Octane rating vs compression ratio

[GDII]

The company I work for has just bought a new 2015 Mazda 2 for us to use to travel around to sites. It has a 13:1 compression ratio and the recommended fuel to use is 91 RON or higher. Also E10 can be used.

I thought I had a basic understanding of what fuel could be used for what compression ratio. This throws my knowledge out the window. I run 95 in the MR2 (GEN2 3SGE)as it has a 10:1 ratio and i have to run 95 in the Starlet (2E) as it gets terrible knock on 91 due to oil burning issues.

How can an engine with 13:1 run on 91? To me that's race car territory and race car fuel like E85 or similar. Anything over 10:1 needs atleast 98.

[Vertigo]

The answer is: new technology.

But for real, I dont have the full answer.

[Mr Ree]

It will have alot to do with combustion chamber shape and piston design, I would imagine.

New cars=new tech

Run the highest octane you can get regardless

[KinLoud]

A couple of things
Direct fuel injection
And/or
Atkinson cycle (vs otto cycle), i.e. Prius https://en.m.wikipedia.org/wiki/Atkinson_cycle

[Flannelman]

Here goes!

Octane is a rating for fuel given for two different components. Octane and Heptane.

Heptane has 0 octane, so its resistant to self ignite when compressed is low. Octane has a high resistance to self ignition. The Octane Number stems from the blend needed to match the test fuel. So, if the test fuel matches a blend of 10% Heptane and 90% Octane, the fuel is said to be 90 Octane fuel.
Heptane is used for Diesel rating. I think (from memory) most diesel is rated at 60 Heptane.

RON is Research Octane Number
RON is determined by a special engine that is designed to changed its compression while its operating.

MON is Motor Octane Number.
MON id determined by an engine with a set 10:1 compression and the timing is adjusted to find the onset of knock.

While both have their controls eg intake temp, ignition timing, this is the main difference between the two. It is this difference which give the different ratings. Eg, America uses RON which give the lower 87/91 octane and we have MON with gives 91/95 octane.

The names we have used and heard of are, Detonation, knock, pre-ignition, ping... all come under one banner. Unfavorable burn character. While all mean different things eg detonation is when the fuel explodes rather than burn and pre-ignition is when the fuel is ignited before the spark plug or by a hot spot elsewhere in the chamber.
All Unfavorable burn character can be traced to one thing, Too much heat. While the engine must be hot, the only way to control the heat in a charge of fresh air and fuel is by having a low compression value.
Less compression means less heat is generated when the fuel mixture is compressed.

This is why MON/RON are different.

While many believe Compression Ratio has an impact on what octane to use, it is the combination of Intake Valve Closing AND Compression Ratio that has the biggest impact on what Octane to use

Before I get jumped on with Combustion Chamber Design, read on.

Intake Valve Closing has the effect of changing DYNAMIC compression ratio. This compression ratio is the one seen when the engine is running. Later intake closing reduces the engine ability to trap intake air at low RPM (called Reversion) With less trapped air, engineers increase the compression to squeeze the smaller quantity of air more to increase the flame speed. Also, by squeezing less air, less heat is created by compression which means all the nasty burn characteristics at low RPM get reduced.
So, it is for this very reason why Race Cams (big ones with the advertised duration starting with a 3**) demand big compression because if they didn't, the reversion they give kills low RPM torque.
Because the Atkinson system has a late intake valve closing event, it not only gets away with high compression, it demands it for the system to work.

Compression is nothing more than a mechanical number. How it effects output is by lifting the engines ability to extract more power from the same quantity of fuel. I'm not surprised automotive engineers are chasing compression to increase power as its what Race Engines have been doing for years. The trick is to get the engine to be well mannered enough to sniff cheap gas, not be loud, lumpy or refuse to operate in the low RPM.

Combustion Chamber design has more to do with how the fuel burns rather than resisting unfavorable burn character. When unleaded fuel was introduced to America, all automotive engineers reduced compression by making their combustion chambers big, wide open spaces. While this reduced the unfavorable burn, it also killed thermal efficiency which resulted in a large drop in power and fuel economy.

Oh, and fuel enrichment effects the compression an engine can use too. making the air/fuel mix richer give a better burn rate than a lean air/fuel mix.

