The Octane Misconception

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#1639367 23-Sep-2016 12:53

ajobbins:

Unfortunately for motorists, it’s often unclear which ‘premium’ fuel (95 or 98 octane) is sold at which service station, as retailers are not required to display the price on the price board – only at the pump. On occasion, that means some motorists may unintentionally spend an extra eight cents per litre buying a higher octane fuel than the one they actually need.

That makes no sense. Why would the fact only the 91 price being displayed on the board someone make them fill their car up with a higher grade fuel than they need? I have never, ever seen a pump in New Zealand or Australia that doesn't have the octane grade displayed at the pump, along with the price. Sure, some may only have one either of 95 or 98, but it always has the octane rating (and the price) at the pump.

Because you pull in, only want 95 but they only have 91 and 98 on the pumps, so you either have to leave or overpay for the 98.

ajobbins:

High-octane mineral fuels do have marginally higher energy levels than lower grades, so fuel economy may improve slightly.

NO. That is the whole misconception and even the AA are perpetuating it *facepalm*

I asked someone who should know about this, and it came down to fuel being sold by volume, not by weight that there were slight differences. This came up with a discussion about the crappy gull stuff they filled with butane as a tax dodge, which is why I wont shop there. It was less dense so same liters would weigh less and have less energy in it was what they explained it as.

ajobbins:

In Europe, 95 and 98 octane are the two most common grades; in Japan it’s 91 and 96/98 octane. In the USA, the fuel grades range between 87-91octane. Australia mostly uses 91 or 95 octane.

No mention of the fact that they aren't comparing things equally (I doubt they even know the difference by the rest of the article). The USA figures are AKI and the rest will be RON.

Yeah the dumb is strong there, but ever since the AA started talking about support of speed cameras, they have lost the plot IMO. Remember how they started out with the badge on the car was so they knew who to warn about speed traps. Good organization then, hopeless now unless all you care about is people talking crap and getting cheaper roadside assistance if you are a paid up member.

Richard rich.ms

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venomio

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#1639375 23-Sep-2016 13:00

ajobbins:

In Europe, 95 and 98 octane are the two most common grades; in Japan it’s 91 and 96/98 octane. In the USA, the fuel grades range between 87-91octane. Australia mostly uses 91 or 95 octane.

No mention of the fact that they aren't comparing things equally (I doubt they even know the difference by the rest of the article). The USA figures are AKI and the rest will be RON.

Holy... Did they actually directly try to compare the rest of the world (RON) with USA/Brazil/Canada (AKI/PON)?

RON 91 = AKI 87
RON 95 = AKI 90
RON 98 = AKI 93

ajobbins

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#1639389 23-Sep-2016 13:19

richms:

Because you pull in, only want 95 but they only have 91 and 98 on the pumps, so you either have to leave or overpay for the 98.

Then it's not unintentional, it's a conscious choice. The AA seem to imply that it's misleading to the consumer in some way, and I disagree.

richms:

I asked someone who should know about this, and it came down to fuel being sold by volume, not by weight that there were slight differences. This came up with a discussion about the crappy gull stuff they filled with butane as a tax dodge, which is why I wont shop there. It was less dense so same liters would weigh less and have less energy in it was what they explained it as.

The difference between 'normal' 91, 95 and 98 octane fuels in this regard are so minute as to be irrelevant. Ethanol or butane blending is another thing entirely as those chemicals have significantly different energy characteristics.

richms:

Yeah the dumb is strong there, but ever since the AA started talking about support of speed cameras, they have lost the plot IMO. Remember how they started out with the badge on the car was so they knew who to warn about speed traps. Good organization then, hopeless now unless all you care about is people talking crap and getting cheaper roadside assistance if you are a paid up member.

Yup, which gives me little confidence in the accuracy of the rest of the article.

