john banks

Conventional wisdom (which is often wrong ) suggests that on a turbocharged car MABT is beyond the knock point hence the lust for octane (but not a pi$$ on your fireworks octane "booster"?)

[Edited by john banks - 7/2/2002 8:45:06 PM]

Pavlo

Evilbevil,

I just used arbitary numbers to give and idea, but yes, as the inlet charge is moving at speed, throught a (slight) resistance there will be a pressure drop between the cylinder and the manifold, fact of life I am afraid.

Andy,

I will have to have another look at my notes, but I pretty sure in thinking that extra power comes from doing things by raising compression, boost, air density, correct *combustion* timing and reducing inlet polution etc.

Even octance boosters should be measured in terms of resistance to knock under increasing compression, that's how it's defined. Increasing advance not only changes the timing of the burn, but does reduce the burn angle, so increased flame speed, possibly back to where it was before.

If you can run more advance, such that the combustion takes place at the best time relative to the motion of the pistion, then that should add power, rather than power coming from a change in flame speed.

If ignition happens too soon, it could be all over before the pistion reaches TDC, and then you might stop the engine in an extreme case, logically enough. Hence there being a sweet spot.

I need to have a look at my notes, and it's been a while....

Paul

[Edited by Pavlo - 7/3/2002 9:22:52 AM]

WREXY

Quote by ANDY F, from the topic Turbo Timers...Are they worth while??

OK Let's do it.

Are they worth the cash then?

Cheers,

Wrexy.

john banks

I have knock correction when I use SUL. No knock correction with SUL + OB. My map is more advanced knowing that I use SUL. Will see how Optimax goes now it is available.

john banks

But then Andy doesn't use a knock sensor

Pavlo

My car definitely runs better, faster, smoother, further on just Shell optimax (98.6).

As for home brew, you may want to try the following additives, all available from your friendly local paint merchant.

Acetone
Toulene
Benzine

check out the following link for more info. Acetone is one of the safest, easiet and cheapest.
http://www.apba-racing.com/Forms/Fue...0ADDITIVES.pdf

Have fun...

Paul

Andy.F

Aah but does it produce any more power Dr Banks than the 'correct' advance for SUL ???

I kid you not when I say that my AP22 (g meter) has recorded the best acceleration when using no octane booster. My top 3 (out of 24) power curves were all with shell SUL, no further additives.
I thought it strange at the time but said nothing as I had attracted a 'mad man' tag already without saying "run your std ecu and injectors at 1.5 bar and don't use any octane booster"

Yes, there was slightly less activity on the Knocklink sensor with NF but certainly no more power, possibly even less

Yes it did run smoother, I assume that's because the combustion chamber, valves etc were a bit cleaner.

I can see how it 'appears' to work though, by slowing down the flame front it delays the combustion pressure rise, this can decrease knock if the ignition is too far advanced. It is effectivly retarding your timing for you by a few degrees.
It even says on the bottle that you may need to advance timing to benefit from this product

As my friend Dr Banks has enlightened us, if the ECU detects knock at one point in the rev range, then it can retard the timing in other ranges. This will make the car feel more responsive with the additive than it was without.

Mapping the car to raw SUL is likely IMHO to give the same result.

There is a very interesting thread running on the GTR forum just now, discussing this very topic. It also has some 'expert' input from a chemist type who's into performance cars.

I'll go find the link and attach it for those interested

I get the feeling this could be a long one ...............

Andy.F

GTR forum Octane booster thread

john banks

I don't have an AP22 but I have to say I can't tell a difference in acceleration times that I logged with the same map on SUL compared with SUL+NF, but the knock correction is less and it runs a bit smoother. Not much in it though, and I will be the first to admit that I can only tell from the butt dyno if the ECU takes away 4 or more degrees, and the diff between NF and SUL is up to 2 degrees. Note Andy as well though that the knock correction on the later cars is more dynamic, and it can actually advance - shock horror - I mapped T-uk's car conservatively on the timing, and around Knockhill it was ADDING +1 degree timing for a lot of the time. Which given the way it is driven...

john banks

The mantra we all get is: If the maximum torque on a turbocharged engine is almost invariably produced at the knock threshold then increasing the octane along with advancing to the knock threshold should give you more torque. But as Andy points out the higher octane fuel effectively retards the flame front for you so how do you gain from higher octane? - it seems that by sparking earlier you do so just because the burn is smoother. There is a factor I am missing here...

