Author Topic: 97 vs 95 RON effect on fuel consumption?  (Read 10612 times)

mikebore

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97 vs 95 RON effect on fuel consumption?
« on: June 29, 2016, 10:56:35 PM »
For all my motoring life (50 years) I have believed that higher octane rating had no effect on fuel consumption, and have used the minimum recommended fuel. Higher octane was only to avoid detonation (pinking), and avoiding it by a bigger margin conferred no benefit.

However I saw an argument somewhere recently that modern engines were able to detect pinking and automatically adjust ignition timing, and therefore a higher octane could enable more favourable timing.

While this argument has some technical plausibility it doesn't mean there is a benefit in actual practice.

Does anyone have any hard data?

Thanks

trebor1652

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Re: 97 vs 95 RON effect on fuel consumption?
« Reply #1 on: June 30, 2016, 11:13:20 AM »
I was thinking along the same lines a couple of years ago when I had my mk1 Jazz used 95 unleaded and regularly achieved high 40's mpg.
So I decided to try super unleaded on a full tank of fuel and the mpg nosedived.
I seam to think this topic has been discussed before but can't remember what the consensus was.

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andruec

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Re: 97 vs 95 RON effect on fuel consumption?
« Reply #2 on: June 30, 2016, 02:35:40 PM »
Many years ago I tried it on a Nissan Sunny 1.4 and it seemed to make the car a bit more peppy. Never tried it since as I'm not very interested in 'peppy' :)

culzean

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Re: 97 vs 95 RON effect on fuel consumption?
« Reply #3 on: June 30, 2016, 05:41:54 PM »
For all my motoring life (50 years) I have believed that higher octane rating had no effect on fuel consumption, and have used the minimum recommended fuel. Higher octane was only to avoid detonation (pinking), and avoiding it by a bigger margin conferred no benefit.

However I saw an argument somewhere recently that modern engines were able to detect pinking and automatically adjust ignition timing, and therefore a higher octane could enable more favourable timing.

While this argument has some technical plausibility it doesn't mean there is a benefit in actual practice.

Does anyone have any hard data?

Thanks

I did the same 70 mile round trip commute  5 days a week for 5 years in both Jazz and Civic and could check like for like during both warm and cold weather,  it is a given that mpg drops during cold and improves during warm weather whatever fuel you use.  What I found was with 97 and 99 RON versus 95 RON you could have more pep  or more mpg,  trouble is when you got more pep you accelerated and drove that much quicker and got same mpg as 95, if you laid of a bit on the loud pedal you could use less throttle to get same performance and get up to 10% better mpg.

Engines have had one or more  'knock sensors' on the engine for many years now,  and ECU's have got cleverer.  The knock sensor is like a microphone listening out for certain frequencies of sound that indicate that pre-ignition is happening (due to octane of fuel and combustion conditions) the fuel air mixture burn actually peaks too soon and is trying to push piston back down the way it came (reverse crank direction) without passing through top dead centre position, this is very damaging to engine and what the knock sensor / ECU will do is retard the ignition (fire the spark later in the stroke until the knocking stops) - this robs the engine of power but is there to protect it from the damage caused by pre-ignition.

High octane fuel contains additives to resist pre-ignition and these additives actually reduce the amount of hydro-carbon (the stuff that burns) in the fuel,  actually lowering the energy content of the fuel,  but allowing the spark timing to be advance more,  which allows the energy in the fuel to be utilised much more efficiently in the engine.

The engines answer when it detects knocking is to lower the engine output to protect it,  if you put a fuel in that resists knocking over a broader range of engine conditions (ie higher octane) the ECU can run with advanced ignition spark timing which makes engine more peppy or allows a lower throttle opening for a given power output = more mpg.

