Okay to run gas empty?

[Ffe]

I want to conduct an experiment to quantify the affect of different octane levels of gas on my Fusion's gas mpg.

 

I know it's safe to run the Energi in EV Now mode until the battery runs out and it will safely and automatically start up the gas engine when the electric goes empty.

 

IS IT OKAY to have a full battery, drive in EV Later mode until I run completely out of gas, and let electric take over?  (Assuming I know I will be within EV range of a gas station of course.)

 

I was thinking this would be the most guaranteed way to know that the tank was as empty as possible before switching to the higher octane.

 

I realise I could also let it get to the 0 mark and give it one gallon at a time for a few days as well but that would take much longer and I'd never know how thorough it was because of the reserve area in the tank.

[jj2me]

It's never a good idea to run the tank down, for at least these two reasons:

1) bottom crud being sucked into fuel filter, requiring replacement,

2) the in-tank fuel pump is cooled by the gasoline in the tank.  Without any such cooling, you risk ruining your fuel pump.

 

[theterminator93]

Most cars are programmed for 87 octane fuel; they will run a spark advance that is programmed to be safe for this octane. Higher octane increases tolerance to spark knock, and allows more spark advance and thereby more power and fuel mileage. However the spark advance is not changed when higher octane fuel is run. There is no performance or economy benefit. The only reason to run a higher octane than what a particular vehicle is programmed for is as a temporary fix for a mechanical fault causing the engine to knock with the base octane (e.g. a dirty MAF sensor - which causes both too much spark and a lean condition - perfect conditions for pre-ignition).

 

Performance-minded engines that are programmed from the factory to run on higher octane fuels (e.g. 91 or 93) have the extra spark advance programmed to take advantage of the higher octane fuels. There are also electronics/sensors to detect when a lower octane fuel is used (such as a knock sensor). When engine knock is detected the computer automatically retards the timing to compensate.

 

No such hardware exists on these vehicles because they aren't programmed for higher-octane fuel in the first place.

 

If you need to make precise fuel measurements, measure the weight of the fuel going into the tank to achieve a top-off condition. As was said, running a fuel pump in a dry (or nearly dry) tank just causes it to overheat and, done chronically, fail prematurely.

Edited June 29, 2021 at 02:39 AM by theterminator93

[Ffe]

19 minutes ago, theterminator93 said:

No such hardware exists on these vehicles because they aren't programmed for higher-octane fuel in the first place.

 

You might want to tell Ford about that, because they wrote in the owner manual that it can take 87 to 91, and 91 is recommended because it will result in better mileage.  What I want to find out is, is that mileage at 91 better enough to justify the higher price of fuel.  Because otherwise I'm a cheapskate like most and would gladly buy the cheap gas forever.  Not that I use much anyway but....

[theterminator93]

Interesting. I looked in my manual and this is what it says:

 

Quote

For best overall vehicle and engine performance, premium fuel with an octane rating of 91 or higher is recommended. The performance gained by using premium fuel is most noticeable in hot weather.

 

However, going from 87 to 93 octane usually results in a minor, barely modest gain in power and economy, maybe on the order of 2-5% depending on application. Considering 91/93 fuels are 30% or so more costly than 87, it's definitely not worth the cost of the improvement.

 

I first tuned my T-bird to run on 93 originally when I built the engine. I detuned it to run 87 a few years ago because the fuel premium was making it hard to stomach; the loss in economy has been negligible.

 

Ford also recommends 91 or higher for my Lincoln. I used to run 93 in it back when the premium was only 5-10% more than regular. Same as with the T-bird, I started using 87 to see what would happen to the mileage when the price difference got crazy. I lost about 1 MPG on average, which in a car that gets mid-high 20s on the highway justified the savings at the pump for sure!

Edited June 29, 2021 at 04:34 AM by theterminator93

[jsamp]

On 6/28/2021 at 9:30 PM, theterminator93 said:

Considering 91/93 fuels are 30% or so more costly than 87, it's definitely not worth the cost of the improvement.

 

30%?????  at $2.50/gal that would be $.75 difference.  I don't know anywhere that the difference is that big.  In expensive CA where I am, the difference is $3.89/87 vs $4.29/91.  that is $.40 difference which is a little over 10% more.

 

Still probably not worth it for the minor improvement in MPG.

[theterminator93]

That's how it is here near Cleveland.

 

When gas was upwards of $4/gal back in 2008, the difference was about 10-15 cents between grades, so it was about a 10% increase in price to go from 87 to 93. In the last 5-8 years stations have skyrocketed from 10-15 cents between grades to between 20 and 40 cents (or even more). So with gas being about $3/gal now, it might be as much as $4 or $4.50 for premium! It's nuts.

[muzicman61]

My experience was it was not worth it.  In a quick Google search I found this:

 

In a consumer notice, the Federal Trade Commission, notes: “In most cases, using a higher-octane gasoline than your owner's manual recommends offers absolutely no benefit. It won't make your car perform better, go faster, get better mileage or run cleaner.”

 

Raising the octane rating (also known as the anti-knock index) doesn't change the energy content of a gallon of gasoline. A higher octane rating indicates greater resistance to knock, the early combustion of the fuel-air mixture that causes cylinder pressure to spike.

 

[Ffe]

Our manual recommends the higher octane though, does it not?

Here's a citation from the actual manual when I searched for octane:

It does show a graphic of 87 octane also, saying a minimum of 87 is recommended. 