Andy.F

I think the point the guy on GTR was making was that although combustion is slightly slower and therefore can be started earlier, there is no difference in the gross energy output of the fuel.
Agree with John that on a properly tuned engine, the closer you come to the knock point then the greater the BMEP and hence torque.
So, if you slow combustion, you travel away from the knock point. Advance the timing and you get close again.....but are you just back where you started with BMEP ?

On my car I do not have dynamic knock correction, that probably explains why i don't gain (and possibly lose ) power.

On the later models, do you gain power or just ignition advance ???

Where's all our S/net experts when you need em ?

Andy

Pavlo

I don't know if anyone has mentioned how RON and MON are calculated, and what they actually mean? It may give an idea to how increased octane can increase power.

Pure Octane (hydrocarbon that is) = 100 octane
This is the base line, but we aren't likely to get pure octane in our tanks.

RON = Research octane number, is measured for a fuel using a variable compression motor, at low load run at 500rpm. Basically, a fuel with 100RON gives the same resistance to knock as pure Octane, 104 RON would give 4% better knock resistance, and I think this means higher compression at the onset of knock.

MON = Motor octance number, typically 89 for unleaded pump fuel. Measured with the same variable compression motor, run at a higher load, and 800rpm. I think that pure octance has a MON of 100 too, and again, fuels are benchmarked against it.

So running higher octane in general should allow higher boost as the same advance and rpm. Also worth noting that MON should give a better indication of how a fuel will perform in our application of interest, ie highload, high speed.

What the Octane doesn't indicate is the calorific value of the fuel, in other words, just how much energy can be released upon buring a unit of fuel. It also doesn't indicate what AFR you need for Stochiometric combustion (that being complete burning of fuel, with no excess oxygen)

In a normally aspirated engine (just to make it simpler) you can run Methanol. Methanol has a calorific value just over half that of petrol. Fortunately you need about twice as much to achieve stochiometric combustion. Net result is about the same. But the latent heat of evaporation means a massive cooling of the charge, something like 55'C on complete evap, this contributes to a high octane value that allows CR in the region of 18:1, but as high as 22:1 in a high load engine. It's this ultra high CR, and cool dense air that leads to high HP. In this case, ignition advance is adjusted to suit the speed of combustion, but doesn't have a major part in the HP increase.

Methanol is a bit extreme, but petrol with additives should give the same effects to a lesser degree.

I'm sure there are some inaccuracies in the above, but I know the general points are correct.

Anyone else?

Paul

nom

Well, not too sure about the rest of things, but it's interesting to see a full explanation of RON/MON for a change!

It does look like, in simplish terms (technically, anyway, rather than practically ) that octane booster is a means to an end rather than a means in itself - chucking it into a tank so raising the RON doesn't have any effect on a 'standard' car unless that car needed the higher octane in the first place, even if it can advance the timing (assuming that this car doesn't suffer any knocking).

However, as high boost produces knocking (or anything other mod that increases knocking), a higher RON fuel allows a higher boost level. i.e., the higher RON does nothing aside from avoid a problem - there's no actual power gain from adding the RON in itself.

This would mean (maybe )that other methods could be used to the same end - John has noted that there can be a lot more advance with a tubular manifold. So would it be possible that on a car that needs 100RON (because of the ECU setup) changing the manifold would mean that 97RON (or whatever) could be used without octane booster, as a permanent solution? Obviously there are other side-effects with this particular mod, but it was just an example of my probably idiotic understanding of what's going on here

john banks

I have not fitted the manifold yet - waiting for VF23, myself, Andy and T-uk all to be in one place

If you fit a manifold to a tuned engine (esp a speed density mapped one) it might det like hell because it would go lean. Nice idea though

There are too many factors to control when discussing any of these things and my brain hurts. Back to the sore throats... (in "summer" too )

[Edited by john banks - 7/2/2002 2:48:38 PM]

[Edited by john banks - 7/2/2002 2:49:06 PM]

Pavlo

Seems logically enough to me. Any mod that increases breathing, should mean less contamination of the inlet charge from the old exhaust gases. If you end up with more new charge, less old gas, you should get a cooler "mixture" in the cylinder prior to ignition. This combined with a little more petrol to evapourate off the piston crown means lower piston temps.

