UK Karting - Notice Board (2-Stroke Karting) : Re: Fuel debate, where is the advantage?

Notice Board (2-Stroke Karting)

Re: Fuel debate, where is the advantage?
Posted by 'ChrisAdams' on 29 Sep 2008 @ 01:22

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ChrisAdamsJoined: August 2001
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There is always a common missconeception between various fuel additives. The additives tested on 5th gear showing a 2% decrease (such as RedEx, and STP fuel treatment) are not designed to 'increase' power, they are designed more along the lines of reducing power lost due to a heavy build up of carbon within the combustion chamber.

The performance enhancing additives (with the exception of Nitrus) increase the octane rating of the fuel. However increasing the octane rating does not result in a bigger bang within the combustion chambre, infact it actually results in the opposite effect. A fuel is given an octane rating relating to its 'resistance to burning', the higher the octane number, the more resistant the fuel is to ignition.

At this point it is necisarry to look at what happens during combustion; the piston draws in the air and fuel mixture, the piston then compresses the mixture and the spark plug ignites the mixture sending the piston back down the bore. With ignition the key question is when to ignite that air/fuel mixture bearing in mind the ignition occurs 80-100 times per second and the air/fuel mixture takes time to burn.

During ignition the spark plug sparks igniting the mixture. The flame then propogates down and across the combustion chamber increasing the pressure that pushes the piston down the cylinder until the air/fuel mixture has been burnt. As the piston nears bottom dead centre the exhaust valve then opens and releases the burnt gasses.

The key to ignition timing is to achieve a peak cylinder pressure after top dead centre (achieving a peak cylinder pressure at top dead centre would just be testing the crank bearings, big & little ends and squashing the con-rod) and time taken for combustion to occur results in the spark plug having to fire before top dead centre (this is called advancing the ignition).

The more advance that can be achieved (up to the point that you would be shooting the piston back down the bore before it had reached top dead centre) generally the higher the torque output of the engine because the flame is given more time to propogate, therefore more time to expand thus the greater the magnitude of the peak cylinder pressure. This pressure is transfered to the reciprocating motion of the pison which through the con-rod is transfered to rotary motion at the crank shaft. The higher the in cylinder pressure, the greater the torque at the crankshaft.

This is the point where octane comes into play, when advancing an ignition we want the fuel to burn throughout the duration of the combustion stroke at an even rate. Given a fuel with a low octane (low resistance to burning) as soon as the spark plug ignites the air/fuel mixture, the majority of the mixture will be burnt at the begining of the combustion stroke, leaving only the inertia of the flywheel to propel the engine through the rest of the combustion stroke.

A second issue is encountered with a fuel that has a low resistance to burning when advancing the ignition in the form of knocking (sometimes refered to as pinking) and auto ignition. This is where the flame does not propogate proportionally through the cylinder resulting in erratic cylinder pressures - resulting in erratic torque outputs and high temperatures resulting in melted pistons, valves and cylinder heads.

Therefore with the low octane fuel only a small amount of ignition advance can be used limiting the torque output potential of the engine.

In short, increasing the octane rating increases the scope for ignition advance. Thus when the ignition is advanced you can obtain increased torque outputs at the crankshaft. Adding the additive on its own will not make any difference whatsoever to the engines performance, unless you were using a substandard fuel in the first place.

Hope this helps (and makes sense - it is rather late!)

Chris Adams

Message Thread:

Fuel debate, where is the advantage? by 'RoadRat' (28 Sep 2008 @ 17:45)

Re: Fuel debate, where is the advantage? by 'QuickOldTimer' (28 Sep 2008 @ 18:38)

Re: Fuel debate, where is the advantage? by 'Stig' (28 Sep 2008 @ 22:01)

Re: Fuel debate, where is the advantage? by 'wildcard' (28 Sep 2008 @ 22:06)

Re: Fuel debate, where is the advantage? by 'RoadRat' (28 Sep 2008 @ 22:09)

Re: Fuel debate, where is the advantage? by 'davidmc' (28 Sep 2008 @ 23:09)

Re: Fuel debate, where is the advantage? by 'ChrisAdams' << You are here!

Beautifully explained, Chris! by 'itpro' (29 Sep 2008 @ 15:10)

Re: Fuel debate, where is the advantage? by 'DavyBoy' (29 Sep 2008 @ 8:23)

1980's and methanol! by 'SteveC' (29 Sep 2008 @ 11:43)

Re: Fuel debate, where is the advantage? by 'NJB' (29 Sep 2008 @ 13:20)

'He' says he wasn't and didn't know! by 'SteveC' (29 Sep 2008 @ 13:34)

Re: 'He' says he wasn't and didn't know! by 'NJB' (29 Sep 2008 @ 16:35)

It makes the difference of about the same as a.... by 'SteveC' (29 Sep 2008 @ 11:38)

Re: It makes the difference of about the same as a by 'davestimpo' (29 Sep 2008 @ 13:49)

Re: It makes the difference of about the same as a by 'Paulhooper45' (29 Sep 2008 @ 16:49)

Re: It makes the difference of about the same as a by 'davidmc' (29 Sep 2008 @ 17:48)

Re: It makes the difference of about the same as a by 'Pop' (29 Sep 2008 @ 22:24)

Re: It makes the difference of about the same as a by 'Pop' (29 Sep 2008 @ 22:28)

Re: It makes the difference of about the same as a by 'neilaustin' (29 Sep 2008 @ 22:44)

Nitro methane by 'bt3962' (30 Sep 2008 @ 4:06)

There are a number of such chemicals ..... by 'itpro' (30 Sep 2008 @ 10:36)

I'm totally with Ian! by 'SteveC' (30 Sep 2008 @ 11:23)

Re: I'm totally with Ian! by 'BanzaiBoy' (30 Sep 2008 @ 13:58)

Re: I'm totally with Ian! by 'RoadRat' (30 Sep 2008 @ 16:44)

Re:Roadrat - Give up and by 'John' (30 Sep 2008 @ 18:15)

Re:Roadrat - Give up and by 'RoadRat' (30 Sep 2008 @ 18:31)

Re:Roadrat - Give up and by 'QuickOldTimer' (30 Sep 2008 @ 20:01)

Re:Roadrat - Give up and by 'BanzaiBoy' (30 Sep 2008 @ 20:22)

Re:Roadrat - Give up and by 'philharrison' (01 Oct 2008 @ 0:36)

Re:Roadrat - Give up and by 'RoadRat' (01 Oct 2008 @ 19:52)

Mrs Savage can't find her chip fat! by 'SteveC' (03 Oct 2008 @ 11:46)

Re: John would never cheat ? by 'John' (03 Oct 2008 @ 13:39)

Ah but did u do it on purpose? by 'SteveC' (03 Oct 2008 @ 17:06)

Re: Ah but did u do it on purpose? by 'chriskasch' (03 Oct 2008 @ 17:22)

Re: Ah but did u do it on purpose? by 'John' (03 Oct 2008 @ 19:26)

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