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Cut and pasted this from a CDI supplier. I also saw the same info' on another CDI.

Key Facts
Performance CDI's advance the timing to provide a better spark at high revs

We (2-stroke lambrettas) try to retard the timing as we rev' higher, but it seems there is a different train of thought out there

MickYork wrote:Cut and pasted this from a CDI supplier. I also saw the same info' on another CDI.

Key Facts
Performance CDI's advance the timing to provide a better spark at high revs

We (2-stroke lambrettas) try to retard the timing as we rev' higher, but it seems there is a different train of thought out there

That's for 4 strokes to make sure that the flame path has enough time to be effective at the optimum moment as revs rise.

Retard on 2 strokes is to protect the engine from overheating.

Baffled me when I first started looking at varitronic etc.

Have a read of this
https://www.f1technical.net/forum/viewtopic.php?t=10315

The fundamental reason that efficient two stroke engines require the ignition to be retarded is that as the revs rise, the molecules of fuel combined with air become greater in number, but smaller in size. Consequently, the surface area of the fuel 'charge' is much greater, so less advance for the ignition is required.

Ideally, the correctly programmed ignition will advance rapidly from zero revs to just a few thousand, then start to retard.

Better still, when over-rev is taken into consideration in conjunction with a good pipe &, say, an electronically controlled power jet being shut off to lean the mixture, the ignition will retard far less @ the point of maximum power (or even 'plateau' by becoming static & start to advance again!) to allow the engine to rev on beyond it's maximum power condition. Such a scenario would be encountered on a hilly track, such as Cadwell Park, where the worst thing would be the engine not being allowed to over-rev going downhill & feeling as though it has hit a brick wall.

With the application of a little science, there are many good, reliable, punchy road engines that have static ignitions still set @ 21 DBTDC & they are a pleasure to ride, simply because they have been built by somebody willing to comprehend the concept of the operating range of the Lambretta engine allied to the better unleaded fuels & a good carburettor set-up with non restrictive air filters.

That said, a programmable ignition is a very good tool to have......but only if used in conjunction with other improvements & the complete understanding of all the concepts involved & the way they interact. Motoplat were making race advance/retard stators decades ago that allowed incredible tractability if set up correctly......

so why is that not applicable to a 4 stroke engine and why do they advance the timing ?

MickYork wrote:so why is that not applicable to a 4 stroke engine and why do they advance the timing ?

Unlike a two stroke engine, the four stroke engine has very distinct, more separate phases:

SUCK SQUASH BANG BLOW

One consequence, is that the fuel air mixture is not like the two stroke, which, as said previously, becomes a much finer 'mist' with the increase in revs, requiring significantly less ignition advance due to the greater surface area meaning more oxygen & the potential to burn quicker.

In terms of potential efficiency of the burn, two strokes can be more efficient than four strokes, but successive legislation & loaded rules have not worked in it's favour to ensure investment to clean up it's act. Formula One banned two strokes many years ago, & the likes of Honda, that threatened to spit their dummies out, lobbied heavily for the effective ban on two strokes in MotoGP, despite their proven ability to produce World beating engines.

It is the naturally occurring phenomenon of two stroke engines that they require less ignition advance as revs rise.

The best two stroke engines have more in common with a jet engine than a four stroke, with the piston acting as the means by which power is extracted, rather than the thrust we ascociate with the jet.

It is difficult to believe, with all of the legislation to outlaw two strokes, but ship's engines are the most fuel efficient, practicable 'mass produced' engines in the World using fossil fuels such as oil. They are supercharged, diesel two stroke engines.

Mark, are you saying that the retard is so that the charge isn't ignited too early at higher revs? I suppose you must be. And is this because it's ineffective at converting explosive energy into kinetic energy? I guess if you ignite too early, you just try to reverse the crank, but as you've but the same quantity of fuel, more of it is converted into heat (because less has been converted to kinetic) so you increase engine temperature?

Sorry if that's a but rambling, but I'm just trying to ask in a way that works for me...if you know what I mean.

dickie wrote:Mark, are you saying that the retard is so that the charge isn't ignited too early at higher revs? I suppose you must be. And is this because it's ineffective at converting explosive energy into kinetic energy? I guess if you ignite too early, you just try to reverse the crank, but as you've but the same quantity of fuel, more of it is converted into heat (because less has been converted to kinetic) so you increase engine temperature?

