DS3 DSport THP155 - Changing to higher octane fuel

[THP]

Only difference I noticed when I started using V Power is that with standard fuel, I'd get a bit of smoke (very light and nothing serious but I can't think of a better word to describe it) on hard acceleration in lower gears which stopped with the better fuel. The car doesn't feel any different at all.

[lfcrule1972]

Now I don't do thousands of miles a year (well not more than 20k anyway) I've been using the V Power Nitro diesel fuel, there may be a small performance difference (probably imagined though) but I'm hoping it will keep the engine cleaner than standard fuels....

That said my previous cars all ran fine on Tesco fuels (petrol and diesel)

[DannyDS3DS]

Only difference I noticed when I started using V Power is that with standard fuel, I'd get a bit of smoke (very light and nothing serious but I can't think of a better word to describe it) on hard acceleration in lower gears which stopped with the better fuel. The car doesn't feel any different at all.

Yeah, my THP 155 is getting a bit of smoke using standard unleaded fuel at the moment. I noticed it when stopped in traffic and my brother also mentioned it when he was driving behind me. Someone mentioned online it could be down to the turbo burning a bit oil? Not sure how serious this is or not though. It's just whitish smoke, not thick clouds of it or anything though.

[Czar]

The carbon buildup is created from oil-vapours - it has nothing to do with the fuel's octane rating in a direct-injection engine. The fuel is injected directly into the cylinder - the intake air and oil-vapour have already passed (and deposited on...) the valves by the time they first meet petrol. It's for this reason that in-fuel-tank cleaning products won't do anything to help.
On an older engine design, there is arguably a case to suggest that "better" fuel could do a better job of cleaning the valves as it washed over them - but we don't have any fuel washing over our valves.

Actually the octane rating of the fuel does have a direct effect on the initial carbon deposit build up, not only in the combustion chamber, piston crown, ring lands and rings, but also on the backs of the inlet valves too, how I hear you ask, very simply, the cleaner the burn in the combustion chamber from the higher octane rating of the fuel, the hotter the flame, resulting in a more efficíent clean burn.

Let's now delve a little deeper, taking into account that most if not all of you reading this, understand that the GDI (gasoline direct injection) engine, does not inject fuel in the same way as regular port injection, so given that the GDI engine has dry inlet port tract runners, which are subject to the PCV suspended oil particle vapour, we all know this vapour is the direct result of heavy sticky gummy carbon residue on the backs of the inlet valves.

Let's focus on the words sticky and gummy, the PCV vapour is just that sticky and gummy, that said, how can increasing the octane rating of the fuel help combat the issue of carbon deposit build up on the backs of the inlet valves, here's how.

Low octane fuel burns at a lower temperature than high octane fuel, during the combustion cycle we encounter a point where both the inlet valves and exhaust valves are open at the same time, this is known as overlap, it's at this point where a poor combustion burn (low octane rated fuel) is less efficient at cleaning the combustion chamber of the suspended oil particles, which enter the combustion chamber from the inlet port tracts, contaminated from the PCV, furthermore, the velocity of the inlet air, adds to the inlet air contamination from the sticky gummy oil residue on the backs of the inlet valves, which are drawn into the combustion chamber during the induction stroke of the 4 stroke cycle.

If your still following this, good, let's carry on, the less efficient burn, can't generate enough combustion heat to dry the PCV oil vapour on the backs of the inlet valves, you see, during the valve overlap period, the combustion flame not only exits the exhaust valves, it also momenterally tries to exit past the inlet valves too, during the valve overlap period, this is why it's very very important to maintain high exhaust gas velocity, so for those of you who think bigger exhausts are best, this is not the case, the larger the diameter exhaust tubing, the slower the exhaust gas velocity, we will discuss that in another thread.

The higher the octane rated fuel, the cleaner the combustion burn, generating a hotter combustion flame, the extra heat generated plays a vital role, not only does it burn most of the combustion chamber contaminants, which won't add to the build up of carbon deposit on the backs of the inlet valves during the overlap period, the extra heat hardens the once sticky gummy PCV vapour, on the backs of the inlet valves

Furthermore the increase in combustion heat adds to exhaust gas velocity too, remember the exhaust tube diameter theory !!

Hope this has shed some light on why running the higher octane fuels, help combat carbon deposit build up.

[THP]

[Nomgle]

Hope this has shed some light on why running the higher octane fuels, help combat carbon deposit build up.

That's an excellent explanation - I hadn't considered Octane with regard to engine temperatures.
Another way to put it, is that higher Octane fuel takes longer to burn. And the longer the burn occurs, the more of the heat can transfer within the chamber.

There's a great article on Road And Track from a few years ago if anyone else is interested - http://www.roadandtrack.com/new-cars...-fuel-futures/

(It's also interesting to note that fuel in the USA is generally of a lower Octane - most BMW MINI owners over there won't even have the option of using 98RON fuel in their THP engines...)

[Superfly Guy]

Those more knowledgeable may suggest otherwise.

Not one to often quote myself but........................