LRP VS Unleaded petrol

Gotta love this place! Keep it up guys, very entertaining & informative.:thumb

well i kind of meant what people usually refer to as "fast burn" which are chambers that are typically have good squish and/or swirl, i.e. not semi hemi. The sort of chambers that usually are suited to high CR and less total advance. Kind of a more modern type of chamber.

sleepe3 wrote:Would someone care to enlighten me what the "resistance to decomposition" is?

Sure.

As the fuel is lit by the spark, the flame front races across the cylinder, increasing the them and pressure as it goes. In the few milliseconds that this process takes, the remaining (not yet burnt) fuel gets heated and pressurised and it starts to break down chemically - decompose. Some of these decomposition products are prone to auto-iginition - they will light up just from the temperature and pressure, so you get the reamining fuel burning before the flame front reaches them, resulting in an uncontrolled burn of the remaining fuel, which produces detonation (an opposing pressure wave in the cylinder and a "hammer blow" to the piston).

Resistance to decomposition depends both on the hydrocarbons in the fuel, and any additives in the fuel. The aromatic compounds like Benzine have very high natural octane numbers (they are chemically stable at high temps). But Benzine compounds are carcenogenic so fuel companies are trying to reduce the amount of these compounds in fuels, which tends to lower the natural octane number.

The additive Tetra Ethyl Lead interferes with the actual decomposition of the fuel - helping it remain "intact" as the flame front approaches, so it burns smoothly. In other words, the fuel does not decompose so readily and retains it's original chemical structure until the flame front reaches it. TEL operates mainly at the high temperature end of the fuel-burn cycle - above about 500c (the temp at the end of the burn cycle reaches about 2000c)

MTBE which is used in the USA, works differently. It reacts at lower temps (around 400-500c) with the fuel to create new molecules which as less prone to auto-ignition. The effect is the same - the fuel is "resistance to decomposition" in this case through changes to the chemical structrue of the fuel itself. This is the case with other oxygenates too - TAME, ETBE and alcohol.

To summarize - the lead used in the past helped prevent the fuel decomposing before the flame front reached it, and MTBE (and other oxygenates) change the chemical structure of the decomposing fuel so that fewer auto-ignition products are created.

The measurement of this resistance to decomposition is the octane number

The octane number is a comparison between the detonation resistance of Heptane and Iso-octane - heptane is zero and iso-octane is 100. So a 91 octane fuel has the same detonation resistance as 91% Iso-octane ad 9% heptane. These two compounds are use as they are far apart in detonation resistance but otherwise physically similar - similar boiling point, molecular weight and so on, very important when creating a "reference" fuel.

These days we also have octane ratings higher than 100 - they do this by extending the scale previously used - for example pure alcohol (ethanol) has an octane rating of over 120 when compared to the 100% iso octane.

A high octane fuel is more resistance to decomposition (into auto-ignition compounds) than a low octane fuel.

Nothing to do with burn speads, as some would argue.

Has anyone tried a PULP / nitromethane mix, if so what percentage of nitro did you use?