General 1.2 High Octane Petrol

[Herts Hillhopper]

Ethanol in fuel also makes the fuel collect water, which then corrodes metal components inside the engine. ...

The chemical reaction resulting from burning any hydrocarbon fuel (eg petrol) produces two products of combustion: CO2 and H2O (water) -- so any car engine has water vapour in it. Since the explosion in the cylinders reaches over 1000C, any water there will be evaporated instantly. And since the cooling system operates at about 120C, water anywhere else in the engine or in parts near to it will evaporate too. In both cases, appearing at the end of the exhaust as steam...

 

[jrkitching]

The chemical reaction resulting from burning any hydrocarbon fuel (eg petrol) produces two products of combustion: CO2 and H2O (water) -- so any car engine has water vapour in it. Since the explosion in the cylinders reaches over 1000C, any water there will be evaporated instantly. And since the cooling system operates at about 120C, water anywhere else in the engine or in parts near to it will evaporate too. In both cases, appearing at the end of the exhaust as steam...

It's the corrosion in the fuel system components that concerns me more.

Just try this simple test:

1 - Half-fill 2 glass jars, one with fuel containing ethanol in it, and one with ethanol-free fuel.


2 - Place a plain steel bolt (not plated) in each jar.


3 - Put lids on the jars.


4 - Punch a single nail hole in each lid to relieve pressure similarly to your car's fuel filler.


Now watch what happens to the steel bolt over the course of the next ten years.

 

[JR45]

Further to my last...
I ran a test on two very similar journeys - same start point, same destination, same weather and traffic conditions - one on bog-standard 95 octane unleaded, one on Tesco Momentum 99 octane. About 25% country roads, 25% fast A roads and 50% motorway. Pretty much one full tank for each trip...
Purely subjectively, the car definitely felt a bit more "lively" on the 99.
Having done the maths - the car got about 7% more mpg on the 99.
As 99 costs about 10% more than 95 I'd have to say yes, it does make a difference; but no, it's not enough of a difference to be financially viable...

 

[Herts Hillhopper]

It's the corrosion in the fuel system components that concerns me more...

That's led me to wonder: how much steel is in the fuel system? The tank is plastic. The pump in the tank is likely to be stainless and plastic. The fuel lines are plastic. And the injector rail too. The injector nozzles are stainless... So, how much steel is in the fuel system? Not much I suspect

(not looking for an argument... but think the parts are designed to cope))

 

[jrkitching]

That's led me to wonder: how much steel is in the fuel system? The tank is plastic. The pump in the tank is likely to be stainless and plastic. The fuel lines are plastic. And the injector rail too. The injector nozzles are stainless... So, how much steel is in the fuel system? Not much I suspect

(not looking for an argument... but think the parts are designed to cope))

I'd hope none at all on a modern car; as you say, the fuel system should have been designed to cope with some ethanol content (but how much ? E5? E10? >E10?).

But even if the fuel system can cope, the car won't run as well on fuels containing ethanol; both performance and economy will be worse as the total energy content of the fuel will be less.

Some folks with older cars are now resorting to washing the fuel before use to remove any ethanol.

 

[DaveMcT]

Further to my last...
I ran a test on two very similar journeys - same start point, same destination, same weather and traffic conditions - one on bog-standard 95 octane unleaded, one on Tesco Momentum 99 octane. About 25% country roads, 25% fast A roads and 50% motorway. Pretty much one full tank for each trip...
Purely subjectively, the car definitely felt a bit more "lively" on the 99.
Having done the maths - the car got about 7% more mpg on the 99.
As 99 costs about 10% more than 95 I'd have to say yes, it does make a difference; but no, it's not enough of a difference to be financially viable...

I recently paid £1.23 for Super where 95RON was £1.15 so its pretty much a break even on costs per improved mpg.

It's more money to fill but you get more mpg so costs per mile are at worst the same. At best the posh is cheaper per mile than "cheap" fuel. The bike especially feels as flat as a fart on plain 95RON so its a no brainer to use the better fuel.

Older cars and bikes that don't have a knock sensor will see no mileage benefit from posh fuels. They can't change the spark timing to benefit from the smoother fuel burn.

Engines that can't self adjust will be set to run safely on 95-RON. Those with a knock sensor will advance the spark timing to extract maximum energy from the better fuels. Result - less heat thrown down the exhaust and more power into the crank.

 

[jrkitching]

Older cars and bikes that don't have a knock sensor will see no mileage benefit from posh fuels.

They will if those posh fuels have less ethanol in them.

 

[DaveMcT]

Who knows. My last bike (900cc, 8 valve, four cylinders, 90bhp) had a fixed timing map and no knock sensors. It consistently gave around 45mpg regardless of the fuel used. I ran Shell and BP posh a few times to keep the system clean but got no mpg or performance difference. Who knows how much ethanol the fuels contained.

The same engine could have the spark timing advanced to benefit from higher octane fuels but it would then knock and pink on cheap fuel so I kept it standard.

 

[JR45]

I recently paid £1.23 for Super where 95RON was £1.15 so its pretty much a break even on costs per improved mpg.

It's more money to fill but you get more mpg so costs per mile are at worst the same. At best the posh is cheaper per mile than "cheap" fuel. The bike especially feels as flat as a fart on plain 95RON so its a no brainer to use the better fuel.

Older cars and bikes that don't have a knock sensor will see no mileage benefit from posh fuels. They can't change the spark timing to benefit from the smoother fuel burn.

Engines that can't self adjust will be set to run safely on 95-RON. Those with a knock sensor will advance the spark timing to extract maximum energy from the better fuels. Result - less heat thrown down the exhaust and more power into the crank.

So, basically, you paid 7% more for "super" - my (admittedly not very scientific) gained 7% more mpg - so no financial benefit...
Older cars and bikes that are always run on higher octane fuel can have the mixture and timing optimised to make use of it - what we used to call "tuning" back in the old days - so can actually see noticeable benefits...

 

[DaveMcT]

People pay a fortune to get a few % extra by engine tuning. So 5% to 10% better drivability and power by simply throwing in a better fuel for no cost per mile has to be a bargain.

Those who drive very slowly or always in town and buy everything on cost over value might not see the point but for the rest of us it's a no brainer.

Older engines can be retimed to run higher octane fuels but they risk damage when run on cheap fuels. Posh stuff isn't always available so not a good option (IMO).

Back in the 1980s I had a Citroen BX 1.9 GT reset for unleaded. It was designed for 4 Star (98 octane). In those days all unleaded was 95-RON. After the sparks were retarded, fuel consumption increased considerably and it ran like the hand brake was on so I soon had it put back to standard and used 4 Star. My next car was a diesel.