Twitter: ajobbins

JonnyCam

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#1639390 23-Sep-2016 13:21

dickytim:

The GDI engines need to run on high octane, unfortunately since it sounds like it has not been there might be quite a bit of coking inside the manifold, this would have a huge effect on the performance, unfortunately the fix is to remove the manifolds and scrape the carbon build ups off.

Unfortunately very few people in NZ know what they have bought and how to run it properly.

My 2016 Tuscon has GDi engine and the manual recommends 91 RON.

Batman

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#1639392 23-Sep-2016 13:29

Coming to think of it, I've fed a 1998 Mitsubishi Diamante GDI 91 fuel and it hasn't died yet. But smells like crap.

PolicyGuy

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#1639394 23-Sep-2016 13:33

And just to make things even less straightforward, RON is 'just a number'

In the "first Formula 1 turbo era" the fuel rules just said it has to be a petroleum-based fuel with a RON not exceeding 100.
I have a family member who was a Tyrell team mechanic at the time, and he said that the Elf-supplied liquid that Renault were pouring into their cars looked and poured like thin treacle, and made your eyes water 100m down wind - the only resemblance between it and the kind of 'petrol' you might put in a road-going car was that it had a RON of exactly 100.

DravidDavid

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#1639469 23-Sep-2016 15:07

joker97: But smells like crap.

Probably because the 91 has mixed with oil by the time it exits the exhaust lol

BruceHamilton

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#1639506 23-Sep-2016 16:12

Just a few things...

Flash point has nothing to do with Octane Rating of a fuel, and the density relationship depends very much of the type of refinery producing the fuel. There is no constant relationship of density with Octane rating, and the same applies for the calorific value ( energy content ) of the fuel. The only property that does correlate with Octane ratings within each chemical family is the autoignition temperature. That's because the autoignition of the unburnt end gases in the combustion are the cause of knock ( detonation ) that Octane rating engines measure.

In NZ the two controlled Octane ratings of petrol fuels are Research Octane Rating ( typical of general driving ) and Motor Octane rating ( typical of sustained motorway/open road driving). Generally, in NZ, the difference ( called Sensitivity ) is about 10, and the Antiknock Index is the average of the two, so a NZ fuel of 96 RON, 86 MON would be considered 91 AKI in the USA. The actual engine tests that determine Octane ratings of fuels are global standards. The octane requirement of an engine can vary with condition, design, load, and environment.

Because of fuel injection some of the properties that affect octane ( volatility, autoignition temperature, ignition energy, etc.) have differing significance, but NZ road fuels still have to cater for carburetted engines as well.

The higher Octane fuels tend to have better quality additive packages ( detergent/dispersants, oxidation inhibitors, stabilizers etc.etc. ) because they have the margin of a premium price. Generally, engines that require higher octane fuels often place more requirements on the fuel, so removing any deposits and staying stable if also used to cool injection systems will help engine performance.

The reason Turbo era F1 fuel smelt was because Honda realised the rules were maximum of 150? litre volume, and more mass would give energy, so instead of typical alkane hydrocarbons ( 0.73 g/ml ), they used more dense aromatic hydrocarbons ( their turbo fuel contained 86% toluene of 0.86 g/ml ) which gave them 18% more energy for same volume. These days, in Formula 1, Mercedes have achieved similar gains by improving efficiency of the engine by using Turbulent Jet Ignition, but others are slowly catching on.

Fred99

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#1639531 23-Sep-2016 16:50

The way that turbulent jet ignition is visualised (below) the fuel efficiency gains would not be what's hoped for with FSI (stratified) systems on normal cars. The TJI is still using port injection for the main fuel charge - so the entire combustion chamber has an even full charge, lean or not.

With direct injected petrol cars, the fuel charge is delivered directly into the combustion chamber, with (as I understand it) two advantages:

The charge can be delivered to the centre part of the combustion chamber and ignited before it's fully dispersed across the combustion chamber, thus the mixture in the fuel-dense centre can be near stoichiometric, but the overall ratio of fuel to air in the combustion chamber is very lean. This is how the cars run under low load - ie most of the time, very economically.

