A short study on ignition timing and combustion

pete.v said:

When does octane adjustment come into the discussion. Doesn't this effect both timing and combustion and concidered a key adjustment tool?

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The higher octane rating means that at any given comp.ratio, more advance or a leaner mixture can be used before pinging occurs. The thing to do is raise the comp.ratio as far as practicable without obstructing the flame front to a level at which the programmed advance is reasonable at peak revs, then jet to that until just before pinging occurs at any part of the usable rev range and any throttle opening. The objective is to maintain a constant optimum combustion condition right through the usable rev range and at all throttle positions when the bike is under load.
This not about fuel economy, more about maximum torque at all revs.

Jim,
'The second way is with a static speed dyno where you hold the rpm at a set speed with a brake and vary the timing looking for peak torque. That is how I do it. Jim'

Is that where the value lies in having a very flat torque curve if you don't have a programmable ignition system ? If you get the timing correct so that you know you are getting peak torque at certain revs, does that mean the rest of the advance curve is in the right place ?
I think methanol is much more forgiving than petrol.

Keep in mind, if you have more octane than is needed for the compression ratio -you can adjust the timing without causing knock to a point where the pressure peak occurs too early after TDC to produce maximum power. This is a common occurrence with a low compression engine. Jim

'The fuel burn rate is determined by the temperature, the cylinder head design and swirl/tumble characteristics. It does not vary with the octane rating. [contrary to popular belief]'

How does tetraethyl lead reduce the tendency for knocking when it is added to petrol, if it doesn't change the flame propagation velocity ?

acotrel said:

Jim,
'The second way is with a static speed dyno where you hold the rpm at a set speed with a brake and vary the timing looking for peak torque. That is how I do it. Jim'

Is that where the value lies in having a very flat torque curve if you don't have a programmable ignition system ? If you get the timing correct so that you know you are getting peak torque at certain revs, does that mean the rest of the advance curve is in the right place ?
I think methanol is much more forgiving than petrol.

Click to expand...

A timing value will need to be determined for every different rpm and throttle position [load] to get maximum performance and economy if the ignition is capable of using a load input.

Or just every rpm in about 500 rpm steps over the used rpm range for full throttle only.

Methanol is more forgiving as far as knock resistance and engine heating is concerned.
However for peak power, getting the peak pressure point in the right place after TDC is just as important as it is with gasoline . Jim

I read somewhere that methanol has unlimited antiknock. My brother uses a Lectrix ignition system on his H2 Kawasaki two stoke using methanol fuel. It popped a lead out of a coil, so he just plugged it in and kept racing. The detonation cracked the cylinder head.

It is very noticeable on those old Indian side valve twins that as you twist the handle bar advance the power reaches a peak then drops off and you have to turn the grip back a bit to get max pulling power.

'In a perfect world timing will be set to burn the mixture to a point where maximum cylinder pressure is achieved at a particular number of degrees after top dead center.'

I've noticed that since Adam was a boy every long stroke 650cc triumph has used 38 degrees full advance for petrol, however the longer stroke Norton uses 28 degrees. Is the difference in location of the pressure peak due to the stroke difference, the reason for this ? Or do you think it has more to do with the desired torque characteristic - lower usable rev range ?

acotrel said:

'The fuel burn rate is determined by the temperature, the cylinder head design and swirl/tumble characteristics. It does not vary with the octane rating. [contrary to popular belief]'

How does tetraethyl lead reduce the tendency for knocking when it is added to petrol, if it doesn't change the flame propagation velocity ?

Click to expand...

Octane increases a fuels resistance to spontaneously ignite [detonate] under high heat and pressure conditions.

http://www.racegas.com/article/10

acotrel said:

'In a perfect world timing will be set to burn the mixture to a point where maximum cylinder pressure is achieved at a particular number of degrees after top dead center.'

I've noticed that since Adam was a boy every long stroke 650cc triumph has used 38 degrees full advance for petrol, however the longer stroke Norton uses 28 degrees. Is the difference in location of the pressure peak due to the stroke difference, the reason for this ? Or do you think it has more to do with the desired torque characteristic - lower usable rev range ?

Click to expand...

It has more to do with the size and shape of the combustion chamber and the high amount of swirl produced by the porting in a Norton head.
That makes for a very fast burn so less advance is needed. Jim

From a Sunoco fuels interview,

The burn rate of a fuel is a measurement of the time required for complete combustion of the air/fuel mixture. The notion that octane ratings affect the burn rate of fuel is about 180-degrees from reality; burn rate is a function of several variables, and the two are completely independent, although there is generally a correlation between octane ratings and burn rates.

