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Re: Funny Fuel

I too use the highest octane (Supreme or VPower) from local independent Esso/Shell stations, but simply because they assure me that it has less ethanol than the lower grade and supermarket fuel. They might be lying of course.....But the bikes run well and ignorance is bliss. Ron

email (option): ronpier@talk21.com

Re: Funny Fuel

The octane rating of a fuel determines its ability to withstand pre ignition/knocking/pinking...

Thus engines with higher compression ratios require a higher octane fuel to resist the tendency to pink under the higher compression values exerted...

The octane rating itself is unrelated to power output..it is the higher compression ratio permitted when using high octane fuel that results in the power increase...

The energy potential of the fuel is unchanged by altering the octane rating. A higher octane fuel has no more air and no more hydrocarbons to burn to produce more energy than a lower octane fuel...

So, except in unusual circumstances there will be no power gain in using a higher octane fuel than that recommended by the manufacturer...

Another factor that affects the ability of an engine to resist pre ignition is the burn characteristics of the combustion chamber so, as usual, it's not just a case applying an octane rating in isolation...

The amount of wasted heat produced by an engine (wasted energy) is indicative of the engines efficiency. To take an extreme and impossible example a 100% efficient engine would be totally cold when running as no heat energy would be lost from the combustion process...

The different levels of inefficiency between OHV and SV engines is the reason the two run at differing temperatures...

So, to achieve more power changes to the engine are required to take advantage of any higher octane fuel used and to reduce running temperatures a change to the efficiency of the engine must be made....Ian

email (option): ian@wright52.plus.com

Re: Funny Fuel

I'm trying to learn here and have no knowledge of this topic at all...

John tried this on 2 bikes he knows well and noticed a difference, so there most likely is an effect. But why? To me, reading the comments here, there are three possible explanations for increased power and cooler running when diesel is added to the fuel:

1. octane rating is reduced. Our old sidevalves were designed to run on lower octane fuel and have a low compression ratio. The engine can not use the extra octane from modern gasoline to make more power so this excess energy is "burned off" producing more heat and the engine temperature gets higher.

2. lubrication is increased because diesel is oilier. the additional lubrication provided by adding diesel to the fuel reduces friction thereby reducing operating temperature. Running at a lower temperature suits the engine and reduces issues with multiple engine components expanding and contracting at different rates.

3. The extra oil content of the diesel/fuel mixture increases compression by compensating for worn rings, resulting in more energy that translates to more power.

It could also be a combination of 2 or 3 of the above theories. We could test these theories at least partially by comparing differences between the diesel/gas mix with adding a bit of 2 stroke oil to the gasoline instead (say at 50:1). This would give us a comparison between 2 "oiler" mixtures. If the 2 stroke and diesel mixes produce similar results, then #2 and #3 are more likely because there would be less octane reduction.

We could also compare how each mix affects temperature, either with a temp gauge on the head or taking temps at the header after an identical length and speed ride. If temperature is lowered, then #1 or#2 would be more likely.

Or we could run the same bike on a dyno, with all 3 fuel options. A higher dyno reading would suggest #3, similar dyno readings would point to #1 or #2.

I would love to learn more about this. I see on the interweb that adding diesel or other oils to fuel mixtures is also a talking point for sidevalve car engines so we are not alone here.

Hopefully the gurus can chip in here. Ian, Trevor, any additional comments?

email (option): cas.vanderwoude@gmail.com

Re: Funny Fuel

['1. octane rating is reduced. Our old sidevalves were designed to run on lower octane fuel and have a low compression ratio. The engine can not use the extra octane from modern gasoline to make more power so this excess energy is "burned off" producing more heat and the engine temperature gets higher..']..

No, the octane rating is only a measure of the fuels ability to withstand pinking or pre detonation...

The additives used to produce this effect do not increase the 'potential energy' of the fuel...Hydrocarbon content remains unaltered between low and high octane fuels.

With the high anti knock values that result from the use of a high octane fuel a higher compression ratio can be utilised and THAT produces extra power...