The timing of fuel delivery can be made when the piston is at nearly TDC to eliminate pre-ignition at the high combustion chamber pressures (either high mechanical compression ratio, or moderate mechanical compression ratio but with forced induction.

But there's a problem, soot / fine particulates are produced at the edges of the stratified "ball" of fuel/air when running in lean mode. In this mode the EGR system is open, to reduce NOx emissions etc. Exhaust gas goes back through the intake port and some of the particulates as well as other crud from the PCV system starts building up on the intake ports and top of the intake valves. With previous generation port injection cars, this didn't happen as the intake ports are being washed continuously by a fuel air mix. If you google, there are some awesome images of late model TSI/FSI audis and VWs with the intake ports so clogged-up it's a wonder they still ran at all. Cleaning these means removing the intake manifold and mechanically scraping or soda blasting it off. If it's a V6 with forced induction, that's a lot of work and cost to look forward to.

The other issue is that those fine particulates are possibly extremely toxic by inhalation - worse than from modern diesels which have DPF. I believe future emissions regulations are going to require particulate filters on FSI petrol cars. That's another nuisance item to service, another nail in the coffin for the ICE in my opinion.

Perhaps that TJI can be used with stratified injection to improve combustion efficiency, lower combustion temps and NOx.

I believe Toyota are (or were) looking at using FSI but with an additional port injection to wash the intake ports.

Does the BP wonder-fuel actually work to reduce fouling in the intake ports? It was a nice article / press release, but lacking any proper data, photos etc.

BruceHamilton

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#1639560 23-Sep-2016 17:46

I was discussing Formula 1 engines. Port injection is not permitted in Formula 1, nor are multiple injectors, only a single direct injection per cylinder. The Mahle TJI system for F1 is apparently a single injector system.

The claims by Mahle for their TJI are that most emissions including NOx and particulates are reduced compared to other injection systems, as well as the obvious efficiency improvement. Mahle purchased Cosworth, and some of the TJI technology is believed to have originated with them.

jcddcjjcd

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#1639602 23-Sep-2016 19:59

I have a Honda CRF250L Dual Sport motorbike that I initially ran on 91. After a couple of months I tried 95 and found I could drive round town quite happily at 50 km/h in 5th gear out of 6. On the 91 fuel this was not possible so for me there was definite benefit in the higher octane fuel.

Fred99

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#1639603 23-Sep-2016 20:05

BruceHamilton:

I was discussing Formula 1 engines. Port injection is not permitted in Formula 1, nor are multiple injectors, only a single direct injection per cylinder. The Mahle TJI system for F1 is apparently a single injector system.

The claims by Mahle for their TJI are that most emissions including NOx and particulates are reduced compared to other injection systems, as well as the obvious efficiency improvement. Mahle purchased Cosworth, and some of the TJI technology is believed to have originated with them.

The diagram I posted above was apparently originally from Mahle - but for production (presumably that means "normal car") engines.

I'm not sure how they balance claims for reduced NOx and (by) reduced combustion temps with increased efficiency (economy). That seems counter-intuitive to me.

Gambit

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#1671850 16-Nov-2016 11:15

Sadly I've filled my Vr4s tank with 91 this morning.
Will anything major happen?
I don't race around so won't be putting the engine under stress.
Hopefully I can use the tank up and put 95 back in.
Any benefit in using 98?

Thanks

gzt

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#1671895 16-Nov-2016 12:03

Don't risk it. Personally I would head to an auto store and get an octane booster. Mixing it is obviously a problem.

Bounce it by hand as far as possible and then bumpy roads? : ).

Batman

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#1671907 16-Nov-2016 12:06

gzt: Don't risk it. Personally I would head to an auto store and get an octane booster. Mixing it is obviously a problem.

Bounce it by hand as far as possible and then bumpy roads? : ).

Nah, just bunny hop.

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