To give you a good example of this, we contacted Jim Wurth from Sunoco Race Fuels. He explains, "A perfect example is Sunoco Maximal, which is our fastest burning fuel, and coincidentally one of Sunoco's highest octane fuels at 116 (R+M) / 2. A lot of Pro Stock teams rely on Maximal for those sub-seven second runs. When they are turning 9,000 rpm or more, the fuel has to burn pretty quickly to achieve complete combustion".

Great thread.....i love this forum

For bench racing fun there are online calculators to plug in engine factoids, static CR, cam intake valve closing crank degree's, iron or steel head, water or air cool, rpm and fuel octane to see if it predicts able to run full spark torque pressure or not. Re-run upping the octane value til it says good enough then decide to detune or supply that octane by a number of ways. Funny Garden tractor pullers around making ~70 hp from their 7000+ rpm 45 cid engines say they run 112 octane that seems to burn the whole power stroke so still burning on exhaust valve opening to push the exhaust out their limited side valves so they keep running at WOT w/o stalling till the engine completely stopped by the load or they finally shut off d/t just digging the tires in going no where. Calculators imply Peel is ok on 91 octane till dynamic CR gets over 12:1 then 112 octane good to ~17:1 w/o dieseling. Cooling the charge also helps detonation tolerance to keep spark at best pressure just after TDC.

motorson said:

Jim's explanation explains why a dual coil is trying to fire to itself instead of ground. So, like hobot said, the spark goes through one plug down, across the head and back through the other plug up. The Lucas Rita fires both coils at the same time but they are both trying to fire to ground. (Thus, wasted spark, but not like a single coil wasted spark.)

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Electrically, the plugs are in a series circuit, not a parallel circuit.

Thanks for the explanation, Jim.

Slick

Sparks to our twins can be gotten by two separate coils packs or dual potted coil pack, firing electrons off center post to ground hook alternately, as our point systems, or by two coils wired in series so one trigger pulse makes current pass both plug gap in opposite directions. The kernal size shape will be the same in the 1st instance but in 2nd may be somewhat smaller/weaker in one gap on its ignition stroke. There is an old lead pencil test to make sure separtate coil wired for best polarity by seeing which polarity blasts across bigger brighter. Jim mentions surface discharge plugs which opens another can of spark worms to pick a juicy one from.

https://www.google.com/?gws_rd=ssl#q=pe ... +plug+test

comnoz said:

acotrel said:

'In a perfect world timing will be set to burn the mixture to a point where maximum cylinder pressure is achieved at a particular number of degrees after top dead center.'

I've noticed that since Adam was a boy every long stroke 650cc triumph has used 38 degrees full advance for petrol, however the longer stroke Norton uses 28 degrees. Is the difference in location of the pressure peak due to the stroke difference, the reason for this ? Or do you think it has more to do with the desired torque characteristic - lower usable rev range ?

Click to expand...

It has more to do with the size and shape of the combustion chamber and the high amount of swirl produced by the porting in a Norton head.
That makes for a very fast burn so less advance is needed. Jim

Click to expand...

I can believe the squish band causes that effect, however the Bonneville head has almost as much down draft as the Norton head, and the ports are splayed. The ports in the Bonneville head are probably the maximum size that can be used with race cams to get acceptable power (torque) with their also limited rev range. Then I've read about certain guys changing the Norton head to full hemisphere - squish band removed. Something doesn't make sense.
In our Australian historic racing, a friend won the championship for a few years with a 750cc Triumph (triton) fitted with the 650 Bonneville head - told me that realistically he could not beat the 750cc Nortons. He was not winning because of his Triumph's performance.

acotrel said:

I can believe the squish band causes that effect, however the Bonneville head has almost as much down draft as the Norton head, and the ports are splayed. The ports in the Bonneville head are probably the maximum size that can be used with race cams to get acceptable power (torque) with their also limited rev range. Then I've read about certain guys changing the Norton head to full hemisphere - squish band removed. Something doesn't make sense.
In our Australian historic racing, a friend won the championship for a few years with a 750cc Triumph (triton) fitted with the 650 Bonneville head - told me that realistically he could not beat the 750cc Nortons. He was not winning because of his Triumph's performance.

