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E: Engine Modern Fuel & Ignition Advance

LoneStar

Well Known and Active Website User
VOC Member
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

Cheers,

Dave
 

greg brillus

Well Known and Active Website User
VOC Member
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.
 

ClassicBiker

Well Known and Active Website User
VOC Member
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.
Steven

http://www.whitfieldoil.com/171.284/vp-racing-fuel-
http://iqlearningsystems.com/ethanol/downloads/Racing Fuel Characteristics.pdf
 

Pete Appleton

VOC Hon. Social Secretary
VOC Member
VOC Forum Administrator
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?
 

ClassicBiker

Well Known and Active Website User
VOC Member
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.
 

bmetcalf

Well Known and Active Website User
VOC Member
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
 
D

Deleted member 3831

Guest
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

VOC Hon. Social Secretary
VOC Member
VOC Forum Administrator
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?
Curves.png
 

Bill Thomas

Well Known and Active Website User
VOC Member
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.
 

davidd

Well Known and Active Website User
VOC Member
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!"

David
 

Chris S

Well Known and Active Website User
VOC Member
I think there a second challenge to consider if you are using standard magneto and ATD setup.
Considering the ATD has a fixed amount of advance, if you set it to 34 or 35 degrees at max advance then you are also setting it to give a very late spark at idle or when starting. I believe the static timing was originally 4 degrees btdc which then gives 39 degrees at max advance. If you set to to 35 degrees at max advance then you will be setting the static timing to top dead centre.
I think a few owners may have done this and then spent ages trying to correct poor starting and irregular idle by forever adjusting the carburettors especially if fitted with Shadow carbs - me included.
I have now set mine to 3 degrees btdc and it starts easily, idles well and revs fine.
Chris
 

ClassicBiker

Well Known and Active Website User
VOC Member
After what I posted(#5) above I got curious what the linear speed of the piston was at 3000 rpm. Well 3000 rpm is 50 rps, which is 314.159 Pi radians a second, with a 90mm stroke is a 45mm throw and equates to 14,137.155 mm per second. As we were talking about originally the reduction in the advance from 38 BTDC to 34 BTDC I went to the table on TheVincent.com under the Magneto tab (by Tom Gross) to see what the linear difference was in how far down the bore the piston was, .11" or 2.794mm. At 3000 rpm the piston covers that distance in .197 milliseconds.
Taking what Grey One posted above, what I read was similar (20 degrees), and realizing that linear distance the piston travels from 28 BTDC to 12 ATDC is .303 inches or 7.6962mm it covers that distance in .544 milliseconds. Doing the same for 24 BTDC to 12 ATDC is .237 inches or 6.0198mm results in .426 milliseonds.
So thinking about it a bit more the E-30 has a burn rate of .36 ms and the piston speed at 3000 rpm puts us at the optimal point with .184 ms to spare, if we begin ignition at 38 BTDC and .066 ms to spare if we begin at 34 BTDC. Of course this is just conjecture on my part, but you can see where those that do the actual practical testing get their jumping off point from. This stuff happens fast.
Steven
 

macvette

Well Known and Active Website User
Non-VOC Member
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?
View attachment 21983
I had a 1963 Corvette 11.25 to 1 comp ratio. It had a factory original blueprinted motor red lined at 6500 rpm, standard engines were red lined at 4000 something.The timing marks were on a harmonic balancer on the crank pulley. If one had an original balancer which was rubber mounted to the pulley flange, the recommedation to set the timing was to diconnect and plug the vacuum advance at the distributor, set the motor to 1500 rpm and turn the distributor so that the motor ran fastest then ease it back a touch and lock it there. This was done after setting the points with a dwell angle meter. Reconnect the vacuum advance and reset idle. This was recommended because of the elimination of lead tetra ethyl anti knock from fuel and to take care of any creep in the rubber mounting of the balancer.
I guess this was done so that the distributor mechanical advance was operating hence the timing was right on the advance curve.
Mac
 

Pete Appleton

VOC Hon. Social Secretary
VOC Member
VOC Forum Administrator
DavidD - You mention starting at 40 degrees then backing off to optimum. Are we talking fixed timing or some form of advance curve? My big question is about these systems that seem to advance right up early then go no further. I am assuming that your tuning was done at far more than 2000 revs. So should the advance curve continue beyond 2000 revs?

Chris S - The ATD is what started me thinking. It is horribly crude and, effectively, driving the magneto through the bob weights, against springs of indeterminate strength must leave questions about when all of the advance takes place. Are you getting away with 38 degrees because you have good ATD springs? Are those retarding their ignition covering up an ATD that is advancing too early?
 

