Super unleaded question

Yes. AFAIK super unleaded contains better antiknock ingredients not better petrol. It won't burn better.

And this is surprising because?

Malc ( snipped-for-privacy@ubht.swest.nhs.uk) gurgled happily, sounding much like they were saying :

Not necessarily.

It may be that the car has a knock sensor and a base map set high enough to allow the ignition to be advanced on 98RON, giving more power.

Most knock sensor base maps are set at the 95RON values, though, so will only retard if pinging is detected, not advance in it's absence.

In message , Malc writes

In engines designed to take advantage of higher octane fuel, you'll get better performance / economy. In engines not so designed, (i.e. most of them) you won't.

Anyone else read that as 'butter'?

Willy Eckerslyke (oss108no snipped-for-privacy@bangor.ac.uk) gurgled happily, sounding much like they were saying :

I can't believe I didn't.

That is because your speed is determined by the position of the accelerator pedal, not on the number indicated by the odometer.

Christian.

Excuse the ignorance, but what exactly is knock?

Tom

Spark plug ignites petrol while piston still on way up on compression stroke. Noise is your big ends having a hard time. Ignition too far advanced. Cars with knock sensors adjust the timing so the engine is *just* on the point of knocking, so always giving max power for a given set of conditions.

I thought that 'knock' is when the mixture self ignites before the spark plug gives off the spark.

I'd just call that a cause of knocking.

Knocking is when the mixture ignites so early that it causes significant amounts of the energy to push down on the cylinder when it is still on the compression stroke.

The cause could be:

(a) Lower octane fuel than required. Low octane fuel burns more rapidly, so can push back on the piston when a high octane fuel hasn't really started burning yet, giving the piston time to get to the top.

(b) Bad ignition timing. The spark occurs too early. For maximum efficiency, the spark is timed to go off before the piston hits the top, as it takes some time for good quality fuel to start expanding, and you want it to do this before the piston has disappeared back down the cylinder. The aim is to light it as early as possible, but not to actually knock.

(c) Pre-ignition. The mixture explodes without a spark, possibly due to hot carbon deposits.

Christian.

The message from "Slurp" contains these words:

That always happens. Knock is when the fuel goes BANG instead of burning smoothly.

The message from Johannes contains these words:

No, that's preignition, or Peter's Pinking as it's known.

It's detonation of the mixture in the cylinder rather than a burn of the mixture. Tends to knock holes in things. The American name for it is pinging which is a far better description of what it sounds like IMHO.

Can be caused by an excessivly hot engine, an excessivly advanced spark, too high a compression ratio or too low a grade of fuel (octane number)

"Knock" is something of a mixture of all the things people have said :-)

In normal combustion, the spark plug lights the mixture and the flame burns smoothly out from there to the edges of the cylinder. This gives a nice smooth release of heat and increase of pressure.

In "knocking" combustion, the rising temperature and pressure due to the flame causes some of the mixture to ignite spontaneously, BEFORE the flame gets to it. As a result you have a sudden very rapid release of energy, which in turn causes a very rapid increase in temperature and pressure. It is this sudden pressure and temperature rise that can cause damage to pistons, head gaskets, etc.

"Knock" is much more likely to occur when the ignition is more advanced, since this causes higher pressure and temperatures. If you light the mixture late enough then the descending piston causes the pressure to fall at the same time as the flame would tend to cause it to rise, thus lowering peak temperatures. But the efficiency is generally less (less work output per unit of fuel burnt) which is why you need to advance the spark as much as you can without causing knock.

I would have to disagree with Christian's Point A ("Low octane fuel burns more rapidly") - my understanding is that the burn rate is similar with all fuels (at least in the "normal" octane range), it's just that high octane petrol is less likely to spontaneously ignite before the flame arrives.

To get back to the OP's question: you might get better MPG with super unleaded, but the benefit will be tiny. Most of the time you are driving at part throttle, where knock is very unlikely and so the ignition is set optimum with both 95 and 98 RON fuel. You are however likely (though certainly not guaranteed) to get more power with 98 RON.

If the timing isn't adjusted where does the extra power come from?

Because on most modern engines the ignition timing IS adjusted when using different octane fuel, thanks to the knock sensor.

The ignition may be optimised for 98 RON, in which case it will always be heavily retarded by the knock sensor when running 95 RON. Or it may be optimised for 95 RON, in which case you will probably still see some change as there will be a little bit of knock and subsequent retard with 95 RON, which would be eliminated if using 98 RON

That was my understanding too, like in the old days with four star and two star. It wasn't the burny bits of petrol that differ, it's the additives that prevent pre-ignition that differ, shirley.

I zeee. I thought that was the same thing. Some performance cars have knock sensors for detecting pre-ignition.

The message from Johannes contains these words:

No, they have knock sensors for detecting knock!