Why does a better spark = more power?
Discussion
Chaps,
Why does a car with a failing ignition system produce less power? Given that the spark is only used to ignite the charge, and the rest of the burn takes place from that point on, why does a smaller spark mean less power?
My reasoning is that the spark is only the start of the process, and saying that it causes a power drop is akin to saying that a smaller match leads to a less-hot bonfire. However, this is not the case, so where am I wrong?
Also, where does it stop? For instance, if a bigger spark = more power (which I would seem to), where is the flaw in the likes of splitfire spark plugs (anyone remember these?)/uprated plug leads etc, which offer a bigger-than-standard spark?
Oli.
Why does a car with a failing ignition system produce less power? Given that the spark is only used to ignite the charge, and the rest of the burn takes place from that point on, why does a smaller spark mean less power?
My reasoning is that the spark is only the start of the process, and saying that it causes a power drop is akin to saying that a smaller match leads to a less-hot bonfire. However, this is not the case, so where am I wrong?
Also, where does it stop? For instance, if a bigger spark = more power (which I would seem to), where is the flaw in the likes of splitfire spark plugs (anyone remember these?)/uprated plug leads etc, which offer a bigger-than-standard spark?
Oli.
Not an expert but, to use your analogy, you'll leave a bonfire going for as long as you have to to get it up to temperature. In a cylinder, you have a very small space of time to get your fuel burnt. If the spark is "weak", it'll take longer to ignite the fuel and thus produce a less impressive bang.
A more powerful initial ignition in the cylinder will produce a better flame front propagation, which should in turn lead to more complete combustion over a shorter duration, hence more power. Even better is of course igniting the mixture in several places at once, as in twin spark designs.
nel said:Hmmm, I think that my understanding is why a better spark produces better flame front propagation. The spark is only the start of the process, once it has finished and the burn started, why should this be slower or faster depending upon the size of the spark that started the process?
A more powerful initial ignition in the cylinder will produce a better flame front propagation, which should in turn lead to more complete combustion over a shorter duration, hence more power. Even better is of course igniting the mixture in several places at once, as in twin spark designs.
Or is it just that is is, and I would do better not to worry about the details?
Oli.
no the flame wont travel faster across the paper once burning but on the paper principal you are burning only a small amount with a match as opposed to a bigger piece with the torch, with a cylinder most plugs are in the centre of the bore so another example, two extemes!! a cruddy plug that does fire very well will only ignite the fuel air mix around that plug it is then upto flame front to do the rest, if lightning was to strike the same place it would burn the whole cylinder instantly and not need the flame front would be zero in time!!
zcacogp said:
Analogy:
How quick is it to light a piece of news paper with
a: A match
b: A blowtorch
If you are lighting the corner of a piece of newspaper with a match instead of a blowtorch, will the flames spread across the rest of the piece any slower?
Not once it's got going, but it'll take longer to get to that point. With the blowtorch it'll get going quicker, so the total time to burn the sheet of newspaper will be less.
Also, if the newspaper is a bit damp, the match might not be able to get it going at all. On my MZ I have a twin-spark head powered from a CDI unit giving about three times as much spark energy as the original coil system. The most noticeable effect is in better running at low speeds and throttle settings, where the two-stroke has most difficulty scavenging and there's a lot of exhaust gas left in the cylinder. This makes the bike a lot nicer round town.
Pigeon said:... which brings me on to the second part of my original question - if better spark=more power (which I accept from experience, and the arguments above), is there potential for better results still by building a bigger spark than standard? Pidge's experience suggests that there is.
On my MZ I have a twin-spark head powered from a CDI unit giving about three times as much spark energy as the original coil system.
This is surely what the (now much derided) splitfire plugs were trying to achieve, as well as numerous gadgets and widgets on sale at most car exhibitions and shows (thinking particularly of things that sit in or replace the king lead from the coil to the dizzy.) Do these genuinely produce better sparks? And, if so, are they a good thing on the basis that better sparks=more power?
Oli.
(FWIW, I once bought one of those thingies that sits in the lead from the coil to the distributor, and never noticed any much difference.)
zcacogp said:
(FWIW, I once bought one of those thingies that sits in the lead from the coil to the distributor, and never noticed any much difference.)
I'd have to ask where it would be getting its extra power from - surely you're not going to get a bigger spark without some more 'leccy getting in there? Also, even if somehow it magically boosted the voltage (some kind of step-up transformer), wouldn't running it through the standard 'plugs (designed for the lower voltage) negate any benefit? You'd probably need to ask a aprk plug manufacturer for the low-down.
Continuing on the match/blowtorch analogy.
Is the blow torch lighting the paper faster because it has a HOTTER flame or a LARGER flame?
And, if the flame was HOTTER/LARGE would the paper light any quicker?
Without knowing the physics/chemistry behind it I would say it was probably yes in all cases BUT you need a magnitude of difference. Presumably once the paper is lit and the 'paper flame' (rather than igniting flame) is more than a few centimetres from the flame source then the whatever the flame source is becomes increasingly irrelevant.
In the CDI versus the normal rubbish MZ coil situation, yes you will get a magnitude better spark and hence it will run better when the intake charge is less than optimal.
