I just returned from a vacation to Williamsburg Virginia from my home in Vermont in my 2006 XJ8L. On this trip I filled up with 93 octane fuel during the fill ups as the places I stopped offered it. The first 300 miles average fuel economy was 27.8 mpg with 91 octane fuel with no ethanol that I have been using locally for the last year or so. On the rest of the trip the fuel economy improved to 30.8 mpg and that was with several traffic delays. The trunk was stuffed with luggage and we had 4 adults in the car. The last trip I took was in October last year and I filled up exclusively with 91 octane fuel. That trip average was 28.6 mpg.
My curiosity is getting the best of me. I am wondering if the octane of the fuel is the factor affecting the economy of the car or is there something else that is improving the fuel economy? Any input would be appreciated so I can better understand the factors that affect the fuel economy of this car.
Thanks,
Jeff
I think you have answered your own question:) if all things stayed the same apart from the fuel used, I.e. Terrain, passengers, speed then the only variable was the fuel type. NZ uses a different rating for fuels we get 91, 95 or 98 all unleaded.
One company supplies a 10% ethanol fuel but a friend with a Subaru WRX Sti used it once and found his fuel economy tanked and he had to fill earlier than he normally would on a regular journey.
Unless the mileage figures were taken on the same route and leg of the journey, it’s hard to compare. I don’y myself see how higher octane fuel helps - it doesn’t have more energy. It just resists detonation more. If the engine is programmed to run leaner with higher octane fuel, this can increase economy. But this doesn’t happen with modern cars because emissions controls don’t permit this (excessive pollutants if the fuel mixture is too lean.)
Assuming these figures weren’t measured on the same leg of the journey, repeated, the best bet is to do your daily driving for a while on your usual routes with one fuel grade, then switch to the other for another while. I am currently on a long (4,000 mile+) road trip with an S-type R, and as I head east my fuel mileage is increasing bit by bit (from 22.7 to 24.9 now.) We’ll so how it goes on the reciprocal leg, as my perception is it’s been downhill so far. 
I was thinking that maybe the ECU learned to be more efficient when taking a trip and made adjustments that improved the economy figures as we drove. The other factor that I was wondering about is the rumor I heard that the fuel companies change the formulation of the gas in the winter which may have been the reason for the lower economy figures in October.
It is next to impossible to fill up and do my normal driving to test the impact of the octane on the economy as this car is used primarily on trips and does not get used as a back and forth to work vehicle, that’s what my truck is for.
We will be taking this trip again next year and I can see if there is a difference then.
It might not have more energy, but it is more knock resistant. The X300 engine should have a knock sensor. So then the engine will run more ignition advance because it can without knocking, thus there will be slightly more power (So far, that is true) and more power at any rpm should mean that less gas pedal input will be required, especially at cruise… and that would mean some very slight savings in fuel consumption even though there is about the same amount of energy in the fuel - does that make sense?
Apart from that, different temperatures, wind and so on will also make a lot of difference, probability way more than fuel octane.
David
IIRC the X308 does have an “Adaptive Capability” where it learns and adapts to the drivers style and journey types. It may be that it plus the higher octane have given your improvement.
I’d put my bet on a changeover of fuel formulation if that took place. I have myself noticed significant (1-2 mpg) improvements in the past between winter and spring when our region used to switch from oxygenated fuel in winter to non-oxygenated in the spring, and in the Seattle Pacific Northwest area the temperature isn’t that much different.
I tend to discount the adaptive aspect of the equation because I’d be skeptical that the emissions programming will allow the engine to run leaner in part throttle, cruising conditions. It would help under load and acceleration.
Well, different octane ratings imply different formulations. And more acceleration means more power, means less gas pedal inputs to hold speed so not leaner, but also less air, so more efficient…
David
Very true, but not quite that simple David.
Less throttle opening means high vacuum, AKA greater pumping losses/lower ‘efficiency’. This is why warm intake air is better for fuel economy (less dense/lower pumping losses) and cold air better for power.
It also explains much of why taller gearing saves fuel.
If a car requires, say, 50hp to maintain a 50 mph steady cruise, a lower-geared car will use a modest throttle opening, as the engine will be running higher revs at that speed so it is throttled back to only deliver the torque/hp needed to overcome air and rolling resistances.
The same car doing the same speed on higher gearing will need the same 50hp but to make that at lower revs means opening the throttle wider to burn more fuel/per bang because it is harder work to generate the same power at lower revs. The wider throttle means lower vacuum/smaller pumping losses/greater ‘efficiency’ for the high geared version.
Fair point, but still not that simple!
His gearing was the same on both trips, that means total revolutions were the same as well.
Our variable here is that the amount of air pumped and correlating amount of fuel injected differed.
For the same speed!
Of course less throttle means less efficiency overall, but more octane provides more advance and thus more efficiency (Torque!). (I say that, it’s just a guess, but I still think it makes sense).
So if you need less pedal input to hold cruise, you get in less air and use less fuel; the distance traveled is the same!
In reality, a car needs about 15hp to hold a 60mph cruise. Pumping losses reduce torque, lesser fuel reduces torque as well - if required throttle is less, then torque gains outweigh pumping losses. The point I wanted to make is that the car won’t have to lean out in order to reduce consumption - YOU remove torque because otherwise there would be more than needed at the given fuel rate, all other factors staying the same… You still need 15hp, but you now have 300hp total instead of 280 total and need less of the total engine capacity.
I think we can both raise our glasses to multifactorial analyses. Only politicians have the luxury of seeing everything in black and white, needing only their personal touch to be solved. Of course, for some of them, the exact placement of said magic touch can depend largely on the gender and eyelash length of the intended beneficiary…
Just curious, how many petrol engines are there currently that have no throttles and thus no pumping loss, besides BMW? Has this caught on?