The best summary that I can give is that an engine is designed around a type of fuel. From cam timing, compression ratio and combustion chamber design. If anyone wants build or modify their engine, consider what fuel it is to run and design the engine around that. Once the fuel is locked in, the expensive parts can be chosen with this in mind.

[Grrrrrrr!]

Here goes!

Octane is a rating for fuel given for two different components. Octane and Heptane.

Heptane has 0 octane, so its resistant to self ignite when compressed is low. Octane has a high resistance to self ignition. The Octane Number stems from the blend needed to match the test fuel. So, if the test fuel matches a blend of 10% Heptane and 90% Octane, the fuel is said to be 90 Octane fuel.
Heptane is used for Diesel rating. I think (from memory) most diesel is rated at 60 Heptane.

No, diesel is cetane rated.

RON is Research Octane Number
RON is determined by a special engine that is designed to changed its compression while its operating.

MON is Motor Octane Number.
MON id determined by an engine with a set 10:1 compression and the timing is adjusted to find the onset of knock.

While both have their controls eg intake temp, ignition timing, this is the main difference between the two. It is this difference which give the different ratings. Eg, America uses RON which give the lower 87/91 octane and we have MON with gives 91/95 octane.

You got that completely wrong. 'murica uses pump octane, which is (RON + MON )/2, we use RON, which give us the higher number, RON is (always?) higher than MON.

The names we have used and heard of are, Detonation, knock, pre-ignition, ping... all come under one banner. Unfavorable burn character. While all mean different things eg detonation is when the fuel explodes rather than burn and pre-ignition is when the fuel is ignited before the spark plug or by a hot spot elsewhere in the chamber.
All Unfavorable burn character can be traced to one thing, Too much heat.

Too much energy, heat and pressure both contribute.

While the engine must be hot, the only way to control the heat in a charge of fresh air and fuel is by having a low compression value.

Or by cooling the intake charge (intercooling, water/meth injection, or injecting excess fuel (rich mixtures) to cool the charge as it vapourises)

Later intake closing reduces the engine ability to trap intake air at low RPM (called Volumetric Efficiency)

[GDII]

That all makes sense. New technology allows better things to happen. Thanks guys.

I'm guessing running it on 98 would be pointless then?

[MAC_HATER]

Id say yeah, if the manual or whatever calls for 91, then 91 it shall get haha

There are a few Direct injection engines that can run on 91 with eye wateringly high compression ratios, I wonder if the various manufacturers have finally gotten around the "Furry intake valve" issue that high mileage older direct injection petrol engines tend to suffer from, I think Toyota got around that at some stage by simply having 2 injectors per cylinder on some engines, 1 in the head for direct injection and one in the intake runner as per normal engines, that would fire up under the correct conditions, and that would keep the problem at bay, even if it wasn't a terribly elegant solution

[sergei]

That all makes sense. New technology allows better things to happen. Thanks guys.

I'm guessing running it on 98 would be pointless then?

No.
The ECU will adjust ignition timing/valve timing/mixture to take advantage of the 98 in any case.
91 will be the minimum they will aim (probably more like 92, as 91 does not exist in Japan).
Most of the cars (jap imports) will be made for minimum of 96. People put 91 in them and complain about fuel economy/being gutless.

[Grrrrrrr!]

I totally disagree with sergei
Its:
1) A NZ new company car AND
2) Manual/sticker says 91 is okay.

So run it on 91. Not sure what the problem here is. If you want you could try to do a 91 vs 95 economy test to see if the car runs any better on 95, but that would require several tanks of 95 to give it time to learn long term trims etc for 95, and a consistent driving style travel pattern to get results, which if its a shard car isn't going to happen.

Is your boss going to ask why you are buying 95 for a car that is designed to run on 91?

Also not sure where Sergei gets the idea that japan doesn't have low octane fuel:

In 2012 the market share in
Japan for regular gasoline (90 RON) was 84.5% and for premium gasoline (100 RON) was 15.5%. In Europe the gasoline market was divided into 91 RON: 8%, 95 RON: 87.6% and 98 RON: 4.4%, making
the average gasoline RON in Japan 91.6 RON and in Europe 94.8 RON, a difference of 3.2 percentage points.