Likewise going to twin plug (two matches etc) is going to give two flames and half the burn time.
On a modern fuel injected car that already has a distributorless coil and a nicely atomising fuel mixture then you need a MASSIVE improvement to measure or feel anything of significance.
I suspect most of the improvement measured with most of these 'better spark' devices is due to the spark occuring fractionally later. Effectively the same as changing the ignition timing a degree or so which most cars like, providing they don't start suffering from pre-ignition.
In other words, try playing with your ignition timing first, you might get the same increase for free.
Is the blow torch lighting the paper faster because it has a HOTTER flame or a LARGER flame?
And, if the flame was HOTTER/LARGE would the paper light any quicker?
Without knowing the physics/chemistry behind it I would say it was probably yes in all cases BUT you need a magnitude of difference. Presumably once the paper is lit and the 'paper flame' (rather than igniting flame) is more than a few centimetres from the flame source then the whatever the flame source is becomes increasingly irrelevant.
In the CDI versus the normal rubbish MZ coil situation, yes you will get a magnitude better spark and hence it will run better when the intake charge is less than optimal.
Likewise going to twin plug (two matches etc) is going to give two flames and half the burn time.
On a modern fuel injected car that already has a distributorless coil and a nicely atomising fuel mixture then you need a MASSIVE improvement to measure or feel anything of significance.
I suspect most of the improvement measured with most of these 'better spark' devices is due to the spark occuring fractionally later. Effectively the same as changing the ignition timing a degree or so which most cars like, providing they don't start suffering from pre-ignition.
In other words, try playing with your ignition timing first, you might get the same increase for free.
I'd say its a case of if you don't have enough power from the coil to jump the gap you won't get a spark. This is why poor ignition systems often casue misfire at high revs. The coil does not have enough time to build the current to produce a spark in the small time it has between firing at high revs.
Also, within the combustion chamber itself the ignition process actually happens relatively slowly as the 'flame front' gradually ignites the fuel/air mixture. Some parts of the combustion chamber will remain unburnt depending on the shape of squish areas and the location of the plug etc. Systems like the Alfa twin spark use multiple sparks to ensure better combustion of the mixture.
Also, within the combustion chamber itself the ignition process actually happens relatively slowly as the 'flame front' gradually ignites the fuel/air mixture. Some parts of the combustion chamber will remain unburnt depending on the shape of squish areas and the location of the plug etc. Systems like the Alfa twin spark use multiple sparks to ensure better combustion of the mixture.
If I had bothered to keep a copy, I could have sent you my Third-Year Paper on the subject.
As others have already spotted, it comes down to a limited time to complete a chemical reaction in a controlled manner. Factors you need to consider are:
Combustion is a complex, multi-stage, exothermic reaction. The burning material needs to create sufficient energy to initiate and maintain the reaction ahead of it ("propogating the flame front"
. To fully understand it, you need to forget the concept of a flame, and start to think of the chemical reactions happening ahead of, through and behind the flame front, at a molecular level. If you did A-level chemistry, you will recall that in a reaction you have to consider not just the chemical compounds involved (which, by the way, are changing through the process as the fuel molecules break down and recombine with other elements), and the energy available as radiant heat (photons) that modifies the molecular energy levels that can make or break atomic links. Techy stuff.
To initiate the flame, you need to create a kernel of energy capable of initiating combustion (remember that a spark is not a flame) in an environment that is not really prepared for combustion (the mixture is relatively cold, and the pre-cursor chemical reactions are not taking place). Looking on a scale of a sphere of about 2mm diameter (around the plug gap), you have a mixture you wish to burn. You create a spark, which in turn creates a plasma column around it. It is this plasma channel that provides the heat that will initiate combustion, and form that stable kernel of fire. You want that plasma to form as quickly as possible (rather than slowly building up), and create sufficient heat (ie input sufficient energy) to initiate and stabilise the kernel against any vagaries and Uncontrollable variables that might be thrown at it - these include on a microscopic level: variations in mixture from the optimum for flame initiation, quality of the fuel, speed of movement of the mixture, chemical state of the mixture (pressure, temperature, any degree of pre-cursor reactions).
Now it follows that a weak input system (not just the plug) may:
- Take time to build up the necessary charge to generate a spark
- Produce a weak spark. Even if the spark does not collapse too soon (creating a misfire), it will take time to input sufficient energy to create the stable kernel. So you are wasting precious time to complete combustion as you have designed it.
It also follows that there is an threshold amount of energy to put into the spark, so that it will always form the kernel promptly and stably (to how the engine was designed), no matter how sub-optimal all the other conditions are (starting from stone cold, acceptably soiled plugs, incorrect gap, low-grade fuel, etc.). Beyond that threshold, you will gain nothing.
The final phase of combustion is that kernel expanding around the combustion chamber. There are a whole load of issues with this, so just accept that it is "designed" to act in a certain way and speed - it is not just a case of doing it as fast as possible, as that can cause other problems (e.g. excessive NOx emissions, energy wasted as heat into the cylinder wall, optimising thermo-mechanical efficiency).