Sorry if that's a but rambling, but I'm just trying to ask in a way that works for me...if you know what I mean.

Firstly, sorry if this appears pedantic, but, there should never be an 'explosion' in a petrol engine. That would be detonation, & a 'burn' is what is required.

As for less ignition advance, that is due to the greater surface area of the fuel charge as the revs increase.

Imagine that low revs assimilates to a combustion chamber filled with lots of balloons of size Large, with the consequential surface area.

Now, imagine those balloons being replaced by more balloons of a smaller size as the revs rise, right down to Extra Small.

Because the surface are of lots of smaller balloons means much more oxygen, much less ignition advance is required, to avoid other problems.

I hope that helps

Yes, it helps but doesn't actually answer what I was asking. It was a bit of a rambling question. The thing I'm trying to get my head round is while I can see why there is a need to reduce the advance (retard) to burn at the optimum moment for power, that doesn't explain why too much advance burns holes in pistons.

Or maybe it doesn't and that's just an old wife's tale?

I think i read somewhere....or a bloke in the pub told me...

When ignition occurs the burning mixture expands and increases in temperature as the expansion increases ...by retarding the ignition it lowers the rate of the expansion/heat as it is closer to TDC and not "active" for as long

makes sense to me but whether or not it's true ...........

dickie wrote:Yes, it helps but doesn't actually answer what I was asking. It was a bit of a rambling question. The thing I'm trying to get my head round is while I can see why there is a need to reduce the advance (retard) to burn at the optimum moment for power, that doesn't explain why too much advance burns holes in pistons.

Or maybe it doesn't and that's just an old wife's tale?

In an ideal state, the ignition occurs prior to the piston having reached the top of the stroke. The flame spread, in a controlled state of burn, does not initially start to become an obstacle to the piston rising, but as it passes TDC, the piston gets an almighty push down toward BDC.

Now, if the ignition occurred sooner than it should, power would be lost because of the force required for the piston to pass TDC.

In addition, we all know that the piston sheds its heat from the crown to the cylinder head (why the squish should be @ the practicable minimum!). If the cylinder head cannot get rid of the heat it has conducted, the piston crown will simply become hotter & hotter.

Just to make the situation even worse, because of the flame taking longer to get the piston down & past the exhaust port opening, there is yet more heat involved in the combustion. In addition, even more heat is generated through having to compress the mixture (already ignited) more than it should, were ignition to be optimised, as in the ideal state first mentioned.

In extreme cases of too much ignition advance, the piston (if not melted) won’t quite get to TDC before going back the same way as it came. That is when engines start to run in reverse. I’ve known it occur on a scooter racing outfit, when the flag dropped, & it roared off in reverse….

Some great tech explanation WT, I hadn't heard of the atomization issue with 2T compared to 4T, is this due to the mixture being forced around the crankcase and back up the transfers?

coaster wrote:Some great tech explanation WT, I hadn't heard of the atomization issue with 2T compared to 4T, is this due to the mixture being forced around the crankcase and back up the transfers?

My understanding is that right from the ouutset, the carburettor delivers, comparably, great globules of fuel into the venturi that has little chance to develop into a fine mist @ low air speed encountered @ low RPM. Racking my brains here, to think of a comparison, but imagine your thumb on the end of a slow running garden hose.

Now, open the garden hose tap up, & the same restriction (via the thumb) starts to cause a fine mist.

(The carburettor venturi, with it's atomiser, slide & needle, is a deliberate constriction to achieve complete combustion. Just how well that juggling act is performed was determined years ago when competition restricted venturi size, but some clever Engineer rose to the challenge with his gamechanging Lectron & Ei Magnum carburettors, which forced the likes of Mikuni & Keihin to sit up & take his priciples on board)

The size & shape of the venturi, the profile of the slide cutaway, & the nozzle (if required @ all) are all important factors. That's why I personally am not a fan of the round slide carburettor so common amongst many on this Forum