Japanese fuel standards Regular actually is minimum 89 RON. It has links to the actual standards if you can read Japanese text

I think its a general internet myth that japanese performance cars are all run only on 100octane, if you believe that you probably believe all NZ cars run on 98

[GDII]

There is no problem. (now) I was just trying to understand how these cars could run 91 on 13:1 compression ratio where older engines wouldn't be able to if they were the same ratio. Clearly I don't have a very good understanding of how engines work with different fuels. If it says 91 then that is fine. Sadly I can't even use 91 in my EP81 but this is not a very healthy engine as it burns a lot of oil causing it to knock. 95 has fixed this and it runs very well at low revs. Different engine, different technology and 25 years of advancement has changed things a lot.

I guess I can keep my mouth shut when I recommend people to run their cars on a higher octane or they will do damage. This was not the case with my mate running his blacktop BZT on 91. He now runs it on 95 as recommened and gets better economy and far less knock.

[sergei]

I totally disagree with sergei
Its:
1) A NZ new company car AND
2) Manual/sticker says 91 is okay.

So run it on 91. Not sure what the problem here is. If you want you could try to do a 91 vs 95 economy test to see if the car runs any better on 95, but that would require several tanks of 95 to give it time to learn long term trims etc for 95, and a consistent driving style travel pattern to get results, which if its a shard car isn't going to happen.

Is your boss going to ask why you are buying 95 for a car that is designed to run on 91?

Also not sure where Sergei gets the idea that japan doesn't have low octane fuel:

In 2012 the market share in
Japan for regular gasoline (90 RON) was 84.5% and for premium gasoline (100 RON) was 15.5%. In Europe the gasoline market was divided into 91 RON: 8%, 95 RON: 87.6% and 98 RON: 4.4%, making
the average gasoline RON in Japan 91.6 RON and in Europe 94.8 RON, a difference of 3.2 percentage points.

Japanese fuel standards Regular actually is minimum 89 RON. It has links to the actual standards if you can read Japanese text

I think its a general internet myth that japanese performance cars are all run only on 100octane, if you believe that you probably believe all NZ cars run on 98

It is not a myth, as you look at owners manual for most of semi-performance JDM cars require premium fuel*. If you look up "JIS K 2202" standard it states that minimum for premium is 96.
Regarding Toyota, anything with G(T)E will be premium only (96+), most of FE stuff can take 91, some will require premium.
Obviously Toyota does not make anything fun anymore (except maybe 2GRFE powered cars) so in a new modern appliance made by toyota you can put 91 and have no problems.

Here is example of modern car:
http://toyota.jp/auris/spec/
120T and 180S requires Premium (even though same 1.8 in a different spec can be OK on regular).
engines like 2GR-FE require Premium.
(google for: 無鉛プレミアムガソリン) .


With older cars it used to be mostly premium only.

Regarding real life experience:
My wife's Toyota IST with 1NZFE:
91 10L/100km (city driving)
95 8.5L/100km (city driving)
98 8.0L/100km (city driving)

with 91 in 1NZFE I can clearly hear knocking on moderate loads.

Another anecdotal evidence: 6000+ km done in rental Camry (2.5L engine) in Australia, about 3 tanks worth of 91 and rest on 98. Far better economy on 98.

* Note: this has change in last years, as most of the manufacturers shifted to regular petrol as standard.

[Flannelman]

Late model cars have a habit of being factory tuned for high advance. It gives good fuel economy under light load.
Biggest thing late model cars rely on is the knock sensor. It's a cheat way of allowing the use of 91-98 octane.

Even Ford Falcon 6 does this. It's rated at 195kw on 91 but goes over 200 if given 98. But, they rate it at the lower value as 98 isnt at every gas station and their own political reasons.

[iOnic]

MPS requires 95. 9.5:1CR + Boost.
No fancy double injectors per cylinder. Just GDI and 2000+ psi fuel pressure from the pump.

[Grrrrrrr!]

It is not a myth, as you look at owners manual for most of semi-performance JDM cars require premium fuel*. If you look up "JIS K 2202" standard it states that minimum for premium is 96.

Yes, 96, not 100. Same as in NZ premium is 95 octane, and there is 98 available at some locations, hence why I said you probably believe all nz performance cars are run on 98.., same applies with 96 and 100 octane in japan

120T and 180S requires Premium (even though same 1.8 in a different spec can be OK on regular).