 

I'm not saying the manual is right.  I'm interested in testing it.  But the FTC probably knows nothing about the ford fusion atkinson and is making a general statement.  Even their statement though, combined with the actual manual, does not indicate we shouldn't be using 91/prem in the 18 Fusion Energi at least.  I don't know if other years used a different ICE maybe.  And I might second guess myself anyway because the weather will be different when I test 91 on the same route.

 

Perhaps to get the most pronounced difference if there is any I should look for 99 octane somewhere for the experiment.

[theterminator93]

The internal combustion engine compresses the fuel/air mixture to a certain degree during the compression stroke. The compression generates heat inside the cylinders - even before the spark ignites the air:fuel mixture. Modern engines use higher compression ratios that used to be impossible with older combustion chamber and fuel delivery designs. Direct injection has really been a game changer with 87 octane being used on 12:1 and even higher static compression ratios on engines today. This is because the higher fuel pressure helps better atomize the fuel and mix it with the air charge as both are brought into the cylinders, creating a more homogeneous air:fuel mixture throughout the combustion chambers. Since leaner A:F is more likely to combust at lower temperatures than richer A:F, this better air/fuel mixing allows for greater predictability and an overall higher temperature (more compression) in the combustion chamber before detonation occurs. Our engines have a 12.3:1 static compression ratio and run on 87, for example.

 

When you run high compression engines, there's a balance between the spark advance, the octane used, the overall efficiency (BSFC - how much fuel required to create 1 hp for 1 hour) and the power output. In general, the higher the compression ratio, the more sensitive the engine is to spark knock (pre-ignition - or fuel mixture combustion as a result of heat buildup during the compression stroke - before the spark plug ignites the mixture). To combat that, higher octane fuels are used as they require a higher flash point temperature and are therefore more resistant to spark knock. Adding half a dozen points of octane can allow you to run 8-10 degrees of additional spark advance in these scenarios. More spark advance means more power production, and, theoretically, better fuel economy.

 

The power and economy benefit therefore is not a direct result of the fuel octane rating, but rather the fuel allowing more spark advance. This is realized only when the vehicle PCM is capable of adding the additional spark to take advantage of this more spark tolerant fuel. Normally this is done via a knock senor. Spark knock (pre-ignition/detonation) has a very different sound than normal combustion. Knock sensors on modern engines "listen" to the sounds of combustion and retard the timing so that knock is avoided. These knock sensors are extremely sensitive to the frequencies and waveform characteristics of knock and can detect the earliest onset of it, well before any human's hearing could. So, Ford programs the "base" spark for a level safe when using 91 octane (since 93 is not available in all areas, but 91 is much more widely available). The knock sensor then allows the PCM to pull timing in case a lower octane is used. Without the knock sensor, running an octane lower than what's been programmed would cause detonation due to the advanced spark. Therefore, using an octane over 91 is not going to yield any benefits, because the car isn't set up to add timing beyond its 91 programming - even when no knock is detected.

 

I've never tuned the engine in these cars so I don't know what the spark tables look like, how spark tolerant they are, or how much the spark advance gets retarded via the knock sensor as a result of the use of 87 vs. 91/93 type fuels. As a rule of thumb 1.5 degrees of spark for every point of octane is what's used in the tuning community, so it's likely not insignificant for an octane boost of 4 points. However, the real world performance and economy gains with ~6 degrees of added spark advance will likely be less than impressive.

 

Should you go through with testing, it's important as many conditions as possible be kept controlled. Remember - barometric pressure, elevation changes, wind speed/direction, ambient temperature and humidity all have an impact on fuel efficiency and power production. Denser, drier, colder air both produces more power but also requires more energy to move the vehicle through. Tail/head winds also have a profound impact on economy - having a tail wind of 15 MPH while driving 45 MPH means you have less than 50% of the wind resistance to overcome than having a headwind of 15 MPH. So you should do your testing for 87 with the same temperature, humidity, wind speed, and course as your testing with 91.

 

1) Measure 87 octane economy in the evening or at night on a cloudy day. Wind speeds tend to be calmer at night, and atmospheric temperature/humidity variations take longer to occur rapid at night - With an overcast cloud cover, this is even better since clouds help blanket the radiant loss of heat at night.

2) Measure the fuel economy at the same speeds, on a closed loop course with the same driving style, acceleration rates, speeds, number/location of stops etc. for both octane types.

 

My recommendation is to do the first course run with 1/3-1/4 (3-4 gallons) in the tank of 87 - make your loop starting from and ending at the same gas station until you have ~1 gallon remaining. Then fill back up using 3 gallons of 93. With ~1 gallon of 87 and 3 gallons of 93, that would leave you with ~91 octane in the tank. Then repeat the same course immediately, considering criteria 1 and 2 above. Try to start and end the trip with the same HVB charge level. Obviously do all the testing in hybrid mode.

 

Ideally you would measure fuel consumption based on fuel required to fill the tank from a previous full condition, but I don't think you want to be driving around for hours without end for this experiment. Using the onboard computer's fuel economy calculations should be accurate enough to yield a result for discussion. Reset the trip odometer at the start of your 87 run, then at the start of your 91 run.

Edited July 24, 2021 at 04:42 AM by theterminator93

[testriderchuck]

I may be a little late to this party, but, if these fords are anything like the '93 Chrysler T&C I had years ago, the fuel return line went into a cup that surrounded the fuel pump, keeping it in a constantly-replenished quart or so of fuel, which overflowed into the rest of the tank. THEREFORE, on that particular vehicle, the engine would starve for fuel before the fuel pump wold lose it's coolant (fuel). I haven't changed any more recent fuel pumps so this may not be the case here, take a look at a replacement pump to be certain.