Robert, last week I was at Mercedes where they assemble M274 engines. 2 litres and up to 250ish hp! That engine uses Schichtladung, which, according to Wikipedia, allows for an ignitable mixture around the spark plug but the rest of the chamber can be at up to lambda 3.
So, very very lean and still ignitable. Apparently zero throttle losses (Well, almost). But they utilize that, also turbos, direct injection, two switchable cam lift profiles (crude!) and very high compression. They have a throttle but I doubt it does much.
Porsche, Honda, VW are said to be using this technology as well. https://de.m.wikipedia.org/wiki/Schichtladung sadly not in English.
Bmw use inlet-and exhaust timing, don’t they?
Fiat, Porsche, Toyota are amongst those who offer variable valve timing and variable lift just as BMW do.
Peter, yes indeed. Which explains a lot sadly, even though I don’t live in the US or DPRK for that matter. We Germans learned to be more diplomatic, so everything is thoroughly multi-factorially analyzed and grey!
Regards,
David
Here’s a suggestion: have you tried these cute, little bluetooth OBD pluggins or the related phone aps? I ask because first they are terrifically fun and can be sourced extremely cheaply. And there are several free aps for android phones. The ones for Apple phones cost a bit I think.
Anyway, with the little bluetooth dongle plugged into your OBD port and a related phone application, you could track and record your speed, RPM’s, etc. Some of the aps even have a track mode (as in “for the race track”). They certainly aren’t sufficient to effectively dial in a real drag car, but they’re certainly good enough to check to see how your daily driver is performing.
So, run the same route with both fuels and record the data. If David is correct, then your RPM’s should drop slightly at the same speed on the same route with the higher octane fuel. I know lower RPM’s doesn’t automatically mean higher MPG (considering fuel trims and all) but if you see a corresponding increase in MPG then it would suggest a cause and effect. By the way, the aps can monitor your fuel trim as well if you’re sophisticated enough to put that data to use.
Try it, have fun with it, report back.
Wait, wait, Jeremy.
I never meant that RPM would drop. I merely suggested that load as a percentage of total engine capability would drop slightly because the timing gets advanced and thus more torque, so less throttle input for same power, same RPM, same speed!
If these OBD dongles have access to ignition timing or throttle position, one might be able to see something. Oh, and maybe the Lambda can be made slightly leaner because the fuel burns slower, but I don’t know whether that’s the case at all.
An Android devices for such purposes would be brilliant, also, there are very cheap endoscope cameras that work via the USB port, and after all it’s not a big deal if one of those phones gets used because they’re not too expensive either…
David
I guess I should have just said, “get a dongle and then chart everything. More and specific data will help you answer the question.”
For what it’s worth, these little dongles are great and everyone should have one. What stats it reports is based primarily on the car, not the dongle. If your OBD system measures and reports it, then the dongle can pick it up.
I bought mine years ago on Ebay for $4. No kidding, $4! You can get them on Amazon now. There are lots of them. Mine is small and clear, blue plastic. Self powered from the OBD port. It came from China and took forever to get here. And lots of aps have warnings that say that the cheap Chinese knock-offs might not work. But mine works great. I even bought one for my elderly father-in-law so that he could clear the Check Engine light on his old truck.
The free Android ap I use is called Torque Lite, but there are others. At first blush, Torque looks like an ap for making a remote dash board. You can organize all kinds of gauges and skins if you’d like. (That reminds me, I also bought one for my brother when the electronic dash on his old Volvo went out. Rather than spend the money to replace, he just uses an old smart phone and dongle as a custom dash.) It also has a “track” function for recording and graphing data. You can even email logged data back to your computer.
I use it mostly as a code reader. I’ve used it so much and for so long that the batteries in my old code reader went dead and corroded. But I’m always looking for other ways to use it, and this sounds like a great one.
Sorry but rpm/speed is a constant governed by the gear ratio, octane will not affect this at all.
Well, it’s not exactly constant. Torque converters and all. But point taken. And again, I should have just said, “Get a dongle and then chart everything. More and specific data will help you answer the question.”
But fuel trim, throttle position, timing advance, etc. over the same trip with one fuel vs. the other certainly would provide some insight. And those are all measured by even the most basic OBD system. So, for $4 and a free ap, it would at least be interesting, if not informative, to log, graph, and then compare the data over the same trip with both fuels. And with the tech available these days, it would be cheap and easy.
At least you’d be able to see if there is a noticeable change in fuel trim or timing advance per a given engine load or RPM. I know that David is correct that the computer will change either or both in response to octane IF NECESSARY. I think the “if necessary” part though is determined primarily by inputs from the knock sensor, and I have no idea what algorithm that decision is based on. But I think there’s a pretty good chance that IF octane is affecting MPG at light highway cruising speeds, then it should show up on the graphs I mentioned.
I would take 91 octane with no ethanol before I took 94 with ethanol.
The 10% ethanol added means LESS power/mileage.
Very noticeable when I’m in the Midwest and that blend is more prevalent.
Noticed the amount saved on purchase of E15 is made up by the loss of mileage.
My findings are that it’s a wash savings wise.
Add 10% Ethanol and you lose about 4% of energy contained in the fuel, so to me it is mostly a question of price difference. In Germany the standard fuel is E5 (up to) or E10, and E10 is usually cheaper by 2-3 cents/liter; thats less than 2%. If it’s more, whether it makes sense or not, i’ll go for E10…
With the BMW. The XJ6 still knocks on that.