Chris S

Well Known and Active Website User
VOC Member
Peter - I replaced the ATD springs to ensure the bob weights were a: pulling back in correctly at low revs and b: controlling the outward movement of the weights to attempt to keep the advance curve correct.
I agree that an ATD is a fairly crude device, but they crop up all over the place in ignition control systems and have done for a very long time and when operating normally give a consistent reliable ignition advance. Arguably, at least you can see and feel if they appear to be operating correctly unlike an electronic unit which needs a lot more kit to check.

You are probably correct that some people will be covering up an ATD that is not functioning properly by altering the timing. However, a lot of owners do report better high speed running at 35 degrees. If using the ATD setup then the advance of the ATD should be restricted to stop a 35 degrees thereby keeping the low speed timing correct rather than simply retarding the whole curve, ending up with retarded low speed timing as per my earlier post.

Chris
 

BigEd

Well Known and Active Website User
VOC Member
VOC Forum Moderator
Peter - I replaced the ATD springs to ensure the bob weights were a: pulling back in correctly at low revs and b: controlling the outward movement of the weights to attempt to keep the advance curve correct.
I agree that an ATD is a fairly crude device, but they crop up all over the place in ignition control systems and have done for a very long time and when operating normally give a consistent reliable ignition advance. Arguably, at least you can see and feel if they appear to be operating correctly unlike an electronic unit which needs a lot more kit to check.

You are probably correct that some people will be covering up an ATD that is not functioning properly by altering the timing. However, a lot of owners do report better high speed running at 35 degrees. If using the ATD setup then the advance of the ATD should be restricted to stop a 35 degrees thereby keeping the low speed timing correct rather than simply retarding the whole curve, ending up with retarded low speed timing as per my earlier post.

Chris
It may be worth pointing out that as Chris states it is very easy to check if the bob weights can move out and be returned by the springs. However, whether you have mechanical or electronic advance you still need to use a timing disc and strobe to get a real picture of what dynamic advance/retard you have.
 

davidd

Well Known and Active Website User
VOC Member
DavidD - You mention starting at 40 degrees then backing off to optimum. Are we talking fixed timing or some form of advance curve? My big question is about these systems that seem to advance right up early then go no further. I am assuming that your tuning was done at far more than 2000 revs. So should the advance curve continue beyond 2000 revs?

Pete,

I was starting with a twin plug BTH with electronic timing. The tuning was done at 7000 rpm. Although I don't know exactly where the advance should stop advancing, I understand that as the speed of the engine gets higher, the combustion becomes much more efficient. This efficiency means the advance is not necessary. I believe that test engines have run over 20,000 rpm with no extra advance necessary.

I could run fixed advance on the racer. I was getting 42 bhp at the rear wheel at 7000 rpm with the timing set at 19 degrees. On this engine I believe I started at 26 or 28 degrees and worked my way down, increasing power while reducing the timing every run. I mentioned 40 degrees above simply because so many owners seem to run a lot of advance.

What surprised me was that Cosworth promoted so much advance. It seemed to me that most tuners describe exactly the opposite, tuning for the most power at the most retarded position possible.

Much of this discussion is above my pay grade. I tried to figure out what the advance/retard timing curve should look like. I think that is an excellent question, but I could not find much on this subject. I was planning on switching the racer to a programmable ignition so that I might get some starting retardation that went to full advance prior to 19 degrees. That can be done easily now, but I think it is more important for street machines. I would guess that retarded timing might help a Vincent's performance up to 3500 or 4000 rpm, but it is a guess.

David
 
D

Deleted member 3831

Guest
I feel I may not have made the delay period of the initial combustion process clear. This delay is not a feature of and not programmable in any ignition system, but is a situation that follows on for about 10 degrees after the initial spark, until such time as the burn really takes hold
T
When the spark occurs at the plug, the situation within the engine is of a moving piston and a varying volume, and if turbulence as a result of swirl and/or tumble is now added into the mix, the burn may be enhanced or hindered by a substantial amount. While most engines respond well to a turbulent combustion chamber arrangement, some equally good performers do not.
 

vibrac

Well Known and Active Website User
VOC Member
I have been staggered by the alterations possible by a programmable ECU when Ben gets on the rolling road with his k100, ignition point and mixture are altered in situ on the laptop with the engine pounding away. Back on the Vincent a lambda sensor plugged in the exhaust and the slots in the BTH body make some running changes possible
 

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