120T is a different motor to 180S... maybe you meant 180S and RS which are both 2zzr-fae? 180S requires regular and RS requires premium.. But you are comparing two different motors (Manual only (RS) vs CVT only (180T)), which probably means different cams (toyota is known to do this eg, gen3 3sge), hence the different torque & power figures... of course they may need different fuel, they are in a different state of tune. Not really sure what point you were trying to make?

And my anecdote beats your anecdote Honda Accord Euro R (K20A), same open road economy (15 -15.2 km/L @ legal limit, minimal overtaking, 13.8- 14.0 km/L at with usual long distance driving) on 98 and 95, unless its that watered down gull E10 stuff. Done over several Akl-Palmy trips, A-B-A testing to minimize variables (still too many to be conclusive.)
I don't even bother trying to get around town economy readings as it takes too many trips (only 3kms to work) and far too many variables to get consistent results, weather, school holidays, fridays, all bring different driving conditions.

Also, google translate hilarity: "Lightly blow up a six-speed manual (RS)" "Super CVT-i to be able to enjoy the sporty driving (180S)"

[sergei]

Regarding the Corolla 1.8 requiring different fuel, my point even for crappy under powered FE motors some models still require premium fuel.
The approach put 91 in everything is wrong. While putting 95+ in almost everything with a knock sensor and more or less modern ECU actually works.
I have seen people put 91 in company Ford Focus, that clearly states min 95 on gas flap. Or people pulling up in a 760iL and chucking 91 in it (sad people, got enough money to buy BMW, but too greedy put right fuel).

When I had my ST205 in stock form on a hot day on 98 it would still get occasional knock. I don't hear/feel knocks in my STI, but because it has smarter ECU.

Speaking of Gull Force10, in my ST205 GT4 it would cause fuel economy go down (consume more), while in my STI it the fuel economy would go up and it would feel more responsive. I bet the difference is on how smart are ECUs on figuring out the good tune.

In anyway, I had to get my STI tuned for 95, as it was not safe to run on 95, especially with the interwarmer on top of the engine. You could probably get away with stock tune and 95 on it, but it rely on knock sensor too much.

Regardless of this, the OP should just use 91 and don't worry about it .

If it was my personal car I still would use 95 (especially with my driving habits). It is known fact that the modern cars will adjust for better fuel, why go for minimum.
If manufacturer recommends minimum 15W40 do you still put mineral 15W40 oil in it, or 5W30 synthetic, providing they cost similar?
Do you always put OEM specified tyres or go for something better?

When I put 98 in that rental 2.5L camry, after first 50kms it noticeably changed in many ways: louder induction noise, better throttle response and better fuel economy. Disregarding economy, the joy of driving is worth the extra money. It was probably thrashed all its very short life on shitty fuel.

[Grrrrrrr!]

The approach put 91 in everything is wrong.

No-ones said that, we've been saying if the manual says use 91, then use 91. Putting 91 in a car that needs premium is just stupid.

[AE82 FXGT]

A couple of things
Direct fuel injection
And/or
Atkinson cycle (vs otto cycle), i.e. Prius https://en.m.wikipedia.org/wiki/Atkinson_cycle

According to Mazda it's closer to the miller cycle with out any FI https://en.wikipedia.org/wiki/Miller_cycle
So, it's not running that high compression all the time.

So easiest explanation is they can do it because they have very fine control over the valve timing.
The exhaust cam is actuated by an oil control valve to advance and retard, and the intake cam control is electric.

Source: I was a Mazda Tech that went to the New model intro training on the Mazda 2 before the car hit our shores.

[AE82 FXGT]

I have seen people put 91 in company Ford Focus, that clearly states min 95 on gas flap.

This kills the focus, not really, but I have heard some knocking Focii in my time; usually manuals that oldies chuck 91 in then complain about a noise when they start off in first as it's knocking it's head off.

[AE82 FXGT]

MPS requires 95. 9.5:1CR + Boost.
No fancy double injectors per cylinder. Just GDI and 2000+ psi fuel pressure from the pump.

You do have variable intake cam control too, the very same system that is used on the diesel cam of the new Mazda2.

Valve timing directly affects the compression ratio, and I feel it hasn't been addressed enough in this thread.

BTW have you been for burn in the newer BL Mazda3, I got to drive a modified example the other day, was insane for fwd.
Shame there isn't a BM Mazda3 MPS in the works.

EDIT: HOLY BUMP FROM THE PAST SORRY I HAVEN"T BEEN HERE RECENTLY