Local garage bins Super Unleaded.
Discussion
Riley Blue said:
kambites said:
I'm not sure I've ever seen anyone using the "super diesel" pumps. I don't even know what the difference is supposed to be; obviously it's not octane rating.
'Cetane' is the diesel buzz word roughly equivalent to 'octane' I believe.Of course the difference is that with a spark ignition engine, you simply want the highest octane rating possible. With a compression ignition engine there is no "right" cetane rating; you just want whatever the engine is tuned for.
Edited by kambites on Thursday 8th December 19:57
PositronicRay said:
kambites said:
Of course the difference is that with a spark ignition engine, you simply want the highest octane rating possible.
Not sure I understand this?Edited by kambites on Thursday 8th December 19:57
amancalledrob said:
PositronicRay said:
kambites said:
Of course the difference is that with a spark ignition engine, you simply want the highest octane rating possible.
Not sure I understand this?But, as you say, in a petrol...
amancalledrob said:
uncontrolled combustion of the mixture otherwise known as 'knock'.
Which, if left unchecked, can cause serious damage to engine internals - burning holes in the piston crown, and other damage.amancalledrob said:
A higher octane rating reduces the tendency of the fuel to detonate under compression, so that the spark plug can ignite it at the right time for a controlled burn.
Indeed. Counter-intuitively, higher octane fuel is harder to ignite.But... so long as there's no pre-ignition, then higher octane fuel will make zero difference. And that's where some engines can take advantage - because they can see no pre-ignition is happening, so can change the map (with a range) until pre-ignition DOES occur, then bring it back a snidge - ensuring that the full advantage is taken from the fuel's resistance to ignition. So a given engine may well run just fine on 95, but can adjust for 97 or 98. Beyond there, though, there's no benefit to using 100 or more if you can find it.
Back in the day, two star leaded was 92 octane, three star was 95, four star was 98, and five star was 101. When unleaded first came out across Europe, in the 80s, there was regular (91), premium (95) and super (97-98) - regular never made it here, and has died out on the continent - so a lot of cars had to be retuned from 98 octane to 95, so they'd run on premium unleaded - that was easy back then, just changing the ignition timing.
AvGas is 100, but measured differently. The US have 87 (regular), 88-90 (mid) and 91-94 (premium), measured differently again.
TooMany2cvs said:
But... so long as there's no pre-ignition, then higher octane fuel will make zero difference.
Indeed but there's no disadvantage in a higher octane rating, even then (assuming the higher octane rating isn't created at the expense of actual energy content). My point was that there is a clear goal for spark ignition fuel - you simply want as much knock resistance as you can get; there may be a point past which it's not particularly useful, but it's never a bad thing. There is no clear "best" Cetane rating for diesel fuel so there's no reason that "super" diesel would have a higher or lower Cetane rating than normal diesel. If 200RON petrol was available at the same price as 95RON, manufacturers would be designing engines to make use of its (presumably running 30:1 compression ratios or something).
Edited by kambites on Friday 9th December 09:50
kambites said:
Indeed but there's no disadvantage in a higher octane rating
...cost aside.kmabites said:
My point was that there is a clear goal for spark ignition fuel - you simply want as much knock resistance as practical.
Kinda. You need just enough knock resistance for the engine's setup.kambites said:
TooMany2cvs said:
Kinda. You need just enough knock resistance for the engine's setup.
Yes but you design the engine around the fuel that's readily available, not vice versa. TooMany2cvs said:
kambites said:
Indeed but there's no disadvantage in a higher octane rating
...cost aside.kmabites said:
My point was that there is a clear goal for spark ignition fuel - you simply want as much knock resistance as practical.
Kinda. You need just enough knock resistance for the engine's setup.kambites said:
Indeed but there's no disadvantage in a higher octane rating, even then (assuming the higher octane rating isn't created at the expense of actual energy content). My point was that there is a clear goal for spark ignition fuel - you simply want as much knock resistance as you can get; there may be a point past which it's not particularly useful, but it's never a bad thing. There is no clear "best" Cetane rating for diesel fuel so there's no reason that "super" diesel would have a higher or lower Cetane rating than normal diesel.
If 200RON petrol was available at the same price as 95RON, manufacturers would be designing engines to make use of its (presumably running 30:1 compression ratios or something).
Higher octane fuel burns slower so there is a trade off its not a case of more is better If 200RON petrol was available at the same price as 95RON, manufacturers would be designing engines to make use of its (presumably running 30:1 compression ratios or something).
Edited by kambites on Friday 9th December 09:50
liner33 said:
Higher octane fuel burns slower so there is a trade off its not a case of more is better
Sort of. Fuel with more Octane in it burns slower; fuel with a higher Octane rating does not necessarily. As with Cetane rating, Octane rating actually has nothing directly to do with the chemical it's named after; it just happens to be what is generally used to control it but there are other ways which don't have the same side-effects. Flame-front speed and knock resistance are not fundamentally related to each other (although of course if the flash-point is higher than the combustion temperature the fuel wont manage to burn at all).
TooMany2cvs said:
I was coming more from the position that the engine's already designed, manufactured, and fitted into your car - and you're figuring out which pump to reach for.
Oh yes I agree entirely. I wasn't trying to claim there was any point in buying higher octane fuel for a car not designed to use it and indeed I run my Elise on 95RON. I was purely trying to understand what theoretical benefits "super" diesel could provide to instantaneous engine performance (as opposed to longevity). I understand what super-unleaded is and why, in the right circumstances, it's better (and why in the wrong circumstances, it's not). I do not have the same understanding for super diesel.
Edited by kambites on Friday 9th December 10:21
TooMany2cvs said:
Likewise. Seems to be 100% marketing guff...
. So let's try the same with diesel, where nobody benefits!"
No.. So let's try the same with diesel, where nobody benefits!"
My car runs like a sack of s
te on regular.As I said in my earlier reply, so much so that when I couldn't get any & used regular I stopped the car after a couple of miles to check my receipt was for Diesel & I hadn't misfuelled.
Jim AK said:
TooMany2cvs said:
Likewise. Seems to be 100% marketing guff...
. So let's try the same with diesel, where nobody benefits!"
No.. So let's try the same with diesel, where nobody benefits!"
My car runs like a sack of s
te on regular.As I said in my earlier reply, so much so that when I couldn't get any & used regular I stopped the car after a couple of miles to check my receipt was for Diesel & I hadn't misfuelled.
So what car is this that won't run on ordinary diesel? And can you explain what and why this difference happens?
TooMany2cvs said:
Sounds more like you got some bad diesel in that fill.
So what car is this that won't run on ordinary diesel? And can you explain what and why this difference happens?
Just a Merc 220. Ive used it from new & now on 107k as a friend, who's current 220 is on over 300k, adviised it because his showed an improvement in economy over standard fuel & was smoother running.So what car is this that won't run on ordinary diesel? And can you explain what and why this difference happens?
Gassing Station | General Gassing | Top of Page | What's New | My Stuff