'Volatile' means more likely to evaporate. Does that mean that it is more likely to pre-ignite when subjected to compression and a spark ?

Whatever it is with modern fuel, you don't see Amals covered in that sticky brown gunge anymore so any evaporation is much more complete than it used to be, even if we're comparing modern 98 octane with 2-star from the 1970s...so as far as I can tell, despite being higher octane, it is, as a whole, more 'volatile'

Modern fuel evaporates like lighter fuel.

Well, I've burnt high octane, low octane and the stuff in between, I haven't noticed any difference in any of them, the thing just starts up, runs like a charm and goes like a dream, so unless it does something beneficial to engine itself, I'll pass on putting anything in the petrol.

First thing to remember is only gasses burn so in order to burn the liquid has to become a gas which is why you have to preheat kerro ( parrafin to some ) blow lamps, lamps & stoves.
Mixing petrol & deisel 50:50 will case the petrol to burn off and in doing so vapourises the deisel which burns slower.

From high school chemistry.
The finer the particle the faster the reaction.
This applies to molecules burning, the smaller the molecule the faster it burns
Diesel is a big molecule typically C 12 H 26 although it varies, light diesel can be as small as C 7 H 16.
Heavier diesel burn a lot slower than petrol of which the biggest molecule is C 8 H 18 ( octane )

Because diesel burns slower and colder than petrol so is more thermodynamically efficient .

Now while deisel & petrol are both mixtures of Carbon & Hydrogen their specific energys are roughly the same, however because diesel is denser than than petrol, you get more energy per unit volume from diesel.

Your M20 gets its fuel by volumes as metered by the carby jets thus by adding some diesel you are getting more energy into the cylinder so providing it all burns you get more power out.

And remember that neither petrol nor diesel are uniform liquids, both are complex mixtures of a variety of hydrocarbon molecules.

Long stroke engines benefit from slower burning so the volume inside the cylinder continues to increase all the way to the end of the power stroke where as short strokes benefit from faster burning fuels

email (option): bsansw1@tpg.com.au

Hi Trevor

I was reading this all on my phone, from the top down, and I'd got to the stage of thinking "I bet Trevor will have something to say"... then I skipped to the end and behold, you had!

This topic is causing me some thought at present with planning my salt bike. I've been looking around at information on the web and trying to see my way through what makes sense and what doesn't. You input here will be added to the knowledge.

One thing is being clear, like you are here, on the terms and processes, i.e. what's happening and why, or at least what are the options for why.

This link was handy in pointing out the differences between detonation (post ignition from spark) and pre-ignition (before spark). It's just a handy reminder and helps to consider the factors, i.e. if the engine is just too hot you have lots of trouble, vs how the combustion chamber shape and burn rate impact knocking.

http://www.contactmagazine.com/Issue54/EngineBasics.html

Elsewhere on the web there's a lot of talk / discussion of 'fast burn' head design, none of which leads to anything as to what the characteristics of that are. Your points here about slowing down the burn rate reminded me of that topic and puzzling around the excepted wisdom of 7:1 limit on side-valves and what caused that, which I guess must relate to the burn rate, i.e. the valves are at the side, therefore the combustion chamber will always have a greater cross-sectional area (horizontal to the piston face) than an overhead valve engine so without some other magic you will reach the point of the mixture starting to burn in the far corner before the flame front has progressed to that corner.

So I guess at a given configuration (say standard) of our beloved M20 engine some diesel in the petrol might be handy, as we get a slower burn and we've a paltry compression ratio, but any tuning of the engine you'd not want to slow the burn down?

There's always so much to try to understand / learn!

email (option): dickie.bobbie@hotmail.co.uk

The problem comes with modern over square engine technology being applied to a long stroke engine.
And most of the stuff on the web is for very much oversquare engines which rev a lot faster.
I am trying ( without much luck ) to fix a K970 concrete saw with a 2.5" bore & a 1.5" stroke so the fuel can just not burn fast enough.
But it does 15,000 rpm.
If you troll around enough you should be able to find a table of flame speeds Vs piston speeds.
Then you do combustion volume increases and table that against cylinder volume increases so you are getting increasing pressure till around 1/2 way down the stroke.
At the very top of the stroke, very little of the combustion pressure is converted to rotation due to the vectors

And of course down here we add a little acetone to replace the high volatile fraction which evaporates out of the fuel very quickly down here so makes starting harder

email (option): bsansw1@tpg.com.au

Hi guys.

First of all- very interesting stuff...

I have been told once, some 10 years ago, by an old BSA mechanic,
That a lower octane is actually better for the SV BSA, as it burns slower,
And the benefit is that instead of combusting almost "at once" in the combustion chamber,
It kind of pressing on the piston along the long stroke, burning slower,
Resulting in more and smoother power.

Maybe the Diesel fuel has something to do with that?

Not really related,
But I have heard of many Taxi drivers, who typically running a Diesel engines,
Which are adding some petrol to every full petrol tank, (Pint, or two? can't remember)
And that also improve performance and keep the injectors clean...

Noam.

email (option): noam10@gmail.com

Hi Trevor

Thanks, that's a good way for me to look at the options. Certainly most focus is over square and high revs. Not where my M21 is heading at all, high revs would be 4,500 to 5,000 and 5k is pushing it, a Long way from the saw engine!

I'll need to check the fuel regs too, there's changes coming due to ban on leaded in motorsport but I don't know what the plan will mean for me or the 'gas' class. I'll have to recheck e web and make it along to a meeting, only every quarter for DLRA, and I've missed everyone so far :-(

Cheers

Richard

email (option): dickie.bobbie@hotmail.co.uk

As a student in the 1970s faced with a 60+ mile round trip to College and little money I ran my M.20 on two thirds petrol to one third paraffin.

I was told that running the bike on part paraffin was illegal so used to drop two naphthalene balls into the fuel tank to kill the paraffin smell.

I was then told following the bike was like opening your Granny's wardrobe!!

The bike showed no difference in terms of performance and as for the valves they were ground in twice yearly anyway along with a de-coke.

Keith, you've now reminded me of the smell of Edwardian grandmothers' wardrobes. I shall wallow in nostalgia for the rest of the evening. :relaxed:

"What do you get if you squeeze a moth ball in each hand ?"







"A bloody angry moth " :grinning:

How interesting. I too had a period of experimenting to see what the M20 would actually run on and the weirdest mix was paraffin /diesel and we dropped in a little old Whiskey that I found in the shed. It ran fine and left a wonerful aroma.

email (option): pauleastwood61@gmail.com