Last year I did fill up at local Tesco with 99 RON (don't normally use their fuel) before taking 4 adults and luggage to North Wales,  I was impressed by the pulling power up the Welsh hills and how cleanly the engine ran.
« Last Edit: July 01, 2016, 03:55:42 PM by culzean »
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jazzaro

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Re: 97 vs 95 RON effect on fuel consumption?
« Reply #4 on: July 01, 2016, 02:39:38 PM »
An engine as the L13B powering the Jazz IMHO can get many benefits running with 97 or 98 RON petrol. It has a high RC  (13,5:1) and it can control the knock also enriching the fuel air mixture (this lowers the internal temperatures) and modifiyng timing and ignition.
Modern engines can use also spark plugs as ion sensors to check the combustion process, they send low voltage currents to the electrodes after the spark  checking how many electrons flow through the spark gaps: this parameter can be used to check if  the combustion was bad or good.

mikebore

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Re: 97 vs 95 RON effect on fuel consumption?
« Reply #5 on: July 01, 2016, 03:35:15 PM »
Thanks! These are the arguments I have seen and which have technical logic, so why do Honda actually recommend 95 RON?

The manual doesn't  say 95 RON is minimum. The stated minimum (page 464) is 91 RON temporarily.

EDIT. The manual does say recommended is "95 or higher" so I suppose this says 95 is the "minimum recommended". But if they believe there is an actual benefit for 97+ I would have expected different wording to make this clearer.
« Last Edit: July 01, 2016, 03:46:13 PM by mikebore »

culzean

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Re: 97 vs 95 RON effect on fuel consumption?
« Reply #6 on: July 01, 2016, 03:46:44 PM »
models for different countries are tuned to local fuel available,  European models will be 95RON minimum and some other parts (Asia and some parts of America) could be 91 - my MKII handbook does warn that 91 will decrease performance.

Direct injection petrol engines can run on lower octane (or with higher compression ratio) without knocking because fuel is only injected at high pressure when compression is almost complete and therefore compression ignition is less likely to happen. The problem with direct injection engines (both Diesel, which is well known but also petrol) is that they pump out more NOx (oxides of nitrogen) which are very harmful to health.

Car makers may be wary of making claims for higher octane fuel because their engine power output is based on the fuel the engine was designed for,  and 97 / 99 RON may not be widely available.  I have a theory that when doing MPG testing they may use a higher octane but not tell anyone (they already use higher ambient temperature and humidity to increase economy).
« Last Edit: July 01, 2016, 03:51:34 PM by culzean »
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madasafish

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Re: 97 vs 95 RON effect on fuel consumption?
« Reply #7 on: July 04, 2016, 06:28:05 AM »
I keep accurate records of miles and fuel used.

I tried Shell V power several times - usually 2-3 tanks at a time.

Satistically it was impossible to see any significant (>1%) or consistent (two tanks the same ) effect vs normal Shell.. (I have a Shell Drivers Card so points )...  So I now just buy 95RON.

I ignore anyone who claims benefits but is unable to produce detailed mpg analysis over seasons..because of course winter MPG is lower than summer. I have records on my Jazz from the day bought#  - sad I know. :-[

# Fuelly and manual


culzean

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Re: 97 vs 95 RON effect on fuel consumption?
« Reply #8 on: July 04, 2016, 07:21:57 AM »
MPG figures are only really valid if measured consistently on like-for-like journeys,  that's why a test track is used for serious tests where gradients, wind and track surface are all known and constant, also important that cars are spaced out so that cars shielding the wind (slipstream towing) other cars are negated - I have seen results on tracks where tanks were emptied and a measured amount of fuel put in,  this was done several times and then the different fuels used to the other cars in the test to even out any differences between the actual cars and engines in the test (binding brakes, tyres etc.).  Its no good just measuring petrol usage if every journey is different because any benefits can be cancelled out by road conditions, that is why 5 years of same journey is good research because I was filling up on average every 4 to 5 days.

As I said before,  I did a blind test on my wife a couple of years ago because she only used supermarket fuel and I started filling it up without her knowledge with 97,  she asked me if I had the car tuned up as it was running better.
« Last Edit: July 04, 2016, 07:25:01 AM by culzean »
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jazzaro

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Re: 97 vs 95 RON effect on fuel consumption?
« Reply #9 on: July 04, 2016, 10:48:33 AM »
models for different countries are tuned to local fuel available,  European models will be 95RON minimum and some other parts (Asia and some parts of America) could be 91 - my MKII handbook does warn that 91 will decrease performance.