I am not sure how piston crown temperature affects knock, unless it was hot enough to pre-ignite the compressed inlet charge? But lower piston crown temps will mean less chance of melting the piston, obviously, which has to be nice for the engine.

Paul

nom

Summer? In Scotland?
heh-heh

TopCat

Andy.F

Pavlo, nom & JB I agree with all your comments/descriptions of octane.........but, not being a scientist I'm guessing a bit here -

Assuming we are talking the same type of CH chain fuel ie Petrol as we know it, the fuels will all have the same flame front speed ? Yes ? So each fuels Octane value is based on its ability to burn 'controllably' under increasing pressure/temperature conditions. If you introduce a compound which reduces the flame speed, you can 'fool' the test rig into measuring a higher octane (i think)
The normal benefit of a high octane fuel is that you can use higher pressures and retain control of combustion, this allows production of more power.
Some of the specialist fuels for high rpm use have increased flame front speed and higher octane values. Slowing the flame is not a power enhancing way of increasing octane. All IMHO - of course !

Plantie

lol @ Moe

Pavlo

Basically, if something is of a higher octane you can do the following:

If it burns slower, add advance to give same power.

Add yet more advance, to get more power than stand OR raise compression with same advance OR raise boost without a penalty of knock.

Octane is C8H18.

Flame speed can be represented thus;

Laminar flame speed Ul
Turbulent flame speed Ut
Actual/Measured flame speed Uf

Ul is measured in a 'bomb' aparatus under laminar (quiescent) conditions. Ul is a function of fuel type, oxidant, AFR, temperature, pressure etc. It is increased as temperature is increase and is at a maximum about 10-20% richer than stoch.

Ut is the flame propagation rate (flame front speed through the unburnt mixture, and relative to it) under turbulent conditions. Ut is higher then Ul;
Ul = some function(Ul, u') where u' is the turbulence intensity.

Uf, the measured flame speed is the rate at which the flam front moves relative to a fixed point. Uf = Ut plus any movement of the bulk fluid.

For a given engine speed, the flame speed ratio (FSR = Ut/Ul ad is directly proportional to engine speed.

FSR us a function of flame radius. Initial the flame propogation is at the laminar speed, then mainly at Ut, reducing as flame approaches cylinder walls.

I could go on....

To sumarise for our needs, things that reduce the total crank rotation required to burn charge, FTA (flame travel angle):

Ignition advance increases delay angle (ramp up to steady burn), and reduces the burn angle. There will be a sweet spot, know as MBT, minimum advance for best torque, but you may get knock before this (so run on the knock limit, easy to find) or you may find knock some point beyond this (too much advance for best torque, had to find without a dyno).

Contamination of inlet charge with exhaust gas residue, water or other steadily increases FTA.

Increased compression ration will reduce FTA with diminishing returns, up until point of knock.

Then you get chamber shape, swirl, tumble, general turbulence.

And before you know it you've opened a big can of worms.

Paul

Pavlo

I would think that's about right. On a turbo car you need to look at dynamic compression ratio. If you are running 8:1 NA, and you then ram another (say) 0.8 bar of pressure (about right at the cylinder), you are looking at (simplistically speaking) 14.4:1 compression, ouch.

That's gonna bring the knock point so close you can taste it.

More octane please sir.

Paul

john banks

Or in Andy's case a 20:1 compression ratio

dingy

Just for the record.

Most OB use the points system, 10 points to a ron so they will appear to do nothing.

NF is very different.

More adavance than ELF 103 RACE fuel.

john banks

Can we have a bit more "meat" to it though Dingy?

EvilBevel

and you then ram another (say) 0.8 bar of pressure

Pavlo, just curious, do we somewhere lose pressure between manifold & cylinder ?

John, there's sales talk & technical talk. Never the twain shall meet

Andy.F

Nice one Pavlo wait till Moe reads that Am I correct in saying that if an engine is running on the (safe) edge of knock, then addition of a flame speed retarder such as some octane boosters or water, would reduce the power output ?

Pavlo

Any more?

z_chromozone

http://www.scoobynet.co.uk/bbs/thread.asp?ThreadID=130443&ForumName=General


You may be interested in this octane boosting thread too.

Z

z_chromozone

Oooops, posted that on the wrong thread. That'll be cos I had too many browsers open.