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E: Engine Modern Fuel & Ignition Advance


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Recently I've seen recommendations to set ignition advance at 34 degrees, as against the factory figure of 38. This is said to be necessary because of the difference between modern fuel and what was available c. 1950. Given that advance allows the mixture time to burn, so the pressure peaks near TDC, this amounts to the claim that modern fuel burns more quickly.

I'm curious as to the basis for this idea. Do we have any petroleum engineers among us? What are the differences in composition and burn rate between

- the 72 octane "pool" petrol available in the UK postwar
- leaded gasoline sold from the 1960s until unleaded replaced it
- unleaded gasoline without ethanol
- unleaded gasoline with 10% ethanol



greg brillus

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Old style combustion chambers.............Hemispherical..........Plus don't forget the effects from an air cooled engine, where temperatures change far more than liquid cooled engines. Most old school engines which are set up at their original full advance will detonate quite badly. It affects carburretion as well, number 4 slides in original 229/289 carb's are almost too lean on modern fuels. It is possible that the specific gravity of modern fuel could have an affect as well. We are finding here in Australia that the thinking of running these bikes on "Premium" fuel that is 98 octane is not so successful on account of that the top end "part" of the fuel evaporates/boils off too quickly and leaves a fuel not unlike kerosene in the tank. Most here are running standard 91 octane unleaded fuels. The 98 octane fuels are suitable for modern fuel injected hi compression liquid cooled cars and bikes.


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A quick look through the internet taught me a few of things.
1) The cost of downloading a true scientific paper on this subject is $50 an article.
2) Octane rating has squat to do with flame speed.
3) Flame speed is dependent on the mixture of the fuel.
4) Pump gas (80-90 octane) has flame speed of .34 ms @ stoichiometric combustion
5) Ethanol (102 octane) .39 ms @ stoichiometric and Methanol (103 octane) .43 @ stoichiometric.

So it would appear that the big brain chemists in attempting to make fuel burn cleaner and more efficiently have slowed the combustion process. So if max pressure is to be achieved at or just after TDC, so that the piston is driven down and not fighting to get to TDC, the process must begin later.
The fly in the ointment is finding out what the flame speed was of the mixture or the actual mixture itself of 1950 pump gasoline.

http://iqlearningsystems.com/ethanol/downloads/Racing Fuel Characteristics.pdf

Pete Appleton

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So it would appear that the big brain chemists in attempting to make fuel burn cleaner and more efficiently have slowed the combustion process. So if max pressure is to be achieved at or just after TDC, so that the piston is driven down and not fighting to get to TDC, the process must begin later.

What am I missing here? Slower flame speed must mean that we want to ignite earlier - doesn't it?


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I think it's a balancing act between several variables. Flame speed is slower but resistance to detonation is increased. Out of curiosity I tried to extrapolate the flame speed of E-30 pdf file I listed from the pure ethanol and pure gas flame speeds. (.3x.39)+(.7x.34) and got .355 to the .36 listed. So the flame speed has been slowed down.
Now if we look at the octane rating pure gas is 80-90, ethanol is 102, and E-30 87-94. Ethanol is less likely to detonate from compression than pure gas. The middle ground of E-30 is just a hair above the best of pure gas.
The higher the actual pressure before ignition the more the power released and more efficient the burn. So while the burn is slower the resistance to detonation is increased allowing the process to begin later and therefore release more power in a more efficient manner.
But if the process is initiated too early the rise in pressure due to early ignition may cause the pressure to rise to fast and cause detonation.


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Something to keep in mind:

"Anti-Knock Index (AKI) or (R+M)/2
In most countries, including Australia, New Zealand and all of those in Europe, the "headline" octane rating shown on the pump is the RON, but in Canada, the United States, Brazil, and some other countries, the headline number is the average of the RON and the MON, called the Anti-Knock Index (AKI), and often written on pumps as (R+M)/2. It may also sometimes be called the Posted Octane Number (PON)."

From https://en.wikipedia.org/wiki/Octane_rating

Deleted member 3831

Combustion is a complicated subject, hardly possible to describe it adequately in a forum, however long the post. However, a couple of features might be worth mentioning as worthy of consideration and thus encourage further comment.
Effective combustion will result from the peak cylinder pressure occurring somewhere between 10 and 15 degrees ATDC on the firing stroke, (average 12.5 degrees), despite the crank not realising it's position to transmit maximum torque until it has moved to approx. 76 degrees ATDC.
From the initial spark at the plug, there is a delay period during which the tiny burn is struggling to take hold, rather as a bonfire would if started from a single match. This delay period is usually considered to be in the region of 10 degrees.
If your starting point is 38 degrees BTDC, the effective burn period will be approx 28 plus 12.5, say 40.5 degrees. Convert 40.5 to milli seconds at the engine speed where max. torque is produced and you will see it is a remarkably short time. At peak power rpm, time gets shorter still.
Now try fitting flame speeds into this and see where it gets to, allowing for the distance of travel being longer on the side of the piston opposite the plug.

Pete Appleton

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Grey one mentions a 'delay period' Comparing the published advance curves for the BTH and Pazon ignition systems it looks to me as though, in the lower part of the rev band, these are set to provide a 2.5 millisecond delay period before tdc.
I understand that this will not be a linear function as we take into account cylinder filling efficiencies etc but it is noticeable that Pazon reach maximum advance at only 2000 revs and do not advance any further.
BTH have a shallower curve and don't reach maximum until 3,000 rpm.
I wonder where these figures have come from. Is it a legacy figure taken from what could once be achieved with a mechanical ATD? Is there a good reason why we stop advancing at not much above mid revs?
I have always suspected that there could be more power available if we actually increased advance up to somewhere in excess of 45 degrees but don't achieve that until much higher revs.
Many years ago I attended a lecture by Cosworth racing engines. In a discussion afterwards with one of their designers he stated that increasing advance was providing corresponding power improvements right up until detonation point. Something along the lines of 'advance it up until it melts and then back it off a bit'
All of this is just my imagining and not backed by the slightest bit of science. has anyone got any dyno figures?

Bill Thomas

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It's Funny, In the 70s, A lot of us with big pistons and short exhaust were going less advance, I was 24/26 "D" Distributor, 12 to 1 pistons, But we had a Sidecar bloke said he was 42 ish, His went well but sounded like a bag of bolts !, And shook like hell. Cheers Bill.


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If we set the timing like the factory did we would go to the dyno, start at 40 degrees and keep retarding until we found maximum power. The hope would be that with all the adjusting we would ultimately find the window for peak pressure occurring circa 12.5 degrees ATC. This would short circuit much of the worry about fuel formulation and flame speed. It also sounds at odds with the water-cooled Cosworth.

I do think there is a lot of motorcycle ignition design that is driven by automotive design. John Healy mentioned that most electronic ignition advance curves were designed with a chip that allowed only small variations from the best water-cooled automotive design. I would note that lots of advance will wear out spark plugs very fast. Many manufacturers retard their ignitions .5 degrees simply to increase plug life.

Of course, as Vincent Speet says "you might have slow spark plugs!"


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