Direct injection petrol engines can run on lower octane (or with higher compression ratio) without knocking because fuel is only injected at high pressure when compression is almost complete and therefore compression ignition is less likely to happen. The problem with direct injection engines (both Diesel, which is well known but also petrol) is that they pump out more NOx (oxides of nitrogen) which are very harmful to health.

Car makers may be wary of making claims for higher octane fuel because their engine power output is based on the fuel the engine was designed for,  and 97 / 99 RON may not be widely available.  I have a theory that when doing MPG testing they may use a higher octane but not tell anyone (they already use higher ambient temperature and humidity to increase economy).
Yes and not.
DI engines can run with higher R.C. because petrol injected inside the combustion chamber evaporates and this causes a cool down both for  the air- fuel mixture and for the pistons/valves and so on. Frankly speaking, this happens also in indirect injection petrol engines, but with less cooling effect for the air mixture.
DI petrol engines does not have the problem of NOx emissions: differently from diesel engines,  the combustion chamber reaches limit temperatures only during knocking events, but usually the combustion becomes with a flame-path propagation, so the process is not an instant bang like the diesel cycle. Lower temperatures means less NOx, because this polluting agent came only at very high temps.
Direct inj. petrol engines will have particulate matter problem, so some of them will have to mount a particulate matter filter, as diesel engines. This because some drops of the petrol spray does not completely evaporate, and they don't burn well.
It's phisics...
I agree with you when you say that models for different countries are tuned to local fuel.
« Last Edit: July 04, 2016, 08:27:37 PM by jazzaro »

edam

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Re: 97 vs 95 RON effect on fuel consumption?
« Reply #10 on: July 09, 2016, 03:51:34 PM »
 I don't understand why this is but the engine is noisier when using Tesco Momentum that the normal grade.
I have been using Momentum since I have had the car but I filled up with the normal stuff this time and its so much quieter.
Very odd

mikebore

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Re: 97 vs 95 RON effect on fuel consumption?
« Reply #11 on: July 09, 2016, 05:13:43 PM »
I don't understand why this is but the engine is noisier when using Tesco Momentum that the normal grade.
I have been using Momentum since I have had the car but I filled up with the normal stuff this time and its so much quieter.
Very odd

I have only had my car one month, and I did two tankfuls with 95 before trying a tankful of 97 which I am now using, and I thought the same. Decided I must be imagining it, but have thought if the ignition is being advanced a bit by the electronics, it could make the engine sound different. 

I don't think I would describe 95 as "much" quieter. The 97 sounds a bit "harder" is how I would describe it. I will try 95 again next time, and re-assess. I am still getting familiar with car.

edam

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Re: 97 vs 95 RON effect on fuel consumption?
« Reply #12 on: July 09, 2016, 06:02:13 PM »
My understanding of the new engine is that the inlet valve is kept open on part of the compression stroke.
Is it possible that the ignition is so far advanced that its firing before the valve is closed
The noise I get with the higher octane could be described as "induction noise"

mikebore

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Re: 97 vs 95 RON effect on fuel consumption?
« Reply #13 on: July 09, 2016, 06:13:29 PM »
My understanding of the new engine is that the inlet valve is kept open on part of the compression stroke.
Is it possible that the ignition is so far advanced that its firing before the valve is closed
The noise I get with the higher octane could be described as "induction noise"

I really doubt this somehow! It would not be very good for the injection system to have combustion products blowing back, apart from anything else  ;)

Not sure what you mean by "induction noise", but to me it would be the rushing air noise I used to hear when running carburettor engines without air filters in my youth.
« Last Edit: July 09, 2016, 06:16:18 PM by mikebore »

edam

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Re: 97 vs 95 RON effect on fuel consumption?
« Reply #14 on: July 09, 2016, 06:33:17 PM »
Your description of "induction noise" is exactly what I mean.
In my case it was from a 350cc single with a Amal carb.

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