That is why running high octane fuel in a side valve that is not constructed to take advantage of it (low compression ratio) leaves the power output effectively unaltered...Ian

email (option): ian@wright52.plus.com

Re: Funny Fuel

Hi all, Very interesting points and quite a subject, Have tried 2 stroke oil at 50/1 and 25/1, Made no difference to performance or engine cooling also tried sae 40, 50,as a upper cylinder lubricant (castrol R40 in my gold star lovely smell only) also tried shell nitro + super unleaded made no difference what so ever' but a pint of diesel made an amazing difference, anyone know a paraffin formula I will give it a try. Regards John.

email (option): rustytomm20@hotmail.com

Re: Funny Fuel

hmmmmm.....

but does adding diesel reduce the "explosiveness" of the mixture I wonder?

I remember when I was a forest ranger we used to use "drip torches" to light up back burns when fighting forest fires. Essentially a drip torch is a 1 gallon metal container with a handle. Sticking out of this is a copper tube that allows the fuel inside to drip onto a big wick. Once lit, one can walk along and disperse little globs of excess burning fuel which would start the back burn. (Stay with me here folks)...

If one used diesel fuel, there would not be enough flammability to keep the thing burning. Using gasoline would make for a mixture that was too explosive, thus no burning globs of fuel would get to the ground. The ideal mixture was a 50:50 mix of standard gas and dieseline.

I wonder if this is the same in a sidevalve. A mixture that is slightly less explosive might work better...

email (option): cas.vanderwoude@gmail.com

Re: Funny Fuel

Hi Cas, Interesting point combustible and slower burning so most heat caused from friction of the piston in the barrel? Talking to my mates dad this evening about this subject he used to use a ex us army ww2 willys jeep in the early 70s as his daily driver and he worked in farming and ran it on TVO (Tractor vaporising oil) paraffin ? (For free) BUT! Didn't do the exhaust valves any favours, so the question is' my M20s and the old boys M20s ok to run on cut fuel, if it break's you fix it,if it works you leave it alone, John.

email (option): rustytomm20@hotmail.com

Re: Funny Fuel

Mike
I have also heard of people who add a little bit of diesel in the m20 tank.
Is it a good idea ?



I was the one rising up this diesel subject.
I do it for years with good results. AFAIK, these engines where originally based on TVO (Tractor vaporizing oil) side valves (farm tractor engines etc.), as the ministry of defense contract wanted the bikes to run on all fuels that where available in the UK, Asia and all other colonies. Some of them quality modern fuels, others a bit lower quality. Also, don't forget that Diesel is actually some kind of oil, so it will help the engine as lubrication is better.

Re: Funny Fuel

Hi Michael, Excellent info' If you've had no problems then neither should i really, I will continue to use it because the results really are very good, Regards John.

Re: Funny Fuel

BSA M20s and the other WD bikes were based on pre-war civilian models and certainly not designed as 'multi-fuel'. Most pre-war owners would have used the cheapest fuel available.

Compresion ratios seem to have been lowered during wartime.

However, there is a puzzling aspect in that I have seen reports of damage to low-comp British engines (in lorries etc.) by the fuels introduced later in the war intended for the high compression US tank engines...so there does seem to be a suggestion that too high an octane rating is not good for an engine that doesn't need it.

I have a suspicion that modern additive packages are extremely volatile and it may be that adding diesel reduces the tendency to vapour-lock and may even make for a more efficient induction. Modern fuels are designed for injection engines, not old side-valves that couldn't suck the skin off a rice pudding.

Adding too much 2-t oil to a two-stroke is a bad thing as it actually causes a lean mixture (there is less fuel being drawn in in relation to the amount of air). Is this also true of diesel or not ?

Re: Funny Fuel

['I have a suspicion that modern additive packages are extremely volatile and it may be that adding diesel reduces the tendency to vapour-lock and may even make for a more efficient induction..']


High octane fuels are LESS volatile than low octane fuels...That's the whole point, to reduce the tendency for the fuel to ignite prematurely under compression before the spark occurs, rather than being ignited by the spark itself...Ian

email (option): ian@wright52.plus.com

Re: Funny Fuel

'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.

Re: Funny 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.

Re: Funny Fuel

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

Re: Funny Fuel

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

Re: Funny Fuel

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

Re: Funny Fuel

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

Re: Funny Fuel

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

Re: Funny Fuel

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.

Re: Funny Fuel

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:

Re: Funny Fuel

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

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