gasoline.

Nope, despite what the EPA test shows, while the 2.0 ecoboost (like all engines) can run on 87 octane, it won't get the advertised power numbers, and will be pulling some timing and richening the fuel mixture at high loads. They don't have to worry about that on the EPA test cycle, because its not a high load test.

Ecoboost 2.0 user manual recommends 91+ octane, if you run less, you will have less peak performance under most conditions.

 

[JASmith]

I cannot explain it but I will tell you why I'm putting 91/93 in while towing: ethanol. I need all the energy I can get while towing and evidently ethanol has less energy than pure gas. For whatever reason 91/93/Premium is the only grade I see at pumps that's not dilluted with ethanol. If a gas station actually had 85/87 grade with 0% ethanol, I'd pump with that instead of premium. So my choice has nothing to do with octanes, compression, knock but because I want pure gas while towing.

I'm in the oilfield industry, and whether or not a fuel has ethanol or not really isn't a matter of octane, and in fact there is no 89 octane gasoline distributed, its merely blending 87 octane and 91 or 93 octane at the pump in the appropriate ratio.

In most jurisdictions, 10% ethanol is on all grades of gasoline because it is required by law as an oxygenate, to replace MTBE that has been banned due to groundwater contamination concerns (well and political reasons to prop up the ethanol/corn industry).

The ethanol content should always be labeled at the pump though, so it shouldn't be a surprise. If the pump itself and not individual octanes have a 10% ethanol notice on it, then all octanes buttons on that pump will have that.

Also note that a vehicle on ethanol will make just as much horsepower as one on gasoline, its merely an issue of energy density. So while the energy density is lower, power is unaffected as the system can adjust how much fuel is delivered. Fuel economy would just decrease, because the energy density per gallon has changed, but with 10% its such an insignificant difference that no one would be able to tell by seat of the pants anyway.

 

[SgtLip]

I don't know anything about all this science stuff spilled out in this thread. I can ONLY speak from experience I've had with my Toyota/Lexus Hybrid's.

I was using Regular 87 octane in my Toyota Avalon after purchasing the car new back in October 2019. The car is rated at 40-44 MPG. I was getting steady 38 MPG. After around 5 months, the friend, who convinced me to buy the car because he had the same one, said to use Ethanol Free as that was what he was using and was getting around 45-46 MPG. What I found is that a few stations have 93 octane Ethanol Free but MOST are running 89 octane Ethanol Free. And believe me, it does cost more.

I started using the Ethanol Free in the Avalon and found after two tanks of fuel, my MPG shifted to 43-45 MPG and stayed there until I traded it for the Lexus ES300h in May of this year.

In the Lexus, which is basically the same car as the Avalon, the dealer had filled the tank with Regular 87 octane. We let it run down to near empty and filled it with the Ethanol Free 89 octane. Again, I have been getting 43-45 MPG as you can see from the screen shot below from my Fuelly app. Notice for 11 fuel-ups $291.35 / 88 gal = average $3.31 per gallon.

I'll continue to run Ethanol Free 89 octane in the Maverick when I get it and while I can afford it. But since 20 January 2021 it's getting tougher to afford. The only time I will put Regular 87 octane in is when I can't get Ethanol Free when traveling. Luckily I live near the Gulf of Mexico and there is a BIG boating community. So a lot of the stations here offer Regular and Ethanol Free. It's when I travel it becomes harder to find, but there is an app for finding it too.

 

[vap0rtranz]

Also note that a vehicle on ethanol will make just as much horsepower as one on gasoline, its merely an issue of energy density.

I want to believe this. The problem is tests keep coming out that say otherwise. Like Car & Driver sticking a dyno on and testing each grade's horsepower:

https://www.caranddriver.com/features/a28565486/honda-cr-v-vs-bmw-m5-ford-f-150-dodge-charger/

Now, Car & Driver's testers don't even mention ethanol but if that's not the reason, and octane grades do nothing more than avoid knock, then where's the extra horsepower coming from? Or is Car & Driver being paid by the Big10 O&G companies?

 

[DesertSweat]

Not always, the maps are often quite complex now, and the engine really has no clue what octane fuel it is being fed, and the same octane at sea level on a humid day that's 120oF outside would have different knock resistance compared to high altitude icy dry air. Heck, even the same engine can benefit from higher octane just based on age, as carbon buildups can reduce knock resistance which higher octane fuel can compensate for.

So sometimes the maps can have a huge range of timing and a/f ratios and what not programmed in, and rely on the knock sensor that are extremely sensitive to "learn" what is appropriate for the particular conditions that day and the fuel. The only way to know for sure would be to throw in some 100 octane and run it on a dyno, but chances are its not worth the effort and just stick with whatever the user manual indicates is the "minimum recommended" fuel, which in the case of the 2.5H is 87. Higher octane can't hurt anything, but probably is a waste of money.

I never said an engine would know what octane is being used. I said engineers use a designated fuel as a starting point for building timing maps. For the 2.0t we would expect premium, and for the hybrid, I would suspect and expect regular.

And yes if you have extreme conditions where timing is being pulled because of the knock sensor, then a higher knock threshold fuel will show benefit.

My only point was in most conditions there is no point to using a higher grade because the timing map is created from what would be regular and ideal conditions. In my experience, knock sensors only detract and don't add. But there could be manufacturers that use some type of close loop knock sensor tuning to try and get better efficiency, but you can start running into issues quickly with engine reliability.

 

[JASmith]

I never said an engine would know what octane is being used. I said engineers use a designated fuel as a starting point for building timing maps. For the 2.0t we would expect premium, and for the hybrid, I would suspect and expect regular.

And yes if you have extreme conditions where timing is being pulled because of the knock sensor, then a higher knock threshold fuel will show benefit.

My only point was in most conditions there is no point to using a higher grade because the timing map is created from what would be regular and ideal conditions. In my experience, knock sensors only detract and never add.

I didn't claim you did say that, but was explaining that the mapping is much more dynamic now thanks to advances in sensors including knock sensors which went from a mere protection device to stop severe damage to a means of calculating its by-the-second operation since they become so very sensitive. The engineers can thus put in maps at extremes, if they choose, that are far too aggressive for average conditions, and use the knock sensor to learn and adjust to more conservative settings as dictated by the specific conditions. Octane aside for example, this can allow them to run mappings that are very lean, leaner than you could expect to run in normal conditions, and just richen as needed. In that way, even a vehicle for which they recommend 87 octane may see a benefit to higher octane.

In any case, as mentioned its overthinking it, as its unlikely to make much of a difference on the Maverick 2.5H one way or another. On a Santa Cruz 2.5T on the other hand that does have 87octane+ as its minimum recommended, it would be interesting to experiment with 93 octane to see if there are aggressive enough maps to take advantage and give you a 15hp or so bump.

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