Now this matters because as we said at the start, you have a fixed amount of time to get that reaction completed. In that time you need to complete three phases:
- Create the spark
- Initiate and stabilise the original kernel
- Allow the combustion to complete in the way you designed it.
In all engine conditions and loads, accommodating all the variables you have accepted, and designed for. The ignition system directly affects two of those phases in both time and quality.
And that's why it can be possible to improve power, efficiency and emissions by optimising the ignition system.
As others have already spotted, it comes down to a limited time to complete a chemical reaction in a controlled manner. Factors you need to consider are:
Combustion is a complex, multi-stage, exothermic reaction. The burning material needs to create sufficient energy to initiate and maintain the reaction ahead of it ("propogating the flame front"
. To fully understand it, you need to forget the concept of a flame, and start to think of the chemical reactions happening ahead of, through and behind the flame front, at a molecular level. If you did A-level chemistry, you will recall that in a reaction you have to consider not just the chemical compounds involved (which, by the way, are changing through the process as the fuel molecules break down and recombine with other elements), and the energy available as radiant heat (photons) that modifies the molecular energy levels that can make or break atomic links. Techy stuff. To initiate the flame, you need to create a kernel of energy capable of initiating combustion (remember that a spark is not a flame) in an environment that is not really prepared for combustion (the mixture is relatively cold, and the pre-cursor chemical reactions are not taking place). Looking on a scale of a sphere of about 2mm diameter (around the plug gap), you have a mixture you wish to burn. You create a spark, which in turn creates a plasma column around it. It is this plasma channel that provides the heat that will initiate combustion, and form that stable kernel of fire. You want that plasma to form as quickly as possible (rather than slowly building up), and create sufficient heat (ie input sufficient energy) to initiate and stabilise the kernel against any vagaries and Uncontrollable variables that might be thrown at it - these include on a microscopic level: variations in mixture from the optimum for flame initiation, quality of the fuel, speed of movement of the mixture, chemical state of the mixture (pressure, temperature, any degree of pre-cursor reactions).
Now it follows that a weak input system (not just the plug) may:
- Take time to build up the necessary charge to generate a spark
- Produce a weak spark. Even if the spark does not collapse too soon (creating a misfire), it will take time to input sufficient energy to create the stable kernel. So you are wasting precious time to complete combustion as you have designed it.
It also follows that there is an threshold amount of energy to put into the spark, so that it will always form the kernel promptly and stably (to how the engine was designed), no matter how sub-optimal all the other conditions are (starting from stone cold, acceptably soiled plugs, incorrect gap, low-grade fuel, etc.). Beyond that threshold, you will gain nothing.
The final phase of combustion is that kernel expanding around the combustion chamber. There are a whole load of issues with this, so just accept that it is "designed" to act in a certain way and speed - it is not just a case of doing it as fast as possible, as that can cause other problems (e.g. excessive NOx emissions, energy wasted as heat into the cylinder wall, optimising thermo-mechanical efficiency).
Now this matters because as we said at the start, you have a fixed amount of time to get that reaction completed. In that time you need to complete three phases:
- Create the spark
- Initiate and stabilise the original kernel
- Allow the combustion to complete in the way you designed it.
In all engine conditions and loads, accommodating all the variables you have accepted, and designed for. The ignition system directly affects two of those phases in both time and quality.
And that's why it can be possible to improve power, efficiency and emissions by optimising the ignition system.
HiRich said:
{clever stuff}
...And that's why it can be possible to improve power, efficiency and emissions by optimising the ignition system.
Great post!
That last scentence sums it all up, really. What we're really talking about is not "getting a better spark improves power" but "getting a good enough spark allows the engine to perform optimally". Anything that degrades the quality of the spark (dodgy plugs, leads, coil etc) loses you power.
I reckon in a fair few cases, it's the other stuff which wears along with the ignition, timing chain, carb jets, dizzy bearings, etc, etc. Spark scatter is a massive cause of power loss.
Also, there is a factor associated with compression ratios and fuel characteristics, how a spark and the timing are critical. This varies between engines but I can't be bothered to explain just now as I'm
knackered.
Also, there is a factor associated with compression ratios and fuel characteristics, how a spark and the timing are critical. This varies between engines but I can't be bothered to explain just now as I'm
knackered.There are a couple of criteria which are pretty much the same for all engines; you want to set your ignition timing by feedback such that peak cylinder pressure occurs 15 degrees after TDC, or maximum acceleration of mass fraction burned occurs at TDC (I read an SAE paper based on the latter approach; they were claiming it was better than the peak-pressure method, but the difference was marginal). Of course the ignition timing required to do this varies between engines and with different operating conditions in the same engine, and the difficulty is that both approaches require some kind of cylinder pressure sensor for the feedback.
At least on a multi-cylinder engine you do... with a single-cylinder engine it strikes me that it should be possible to detect variations in the cyclic acceleration/deceleration of the crank, and I intend to make some experiments on my MZ.
At least on a multi-cylinder engine you do... with a single-cylinder engine it strikes me that it should be possible to detect variations in the cyclic acceleration/deceleration of the crank, and I intend to make some experiments on my MZ.
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