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I don't like removing the flat squish band either even if the valve flow rate may be a little higher with a full hemi head. A domed piston with a spherical squish band is better than no squish band but a flat squish band seems to work a little better in my experience.

A lot of the reason a Norton motor burns fast is the horizontally offset angle at which the port enters the combustion chamber plus the ability to get a decent compression ratio without resorting to high dome pistons makes for a lot of swirl. Jim

acotrel said:

comnoz said:

acotrel said:

'In a perfect world timing will be set to burn the mixture to a point where maximum cylinder pressure is achieved at a particular number of degrees after top dead center.'

I've noticed that since Adam was a boy every long stroke 650cc triumph has used 38 degrees full advance for petrol, however the longer stroke Norton uses 28 degrees. Is the difference in location of the pressure peak due to the stroke difference, the reason for this ? Or do you think it has more to do with the desired torque characteristic - lower usable rev range ?

Click to expand...

It has more to do with the size and shape of the combustion chamber and the high amount of swirl produced by the porting in a Norton head.
That makes for a very fast burn so less advance is needed. Jim

Click to expand...

I can believe the squish band causes that effect, however the Bonneville head has almost as much down draft as the Norton head, and the ports are splayed. The ports in the Bonneville head are probably the maximum size that can be used with race cams to get acceptable power (torque) with their also limited rev range. Then I've read about certain guys changing the Norton head to full hemisphere - squish band removed. Something doesn't make sense.
In our Australian historic racing, a friend won the championship for a few years with a 750cc Triumph (triton) fitted with the 650 Bonneville head - told me that realistically he could not beat the 750cc Nortons. He was not winning because of his Triumph's performance.

Click to expand...

What Jim is saying is true.

Four things the Norton big twin cylinder head has going for it compared to other period motorcycle engines:
1.) The plane that contains both the intake and exhaust valve has been rotated around the cylinder axis. This spreads the exhaust valves further apart from eachother, allowing more cooling vanes for the exhaust area. This also causes the intake tracts to potentially clash so Norton had to rotate each intake tract around the intake valve stem - this allowed for a more down draught intake than that of a Triumph, this is not easy to visualize and is not that apparent until you start digging into it a bit. The more down draught arrangement allows the Norton head to exhibit extraordinarily good intake flow coefficients.
2.) As a direct result of rotating the intake tracts around their respective stems, the tracts, ports and throats are pointing more tangential to the cylinder rather than straight to the center as is found in a Triumph. This promotes swirl - big time.
3.) The inlcuded angle between the intake and exhaust on a Norton twin is less than that of Triumph, allowing for a more moderate dome to achieve a given compression ratio whereas a Triumph has a big honking dome, not too friendly for flame propagation.
4.) The Norton hemispherical design with more or lesss flat top pistons allows the kinetic energy to sustain; swirl velocity increases upon compression (closing up of the effective chamber size with the nominal quench area, ie. conservation of momentum). The Triumph head has a big honking piston dome in the middle.

Getting back on topic - So the Norton intake tract orientation promotes swirl and with the combustion chamber shape this supports less ignition advance required to achive max torque for various engine speeds.

Dances with Shrapnel said:

Getting back on topic - So the Norton intake tract orientation promotes swirl and with the combustion chamber shape this supports less ignition advance required to achive max torque for various engine speeds.

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I think I heard that somewhere before..

Thanks for the more technical explanation. Jim

This article is very much in line with what the fuel companies will tell you. Yet the old fuel burning speed vs octane tale lives on. Jim

http://motorcycleriderz.motionsforum.co ... fuel-burns

By the way. When your looking at conventional pump fuel, the time difference between what would be considered a fast burning fuel and a slow burning fuel is so small that on a low revving engine like a Norton, the crank angle for maximum pressure is hardly even affected.

I can accept the free radical theory, and that TEL might suppress it, however the article mentions the speed at which the flame front proceeds in the combustion chamber. I am used to working with substances which have a 'critical height' i.e. the flame front accelerates and does not proceed at constant velocity. Some guys have claimed that nitromethane is safe because when you light it in a dish it burns quietly. The fact is that if you stack it high enough you will get the big bang. If you've ever watched gun propellant burn, it burns quietly for a while, then you can watch it accelerate. If it has been stacked up too high on the burning pad, you can be bowled over and deafened. So if you have a long 8 inch diameter pipe full of petrol and light one end does it accelerate and go bang at the end ?